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silicone vs. urethane rubber ? Answered

what differences would arise in the process if making a mold from the much less expensive urethane rubber as opposed to silicone? If i was planning to cast with resin ,are there any advantages to using one material over the other when making my mold?


Paige Russell

2 years ago

Hi Matt,

Polyurethane rubber is a great material that can be used instead of silicone rubber in most mold making applications, which can offer cost savings for sure. However, I choose silicone over urethane when making smaller molds for the following reasons. Polyurethane rubber:

- Is slightly more hazardous to work with, requiring more serious ventilation and safety precautions
- Will stick to most materials, making mold release a must instead of a 'just in case'
- Is incompatible with water or moisture. Any amount of water on a model or mold wall will cause that area to foam and prevent curing. That spot will be sticky FOREVER.

NOTE: Even the moisture content in DRY wood can cause this. So every model that has ANY moisture content (wood, plaster, water based clay) MUST be sealed with several coats of spray lacquer (water based clay) or acrylic (dry plaster and wood) which adds extra work/steps. You must additionally add a release agent before pouring the mold.

NEVER: Use polyurethane to make a cast in a gelatin/alginate mold (gelatin contains water).

As for casting in resin, the only advantage to using silicone over urethane is that nothing sticks to silicone. So if you miss a tiny spot when applying a release agent in a silicone mold, no big deal. But if you do the same on a urethane mold, that area will stick to the cast, making it hard to remove the piece from the mold.

If you're making a large mold, urethane will keep the cost down, so in this case it's worth dealing with its cons.

Hope this helps! Happy making. :)