What does Hollowing mean?

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.

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).

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?

I've heard that objects with the detail support material inside are more likely to crack when dropped: https://www.shapeways.com/forum/index.php?t=msg&goto=8898

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.

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.

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

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.

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.