I need to make a mold

Hello all:)

I am struggling to figure out how to make a mold of an object i made in SUpro. I need to 3d print the mold, then use it to fill with resin or silicone. I have been asked to make a medical protoype of a device and they like my remixed design, i just need to cast one for them.

How do i take my sketch and turn it into a mold i can 3d print?

do i just cover it with a box then use solid tools to subtract it out?


Depends exactly what the model is. If it’s more than a single component or group, use Outer Shell to make it a single group with no interior geometry. Then stick it inside a box. Or make two separate boxes for halves of the mold and use Trim to create the cavities in each of them.


its a single group:) only issue is, i will need to cover the eyelets on it so that when i pour resin or silicone, it wont mold lock on me haha, but i can make that in su no problem.

See my image above. I edited the mold component and removed those parts that would lock the part in.

awesome, Just to make sure i understand you, you made the mold by using solid tools, did a subtract, then removed the mold locks? I will be doing a one part mold.


I actually use Trim from Eneroth Solid Tools so I could keep the origin. Then I edited the mold to remove the cores for the mounting holes and the vertical hole at the front.

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i only need a one part mold, but it does have walls. Its a prototype of a mask. so i need the hollow face part to be solid. That is tripping me up

How do you plan to get the material into the mold and how do you plan to get the molded object out of the mold?

ohhh crp. sorry i was still thinking of a silicone mold… i got to switch to 3d printed mold.

I would need a two part mold then. Its either going to be cast in resin or silicone. I need to add the pouring hols to the mold as well

You can add the geometry for the sprue to your object or just make a separate solid component to use to trim that out. Figure out where the part lines will be so you can split the mold along them. Are you going to need to add draft? Are you going to need to add ejector pins to get the part out?

my part lines will probably be down the center vertically. The final material is going to be a skin safe silicone that is more rigid than the softer stuff.

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Consider that you might also add a core that locks into the mold when you assemble it. I did that for the long hole through the part here. Note that the mold halves are designed to hold the core in place. The core comes out when unmolding the part.

Also don’t forget when adding the sprue to add vents so air can escape when you pour in the resin.


i am having some crazy issues with this. One, the original STL file that i found and remixed, is crapy, two, im botching things up like crazy and dont know what is what haha

What units did you use to import it? Did you run CleanUp 3 on it before doing anything else?

when i first got the stl 53 days ago, i just imported it right into SU pro. I then made my changes, got rid of the non NIH approved stuff, added what neded to be added and then exported the stl. It was a little tough because of how many face, etc it has.

When i imported it, i looked for a units option and found none, because i usually work in the metric system.

I have not run clean up 3 on it. Is that an app or an extension?

I have the idea in my head of HOW i want to do this, but i cant seem to grasp the concept in SU. In the real world when i have seen it done, they cover the sides of the mask with clay, then fill int inside cavity with either resin or some sort of plaster. this makes the inside part of the mold, then they flip it over, pour resin or plaster on the outside of the mask. this makes the two pieces.

Im not gonna give up on this because i need to learn this for other applications i may run into and i dont like to give up either haha

below is an image from smooth-on. this is not my mask but the concept im going for.


There is an option for choosing the units in the Import options panel.
Screenshot - 5_18_2020 , 7_53_43 PM

Imported .stl nearly always have way more geometry than needed in SketchUp. Leaving it can create problems when you start to modify it. CleanUp3 (an Extensions available in the EW) will get rid of the unneeded edges and faces.

ok, i just checked and it was imported in CM, and i just ran cleanup 3 on it. it reduced face count by 356. Anytime i do any type of solid tools, i scale it by 1000, just to play it safe. like you taught me:)

i think what my issue is, i cant seem to “fill” in the cavity of the mask with a solid. ANy time i use solid tools it cuts the entire mask shape out. How do i leave just the inside shape? or am i going about this all wrong?

i pm’ed you the file

You could save yourself some possible trouble if you import using meters.

Not seeing the file.

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i will start doing that from now on. Luckily, when i imported it, i immediately scaled it 1000 times. Did my NIH approved modifications and then called it good.

Apparently you scaled this one down.

Do you mean you want just a core for the inside of the mask? If so, you need to do something to divide the space between inside and out. I don’t know exactly how you want to do that since the edge of the mask isn’t flat. Maybe you need to start with something like this (I only did half of it for the example) and then cut the center free. That will be tedious but you should be able to do it.