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Jet array driven flow on the nozzle plate of an inkjet printhead in deposition of molten nylon materials
Journal of Materials Processing Technology 213 (2013) 383–391
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Jet array driven flow on the nozzle plate of an inkjet printhead in deposition of molten nylon materials Saeed Fathi ∗ , Phill Dickens Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire LE11 3TU, United Kingdom
a r t i c l e
i n f o
Article history: Received 18 March 2012 Received in revised form 30 September 2012 Accepted 22 October 2012 Available online 1 November 2012 Keywords: Particle tracking velocimetry Flow field Nozzle plate Inkjet printing Additive manufacturing
a b s t r a c t During research into an inkjet-integrated manufacturing process, jetting of molten caprolactam was investigated using a piezoelectric drop-on-demand printhead. Due to the start-up purging step and the surface energy differences, a wetting melt layer on the printhead’s nozzle plate was formed. With appropriate parameters, a stable jet array was made. However, contamination on the nozzle plate disturbed the jet stability resulting in jet trajectory errors and even jet failures. Particles were used to characterise the melt flow field on the nozzle plate during jetting when multiple nozzles were actuated. Particle tracking velocimetry revealed that movement of the particles followed a specific pattern when the jet array was developed. Flow pattern driven by an actuating nozzle influenced those of adjacent nozzles. The movement of particles towards and from the actuating nozzles was observed at the same time and position with velocities up to 2 mm/s. This showed that a complex flow system was generated on the nozzle plate during jetting with multiple nozzles which influenced the reliability of the inkjet printhead. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Inkjet technology is classified into two main modes based on the jetting head used: continuous and drop-on-demand (DoD) as reported by Le (1998). In continuous mode, the ink is pressurised through a feeding system and then by vibrating a piezoelectric element, a train of droplets is made. The droplets either impinge onto a substrate or are deflected into a recirculation system. In a DoD mode inkjet printhead, a voltage signal is sent to a transducer that forces liquid material out through a nozzle and a droplet is generated to hit the substrate when needed. Advances in inkjet printing have increased the range of materials being deposited and so it is used in non-graphical applications for a number of years. Inkjet-based processes as a method of manufacturing objects were reviewed by Hon et al. (2008). Mironov et al. (2006) reported on the applications of the technology in bioprinting of tissues and organs. Advances in inkjet printing of electroactive polymers were reviewed by de Gans et al. (2004) and more recently Perelaer et al. (2010) reported on the progress and challenges of inkjet printing of electronic devices. Developing inkjet-integrated additive layer manufacturing processes requires a high level of reliability in material deposition as the process may take several hours to fabricate hundreds of thin layers. de Jong et al. (2006) and Wijshoff (2008) reported that the droplet formation process
∗ Corresponding author. Tel.: +44 7912 618 177. E-mail address: [email protected] (S. Fathi). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.10.012
instability (inconsistency in the droplets characteristics and jet trajectory deviations) could occur due to external sources such as contamination or air motion causing a trajectory error or jet failure. Contamination may come from impurities in the ink or from the surrounding environment and could affect droplet formation by disturbing the oscillating meniscus. Jetting of particulate suspensions could be particularly challenging where the nozzle cloging is to be avoided by tailoring the ink formulation and also jetting parameters as reported by Lee et al. (2012). de Jong et al. (2006) studied the mechanism behind the failure of a single jet in the presence of a thin wetting ink layer on the nozzle plate of a DoD printhead. In their study, the jet failure was found to be affected by formation of an air bubble due to presence of contamination in the ink layer around the actuating nozzle. An increase of the ink layer thickness was found to be responsible for air ingestion during the meniscus oscillation. There is a lack of literature in studying the flow behaviour on the nozzle plate driven by an array of nozzles in inkjet printing of functional materials. The graphical ink flow on the nozzle plate induced by a single nozzle actuation in a piezoelectric DoD printhead was studied by Beulen et al. (2007) and de Jong et al. (2007). In their study, excess ink including tracer particles was placed on the nozzle plate around the actuating nozzle, and particle tracking was used to visualise the flow motion on the nozzle plate. It was found that the particles were attracted towards the actuating nozzle during jetting. Two main origins of flow motion were identified by Beulen et al. (2007); the meniscus oscillation and air motion induced by ejecting droplets. By applying jetting voltages low enough to have
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S. Fathi, P. Dickens / Journal of Materials Processing Technology 213 (2013) 383–391
meniscus oscillation without expelling a droplet, fluid flow around the actuating nozzle was observed which showed the role of meniscus oscillation as the origin of the flow on the nozzle plate. de Jong et al. (2007) suggested that the flow pattern was affected by a Marangoni flow due to a gradient in the surface tension of the ink across the ink layer. The authors of this paper are researching a novel inkjet additive manufacturing process for 3D printing of nylon parts. The concept is to deposit two molten mixtures of caprolactam with initiator and catalyst separately on top of each other and then use radiation heating to start the nylon anionic polymerisation reaction. The mixtures are dominated by the monomer, caprolactam, with a volumetric proportion of more than 98%. The stable range of jetting parameters for molten caprolactam, namely the melt temperature, jetting voltage and the vacuum level, were found (Fathi et al., 2012). Within the stable window though, contamination on the nozzle plate influenced the stability of jets in the form of trajectory errors, single jet failure and occasionally failure of the whole jet array in a behaviour similar to falling domino. This led to a study of the interaction between the molten jetting material and the nozzle plate to determine the behaviour of contamination when multiple nozzles are actuated. 