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Research Papers

Nozzle Wetting and Instabilities During Droplet Formation of Molten Nylon Materials in an Inkjet Printhead

[+] Author and Article Information
Saeed Fathi1

Phill Dickens

Wolfson School of Mechanical and Manufacturing Engineering,  Loughborough University, Leicestershire LE11 3TU, UK

1

Corresponding author.

J. Manuf. Sci. Eng 134(4), 041008 (Jul 18, 2012) (8 pages) doi:10.1115/1.4006971 History: Received November 02, 2011; Accepted May 01, 2012; Published July 18, 2012; Online July 18, 2012

Inkjet technology can offer exciting benefits in new additive manufacturing processes, including deposition of multiple materials. This in turn requires high reliability in terms of droplet formation consistency and placement accuracy. This paper presents research into such requirements during a research where drop-on-drop deposition of two reactive molten mixtures of caprolactam is used to print nylon 6. Using an inkjet system based on a graphics industry printhead, reliable jetting of the nylon materials through an array of nozzles has been established and as the subject of this paper, droplet formation instabilities (abnormalities in consistent formation of droplets train) initiated by the process conditions were investigated using high speed imaging. With image analysis, nozzle wetting around the actuating nozzle and the droplets train trajectory error were studied. High speed imaging revealed that the nozzle wetting decreased with increasing jetting frequency. Asymmetric development of the wetting area was observed in two situations: (1) when contamination existed near to the actuating nozzle and (2) when air motion changed the trajectory of low kinetic energy droplets formed by low jetting voltages. In both, separation of the droplet tail was observed to occur toward the asymmetric side of the wetting area. The droplet instability behavior initiated by asymmetric nozzle wetting and air motion were understood. For a reliable droplet placement, use of a uniformly wet nozzle plate and higher jetting voltages were recommended to avoid trajectory errors and jet failures as seen in this paper.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Concept of jetting of nylon

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Figure 2

Digital microscope camera and high speed camera positions

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Figure 3

A caprolactam droplet being ejected from a nozzle (15.0 V, 3 kHz)

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Figure 4

Nozzle wetting in a trial with caprolactam after jetting (a) 2 droplets, (b) 3311 droplets (25.0 V, 3 kHz)

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Figure 5

Nozzle wetting area development for first 1000 droplets ejected versus voltage at different frequencies in jetting caprolactam

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Figure 6

Jet trajectory error made by asymmetric development of wetting around the nozzle due to contamination on the nozzle plate in a trial with molten caprolactam (15.0 V, 4 kHz)

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Figure 7

Trajectory error during tail separation in a trial with the catalyst mixture (20.0 V, 3 kHz)

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Figure 8

Increase of the trajectory error over time in a trial with the catalyst mixture as shown in Fig. 7 (20.0 V, 3 kHz)

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Figure 9

Jet failure initiated from a trajectory error in a trial with the catalyst mixture (17.5 V, 3 kHz)

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Figure 10

Rapid increase of the trajectory error over a short time (during the trial shown in Fig. 9) compared with Fig. 8 (17.5 V, 3 kHz)

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Figure 11

Air motion visualized by solid particles remaining from previous jetting trials. Arrows show the change of air motion direction.

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Figure 12

Wetting of the nozzle plate by a jet failure in a trial with catalyst mixture due to improper setting of the parameters (17.5 V, 3 kHz)

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Figure 13

Jet failure and nozzle wetting of a trial with the activator mixture due to improper setting of the parameters (17.5 V, 3 kHz)

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