shrink factors for rubber that is injected

[captam21]

I am a tooling designer primarily for injection presses of the REP brand and am looking for any input on how to reliably obtain shrink facters for compounds since a lot of our venders are not wanting to pay for prototype tooling any guidance would be appreciated ........thanks

 

[ElastomerXpert]

Industry practice has been to make a simple mold (one I had was 1"x0.25"x6") and make specimens from it and measure shrinkage directly. However, molding can result in anisotropy (especially injection molding) so really tight numbers on shrinkage are hard to come by. You can have a shrinkage test indicate 2.5% shrink, but one dimension of the actual product shrinks 3+% and another shrinks very little. Good luck... You may want to get the super authoritative book on Molding by a guy named John Sommer, available from the Rubber Div of the American Chemical Society.