2. Experiments 2.1. Jetting material The main material used was caprolactam supplied from Sigma–Aldrich GmbH. It is white in the solid state but melts at 68 ◦ C and is colourless when molten. The appropriate jetting temperature was found to be 80 ◦ C at which it has a dynamic viscosity of 9 mPa s at shear rates higher than 100 s−1 measured by a rheometer (MCR101 – Anton Paar Ltd). The surface tension was also found to be 35 mN/m measured at 80 ◦ C using the pendent drop method (OCR 20 – DataPhysics GmbH). 2.2. Experimental setup Two jetting assemblies were developed for the two mixtures to avoid pre-deposition reaction and consequent printhead blocking. A Xaar 126 piezoelectric DoD printhead was used and for each jetting assembly, a melt supply unit was developed. This was to melt solid caprolactam provided in form of a bar cartridge and then supply the melt to the printhead through a filtration sub-unit. Fig. 1 shows the melt supply unit attached to the printhead. It consisted of an aluminium syringe with 25 ml capacity, a flexible rope heater insulated with cotton wool and a filtration unit attached to the syringe with a luer type connection. The connection on top of the syringe cap, as shown in Fig. 1, allowed a pneumatic tube to provide pressure or a vacuum for melt flow control. Thermal control of the rope heater was achieved via a multi-channel controller (CN1507TC1 from Omega Engineering Inc.) and a T-type thermocouple (RS Components Ltd.) attached to the syringe at its conical end. The printhead was heated by two resistors (HS25, 25 W, from Arcol UK Ltd.) attached via two aluminium holders and all fixed to a mounting plate. A T-type thermocouple embedded inside the holders near the printhead’s nozzle plate was used to feedback the temperature for resistor control. The nozzle actuation in the printhead was controlled by software though an electronic peripheral which could initiate individual nozzles as well as full nozzle array. Nozzle plate monitoring was achieved via a digital microscope camera (Dino-Lite AM211 – ANMO Electronics Corp.) by placing it underneath the printhead.The printhead had an array of 126 nozzles of 50 m diameter in an array of 17.2 mm length and therefore a nozzle spacing of 137 m. The nozzle plate of the
Fig. 1. Melt supply unit with luer connection joined to the printhead.
printhead was made of a heat resistant polyimide thin film. The surface tension of the nozzle plate was 40 mN/m as reported by the printhead manufacturer. A view of the nozzle array is seen in Fig. 2. The black rectangular channel seen in this figure where the nozzles were placed had a dimension of 430 m × 78 m and was made by removing material from the nozzle plate surface to ease the droplet separation from the nozzle. 2.3. Experimental procedure The start-up strategy ensured a supply of the melt to the printhead before nozzle actuation. Pneumatic pressure was applied to purge out the melt through the nozzles which provided dripping which continued even after removing the applied pressure. In this state, the nozzle plate was cleaned with a lint-free cloth in order to remove any contamination. A vacuum was applied to stop the dripping and retract the large meniscus as shown in Fig. 3(a). To obtain the appropriate meniscus on the nozzles for droplet generation, the applied vacuum level was increased to more than 10 mbar. The vacuum retracted the large melt meniscus leaving only a thin wetting layer on the nozzle plate (having a higher surface energy than the molten caprolactam) as shown in Fig. 3(b). The jetting voltage and vacuum level were the two main parameters. A jetting voltage above 12.5 V and 10 mbar was required to obtain a stable jet array as reported by Fathi et al. (2012). Fig. 4 shows a typical stable trial with 126 jets of molten caprolactam. Even within the range of parameters expected to give stable jetting, there were cases where instability still occurred. As the instability did not always occur, it was assumed to be due to contamination within the melt.
Fig. 2. Nozzle array on the nozzle plate of the printhead.
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Fig. 3. Formation of molten wetting layer on the nozzle plate during the start-up for jetting trials.
Fig. 4. Jet array of melt caprolactam (17.5 V, 5 kHz, 25 mbar) (Fathi et al., 2012).
Particle tracking was used to monitor the thin layer of molten caprolactam on the printhead nozzle plate. For this, a normal black graphical ink (Xaar 861, an oil-based ink) was initially purged into the printhead and then molten caprolactam was used to startup the jetting trials. Some particles (the agglomerate of the black graphical ink) remained inside the printhead during subsequent trials with molten caprolactam and provided an opportunity to study the melt flow on the nozzle plate. Formation of the thin molten layer as demonstrated in Fig. 3(b) was observed via movement of the black particles on the nozzle plate as shown in Fig. 5. Further purging steps were undertaken to reduce the number of residual black particles to less than 10 (at a time during jetting trials) to avoid high level of jet array instabilities during the trials for the melt flow study on the nozzle plate. This was because the particles were as large as 50 m which could block the nozzle while
Fig. 5. Black particles in the thin molten layer on the nozzle plate with a vacuum level of 20 mbar.
Fig. 6. Viewing direction of the microscope camera in relation to the nozzle plate.
actuation (recommendation is to filter contamination, in the ink supply, larger than 5% of the nozzle size to avoid disturbing the droplet formation process; Pique and Chrisey, 2002). Two situations are reported here, trial with all nozzles jetting and trial with five nozzles jetting. Both trials were undertaken at jetting voltage, frequency and vacuum level of 15.0 V, 5 kHz and 30 mbar. Earlier research by Fathi and Dickens (2012) using high speed imaging of the droplet formation process of molten caprolactam showed that the droplets at such jetting condition were stably expelled at a velocity of 2 m/s. This was to compare the melt flow in the two situations when a higher distance between two adjacent jets existed. Therefore, with the five jet trial, the actuating nozzles were chosen in middle of the nozzle array and there were two non-actuating nozzles in between providing a jet spacing of 410 m (compared with the normal spacing of 137 m). In addition, in both situations, five periods of nozzle actuation (jetting) were undertaken of about 25 s to monitor the particles. To protect the microscope camera against depositing droplets, the viewing direction was angled with the arrangement shown in Fig. 6. The first nozzle in the array (to the left) was the reference point with X and Y directions as shown in Fig. 7.
