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What does Hollowing mean? [message #16673] Wed, 25 August 2010 20:39 UTC Go to next message
avatar Aquillyne  is currently offline Aquillyne
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I've heard lots of talk about hollowing but what does this actually mean?

If I upload a normal shape, e.g. a cube, Shapeways thinks it's solid, and prints the material right through - is that correct?

So instead I need to upload what's effectively a box instead of a cube - with internal walls - hence making it hollow. And then Shapeways prints it as a box and it's hollow.

But then I hear talk about leaving holes and so on. So I'm very confused. Do you need to leave a hole? What's that for?

Thanks anyone who can help.
Re: What does Hollowing mean? [message #16677 is a reply to message #16673 ] Wed, 25 August 2010 21:18 UTC Go to previous messageGo to next message
avatar TomZ  is currently offline TomZ
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The printers cannot produce hollow parts. In place of the hollow, they print a support material. This support material is removed later.
If your model does not have a hole connecting the inner and outer cubes, Shapeways cannot access the support material and they have to leave it in place. This is not a problem as this material has to be discarded anyway. The real problem is in the software.

The model you are describing is a "positive" cube, in which a smaller "negative" cube is nested.
The Shapeways software has a problem with negative solids and ignores them. The result is you end up paying for the solid cube, which is very uneconomical.

The solution to this problem is to connect the inner and outer cubes, by putting in a hole that makes the negative cube accessible from the outside. This hole does not have to be big enough to allow removal of the support material, it just has to be there to make the software see the hollow.
Of course, if you want to get an actual hollow model you need to make the hole large enough for support removal. Trapped support material will increase the weight of your model, which in some cases is beneficial, in others, not. In some cases, having support material trapped will decrease the material's performance (this happens with the detail materials).
Re: What does Hollowing mean? [message #16716 is a reply to message #16677 ] Thu, 26 August 2010 12:21 UTC Go to previous messageGo to next message
avatar clsn  is currently offline clsn
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TomZ wrote on Wed, 25 August 2010 21:18

In some cases, having support material trapped will decrease the material's performance (this happens with the detail materials).


OK, this is news to me, and I intend to do a fair amount with detail material and trapped support material. "Decrease performance" in what sense? Make the whole thing weaker, more likely to break? Less stiff? Shorten its lifespan somehow?
Re: What does Hollowing mean? [message #16719 is a reply to message #16673 ] Thu, 26 August 2010 13:40 UTC Go to previous messageGo to next message
avatar TomZ  is currently offline TomZ
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I've heard that objects with the detail support material inside are more likely to crack when dropped: http://www.shapeways.com/forum/index.php?t=msg&goto=8898
Re: What does Hollowing mean? [message #16750 is a reply to message #16677 ] Thu, 26 August 2010 21:05 UTC Go to previous messageGo to next message
avatar Aquillyne  is currently offline Aquillyne
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How big does the hole have to be to let the support material out? Thanks. (Do you need 2 holes?)
Re: What does Hollowing mean? [message #16759 is a reply to message #16750 ] Thu, 26 August 2010 22:14 UTC Go to previous messageGo to next message
avatar gibell  is currently offline gibell
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Aquillyne wrote on Thu, 26 August 2010 21:05

How big does the hole have to be to let the support material out? Thanks. (Do you need 2 holes?)


Check out the Hollow Object Tutorial. They recommend multiple 2mm diameter holes. If you only have one hole, I'd make it bigger, at least 3mm.

If your object is hollow and convoluted with small passageways inside, Shapeways won't be able to get all the powder out (assuming White Strong and Flexible). Then you model can end up leaking small amounts of powder all the time, NOT GOOD. So you need more holes in that case.
Re: What does Hollowing mean? [message #16807 is a reply to message #16759 ] Fri, 27 August 2010 12:22 UTC Go to previous messageGo to next message
avatar Aquillyne  is currently offline Aquillyne
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How small does the hole need to be if you want to keep the support material in? Can I make the hole 0.000001mm thick so that the software understands it but in practice no hole is actually formed?

Thanks.
Re: What does Hollowing mean? [message #16811 is a reply to message #16673 ] Fri, 27 August 2010 13:03 UTC Go to previous messageGo to next message
avatar Drawn-SteelHero  is currently offline Drawn-SteelHero
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It might be best to keep the hole at the minimum detail size, so about 0.2mm diameter (or whatever the minimum stated detail size is for the material you want to print in); I would expect the printer to recognise the hole, but I don't think it'd even be able to guarantee that the hole went through anyway.

Andy
Re: What does Hollowing mean? [message #16813 is a reply to message #16811 ] Fri, 27 August 2010 13:42 UTC Go to previous messageGo to next message
avatar gibell  is currently offline gibell
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If I want the object to be hollow but trap the support material inside I use a hole of diameter 0.05mm. Anything less than the minimum clearance should fuse over, so I would think you could make it as small as you like. You don't really want to see the hole anyway.
Re: What does Hollowing mean? [message #16814 is a reply to message #16673 ] Fri, 27 August 2010 13:43 UTC Go to previous message
avatar clsn  is currently offline clsn
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I haven't yet received prints from anything I did this with, so I might be wrong, but when I'm after a hole-that-isn't-a-hole, i.e. a "hollow" object filled with support material, I make it as small as possible. I mean really really tiny. In my hollow 60-sided die, the hole leading to the center is in the dot under the 6 in "60", and isn't even the size of the whole dot. What I'm essentially expecting is that the printer will ignore the hole altogether, since it's way too small to affect anything. The pricing machinery properly registers its existence; I can tell by the volume calculation and prices.

 
   
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