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

Tom Madden

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

--- In STMFC@yahoogroups.com, "Gene" <bierglaeser@...> wrote:

I've been thinking about my questions below. What I should have asked is, "Who can tell me about shrinkage and what should I know?"

--- In STMFC@yahoogroups.com, "Gene" <bierglaeser@> wrote:

You mentioned shrinkage in the RTV rubber mold and resin casting processes. Others have mentioned shrinkage as well. It must be significant if it is mentioned so often.

Is there a given size or volume above which shrinkage matters?

Does the rubber mold shrink?

Do resin castings shrink a predictable amount or are we talking trial and error?

Is shrinkage expressed as a per cent?

Do various resin concoctions shrink at different rates or all the same?

Gene Green

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