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Fig. 7. Coordinate system for particle tracking and some of the particles on the nozzle plate.
Fig. 10. Path history of Particle 1 tracked in a trial with all nozzles actuating as seen in Fig. 9 (15.0 V, 5 kHz, 30 mbar).
temporarily affected by a particle. Movement and tracking of these particles are shown in the following sections. 3.2. Particle tracking with all nozzles jetting
Fig. 8. Jet instability during the five nozzle jetting trial containing particles on the nozzle plate (15.0 V, 5 kHz, 30 mbar).
3. Results and discussions 3.1. Melt flow and jet disturbance by contamination Monitoring the nozzle plate showed that the particles moved within the molten layer towards the actuating nozzles during jetting periods. This indicated the melt flow on the nozzle plate during both trials (with all nozzles and five nozzles jetting). Interestingly, in the trials, jet instability was also observed concurrently with the particles movement. Fig. 8 shows the five nozzle jetting trial before, during and after instability of one of the five jets (jet 1 of 5). The instability was a temporary change of jet trajectory. Some of the particles on the nozzle plate are also shown in Fig. 8(a). The instability could have been due to the jet being
Particle tracking showed that not all particles moved similarly. Fig. 9 shows four of the particles tracked on the nozzle plate labelled as Particles 1–4. The dots in Fig. 9 represent the centre position of the particles at 0.65 ± 0.05 s intervals. Only Particle 3 was on the nozzle plate when the trial started while the other particles moved onto the nozzle plate due to the flow of the melt. Particle 3 behaved differently as it moved parallel to the nozzle array and was not attracted to the nozzles. Fig. 10 schematically depicts Particle 1’s movement on the nozzle plate. It is clearly seen that the particle motion was dominated by the four jetting periods. Relatively large displacements occurred during the first two jetting periods. The first period influenced the particle most. The particle continued to move after each jetting period. Possibilities are inertia of the melt flow and Marangoni effect of surface tension gradient across the melt layer. Fig. 11 shows the result of particle tracking velocimetry for the four particles in Fig. 9. It indicates how melt flow behaved during and after jetting periods. As a general indication, a higher velocity was seen during the jetting periods. This suggests that the flow motion could be driven by droplet generation. Entrance of Particle 2 from the side onto the nozzle plate occurred during the third jetting period. Its velocity was as high as 0.9 mm/s when entering the nozzle plate.
Fig. 9. Particle tracking with all nozzles actuating (15.0 V, 5 kHz, 30 mbar).
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Fig. 11. Particle tracking velocimetry during the trial with all nozzles jetting as seen in Fig. 9.
The particles behaved differently as seen in Fig. 9. The velocity of Particle 3 was not significantly affected by the jetting periods. Although all other particles entered onto the nozzle plate from the outside during the jetting periods, when they reached the nozzles, they then had limited motion. It was not clear what
influenced this. Possibility was to have a complex flow regime due to superimposition of the flow patterns initiated by each nozzle. Therefore, a higher distance between the actuating nozzles was used in the trial with five jetting nozzles to investigate this possibility.
Fig. 12. The first jetting period of the particle tracking trial with five jets of caprolactam (15.0 V, 5 kHz, 30 mbar).
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Fig. 13. Particle tracking of the second jetting period.
Fig. 14. Particle tracking in the third jetting period.
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Fig. 15. Particle tracking in the fourth and fifth jetting periods.
3.3. Particle tracking with five nozzles jetting Figs. 12 and 13 show images from the first and second jetting periods of the trial with five nozzles jetting. As seen in Fig. 12(a) and (b), Particle 1 (P1), P2 and P3 had no displacement until the first jetting period started at t = 3.04 s. P3 was seen to move more than the other two during jetting. At the end of the first jetting period, all the particles continued to move, possibly due to inertia as seen in Fig. 12(g and h). Fig. 12 (frames d–h) also shows a particle (P8) being repelled from the actuating nozzles. This was while P1, P2 and P3 were being attracted. This behaviour was also observed with P9 just before the end of the second jetting period when P1 and P6 were being attracted as shown in Fig. 13. The simultaneous attraction and repelling were observed with the other particles during the third, fourth and fifth jetting periods as shown in Figs. 14 and 15. P1 for instance was attracted towards the jets from far left side of the nozzle plate and reached the nozzles just after the end of the third jetting period (Fig. 16(g)). Then, during the fourth and fifth jetting periods, P1 was repelled from the nozzles as shown in Fig. 15 whereas P14 moved towards the nozzles at the same time. Plotting the particles paths during the five
jetting periods helped to visualise the melt flow on the nozzle plate. 3.4. Melt flow with five nozzles jetting Fig. 16 shows the results of the particle tracking with five nozzles jetting. This figure shows the position of the actuating nozzles and the tracked particles. The dots in the graph represent the centre position of the particles (with ±50 m accuracy) for every 0.65 ± 0.05 s of the trial. This could also represent the velocity variation in terms of the distance from the actuating nozzles. Fig. 17 shows the particle tracking velocimetry results (corresponding to Fig. 16) versus time to provide information on the effect of start and end of jetting period on the melt flow. It can be seen that the velocity was higher during the jetting periods. Overall, particle velocity was higher when they entered onto the nozzle plate from the side when a maximum velocity of 2 mm/s was recorded. Fig. 16 suggests that the particles moved in a kind of radial pattern centred on the middle nozzle (in the array of actuating nozzles) and symmetric to the line perpendicular to the nozzle array and passing through the middle one. It also shows that some of the particles moved towards the actuating nozzles and then moved in the
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Fig. 16. Particle tracking in a trial with five individual actuating nozzles in four jetting periods (15.0 V, 5 kHz, 30 mbar).
