Micro-lavorazioni meccaniche e micro-misure per componenti meccatronici

Transcript

1 POLITECNICO DI BARI DOTTORATO DI RICERCA IN IGNEGNERIA MECCANICA E GESTIONALE XXVIII CICLO DIPARTIMENTO DI INGEGNERIA MECCANICA MATEMATICA E MANAGEMENT Micro-lavorazioni meccaniche e micro-misure per componenti meccatronici PhD Student: Marta PESCE Coordinatore: Prof. Ing. G. PASCAZIO Tutor: Prof. Ing. Luigi M. GALANTUCCI ANNO ACCADEMICO

2 2 Introduzione Il micromachining può essere definito in diversi modi: a seconda della dimensione o della funzione del componente o in base al settore o area di interesse. Il processo in genere comporta la definizione di processi di rimozione di materiale, come il taglio, la lucidatura, incisione, sputtering, processi di rimozione termici e altri. Molti di questi processi sono utilizzati nel settore dei semiconduttori e Micro Electro-Mechanical Systems (MEMS) e tendono ad essere indicati come micro o nano lavorazioni. Per questo studio, il micromachining è strettamente definito come le caratteristiche meccaniche di taglio per ottenere in forma con l'uso di strumenti di dimensioni inferiori a 1 mm con taglienti geometricamente definiti. Tra tutti i processi di microlavorazione, la microfresatura e la microforatura dei metalli sono molto importanti, per applicazioni in diversi settori quali il biomedicale (fresatura di stent coronarici), la fotonica ed ovviamente, la meccatronica (ugelli e iniettori dei motori diesel o altri MEMS ad alto contenuto tecnologico). La lavorazione di geometrie complesse con elevata precisione dimensionale in alcuni materiali, diventa estremamente difficile con i metodi convenzionali di lavorazione ed in particolare quando si parla di componenti miniaturizzati. La produzione di parti in miniatura richiede quindi, l'applicazione di tecniche di micromachining avanzate. Ci sono diversi processi di microlavorazione che possono essere utilizzati per la lavorazione dei metalli. Tuttavia, ulteriori progressi sia per quanto concerne l'efficienza di rimozione, che per quanto concerne la qualità del componente lavorato, sono limitati dalla comprensione incompleta della meccanica di base del processo[1], [2]. Si consideri, ad esempio, la criticità del processo di microforatura degli iniettori dei motori diesel che attualmente vengono realizzati con EDM o la difficoltà di realizzare un micro stampo di forma complessa, per processi di micro-stampaggio ad iniezione. Il micromachining non è semplicemente il ridimensionato delle tecniche di lavorazione convenzionali, ma ha le proprie caratteristiche come l effetto di scala, il dimensionamento del raggio della punta del tagliente, lo spessore minimo del truciolo, produzione di oggetti con superficie di scarsa qualità, formazione di bave ed usura precoce ed improvvisa rottura dell'utensile [3], [4], [5], [6]. Di conseguenza, molti fattori che sono spesso ignorati nei processi di lavorazione tradizionali, diventato significativi per le microlavorazioni e quindi, ottenere le prestazione desiderate in micro-scala, risulta più complicato [7], [8]. La difficoltà diventa ancora più pronunciata quando il pezzo è di un materiale di difficile lavorazione, come ad esempio una particolare lega di titanio molto utilizzata in applicazioni aerospaziali e mediche per il suo alto rapporto durezza/peso e per le caratteristiche di resistenza alla corrosione e biocompatibilità [9]. Lo studio e la comprensione dei fenomeni che regolano il processo di taglio a livello micro, sono dunque la base per migliorare la produzione di componenti meccatronici sia a livello qualitativo che quantitativo. L obiettivo ultimo del presente lavoro di ricerca è quello di realizzare parti con geometria complessa, con caratteristiche di lavorazione aventi dimensioni comprese tra m, ottenute per asportazione di truciolo con un utensile di diametro inferiore ad 1mm e con taglienti geometricamente definiti. Con lo scopo di perseguire l obiettivo prefissato, si è deciso di focalizzare lo studio di questo primo anno di ricerca, sul processo di micro-foratura meccanica e sul tema delle misure di alcuni componenti micro-lavorati, attraverso l utilizzo di macchine di misura commerciali ed home-made.

3 3 Stato dell arte Per le applicazioni nel campo della meccatronica, EDM (Electro Dischange Machining) e laser drilling sono le tecnologie attualmente più utilizzate per la produzione di micro-fori, malgrado presentino problemi relativi a zone termicamente alterate, che facilitano la formazione di bave all ingresso e all uscita degli orifizi. In particolare, l EDM è ampiamente utilizzato per applicazioni nell'industria aerospaziale per la produzione di fori di raffreddamento sulle palette delle turbine. Questa tecnologia, tuttavia, presenta alcuni problemi legati alla rifusione del materiale nelle zone colpite dal calore e gli alti costi degli utensili. Inoltre, la velocità di perforazione dell EDM è molto bassa (0,0125 mm/s) se confrontata con la foratura meccanica (>0,25mm/s) ed il laser drilling (>1000mm/s). KUPPAN et al. [10] hanno sperimentato un aumento del tasso di asportazione (MRR) quando, durante l operazione di foratura, l elettrodo è messo in rotazione: all aumentare della velocità di rotazione dell elettrodo il MRR aumenta, mentre la finitura superficiale raggiunge un punto di ottimo, e poi peggiora. Ottimizzando i parametri di processo, per la realizzazione di micro-fori (diametro medio 130μm) su Inconel 718, il massimo MRR è stato ottenuto con una rugosità superficiale di circa 3-3,5 μm (Fig. 1). Pertanto il tempo di micro-foratura con EDM può essere ridotto, ma a scapito della finitura superficiale. Fig. 1. Immagini SEM dell entrata di micro-fori ottenuti con EDM, per differenti velocità di rotazione dell elettrodo. L applicazione della tecnologia laser per la lavorazione di micro-fori, invece, presenta problemi di scarsa qualità superficiale, causata dalla presenza di uno strato di materiale rifuso ed una zona termicamente alterata (Fig. 2). Il processo di micro-foratura laser diventa ancora più compresso se la parte in lavorazione è cava e si rischia, dunque, il danneggiamento della parete posteriore [11]. Fig. 2. Micro-fori realizzati su Inconel 718 con laser pulsato Nd:YAG (lunghezza d onda 1064 nm), durata della pulsazione 0.3 ms, assistito con gas alla pressione di 5 bar. Evoluzioni della micro-foratura laser in continuo verso tecniche ad impulso a percussione o trapanazione, hanno permesso di migliorare la qualità superficiale dei componenti lavorati [12]. Tuttavia, pur operando con durate dell impulso dell ordine del nanosecondo, non è possibile trascurare i fenomeni di alterazioni termiche.

4 4 È stato dimostrato che nella foratura di materiali metallici, la profondità di penetrazione dipende dalla diffusività termica e dalla durata dell'impulso e quindi, che è possibile migliorare la finitura superficiale, riducendo la durata dell'impulso, fino ad arrivare ad impulsi ultra-corti [13, 14]. Infatti, come è possibile notare in Fig. 3, i fori realizzati con tecnologia di micro-foratura laser ad impulsi dell ordine del nanosecondo, presentano una maggiore quantità di materiale rifuso rispetto a quelli prodotti con tecnologia di micro-foratura laser ad impulsi dell ordine del femtosecondo. Fig. 3. Altezza delle bave di lavorazione per processi di micro-foratura laser ad impulsi su acciaio [13] Per la lavorazione di micro-fori con aspect ratio maggiore di 10, la tecnologia ad asportazione di truciolo risulterebbe essere più adatta rispetto alle tecnologie EDM e laser drilling, poiché permetterebbe di ottenere una migliore qualità della superficie lavorata e maggiori precisioni dimensionali. Tuttavia anche la tecnologia di micro-foratura meccanica non è esente da problemi: l oscillazione dell utensile o le vibrazioni di lavorazione, potrebbero causare un diverso punto di contatto tra l utensile ed il pezzo (rispetto a quello programmato). In una tale situazione, forze di taglio non previste durante la progettazione del processo, potrebbero dare come risultato una superficie di scarsa qualità e/o la rottura prematura dell utensile. Vi sono, infine, soluzioni ibride al problema della micro-foratura, che combinano le tecnologie appena descritte. In [17] gli autori propongono un processo ibrido per la realizzazione di micro-fori, combinando la tecnologia di laser drilling con la tecnologia ad asportazione di truciolo, eseguendole in modo sequenziale. Il risultato è un foro esente dai tipici difetti di alterazione termica associati al processo di foratura laser, e con bave di altezza minore rispetto a quelle ottenibili con un processo di micro-foratura, che utilizza esclusivamente la tecnologia ad asportazione di truciolo. La vita dell utensile aumenta, ma aumentano anche i tempi di lavorazione. Un ulteriore soluzione ibrida, che combina le tecnologie laser ed EDM, è stata proposta da Li et al. [18]. Per la realizzazione di fori di diametro inferiore a 140 μm, gli autori hanno riscontrato una riduzione del 70% nel tempo di foratura ed un aumento, quindi, della capacità di produzione del 90%, rispetto ad un processo di micro-foratura che utilizza esclusivamente la tecnologia EDM. Principale problema che accomuna le soluzioni ibride sono gli elevati costi.

5 5 Il processo di micro-foratura meccanica Capire e controllare un processo di micro-foratura meccanica è molto complesso poiché esistono una moltitudine di fattori che influenzano il risultato. Il primo problema da affrontare nello studio di un processo di micro-foratura meccanica, riguarda la complessità dell interazione utensile/pezzo, nel momento in cui questi vengono a contatto. Quando si opera su scala microscopica, il problema è più critico che in un tradizionale processo ad asportazione di truciolo a livello macroscopico. Esistono, infatti, problemi legati alla prematura rottura della punta ed al movimento oscillatorio della stessa, causati dalla geometria (utensile snello) ed alle elevatissime velocità di rotazione del mandrino (min giri/min) [15]. Il problema della rottura prematura dell utensile è ancora più accentuato per operazioni di micro-foratura su superfici non planari, poiché in queste condizioni, l utensile è sottoposto a una forza con una elevata componente nella direzione ortogonale al suo asse, applicata nel punto di contatto con la superficie. Questa forza tende a far slittare la punta dell utensile che si troverà, quindi, in una posizione diversa da quella programmata e che causerà una sua inflessione, nonché una sua probabile rottura[16]. Tra i parametri che influenzano l interazione utensile/pezzo, e quindi la qualità della lavorazione, l angolo di inclinazione tra l asse dell utensile e la superficie da lavorare, risulta di particolare importanza. In [19] Weinert and Surmann hanno proposto un algoritmo di simulazione basato su analisi geometriche delle varie condizioni di interazioni del tagliente con la superficie, per ottimizzare tale parametro. Fig. 4. Confronto tra i risultati ottenuti con un centro di lavoro CNC a 3 assi comandati ed un centro di lavoro CNC a 5 assi comandati Gli autori hanno riscontrato che al variare dell angolo di inclinazione dell utensile, si riscontra una variazione dello spessore del truciolo. Viene quindi generata una variazione del gradiente delle forze di taglio ed una variazione del profilo delle velocità di taglio. Biermann et al. [20] hanno verificato l algoritmo di ottimizzazione di fresatura meccanica in un processo di foratura, su barre a sezione circolare (Ø34.1 mm) in NiTi. Gli esperimenti sono stati condotti su un centro di lavoro CNC a 5 assi comandati, utilizzando utensili sferici Ø1.0 mm in carburo cementato, rivestiti di TiAlN. La strategia tipica di un programma a controllo numerico per una

6 6 fresatura a 3 assi produce una bassa finitura superficiale (profilo con elevata rugosità e zone di incollaggio tra utensile e pezzo), dovute ad un crescente angolo di inclinazione tra l utensile e la superficie da lavorare. La strategia a 5 assi produce un foro con una qualità superficiale decisamente migliore (Fig. 4) e questo è la prova dell influenza dell angolo di inclinazione utensile/pezzo e della validità dell algoritmo di ottimizzazione presentato in [19] per controllare la qualità della superficie controllando l inclinazione dell utensile durante la lavorazione. La qualità del componente lavorato e l'affidabilità del processo di microlavorazione sono fortemente dipendenti dal dispositivo che si utilizza. Utensili di taglio di precisione e macchine utensili sono fondamentali nei processi di microlavorazione meccanica, poiché piccole vibrazioni e forze eccessive, influenzano fortemente la qualità della superficie lavorata. La maggior parte dei centri di lavoro per microlavorazioni meccaniche sono simili alle macchine per lavorazioni meccaniche convenzionali ultra-precise ad alta rigidità, ed operano in ambiente controllato [21]. Fig. 5. Centro di lavoro ultra preciso KERN Evo a 5 assi controllati I centri di lavoro per microlavorazioni meccaniche disponibili attualmente sul mercato, hanno un controllo multi-asse, con vari gradi di libertà, che consentono di produrre piccole parti con texture e geometrie complesse, nonché modelli di superfici in micro scala come stampi e matrici. Le lavorazioni per condurre la sperimentazione, saranno realizzate utilizzando un centro di lavoro CNC ultra preciso Kern Evo a 5 assi controllati (Fig. 5) che sarà disponibile presso il laboratorio Microtronic del DMMM del Politecnico di Bari. Questo centro di lavoro può sviluppare una potenza di 6,4kW, sufficiente per lavorare acciaio temprato e altri materiali caratterizzati da elevata durezza. Le elevate accelerazioni (8m/s2) e velocità di avanzamento (fino a mm/min) consentite dalla Kern Evo consentono di lavorare con ridotti tempi di processo. Il centro di lavoro consente di lavorare oggetti aventi dimensioni massime di 300 X 280 X 250 mm, ed è corredato da un sistema per il caricamento automatico del materiale, con una precisione di posizionamento di ± 0,5µm. Infine la qualità superficiale certificata da Kern per questo centro di lavoro è inferiore a 0,1 µm.

