ExOne’s ProMetal 3D Printing

One of my favorite types of 3D printing is the 420Stainless/bronze composite from ExOne. It’s a powder-based process that is very similar to the Z-corp (Now owned by 3D Systems) plaster printers. The processes fall under the same 3D printing patent licensed from MIT. An organic binder “glues” the powder together, and a series of carefully developed processing steps burn out the binder and replace the voids in the powder with bronze. This is an odd mix, since these metals are not very similar. You could not weld them, and this is not an alloy, but a mottled mix of about 60% steel and 40% bronze. Here is a close up of a polished ring:

The metal grains vary in size, but they look about about 25-50 microns in diameter. The crevice in the image is a layer boundary. The polishing process removes projecting roughness, but is not aggressive enough to remove depressions. The material and process produce a very nice aged, textured look. There is always a small flat spot on a model where a stem allowed bronze to flow into the part, but they are much more free of defect than raw DMLS parts, and the price is far less. Aside from the pleasing appearance, the material is resistant to oxidation and very strong (about the same as structural steel).

The metal printed Time Keeper, available customized on Shapeways

The mixed bronze gives the metal a slightly warmer hue than regular steel, and depending on some variables in the process, can come out mildly golden in color. I was curious about the composition of the metal, and measured it with an X-ray spectrometer. The elemental composition in the sample I took was:

Iron 45.00%, Copper 40.30%, Chromium 9.97%, Tin 3.58%, Cobalt 0.58%, Manganese 0.31%, Silver 0.17%, Palladium, 0.05%, Nickel 0.02%

Not much nickel, surprisingly. This mixture has a lot of chromium, but not more than typical stainless steel. The lack of nickel interesting to me partly because, apart from being a common component of stainless steel, nickel works well with copper, the main non-ferrous component in the material, so this is the opposite of what I would have guessed. ExOne has a lot of variations on their process, and can do huge parts up to a meter square, weighing hundreds of pounds. Using this material for years, I keep finding more and more good uses for it. Unfortunately this process needs huge vacuum ovens, so we won’t be doing this at home, but the number of installations is increasing, and even Shapeways has plans to purchase their own setup eventually.

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