[Katherine Swift]

This question comes through email by P:

Hello,

I have watched some of your wonderful resin casting tutorials on YouTube and they have  been of great help and simple to follow. However, I have  some questions about polyester clear-casting resin that have  been nagging me. I would really appreciate if you could give  me some insights.

1. When  the polyester resin starts to gel and harden in the silicone  mold (approx. 2-3 hr.), the resin edges receded from the  silicone mold edges. I understand that resin shrinks while  curing, but in this case the shrinkage left the sides of the  resin with a coarse surface and scratches, much like  scrunched up paper. How can I make the sides of the resin  smoother?

2. I have both silicone molds shiny surface and those with matte surface. Does this factor  affect what I wrote in 1.? It looks like the one with a  matte surface, which happens to be a large circle, is most  prone to scratches.

3. I have experimented  with both plastic and silicone molds. It seems the plastic  molds creates smooth resin sides. I am puzzled as to why has  not been the case for silicone molds.

4. When working with polyester resin, Ive heard that you  should prevent air from entering for proper curing, and I  saw you cover the silicone mold in one of your videos. Does  this also help make the sides of the resin smooth, rather  than scratched?

5. Does mold release agent/spray help maintain smooth resin edges in silicone  mold? Or, does it only prevent resin from sticking and  extending the life of the silicone mold?

Thank you very much in advance,

[Katherine Swift]

Hi P,

Here are a few of my thoughts:

The scratchy surface you mention is something I have had happen as well when casting polyester resin.  It sounds like the polyester resin is getting too hot.

The surface of your mold is going to impart onto the surface of your casting, so a mold made from a matte finished model is going to make castings with a matte finish.

Covering the molds while curing or whether or not you use a mold release will not impact it. 

I’m also going to send a note to our polyester resin manufacturer to see if they can shed some insight on whats going on.

Here’s an article about polyester casting resin: https://resinobsession.com/resin-resin-resin/polyester-casting-resin/

[Katherine Swift]

Hi P,

Here a couple of comments in ALL CAPS from the manufacturer:

1. When  the polyester resin starts to gel and harden in the silicone  mold (approx. 2-3 hr.), the resin edges receded from the  silicone mold edges. I understand that resin shrinks while  curing, but in this case the shrinkage left the sides of the  resin with a coarse surface and scratches, much like scrunched up paper. How can I make the sides of the resin  smoother? THE SIDES OF THE CASTING WILL BE DRY WHEN ENOUGH CATALYST IS USED SO CONSIDER INCREASING CATALYST BY ABOUT 20% AND KEEP NOTES OF WHAT YOU DO TO BE ABLE TO REPRODUCE THE SUCCESS 

2. I have both silicone molds  shiny surface and those with matte surface. Does this factor  affect what I wrote in 1.? It looks like the one with a  matte surface, which happens to be a large circle, is most  prone to scratches. MATTE SURFACE ON A MOLD RESULTS IN MATTE SURFACE ON THE CASTING. A GOOD QUALITY POURABLE SILICONE LIKE EASYMOLD BRAND SILICONE WILL RESULT IN A GLOSSY SMOOTH SURFACE WHEN THE ORIGINAL ITEM IS GLOSSY.

3. I have experimented with both plastic and silicone molds. It seems the plastic  molds creates smooth resin sides. I am puzzled as to why has  not been the case for silicone molds. GOOD QUALITY PLASTIC MOLDS ARE POLISHED 

4.  When working with polyester resin, Ive heard that you  should prevent air from entering for proper curing, and I  saw you cover the silicone mold in one of your videos. Does  this also help make the sides of the resin smooth, rather  than scratched? COVERING THE MOLD HELPS ALL OF THE CASTING REMAIN A BIT WARMER DURING THE CASTING PERIOD – WORK AT 70-75*F.

5. Does mold release agent/spray help maintain smooth resin edges in silicone  mold? Or, does it only prevent resin from sticking and  extending the life of the silicone mold? RELEASE SPRAY HELPS PREVENT RESIN FROM PULLING BITS OF SILICONE FROM THE MOLD WHEN REMOVING THE RESIN CASTING

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