7 7 Fig. 6. Pattern di micro-fori Nell attesa che il centro di lavoro sia disponibile presso il dipartimento, si è pensato di sfruttare il mandrino a giri montato su una tradizionale fresatrice a tre assi controllati, per realizzare delle operazioni semplici di micro-foratura su alluminio anodizzato. Le lavorazioni che si intendono realizzare consistono in due pattern di fori: uno con fori aventi diametro Ø0.60mm ed un altro con fori aventi diametro Ø0.30mm, come quello presentato in Fig. 6. La profondità di penetrazione è pari allo strato di ossido deposto sulle piastre (~20µm). A tale scopo sono state scelte delle frese cilindriche con codolo rinforzato in metallo duro, rivestite con TiALN, con le seguenti specifiche: D1 rispettivamente di 0.60mm e 0.30mm, D = 3.00mm, L = 38mm, Z = 2 (Fig. 7). Queste punte sono in grado di lavorare una moltitudine di materiali tra cui acciai legati ed inox austenitici, ghise, titanio e sue leghe, alluminio e sue leghe, plastica, leghe di rame difficili da lavorare e metalli nobili. Fig. 7. Utensile per la realizzazione del pattern di micro-fori Per ogni materiale esiste un valore di velocita di taglio Vc espressa in m/min, dalla quale dipende la velocità di rotazione del mandrino. Supponendo una durezza dello strato di ossido da asportare > 250HB, la velocità di taglio Vt suggerita dal fornitore per questo utensile è 70 m/min - 90m/min. Tuttavia questo, risulta un valore esageratamente elevato, considerata la piccola quantità di materiale che verrà asportata. Si è deciso pertanto, di scegliere una velocità di taglio <40m/min. La velocità di rotazione del mandrino la si può calcolare come: n = vt x 1000 / d x π [giri/min] Indicando con Fz l avanzamento al dente e con Z il numero di taglienti dell utensile, la velocità di avanzamento al giro Vn è il prodotto tra la velocità al dente per il numero di denti. La velocità di avanzamento Vf la si ottiene moltiplicando la velocità di avanzamento al giro per il numero di giri, o equivalentemente come: Vf = Fz X Z X n Infine, il tempo di taglio per un foro, può essere calcolato come: Tc = Im / Vf [min]

8 8 avendo indicato con Im la profondità di penetrazione del foro. In Tab. 1 sono indicati i parametri di taglio scelti per realizzare le due lavorazioni. Parametri di taglio Fori Ø0.60mm Fori Ø0.30mm Velocità di taglio Vc 40 m/min 20 m/min Rotazione del mandrino n giri/min giri/min Avanzamento al dente fz 0,0060 mm 0,0030 mm Numero di denti Z 2 2 Avanzamento al giro Vn 0,0120 mm/giro 0,0060 mm/giro Velocità di taglio Vf 255 mm/min 127 mm/min Tempo di taglio Vc 0,005 s 0,009 s Tab. 1. Parametri di taglio per la realizzazione dei micro-fori Le operazioni di micro-foratura che si intendono realizzare per asportazione di truciolo, sono già state realizzate con tecnologia laser. L intento è quello di confrontare il risultato di una micro-foratura meccanica con quello di una micro-foratura laser, e verificare quindi quanto studiato in bibliografia circa la qualità delle due tecnologie di lavorazione. Realizzazione di un componente con caratteristiche microscopiche Principale obiettivo della scuola estiva frequentata a Copenaghen presso la DTU, è stato quello di progettare e realizzare un micro-dispenser che funga da cartuccia per una stampante 3D a polimero fotosensibile. Le specifiche del dispositivo da realizzare sono riportate di seguito: Viscosità del polimero Pa s. Deposizione di gocce con volume massimo di 0.1mm 3. Frequenza di dosaggio minima 1 Hz. Ingombro Massimo del dispositivo 17 mm x 17 mm Temperatura del polimero all ingresso C e temperatura ambiente 25 C La tecnologia di fabbricazione deve risultare economicamente conveniente per produzione su larga scala Fig. 8. Modello CAD del prototipo di cartuccia per stampante 3D a polimero fotosensibile. In Fig. 8 è riportato il progetto del prototipo: si tratta di un dispositivo in policarbonato con tre diversi circuiti di pompaggio, realizzabile tramite tecnologia di micro stampaggio ad iniezione. I circuiti contengono ognuno tre camere di pompaggio, profonde 200 μm e con diametro Ø 3mm. L ultima camera è passante, ed ha una rastremazione conica che termina in un piccolo foro che funge da ugello. I tre circuiti sono caratterizzati da ugelli con diverse dimensioni (Ø 120 μm, Ø 220 μm, e Ø 320 μm), poiché questo consente di utilizzare lo stesso

9 9 dispositivo per diverse viscosità di fluido, in accordo alle specifiche richieste. Il foro più piccolo, cieco, ha una geometria a scalino, che funge da camera di raccolta del fotopolimero in ingresso. Il progetto prevede l incollaggio di una sottile e flessibile membrana plastica (spessore 200 µm) sulla superficie superiore del dispositivo, e l inserimento di una cannula Ø 1.2mm attraverso la membrana, al fine di realizzare un collegamento meccanicamente stabile e privo di perdite, per far fluire il polimero dal serbatoio al dispositivo. Fig. 9. Stampo ottenuto con tecnologia di micro-lavorazione meccanica. Lo stampo utilizzato per il processo di micro stampaggio ad iniezione (Fig. 9), è stato realizzato con tecnologia di micro-lavorazione meccanica ad asportazione di truciolo, utilizzando la Mikron UCP 600. Come materiale si è scelto un acciaio indurito (32-36 HRC) per le sue buone proprietà termiche e di resistenza all usura. Il grezzo di partenza aveva dimensioni 19X19X6mm. Il ciclo di lavorazione prevedeva dunque una operazione di sgrossatura per la rimozione del sovrametallo, una fase di fresatura della superficie progettata, foratura e alesatura dei due fori centrali, barenatura per ottenere la rastremazione conica ed infine una operazione di finitura. In Tab. 2 sono riportate le specifiche degli utensili e dei parametri di taglio utilizzati. Operazione Utensile Spindle Speed [rpm] Avanzamento [µm] Sgrossatura Fresa end mill Ø1 mm Fresatura Fresa end mill Ø0.8 mm Finitura Fresa sferica Ø0.8 mm Foratura Punta a forare Alesatura Alesatore Ø2 mm Barenatura NC Boring Tab. 2. Utensili e principali parametri di taglio utilizzati per la lavorazione dello stampo.

10 10 Il prototipo realizzato, completo di membrana elastica e cannule, è riportato in Fig. 10: il polimero viene fatto confluire nella camera di raccolta attraverso una cannula, e viene condotto all ugello tramite un sistema di pompaggio a micro-attuatori piezoelettrici. Regolando la frequenza di oscillazione dei micro-attuatori, è possibile regolare il flusso di polimero. In appendice è riportato il report completo della progettazione e realizzazione del prototipo. Misure di micro-componenti Fig. 10. Prototipo completo di cartuccia per stampante 3D a polimero fotosensibile Le operazioni di misurazione dei componenti che saranno lavorati con il centro di lavoro ultra preciso, dovranno essere realizzate con attrezzature che rispondono a specifici requisito. Pertanto, è stato effettuato uno studio per la definizione delle procedure e dei metodi per la misura, il rilievo superficiale ed il controllo delle superfici micro lavorate, al fine di individuare le necessità di misura dei componenti micro-lavorati. Quando si opera su scala microscopica, le necessità metrologiche possono essere suddivise principalmente in tre gruppi: Strutture 2D (aspect ratio < 1), strutture 2½D (aspect ratio >= 1), e strutture tridimensionali 3D (aspect ratio >> 1). Fig. 11. Strumenti per la misura di parti micro-lavorate

11 11 Fra tutte le tecnologie disponibili per la misurazione di parti micro-lavorate (Fig. 11), coerentemente con le necessità degli oggetti che verranno misurati, si è scelto ti utilizzare strumenti che fossero in grado di realizzare misure verticali ed orizzontali comprese nel range di m e aspect ratio da 1:1 a 1000:1. Pertanto, per la misurazione delle parti micro lavorate, è stata utilizzata la tecnologia della micro topografia a scansione laser confocale. (a) (b) Fig. 12. Provini utilizzati per le operazioni di misurazione Misurazioni con tecnologia micro-topografica In Fig. 12 sono riportati i provini scelti per le operazioni di misura. La Fig. 12 (a) riporta un provino realizzato mediante tecnologia Fused Deposition Modeling con la FDM Il file Stl utilizzato per la fabbricazione del provino è stato ottenuto attraverso un operazione di reverse engineering di un benchmark, tramite sistema di scansione a testa laser e braccio articolato. In Fig. 12 (b) invece, è riportato un prototipo di cartuccia per stampante 3D a polimero fotosensibile, realizzato mediante tecnologia di micro injection molding, durante la scuola estiva frequentata a Copenaghen. I provini risultano particolarmente interessanti e coerenti con il tema della presente ricerca, in quanto presentano caratteristiche di dimensione microscopica, nonché superfici tridimensionali complesse (free-form). Fig 13. Ricostruzioni 3D dei provini ottenuti con scansione micro-topografica Le misurazioni con tecnologia micro-topografica sono state realizzate con uno scanner Optimet, allo scopo di caratterizzare forma e dimensione dei provini. Questo strumento utilizza la scansione ottica con microscopia

12 12 confocale e fornisce informazioni di tipo quantitativo sulle altezze, rispetto alla posizione della dimensione misurata. La microscopia laser a scansione confocale è una tecnica ottica basata sul principio di rilevazione mediante messa a fuoco, grazie al quale ogni singolo elemento della superficie è soggetto a rilevazione. In questo modo è possibile misurare fino a 500 punti al secondo in modo dinamico. È uno strumento adatto per realizzare misure con elevata precisione, accuratezza e ripetibilità, senza contatto, su superfici tridimensionali in metallo, plastica, e gomma. Cambiando la lente sulla sonda, si possono misurare con esattezza, lunghezze che vanno da 1µm ai 200mm. In Fig. 13 sono riportate le ricostruzioni tridimensionali dei provini mostrati in Fig. 12. Provino (a) La scansione del provino (a) è stata realizzata con il sensore ConoProbe Mark 3.0, utilizzando una lente da 50mm che consente di ottenere punti con una precisione di 6µm ed un discreto working range (8mm). Allineando il modello tridimensionale al piano XY del Sistema di Coordinate, e sezionandolo con un piano parallelo al piano XZ, si è ottenuta la Sezione 1. Il provino è stato così caratterizzato per forma e dimensione: l altezza misurata per i gradini è di circa 250 µm, mentre l altezza complessiva è di 2mm nella parte più bassa, e di 4.3 mm nella parte più alta. Fig. 14. Sezione 1 del modello del provino (a) ottenuto con tecnologia micro-topografica

13 13 Sezione 1 Nome Altezza [µm] Nome Altezza [µm] H2 250 H7 267 H3 244 H8 265 H4 254 H9 268 H5 266 H H6 245 Tabella 3 Misura delle altezze dei gradini del provino (a) per la Sezione 1. Sezionando il modello con un piano parallelo al piano YZ, si è ottenuta invece, la Sezione 2. Anche per questa sezione, l altezza de i gradini è di circa 250 µm. L altezza complessiva è di 2mm nella parte più bassa, e di 4.3 mm nella parte più alta. Fig. 15. Sezione 2 del modello del provino (a) ottenuto con tecnologia micro-topografica Sezione 2 Nome Altezza [µm] Nome Altezza [µm] H H H H H H H H H Tabella 4. Misura delle altezze dei gradini del provino (a) per la Sezione 2. Provino (b) La scansione del provino in Fig 12(b) è stata realizzata utilizzando il sensore NanoCono Probe, adatto per l acquisizione di oggetti con superfici riflettenti. In questo caso è stata scelta una lente 25 mm che consente di

14 14 rilevare i punti con una precisione di 0.5µm e risoluzione di 0.1µm, a scapito di un ridotto working range (1mm). Di seguito sono riportate le misure di alcune caratteristiche micrometriche del provino. Allineando il modello tridimensionale al piano XY del Sistema di Coordinate, e sezionandolo con un piano parallelo al piano XZ, si è ottenuta la Sezione 1. Il provino è stato così caratterizzato per forma e dimensione: l altezza misurata per gli alloggiamenti dei micro attuatori piezoelettrici è di circa 200 µm (Tab. 5). Fig. 16. Sezione 1 del modello del provino (b) ottenuto con tecnologia micro-topografica Sezione 1 Nome Altezza [µm] Nome Altezza [µm] H1 201 H4 200 H2 198 H5 190 H3 187 H6 202 Tabella 5. Misura della profondità degli alloggiamenti per i micro attuatori del provino (b) per la Sezione 1.

