Pan Pacific Symposium Conference Proceedings

A Fast Pneumatic Droplet Generator For The Ejection of Molten Aluminum

Authors: Dominik Rumschoettel, Benjamin Griebel, Franz Irlinger and Tim C. Lueth
Company: Chair of Micro Technology and Medical Device Technology Technical University of Munich
Date Published: 2/6/2017   Conference: Pan Pacific Symposium

Abstract: Contactless dosing of minute amounts of molten metals in the form of micron sized droplets is a promising technology with applications in the area of three dimensional printing (3DP) and electronics manufacturing. However the generation of droplets of higher melting point metals, such as aluminum and its alloys, has proven to be a challenging task. Difficulties arise mainly due to the high temperatures needed to get the metal into its liquid state and the inherent chemical aggressiveness of fluids like molten aluminum with its high reduction potential. Those conditions preclude the use of most of the common drop-on-demand (DoD) operating principles for the generation of metal droplets.

One promising way to drive a droplet generator at high temperatures is by pneumatic actuation. The basic set-up for a printhead based on this drive principle consists of a crucible with a nozzle attached to the bottom. A short pneumatic pressure pulse is applied to the surface of the metal melt inside the crucible and causes the ejection of a droplet. Therefore the temperature sensitive parts of the drive mechanismcan be kept far away from the metal melt because theenergy needed for droplet ejection is transmitted by the gas cushion between the pneumatic valve and the hot working fluid. The inherent robustness of this approach allows working temperatures in excess of 1000 °C. Droplet generators based on this working principle have been used for the ejection of molten metals for research purposes by Chandra and Jivraj [1], Fang, et al. [2], [3], Cao and Miyamoto [4], [5], Tropmann, et al. [6], Lass, et al. [7] Zhong, et al. [8] and Luo, et al. [9](see Table 1). A disadvantage of this actuation principle is the high compressibility of the drive gas. In combination with the relatively high volume of the crucible and the supply lines of existing designs this leads to a low acoustic bandwidth and thus limits the maximum achievable droplet repetition rate to a few tens of Hz. In addition the comparatively long duration of the pressure pulses makes it difficult to achieve droplet diameters below 500 µm. Those properties limit the usefulness of common designs for practical applications like three dimensional printing, where a very high number of small droplets have to be dispensed in an acceptable amount of time.

In the present paper a new droplet generator design is proposed which tries to overcome those limitations by consequently minimizing the acoustically effective volume of the crucible and the supply line. A model of the proposed design is fabricated by 3DP and operated at room temperature with a eutectic Gallium-Indium (GaIn) alloy. The successful operation of the model printhead is demonstrated up to a droplet repetition rate of 500 Hz, which is the highest working frequency reported for a pneumatic DoD printhead so far. Based on the experiences of the model a heated prototype printhead is designed and operated with the aluminum alloy AlSi12(A) at temperatures up to 800 °C.

Key Words: 

Metal Jetting, Pneumatic Droplet Generator, Drop-on-Demand (DoD), Additive Manufacturing

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