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# How to make solid shells

How to make solid shells [message #40487] Mon, 19 December 2011 19:23 UTC
Suppose I design a cube and choose a material that has a minimum thickness of x. As I understand it, if I put a hole in the cube, it will be fabbed hollow with all its faces x thick. But suppose I want each face the be 3x thick and solid (no powder between the inner and outer faces)? I can design an inner and outer cube, with a 3x gap between them and a hole through both so the inner-most powder can drain out, but why wouldn't that just create two cubes, one inside the other, both of whose faces are x thick and a gap of x between them? I assume the gap will either be powder or empty (causing the inner cube to rattle around slightly inside the outer one). How can I ensure this gap is actually solid, fused material?

I asked this question of customer service and they referred me here. I use Alibre Design.

Thanks.
Re: How to make solid shells [message #40498 is a reply to message #40487 ] Mon, 19 December 2011 22:22 UTC
Hi Involute, and welcome to Shapeways.

Actually your first assumption is wrong: Shapeways will not add a minimum thickness to a shape with 0 thickness. So if you design only a cube with a hole, it will not print.

If you want a cube to have x thickness you must design two cubes: a outer of size say a * a * a shell and an inner shell of size (a-x) * (a-x) * (a-x), linked by a "tunnel".

For a 3x thickness same principle but the inner cube will be smaller: (a-3x)*(a-3x)*(a-3x). Do not forget the tunnel, so that the inner shell and the outer shell are a single surface (not two separate shell).

Is that clear?

[Updated on: Mon, 19 December 2011 22:22 UTC]

So many things to design, so little time...
Re: How to make solid shells [message #40501 is a reply to message #40498 ] Mon, 19 December 2011 22:59 UTC
Thanks, Magic. What you say makes sense, but I could swear I sent Shapeways the equivalent of a cube with no hole (a few months ago), and when I drilled a hole out came a ton of powder. Are you saying that shouldn't have printed either? Or it should have printed as a solid with no powder?

In your example, what would you get if, instead of using the tunnel you mention, you just use two concentric holes, one in the inner cube and one in the outer, with no connection between them?
Re: How to make solid shells [message #40517 is a reply to message #40501 ] Tue, 20 December 2011 01:26 UTC
the concentric holes, unless linked with polygons, will not make any difference.

i am not sure where you got the cube with"powder" on the inside but i can assure you this was not how it usually works. the powder, i can imagine, didn't cure and thus you have it in it's raw state.

a hollow design is on the designer not on the printer. the printer is ignorant to these things unless the designer puts it in there... if you want further confirmation you should post the 3d file and someone will better analyze it for you.

chris

please visit my blog at...
http://chrisshopofmodels.blogspot.com
Re: How to make solid shells [message #40531 is a reply to message #40517 ] Tue, 20 December 2011 06:42 UTC
Basically the - stupid - printer has to know what is the inside of the object (and must be solid) and what is the outside. This implies that you can go continously from one polygon to anothe without having to "jump" any hole (thus the polygons connecting the two holes). So a cube with a hole (or two cubes with two holes that are not connected) will be rejected as being "non-manifold".
The fact that you find powder inside a model, as Chris stated, is probably an artefact of the printing process (the binder could not dry because it was too deep inside the model or something). It should not have happened.

So many things to design, so little time...

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