15 15 Sezionando il modello con un piano parallelo al piano YZ, si è ottenuta invece, la Sezione 2. Anche per questa sezione, l altezza degli alloggiamenti dei micro attuatori piezoelettrici risulta di circa 200 µm (Tab. 6). Inoltre, nell analisi di entrambe le sezioni, la misura dei diametri degli alloggiamenti dei micro attuatori piezoelettrici risulta di 3mm coerentemente con le specifiche di progetto. Fig. 17. Sezione 2 del modello del provino (b) ottenuto con tecnologia micro-topografica Sezione 2 Nome Altezza [µm] Nome Altezza [µm] H7 200 H H8 199 H H9 193 H Tabella 6. Misura della profondità degli alloggiamenti per i micro attuatori del provino (b) per la Sezione 2.

16 16 Misurazioni con tecnologia Micro-fotogrammetrica Dai risultati ottenuti, è evidente che la tecnologia micro-topografica incontra in modo soddisfacente, le necessità di misura dello studio condotto nel presente lavoro di ricerca, ma ad un costo relativamente alto (Optimet Miniconoscan 3000, con un costo di , risulta essere tra gli strumenti di misura più convenienti). A tal proposito si è pensato di realizzare un sistema di misura 3D, basato su una tecnologia capace di dare buoni risultati in termini di precisione, bassi costi, con discrete capacità dimensionali di scansione, e buone profondità di campo. Sfruttando le competenze nel campo fotogrammetrico, acquisite grazie a lavori in campo meccanico e biomedicale, si è pensato di sfruttare tale principio per costruire un microscanner composto da: Il sistema realizzato si compone dei seguenti principali elementi: Tavola rotante motorizzata tramite un motore elettrico a corrente continua, e controllata attraverso un controllo numerico e retroazione ad encoder ottico; Fotocamera Canon EOS 40D, con obiettivo macro 56mm; Un computer per controllare sia la fotocamera che il movimento della tavola; Un flash anulare ed una struttura a 16 led, montati a sbalzo sulla tavola rotante e solidale ad essa, che forniscono una adeguata ed uniforme illuminazione dell oggetto da scansionare. Fig. 18. Modelli 3D dei provini, realizzati con scansione micro-fotogrammetrica, senza texture (in blu) e con texture.

17 17 Si posiziona l oggetto da scansionare sulla tavola, e si avvia la rotazione mediante il controllo numerico. Il sistema di acquisizione cattura i fotogrammi mentre l oggetto è in rotazione. I led, solidali alla tavola rotante, assicurano una illuminazione uniforme dell oggetto in tutte le immagini, mentre il flash anulare, offre una perfetta esposizione durante lo scatto. Tramite un programma in codice G ed un software di gestione della fotocamera, è stato possibile rendere il processo di acquisizione completamente automatico: programmando una sosta per ogni rotazione pari a 10 della tavola rotante, è quindi possibile realizzare un acquisizione tridimensionale dell oggetto a 360. Provino (a) La scansione del provino in Fig 12(a) è stata realizzata con obiettivo macro 65mm. Le immagini sono state scattate con un tempo di esposizione pari a 1/100 ed una sensibilità ISO 200. La risoluzione delle immagini è di 3888X2592 pixels. Analogamente a quanto fatto in precedenza, anche in questo caso le ricostruzioni tridimensionali sono state sezionate e caratterizzati per forma e dimensioni. Fig. 19. Sezione 1 del modello del provino (a) ottenuto con scanner micro-fotogrammetrico

18 18 Sezione 1 Nome Altezza [µm] Nome Altezza [µm] H2 223 H7 267 H3 259 H8 250 H4 255 H9 259 H5 259 H H6 255 Tabella 7. Misura delle altezze dei gradini del provino (a) per la Sezione 1. Fig. 20 Sezione 2 del modello del provino (a) ottenuto con scanner micro-fotogrammetrico Sezione 2 Nome Altezza [µm] Nome Altezza [µm] H H H H H H H H H Tabella 8. Misura delle altezze dei gradini del provino (a) per la Sezione 2. Provino (b) La scansione del provino in Fig 12(b) è stata realizzata con obiettivo macro 65mm. Le immagini sono state scattate con un tempo di esposizione pari a 1/100 ed una sensibilità ISO 400. La risoluzione delle immagini è di 3888X2592 pixels. Di seguito si riportano le misure delle altezze degli alloggi per i micro-attuatori piezoelettrici, così come è stato fatto anche precedentemente sul modello ottenuto con scansione micro-topografica. Da specifiche di progetto la profondità di tali alloggi è di 200 µm, mentre il diametro è di 3mm.

19 19 Fig. 21. Sezione 1 del modello del provino (b) ottenuto con scanner micro-fotogrammetrico Sezione 1 Nome Altezza [µm] Nome Altezza [µm] H1 197 H4 207 H2 187 H5 155 H3 158 H6 233 Tabella 9. Misura della profondità degli alloggiamenti per i micro attuatori del provino (b) per la Sezione 1.

20 20 Fig. 25. Sezione 2 del modello del provino (b) ottenuto con scanner micro-fotogrammetrico Sezione 2 Nome Altezza [µm] Nome Altezza [µm] H7 191 H H8 202 H H9 197 H Tabella 10. Misura della profondità degli alloggiamenti per i micro attuatori del provino (b) per la Sezione 2.

21 21 Analisi dei dati Per testare l accuratezza dello Scanner Micro-Fotogrammetrico realizzato, sono stati condotti confronti bidimensionali e tridimensionali, delle ricostruzioni ottenute con i due sistemi di scansione. Analisi bidimensionale È stata condotta quindi, un analisi della deviazione delle misure, realizzate sulla ricostruzione tridimensionale dei provini, ottenuti con i due sistemi di scansione. Come è possibile notare, le misure presentano una deviazione media di circa 10 µm - 20 µm su entrambi i modelli, con deviazione massima di 35 µm. Il risultato è alquanto soddisfacente se si considera che, in questo primo stadio di ricerca, l ottica è stata utilizzata senza calibrazione. Le distorsioni sono state compensate tramite un algoritmo di compensazione utilizzato dal software di elaborazione delle immagini, impiegato per la costruzione dei modelli tridimensionali. Fig. 26. Scostamenti dei valori misurati sui modelli 3D del provino (a) nella Sezione 1 e nella Sezione 2. Fig. 27. Scostamenti dei valori misurati sui modelli 3D del provino (b) nella Sezione 1 e nella Sezione 2.

22 22 Analisi tridimensionale Una ulteriore analisi per testare l accuratezza dello Scanner Micro-Fotogrammetrico realizzato, è il confronto 3D dei modelli ottenuti con i due sistemi di scansione. Come si può vedere in Fig. 28, lo scostamento medio misurato complessivamente sul modello tridimensionale del provino (a) è di 120µm. Essendo questo un valore medio, è fortemente influenzato dai valori minimo e massimo misurati. In particolare, si vuol far notare che le zone in blu scuro ed in rosso scuro, indicano rispettivamente, gli scostamenti massimi negativo e positivo del modello ottenuto con il Micro-Scanner Fotogrammetrico, rispetto al modello ottenuto con l Optimet (100µm). Queste zone sono maggiormente concentrate nella periferia del modello. Infatti, per il basso overlap tra i fotogrammi (Fig. 29), la ricostruzione è meno precisa in questi punti. Inoltre, è possibile notare che l errore maggiore viene commesso sugli spigoli dei gradini, indice di un operazione di smoothing nella fase di costruzione del modello, con acquisizione micro-fotogrammetrica. Ad ogni modo, ignorando le zone periferiche del modello, l errore commesso nella ricostruzione del modello, è inferiore a 40 µm. Fig. 28. Mappa di colore del provino (a) effettuata con Optimet 3000 e con lo Scanner Micro-Fotogrammetrico Fig. 29. Schema delle posizioni assunte dalla fotocamera e di overlap delle immagini

23 23 Lo scostamento medio misurato complessivamente sul modello tridimensionale del provino (b) invece, risulta leggermente superiore (200µm). La zona in verde, che indica uno scostamento <10 µm è piuttosto estesa. Tuttavia la ricostruzione ottenuta con lo scanner micro-fotogrammetrico risulta imbarcata lungo una diagonale. Infatti nella parte centrale del modello è evidente una zona in azzurro chiaro (circa -25 µm) con picchi di 100 µm in alcuni punti, mentre due dei quattro spigoli sono in giallo (25 µm) e arancione (40 µm). inoltre, anche per questa ricostruzione, come nel caso del provino (a), i bordi risultano leggermente arrotondati. Fig. 30. Mappa di colore del provino (b) effettuata con Optimet 4000 e con lo Scanner Micro-Fotogrammetrico

24 24 Bibliografia [1] X. Lai, H. Li, C. Li, Z. Lin, J. Ni, Modelling and analysis of micro scale milling considering size effect, micro cutter edge radius and minimum chip thickness, International Journal of Machine Tools & Manufacture 48 (2008) [2] T. Thepsonthi, T. Özel, Experimental and finite element simulation based investigations on micromilling Ti-6Al-4V titanium alloy: Effects of cbn coating on tool wear, Journal of Materials Processing Technology Volume 213, Issue 4, April 2013, Pages [3] M.C. Shaw, Precision finishing, CIRP Annals 44 (1) (1995) [4] C.-J. Kim, J. Rhett Mayor, J. Ni, A static model of chip formation in micro scale milling, ASME Journal of Manufacturing Science and Engineering 126 (2004) [5] X. Liu, M.P. Vogler, S.G. Kapoor, R.E. DeVor, K.F. Ehmann, R. Mayor, C.-J. Kim, J. Ni, Microendmilling With meso-machine-tool system, NSF Design, Service and Manufacturing Grantees and Research Conference Proc., Dallas, TX 1 (2004) 1 9.M.P. [6] Vogler, R.E. Devor, S.G. Kapoor, Microstructure-level force prediction model for micro-milling of multi- phase materials, ASME Journal of Manufacturing Science and Engineering 125 (2004) [7] A. Dhanorker, T. Özel, Meso/micro scale milling for micro-manufacturing, International Journal of Mechatronics and Manufacturing Systems (2008) [8] C.D. Torres, P.J. Heaney, A.V. Sumant, M.A. Hamilton, R.W. Carpick, F.E. Pfefferkorn, Analyzing the performance of diamond-coated micro end mills International Journal of Machine Tools and Manufacture (2009) [9] S.I. Jaffery, P.T. Mativenga, Assessment of the machinability of Ti-6Al-4V alloy using the wear map approach, International Journal of Advance Manufacturing Technology (2009) [10] Kuppan P, Rajadurai A, Narayanan S (2008) Influence of EDM Process Parameters in Deep Hole Drilling of Inconel 718. International Journal of advanced Manufacturing Technology 38(1-2): [11] Li L, Low DKY, Ghoreshi M, Crookall JR (2002) Hole Taper Characterisation and Control in Laser Percussion Drilling. CIRP Annals Manufacturing Technology 51(1): [12] C. Fornaroli, J. Holtkamp, A. Gillner (2013) Laser-beam helical drilling of high quality micro holes. Lasers in Manufacturing Conference 4:

