Talk Composites - The Forum for Advanced Composites

Silicon mould stuck to polyurethane resin cast?

http://www.talkcomposites.com/Topic29179.aspx

By maggie8magpie - 8/31/2018 12:27:16 PM

Hey guys,
I'm kind of very fresh to resin casting, doing some personal mini projects only.
The story is: About a month ago I made 2 silicone moulds out of my clay sculptures using CS25 Condensation Cure Silicone Rubber.
It served me great, made about 40 casts (using Xencast P2 Fast Cast Polyurethane Casting Resin) from both of these. Yesterday I decided I need to cast a few more aaaand
One of these two turned out normally, just mixed the resin in equal parts, poured it in and in half an hour I had a ready sculpture to take out.
But the second one... the silicon basically got stuck to the resin? I couldnt take it out, mostly had to cut it out to see what has happened. 
On the first picture it's the silicon stretching, not the cast that has poured into the walls of the mold. On the second photo, once I got it out, its all covered in little silicon bits... 
I'd like to know what have I done wrong? Any ideas? both moulds were identical and made together, same with the resin I used - poured it from the same cup... and one is fine, the other got ruined. Wish I could prevent that in the future!

Thanks!
maggie 

  
By maggie8magpie - 8/31/2018 6:01:00 PM

 
Warren (Staff) - 8/31/2018 3:31:36 PM
Hi Maggie,

Thanks for the picture.  You will be pleased to know you haven't done anything wrong per se.  What has happened is the moulds have worn out from all the casts you have taken. 

Silicone does not last forever when casting from.  Depending on the conditions, type of resin, complexity of the mould/part etc the mould will have a lifespan.  Certain resins will wear the mould out quicker - PU being one of them.  It tends to dry the natural oils out of the silicone which degrades the surface of the mould and it tends to stick then rip off in chunks.  15-20 pulls is typically what you would expect with more aggresive resins. 

You can prolong the life of the moulds by a few pulls by wiping  the mould after each pull with pure silicone oil to help restore the natural oils lost.

Obviously you will need to remake the mould, so  i would use one of the good casts as a "master" pattern for making future moulds from if you plan to make a lot of these in the future.

oh, thanks Warren! Good to hear that. Happy they lasted a few more pulls then Smile Yes, I'll use one of the previous casts to remake it! Cheers!