Fig. 17. Particle tracking velocimetry during the trial with five nozzles jetting corresponding to Fig. 16.
opposite direction. However, not all particles returned from the nozzles (such as Particle 2) and were possibly taken into the ink channel providing opportunity for air ingestion and consequently the jet instability according to de Jong et al. (2006). The effect of multi-nozzle jetting on the overall flow pattern is highlighted with movements of some of particles. Looking at Particles 1, 2, 3 and 9, it is seen that particles introduced to the nozzles from side (with typically 45◦ angle) were expelled out in a direction perpendicular to the nozzle array. This shows a superimposition of flow fields generated by the individual nozzle actuations. Observing the movement of a number of particles at the same time was easy in the video clips and limited images were shown for this in Figs. 12–15. One could relate the particle positions and timings by observing Figs. 13 and 14 respectively, to find out how particles moved in relation to each other. It was found that attraction and repelling of some of the particles occurred at the same locations and times (e.g. Particles 7–12 in Fig. 16). In addition, these particles were recorded with different velocities as seen in Fig. 17. This indicated a complex flow motion in the molten layer and suggests a velocity profile existed across the melt layer thickness where the melt in the air interface moved in opposite direction to the melt closer to the nozzle plate. The particle tracking study with the molten caprolactam showed how the flow on the nozzle plate could take particles to the actuating nozzles resulting in potential for disturbing the droplet formation process. This research helped in understanding the random jet array instabilities that occurred with one of the catalyst mixtures used when attempting to produce nylon. This was because the catalyst complex did not completely dissolve in caprolactam at 80 ◦ C and was found in form of microcrystals which had a tendency to agglomerate which could then disturb the jets.
4. Conclusions Flow on the nozzle plate of a piezoelectric DoD printhead was investigated during multi-nozzle jetting, as a part of a reliability study to develop a new additive manufacturing process. Nozzle plate monitoring showed how nozzle actuation initiated flow on the nozzle plate and could move particles to the nozzle which in turn could lead to jet instability. Quantitative visualisation of the flow was undertaken via particle tracking velocimetry. Different particle motion behaviours were observed that indicated the melt layer on the nozzle plate had a complicated flow regime while jetting with an array of nozzles. Studying the situation with a reduced number of jets with higher distances between them indicated a radial pattern of flow on the nozzle plate and centred to the middle nozzle in the array of the actuating nozzles and symmetric to the line perpendicularly passing through the middle nozzle. The general melt flow seems to be affected by the combination of multiple jets. This means that the interaction of the flow generated by each actuating nozzle could have affected the overall flow which was seen with particles on either side of the multiple nozzles. This could have made a more complex velocity profile than that of a single jet where a superimposition of flow fields generated by the individual nozzle actuations defined the overall pattern. Movement of particles in opposite directions at the same position and time suggested a velocity profile across the thickness of the melt layer. This could provide a recirculating flow across the melt layer and introduce the contamination from the surrounding of the nozzle plate to the actuating nozzles causing jet instability. Studying the melt flow on the nozzle plate could help in understanding how jet array instability is initiated by contamination. As a future work, simulation of such flow behaviour along with the
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experimental analysis could contribute towards reliability in developing the novel inkjet additive manufacturing process. References Beulen, B., de Jong, J., Reinten, H., van den Berg, M., Wijshoff, H., van Dongen, M.E.H., 2007. Flow on the nozzle plate of an inkjet printhead. Experiments in Fluids 42, 217–224. de Gans, B.J., Schubert, U.S., Duineveld, P.C., 2004. Inkjet printing of polymers: state of the art and future developments. Journal of Advanced Materials 16, 203–213. de Jong, J., de Bruin, G., Reinten, H., van den Berg, M., Wijshoff, H., Versluis, M., Lohse, D., 2006. Air entrapment in piezo-driven inkjet printheads. Journal of the Acoustical Society of America 120, 1257–1265. de Jong, J., Reinten, H., Wijshoff, H., van den Berg, M., Delescen, K., van Dongen, R., Mugele, F., Versluis, M., Lohse, D., 2007. Marangoni flow on an inkjet nozzle plate. Applied Physics Letters 91, 204102. Fathi, S., Dickens, P., Hague, R., 2012. Jetting stability of molten caprolactam in an additive inkjet manufacturing process. International Journal of Advanced Manufacturing Technology 59, 201–212.
