How to make a mold of an object

I have been wondering if it is possible to somehow make a mold of an object. Take for example, a lower receiver of a AR15 style paintball gun that has been modeled in SU. I currently do not have a model yet, but hopefully i will once i can somehow get the stl to be edited in SU.

My goal is to make a mold of the receiver, then print the mold, pour resin in it and have a new reciever for my paintball gun.

It narrows down to these two questions:

  1. How to make a mold?
  2. Can i convert a .STL file in SU to be editable so i can use that as my piece for the mold?


What kind of mold do you want? For resin, I expect a silicone mold would make sense. Instead of modeling the mold, you should model the receiver, suspend it in a box and pour the silicone mold material in to fill the spaces.

If you really want to model it, figure out how you would split the mold, draw boxes to that shape and use Trim with the model of the receiver to create the negative space.

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probably resin. I was going to go the route you mention, but someone told me that would be very difficult because i would have to add keys, do multiple molds because of the negative space etc etc. But to me, it seems easy to just suspend it in a box, pour silicon over and call it good.

I have never cast or done any type of molding ever. its all new to me haha

For experience, modeling it would be cool. just to say that it can be done.

Adding keys to register mold halves is trivial. You’d need to create the mold pieces for the interior. That shouldn’t be difficult, though. You just have to think about how you’ll get the pieces out.

Silicone will be easier. Just make sure you only have the inner part of the mold connected to the outer part in one place.

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ok, ill keep researching it on the web. maybe ill call the univeristy art department i go to just to see it done ine person. New things means i need to see it to get it. My brain is amazing that way:)

Switching gears for the moment, there are alot of stl files i get from the web that are kind of in bad shape, can they be imported into SU to be edited with the native tools?

thanks for the help and ideas. I have never done this. Its interesting how much i DONT know

Yes. You can import STL files and clean them up. Depending on the file, it might be easier to model them from scratch, though.

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Just made this mold but not sure if i understand it. The part is “floating” in it. I used trim from eneroth to make the part inside.

where would i pour the resin haha? Did i do the trim wrong? should it have left a “hole” of the part rather than adding the part into the block?


You’ll need to add a sprue and some vents so the air can escape. To save time, i would add those things to the positive model before running Trim.

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ok cool. i will add the vents and the sprue.

I feel like i did the trim wrong. as in, i selected the box first, then the part, rather then the part and then the box haha.


Never mind… i tried it the other way and it didnt work haha.

Ok, for the sprue, thats what attaches to the slot shape so i can pour the material right? Once i pour it, i guess i just cut away the mold to get the part out?

this is making my brain think backwards and i dont like it haha

Agree with everything Dave has said. Make the part itself first, then pour the mold around it. This allows you to first refine the part with hand tools, use clay to change the shape, etc. and really see how it fits in your hand before you mold it.

At this point it sounds like you are having trouble visualizing the process, you should go to youtube and start watching videos.

I would start with something simple first as a test and experiment. Maybe try taking an existing simple object and molding it just to understand the process and not waste materials.

If your part has any areas with critical mating surfaces or hole tolerances you will want to leave extra material and then finish later with proper machine tools. It’s probably not a huge issue but there is shrinkage which can affect finished part size and shape.

I have been watching videos, alot of videos, but all the ones i find are of solid objects, like a bird, a helmet, a blade, model car, rings, etc. Nothing that is like the shape i have.

Ill have to look at the post again, im replying via email, but in case i didnt state it, this is for a Ar style lower receiver for a paintball gun. Yes, pla, and nylon will work, but thinking for learning, that resin would be nice.

My issue is, i know how to add vents, a sprue, and what not, but once i cover the entire thing in silicone, getting the part out is the problem i foresee. Since it is thinwalled and hallow, cutting the mold and extracting the reciever without damaging the mold seems difficult haha

I’m sure you can find videos that will show you this stuff.

One option you might consider for separating the mold is to add a thin wall to your positive to split the mold in half. If would look a bit like flashing–the excess material where the parts of the mold or die come together. You’ll likely wind up with flashing somewhere on the part and it’ll need to be cleaned up after the resin has cured.


If you’re only making one, then damaging the mold won’t be an issue. The more durable silicone molds can withstand some very complex geometry and undercuts, there’s a great instagram account I wish I could find right now.

For some hollow areas you may want a separate removable core which takes up some of the interior space, resulting in your mold itself having a thinner wall. After molding your part, you can pull the core out first, which will allow the mold walls to wiggle and collapse a bit and make removal easier.

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If you search You Tube for “lure mold making” there are lots of videos out there covering both solid and flexible products. I don’t know whether any would suit you but there’s a good selection.

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