Use of silicone bags in an autoclave 1

[DB-TACG]

Has anyone used silicone bags for curing prepregs in the autoclave? Right now the cure temps we are using are on the lower end at 250F for the prepregs. Im curious about the pressures it can handle and if the life cycle of the silicone is worth the use. Any experience with this material wil be helpful. Thanks

 

[Compositepro]

Pressure is rather irrelevant, because the the pressure should be equal on both sides of the film. Silicone bags are made to the shape of the part so that there should be little stretch required during normal use. There are a number of resins that are corrosive to silicone, so FEP film should be applied so there is no contact between the two.

 

[DB-TACG]

thank you for the info Compositepro. Let me be a litle more specific about the silicon bag(s)and how I'm looking to use/test it. The product being built is in two parts. Similar to a hat stiffiner and skin cosntruction. These are made on heated tooling in a presure vessel and have run numerous successful cycles with and without pressure. Since this part is for R&D and demonstration purposes an effort made to use silicon bags, heated cauls and a combination of heated caul and silicon/Airpad (Airtech) are all going to be tested. The approach with silicon is to test for longevity and if it has a pressure intesifier effect on the areas with tighter radii. We are looking to use cheaper materials to build cheaper parts for these tests. Basically higher quality of the cured lamiantes without the cost normally associated with the autoclave cure cycle. Nothing is being made with qualified materials or required certs and there is less scrutiny of the finished product. The attempt is to make a common product as inexpensive as possible with pressure so a lot is on the table for us in terms of materials.

I have only used silicon bags on flat tools. But I've used plenty of regular bagging materials. Im curious about:

1 - The number of pulls off a silicon bag before replacement. This approximate number would be if the silicon is kept from contact with the resin. I believe the exposure to lower cure temps may extend the life a little...maybe
2 - Has the silicone bag shown some benfits of being a pressure intensifier in tight areas. If made thicker or etc. in those areas.
3 - In dealing with bridging in tight corners, how does the handling of the silicone bag vs standard bagging techniques used compare to each other.
4 - is there a preferred method of sealing the bag to the molds for prepregs and pressurized cures.
5 - a thought is to make numerous silicone bags on a seperate tool and replace them on a caul as needed.

Any comments or suggestions even relating to the use will be helpful.

 

[Compositepro]

It does not sound like you are doing anything new, just that you are on the early part of the learning curve of how to cure composite parts. Your vendors should be able o help you solve any issues. Just be aware that the value of their advice depends entirely on the experience of the individual giving the advice. And there is a lot of bad advice out there. Be skeptical and apply basic principles. Vacuum bagging is a very simple process but very few people actually understand it very well. The primary misconception is that a vacuum-bag creates a vacuum chamber type environment. A vacuum-bag is just an impermeable membrane (relatively). The "pressure" is the same on both sides.