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Fathi, S., Dickens, P., 2012. Droplet characterisation of molten caprolactam for additive manufacturing applications. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 226, 1052–1060. Hon, K.K.B., Li, L., Hutchings, I.M., 2008. Direct writing technology – advances and developments. CIRP Annals – Manufacturing Technology 57, 601–620. Le, H.P., 1998. Progress and trends in inkjet printing technology. Journal of Imaging Science and Technology 42, 49–62. Lee, A., Sudau, K., Ahn, K.H., Lee, S.J., Willenbacher, N., 2012. Optimization of experimenttal parameters to suppress nozzle clogging in inkjet printing. Industrial and Engineering Chemistry Research 51 (40), 13195–13204. Mironov, V., Reis, N., Derby, B., 2006. Bioprinting: a beginning. Tissue Engineering 12, 631–634. Perelaer, J., Smith, P.J., Mager, D., Soltman, D., Volkman, S.K., Subramanian, V., Korvink, J.G., Schubert, U.S., 2010. Printed electronics: the challenges involved in printing devices, interconnects, and contacts based on inorganic materials. Journal of Materials Chemistry 20, 8446–8453. Pique, A., Chrisey, B., 2002. Direct-Write Technologies for Rapid Prototyping Applications. Academic Press, San Diego, CA, p. 184. Wijshoff, H. Structure and fluid dynamics in piezo inkjet printheads. Ph.D. Dissertation. University of Twente, 2008.
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Jet array driven flow on the nozzle plate of an inkjet printhead in deposition of molten nylon materials Saeed Fathi ∗ , Phill Dickens Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire LE11 3TU, United Kingdom
a r t i c l e
i n f o
Article history: Received 18 March 2012 Received in revised form 30 September 2012 Accepted 22 October 2012 Available online 1 November 2012 Keywords: Particle tracking velocimetry Flow field Nozzle plate Inkjet printing Additive manufacturing
a b s t r a c t During research into an inkjet-integrated manufacturing process, jetting of molten caprolactam was investigated using a piezoelectric drop-on-demand printhead. Due to the start-up purging step and the surface energy differences, a wetting melt layer on the printhead’s nozzle plate was formed. With appropriate parameters, a stable jet array was made. However, contamination on the nozzle plate disturbed the jet stability resulting in jet trajectory errors and even jet failures. Particles were used to characterise the melt flow field on the nozzle plate during jetting when multiple nozzles were actuated. Particle tracking velocimetry revealed that movement of the particles followed a specific pattern when the jet array was developed. Flow pattern driven by an actuating nozzle influenced those of adjacent nozzles. The movement of particles towards and from the actuating nozzles was observed at the same time and position with velocities up to 2 mm/s. This showed that a complex flow system was generated on the nozzle plate during jetting with multiple nozzles which influenced the reliability of the inkjet printhead. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Inkjet technology is classified into two main modes based on the jetting head used: continuous and drop-on-demand (DoD) as reported by Le (1998). In continuous mode, the ink is pressurised through a feeding system and then by vibrating a piezoelectric element, a train of droplets is made. The droplets either impinge onto a substrate or are deflected into a recirculation system. In a DoD mode inkjet printhead, a voltage signal is sent to a transducer that forces liquid material out through a nozzle and a droplet is generated to hit the substrate when needed. Advances in inkjet printing have increased the range of materials being deposited and so it is used in non-graphical applications for a number of years. Inkjet-based processes as a method of manufacturing objects were reviewed by Hon et al. (2008). Mironov et al. (2006) reported on the applications of the technology in bioprinting of tissues and organs. Advances in inkjet printing of electroactive polymers were reviewed by de Gans et al. (2004) and more recently Perelaer et al. (2010) reported on the progress and challenges of inkjet printing of electronic devices. Developing inkjet-integrated additive layer manufacturing processes requires a high level of reliability in material deposition as the process may take several hours to fabricate hundreds of thin layers. de Jong et al. (2006) and Wijshoff (2008) reported that the droplet formation process
∗ Corresponding author. Tel.: +44 7912 618 177. E-mail address: [email protected] (S. Fathi). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.10.012
instability (inconsistency in the droplets characteristics and jet trajectory deviations) could occur due to external sources such as contamination or air motion causing a trajectory error or jet failure. Contamination may come from impurities in the ink or from the surrounding environment and could affect droplet formation by disturbing the oscillating meniscus. Jetting of particulate suspensions could be particularly challenging where the nozzle cloging is to be avoided by tailoring the ink formulation and also jetting parameters as reported by Lee et al. (2012). de Jong et al. (2006) studied the mechanism behind the failure of a single jet in the presence of a thin wetting ink layer on the nozzle plate of a DoD printhead. In their study, the jet failure was found to be affected by formation of an air bubble due to presence of contamination in the ink layer around the actuating nozzle. An increase of the ink layer thickness was found to be responsible for air ingestion during the meniscus oscillation. There is a lack of literature in studying the flow behaviour on the nozzle plate driven by an array of nozzles in inkjet printing of functional materials. The graphical ink flow on the nozzle plate induced by a single nozzle actuation in a piezoelectric DoD printhead was studied by Beulen et al. (2007) and de Jong et al. (2007). In their study, excess ink including tracer particles was placed on the nozzle plate around the actuating nozzle, and particle tracking was used to visualise the flow motion on the nozzle plate. It was found that the particles were attracted towards the actuating nozzle during jetting. Two main origins of flow motion were identified by Beulen et al. (2007); the meniscus oscillation and air motion induced by ejecting droplets. By applying jetting voltages low enough to have
384
S. Fathi, P. Dickens / Journal of Materials Processing Technology 213 (2013) 383–391
