PrePreg Molding

General Composites Discussions Composites Talk
#1

I’m changing to a 250* F cure prepreg, and need to re-create my tooling. At at 250deg F temp range, is it really necessary to create the tooling out of carbon? Could I get away with maybe 1-2 layers of carbon, and then backed with glass matting for strength/thickness? I am concerned about the thermal expansion differences, but I’m not sure if I’m over-thinking this or not…

If the mold did need to be made of full carbon, any idea on thickness? I have access to 20oz carbon twill at a decent price, that I could use to back up the mold with.

#2

Carbon tooling will be much better, but you can get away with fiberglass. Problem with fiberglass is CTE as you mentioned, and also heating up the moulds. You will likely find that the carbon tooling will make your parts have a better finish, be more tolerance tight, and do not need to demould hot. Not sure the size and shape of your mould, but for small moulds 4-6mm is generally ok. larger moulds generally should have a support structure, ideally also made from carbon

#3

we use some molds that are fiberglass. I believe they’re infused fiberglass molds. We lay up carbon parts. I think the enineers account for thermal expansion in their designs, definitely so with tooling board molds… those expand a lot.

Haven’t had any issues with the glass molds. Parts demold fine and surface quality is fine. The molds have metal support structure with casters to they can move around. And these are fairly thick molds and pretty long, like wing size.

I’m not sure about mixing glass and carbon for the mold? The carbon molds i’ve used are all carbon no glass. Ultimately carbon is best for the previous listed reasons but, more costly.

Are you using pre preg tooling fabric?

#4

No I was planning on using bi-axial carbon fabric, and infusing it to create the molds. I won’t be mixing carbon and glass into the mold though, it would either be 100% glass or 100% carbon. And regardless of the fabric I would be infusing these molds. My other concern will be whether or not to infuse all 4 layers of fabric at once. Not sure if the exotherm would get me into trouble or not.

#5

No I was planning on using bi-axial carbon fabric, and infusing it to create the molds. I won’t be mixing carbon and glass into the mold though, it would either be 100% glass or 100% carbon. And regardless of the fabric I would be infusing these molds. My other concern will be whether or not to infuse all 4 layers of fabric at once. Not sure if the exotherm would get me into trouble or not.

#6

In my opinion, for small moulds, the cost difference between an infused fiberglass mould, and an infused carbon mould is minimal, and definitely worth the slightly higher cost.

If you do it right, there is not many reason why you shouldn’t infuse all at once. If the shape is extremely complicated it may be worth considering, but single shot should be achievable. Just be sure to strategically cut your fabric so that there is no bridging in the corners. This is most important. That and high vacuum integrity. Be sure to check your vacuum at inlet using a digital absolute pressure gauge like the GDH200-14.

#7

I’ve been looking into infusion molding but have yet to come up with a good way to make my flanges strong enough to hold and infuse the part? How are you guys attaching the part or flanges and making things strong enough to hold shape while infuse mold making?

#8

in the case of copying an original part that has no flanges, you can do it a few ways; 1. use MDF or other rigid material to make the flanges, make sure it is vacuum tight. 2. Splash fiberglass off the originals with traditionally made flanges, then splash a hard plug off the splash moulds. 3. 3D scan and CNC your plug.

#9

I’m going to attach the plug to a flat board (what hojo described), then pull vacuum against the board with plug attached to it.

#10

Always good posts man! Thank you.

#11

You’re welcome. Also, on another note, if you are using epoxy surface coat, this helps to seal any possible leaks. Good luck