Facilitating post-processing of printed parts using multi-material printing

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To many, 3D-printing is still an almost magical technology that can create objects seemingly out of thin air, much like the replicators in Star Trek. And indeed, with proper design, almost ready-to-use parts can be pulled directly from a hobbyist fused deposition modeling (FDM) printer after the print run is finished. Though more often a significant amount of manual effort is needed to make the part usable, and a lot of this is support removal.

It is surprising how much of this also applies to industrial metal printing using laser powder bed fusion (L-PBF) technology. Indeed, a surprising amount of supports are used and for mostly the same reasons. In L-PBF producing large overhangs can be equally challenging as in FDM, and support are used to facilitate this. Like in the printing of ABS, metals are subject to thermal warping. Though the stresses involved with this are of a different order of magnitude, requiring much stronger supporting structures. To ease thermal warping support are also used to connect parts to the build plate and allow it to act as a heat sink.

And as in FDM the orientation of the part and its design can greatly influence the number of supports needed but cannot always be avoided. In other words, to get the most out of the design freedom that 3d printing offers supports are a necessary compromise. And even then, one needs to keep the support placement in mind during design to be able to tear them off afterwards.

It therefore is not surprising that in the additive manufacturing industry alternate support strategies have been devised. In industrial FDM a secondary print head deposition water soluble polymer is a common solution to the hassle manual support removal. This technology is now even available on some consumer grade machines. 

Figure 1 Schematic representation of material jetting, as presented on the 3d hubs knowledge base

No such approach is currently available for L-PBF, despite this being one of the AM technologies where the strongest support structures are built. This results in a significant amount of often manual post-processing, giving the technology an artisanal flavor. And similar to the hobbyist FDM, The need for supports has to be taken into account in the design of internal features, such as complex cooling channels and chambers. Typically one limits the diameter of these internal features and changes their shape to optimize for building without support. Even then, powder removal can be challenging, certainly is parallel flow parts create dead zones which cannot be flushed out easily.

This cannot be achieved using water soluble polymers, as these have little chance of withstanding the temperatures associated with liquid metal. Furthermore, they would not be nearly strong enough to withstand the forces associated with thermal stresses. While many ceramics, including salts, might withstand the high temperature associated with liquid metal, they also would not be able to handle the thermal stresses which are often tensile in nature and are, furthermore, are thermal shock sensitive prone to cracking. Instead we’d need to look towards other metals. But can these be selectively dissolved?

The short answer is yes, but only to an extent. Metals have the interesting chemical property that one in contact with another metal, and exposed to a corrosive atmosphere, the less noble of both metals will dissolve first. This is in nature electrochemical process has been used to protect sensitive parts of sea going metal ships by strategically mounting sacrificial electrodes, block of a less noble metal, in their vicinity. The process can be sped up by application of an electrical current or by altering the composition of the medium. The metals don’t even need to be too dissimilar. A stainless steel in contact with a regular mild steel can speed up the corrosion of the latter when exposed to a corrosive atmosphere.

Figure 2 Sacrificial Zinc electrodes (arrows) placed on the nose and near the propeller on the tail of a Danish Tumleren class submarine

As this is an electrochemical process, it will also work within blind holes and internal channels, even when there is no line of sight, if the dissolution products get the chance to be transported away. And this is not limited to actual printed support. If a narrow channel is filled with less noble material in powder form this will also be dissolved, making it possible to free up blocked channels. As such building more complex, parallel and/or larger internal channels and chambers might be feasible as long as liquid can be flushed through them. This automated support removal of course also holds for easily reached zones, such as open overhangs

Hence even partially or loose powder might be usable as support material. And this can go to very fine details, finer than we’ll ever be able to deposit. In fact, selective electrochemical leaching has been used to make metallic sponges with micron sized pores.

In summary, all that might separate industrial powder bed-based AM from the convenience of dissolvable supports is the ability to selectively deposit material and some high school level chemistry. Now we have demonstrated multi-metal printing, there is little holding us back from exploring this innovation.

by Bram Neirinck, Ph.D. Senior R&D / Applications Engineer @ Aerosint

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