Is UNI directional okay for a surface layer?


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lovethepirk
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I'm new to composites. I'm doing a wing that is just over 1 foot in wingspan. There will be no core, so it is 100% solid carbon fiber...joining both halves with a 1mm layer of very high strength epoxy and chopped/milled fiber.

I don't need torsional strength at all for this part, I need strength from tip to tip so the wing doesn't snap in half.  My plan is to use mostly  Intermediate Modulus(830 tensile 43 Modulus) UNI directional 12K 8oz/300g for the skin or surface layer.

I don't care about appearance, so is  UNI directional okay for a skin layer?

If each half requires 10 layers of fabric should I vacuum bag each layer with the first 5 layers and then rough up the surface and do the other 5 layers as a secondary process? A rep at the composite store said this may be a preferred method? Below is the pic of a generalized layup I have in mind....thanks in advance.




Edited 5 Years Ago by lovethepirk
Hanaldo
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Yeh no problem, nothing wrong with uni on the surface. 

Personally I don't think 10 layers of a 300g is too much to layup in one hit, but I would use a slower resin system to give yourself a bit more time and reduce the peak exotherm. 

lovethepirk
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Hanaldo - 7/5/2019 11:39:07 PM
Yeh no problem, nothing wrong with uni on the surface. 

Personally I don't think 10 layers of a 300g is too much to layup in one hit, but I would use a slower resin system to give yourself a bit more time and reduce the peak exotherm. 


Thanks for the reply...Interesting you mentioned the resin...I was going to use the slowest resin research has which is the 2100X with a Gel time of 140min and set time of 14hr.  My question is why do you mention the peak exotherm?  It is at 158F vs the faster stuff at 200F.  I choose the slow stuff b/c I need as much time as a rookie in case of mistakes.  If I can get a solid leak free vacuum, I was actually considering putting the setup in my car in the direct sun in south florida for a nice hot box Smile   I read that increased temperature around the cure strengthens the part.

Btw, the main wing is 2x as large and requires 15 or more layers....when would you personally consider drawing the line and doing a secondary vacuum?  I have a very stong vacuum pump, but my plan was to not utilize its complete strength b/c I didn't want a dry surface layer even though I will be using plenty of resin, peel ply, and a layer of breather.


Edited 5 Years Ago by lovethepirk
Hanaldo
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Peak exotherm is important because you want to avoid a run away if you're doing a thick laminate. Building too much thickness in one hit can cause the heat build up to get so hot that you get uncontrollable shrinkage, potential cracking, and possibly damage the mould if it cant take the temperature. 

Honestly, 10 layers of a 300g uni is not really that thick, so this shouldn't be too big a worry for you. But using a slower resin system with a low exotherm is a good idea regardless. By all means, apply heat AFTER the exotherm has died down again.

For me personally, with the resin systems I use - 15 layers in one hit is no issue. However it may be an issue with your materials and equipment, so I would always recommend doing a smallish test first.
oekmont
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Is the wing intended for underwater use, or supersonic applications? 10 layers of 300g seems unnecessarily heavy for a 1 foot wing. The inner layers do not carry any load anyways. Except it's a part for a turbine or something similar, where the wing will face high centrifugal forces.

GO

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