meniscus oscillation without expelling a droplet, fluid flow around the actuating nozzle was observed which showed the role of meniscus oscillation as the origin of the flow on the nozzle plate. de Jong et al. (2007) suggested that the flow pattern was affected by a Marangoni flow due to a gradient in the surface tension of the ink across the ink layer. The authors of this paper are researching a novel inkjet additive manufacturing process for 3D printing of nylon parts. The concept is to deposit two molten mixtures of caprolactam with initiator and catalyst separately on top of each other and then use radiation heating to start the nylon anionic polymerisation reaction. The mixtures are dominated by the monomer, caprolactam, with a volumetric proportion of more than 98%. The stable range of jetting parameters for molten caprolactam, namely the melt temperature, jetting voltage and the vacuum level, were found (Fathi et al., 2012). Within the stable window though, contamination on the nozzle plate influenced the stability of jets in the form of trajectory errors, single jet failure and occasionally failure of the whole jet array in a behaviour similar to falling domino. This led to a study of the interaction between the molten jetting material and the nozzle plate to determine the behaviour of contamination when multiple nozzles are actuated. 2. Experiments 2.1. Jetting material The main material used was caprolactam supplied from Sigma–Aldrich GmbH. It is white in the solid state but melts at 68 ◦ C and is colourless when molten. The appropriate jetting temperature was found to be 80 ◦ C at which it has a dynamic viscosity of 9 mPa s at shear rates higher than 100 s−1 measured by a rheometer (MCR101 – Anton Paar Ltd). The surface tension was also found to be 35 mN/m measured at 80 ◦ C using the pendent drop method (OCR 20 – DataPhysics GmbH). 2.2. Experimental setup Two jetting assemblies were developed for the two mixtures to avoid pre-deposition reaction and consequent printhead blocking. A Xaar 126 piezoelectric DoD printhead was used and for each jetting assembly, a melt supply unit was developed. This was to melt solid caprolactam provided in form of a bar cartridge and then supply the melt to the printhead through a filtration sub-unit. Fig. 1 shows the melt supply unit attached to the printhead. It consisted of an aluminium syringe with 25 ml capacity, a flexible rope heater insulated with cotton wool and a filtration unit attached to the syringe with a luer type connection. The connection on top of the syringe cap, as shown in Fig. 1, allowed a pneumatic tube to provide pressure or a vacuum for melt flow control. Thermal control of the rope heater was achieved via a multi-channel controller (CN1507TC1 from Omega Engineering Inc.) and a T-type thermocouple (RS Components Ltd.) attached to the syringe at its conical end. The printhead was heated by two resistors (HS25, 25 W, from Arcol UK Ltd.) attached via two aluminium holders and all fixed to a mounting plate. A T-type thermocouple embedded inside the holders near the printhead’s nozzle plate was used to feedback the temperature for resistor control. The nozzle actuation in the printhead was controlled by software though an electronic peripheral which could initiate individual nozzles as well as full nozzle array. Nozzle plate monitoring was achieved via a digital microscope camera (Dino-Lite AM211 – ANMO Electronics Corp.) by placing it underneath the printhead.The printhead had an array of 126 nozzles of 50 m diameter in an array of 17.2 mm length and therefore a nozzle spacing of 137 m. The nozzle plate of the
Fig. 1. Melt supply unit with luer connection joined to the printhead.
printhead was made of a heat resistant polyimide thin film. The surface tension of the nozzle plate was 40 mN/m as reported by the printhead manufacturer. A view of the nozzle array is seen in Fig. 2. The black rectangular channel seen in this figure where the nozzles were placed had a dimension of 430 m × 78 m and was made by removing material from the nozzle plate surface to ease the droplet separation from the nozzle. 2.3. Experimental procedure The start-up strategy ensured a supply of the melt to the printhead before nozzle actuation. Pneumatic pressure was applied to purge out the melt through the nozzles which provided dripping which continued even after removing the applied pressure. In this state, the nozzle plate was cleaned with a lint-free cloth in order to remove any contamination. A vacuum was applied to stop the dripping and retract the large meniscus as shown in Fig. 3(a). To obtain the appropriate meniscus on the nozzles for droplet generation, the applied vacuum level was increased to more than 10 mbar. The vacuum retracted the large melt meniscus leaving only a thin wetting layer on the nozzle plate (having a higher surface energy than the molten caprolactam) as shown in Fig. 3(b). The jetting voltage and vacuum level were the two main parameters. A jetting voltage above 12.5 V and 10 mbar was required to obtain a stable jet array as reported by Fathi et al. (2012). Fig. 4 shows a typical stable trial with 126 jets of molten caprolactam. Even within the range of parameters expected to give stable jetting, there were cases where instability still occurred. As the instability did not always occur, it was assumed to be due to contamination within the melt.
Fig. 2. Nozzle array on the nozzle plate of the printhead.
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Fig. 3. Formation of molten wetting layer on the nozzle plate during the start-up for jetting trials.
Fig. 4. Jet array of melt caprolactam (17.5 V, 5 kHz, 25 mbar) (Fathi et al., 2012).
Particle tracking was used to monitor the thin layer of molten caprolactam on the printhead nozzle plate. For this, a normal black graphical ink (Xaar 861, an oil-based ink) was initially purged into the printhead and then molten caprolactam was used to startup the jetting trials. Some particles (the agglomerate of the black graphical ink) remained inside the printhead during subsequent trials with molten caprolactam and provided an opportunity to study the melt flow on the nozzle plate. Formation of the thin molten layer as demonstrated in Fig. 3(b) was observed via movement of the black particles on the nozzle plate as shown in Fig. 5. Further purging steps were undertaken to reduce the number of residual black particles to less than 10 (at a time during jetting trials) to avoid high level of jet array instabilities during the trials for the melt flow study on the nozzle plate. This was because the particles were as large as 50 m which could block the nozzle while
Fig. 5. Black particles in the thin molten layer on the nozzle plate with a vacuum level of 20 mbar.
