Talk Composites - The Forum for Advanced Composites

Compression molding prepreg

http://www.talkcomposites.com/Topic29365.aspx

By quinn - 9/11/2018 3:06:29 PM

I think I wanna start playing with some compression molded prepreg parts using aluminum molds. I've already done a couple using wet layup. The male and female mold had calculated offset figured between them based on lay up thickness. Worked out great. One of them was a cap shape that had vertical walls around the outside, so those areas had to completely squeeze down the wall thickness as the male mold was being pushed into the female mold. Didn't seem to be an issue with wet lay up. Here's a pic of the molds and finished part.



Question is, how would something similar to this work out with prepreg? Can it squeeze down to final thickness before curing begins? Wouldn't be a big deal to design the male mold with a slight wall taper to make it a bit easier, but I'm just not sure how uncured prepreg behaves. Aside from dealing with parallel vertical walls, I'm also curious how a more simple part needs to be compressed. I can design molds to bolt together before curing, but can it be clamped to final thickness and stay there? Or does some of this need to happen while curing? Like start to heat and then keep tightening bolts? Or can it fully consolidate before starting cure? Maybe just slight warming while bolting molds together? Just not sure what is the best method here.
By Fasta - 9/14/2018 7:38:14 AM

quinn - 9/12/2018 1:43:38 PM
Warren (Staff) - 9/12/2018 1:29:32 PM
The basic principle is the same except that expansion and contraction due to the heat cycle are very important here.  Ideally you need to minimise or take into account the difference in CTE of the mould and part.

Imagine your mould above in aluminium, the aluminium and carbon would expand as it is heated, then the carbon cures and the heat is turned off.  As the temperature drops, the aluminium will contract more than the carbon meaning the inner mould will probably self release but the outer mould will grip the part making the release very hard if not impossible.   With a composite mould, for example, that issue would be significantly reduced. You could tweak the mould design to aid release and take this into account.

Ah, didn't think about that. Couple questions, is it a bad thing for the male mold to shrink and release during ramp down? Will it affect the surface of the part, or is it already fully cured at the point you start ramping down and won't really cause harm? Second question, for addressing female mold shrinkage onto a part, can it be demolded at a higher temp to try and avoid the mold shrinking down on it? Or is it just too hard on the part to not have a proper slow ramp down inside both molds?

As long as the outer mould also has an appropriate release angle it will also self release as the shrinking mould pushes it out.
I have the same issue with building pre preg carbon boat hulls from fibreglass tooling. Cheap tooling but the parts just self release every time, I leave them overnight and in the morning everything is cool and released, it just lifts out easy. Shrinkage was about 1mm per 1m length.