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


Gene – I agree with Tom; it’s trial and error. For my primary production master I used a very hard rubber to reduce sink. However, the mold would shrink between the first and second casting and die at 4 or 5. So I would use the second and third castings to match for the production master pair of sides, etc. For production molds I found one that had very little shrink. Since I used heat to cure the castings in the molds to reduce turnaround time, and a controlled environment, there was little variation between castings until the molds were nearing there end-life when stretching of the molds from repeated casting removal became a factor. As I’ve said before, I cannot recommend particular products because they’re trade secrets. – Al

From: pullmanboss
Sent: Wednesday, July 24, 2013 12:54 AM
To: STMFC@...
Subject: [STMFC] RTV Rubber Mold/Resin Casting shrinkage (was Re: 3D Printed ATSF Tank Cars)

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, "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, "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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