Annoying Prepreg curing issue

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#1

Hi everyone,

I seem to be experiencing a technical issue I’ve not had before with our prepreg parts. When I cure them they come out spot on… No pin holes or blemishes anywhere, however when I post cure them they come out looking like crap. They get white areas everywhere and I’m struggling to get my head around it. The post cure is done gradually and the issue happens before it exceeds the initial cure. I know that the tg doesn’t necessarily be what the end cure was, so I start the post cure around 70% of what I cured was and ramp 10 degrees every couple of hours. To me it looks like moisture or contamination of some sort but I can’t understand how it can be as I’m extra careful with the thawing and I even have dehumidifiers running in the shop at all times. Also I would expect to see moisture kick in when the initial cure was done

I’m trying to add before and after pics but the files seems to be too big but il try and upload also.

Any ideas or potentials would be much appreciated!

Thanks in advance

#2

Here is how the part came out the mould.

#3

And here’s what happened after post cure! …

#4

I would contact the prepreg company, they would know best. I would also suspect moisture as well if you didnt have this issue before. How long is it under vacuum before post cure? Have you changed anything with process, or resin content? Are you using a perforated, or non perforated release film? Perhaps your change in process has increased RC, and now the resin between fibers has become more visible. I have seen resin systems that turn white after a full postcure, however it seems that you have been using this prepreg system for some time with no issues.

#5

Hi Hojo,

Thanks for the response!

Yes I’m going give the company a call to find out if it could be tougheners or some sort of additive in the resin as this has happened now on 2 separate fresh rolls. This is a new material I’m using so maybe it is a material issue? I’ve allowed the prepreg to thaw out over 48 hrs @22-25 degrees before opening the packaging up. Last night I decided to put the item back into the clave for few hours and ramped heat upto 120 degrees just to see what would happen and the whiteness did reduce in intensity on the surface but it did become uniform all over the part.

As for the process, I’m very particular and everything I do from releasing of the mould - drop testing of the vacuum bag is identical. I only use a bag if it’s a full bar and doesn’t drop below 0.9 over 20mins on drop test. I also keep the item under full vacuum during cure and will run the vacuum for an hour @ 35-40 degrees (heated debulk) before putting the parts in the clave for full cycle.

As this is a new material I’ve started using ( using it for a specific client who requested it) I’m wondering if any other contamination could of caused it? Non compatible with something in the release agent? I know it might sound daft but just trying to cover all potentials…

#6

Keep us posted on this one will you. I couldnt imagine it being moisture to be honest given the process you have described and also due to the fact that you have no problems on initial cure - even if there was moisture in the part after initial cure, could it migrate to the surface during one post-cure below the cure temp?I doubt it.
I assume its epoxy and post cured out of the mould is it? At that heating rate it shouldnt really be a shrinkage issue so yes perhaps it is an additive or the likes. Does the data sheet recommend post-curing?

#7

I would contact the prepreg company. Looks like something has precipitated out of the resin during the postcure. It might be normal if its a toughener or something like that. Depends why the prepreg has been specified for the job, if its for mechanical properties it might be acceptable but if its a cosmetic part then obviously its an issue.

#8

Ive seen a lot of tooling epoxies turn white like that. I think it has to do with the formulation, or fillers in it.

#9

I"d vote for moisture… but if it’s fresh material and thawed well, then it’s probably not moisture.

If this is the first time you’ve used the material, than this could just be a property of the material. Also is this material design for cosmetic use? If not than i’d not worry about that and move on. Though it sounds like you want a naked carbon look?

#10

If you have been using this prepreg for awhile, and only these two rolls are doing this…I’d say material issue.
If this is a new system entirely for you, then it’s what that resin system is. There are many whiteish resins. Odd that it happens only after post-cure, but…it is what it is.

There shouldn’t be a change from cure to post-cure in terms of say, moisture, or particulates.

#11

Who’s the maker of this prepreg?