A novel piezoelectric printhead for high melting point liquid metals

Additive manufacturing technologies have recently attracted a lot of attention as a new highly flexible prototyping and fabrication method. Machines that use polymers as building materials are now a common occurrence in laboratories and production settings across the industry. Despite their superior strength, metals are a far less common building material for three dimensional printing (3DP). Although there are available commercial processes for the additive manufacturing with metallic materials, the high cost of those processes impede their widespread adoption. The most common group of commercially available processes are the so called beam-melting processes, where a focused energetic beam of either photons or electrons is used to selectively melt the surface of a bed of metal powder. The need for a high powered laser or electron beam and the related scanning device is the prime reason for the high equipment cost. Due to this reason several authors have proposed processes which are based on the direct deposition of droplets of molten metal by drop-on-demand (DOD) printheads, similar to inkjet printing. Dispensing minute amounts of fluids, such as adhesives, by DOD printheads is a technique which is now routinely used for electronics manufacturing and packaging. Piezoelectrically driven droplet generators for the dispensing of low melting-point alloys, such as Sn-based solder, are already commercially available. However, the implementation of 3DP by direct deposition of droplets of molten metal has so far been hampered by the lack of availability of a droplet generator suited to the operation with higher melting-point metals such as Zinc (Tm=420 °C), Aluminum (Tm=660 °C), Silver (Tm=962 °C) or Copper (Tm=1084 °C). Most piezoelectric droplet generators are thus far limited to temperatures significantly lower than the Curie-temperature of the piezoelectric material used, usually not more than 250 °C. Therefore, a new working principle for a piezoelectric DOD droplet generator which is capable of operation temperatures far higher than the Curie-temperature of the piezoelectric material is proposed. The proposed droplet generator consists of a tube of fused silica glass (di=2 mm, do=3 mm, l=50 mm). On one end of the tube, a small nozzle (d= 150μm) is formed by melting with a propane-oxygen flame. On the other end a tubular piezoelectric actuator is attached with an epoxy adhesive. The actuator is then bonded to an aluminum base plate. During operation the vertically oriented tube is partly filled with molten metal. On application of an electric pulse the piezoelectric actuator at the upper end of the tube contracts, generating a longitudinal acoustic wave in the tube wall. The empty part of the tube acts as a waveguide which transmits the pulse from the cold actuator region into the hot part of the droplet generator where it is transferred to the metal melt. Due to the inertance of the melt a pressure pulse is created, which leads to the ejection of a droplet. A functional model of the printhead has been built and successfully operated with Sn63Pb37 solder at temperatures of up to 600 °C. To the authors knowledge this is the highest working temperature so far achieved by a piezoelectric drop-on-demand droplet generator.