My Rubber Molds Are Not Lasting

I received an email a couple of days ago from someone saying that the rubber molds they where using did not last more than about 25 pours.

They said that after a while the mold started to stretch and soon after that it would be soft and useless.  (no smart comments here please)  The molds where homemade using a latex rubber compound and had a fiberglass backup.

The concrete that was used for pouring the castings was the fast drying type with fibers added.  The release agent that was being used was vegetable oil "because that is what I have at hand".  There was nothing unusual there except the fast drying concrete, not a lot of people use it because of the cost.  The writer was using it because it makes a nice white casting.  I didn't know that and had a look at some in the local Home Depot, sure enough it says it makes a lighter colored concrete.

Back to the problem...  I at first wondered if the heat from the fast dry concrete was the problem so I heated a couple of old molds with a heat gun (this is like a hair dryer on steroids).  The molds got soft and stretchy but once they cooled off in the shower they where OK.  So I decided it was not the heat.

That night I was thinking about it while having a couple of beers and watching a ball game when it came to me!  It was the vegetable oil.  It will break down rubber.  I have a few friends that dabble in using biodiesel in their cars as fuel and one of the things that happens on some cars and trucks is the biodiesel breaks down the rubber hoses and seals in some of the older cars.  They have to be replaced with hoses and seals made from newer synthetic materials.

I sent an email to the person who inquired and advised them to buy cheap engine oil and use it as a release agent.  I have used it for years and never had any trouble.  I just go to whoever has it on sale and buy a couple of quarts when I see that it is cheap.  It goes a long way so is not a huge expense.

Another good idea is to wash your molds with soap and water and store them carefully when you are done with them.  I have seen so many molds laying around folded up or just dropped where they where used last.  It makes me sad to think that someone paid big bucks for them and they are being destroyed because someone will not look after them properly.