Fig. 6. Viewing direction of the microscope camera in relation to the nozzle plate.
actuation (recommendation is to filter contamination, in the ink supply, larger than 5% of the nozzle size to avoid disturbing the droplet formation process; Pique and Chrisey, 2002). Two situations are reported here, trial with all nozzles jetting and trial with five nozzles jetting. Both trials were undertaken at jetting voltage, frequency and vacuum level of 15.0 V, 5 kHz and 30 mbar. Earlier research by Fathi and Dickens (2012) using high speed imaging of the droplet formation process of molten caprolactam showed that the droplets at such jetting condition were stably expelled at a velocity of 2 m/s. This was to compare the melt flow in the two situations when a higher distance between two adjacent jets existed. Therefore, with the five jet trial, the actuating nozzles were chosen in middle of the nozzle array and there were two non-actuating nozzles in between providing a jet spacing of 410 m (compared with the normal spacing of 137 m). In addition, in both situations, five periods of nozzle actuation (jetting) were undertaken of about 25 s to monitor the particles. To protect the microscope camera against depositing droplets, the viewing direction was angled with the arrangement shown in Fig. 6. The first nozzle in the array (to the left) was the reference point with X and Y directions as shown in Fig. 7.
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Fig. 7. Coordinate system for particle tracking and some of the particles on the nozzle plate.
Fig. 10. Path history of Particle 1 tracked in a trial with all nozzles actuating as seen in Fig. 9 (15.0 V, 5 kHz, 30 mbar).
temporarily affected by a particle. Movement and tracking of these particles are shown in the following sections. 3.2. Particle tracking with all nozzles jetting
Fig. 8. Jet instability during the five nozzle jetting trial containing particles on the nozzle plate (15.0 V, 5 kHz, 30 mbar).
3. Results and discussions 3.1. Melt flow and jet disturbance by contamination Monitoring the nozzle plate showed that the particles moved within the molten layer towards the actuating nozzles during jetting periods. This indicated the melt flow on the nozzle plate during both trials (with all nozzles and five nozzles jetting). Interestingly, in the trials, jet instability was also observed concurrently with the particles movement. Fig. 8 shows the five nozzle jetting trial before, during and after instability of one of the five jets (jet 1 of 5). The instability was a temporary change of jet trajectory. Some of the particles on the nozzle plate are also shown in Fig. 8(a). The instability could have been due to the jet being
Particle tracking showed that not all particles moved similarly. Fig. 9 shows four of the particles tracked on the nozzle plate labelled as Particles 1–4. The dots in Fig. 9 represent the centre position of the particles at 0.65 ± 0.05 s intervals. Only Particle 3 was on the nozzle plate when the trial started while the other particles moved onto the nozzle plate due to the flow of the melt. Particle 3 behaved differently as it moved parallel to the nozzle array and was not attracted to the nozzles. Fig. 10 schematically depicts Particle 1’s movement on the nozzle plate. It is clearly seen that the particle motion was dominated by the four jetting periods. Relatively large displacements occurred during the first two jetting periods. The first period influenced the particle most. The particle continued to move after each jetting period. Possibilities are inertia of the melt flow and Marangoni effect of surface tension gradient across the melt layer. Fig. 11 shows the result of particle tracking velocimetry for the four particles in Fig. 9. It indicates how melt flow behaved during and after jetting periods. As a general indication, a higher velocity was seen during the jetting periods. This suggests that the flow motion could be driven by droplet generation. Entrance of Particle 2 from the side onto the nozzle plate occurred during the third jetting period. Its velocity was as high as 0.9 mm/s when entering the nozzle plate.
Fig. 9. Particle tracking with all nozzles actuating (15.0 V, 5 kHz, 30 mbar).
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Fig. 11. Particle tracking velocimetry during the trial with all nozzles jetting as seen in Fig. 9.
The particles behaved differently as seen in Fig. 9. The velocity of Particle 3 was not significantly affected by the jetting periods. Although all other particles entered onto the nozzle plate from the outside during the jetting periods, when they reached the nozzles, they then had limited motion. It was not clear what
influenced this. Possibility was to have a complex flow regime due to superimposition of the flow patterns initiated by each nozzle. Therefore, a higher distance between the actuating nozzles was used in the trial with five jetting nozzles to investigate this possibility.
Fig. 12. The first jetting period of the particle tracking trial with five jets of caprolactam (15.0 V, 5 kHz, 30 mbar).
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Fig. 13. Particle tracking of the second jetting period.
Fig. 14. Particle tracking in the third jetting period.
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Fig. 15. Particle tracking in the fourth and fifth jetting periods.
