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Silicone Mold to Modify a Fragile or Broken Original Part (Post 1 of 2)

June 8, 2017

Sometimes, I have an original part that is either broken or that I want to modify in different ways. In this 2 part blog post, I am going to make several slightly different variations of this part. However, the original is a bit fragile. I don’t want to damage it by sculpting clay on it and then making a mold from it several times over. So, I’m going to make one mold, cast a piece in urethane resin, and use the urethane resin part to sculpt on and make new molds from it. If the part was broken, I would mold and cast the original and then use clay to fill in the broken areas of the original part.

Materials needed:

Your work area:

  • Clean, level work surface, covered in paper or plastic for easier clean-up.
  • All materials comfortably within reach.

Project steps:

  • If your original is unpainted clay, old concrete or plaster, or another similarly porous material, seal it before making the mold. You can do this by painting the part, or using a sealant type material. For my clay original part, I used an clear aerosol urethane sealant.

  • Anchor the original to the bottom of the mold  box with a small amount of modeling clay. For this mold box, I used a plastic food storage box because it happened to be just about the perfect size. You can always make a custom mold box using cardboard and a hot glue as well. This blog post covers how to make a mold box,


  • I weighed out about 900 grams of the silicone Part A base and 90 grams of the Part B catalyst and mixed well until it was a nice uniform pale purple color. The mixing took about five minutes.
  • Since this piece has a lot of fine detail, I used an inexpensive chip brush to brush the silicone into the detail of the piece before pouring the rest of the silicone. This AeroMarine Products AM128 Pourable Silicone has a pot life of 45 minutes, so there was plenty of time to brush some on before slowly pouring the rest into the mold box.

  • 20 hours later, I removed the mold from the mold box and the original part from the mold. It took a little wiggling, because of the undercut of the beard, but it all came out in one piece.

  • Then, I ran into a problem. I will cast the piece in a white urethane. The weather here in San Diego was very uncooperative for about a week. It was overcast and muggy (June Gloom, we call it). The humidity didn’t get below 50%, which is the threshold for working with urethane. Urethane is very moisture sensitive before it is cured.  Mixing and pouring a urethane on a humid or rainy day will result a lot of bubbles in the piece. So, I put this project on hold until the weather cleared up.
  • When the weather finally cleared, I washed the mold with warm water and dish soap, patted it dry and then let it air dry as well. I did this because it had been sitting a dusty warehouse for over a week. Once it was completely dry, I sprayed some Urethane Mold Release into the mold and let it sit for a few minutes. This lets the propellants evaporate and leave behind just the mold release.

  • I mixed my AeroMarine Products Urethane Casting Resin 1:1 by volume, stirring vigorously for about a minute and then poured slowly into the mold. Not super slow because the pot life for the Urethane Casting Resin is only about 3-5 minutes, but slowly enough to let it flow through mold without causing air bubbles.


  • It’s hard to tell from the second picture, but all the little bubbles from pouring the material have popped and the AeroMarine Products Urethane Casting Resin is smooth and bubble free.
  • After thirty minutes, I popped the cast out of the mold. Because of the mold release, it came out very easily.

  • An excellent, sturdy replica of my fragile, original part.

In the next blog post, I’ll go over sculpting, re-molding, and using the urethane casting resin and colorant to create new pieces.

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