RTV Rubber Mold/Resin Casting shrinkage (was Re: 3D Printed ATSF Tank Cars)

Gene <bierglaeser@...>

Thanks, Tom. Very interesting. I've seen "consumer" sized packaging of the rubber and resin for modelers but I don't recall specifications. Maybe inside the package, huh? May I assume that the specific rubber (Silicone Inc.'s GI-1000) mentioned must be purchased in "industrial" quantities?

In my situation I can "dial in" the desired room temperature and humidity. That came about merely because I didn't want wood bench work to expand and contract with changes in humidity, not that such changes are extreme in this area anyway.

Right now it appears that in my situation trial and error will likely get me to my goal sooner than anything else. I had no idea resin could get so hot in the curing process. One could get burned.

Gene Green

--- In STMFC@..., "pullmanboss" <pullmanboss@...> wrote:

Gene, it depends on the rubber/resin combination. They're all different, and it will be on the materials' spec sheets. I use a low-shrink resin that's more expensive than what many use, and it and the rubber I use (Silicone Inc.'s GI-1000) have a combined shrinkage of 0.004" per inch. That's less than half a percent. A fast resin that cures hot will have much greater shrinkage. It may reach 200 degrees F when it "kicks" and cures, then shrink as it cools down to room temperature. (Coefficient of thermal expansion and all that.) The temperature(s) at which you make and use a mold can have a greater effect than mere resin & rubber shrinkage. Cured silicone rubber has a larger coefficient of expansion than cured urethane resin. If you make a mold in the winter when your room temp is 68 and use it during the summer when it's 80, the castings may well be bigger than the master.

Nothing is as simple as it looks....

Tom Madden

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