Air trapped - Prepreg - Enthalpy

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#1

Hello,

I have some issues with air trapped inside my laminate (epoxy / carbon, 10+ layers). Fibers aren’t dry, however I get some layers not sticking together. I understand that could be due to a few things. If my curing process is too fast then I will cure the outer layers first and trapped the air in the inner layers. I didnt change the curing process, so it cant be this. If my resin system has different ratio, then it could cure faster, gel faster and then trap air. Can I relate a higher enthalpy to faster curing resin system? My reasoning is as follows: if the enthalpy is higher than my reaction is more exothermic and then the outer layers of my laminate will increase the curing rate since they produce more energy… Does it make sense?

Comments are welcome. Thanks!

#2

how long you debulking for before post curing? perhaps you have trapped moisture.

#3

are you de-bulking mid way through you lamination? 10 ply’s is a large number to not be de-bulking, as a general rule I would de-bulk say every 4 or 5 ply’s. i
Is the problem causing the end result to delaminate? are the cured ply’s are pulling apart?
Check the out life of your prepreg?

#4

1: Your material can be the issue. Moisture uptake, dirt, old material, or even you left the plastic on (did that once…)
2: Make sure your cure conditions are following the suggested cycle. Make sure to do a test with a thermocouple mid-stack, to see if the inside is getting hot enough. If your ramp is too quick, the outside can cure before the middle.
3: What is your bagging method? If you are taping solid release over the part, air and VOC’s can’t get sucked out. If your part is large, make sure you have paths the air can get out!!! Perforated release helps with this, or overlapping release layers (2-3" overlap).

10 plies isn’t a large stack if it’s uni. I do 24 layer plates all the time (which…SUCKS), and I haven’t noticed any major changes between when I debulk, and when I don’t.

#5

thank you all for your responses.

by debulking you mean holding the vacuum for a while before starting the cure?

if yes, i guess i should do a better job and do it, is there any rule of duration vs laminate thickness? i saw that one of you said every 4-5 plies but that is not gonna work (too much trouble for my process).

there is delam but it doesnt cause the part to break, it is just not good enough for customer to have any delam.

the vacuum bagging setup is strong and shouldnt be any problem, my stack is a combination of uni and biax.

any advice on the enthalpy indication? diff. enthalpy in batches between 5% to 20%

#6

[QUOTE=drougerie;29763]thank you all for your responses.

by debulking you mean holding the vacuum for a while before starting the cure?

Yes, remember that any moisture present in your laminate and bagging materials will start boiling a lower temperature in vacuum which turns to water vapor. not saying that thats whats causing it but be sure to give your laminate a good time under full vacuum(Do vacuum test) to remove all water vapor. Also be sure to pull your prepreg from the freezer, let full thaw before removing from the bag, otherwise moisture will condense on the prepreg.

#7

heat guns and wedges are your friend…

maybe warm during debulk as well under vacuum… just to get the resin moving…

#8

i also forgot to add that… warming the mold helps greatly while laying up… and is very very helpfull on aluminum tooling…

look at the past part and determin wich layers are not bonding… try adding a layer of film adhesive and see if you get a nice bond…

other than that i have had problems before with old and new laminates not bonding…
different vendors not bonding together… and incomptatible resins… essentially the faster epoxy will kick and the exotherm will make a barrier between plys… hope this makes sense.

#9

This will also help. Just make sure you dont get too close to the curing temp. Say if its a 120’c cure. Debulking at say 60’c but give it a while under vac prior to warming and make sure your bad does not have any trapped air in it while it is warmed.
Ie use wedges etc to get your bag into corners as the prepreg will push into corners easier whilst warm in the same way as with your heat gun whilst laying up.

#10

I’ve read and tested that air traped voids (pinholes) in laminates appear not only due to trapped air and Voc’s but also due to low resin percentage on the fabric/laminate. I wonder if there is a way /a material or sth else to help remove the air/vocs while bagging without removing the resin?

#11

thanks you all for he input.
nothing on the enthalpy?

#12

1: not sure about the enthalpy part. You mean change the cure cycle?
2: debulking can be done every few layers. I tend to do every 3-5 layers for unidrectional. I hold between 1-5min.
3: is this home-made prepreg by the way?

#13

no its not home made prepreg

enthalpy is the amount of energy released during the curing reaction of the resin

#14

there are lots of problems when curing pre preg…

sometimes when its gassing out during kick and you dont have pressure or enough vacuum “things” can happen. if your having problems i suggest checking your data logs and look and see when and at what temperature your getting this reaction… chances are your curing too fast.

alot of the cure cycles ive done had a window that while we brought temp up at a certain rate we would level off… generally the cycle would hold at gas and allow the resin to flow and “gass out” (enthalpy i guess). sometimes the total opposite.

keep in mind sometimes we would purge at “kick” or lower pressure and so on.

if your baking in just an oven and dont have any data on whats going on there is no telling other than playing with it…

your kinda asking alot of us to guess in the dark with so little information… if you do have all the data post it up and we might be able to spot problems in your cure…

a nice big pleated bag giving plenty of grow room and a nice and slow cure can help alot… on even the most simple parts ive spent hours watching a computer screen monitoring the cure… as have most in this board im assuming

#15

If you are going to the World Cafe Live Beerfest in downtown saturday, we can talk more then in detail about how to fix things.
I would try to talk to the manufacture if it’s purchased prepreg. They might have had issues with a certain batch.