prepreg resin colour differences showing through

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#1

I’ve only had this twice. First time I ignored it as a random oddity. But its happened again. Ive had “print through” (if you want to call it that) from the backing layer through to the surface. Its not print through as in the carbon fibres showing through the resin, but the patching of backing shwing through. I know I could just use a single layer of backing, but the offcuts need to be used somewhere!

Im guessing its the difference in clarity between the two resin systems. Where the backing layers overlap, the part is slightly thicker there, creating a resin build-up. Another part was cooked at the same time alongside it with the same layup yet that one doesn’t show this effect.

Forgot what the cooking times were the first time. This time it was 65 for 1 hour, 75 for 10 hours. Using VTF261 and PRF 464.

I’ve edited the photos by upping the contrast to highlight the effect. In real life its a bit less noticeable.

You can clearly see the corresponding lines that came through from the backing:

#2

Whats that? a caterham/lotus super seven sort of cyclewing?
Do you use tinted resin?
I’ve had the same problem too. I even tried to order black gluefilm once, because I had several pink spots in my products, haha.
Curing the layer first would solve the problem, I don’t know any other way. I used a black tinted resin in the outer ply, clear resin on the inners, that seemed a little more subtle.

Or you could use the scraps for parts which get painted :wink: I only used scraps for the carter protection shields, or smaller parts. Cooked the others together with another curecycle, so it wasn’t chemical waste anymore.

#3

Yea, caterham cycle wing.
Its prepreg, so the VTF surface has a darker resin than the back.
I’m still scratching my head as to why it happens.

If I kept scraps back for other bits, I’d have a fridge full of scrap carbon!

#4

I’ve had it when the salvage edge, which is dry, sucks resin from the first layer.

#5

I am at a loss too… Not had the same problem, but then I have only use either scraps of VTF or SHD’s LTC250 for back up and not had a problem… Odd as the PRF material is really resin rich too… could be diluting the resin of the VTF??? But it is an odd one…

#6

Im pretty sure its a resin cure problem. I would re-adjust the cure cycle and possibly try and cure get the surface to gel faster as PRF backing looks like its going off alot faster than your surface . I think if you try and do this with your cure cycle it will solve the issue

#7

If the prepreg has pigmented resin, than that will occurr any tima there will be an uneven resin distribution in the first layer: wherever there will be zones with different compaction, it’s likely this problem will show through.
Next time you should try to put the inner layers butt jointed (precisely).

#8

butt joining precisely takes time. What is more expensive, throwing away scraps, or put a lot of time in using them? That depends off course on a lot of things and you can only decide for yourself …

#9

This. butt the joints. I’ve had issues with foam cure edges lifting the tooling layer of fabric up (not the resin, just the fabric), causing odd formations on the colouring. If you overlap so close to the surface, the space between each layer can pull up the surface layer just enough to notice.

#10

On the foam core: round of the edge a little bit on the mould side. Not too much. the glue film will fill up the tiny cavity, and pressure from the core is not concentrated on the edge that much any more. Worked on relatively flat surfaces.

#11

I fillet all my edges, even with 1/8" core. I’ve given to just curing the first skin, then core/everything else.