I am going to make a mold for prepreg, and from my understanding, you need a mold that can withstand the temperature of oven/autoclave and the mold need to be post cured. My question is, for people that who build high temp mold with post cure, how do you keep the mold in exact shape and no wrap during the post cure process (I cannot post cure both plug and mold during post cure since the plug will deform at high temperature).
Another question: this mold is going to be use for bladder method, I’ve seen people simply made their own bladder with vacuum bag, what is the maximum pressure I should watch out before I get the leak from the bag and/or break the mold?
Cure the resin as normal, praying you can do a RT temp cure. Make a full back structure out of the same material (like ribs, and beams, etc) so coefficient of thermal expansion between materials won’t be affected during cure.
Then post-cure at a VERY VERY slow ramp. This will allow slow cross linking in the resin evenly. If possible, do one post-cure at the top most temp the plug will withstand. Then take out plug, and do another post cure.
Second question depends on everything. Your mold thickness/materials will determine the pressure it can take, and the bagging material, part, sealing methods, etc will determine that aspect. If your bladder is net shape, it shouldn’t bridge, and can handle higher pressures. If the bladder isn;t, and it has to stretch to get into the corners, it risks blowing out.
Choose a material that is suitable for a freestanding postcure.
Just as an example:
http://catalogue.airtech.lu/product.php?product_id=513&lang=EN
I must also admit that I am stubborn enough to disagree with the postcure cycle as advertised. As long as tool building is highly important and not core business, I prefer taking my time with the temperature ramp. Basicly what Riff said.
RT temp cure (room temp cure?) I am planing to use high tempt epoxy with epoxy clay for the backing and hope that will work.
The bladder will be made a bit bigger than the mold itself so I won’t have to worry it will rip during the cure process. But what pressure should I set to? 30 psi 50psi 100psi? which pressure is enough for prepreg to get very low void content?
You need a very stiff mold to handle higher bladder pressures. This requires a thick mold (think sandwich of some sort). All my bladder molds are at least 1" thick. I often run 80 psi.
The thing that you will have to watch out for is mold surface damage at the higher temps. High bladder pressure can “dent” the part fabric pattern into the surface of a hot mold…even with high temp resins. For this reason if often use high temp resin that will set at room temp. I’ll do the bladder work at lower temps (110*F) until the resin sets, release the bladder pressure, and then slowly ramp the temps up to post curing temps all while the part is in the mold.
It’s always best to use a “juicy” prepreg and let the bladder press the excess resin out of the mold cavity. You’ll want to use slow ramp rates when making the part so the resin remains migratable for at least 30 minutes. I crack my molds open a few thousands during the resin migration period and then close the mold up nice and tight. This is hard to do with high temp pre-pregs so I rarely use them. If you use dryer prepregs then you have to migrate all the air traped in the weave out of the mold… which is typically impossible and the result is a laminate with a high void content. I often start with laminates that are only 40% fiber. The end result is laminate with a Vf around 60% and nearly voidless.
The key to a fail-free bladder system is to make the bladder the correct size and create a porting method that fully supports the bladder. If the bladder isn’t supported by the mold it will fail at very low pressures.
Do not hope, but read the datasheet. That way you know for sure…