Of all the various methods and materials used in 3D printing, metal printing remains one of the most difficult, expensive and time-consuming specializations. Not only are its components and printers very expensive, only a few metal materials currently lend themselves to this process, while the material's melting and solidifying phases are all very time-consuming. All this makes it's applicability rather limited and its energy consumption extensive.

A new Chinese research paper, that has recently been published in Research China last September, is therefore particularly intriguing. Entitled 'Liquid phase 3D printing for quickly manufacturing conductive metal objects with low melting point alloy ink', this article is the fruit of the labours of Beijing engineers Lei Wang (Chinese Academy of Sciences) and Jing Liu. (Tsinghua University). 

In it, they are sharing the very interesting results of their own research into a 'liquid phase' printing method, an experimental metal printing method that uses metals that melt at a low temperature to greatly reduce printing time and costs. Furthermore, it utilizes particular print nozzles and cooling methods to optimize the necessary time to develop shapes.

While a number of research teams are looking, or have looked, into this particular method, it comes with a number of drawbacks that Wang and Liu are trying to overcome. Their particular concept could very well be changing the field of metal 3D printing.

Liquid phase printing above all seeks to overcome the lengthy cooling time of traditional metal manufacturing by working with metals that melt at room temperature. While certainly diminishing printing time, this also restricts their functionality as the finished products can easily melt.

Wang and Liu have therefore instead limited themselves to printing metal inks whose melting points are above room temperature and less than 300°C (which traditional metal printers require). For the purposes of their research, they have focused solely on one particular alloy: the Bi35In48.6Sn15.9Zn0.4 alloy, which demonstrates 'the basic working principle of liquid phase 3D printing method.'

This mixture of bismuth, indium, tin and zinc has a melting temperature of slightly above room temperature, making it an 'ideal liquid metal printing ink to implement the liquid phase 3D printing as proposed in this paper.'

However, their method can also be applied to a number of other allows. As they explained in their published paper, 'these include gallium-, bismuth- and indium-based alloys. Addition of nanoparticles such as copper, silver particles into such metal fluids also offers a method to fabricate functional inks as desired. Besides, combination of metal and nonmetal material together can be adopted to make diverse printing inks.'

And this is how fabrication in liquid phase printing takes place. Much like regular printing, 3D objects are first rendered and converted into STL files and sliced into horizontal layers. Then 'The printing ink is dropped into the room temperature ethanol cooling fluid from the injection needle whose movement is controlled along the tool paths and the object is finally printed layer by layer.'

The printing phase itself actually consists of the deposition of fused droplets. Because of the low temperature at which these metal particles become liquid, they easily return to their solid form again. 'When one droplet falls onto the solid column below, the top of the column absorbs the heat and fuses with the droplet. As the droplet is cooled to the temperature of the cooling fluid because of heat transfer, it becomes a part of the column which will then grow into certain structures as desired.'

This is done at an exceptionally high speed and allows for very quick metal prototyping. The innovation in this research is above all in, as they stated, 'the high thermal conductivity and heat capacity of the liquid phase cooling fluid compared to other cooling mediums.'

While the results currently consist of low resolution prints, optimizing nozzles (well, they are actually air-pressured syringes), cooling fluids, and droplet sizes can potentially lead to highly-detailed prints of whatever is needed. And of course, as a large variety of alloys (and mixtures) can potentially be used, there is still some wiggle room to be found there as well.

This figure shows the injection needle array of a future liquid phase 3-D printer. Credit: ©Science China Press 

Nonetheless, Wang and Liu convincingly argued for the benefits of this printing method throughout their paper.

Especially interesting is the diversity of the printing materials that could be used in this particular method. As they argued, '3D electromechanical systems can be printed. Conductive liquid metal can be used in conjunction with nonmetal materials (e.g. plastic) to form 3D functional devices which include supporting structures and conductive devices. The combination of liquid phase 3D printing and conventional printing method can better meet all kinds of printing needs.'

Furthermore, this could potentially be done at a fraction of current prices. Even in the research phase, their droplet-printing method is already proving to have an exceptionally fast manufacturing speed. They have also noted that 'the manufacturing energy consumption of metal components will be reduced. Due to introduction of low melting point liquid metal ink, the energy required for the solid ink to melt is small.'

While by no means a functional printing method yet, all this is nonetheless looking very promising. Could this paper be pointing at a next-generation of affordable and efficient 3D metal printers?

( www.3ders.org   )