How to avold resin shrinkage?

[aye_captain]

Hi, I've casted some headsculpts with polyester resin recently. But i noticed a significant amount of shrinkage. How to avoid this problem? I read it somewhere that you need to use less catalyst, or work in warm environment.. Please help.

 

[logan25]

Use polyurethane resin, it has practically no shrinkage, polyester resin always shrinks a bit, usually between 3 and 5 % i think

 

[aye_captain]

Thank you logan25, but it seems that it is hard to find polyurethane resin in my local. But i'll keep looking.

 

[Cheungkinmen]

Polyester resin will shrink no matter what you do. I've tried adding talcum powder or calcium carbonate as but no luck.

Less catalyst means longer cure time, but that depends, the part you are casting might get all gooey because of the lack of catalyst. So there is a balance.

When the resin gets too hot it's when the part starts to shrink because the styrene "evaporates". This is just a guess. So, some people put the mold in the freezer when they finished pouring to soften the kick. I haven't tried it because I gave up on polyester resin a long time ago, and also because I have food in my freezer.

 

[Cheungkinmen]

When you are casting solid pieces the heat would be quite a lot, resin starts to radiate heat from the center outwards, so yes, use less catalyst and try to use a filler like the ones I mentioned, talcum powder or calcium carbonate, even poly fiber and see how it behaves.

Depending on what you are doing, another would be rotocasting the piece.

 

[aye_captain]

Thank you for your explanation Cheungkinmen. Today I managed to get my hands on Polyurethane resin. More expensive than Polyester resin, but worth every penny. I'm very satisfied with the result. Thank you guys

 

[Fangs of Death]

This thread cropped on Google when I was trying to stop my damn heads from shrinking, using polyurethane resin (fastcast)--wanted zero shrinkage. One of my historical figures in particular was curing too narrow in the face, almost ruining the likeness.

Anyway--fixed it! After about ten casting attempts, different methods. And what a game--ended up having to rotocast it (roll around a partial amount in the mould until it stiffens, making it hollow), then apply pressure with powdered bronze (for weight and fineness) poured into the nearly cured cast, then push a cone (mastic nozzle with the end heated shut) into the metal powder with some pressure...to PREVENT the resin from shrinkage. After a first ballsup, it worked!
See the head on the right, too narrow for a reasonable likeness. Head in the middle--full size! Ahhhhhhhh relief.
***

 

[Cheungkinmen]

This thread cropped on Google when I was trying to stop my damn heads from shrinking, using polyurethane resin (fastcast)--wanted zero shrinkage. One of my historical figures in particular was curing too narrow in the face, almost ruining the likeness.

Anyway--fixed it! After about ten casting attempts, different methods. And what a game--ended up having to rotocast it (roll around a partial amount in the mould until it stiffens, making it hollow), then apply pressure with powdered bronze (for weight and fineness) poured into the nearly cured cast, then push a cone (mastic nozzle with the end heated shut) into the metal powder with some pressure...to PREVENT the resin from shrinkage. After a first ballsup, it worked!
See the head on the right, too narrow for a reasonable likeness. Head in the middle--full size! Ahhhhhhhh relief.
***

Awesome!

 

[Fangs of Death]

Cheers mate!

BTW--interesting that hollow resin still shrinks the sculpt when left to its own devices...I wouldn't have thought it would. Because it shrinks in every direction--not only thickness, but the vertical (a distance from the rim, up the head and down the other side to the other rim), and circumference (all the way round the head). The last two (curved) distances are considerable, and shrinkage noticeable! Live and learn.