Need to create an inverse or shell of a 3D surface

I am new at SU and am trying to create model to 3D print in plastic, which I can then use as a mold to cast acrylic. (Think: The inside surface of a mask.) So, I’ve created an example of the object I want to cast in acrylic. Now, I need to make the inverse of that (or in other words the negative space of the object or a shell of the object’s surface) to print in plastic- so that when I fill the 3D printed plastic with liquid acrylic, it will cure into the shape I’ve created in this file:test.skp (398.1 KB)

How can I make something that is simply the shell of the top surface of the object? Please help! Thanks in advance!

It’s easier to do than to explain.
You basically need to intersect the face with the surface, then reverse everything and make a box around it.

Mold.skp (244.5 KB)


I love the good old positive/negative illusion that comes from looking at the face part of your “mold”

Using SketchUp Pro makes this process simpler: you can create the box and use the Solid tools to substract the positive from the mold.


@Box’s and @Anssi’s answers were correct, of course, but I think perhaps we have still not hit bottom conceptually.

SU is a surface modeler. There is no mass or material “inside” of an SU model. There is only the thin, thin skin of the modeled surface, and nothing on either side.

A surface representing a positive volume in SU is not merely similar to the surface representing its negative or figure-ground alternate; it is identical. The positive and negative are the exact same geometry, just seen from opposite sides.

To derive the negative counterpart of a positive surface, you don’t really have to subtract the positive feature from a block; just turn the surface around and you have the corresponding negative.

By convention, we designate the side of the surface we are going to look at as “front” and give it a white color attribute, and we call the other side “back” and show it as light blue. So to obtain the figure-ground alternate of a surface, just turn it around and Reverse Faces.


Just to clarify @Gully_Foyle, what you say is absolutely correct, but in the case of this model the face wasn’t part of the surface so first needed to be intersected with it and cleaned up before it could be used as a negative.

Yes, of course. As I mentioned, I’m speaking mainly conceptually. I simply recall that for me, I was able to obtain a figure-ground alternate procedurally (as though I were dealing with solid material) before it really dawned on me what I was dealing with.


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Thanks for the help everyone. @Box maybe you could explain how you intersected the face with the surface, reversed everything, and “cleaned it up”? Sorry, I’m new at this and don’t know what that means exactly. But the model you made was exactly what I was looking for! Now I just need to learn how to do it myself :slight_smile:

He is a simplified version of how I did it. But your face was more difficult to intersect as it has a very complex structure and I needed to find the little edges that didn’t cut.
Try doing it with more basic shapes until you get to understand the process. Then you can battle the nightmare of complex organic shapes.

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@Box A bit late to the party, but I’ve been trying to make a 3D mold and your instructions and gif helped so much. I literally made this account just to thank you because this would have been so hard without your help. Thank you!


Thanks for that, it’s good to know that even four years later the gifs still help.
It’s the gif that keeps on gifing.


Haha! Keep up the great work!

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