oven cured fiberglass? 2

[bloodfart]

Hi,

i would like to know if you can cure vacuum bagged fiberglass parts in an oven, at about 100 degrees celcius. I thought that you can cure them in the oven just like with carbon fiber, as long as you use epoxy resin, not cheap polyester. Am I right, you can cure them without any problem in an oven, or are there warping issues? I thought that the warping was an issue only with polyester, but someone i know insists that that's not the case. I haven't found any information on google, but this forum seems pretty advanced, so figured i would give it a shot on here.

Thank you in advance

 

[Compositepro]

You can cure any resin in an oven. Heat simply accelerates the chemical reactions and forces the reactions to a greater degree of completion.

Warping is caused by resin shrinkage during cure. Shrinkage occurs due to a change in volume during the curing reaction and also due to cooling after cure. So higher cure temperatures often result in more warping problems.

Polyester resins generally exhibit significantly higher cure shrinkage than epoxies. Polyesters also are free radical cures which are chain reactions. This means that after catalyzing they have a certain pot life and then the reaction takes-off and occurs very quickly. Heat is released during cure and this is called exotherm, and is a normal part of polyester cures. Thick sections will get much hotter than thin sections and this too leads to warping. Oven heat will increase the exotherm and could ruin the part.

Epoxies react at a fairly unifom rate with time and the rate depends on temperature. The rate is usually designed to be fairly slow so that there is no signifcant temperature rise during cure which could lead to an uncontrolled exotherm. Exotherming epoxies can smoke and catch fire.

All type of thermosets can be post-cured - meaning that after most of the cure is done at low temperature the parts are heated to a high temperature to make sure that the chemical reactions are compete. In solids molecules are highly restrained and temperature shakes them up.

There is no difference between fiberglass and carbon fiber regarding oven cure.

 

[rerig]

Compositepro

Could the warping also be a function of a non-symmetrical Laminate Stacking Sequence? (i.e. too many 45 deg plies)

(I Don't mean to hijack this thread. I thought this might pertain)

 

[Compositepro]

Rerig, you are certainly correct, but stacking sequence merely affects how resin shrinkage warps the part. Balanced lay-ups are used so that shrinkage in different layers cause an equal and opposite "warp" so there is no actual warp. Instead there is stored internal stress. If these get high enough, cracks can occur.

 

[bloodfart]

but since epoxy has no shrinkage, that's not an issue right? that's what i thought, as long as you use epoxy, you're fine... am i correct?

 

[Compositepro]

All resins shrink when cured, it is just a question of degree. There are some exotic ingredients that expand when they react (due to ring opening) but I'm not aware of any actual commercial applications. Most epoxies have about 2% reduction in volume. I believe polyester is more like 5 to 6%. Of course this can vary quite a bit with exact formulation of the resin. Low cross-link density resins will shrink less but will also have low glass transition temperature (Tg) which means low use temperature.

 

[QuentinNZ]

to limit warping you should balance your laminate stacking and could also consider non crimp fabric (stitched fabric rather than woven, or satin weave rather than plain weave)

Other than than any resin can be post cured in an oven. You should refer to the manufacturer for the best post-cure temperature.

Also Epoxy tends to shrinks less than Polyester but there is still some shrinkage during curing. thus the need to balance your laminate.

 

[Compositepro]

QuentinNZ, why do you suggest that non-crimp fabric or satin weave will have less warp problems than plain weave?
They both have a "warp face" and a "fill face" which has to be nested properly to have a balanced lay-up and avoid warping. Plain weave is a balanced weave that does not have this issue.

For those who are unfamiliar with these terms "warp face" is the side of a fabric that consists mainly of warp yarns which run the length of the fabric. Fill yarns run across the width of a fabric. Non-crimp fabric is two layers of yarn (actually "tows") in cross directions that are held together by stitching or technique other that weaving.

A single ply of cured plain-weave can be flat. A single ply of cured satin weave or non-crimp fabric will warp severely.

 

[RPstress]

Probably worth mentioning a couple of slight subtleties: the resin typically expands more when liquid on heat up than it shrinks after gelation and on cool down as a solid (these are physical CTE effects). This means that the shrinkage on gelation at the cure temperature (a chemical effect) is usually more than you'd think from the difference in volume between RT pre-cure and RT post-cure.

Even perfectly symmetric laminates will warp if they are curved (especially sharp curves like those when making L-shaped features). This is due to shrinking much more through thickness than in plane, which causes curved material to distort in through-thickness shear. This causes spring-in.

 

[Compositepro]

Those are really good points, RPstress. While I was aware of the facts in your first paragraph, I had to really think in order to understand the point you make. And yet I can't explain it any more clearly than you did.

It still amazes me how significant cure shrinkage and CTE are in making composite parts. The subtle points are very important to understanding the issue. Vacuum-bagging is far more forgiving of shrinkage because it will primarily be seen as a very slight thickness change which is very difficult to even measure. Closed cavity molding has far more difficulty in dealing with shrinkage where it often shows-up as voids inside the part. These are easily mis-diagnosed as having some other cause.