silicone bladder question

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#1

I want to make an inflatable mold for a carbon handbrake lever i plan on making in pre-preg. For the mold itself, i’m using PTM-W 2520 with a 1945 surface coat, which is rated for about 330F but the prepreg i’m using is 250F cure. I have some donated uncured silicone sheeting that AirTech donated to class. Its about 1/8" thick, and probably overkill for the size of this part (~14").

I plan on first doing a vacuum bagged part of each half, and then using that offset to vacuum bag and cure each half of the silicone. HOOOOOW do i combine the cured silcone halves?

Here’s a pic of of the part for reference. Its a wetlayup part that I bought…that immediately broke. -_-* Any help would be appreciated.

#2

I would combine them in the mold before they have cured.

The problem with doing it after is that hardly any adhesives will reliably stick two pieces of silicone together.

I only know of two products that will adhere to silicone. One is more silicone and you could use that as an adhesive. The other is Silpoxy sold by Smooth-on. Personally, I wouldn’t rely on it to hold under the pressure when you inflate. Making it as one piece is the way to go.

I use the brush-on vacuum bag silicone (also from smooth-on). The problem is that you need to apply it in multiple layers. The way to get around the problem is to apply the first two layers in each mold half separately, then join the two mold halves after applying the 3rd.

If you use the spray on bagging silicone, you can apply it over a smaller plug which you can cast with a layer of sheet wax in the mold.

Either way, you will want to buy some silpoxy to adhere the inflation tube and valve to the silicone bladder.

#3

You have to put your original parts and the silicone “halfs” back in your mold, and run a bag through the tube. Cure the silicone in the oven while under vacuum. You will have to design a cap or cover on both ends of the tube so you can apply pressure with an air hose. Pull vacuum on your air fitting and pull your silicone bag out of your part.

#4

Another consideration when making any bladder system is taking into consideration the thickness of the laminate. The bladder needs to be the same as the interior dimension of the finished part, not the exterior dimension or interior mold dimension. Silicone will take those temps and more, no issue there. and releasing is easy. I layup silicone bladders the same way Zebra mentioned, 2-3 layers brushed into each mold half with about an hour between coats. For bonding the two halves, I use a small syringe to apply a bonding bead to the upper edge of each half followed by closing the mold and setting aside to cure. This all needs to be done before the silicone is fully cured to bond properly. Also take into consideration how you intend to attach your connector for inflation, you want to do everything in one shot. Post curing after air curing overnight is recommended for a higher sheer strength.