25 25 [13] Friedrich Dausinger, Helmut Hügel, Vitali Konov ( ) Micro-machining with ultrashort laser pulses: From basic understanding to technical applications a Stuttgart University, Institut für Strahlwerkzeuge (IFSW), Pfaffenwaldring 43, D Stuttgart, Germany1 b General Physics Institute, Moscow, ,Vavilova str., 38, Russia [14] Pal Molian, Ben Pecholt, Saurabh Gupta (2009) Picosecond pulsed laser ablation and micromachining of 4H-SiC wafers. Applied Surface Science 255: [15] Man Sheel C, Dong-Woo C, Ehmann KF (1999) Identification and Control for Micro-Drilling Productivity Enhancement. International Journal of MachineTools & Manufacture 39(10): [16] Chae J, Park SS, Freiheit T (2006) Investigation of Micro-Cutting Operations. International Journal of Machine Tools and Manufacture 46(3-4): [17] M.M. Okasha, P.T. Mativenga, N. Driver, L. Li (2010) Sequential laser and mechanical microdrilling of Ni superalloy for aerospace application. CIRP Annals - Manufacturing Technology 59 (2010) [18] Li L, Diver C, Atkinson J, Giedl-Wagner R, Helml HJ (2006) Sequential Laser and EDM Micro-Drilling for Next Generation Fuel Injection Nozzle Manufacture. CIRP Annals Manufacturing Technology 55(1): [19] K. Weinert and T. Surmann, Geometric (2003) Simulation of the Milling Process for Free Formed Surfaces. Simulation Aided Offline Process Design and Optimization in Manufacturing Sculptured Surfaces, K. Weinert, Ed., p [20] D. Biermann, F. Kahleyss, E. Krebs, and T. Upmeier (2010) A Study on Micro-Machining Technology for the Machining of NiTi: Five-Axis Micro-Milling and Micro Deep-Hole Drilling. Journal of Materials Engineering and Performance 20(4 5): 751. [21] Y.B. Bang, K. Lee, S. Oh (2004) 5-Axis micro milling machine for machining micro parts. Advanced Manufacturing Technology

26 26 Appendice Design and production of a microfluidic dispenser for additive manufacturing using photopolymers Micro mechanical system design and production (PhD summer school)

27 27 1 Introduction Project management Design process Concept development Functional requirements Constrains Selection criteria System-Level Design Design solutions Concept screening First iteration Second iteration Third iteration Fourth iteration Fig.3.5 Fourth iteration device design Design considerations Calculations Critical features and requirements Dimensions and tolerances References Tooling Design of the Tool Process Chain Direct tooling vs. indirect tooling Selection of Tool Material Selection of Manufacturing Method CAM & Production References Polymer Introduction... 47

28 Polymer selection Requirements to the polymer Theoretical aspects of PC Shrinkage of molded PC Demolding Force Injection molding of the part Parameter optimization References Additive manufacturing Introduction D Printing for actuators Actuator 1: camshaft Design and construction of the 3D printed actuator Actuator 2: push/pull solenoid with piston Introduction to linear variable differential transformer Piston displacement measurements with LVDT Functional tests on the device References Joining Techniques and LISA Methods Selection of method Experimental setup Results from the experiments on the final structures Comments from the actuation results Conclusion References Metrology Introduction Critical features and metrological equipment Conforming the measurements tools to the standards... 73

29 Statistical Process Control and monitoring Measurements Part measurements The mold Discussion of the results References Conclusion and outlook Conclusion Outlook: Appendix Appendix Metrology Calculation of the uncertainty budget Measurements results Appendix

30 30 1 Introduction This report was made for course Micro mechanical system design and production. The main objectives of the project work were the design, manufacturing and test of a micro fluidic dispenser for application as a cartridge in 3D photopolymer printer. The initial demands to the device were: Should dispense fluid with a viscosity in the range of Pa s. Allow fluid supply in finite volumes of maximum drop volume 0.1mm 3. Minimum dosage frequency of 1 Hz. Should consist of passive/active elements Produced by means of low cost replication technologies (polymer injection molding) Maximum area of the device is 17 mm x 17 mm Temperature of fluid at inlet of C and ambient temperature of 25 C 2 Project management The preliminary work on the design was performed by varying between work in random groups (diverging) and work in plenum (converging). After deciding on an overall design concept the class was divided into 6 groups, each with their area of responsibility: 1. Design coordination, modeling and project management a. Design optimization and incorporation of input from other groups (after the initial determination of overall design performed in plenum) b. Determination of critical features and tolerances c. Coordination of project 2. Tooling a. Evaluation on possibilities for fabrication of injection molding insert b. Define tooling process chain c. Select process parameters and machining strategy 3. Polymer a. Selection of suitable polymer for device fabrication b. Determine optimal injection molding parameters c. Determine shrinkage and impact of this on the tool design d. Ensure de-molding by estimating the de-molding force and changing tool design accordingly e. Mold devices 4. Additive Manufacturing a. Set up test solution for prototypes and the final device b. Fabrication and test of prototype for validation of device functionality c. Validate functionality actuation system d. Design and fabricate new mechanical actuation system using additive manufacturing if primary actuation system fail

31 31 5. Joining and LISA a. Evaluate different solutions for joining the various parts of the device b. Select the best suited joining solutions c. Optimize and validate the selected solutions d. Join the devices e. Verify the use of LISA for sensor and/or actuators integration 6. Metrology a. Identify features characteristics critical for the functionality/performance b. Selection of suitable metrological equipment for characterization of critical features c. Define a suitable measuring strategy for the quality control of parts and insert d. Conduct performance measurements for qualification of insert and parts The groups worked individually on their tasks every afternoon from to In this timespan communication between the groups were conducted directly for optimal work efficiency. In order to summarize and inform on the work progress of all the groups a common meeting was held at every day. At the meeting the latest changes to the device design was furthermore presented, evaluated and updated to fulfill the demands set forward by all the groups. In order to facilitate knowledge sharing the internal network at DTU (CampusNet) was used for data exchange. Furthermore the blackboard in the class room was converted to an information board were the current status and work tasks of the groups were updated regularly. This made it possible for everybody to be updated, even if working most of the time at a different location. 3 Design process During the course of this project a photopolymer dispenser has been designed and produced in accordance with the design process illustrated in Figure 3.1. Figure 3.1: Design process [3.1]. The planning was mainly done by the course responsible before the beginning of the course and thus the work presented in this report starts with the concept development. During this phase the functional requirements along with constrains and selection criteria were considered and reviewed continuously in order to obtain a general design concept. Afterwards, the system-level Design was done by performing a concept screening, see section 1.4. The concept screening resulted in a single product design that was utilized for the initial considerations in the work groups. Following the initial group work, it was adjusted continuously until a final design was made (design phase) Friday evening and sent for production (initial build phase) Monday morning. The building phase stretched from Monday to Wednesday when the devices were assembled. The tests were

32 32 initiated as the first devices were finished and continued Thursday. As the course is only two weeks there was no room for iterations and thus no production ramp-up. 3.1 Concept development In the following a short description of the functional requirements, constraints and selection criteria are presented. These were used for developing a general concept before moving on to the design phase Functional requirements The functional requirements are a set of requirements that describes the input, behavior and output of a system. In the case of the microfluidic dispenser considered in this project the primary functional requirement is that the cartridge should be able to facilitate a flow of photopolymer and in the end divide the flow into individual droplets of a finite size and at a constant frequency. Open system o Flow in o Flow out Droplet formation/flow control o Actuation Local pressure increase Non-capacitive device polymer o Leek prove channel o No backflow Valve Droplet control o Frequency o Volume Block UV-light Constrains After having determined the initial functional requirements of the microfluidic dispenser, the constrains were considered. Most of these were set beforehand and were mainly used to revisit the functional requirements and add the feature of not allowing UV light to enter the device. Polymer device Handle photopolymer Viscosity: Pa s Max. drop volume: 0.1 mm³ Minimum dosage frequency: 1 Hz Maximum area: 17x17 mm Temperature of fluid at inlet: C

33 33 Ambient temperature: 25 C Selection criteria After having determined both the functional requirements and the constrictions of the dispenser an initial design proposal could be proposed. However, some selection criteria had been found during the previous work. These are listed below and mainly affect the material choice and method. Cheap large scale fabrication method (e.g. polymer injection molding or hot embossing) Cheap polymers Availability of tools Ease of manufacture Time taken to manufacture Robustness of the device 3.2 System-Level Design After having determined the functional requirements, constraints and selection criteria for the device a design had to be found. Initially different solutions were found to support the demands to the device, see section Next a concept screening was performed by, in turn, brainstorming in groups (diverging) and discussing in plenum (converging) Design solutions In order to fulfill the functional requirements several solutions were proposed for each individual requirement. These were the basis for the following concept screening. Table 3.1 Morphological table for different functional requirements Functional Requirement Means to achieve the functional requirement Flow in Leur connector Needles Flow out (design of outlet) Conical Constriction of walls Cylindrical Rectangular Flow out (placement of Side Bottom outlet) Actuators Piezoelectric Mechanical deflection Electrostatic Electro magnetic Leakage proof Peristaltic pump External pump Droplet frequency control Actuators External pump Droplet volume control Deflection of thin film Hydraulic resistance based Pressure in the External pump on channel dimensions channel Blockage of UV light Right device material Packaging Painting Protective layer Based on the functional requirements, a morphological table (Table 3.1.) was made where different methods to accomplish each requirement were discussed. Then based on the selection criteria, one or a combination of methods was selected (green in Table 3.1.). Keeping this in consideration, the final design was made. For example, in order to pump the liquid inside the device, two methods were proposed: peristaltic pump (left) and external pump (right) as shown in Fig Considering the ease of manufacture and robustness of the device, the peristaltic pump design was chosen.

34 34 Fig Two different pump designs. 3.3 Concept screening During concept screening, various rounds of designing the device were conducted. During the fourth and final iteration of the designing and screening process, final device design was chosen. After the screening, the device was further tuned to get the exact design that fulfills the functional requirements along with critical features, their dimensions and tolerances. Here is the concept screening process: First iteration During the first iteration, brainstorming of the main functional requirements estimate of the design was performed and an initial estimate of the device design was made. Some of the designs that came out of this iteration are illustrated in Fig Fig Initial device designs.

35 Second iteration Fig Second iteration of device designs. The second iteration was done after defining the functional requirements in detail and brainstorming on the various selection criteria and device constraints. This time the class was divided in the group of four to come up with five various designs, which are shown in Fig. 3.3 After each design was presented, the most complicated design (Fig. 3.3 (bottom)) was discarded as it did not meet the selection criteria of ease and time of manufacturing. Then the other four designs were basically group into two basic designs of 1) use of pump and no thin film membrane (Fig. 3.3 (top))and 2) use of no external pump to control droplet volume and flow but use of actuators to do so with a thin film membrane (Fig. 3.3 (center)) Third iteration After the five designs made in the second iteration, one was rejected and rest four were divided in two groups. The class was divided into two groups working on each concept: 1. External pump group and 2. Actuator and thin film group. Three designs came out from this iteration (Fig.1.4.3). One (Fig (bottom left)) was rejected based on the complexity of the design failing the ease and time of manufacturing criteria. The rest two designs were discussed in detail and finally the actuator and thin film design was chosen.

36 36 Fig. 3.4 Third iteration of device designs Fourth iteration The fourth iteration gave us a benchmark for final design (Fig ). The design consisted of an inlet through-hole with a channel designed to align three actuators. The channel is supposed to be sealed by a thin film membrane and actuation needs to be done on this membrane to control the volume, flow and leakage of the photopolymer fluid. Fig.3.5 Fourth iteration device design The fourth iteration design went through many rounds of re-designing. It was refined further and various features and parameters of the design were explored and discussed. Finally, Fig was fixed as the final design. The final device consists of one molded plastic part with actually three different pumping circuits. A 200 µm thick thin film membrane is supposed to be used to seal the channels of 200 µm depth and 400 µm height between the pumping sites. The inlet is realized by 1.2 mm thick cannulas which will be punched through the polymer thin film and glued to the device for leakage-free and mechanically stable connection. For connecting the cannulas the device is equipped with matching cavities. Each cavity contains a stop, so that a fluidic connection between the cannula and the first pumping chamber is ensured. The cannulas can be equipped with flexible plastic hoses for connecting the reservoir. Peristaltic pumping was considered to control the flow of the fluid in the channel. Therefore, the circuits contain three pumping chambers of 3 mm diameter

37 37 each. The pumping chambers are 200 µm deep. The last pumping chamber is conically tapered through the entire device ending in a tiny through hole which functions as the actual nozzle. The nozzle diameters are chosen as 120 µm, 220 µm, and 320 µm. This enables to try the behavior of three different sizes on the same chip. Since the nozzle is by far the narrowest fluidic structure, it determines mainly the fluidic resistance of the device and the necessary pressure to shot out the drops. Having three different nozzles provides flexibility in the hydraulic resistance of the nozzle, so that the nozzle can be changed according to the viscosity of the handled fluid. The mold insert can be derived as the negative of the part structure and will be discussed in more detail in the tooling chapter (chapter 2). The drawings of the insert for production can be found in the appendix (Appendix A). Fig. 3.6 Final device design after fine tuning of fourth iteration device design 3.4 Design considerations To realize the functionality of the device it is necessary to avoid the leakage but also realize the volume flow by using the activators. For the first the burst pressure of micro valve has to be greater than hydrostatic pressure. For the second the pressure realized through the activators has to be over the burst pressure Calculations Micro capillary burst valve Two pressures act on the liquid/air interface at the ejection nozzle. On the one hand there is the pressure of liquid column that presses on the liquid surface and on the other hand the pressure of air due to surface curvature, resulting out of surface tension and contact angle. Assumptions for calculating the hydrostatic pressure that presses onto the liquid surface at the ejection nozzle. Assumptions: - Volume flow is vertical (Z-direction -> worst case) - Nozzle is fixed in height - Outlet of the monomer tank is on the same height as the device inlet) - Reservoir filling level h f = 30 mm - Channel height h c = 3 mm - Density of water = 1200 kg/m 3

