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Blender Newbie.

I read through these related threads, but they don't provide an answer or fix. Also watch several YouTube videos. Nothing seems to work.

What is the most efficient way to invert a mesh so as to inform a key, coupler or mold?

https://blender.stackexchange.com/a/15548

I'm trying to make a mold of a muscle mesh I downloaded and cut into segments for 3d printing.

Muscle mesh to become a mold

Muscle mesh to become a mold

I tried to find a way to just invert the mesh from a positive to a negative. The mold doesn't need to be thick so the outside surface can be a few millimetres.

I also tried using a shrink wrap modifier on a plane over the mesh and this is what happened.

before shrink wrap

After shrink wrap

I tried Booleans and nothing happens. Some of the Boolean fixes I found on YouTube suggested adding a solidify modifier to the mesh, so I have one added to it. Another suggestion was to add vertices to the cube, that didn't work. I've also played with Boolean settings (Fast, exact, self intersect, hole tolerant, etc.)

Boolean modifier added to cube, mesh as target

wire view showing nothing happened

Is there a way to just invert the mesh from a positive to a negative? Or any suggestions for the shrink wrap or boolean methods to work? Maybe something wrong with my mesh settings?

Duarte Farrajota Ramos
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comicreplicas
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  • Hello could you please share your file? https://blend-exchange.com/ – moonboots Mar 27 '23 at 15:57
  • Here's a link to the stl I'm trying to invert or make a boolean with.

    https://drive.google.com/file/d/1L8Eh_eMfMe01wAxOsUD8mmWwoPm29G2J/view?usp=sharing

     – comicreplicas Mar 27 '23 at 17:48
  • it needs an authorization, could you please fix that or use another platform? – moonboots Mar 27 '23 at 17:57
  • Apologies, fixed:

    https://drive.google.com/file/d/1L8Eh_eMfMe01wAxOsUD8mmWwoPm29G2J/view?usp=sharing

     – comicreplicas Mar 27 '23 at 18:02
  • I'm not sure why you say that the boolean doesn't work, it works fine, are you trying to create a plaster mold or something like that? – moonboots Mar 27 '23 at 18:18
  • Nothing happens when I add a Boolean. Yep, I’m just trying to create a one sided negative mold, no keys or border necessary. I think I found a fix. I just added a solidify modifier and then set the thickness to a negative number. this inverted the front face. I think flipping the normals does the same thing. – comicreplicas Mar 27 '23 at 19:12

2 Answers2

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Mesh is not really solid, it just has a little marker on each face called a normal that tells that face which way is out. If you invert the normals, technically, the mesh is now “negative”, but then you need to add something around it to be the outside wall. That’s simple enough, add a cube mesh around the cavity. For a mold, you also need to add some kind of tubing so that Stuffs can get in and air can get out. That’s where a Boolean operation might be helpful, unifying a cylinder (with its normals also inverted) with the cube and the cavity.

TheLabCat
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  • Would you mind sharing a link explaining the first part? I’m a bit lost about the outside wall? Like create a border around the mesh with planes? How would I invert the mesh, because I actually just need this to be a negative? I won’t need the tubing, because I’m just pouring latex directly into the mold in layers, it’s not like a traditional mold. – comicreplicas Mar 27 '23 at 17:00
  • If you make the shape you have a negative, then it’s an air bubble in an entire universe of solid mold material. So, you add an outside wall where the mold stops and air starts again. – TheLabCat Mar 27 '23 at 21:50
  • I say again, mesh is not solid. It’s only a surface, and each face has a marker that tells it which way is out. You need to flip those. They’re called normals. – TheLabCat Mar 27 '23 at 21:51
  • You will need at least one hole in the mold, or you’ll have nowhere to pour into! – TheLabCat Mar 27 '23 at 21:51
  • I will try to make a visual step-by-step… – TheLabCat Mar 27 '23 at 21:52
  • Hmm, I'm having a bit of trouble... – TheLabCat Mar 27 '23 at 22:18
  • All good thank you, I think I got it to work! – comicreplicas Mar 28 '23 at 13:29
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You could try this method. First make sure that your topology is clean (you can use the Mesh: 3D-Print Toolbox addon:

enter image description here

Then create a plane that you stick to the surface of your object with the help of the Snap to option:

enter image description here

Extrude the plane a bit (I guess the shape will depend on how you want to cut the mold afterwards):

enter image description here

Extrude to give thickness to this plane so that it contains your original object:

enter image description here

Give a Boolean modifier to your plane:

enter image description here

Apply the modifier, you get your mold, again check its topology with the Mesh: 3D-Print Toolbox addon:

enter image description here

If you need your mold to be in several pieces, you need to separate the plane in different pieces before the extrusion.

You could also create your mold with a fattening operation of your original object, then remesh in Sculpt mode then remesh in Object mode, but it will need more steps.

moonboots
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  • Thanks for this! I'm using this method now! – comicreplicas Mar 28 '23 at 13:27
  • Oh OK, glad it works, do you have to create a mold in separate parts? – moonboots Mar 28 '23 at 13:29
  • No just the front of the surface. So I wouldn't need to make the backside. What I did was run the checks, and nothing came back bad on cont. edges or non-flat faces, but it did have some high values on non manifold edge and zero faces. So I'm watching tutorials on how to clean up topology. Then I'm doing what you suggested by creating a plane, give it some thickness and snapping it. I'm also going to try applying a shrinkwrap mod and then doing the boolean. I'll post an update tomorrow hopefully. – comicreplicas Mar 29 '23 at 00:15