Problem with flat panels bowing

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#1

I have tried a few times now to make some flat panels but always seem to have the same end problem, once I remove the bag and lift the panels they are bowed and not perfectly flat.

I’m can’t figure out what is causing this but it’s really getting to me. They are made with 3 layers 6 oz carbon and 3 layers of 6oz glass. I have tried to lay laminates 0,45,90 to each other but still I have the problem, I have layed them all 0 to each other and the same thing happens. The surface I am using for is sheet of glass about 1/2" thick, so it’s not because of the glass that it’s bowing.

Any help, or suggestions? thanks.

#2

I could think of a couple things that could cause that.

1- resin shrinkage, demolding too soon
2- resin pooling your resin isn;t distributing as evenly as you might think
2- differing layers weaves of fiber are twisted. Try to get the strands of fibers as straight as possible

I’ve done alot of flat panels, epoxy, VER and PER. Fiberglass and carbon and have never had major bowing problems. With std poly I’ve had some issues but nother really Major or to be overly concerned about, even on larger sheets of 4’ or more in length

#3

Are you laying three layers of carbon on top of one another, then three layers of glass behind that? If so that is the reason why.

Gotta go: carbon,glass,carbon,glass,carbon. 0-90-0-90 etc or 0-45-90-45-0

#4

I use epoxy and I had the panels in question under heat lamps for a whole day then I had the part sit for at least 24 hours at room temps. Only then did I remove them did I demold.

I did infact lay 3 layers of carbon and the 3 layers of glass, so you are right. Next time I will try the way you mentioned. Why does it make them bow ?

#5

yep that is why they bowed.

#6

what’s the reason behind it? any idea?

#7

probably they shrink in a different way. one material is more prone to shrinking than the other. just like thermal switches. two metal rods with different expansion characteristics glued together, so when temperature changes, it bends, thus opening or closing the circuit. what was said, using a different material one layer after another might be your solution. i would say that you should start and end with the same material though… like: C F C F C

#8

cfcfc sounds like good advice…

#9

To put it simply, your laminate is not balanced.

#10

you can also do, carbon/glas/carbon, as long as there are equal carbons on both sides. also your laminate may be too thin, if you go thicker it will help the problem

#11

I’ve done 16 layers of 0/90 glass before, and it bows. I can only imagine that 1: I didn’t do 0/90/0/90/90/0/90/0 schedule, or somewhere there was a mismatch in the fibers, having more fibers along one direction, or even offset a bit.

Hell, I’ve done 100 layer panels, and they warp and bow.

flat panels suck, no matter what it is!
Remember, most fabrics, the warp and weft are different weights!!!
One layer of 0/90 can warp due to more fiber pulling in 0 than 90 (or whatever)

#12

jeez, 100 layer panels? what are you gonna do, hit somebody to death with it? :smiley:

#13

Every book on composites that I’ve read said, flat panels bow, or distort.

So I put a slight curve to my panels to start with.

#14

Those techniques aren’t much better. You are still missing a lot of reinforcement in the +/-45*. Also, the plies need to be inverted from the middle due to warp and weft differences.
There’s a lot more to a balanced laminate than just turning the plies 90 degrees!

#15

TET, you made a couple of references to the laminate balance and inverting the material from the middle. I would appreciate any more in-depth comments. When you say invert, I assume you mean turn upside down? I guess this would be putting the “crowns” that two seperate panels would form into opposition to each other?

#16

Yes, it’s pretty simple. But can get complicated/confusing when dealing with certain types of materials, weaves, and combinations.
Every thing needs to be mirrored. Let’s say you have a panel using 4 plies. You roll out the material, and cut the plies. The first two plies go down just the way they were cut. But the top 2 plies get flipped over so that the original “top” side is now facing down.
Same thing applies when using core materials. The core is the middle. Top and bottom skins need to be mirrored.

#17

good info tet thanks.

#18

Save lives with it, actually :slight_smile:

#19

Oh, even with a symmetrical 0/90/±45 layup, they warp. But normally, I just do what they tell me to do! I always make them as symmetrical as I can.
Each layer of the last one was made from six 4.5" uni tapes. all soldered together in 0/90 for stability. if I had to cut 45deg layers, omg, I would have quit!!!
Even cutting the polystrand material was a pain. but at least the 45s would have been easier.

#20

The layup would be like this: ||||||||||
each half of the laminate would have a starting point, and then in the middle, flip the order you lay the plys down.