38 38 - Gravity g = 9.81 m/s 2 - Contact angle β = 90 (output of the nozzle) - Diameter of the nozzles: d 1 = 120 µm, d 2 = 220 µm, d 3 = 320 µm - Thermal expansion = 70*10-6 1/K Measured values: - Surface tension = 73 mn/m (typical value for water to PC in literature 31.0 mn/m [xy]) - Contact angle = 72 (typical value for water on PC in literature 87.2 [xy]) Results: Hydrostatic pressure can be calculated to be p hyd = 390 Pa. The calculated burst pressure for assumed nozzle diameters is between approx to 2300 Pa. So the compliance is ensured. For further optimization a diagram of burst pressure to channel diameter is shown in Appendix Test of the surface tension effect of fluid in micro channel As the surface tension affects the fluid flow greatly in micro size, surface tension effect is tested. The experimental setup is shown in Fig Bottom Plate Flow inlet Gravity direction Covering tape Channel Fig. 3.7 Experimental setup for test Since the fabrication of the channels with the same dimensions as the final target is not accessible, the test was done with the available PC fluidic channel chips. Tape was used to cover the channels. The channel width of 0.8mm with channel height of 0.3mm is tested. A syringe is used to insert the liquid into the channel through the inlet. The result shows that the liquid coming through the channel will not fall down under gravity force due to the surface tension. However, if pressure is applied in the inlet, there will be flow to the outlet. Therefore, we can reach to the conclusion that as long as there is no actuation, no liquid will leak out through the channel with a channel width of 0.8mm. Furthermore, channel width smaller than 0.8mm will also hold the liquid [3.3] Deflection of membrane The peristaltic pump system is based on three different chambers with flexible membranes realized as polymer thin film. The membranes are deflected by applying an external force which is provided by an actuator. Due to the requirements for the amount of pumped fluid, the necessary membrane deflection can be estimated. Assumptions:

39 39 - The deflected membrane has the shape of a spherical cap. - The whole displaced fluid will be pumped in forward direction. Results: The necessary deflection for displacing the maximum target volume of 0.1 mm³ can be calculated as w = 30 µm. Since not all displaced volume is pumped in forward direction, the respective deflection must be higher to meet the requirements. For a deflection of 100 µm, the displaced volume is about V = 0.35 mm³. Nonetheless, the given target volume is an upper limit. As a result, smaller displacements are acceptable, too. Moreover, the actually displaced volume might be smaller, because the membrane does not describe a perfect spherical cap Critical features and requirements There are multiple critical features of the nozzle device which are either crucial for the functionality of the system or the success of the process chain. The following features must be particularly considered for the design of the part and mold insert: Ejection nozzle (diameter, shape) Channels (depth, width, shape of cross-section, roughness) Pumping chamber (diameter, depth, spacing) Edges perpendicular to flow direction Ejector holes (diameter, roughness) Elevated plateau (dimensions) The ejection nozzle is the core element of the device. The diameter is very important as it decides about the burst pressure of the capillary micro valve. Moreover, the shape of the nozzle influences the release and shaping of the droplet. The pumping chambers of the peristaltic pump must be large enough to allow the membrane to deflect without breaking and to provide enough space to couple the actuators. The deflection is again linked to the volume that must be pumped. The spacing between the chambers should be short to prevent from high hydraulic resistance of the channels in between. Any edges being perpendicular to the flow direction must be rounded. Sharp edges are favorable locations for the formation or pinning of bubbles inside the channel which might block or tremendously hinder the flow. The mold insert is fixed in the mold by a frame with an open window for the plastic of 17 x 17 mm². The protruded plateau of the mold insert must meet this size very precisely. If the gap due to alignment between the frame and the insert is too big, polymer will be able to flow inside. This results in flash which might affect or hinder the bonding. The upper tolerance is chosen as 0 µm. If the plateau is larger, the frame cannot be mounted on the insert. The lower tolerance is chosen as -40 µm. This will ensure on the one hand the mounting and on the other hand a very tight fitting. The ejector holes are not a functional part of the nozzle device, but they contribute to the robustness of the system. They must meet requirements regarding their diameter, because they have to fit very precisely to the

40 40 ejector pins. If the gap is too large, polymer could flow into the gap during injection. This results in flash which can negatively influence or completely prevent from bonding. The roughness of the ejector holes must not be too large. Otherwise, the friction between holes and pins will increase significantly. A larger ejection force will be thus necessary, so that a higher risk of damaging or deforming the part arises Dimensions and tolerances Tolerances state in general the range of a property of an object in which changes caused by a disturbing influence does not affect the functionality of the object. Some of the aforementioned critical features must therefore be equipped and manufactured with tolerances in order to keep correct functionality and meet the actual design. Uncritical features do not need any tolerances, as the machining precision can be assumed as being sufficient. Introducing too tight tolerances will be very costly and time-consuming. The diameter of the ejector nozzle will become larger due to the shrinkage of the plastic. Here, very tight tolerances are necessary to stay within the calculated specifications. To counteract the shrinkage the upper and lower tolerances of the nozzles are chosen as +0 µm and -10 µm. This will result in high demands on the machining precision. Nonetheless, the time consumption is little as the structures cover only a small part of the whole machining. The roughness should be R z =1 µm to prevent from big changes in contact angle and thus in the valve capillary pressure. The depth and width of the channels are not as crucial as the size of the nozzle. It can be assumed that the machining precision is sufficient for providing a tolerance that still enables the capillary filling of the channels. The viscosity is given in a range of Pa*s (water like) to 0.2 Pa*s (maple syrup like). The higher viscosity is more critical with regards to the hydraulic resistance. However, it can be assumed that the actuators provide enough pressure to drive the flow even with a higher hydraulic resistance of the channels. Furthermore two The usually achievable tolerances of few micrometers are small compared to the size of the channels. The roughness is however very important and hence chosen as R z =2 µm. The risk of large changes in contact angle and thus in capillary pressure increases with increasing roughness. The diameter of the pumping chambers was chosen to be 3 mm. This size is necessary for sufficient deflection of the membrane. Moreover, it provides enough space to avoid the necessity of a very precise and elaborate alignment of the actuators. The safety margin is large enough, so that the usual achievable machining tolerance is sufficient. It will be negligible compared to the large size of the chamber. The spacing is chosen as 4 mm to allow the rounding of the channel edges between the chambers. On the other hand, the channel length and hydraulic resistance is minimized. The narrowing between the chambers is favored over one broad channel with three different actuation sites to guide the flow. The diameter of suitable standard ejector pins is given as 2 mm g6 [3.2]. Due to the possible flash issues, the diameter of matching ejector holes was chosen as 2 mm G6. Roughness requirements to the ejector holes were not stated in particular, as it can be assumed that the machining process will yield a roughness that will not tremendously increase the friction.

41 41 The radius of the fillet of the edges is chosen as 0.5 mm. This allows using only one tool diameter for the fine machining and it will not be necessary to change tool and realign. The tool diameter of 1 mm is still large enough to provide sufficient mechanical stability to prevent from bending of the tool leading to a loss of machining precision. 3.5 References [3.1] Ulrich, K. T. and Eppinger, S.D. Product Design and Development. McGraw-Hill, 2008 [3.2] accessed [3.3] [Ref] D.S. Kim, Micro-channel filling flow considering surface tension effect, J. Micromech. Microeng, vol. 12, pp (2002)

42 42 4 Tooling Tooling is an important part of many manufacturing processes. While producing micro-components using micro-injection moulding, the designing and production of an appropriate tool insert is essential for ensuring accurate feature generation on the moulded part. However, unlike traditional macro-scale manufacturing, tooling for micro-components have a tighter tolerance and require better surface quality. In this section we discuss the strategies adopted for creating the tool for micro-injection moulding a micro-fluidic droplet dispenser for an additive manufacturing device. 4.1 Design of the Tool The tool for micro-injection moulding was designed using the CAD model of the desired product. The microinjection moulding setup allowed a maximum tool size of 19mm X 19mm X 6mm. However, the clamping mechanism for the tool reduced the available area to 17mm X 17mm only. Appropriate tolerances and roughness values were prescribed after identifying the critical functional features of the final component. The critical features and the associated tolerance and/or roughness values were: Roughness value of bottom wall of micro-channel: The micro-channel bottom surface needed to have a good surface roughness to allow the product to function as designed. Thus, a maximum roughness (Rz) of 2 microns was planned. Tolerance of output nozzle: The output nozzles were designed to allow outflow of the fluid under certain pressure. As the micro-channel device would undergo shrinkage when cooling after moulding, the nozzle is expected to increase in diameter. However, the amount of tolerance to accommodate this phenomenon is small and thus considerations were made based on the manufacturability and the time required for achieving the tolerance. Thus only a negative tolerance of 10 microns was allowed on the tool, which would correspond to a negative tolerance of the nozzle diameter. Roughness of nozzle wall: The output nozzle as mentioned earlier is a critical functional feature. Thus, a maximum roughness (Rz) of 1 micron was prescribed for the region of tool corresponding to the inner walls of the nozzle. The micro-injection moulding setup makes use of two different sets of ejector pins for removing the product after moulding. However, based on preliminary simulations of demoulding process (Section), the inner ejector pins were chosen to be used. This implied a need for having two through holes of 2mm diameter near the middle region of the tool insert. As flashing of polymer is an undesired phenomenon, tight tolerances were given for the ejector pin holes. The 3D CAD drawing of the tool is shown in Fig 4.1 and the corresponding engineering drawing with dimensions and tolerances is shown in Appendix 2.

43 43 Figure 4.1 3D CAD drawing of the tool. 4.2 Process Chain After the tool design was made, the manufacturing process chain for achieving it had to be planned. The key steps involved in producing the tool were the selection of tool material, selection of tool manufacturing method and the planning of the production steps Direct tooling vs. indirect tooling While producing a tool for micro-injection moulding, an important decision is whether to use direct tooling or indirect tooling. Direct tooling is when the tool is manufactured directly from a CAD drawing [Fig 4.3] and has a negative geometry with respect to the original part to be manufactured. This sort of tooling can be achieved either with an additive process or subtractive process. The additive manufacturing processes, such as Direct Metal Laser Sintering or Fused Deposition Modeling, add material in a layer by layer manner to build the tool. The subtractive processes, such as milling and Electro-Discharge Machining, involve removing material from a block of bulk material. Indirect tooling is where a tool is made by an in-between step. First a positive master is produced. From this master, a negative mold is formed, which is then used as a tool for the polymer replication process, see Fig 4.2. The advantage of using direct tooling is the lower manufacturing time. Direct tools are thus preferred when the structures on the tool are easy to manufacture using standard techniques such as micro-milling, micro-forming or micro-edm The disadvantage is the limited robustness of the tool; direct processes gives tools that can be used approx. 5,000 times [4.1]. A second disadvantage is the limitation on the materials that can be used for direct tooling, which might be unsuitable for injection moulding certain polymers. The primary advantage of using indirect tooling is that the tools are stronger and more robust. Therefore, typically a significantly larger volume of injection-moulded units can be produced with each tool. The most

44 44 apparent disadvantage is that it involves extra manufacturing step. This increases the cost of production for the parts and is thus only suitable when the number of parts is high or when the tool cannot be manufactured using direct tooling. Since the requirement for the current micro-fluidic device is only 100 pieces, direct tooling is sufficient for this project. Figure 4.2: Direct tooling and indirect tooling Selection of Tool Material The material for the tool should have the following characteristics: 1. Good heat conductivity: The tool would need to transport heat away from the process so the polymer repetition process is not influenced by heat. 2. Thermal stability: The tool should not undergo any phase transitions during the polymer repetition process. 3. High wear resistance and surface hardness: The tool should be able to withstand high and repeated loadings. For polymer device replication the hardness of the tool needs to be between approx. 80 and 550 HV. The material for the tool was chosen to be hardened steel (Impax supreme, Uddeholm) [4.2]. This kind of steel has a high degree of homogeneity and isotropic hardness. The hardness is listed in Table 4.1 and shows a sufficient hardness for injection molding. Furthermore, the density is only changing from 7,800 kg/m 3 to 7,750 kg/m 3 when the temperature changes from 20 o C to 200 o C, and similarly the elastic modulus is only changing from N/mm 2 to N/mm 2 over the same temperature range. This indicates high thermal stability. The heat conductivity of the steel is 28 W/m K ± 15%, which is intermediate, but possible high enough to transfer heat in the injection molding process away from the polymer. Table 4.1: Hardness of the tooling material hardened steel (Impax supreme) from Uddeholm. Hardness Brinell HB Vickers HV Rockwell HRC Rockwell HRB Impax supreme ~ ~32-36 ~

