Is it possible to get good surface details with silicone caulk molds?

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helix_3

Active Member
I have been using this procedure to make open-back silicone molds for a few miniature items. I can't modify the master objects, so my procedure goes master -> mold 1 -> resin cast -> modify resin cast -> make final poured silicone mold for production.

Since the first mold only needs to be used once, it would be great if I could use the cheaper silicone caulk option. However, I have found the surface finish of objects made using the silicone caulk molds to be inferior to both poured silicone and pressed putty options from Alumilite. Small shallow details are sometimes lost, flat surfaces have slight distortions and defects, etc.

Is there any way to get high-quality surface casts from a silicone caulk mold? Or should I just bite the bullet and buy a bunch of Alumilite putty?
 

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PlanetAlexander

Active Member
I've been wondering about using that technique, since it's easier and I've already got everything, but I'm thinking of just putting in the extra money and getting some proper sillicone. For me I can get it at around $100 (AUD) but that might mold a couple of objects, maybe even ones I can sell off casts of.

I also think it's important to get as much detail from the first mold as possible so you don't lose all that detail instantly. Haven't done any mold making yet, but seen lots of others do it.
 

renaissance_man

Sr Member
RPF PREMIUM MEMBER
In a word, no.
If you really want the best possible replica with high detail capture then a low viscosity, ideally degassed, RTV addition cure silicone mold is your best option.

It's the old adage "buy cheap, buy twice".

Good luck with your project.
 

jarvis

Sr Member
You can do a "print coat" with the caulk. Paint a very thin layer directly from the caulk tube on your model with a brush. Some people add a little acrylic paint to make it easier to see your coverage. Wait until it starts to set a bit where it is still sticky but won't come off on your fingers then follow up with the soapy water blobs.
That being said I agree with everyone else. Buying purpose made silicone will yeild better results and save you time and frustration.
 
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Gerard2567

Active Member
I've used silicone caulk once for molding. ONCE.

It was the black stuff used for windows that smells oddly like vinegar. (Yes, the silicone GLUE)

And I only used it to mold a small clay piece that I wanted a resin copy to add to a helmet.

Long cure time, like, 48 hours. Mold didn't lose any detail or have chunks missing out after a cast, but I'm not too sure on the long term use.
 

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helix_3

Active Member
Thanks for the advice everyone. I did some tests using both the soapy water and cornstarch method, and I can confirm that you'll only get mediocre results. I tried molding both small toys and coins. The toys did OK because you get good detail on the ~1mm scale geometry, but the smooth surfaces were rough and ugly. The subtle details of a US quarter were almost completely destroyed by the rough finish.

I still think silicone caulk is a useful material, however. For around 30% the cost of Alumilite putty and 50% the cost of pourable silicone, you could make large, decently detailed items that can be sanded to improve surface finish.

The best outcome I could get was using this procedure:

Mold step
  1. Prep master object by washing with soapy water, then dry. Place onto a flat surface, like a piece of glass
  2. Add 5 drops of food dye per pump of silicone caulk. Stir until consistently mixed. Now that moisture has been introduced, there is a limited time window until the caulk starts to set, so make sure to move somewhat quickly.
  3. Put caulk into a bed of corn starch. Cover hands with corn starch, and knead until it’s a consistent doughy texture. Re-powder hands if it starts to stick.
  4. Put ball of caulk aside. Wash hands, and put on gloves
  5. Rub mineral oil onto gloved hands until they’re slippery, but not dripping
  6. Knead caulk until it’s a consistent color, with no hints of powder
  7. Roll into a ball, and press onto master object
  8. Let cure for around 30 minutes (I didn’t carefully time this step). Gently press your fingernail into the back of the mold to check if it’s done- any stickiness or permanent scratching means it needs more time. Also leave it alone if there’s any vinegar smell.

Resin casting step- I had some issues with resin sticking to the mold. Cornstarch seems to do alright as a release agent, but there might be better options out there
  1. Gently dust some cornstarch into the mold. You want as thin a layer as possible, and no clumping/ accumulation in the corners
  2. Level the mold, and pour in your resin of choice.
  3. Allow to cure like normal, then pull and enjoy your cheaply made but mediocre surface-d object.
 

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