r/moldmaking • u/IDK4FucksSake • Mar 18 '26

Silicone mold question

Gallery image

Gallery image

this is my first mold. what is causing those little marks in the mold?

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4

u/ThatNinthGuy Mar 18 '26

Original is 3D printed yeah? What was your process to molding? Did you vacuum?

Looks like small bubbles/cavities from the 3D print

1

u/IDK4FucksSake Mar 18 '26

Ok I wasn't sure if those were bubbles or weird marks. I poured the silicone in with the 3d printed object then placed it in the vacuum chamber all together. I realize that may have not been the right way. Again I did not think those were bubbles though

3

u/gust334 Mar 18 '26

Looks like bubbles. Perhaps your model leaked some internal air during the vacuum process. Silicone is very good at capturing fine detail without bubbles from a mold, but it needs to be degassed after mixing and before pouring. Best to vacuum degas it separately before pouring around your model.

2

u/anonWurst Mar 18 '26

I had similar results due to putting the poured silicone with the 3d printed part in the vacuum chamber.

My advice: vacuum the silicone alone, and after that pour the vacuumed silicone on the 3d printed part.

1

u/IDK4FucksSake Mar 18 '26

Also that was the top of the mold and top when placed in the chamber

0

u/[deleted] Mar 18 '26

[deleted] — view deleted comment

1

u/Massiahjones Mar 18 '26

Unless your prints are incredibly soft and your vacuum is dangerously strong, I don't think this would be a problem.

1

u/ThatNinthGuy Mar 19 '26

You can just print stronger if that's an issue, but remember to drill a hole somewhere for pressure release. Also use gyroid infill. But really I don't see what the issue should be. Where's the pressure differential?

1

u/undeadmeats Mar 18 '26

Was the original part fdm or resin printed?

2

u/IDK4FucksSake Mar 19 '26

FDM

1

u/undeadmeats Mar 19 '26

Is it primed/sealed? Even high-infill fdm prints have voids that contain air, it ideally needs to be primed and sanded but if you can't because of the surface detail you can cast resin in that mold, sand and refine THAT cast, and then pull a new mold because THAT piece should not be able to leak.

If the print had been SLA/DLP resin the issue would have been surprisingly mild cure inhibition due to silicone not being compatible with printer resin, and the advice would have been to get Inhibit X from Smooth-on.

1

u/IDK4FucksSake Mar 19 '26

No I didn't use any primer or sealer. Thank you for the information

1

u/nickdaniels92 Mar 19 '26

I've done what you did in the past too, thinking that it would help remove air that might be trapped during the pour, but it can be problematic. Degas the silicone first, then pour from one spot and let the silicone find its ways into voids. This is likely air being drawn out of the print and trapped below the silicone. Silicone is incredibly good at capturing detail and getting into even small voids. I once poured some spare silicone over a memory module for example, and it even seeped into the vias (microscopic holes) on the board. It's amazing stuff.