45 Selection of Manufacturing Method The choices for primary manufacturing method involved micro-milling and micro-edm. The considerations for choosing between the methods are provided below. 1. High geometrical freedom: When the tool can be made can be in 3 dimensions, the need for multiple tools are less likely. 2. High accuracy: The tolerance of the tool is partly defining the tolerance of the injection molded device. 3. High surface quality: The surface quality requirements are dependent on the component, the repetition process and the joining method. 4. High surface integrity: The integrity means the amount of micro cracks, tensile stresses and heataffected zones. These effects should be in a low amount as possible. 5. Capability to machine hard materials: Since the tool material should be of a highly hard material, the tooling process should be able to work with these kinds of materials. The not so important considerations of the tooling methods include productivity and cost. This is because the tool is only made once and is used over and over again. This is in contrast to the polymer replication process which needs to be highly productive and cheap, since the polymer device needs to be made in a high number. The tooling method chosen is micromilling. This is a subtractive method, where a rotating screw is removing one layer at a time and thereby revealing a structure. This structure will be the tool for the injection molding. Micromilling is chosen because the structures that we want are in the hundreds of micrometer to millimeter, and the tolerances are not too strict. Also since there are structures that go all the way through the polymer, a lot of material needs to be removed to form the tool. For this, the micromilling method is optimal. 4.3 CAM & Production Upon selection of micro-milling as the manufacturing method, the appropriate machining tools and the processing sequence had to be chosen. The CAD model for the tool was sent to the micro-machining workshop for development of process plan and production. The CAD geometry was used to create a processing sequence involving 6 different tools. Based on the tools used, appropriate processing parameters were chosen using a database which provided the operating ranges. This information was used to create a final processing plan which was simulated and validated using a CAM software. The six different tools and their operating parameters are listed in Table 4.2. Table 4.2 Tools used during micro-milling of insert. Tool Type Tool Speed (rpm) Tool Feed (μm) NC Boring Drilling Reaming Φ2H End Mill Φ

46 46 End Mill Φ Ball Mill Φ The 1 mm diameter end mill is used for arbitrary stock roughing operation where a 19 mm X 19 mm X 6 mm block of Impax is machined to remove a large portion of material. The roughing tool path is generated with a roughing offset of 1 mm. Subsequently, the 0.8mm diameter end mill is used for contour milling of the designed structures and for finishing operations. The 0.8 mm ball mill is used for z-level finishing operation. The holes for the ejector pins are created with the drilling tool with peck drillling and then produced to accurate size with a 2mm diameter reaming tool. The boring tool is used to create a tapered hole for the nozzle. The tool insert was finally created on a Mikron UCP 600 milling machine. The manufactured tool is shown in figure 4.4. Figure 4.4: Manufactured tool insert for the micro-injection moulding process 4.4 References [4.1] Patrick Grant and Stephen Duncan. Spray-on-steel. Ingenia, 2005, Issue 25, pp [4.2] Data sheet from Uddeholm - Impax supreme.

47 47 5 Polymer 5.1 Introduction Injection molding is one of the most widely used processes for large scale plastic manufacturing. The master can be repeatedly used with faithful replication. Injection molding utilizes a ram or a screw-type plunger to force molten polymer into a cavity. This molten polymer will conform to the contour of the mold. Parts that are to be injection molded have to be carefully designed to facilitate the process. The material used for the part, the desired shape and features of the part, the material of the mold, and the properties of the molding machine must all be taken into account. Often, thermoplastics polymers are used in this process. The process itself is a combination of time, temperature and pressure variables with a multitude of manufacturing defects that can happen without the correct processing parameters and design components [1]. In this section, the details of the polymer replication of the designed component using injection molding will be covered. Both the requirements to the polymer and the processing parameters are covered using analytical, numerical and empirical tools. 5.2 Polymer selection The microdevice will be molded by injection molding on a suitable thermoplastic polymer. The following polymers are of interest to us, due to their impermeability to visible (and thereby probably also UV) light and their suitability in micro injection molding: (i) Acrylonitrile butadiene styrene (ABS) (ii) Polycarbonate (PC) (iii) Polypropylene (PP) (iv) COC (TOPAS) Other polymers could possibly also be used, but only the four listed candidates are considered. 5.3 Requirements to the polymer The four candidates must be analysed based on the requirements for the final device. Table 5.1 shows the list of main requirements that the polymer is required to have. The requirements have been ranked according to the importance to the final device functionality, 5 being an essential requirement and 1 being less important. A few of the requirements should be elaborated on: The heat deflection temperature is meant as a lower limit, ensuring that the device will not deform during normal operation. The requirement on electrical conductivity is stated in order to facilitate integration with an electrical actuation mechanism. Finally, amorphous polymers are preferred over semi-crystalline polymers due to their lower shrinkage during cooling. Based on the results obtained, PC (black colour) is the most preferred material for the bulk of the device. The molded structures on the PC will then be bonded to a thin foil, which does not necessarily need to be of the same material. The material of choice will be investigated in the bonding section of the report.

48 48 Table 5.1 Tabulation of the necessary requirements for the four different kinds of polymers often used in injection molding. 5.4 Theoretical aspects of PC In injection molding processes, software packages are often used to predict the process [2]. Often, the molded parts are at the risk of being subjected to defects at the ejection stage. Numerous authors have investigated the parameters that influence the ejection forces. Shen et. al. have reported that increasing diameter and thickness of the part will result in higher demolding forces. Packing pressure [4], demolding temperature [4, 5], cooling time and roughness [6] are all interlinked. These parameters not only contribute to the ejection forces, they also influence them. Charmeau et. al. had also reported on investigating the application of different mold surface coatings at the ejection process. In this section, we will also be discussing the demolding force based on mathematical calculations and simulations of the deformation of the molded PC part. The software, Comsol Multiphysics (ver 4.3), was used to perform the simulations. First, the expected contraction of the polymer was estimated to evaluate the potential impact on the design of the mold tool. Then, approximate analytical estimates and detailed simulations of the actual tool and polymer part were performed to investigate the impact on the demolding on contraction. The former is used to supply order-of-magnitude estimates of the demolding force, while the latter is used to evaluate the optimal ejector pin positioning and potential weak points of the tool Shrinkage of molded PC In order to evaluate the requirements of the mold, the expected shrinkage of the polymer needs to be taken into account. According to the polymer specifications, the shrinkage should be in the order of 0.7 % at maximum. Thus, at a distance,, from the center of contraction, the shrinkage will be The relative change in channel dimensions will therefore be a function of the channel location with respect to the center of contraction. Assuming that the center of contraction coincides with the geometrical center of the

49 49 part, the channel will experience the largest distortion if it is placed near the edge. For a channel of width 400 µm, the relative geometrical distortion for a 17 mm wide part will thus at the most be Assuming the channel is placed in the center of the mold, the estimated relative geometrical distortion will be: For the worst case scenario of 14 %, the channel distortion is 56 µm. However, as the channel functionality is not critically impaired by such a change, it is decided not to compensate for the shrinkage in the tool manufacturing. This would also require very accurate simulations of the molding process Demolding Force The shrinkage can also be used to evaluate the demolding forces. According to the polymer specifications, the stress at 85 C (the molding temperature) and a strain of 0.7 % (the maximum expected strain) is in the order of 10 MPa. The corresponding normal force on a channel of height h and length L is thus Note that this is independent of the channel position,, relative to the center of contraction. Note also that the force is calculated only on one channel wall. Only walls with surface normals pointing away from the center of contraction will be affected. Assuming a static friction coefficient of 0.3, the force on the channel walls during demolding will therefore be The order of magnitude for the demolding force as a function of channel height and length is sketched in Fig To estimate the optimal position of the ejector pins, a simulation of the final designed part was set up in Comsol, see figure 5.2. It was found that when the ejector pins are closer to the center of the part, the simulated maximum stress on the part is much lesser than when they are located at the ejector parts. Comsol was also used to simulate the same stresses acting on the tool insert. It was observed that the stress was no higher than 34 MPa, i.e. lower than the typical yield strength of steel. Thus, it was estimated that even when relatively fragile pins were to be made for the through-hole connections, the tool insert would be able to withstand the demolding forces. Figure 5.3 shows the simulation of the eventual part. No investigations on the effect of clamping force on the pins were made, as it was assumed that the tool is made is such a way that it will not collide with the mold.

50 50 Fig. 5.1 Estimation of the required demolding forces as a function of channel height displayed for three different channel lengths. Note that the simulations are approximate as they use a rough estimate of the friction forces. In order to achieve more accurate results, the actual stresses in the molded part should be evaluated using numerical software such as MoldFlow. However, in order to model micro injection molded parts, precise knowledge and modeling of the entire mold and injection nozzle is a requirement to get usable results. Therefore, this has not been done. Fig 2.2 Simulation of the demolding forces for two different positions of the ejector pins. The left figure shows the ejector pins located at the ejector parts, and the right figure shows the situation with the ejector pins located at the center of the part.

51 51 Fig 5.3 Simulation of the demolding forces on the mold tool. 5.5 Injection molding of the part Parameter optimization As mentioned earlier, the injected molded part is dependent on several parameters during injection molding. Therefore, it is necessary to optimize the following parameters: the injection volume, packing time, packing pressure and molding temperature. Below is the flow of the optimization process, we will be following when molding the desired part. Optimisation and flow process of injection molding: 1. No packing. Increase the injected volume until 98 % filling. This defines the final injection volume. 3. Set the packing pressure to 40 % or 80 %. 4. Increase packing time until the weight of the piece becomes constant. 5. Check the surface/channel quality. 6. If channels are not replicated well, increase the mold temperature and repeat the process. The ENGEL machine was used in the injection molding process.initially, an injection temperature of 75 C was experimented and the surface quality of the part was poor. However, once the mold temperature was increased to 85 C, the surface quality improved hugely. This was therefore used for molding the parts. The next parameter that we had to optimize was the injection volume by adjusting the stroke length. Effects of injection stroke length on the injected molded part The injection molding machine was operated at a cooling time of 15 s and an injection speed of 50 mm/s. To investigate the effect of the injection stroke length, packing pressure was not applied. The injected molded pieces from different injection stroke lengths were collected and compared visually.

52 52 (a) Figure 5.1 (b) Effects of injection stroke length on the eventual part. (a) Stroke length = 10mm; (b) Stroke length = 10.5mm; (c) Stroke length = 11mm (c) As seen from figure 3.1, the volume and shape of the injected molded part is highly dependent on the injection stroke length. With increasing injection stroke length, the part will be filled to a higher degree. The optimal stoke length fills roughly 98 % of the part, leaving a small portion for the subsequent packing. Our investigations had showed that the optimized injection stroke length is 11 mm as seen in Fig. 3.1 c. Effects of packing time on the injected molded part Having optimized the injection stroke length, the next step was to optimize the packing time. Keeping the cooling time at 15 s, injection speed at 50 mm/s, we added a 40 % of packing pressure which corresponded to 44 bars. The percentage is taken with respect to the maximum injection pressure, being 110 bars. 3.2 Injection molded device (a) (b) Figure 3.2 Effects of packing time on the surface finish of the injected molded part. (a) Packing time = 0.5 s; (b) Packing time = 2 s The packing time was increased gradually and the molded parts were weighed. Once the weight of the molded parts stagnated, the time that the part first reached the plateau weight was chosen. As can be seen from figure 3.3, this corresponds to a packing time of 2 seconds. The packing time also affected the finished surface of the injected molded part. As seen from figure 3.2, with a shorter packing time, the surface of the part showed shrinkage voids, thereby affecting the flatness of the surface. This shrinkage, even in non-visible amounts, is easily verified by using the before mentioned weighing procedure.

53 53 The last parameter we optimized was the packing pressure. The packing pressure was increased to 80 % which corresponds to 88 bars. We also varied the packing time for this pressure to obtain the optimum packing time. Figure 5.3 Effect of packing time with respect to the measured weight. As seen from Fig 5.3, for both packing pressure, the optimum packing time was at 2 s. Fig 5.3 also showed that with increasing packing pressure, the weight of the part also increased. With an optimum packing time of 2 s, at 80 % packing pressure, the weight of the part was g whilst at 40 % packing pressure, the weight was g. The 80 % packing pressure was chosen, as the injected molded part showed a better and smoother surface finish and better filling as evidenced by the higher weight. Table 5.1 shows the tabulated optimized injection molding parameters that were used to produce the parts. Table 3.1 Optimised parameters for the injection molding process.

