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

soolinehistory <destorzek@...>

--- In STMFC@..., "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?"

It's been a very long time since I've had to deal with this, and you've gotten some good answers already, but I'll throw out some thoughts...

All the "industrial grade" mold and casting materials will list their average shrinkage in their tech literature, but this is usually not the case for hobby grade materials. As I recall, shrinkage for each material was <1%, but this can add up when using multiple generations of cast sub-masters.

The typical hobbyist makes a mold from a pattern, compares a part cast in the mold with the original master, can't see any real difference, and says, "the process has no shrink." They don't log the dimensions over the life of the mold, so never see the variance that is possible.

As I recall, most of the mold making materials expressed their shrinkage as as a value at a specific time interval after the mold was made. That is not to say that the material stops shrinking, far from it. I learned early on not to try to save rubber molds; make them, then run them to death over the normal production cycle, and store the parts.

Certain solvents also cause certain RTV rubbers to swell, which causes the mold cavity to grow in size. This was definitely a problem when using the polyester resins that were used to produce early resin kits. I ran some test parts early on, cycling the mold as fast as the parts solidified, and found the third part was one scale foot longer than the first. Given the part was 26 scale feet long, that's almost 4% GROWTH. The polyurethane resins everyone adopted shortly after are not as bad, but I've always been suspicious of Alumilite, since it has volatile fractions that tend to boil off under vacuum.

Given all the variables, it's not so much that the process has no shrink as that the shrinkage is indeterminate. If you can't quantify it, you can't allow for it, so people build masters full size and live with the result.

Dennis Storzek

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