Tight Corners With Carbon Fibre?

General Composites Discussions Composites Talk
#1

On my moulds, the edges have little lips on them. When I make a fiberglass part, this is no problem, because I can fill that is with whatever I want. But with carbon has to look pretty and isn’t as flexible. So it rides up like the diagram below

The red is the carbon, black is the mould, and green is the gap I need help with. I’ve seen parts that seem to have just a black resin or fiberglass in these hard corners. Is that my imagination or do they use a black resin or somthing?

Now, even my vacuum bagging attempts have not been able to make this comform to the “lip” and I don’t want to bag everything I do. So I need other tips, please.

#2

Try using a mandrel on the inside with some peel ply?

I think if you had a positve mold of that corner you could lay on the back side of the carbon it would help.

#3

So use a positive mould or some rounded piece to push it down into the corner?

Like this?

#4

man, that’s a high tech drawing :smiley: yes good idea on the “mandrel”. use something as close to the shape of the mold corner as possible. Maybe even cast some plaster of paris to the mold prior, remove it and use it as the mandrel or rtv silicone perhaps. I’m not sure because i haven’t tried either of those ideas myself. Although i need something similar.

#5

RTV silicone is actually a realy good idea, hard enough to push down, but not hard enough to really imprint anything too badly.

#6

Thanks all.

I was kind of losing my interest in this hobby because of this issue. I think this re-spurred my passion. :smiley:

… and I only have MS Paint at work, since I’m a database programmer. So my graphics will be a little lacking unless I’m posting from home. :wink:

#7

if your vacuum bagging you sghouldnt have this problem, and if you are then use a rubber squeegie after the part is bagged to push in to the tight corners

#8

Another good idea :slight_smile: ^^

#9

Not knowing your bagging technique, I would have to say this is the cause of laying the bag down, and pulling the vacuum, with no extra slack in the bagging or even fabric. This will cause everything to stretch across, and not DOWN.
SO, if that is the case. Make the bag bigger, and as you are pulling the vacuum, push in the corners with fingers, et at, and make sure there is no bag/fabric/etc stuck on anything.

Yes, you can use a silicone helper tool, that you would put on back of the release film, and that will give you a more hydrostatic pressure in the corner.

#10

the usual solution is what riff said, its always the bag at fault… it has to have plenty to go in the crevices… its a bitch when you have a bag that doesnt strech (like mine)… anyhow, check if you have full vacuum (around 27mmhg) and help the bag get in there with your fingers. also, keep the bag under vacuum for a long time (i keep the pump running for about 4 hours)

i had the same problems, with a part that has some hard edges , and this weekend im gonna try something new. i made new molds , male ones, and im gonna overlay the carbon on top of them. this way, there will be no bubbles for sure, but im curious about the surface quality. i’ll just wet it and bag it, no infusion. :rolleyes:

#11

I’ve dealt with almost no radius (~ R0.125") and can still get almost no bridging! Then again, depending on the composite material, I can get briding with R0.5"!

#12

yeah a male mold would take care of many bridging issues. I wonder if a smooth non perforated release ply would leave a good finish in that situation?

I’m doing a pair of panels now that i know are bridging. sucks and i have tons of pleats in the stretchlon bag, the pleats were placed in the right areas too. darn problem is i’m not pulling enough vacuum inside the mold. ( i need to switch to 5/8" I.D. vacuum hose and connector in the bag.

One thing that concerns me is that if i cast a silicone rtv part first… will that rtv part be pushed down hard into the flow medium and block the flow of resin during infusion?

What about using clear casting resin to make a tool with to press down the fabric into the corners and over the whole area of the part? I know you need a small space (say 25 mils for cloth and resin). I think i could take care of that by sanding away 25 mils from the “bridging tool” surfaces. If this is a bad idea just say so lol.

#13

I found some info on what is available out there for making silicone/rubber mandrels.

Check out www.fiberlay.com in their mold making section for “Pour-A-Mold” products.

Also www.airtechonline.com has rtv silicone for making mandrels.

www.jgreer.com also sells rtv silicone.

#14

well, you will have issues casting a “part” in silicone, and then using it WITH fabric. Because the silicone part will be the correct size, and then you add in more material it has to squeeze in. You can make a actual part, and then make the silicone pressure tool, that way you don’t have to worry about it being too large.
It shouldn’t cause flow issues unless you use a REALLY soft silicone. If you try the above method, you can use a harder, even glass reinforced silicone tool. THen you have a closed mold…almost.

Might not even NEED flow medium. introduce the resin with a slight pressure, along with the vacuum. Then you got RTM molding!~

#15

RTM really interests me. The thing is unless i can engineer and build an Light RTM machine… i can’t really afford $5,000-20,000 for the set up.

I think i’ll try what you suggest, mold the mandrel tool after i have a part made inside the female mold. That RTV silicone is darn expensive … like $180 a gallon

#16

what about a “casting” polyurethane, it wouldn’t need to be an excessively thick layer as you’d (or I) would still vac. bag, so the silicon/urethane part wouldn’t need to be super hard, or big.