54 References 1. Pantani, R. and Titomanlio, G., The Simulation of Post-Filling Steps in Injection Molding, in Injection Molding Fundamentals and Applications, M.R. Kamal, A.I. Isayev and S.-J. Liu, Eds., Carl Hanser Publisher, Cincinnati, OH (2009). 2. Vietri, U., Sorrentino, A., Speranza, V., Pantani, R., Improving the Predictions of Injection Molding Simulation Software, Polymer engineering and science, 2009, 51, 12, Shen, K., Chen, L-M., Jiang, L., ʻCalculation of ejection force of hollow, thin walled, and injection molded cones, Plastics, rubber and composites, 1999, 28, 7, pp (5) 4. Pontes, A. J. and Pouzada, A. S., ʻEjection Force in Tubular Injection Moldings. Part I: Effect of Processing Conditions, Polymer engineering and science, 2004, 44, 5, pp Pontes, A. J., Brito, A. M., Pouzada, A. S., ʻAssessment of the ejection force in tubular injection moldings, Journal of injection molding technology, 2002, 6, 4, Sasaki, T., Koga, N., Shirai, K., Kobayashi, Y., Toyoshima, A., ʻAn experimental study on ejection forces of injection molding, Journal of precesion engineering, 2000, 24, 3, Charmeau, J.-Y., Chailly, M., Gilbert, V., Béreaux, Y., ʻInfluence of mold surface coatings in injection molding application to the ejection stage, International Journal Material Form, 2008, 1, 1, Additive manufacturing 6.1 Introduction The first form of creating layer by layer a three-dimensional object using computer-aided design (CAD) was rapid prototyping, developed in the 1980 s for creating models and prototype parts. Rapid prototyping is one of the earlier additive manufacturing (AM) processes and it allows for the creation of printed parts, not just models [6.1]. It is also known as 3D printing or digital fabrication and its primary distinguishing feature is to create parts by the controlled addition (rather than subtraction) of material [6.2]. Fig. 6.1 MakerBot Thing-O-Matic

55 55 Among the major advances that this process presented to product development are the time and cost reduction, human interaction, and consequently the product development cycle, also the possibility to create almost any shape that could be very difficult to machine. AM processes take the information from a computer-aided design (CAD) file that is later converted to a stereolithography (STL) file. In this process, the drawing made in the CAD software is approximated by triangles and sliced containing the information of each layer that is going to be printed. Recently, AM technologies have been standardized and classified by the American Society for Testing and Materials (ASTM) International Committee F42 on Additive Manufacturing Technologies. The committee has classified AM processes and their variants into seven main categories including: photopolymer vat, material extrusion, powder bed fusion, directed energy deposition, sheet lamination, material jetting, and binder jetting [6.1, 6.3]. They differ in the way layers are deposited to create parts and in the materials that can be used. Some methods melt or soften material to produce the layers, e.g. selective laser melting (SLM) or direct metal laser sintering (DMLS), selective laser sintering (SLS), fused deposition modeling (FDM), while others cure liquid materials using different sophisticated technologies, e.g. stereolithography (SLA). With laminated object manufacturing (LOM), thin layers are cut to shape and joined together (e.g. paper, polymer, metal, ). Each method has its own advantages and drawbacks, and the costumer should find the best solution to be applied in each specific case D Printing for actuators An actuator is a device that takes energy in some form and converts it to perform some type of mechanical work, which is typically motion (i.e. blocking, clamping or ejecting). It also can be used to apply a force. One of the most challenging and exciting frontiers in self-replicating machine research is the development of printable actuators: actuators that can be built using the constrained set of materials and processes available to a 3D printer. Smart materials exhibit changes in their physical properties, such as size or shape, in response to external stimuli (e.g. temperature, or electric current). Smart materials can function as actuators i.e. devices that provide movement or changes in shape, for robotics and related applications. Some examples are: piezoelectrics, that produce a voltage when stress is applied; shape memory materials, in which large deformation can be induced and recovered through temperature changes, stress changes (shape-memory alloys and polymers) or a significant variation in the magnetic field (magnetic shape-memory alloys); temperature-responsive polymers, which undergo changes upon temperature; photomechanical materials, changing shape under exposure to light; Complete actuators including active membranes and support structures can be 3D printed in one go, resulting in a great improvement in fabrication speed and increases in accuracy and consistency A major advantage related to the employment of this technology is that more complex 3D structures can be constructed if more flexibility in layer deposition is possible, for example by enabling the forming of variable thickness layers and the deposition of actuator layers at different angles [6.4]. In 2011 Walters et al. [6.5] presented a 3D-printed bio-actuator comprising a pressure inlet, a diaphragm fabricated from soft silicone elastomer, a magnetic switch, a smart control valve incorporating a NiTi fibre actuator, and an outlet. The 3D printing process employed in the fabrication was photopolymer jetting in which

56 56 a liquid photopolymer resin was deposited by inkjet printing and immediately cured by ultraviolet light. This biologically-driven actuator will serve as a proof-or-concept artificial heartbeat for future use within biorobotic art and design. Fig 6.2 Bio-actuator 3D printed working prototype. 6.3 Actuator 1: camshaft Design and construction of the 3D printed actuator For the actuation of the nozzle- device a camshaft-device was designed and 3D-printed. The single cams are actuating with a 60 degree phase-shift to each other to follow the actuation principle suggested by Tan, et al. [6]. The actuation principle is shown in Figure 6.3. Fig. 6.4 Actuation principle of camshaft actuator. Grey: Cam 1, Purple: Cam 2, Blue Cam 3. In Figure 6.3 each line symbolizes the movement of a certain point on one of the cams. If the value is higher than zero the cam is pushing on the membrane and pumping the chamber underneath. For values lower than zero there is no contact between the membrane and the actuator. To enable the actuation of all three channels of the nozzle-device a x-y-stage was implemented in the design. This stage enables the positioning of the nozzle-device underneath the cams using M2-screws. This leads to a

57 57 position accuracy of 400 µm per 360 degree revolution of the screw. The position in z-direction can be defined by four M2- screws in the corners of the device (Figure 6.4). Fig. 6.5 Left: Camshaft-device after 3D-printing and rinsing of the support material, right side: Assembled system. The camshaft can be connected to a Como Drills 951D10216V DC-Motor for the necessary actuation speed of the nozzle device. For first prototypes of the camshaft-device 3D-prints with different tolerances (50 µm, 100 µm, 150 µm, 200 µm, 250 µm) for the axle clearance bearing were printed on a Objet 30 3D-printer. The tolerance range was choosen because it was not possible to predict the minimum clearance that could be used. Even if the resolution of the printer is higher than the chosen tolerances, the limiting factor for the clearance was the removal of the support material which is necessary to produce a rotating axis in a 3D-printed device. This was possible for all the clearances without the 50 µm tolerance. The material used by this printer is an acrylate like photopolymer with the Tradename VeroWhitePlus RGD Actuator 2: push/pull solenoid with piston The actuator consists of a push/pull solenoid which displaces a piston located through it. It is an on/off type actuator and develops force in one direction when energized with electric current. The return force is provided externally by a return spring. Spring Piston Figure 6.5 Actuator (RS code: ). In order to determine how much current should be applied the get the right piston displacement for actuating the fabricated device, a linear variable differential transformer (LVDT) transducer was used as sensitive electromechanical detector of linear movement with appropriate electronics.

58 58 Figure 6.6 LVDT measurement setup for piston actuator. 6.5 Introduction to linear variable differential transformer The basic LVDT design consists of a cylindrical array with a primary winding centered between two identically wound secondary windings. The coils are wound on a hollow glass-reinforced polymer (GRP) former, which is surrounded by a high-permeability magnetic shield. This assembly is then secured into a stainless steel tube that is sealed at both ends. This tubular element is the static part of the LVDT construction. [6.6] Figure 6.7 LVDT LVDT produces a linear electrical output that is directly proportional to the position of a movable ferromagnetic core of nickel-iron alloy. Typically, the LVDT body is fixed in place and the core is attached to the object whose position is to be measured, slides along the axis of the tube. Inside the LVDT are three windings: one primary and two secondary coils that are 180 out of phase. A sinusoidal RMS voltage between 2.5 and 10 khz excites the primary winding. The two secondary coils share the magnetic field generated by the primary coil, inducing a voltage in the secondary windings similar to a transformer. However, the position of the ferromagnetic core determines how much magnetic flux each winding senses and thus the resulting output voltage. At an end-of-travel position, one secondary senses almost all of the magnetic flux while the other winding senses almost none. The two secondaries flip division of the magnetic field when the core moves to the opposite extreme. When the core is in the center (null position), both secondaries receive equal amounts of the primary magnetic field, producing output voltages that are identical, though 180 out of phase [6.7, 6.8]. That signal is easily collected by any data acquisition device. In machining centers LDVTs are used for positional feedback on the tool and tool holder since they can make measurements directly inside the machine during the machining process.

59 59 a. b. Fig. 6.8 a. Cutaway view of an LVDT. Current is driven through the primary coil at A, causing an induction current to be generated through the secondary coils at B. ~ b. LVDT schematic. 6.6 Piston displacement measurements with LVDT LVDT sensor was used for displacement measurement to assess how much the piston moves when applying a force. To do that the piston was used to place the sensor in the null position. This is crucial for measuring through the most linear portion of the transducer range. Fig. 6.9 Illustration of measuring principle for LVDT sensor. Null position could be found by varying the applied force on the actuator; however this was not achieved due to spring softening and collapsing. This resulted in lack of repeatability for each measurement and could be explained taking into account different phenomena: 1. change of magnetic properties of the piston due to the presence of different magnetic fields in the measurement setup ; 2. heating of actuator and piston; 3. variability in aligning the piston on LVDT sensor. To contrast the first problem, the piston was removed from the magnetizing actuator and heated in order to demagnetize it. Nevertheless the approach had only a small effect and was not sufficient to overcome the problem. In order to overcome the overheating, nitrogen gas was applied on the actuator to cool it down. Since it showed only a little effect, another approach was tried by lowering the current flow through the actuator: this allowed measurements on the system. This suggests that overheating was a more influencing problem if compared to magnetization.

60 60 For that concern variability in aligning the piston on LVDT sensor, the issue could not be solved because of the huge space of movement of the piston inside the actuator. Even though the problems stated above were minimized as much as possible, the null position for the sensor was not established. Therefore displacement measurements were done somewhere within the LVDT linear range, with a differential approach. Measurements were done 5 times for each of the three pistons to check system reproducibility and data are reported in the Table 6.1. Piston #2 was not able to get stable positions and therefore measurements were not taken for it. Pistons #1 and #3 showed an acceptable reproducibility but they did not allow the displacement of maximum 100 µm needed for the device actuation. Table 6.1 LVDT measurements for minimum displacement. # Piston Upper value [µm] Lower value [µm] Δ Displacement [µm] However since the force applied to displace the piston inside the actuator was also used to put the LVDT within its measuring linear range, this force cannot be directly translated to the force needed for displacing the membrane of the fabricated device. Therefore the measurements here reported do not coincide with the real piston displacement provoked by the actuator, thus functional tests were done directly on the device. 6.7 Functional tests on the device Figure 3 shows the testing setup exploited to assess the functionality of the device. The device with thin film bonded to the injection molded component was tested as first and it turned out that the maximum displacement force which could be produced by the actuator was not high enough to move the membrane. Therefore testing experiments were done with the backup solution consisting of a tape sealed chamber. A syringe was used to put water through cannels until a droplet was seen at the nozzle output to verify that the device was completely filled. Then, the needle was disconnected from tubing leaving it filled with water to create an open reservoir. To control the actuator, provided software and code were exploited and modified so that the three actuators were moving in the right sequence, with a frequency of 1 Hz. Liquid moved through the tubing. After approximately 20 seconds the tape was punctured and leakage at the inlet was observed, therefore it was not possible to determine if the system was pumping in an effective way the liquid through.

61 61 Actuator #1 Actuator #3 Actuator #2 A. Fig a. Test setup for the fabricated device. ~ b. Zoom in showing actuator pushing chambers. 6.8 References 1. A Review of Additive Manufacturing KV Wang et al. ISRN Mechanical Engineering Three-DimensionaI Printing: The Physics and ImpIications of Additive Manufacturing E Sachs et al. CIRP Annals - Manufacturing Technology Multiple material additive manufacturing - Part 1: a review Vaezi et al. Virtual and Physical Prototyping Printing 3D dielectric elastomer actuators for soft robotics J Rossiter et al. EAPAD Digital fabrication of a novel bio-actuator for bio-robotic art and design P Walters et al. IS&T Digital Fabrication Technical article: Applied Measurements discusses the LVDT operating principle Applied measurements New Uses for Linear Variable Differential Transformers (LVDTs) R Rapas Machine Design The LVDT: construction and principles of operation Technical Paper by Measurement Specialties Inc. B.

