A couple of weeks ago I pulled out an old casting I had done several years ago made out of RTV-2 silicone and decided to lay some glass and epoxy into it for kicks. The result was a really nice part that virtually fell out of the silicone mold with no release added! I think I’m going to look into this further and try to make a high durometer silicone inner mold with a hard mother mold and possibly a softer silicone intensifier with a pressure plate, no vacuum used. Am I wasting my time or is this a practice sometimes used?
sounds good.
silicone is great for this sort of stuff. It’s self releasing and works great with epoxy. A silicone mold with a reinforced back, usually plaster or fiberglass, is called a pocket mold. There are many kinds of silicone in varying durometers that work great. If you don’t have undercuts, low draft angles, or very intricate parts, a high durometer works well with good durability and good expansion vs. a lower durometer that helps with any draft/negative draft issues.
Definitely not wasting your time! 
I have spent some as well with some types of rtv silicone and i think that if you dont plan on using them with prepreg cf then it is an ecselent choise, but if uou do have in mind that you will have to consider factors such as thermal expansion and the epoxy is attacking the silicon in hot temperatures…
I’ve use silicone for prepreg, mostly for prepegs actually. They work fantastic for prepreg and would highly recommend their use in the right situation. The best thing about the silicone is the thermal expansion. You can get autoclave type results using silicone and the expansion to compact the laminate. They’re not cheap but, effective. They are good for room temp as well, it’s nice to not use release on the mold and have a flexible mold that can allow for some undredraft or tight details.
I don’t think the epoxy ‘attacks’ the silicone per se. The silicone is pretty resistant to chemicals and if used in RT situation would last a long time. The heat of the cure cycle is really what does the silicone in over time. even in a room temp cure the exotherm could degrade the epoxy. In an elevated cure, the temps will degrade the silicone over time for sure. And the hotter the cure the faster the degradation. Also it’s probably a good practice to do a post cure on the epoxy as well.
Your response sammy and creator is much appreciated and has further sparked my intrigue of silicone molds. I am now giving thought to the issue of thermal expansion using a silicone mold with a silicone intensifier. My questions are as follows.
- Would an initial force cure of say 100 to 120 degrees F have much of an effect on expansion of the silicone? Hopefully I can do this to cycle my molds 2 times daily.
- Is it possible to do an in mold post cure to facilitate an epoxy with a higher TG, say 220 F?
- Would you use the same silicone for the intensifier as the mold for controlled expansion in post cure? Accuracy of the part is very important.
Jon
http://www.compositescentral.com/showthread.php?t=9484
this is a post of mine. It will help you.
For example i was coking at 120 celcius and i had significant expansion - about 3-4 mm for 120 mm part, but at 50 degrees celcius it will be be minor. and the attack on the silicone will be less as well
Jon,
Some silicones can be cured at elevated temps, some maybe not. Depends on the silicone, always check the tech data sheets or contact the manufacturer to ask. Usually it’s a good idea to do a post cure.
You’re talking about post curing the part in the mold right? Most silicones have very good high temp ratings. 220f is not really high temp. Silicones are usually resistant to temps upward of 400f.
You should be able to use the same silicone intensifiers for intial cure and for post cure, but i guess like most things, there are more ways, than one to do the job. I guess the main thing is to design the molds accommodating for the thermal expansion of the mold and materials during cure/post cure.
You should probably figure out exactly what you’re trying to accomplish and then maybe get any fine details about the various silicones from whomever you’re planning on buying from. There are many fruits and flavors to choose from.
Again, I would like to express my gratitude for your response, hobbyist and professional alike. Sammy, your professional knowledge seems to be fairly extensive, and the info I have gleaned from your’s and creator’s posts has driven me further to explore in much more depth the application of silicone molds. Silicone just seems to be the perfect fit for my project. The beauty of my process is that if one silicone is not optimal I can peel it from the mother mold, assemble my master plug and mother mold and do a cavity pour for a new one. I understand that the higher durometer platinum silicones have a very high viscosity(and require a post cure to achieve their highest durometer rating), but in my case I can accommodate a fairly large channel in my mother mold to facilitate a re-pour. Mixed viscosity in my case becomes less of an issue. More thoughts later.
Jon