# Problem creating hollow solids with a wall thickness

I know that using the ‘Offset Surface’ command with solid option checked you can create a hollow solids with a wall thickness. But this method failed when I have complex geometry. I then modified the geometry of the offset surfaces and joined them together to create a polysurface and tried to run the ‘CreateSolid’ command and the ‘Boolean Difference’ command to see if it will work. As a result, neither of them worked for me to create a hollow solid with a wall thickness because there is no intersection geometry. Does anyone have any idea how I should approach this?

Try the new V5 “Shell” command.
It’s not perfect, but it’s a lot better than the old V4 method you described.

As I understand, ‘shell’ let’s you create an ‘open’ solid by having to remove a face from the datum surface. What I need instead is a ‘closed’ solid object with wall thickness. Like those rotational moded plastic objects that you might see around.

An easier explanation of my problem would be: if you create a sphere primitive, ‘OffsetSRF’ by the thickness you want with ‘solid=yes’, you get a hollow sphere solid with wall thickness as output. Then say if I explode the solid, rhino will give me to surfaces as the output (two spherical surfaces of different diameter). So the problem is: how to unite those two surfaces back into the original output of a hollow sphere solid with wall thickness. Hope this example illustrates my question well.

Like in real world a hollow sphere it’s impossible to get, and Rhino3D does exactly the same.
You can achieve something similar by make a small hole connecting the inside to the outside… like real world.

Well, it’s not exactly impossible, otherwise we would not be able to play tennis or ping-pong…

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Technically tennis ball are made of 2 halves glued togheter, and pingpong have the moulding part division line. So it’s Impossible

Soap bubble anyone?

Auch!!!

… but we can consider it as a thin single spherical shape, or not?

With infinitesimal thickness?

Doesn’t really matter how it’s made, the end product is still a hollow sphere. Try Boolean Unioning two half spheres with wall thickness together (as the tennis ball is made) in Rhino…

Hollow objects can be made in one piece via roto molding or 3D printing…

–Mitch

Exactly Mitch!

I think the problem here is one of the conceptual problem people have of describing a solid.
3D CAD geometry does not have a way of defining the “solid” material between the inner and outer surfaces. It doesn’t matter whether you are milling or 3D printing. All that the output device cares about are the inner and outer surface definitions - the stuff in between can be considered “dark matter”

If we imagine a model of a plastic drink cup lid, no one has a problem with this as it’s one fully enclosed surface and all the normals point outwards.

It’s when we have hollow objects such as a sphere with wall thickness where the inner and outer surfaces are separate and do not touch that the conceptual difficulty always seems to come up. All I can say is, “Don’t worry about it!”.
As long as the inner surface/mesh has normals pointing IN and the outer surface/mesh has the normals pointing OUT, it WILL print. Some software will ignore the inner surface though, so in those cases it’s best to add a small hole somewhere to join the whole thing into a single surface. Obviously you can’t mill such a thing as it stands but that’s a whole different story.

So, to reiterate: Don’t worry about the “dark matter” in between, all that matters are the inner and outer surface definitions.

hopefully that helps, Steve

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Well, I personally don’t worry about it, but… as an experienced user I understand how Rhino works and it’s particularities. For example, I have the two concentric spheres as we’ve been talking about - which don’t form a solid for Rhino, but I want them to act that way - I can group them or something like that… But if I run a cut plane through them and then do a BooleanSplit, I would want to get two “solid” halves. Which I don’t, I get 4 separate open shells and no end caps…

I can do this correctly with NonManifoldMerge and CreateRegions as Chuck suggested in another thread, but these are kinda hard to find by the average user…

–Mitch

Now we can 3D print objects, hollow solid is a must-have feature!