Compared with the air-cooling in a conventional 3D printing, the liquid-phase-manufacturing offers a much higher cooling rate and thus significantly improves the speed in fabricating the target metal objects. This unique strategy also efficiently prevents the liquid metal inks from air oxidation, which is hard to avoid otherwise in an ordinary 3D printing.

The paper by researchers Liu Jing and Wang Lei is published by the journal SCIENCE CHINA Technological Sciences.

In recent years, these scientists state, metals with a low melting point, especially metals that melt at room temperature, have attracted extensive attention in the areas of computer chip cooling, thermal interface materials, and microfluidics. In their new study, a four-element alloy, Bi₃₅In_48.6Sn₁₆Zn_0.4, was developed and adopted as the printing ink.

The scientists likewise developed a streamlined fabrication process. First, a 3D object is generated as a computer-aided design (CAD) model, and then converted into an STL (STereoLithography) file. The STL file is imported into an open-source software program that generates slices of the object as a set of horizontal layers and that generates tool paths for each layer. The printing ink is dropped into a liquid phase cooling fluid via an injection needle; the object is printed layer by layer.

Droplet deposition process (from A to F) in an ethanol cooling fluid. Credit: Science China Press. Click to enlarge.

During the process of liquid phase 3D printing, several factors affect the final printing quality.

The types and properties of the printing ink dominate the fabrication process. In principle, any metal with a low melting point (or less than 300°C) can be selected as a printing ink on condition that an appropriate cooling liquid is available. The ink material can be an alloy based on gallium, bismuth, or indium, or even a mixture of these alloys and nanoparticles.

Compared to conventional metal prototyping techniques, liquid phase 3D printing offers several distinct advantages, the researchers said:

1. At a relatively high speed of manufacturing, the process of printing metal objects in a liquid phase can be used to form three-dimensional structures. The temperature field and flow field of the cooling fluid can be flexibly controlled. Through regulating the flow velocity and direction of the cooling fluid, some unique 3D metal structures can be realized, e.g. a 3D rotating body.

2. 3D electromechanical systems can be printed. A conductive liquid metal can be used in conjunction with nonmetal materials (e.g. plastic) to form 3D functional devices that include supporting structures and conductive devices. The combination of liquid phase 3D printing and conventional printing can meet all kinds of objectives.

The injection needle array of a future liquid phase 3-D printer. Credit: Science China Press. Click to enlarge.

In the new study, researchers at the Beijing Key Laboratory of CryoBiomedical Engineering also describe a possible future liquid-phase 3D printer. To optimize the accuracy and speed of 3D printing, they propose adopting a combination of a syringe pump array and a syringe needle array.

In this system, the syringe pump array is used to extract the liquid metal solution, while the syringe needle array is deployed to inject the liquid metal ink into the cooling fluid. The injection needles can be replaced conveniently with others of different sizes to meet various printing objectives.

Transforming digital 3D models into printed structures and controlling each needle's injection speed are completed through a computer-implemented process. In this way, 3D metal objects are printed on the bottom of a trough holding the cooling fluid, formed of water, ethanol or other substance.

This work was partially supported by the Key Research Program of the Chinese Academy of Sciences (Grant No. KGZD-EW-T04).

Resources

• Wang L, Liu J (2014) “Liquid phase 3D printing for quickly manufacturing conductive metal objects with a low melting point alloy ink.” SCI CHINA TECHNOL SC, Vol. 57 (9): 1721-1728 doi: 10.1007/s11431-014-5583-4