62 62 7 Joining Techniques and LISA When a microfluidic part has been designed and fabricated, a major concern is often how to seal the channels on the chip. Several processes have been developed for joining of two polymer devices without collapsing the small channels. These include laser welding, thermal bonding, tape and glue. Each of these methods will be presented below and a preferred procedure for the fabricated device will be chosen. Also the method of Laser- Induced Selective Activation (LISA) is introduced. 7.1 Methods Laser welding Welding is extensively used in the metal industry for its ability to locally heat the substrate above the melding temperature, and thereby creating a joined point of two parts when cooled. It is however not possible to directly transfer the method into polymer fabrication. Since many polymers are transparent to the wavelength of the laser, only a small portion of the energy will be converted into heat in the polymer. Therefore this method has a set of requirements for it to work: 1. One of the materials to be joined must have a higher absorption of the laser energy compared to the other. 2. The other material should be transparent to the laser. 3. The two materials should be compatible for each laser welding to each other [1]. The first requirement is set since the laser energy has to be absorbed at the interface between the two materials in order to create a strong bond. This also means that the top material should be transparent (hence the second requirement). Thirdly a list of compatible materials for laser welding has been made by [1], which shows how well a joint can be made between two materials. This method is very fast as it only relies on positioning of a laser beam. Thermal bonding Bonding of two polymers by applying heat and pressure has been a much used method for sealing microfluidic channels [2,3]. By heating the polymer to around the glass transition temperature it is possible to form an invisible, seamless seal. However the process is slow as often many minutes (or even hours) have to be used in the bonding press. By keeping the temperature at the glass transition it is possible to join the polymers without destroying the structure of the channels. Y. Wang et al. reported thermal bonding of two PC parts by treating the chip with air plasma for 5 minutes, followed by a 5 minute bonding procedure. Taping of channels An easy solution for sealing a surface is to use tape. The tape will then act as the last part of the channel, and if the seal is strong enough to withstand the pressure of the fluid inside the channel, this can be a suitable solution for many devices. However one side of the channel will be covered in the glue from the tape, which

63 63 can be a severe contamination of the fluid sample. Also the tape has to be compatible with the use of the device. Gluing of parts Another solution is to use glue (e.g. epoxy) between the materials. This however has to be implemented in the design of the device, as grooves have to be made where the glue should be placed. The glue also suffers from some of the same drawbacks as taping when it comes to compatibility and contamination of the fluid sample. 7.2 Selection of method Laser welding was quickly tested for joining of the black PC template chip and a transparent PC thin film, since this method should be the fastest to conduct of the presented methods. Unfortunately the test showed that the amount of heat created at surface of the black structure, was too high and basically just cut straight through the thin film. Another problem was also that the laser left a track of burned polymer around the laser path. Therefore this method was disregarded. Through further discussions within the group it was decided to use thermal bonding of the parts, as this method should provide the strongest seal of the channel. Even though a plasma oven was not available the method was still chosen. Taping and gluing was disregarded due to the contamination issues. Taping was however kept as a backup plan, if there would be troubles with the thermal bonding. LISA Laser-Induced Selective Activation (LISA) is a method that can incorporate small electric circuits on polymer films. This is done through metallization of specific parts of the polymer. By scanning the surface of the polymer in the desired pattern, it is possible make the surface porous, which enables autocatalytic electroless plating of copper [4]. Metal lines on the order of 80 µm [5] can be produced by this method, which makes it useful for implementation in micro devices. The fabricated circuits can serve as e.g. sensors or heating devices, which increases the functionalities of the final device without decreasing the portability. In this project we have chosen not to implement LISA elements, as temperature control was considered not to be a need to have feature. Future version of the design could possibly benefit from the inclusion of LISA elements, if e.g. temperature control proves to be an important factor. 7.3 Experimental setup For the experimental part we used a thermal bonding press. As can be seen in Figure X.1, it is mainly composed by a hydraulic unit (max 12 ton), polished pressing plates, dial face thermometers, and pressure gauge. By placing the two polymer parts in the press, the force exceeded by the plates can be manually adjusted and the parts bonded. An external temperature control was implemented in order to ensure that the desired temperature was reached and maintained throughout the bonding process.

64 64 Fig Experimental setup with thermal bonding press, which includes two parallel plates for bonding, hydraulic unit for controlling the pressure and a temperature control. An external temperature control (green wire) was used to monitor the actual temperature. The internal temperature control (right next to the press), was first set to 128 C on both the upper and lower plate, see Figure 7.2. This is close to the glass transition temperature (138 C) of the PC used in this device (see Table 7.1). A closer margin could be used, but the temperature was only guaranteed to be within ±5 C. Unfortunately the external control showed very varying temperature (up to 150 C), which made the first set of experiments unpredictable, but nevertheless bonding was attempted.

65 65 Fig The Internal temperature control was set to 128 C, which is just below the glass transition temperature of the PC used in the device. Table 7.1. Thermal properties of the polycarbonate used for bonding. PC Sabic Lexan 141R Glass Temperature, T g 138 C Melt Temerature, T m 320 C For testing the strength of the bond achieved in the bonding press a mass was applied in one end of the bonded thin film. By increasing the total mass it was possible to determine a force required to break the bonding. A custom made cup holder (Figure 7.3a)) was made which could contain smaller weights (Figure 7.3b)). Fig. 7.3 a) b) a) Picture of the cup holder, which should be attached to an extruding part of the bonded thin film, and serve as a holder for smaller weights (b)). The cup holder was attached to a small extruding part of the thin film through the use of a paper clip, see Figure 7.4. It was then examined how much weight it would take to make the film start delaminating from the bulk surface.

66 66 Fig. 7.4 The cup holder was free hanging only in a small extruding part of the thin film. This was achieved by using a paper clip. Materials Two sets of experiments were performed on the bonding press; one using the template structure and one using the real device. The first was done, since the real device was not yet available for testing. A small piece of the template structure was cut in order to mimic the size and shape of the final device, see Figure 7.5. The dimensions are roughly 25 mm x 28 mm compared to the final 17 mm x 17 mm (Figure 7.6). Fig The test pieces for the first set of experiments were cut from the template structure provided. The dimensions are roughly 25 mm x 28 mm. Fig Picture of the final device and thin film to be bonded. The dimensions are 17 mm x 17 mm. Preliminary results (first set of experiments) A DOE analysis was conducted to determine the influence of bonding time and pressure on the bonding process between thin film and substrate. For this preliminary analysis a microfluidic component in PC Sabic Lexan 141R 25 x 27 mm was used. The thin film was Makrofol DE The temperature of the plates was kept constant at 128 ±5 C for both plates.

67 Mean of Breaking force Mean 67 A factorial plane 2 2 was generated and for each set of values 3 replications of the experiments have been performed. The values are summarized in Table 7.2 and Figure 7.7 and 7.8. Table 7.2 Experimental results gathered for the first set of experiments with the template structure. n Time Pressure Breaking force [min] [Mpa] [N] 1 4 7,0 (0.5 ton) 0, ,0 (1 ton) 0, ,0 0, ,0 0, ,0 0, ,0 0, ,0 0, ,0 0, ,0 0, ,0 0, ,0 0, ,0 0, ,5 0,7 Main Effects Plot (data means) for Breaking force Interaction Plot (data means) for Breaking force 0,54 Time Force 0,65 Time 4 8 0,52 0,60 0,50 0,55 0,48 0,46 0,50 0,45 0,44 0, ,5 1,0 0,40 0,5 Force 1,0 Figure 7.7. Main Effects Plot and Interaction Plot

68 Force 68 Contour Plot of Breaking force vs Force; Time 1,0 0,9 0,8 Breaking force < 0,45 0,45-0,50 0,50-0,55 0,55-0,60 > 0,60 Surface Plot of Breaking force vs Force; Time 0,60 0,7 Breaking force 0,55 0,50 0,6 0, Time 8 0,50 0,75 1,00 Force 0, Time 7 8 Figure 7.8. Contour Plot and Surface Plot It should be noticed that breaking force increases a lot for both higher pressure and longer bonding time. This was expected as both parameters ensure a tighter bond between the thin film and the chip. However manual inspection of the chip was needed, in order to verify that the channel and other structures have been properly sealed and not collapsed. The parameters tested in this experiment did not show any sign of collapsing channels, but a full seal between the channels was not always achieved. This can be attributed to the very varying temperature, which was hard to keep steady. Some experiments resulted in almost melted chips, which have not been included in the data shown. 7.4 Results from the experiments on the final structures For the actual bonding experiments of the final device, a series of optimized parameters was chosen according to the previous test. As is mentioned above, it is preferable to choose a larger pressure and longer bonding time to obtain a tighter seal. Since we did not observe any collapse of the channels, it was decided to increase the parameters a slight bit. In this experiment, we used 1.5 tons, 8 minutes and 128 C. As is shown above in Figure 7.6, the PC thin film now has the same area as the PC plate. The first experiments on the final device showed, that after thermal bonding, some regions of the joined part were not united. This meant that the channels where not fully separated and sealed. This was attributed to the fact that the pressure distribution of the bonding press was not very uniform. To fix this, two silicon wafers where tried to serve as pressure distributors on both of the bonding plates. These however shattered immediately due to the high pressure and large amount of dirt in the bonding press. Consequently, an assisting sheet was added around the chip (see Figure 7.9). This piece of metal had roughly the height of the chip, but a small metal piece had to support the chip to align it fully.

69 69 Fig A metal sheet was placed between the bonding plates in order to even the pressure load on the final device. A smaller metal piece have been inserted under the chip in order to align the two structures. After this adjustment a nice seal was obtained, except for some small bubbles between the channels. These were vented by making two holes in the thin film between the channels. This resulted in a bonding as shown in Figure As can be seen, the middle channel has a lesser tight seal than the two edge channels; however a sealing is still obtained. This proves that thermal bonding of the device is possible, but it requires a lot of optimization before a satisfying result can be obtained. Fig The final bonded device. It can be seen, that a tighter seal is achieved for the two outer channels compared to the middle one. This can be further optimized in future devices. 7.5 Comments from the actuation results When the bonding was completed, the final device was handed on to the testing facilities. It however quickly became evident that the 200 µm thick PC thin film was too stiff for the actuation to work. Therefore the backup solution of using tape for sealing was initiated. In order to provide the best device for testing, two different tapes were used, see Figure Also three inlet tubing were glued (Locktite super glue power easy gel) onto the device for easy handling.

70 70 Fig The two different taped chips with three inlet tubing glued on. 7.6 Conclusion This part presents the influence of bonding time and pressure on the breaking force. Test results show that a larger pressure and a longer bonding time lead to a higher breaking force. By adjusting the pressure distribution across the bonding plates is was possible to achieve a satisfying bonding. Further optimization would however be needed in future devices. 7.7 References [1] S. E. Nielsen, J. K. Kristensen, M. Strange, Laser welding of plastics weld compatibility investigations, online at [2] C.-W. Tsao, D. L. DeVoe, Bonding of thermoplastic polymer microfluidics, Microfluid Nanofluid, , p [3] Y. Wang, H. Chen, Q. He, S. Soper, A high-performance polycarbonate electrophoresis microchip with integrated three-electrode system for end-channel amperometric detection, Electrophoresis, 29, 2008, p [4] Y. Zhang, H. N. Hansen, P. T. Tang, J. S. Nielsen, Verification of a characterization method of the laserinduced selective activation based on industrial lasers, Int J Adv Manuf Technol, [5] IPU homepage regarding the LISA process,

71 71 8 Metrology 8.1 Introduction In this section the topics related to the measurement process with the aim to control and improve the manufacturing process quality are discussed. In the first section, the critical features for the functionality of the part are identified and described, as well as the most suitable measurement systems. In the second chapter the issue related to the uncertainty of measurements is handled presenting also the strategy used for obtaining precise measurements on the parts, to make sure that those are not influenced by external factors. A statistical process method to keep the manufacturing process under control is introduced on a theoretical way in the third chapter. Finally, in the fourth and in the fifth chapter, the measurements performed and the discussion about the results are presented. 8.2 Critical features and metrological equipment After a careful study of the design of the part and of the mold, and considering all the functional characteristics, the main critical features are detected and reported in Table 8. The ID Numbers are shown also in Fig ID NUMBER 1 OVERALL DIMENSION OF THE PART 2 CHANNELS CRITICAL FEATURES Height Width Diameter Depth Roughness < 2 microns Profile shape MEASUREMENT MACHINE 3 RESRVOIRS Diameter (B) Depth A 5 NOZZLE Diameter of the tip A Outlet hole Roundness A Roughness < 1 microns B Table 8.1 Critical features of the part a the related more suitable measurement systems (A = DeMeet Optical System, B = Alicona InfiniteFocus). A A A B B B

72 72 Fig. 8.2 Technical drawing with the identification of the critical features. Considering the critical features identified, two measuring instruments are chosen: DeMeet Optical System and Alicona InfiniteFocus (Fig. 8.2). Fig. 8.3 DeMeet optical system on the left of the picture and Alicona Infinite Focus on the right of the picture DeMeet 220 is a micro optical system of 3D coordinates (CMM) used for 2D measurements. Data are collected one point at a time with each point having a discrete X, Y, Z location. The measuring area (image view) is defined by the magnification of the applied lens (from 40X to 400X). For the measurements of the part it was used a 5x magnification.