3.3. Particle tracking with five nozzles jetting Figs. 12 and 13 show images from the first and second jetting periods of the trial with five nozzles jetting. As seen in Fig. 12(a) and (b), Particle 1 (P1), P2 and P3 had no displacement until the first jetting period started at t = 3.04 s. P3 was seen to move more than the other two during jetting. At the end of the first jetting period, all the particles continued to move, possibly due to inertia as seen in Fig. 12(g and h). Fig. 12 (frames d–h) also shows a particle (P8) being repelled from the actuating nozzles. This was while P1, P2 and P3 were being attracted. This behaviour was also observed with P9 just before the end of the second jetting period when P1 and P6 were being attracted as shown in Fig. 13. The simultaneous attraction and repelling were observed with the other particles during the third, fourth and fifth jetting periods as shown in Figs. 14 and 15. P1 for instance was attracted towards the jets from far left side of the nozzle plate and reached the nozzles just after the end of the third jetting period (Fig. 16(g)). Then, during the fourth and fifth jetting periods, P1 was repelled from the nozzles as shown in Fig. 15 whereas P14 moved towards the nozzles at the same time. Plotting the particles paths during the five
jetting periods helped to visualise the melt flow on the nozzle plate. 3.4. Melt flow with five nozzles jetting Fig. 16 shows the results of the particle tracking with five nozzles jetting. This figure shows the position of the actuating nozzles and the tracked particles. The dots in the graph represent the centre position of the particles (with ±50 m accuracy) for every 0.65 ± 0.05 s of the trial. This could also represent the velocity variation in terms of the distance from the actuating nozzles. Fig. 17 shows the particle tracking velocimetry results (corresponding to Fig. 16) versus time to provide information on the effect of start and end of jetting period on the melt flow. It can be seen that the velocity was higher during the jetting periods. Overall, particle velocity was higher when they entered onto the nozzle plate from the side when a maximum velocity of 2 mm/s was recorded. Fig. 16 suggests that the particles moved in a kind of radial pattern centred on the middle nozzle (in the array of actuating nozzles) and symmetric to the line perpendicular to the nozzle array and passing through the middle one. It also shows that some of the particles moved towards the actuating nozzles and then moved in the
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Fig. 16. Particle tracking in a trial with five individual actuating nozzles in four jetting periods (15.0 V, 5 kHz, 30 mbar).
Fig. 17. Particle tracking velocimetry during the trial with five nozzles jetting corresponding to Fig. 16.
opposite direction. However, not all particles returned from the nozzles (such as Particle 2) and were possibly taken into the ink channel providing opportunity for air ingestion and consequently the jet instability according to de Jong et al. (2006). The effect of multi-nozzle jetting on the overall flow pattern is highlighted with movements of some of particles. Looking at Particles 1, 2, 3 and 9, it is seen that particles introduced to the nozzles from side (with typically 45◦ angle) were expelled out in a direction perpendicular to the nozzle array. This shows a superimposition of flow fields generated by the individual nozzle actuations. Observing the movement of a number of particles at the same time was easy in the video clips and limited images were shown for this in Figs. 12–15. One could relate the particle positions and timings by observing Figs. 13 and 14 respectively, to find out how particles moved in relation to each other. It was found that attraction and repelling of some of the particles occurred at the same locations and times (e.g. Particles 7–12 in Fig. 16). In addition, these particles were recorded with different velocities as seen in Fig. 17. This indicated a complex flow motion in the molten layer and suggests a velocity profile existed across the melt layer thickness where the melt in the air interface moved in opposite direction to the melt closer to the nozzle plate. The particle tracking study with the molten caprolactam showed how the flow on the nozzle plate could take particles to the actuating nozzles resulting in potential for disturbing the droplet formation process. This research helped in understanding the random jet array instabilities that occurred with one of the catalyst mixtures used when attempting to produce nylon. This was because the catalyst complex did not completely dissolve in caprolactam at 80 ◦ C and was found in form of microcrystals which had a tendency to agglomerate which could then disturb the jets.
4. Conclusions Flow on the nozzle plate of a piezoelectric DoD printhead was investigated during multi-nozzle jetting, as a part of a reliability study to develop a new additive manufacturing process. Nozzle plate monitoring showed how nozzle actuation initiated flow on the nozzle plate and could move particles to the nozzle which in turn could lead to jet instability. Quantitative visualisation of the flow was undertaken via particle tracking velocimetry. Different particle motion behaviours were observed that indicated the melt layer on the nozzle plate had a complicated flow regime while jetting with an array of nozzles. Studying the situation with a reduced number of jets with higher distances between them indicated a radial pattern of flow on the nozzle plate and centred to the middle nozzle in the array of the actuating nozzles and symmetric to the line perpendicularly passing through the middle nozzle. The general melt flow seems to be affected by the combination of multiple jets. This means that the interaction of the flow generated by each actuating nozzle could have affected the overall flow which was seen with particles on either side of the multiple nozzles. This could have made a more complex velocity profile than that of a single jet where a superimposition of flow fields generated by the individual nozzle actuations defined the overall pattern. Movement of particles in opposite directions at the same position and time suggested a velocity profile across the thickness of the melt layer. This could provide a recirculating flow across the melt layer and introduce the contamination from the surrounding of the nozzle plate to the actuating nozzles causing jet instability. Studying the melt flow on the nozzle plate could help in understanding how jet array instability is initiated by contamination. As a future work, simulation of such flow behaviour along with the
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experimental analysis could contribute towards reliability in developing the novel inkjet additive manufacturing process. References Beulen, B., de Jong, J., Reinten, H., van den Berg, M., Wijshoff, H., van Dongen, M.E.H., 2007. Flow on the nozzle plate of an inkjet printhead. Experiments in Fluids 42, 217–224. de Gans, B.J., Schubert, U.S., Duineveld, P.C., 2004. Inkjet printing of polymers: state of the art and future developments. Journal of Advanced Materials 16, 203–213. de Jong, J., de Bruin, G., Reinten, H., van den Berg, M., Wijshoff, H., Versluis, M., Lohse, D., 2006. Air entrapment in piezo-driven inkjet printheads. Journal of the Acoustical Society of America 120, 1257–1265. de Jong, J., Reinten, H., Wijshoff, H., van den Berg, M., Delescen, K., van Dongen, R., Mugele, F., Versluis, M., Lohse, D., 2007. Marangoni flow on an inkjet nozzle plate. Applied Physics Letters 91, 204102. Fathi, S., Dickens, P., Hague, R., 2012. Jetting stability of molten caprolactam in an additive inkjet manufacturing process. International Journal of Advanced Manufacturing Technology 59, 201–212.
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