Forgotten heat resistance - curing laminate

General Composites Discussions Composites Talk
#1

Hey guys I was wonder what are your thoughts about curing carbon laminates. Some of you may dont realize that the temperature in your cars at suunny weather can reach even 75* celsius degrees, that mean most every laminate without curing wont stand long in such condition, it will simply starting to deform and become soft.

The question is if someone has experience in proper cure? I have to say I made many many trials with my oven and I found one thing - Thickness of laminate makes it resistant to heat, for instance 10 layers of carbon fiber flat sheet ( non-oven cured) didnt deform when exposed on 75degrees, but 4 layers did deform.

I also tested 3 layers but cured in oven in 80 degrees, but it also deform…its strange because 80 degrees cycle should give resistance up to around 90 degrees.

I writting that because Im about to make my own hood, and I know that heat resistance is very important, otherwise hood will roll up

#2

The answer is simple: Carbon fiber is a good heat insulating material with low thermal inertia. So the inner layer are exposed in a lower temperature when exposed to sun. (heating from one side)
If you heat the thick panel in an oven and wait for enough time for the heat to reach the inside of the panel, it will bend anyway.
All of thet assuming (it’s your case) that you are not using an heat resistant resin.
For example, the cheapest resin I use, the Axson 2015, is able to whitstand the summer sun also with a black (carbon) surface with no problem at all.
For most of the resin manufacturers, the data sheet turn itself to be just a piece of paper.

PS: I assume also that you are using proper heating increase rates and dwell times during postcure.

#3

The likely reason that the thicker laminates are not warping is that flexural stiffness is proportional the thickness to the power of 3.

The increased thickness allows the thicker laminate to resist warping.

#4

I’m sorry, but I already explained the reason for that.
Cold laminate stiffeness has nothing to do with heat resistance.

#5

Question from the original post,

“The question is if someone has experience in proper cure? I have to say I made many many trials with my oven and I found one thing - Thickness of laminate makes it resistant to heat, for instance 10 layers of carbon fiber flat sheet ( non-oven cured) didnt deform when exposed on 75degrees, but 4 layers did deform.”

The original post leaves more unknown than known, i.e. what type of resin was used? How long was it allowed to cure initially? How was it “exposed to 75 degrees”? What is meant by it “deformed”?

My initial thought on it “deformed” was the the panels warped due to a non-symmetric layup and resin shrinkage when exposed to an elevated temperature.

Roberto, I don’t see where you came up with the part about uneven heating due to sun exposure from the original post regarding the test panels. Depending on where they are located in a vehicle, and there was no mention of the test panels being put in a vehicle, they may or may not have seen direct sunlight/uneven heating.

In any case, those who choose to can take my thoughts for what they are, my thoughts, and others can and should simply ignore them.

#6

The cold stiffness and the stiffness when hot definitely correlate. The stiffer it is when it’s cold the stiffer it will be when it’s hot…even when the Tg is exceeded. Every composite laminate loses flexural stiffness as it become warmer. It is a fact of composites. The resin choice and post-curing process will determine how much the laminate will lose it’s flexural stiffness as it become warmer.

The 10 layer laminate will always be stiffer than the 3 layer laminate regardless of the temperature.

I will second infusible’s question: When you refer to deformation are talking about a loss of stability or deformation in terms of stress (elastic/plastic deformation)?

If you are referring to a loss of stability then the thicker laminate will be more stable than the thin one. A properly constructed laminate is necessary to insure that the laminate is stable as the resin cures at the elevated temperatures. It’s not the heat that is bringing about the lose of stability but rather the non-symmetric tension within the laminate due to resin shrinkage.

#7

Panels i tested was made from CF only and CF with Soric. I put test panels behind front window of a car where it was 75 degrees in sunny weather.

When i said the panels deform I mean that they become soft - easily to break and change from flat sheet to curve about 3mm , and test panel was like 30x40centimeters, now imagine hood, with that shrinkage it will look dramatic.

Resin I used was LG700 as I know its MGS system and it wasnt cheap.

#8

Oh , and What do you guys mean by symetrical laminate ?
On soric I used CF, GF , GF , GF, Soric , CF