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Old 05-05-2015, 01:25 PM   #11
Crash71

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I know it's been a few years since this thread was created, and a little more than a year since the last post, but I wanted to give a bit of information so that others can find it useful.

Vacuum bagging onto a rubber tool is difficult but the results all depend on the durometer of the rubber. A low durometer, up to 45A, will distort under vacuum pressure. A higher durometer, over 55A, will hold up much better. 60A hardness or higher will likely not distort.

I choose the hardness of my silicone molds based on the detail of the part being molded. If there is a lot of negative draft, I need a lower durometer silicone because the elasticity is much higher, so its resistance to tear is higher than a high-durometer silicone. If I need to remove the mold from the part like a wet sock by turning it inside out, a lower durometer rubber is needed because it is harder to do with a high durometer rubber. How the silicone is removed from the part, and the amount of detail on the part being created, should be the first thing to consider when choosing the hardness of the silicone to use for the mold.

If a lower durometer silicone is needed but you still want to vacuum bag or infuse the fiber reinforcement, adjust the vacuum pressure to about 6 inches. Six to eight inches of mercury is all that is needed to infuse a Light RTM part in the cavity of the mold system when the B mold is a shell laminate. If the mold has no negative draft, create a smooth caul plate so that the pressure distribution is equalized across the bagged surface, and infuse at the lower pressure.

Also, consider the type of silicone you want to use. RTV silicone is softer and does not hold up as well as Platinum cure silicones but platinum silicones are very sensitive to contamination, so an inhibitor is needed.

Lastly, I have had students use silicone molds for vacuum bagging and the parts turned out great because we followed the principles listed above. I also know of a company that makes faux rock fiberglass window wells using the VIM process with a high durometer silicone A mold and a silicone bag system, and the surfaces are not distorted, also because the principles listed are used.

Hopefully this helps someone.
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