Re: Another Shapeways report


Tom Madden
 

Dennis wrote:

I am now realizing that the process used by Shapeways has another pitfall; the basic process can't build parts with overhangs, since there would be nothing to support the first layer of resin that hangs past the previous layer, so the process used by Shapeways builds a wax structure to support this first layer. Unfortunately, it appears that the wax construction is done at a considerably coarser resolution than the actual part, and any surface built against the wax picks up this coarser resolution.
Time to clear up some misconceptions. There are two inkjet-type heads mounted back to back. One contains body material, the other contains support material, in this case wax. Each head is capable of dispensing its material onto any pixel in a layer in the same pass. Same resolution, same layer thickness. On our PolyJet machine, a high-intensity UV light is mounted on the print head in the trailing position and cures the material almost as fast as it is deposited, "almost" being the key word. More on that shortly. I don't know if Shapeways' MultiJet FUD process uses a moving lamp or flashes each layer as it is completed, but the effect is the same - deposit first, then cure.

Think of your part suspended above the build platform and casting a shadow. A few layers of wax are laid down in the exact pattern of that shadow. This permits the part to be removed from the platform. If your part is, say, a cube, no more wax would be required - every layer from there on up would be body material only. If your part was a pyramid, built with the point up, that too would require no more wax. But if you built your pyramid with the point down, the first layer above the wax base would be all wax except one lonely spot of body material in the center. (Keep thinking of the shadow of the upside-down pyramid.) In the next layer the center spot would be larger and recognizably square, but the rest of the layer would be wax. And so on, until the top layer, which would be all body material. The point is, there has to be something underneath the body material on every layer. If it's not other body material, it has to be wax. If you were building a pyramid with a cube balanced on the tip, the pyramid would be completely encased in the wax that provides support for the bottom of the cube. Every overhanging portion of an object will have wax support material under the overhang.

Back to that "almost" issue. Uncured body material is a gel. In the few milliseconds after a pixel of body material is laid down and before it is cured, it conforms to the underlying surface and to its neighbors. Curing the material bonds it to that underlying surface, if that surface is body material. The problems come at the interfaces. An uncured pixel of body material in the middle of a row will merge with its neighbors, while the end pixels in contact with air will pretty much hold their shapes. But if there's wax at the end of that row, the end pixel will merge with the wax pixel and lose its shape. Think of a vertical sidewall as a matrix of tiny marbles. In the absence of wax, under magnification the sidewall will appear uniformly granular. But where there was wax, those tiny marbles that were in contact with the wax will have oozed out from the surface, leaving a rough patch. You can see that in the faces of the double steam control box that was built standing on one end:
http://www.pullmanproject.com/Shapeways10.jpg

The three wax tracks go all the way up to the overhanging cleats on the mirrored component. Fortunately, I only needed one of each (left & right) to serve as resin casting masters, and the part that was built face up provided those.

Hope this all helps understand the process.

Tom Madden

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