Warped Epoxy Fiberglass 1

[dereklola]

Last year I had some lightweight epoxy-resin fiberglass panels made for a project - details below. They were in unheated, dry storage for about 12 months, sitting "on end" (which should have caused no external deforming forces) but have now warped to a small, but significant extent - presumably from residual internal stresses.

My question is would heating them while "blocked" to the right shape be successful in restoring them to their original "out of the mold" shape? Would this be a permanent fix or might it have to be repeated from time to time. (It would be possible to add some internal webbing if that would help.) What sort of heating arrangement is appropriate (heat lamps, fan heater blowing into an insulated box)? What sort of temperature? I've had one non-expert suggest 120-130degF should be about right - is it? Note that they were made from epoxy not polyester resin - and that they are foam sandwich construction which I presume means heat is needed on both sides.

Details: - each part is about 5ft x 5ft with some significant 3D shape and intermittent edge conditions - by that I mean there is not a consistent perpendicular wall all round - in some places it is a 12"-15" deep and in others almost non-existent. Construction is a 1/4" foam core covering about 90% of the full surface area - then skinned both sides with "thin" vacuum bagged epoxy/fiberglass - sorry I don't have exact details - I didn't make them myself - but as a guidance each part weighs only about 15 lbs.

The distortion is basically (1) one corner being about 1" low and (2) a central hump is now offset about 1" (was originally symmetrical).

Appreciate any help anyone can offer - Derek

 

[berkshire]

Need a bit more information about the way these panels were made.
Were they room temperature cured or were they autoclaved?
Generally to get a distorted panel to come back to where you want it to be, it has to be gently heated over a long period of time (several hours), just above either its molding temperature, or the highest temperature it has been stored or post heat treated at, whichever is the highest.
Restraints or blocking really should be added on a trial basis. The ideal is to clamp it firmly into its original mold and apply heat.
A cruder but effective form , is to take a heat gun to the part and apply local heat and pressure, then remove the heat while still retaining the pressure until the part is cool. It should be noted that epoxy resins are more amendable heat forming like this than polyester resins.
B.E.

You are judged not by what you know, but by what you can do.

 

[dereklola]

Thx Berkshire for your encouraging reply.

The parts were not autoclaved - just hand laid up in heavy old fiberglass molds which unfortunately no longer exist. All this was done at shop temperature plus whatever minor amount of resin curing heat might have been generated - as I said the skins are very light/thin and were vacuum bagged in the molds - so not much mass to generate curing heat.

They were in unheated storage over winter and into summer - so I guess the original manufacturing process would be the appropriate reference temperature you mention - say 65-75degF. That being so what temperature should I aim for to re-shape them - 80? - 100? - 120? And you said to do this for several hours.

I doubt a heat gun would work but I could experiment. If not, and thinking of the corner distortion first, would a couple of heat lamps on each side (to get both sides the same temp) be appropriate? Would it be better to try to make a 'tent' over the corner - maybe some plywood lined with 1" insulation foam? I'm thinking I'll support the non-distorted area on a wooden frame and then one or more adjustable supports to gradually encourage the distorted corner back to shape. Then let that cool and (as mentioned before) see if I can add some internal webs to discourage future distortion.

Looking forward to your further reply - and maybe comments by anyone else who knows more than I do (not difficult!). FDerek

 

[berkshire]

The reason for mentioning several hours of gentle heat is because you have foam in there. You have to get the foam warm enough to reset and of course it is an insulator. You will do better with the two heat lamps , because then you will heat the resin without having to worry so much about the foam. The characteristic you are trying to take advantage of , is that epoxy resin cures in a exponential fashion, never fully curing. So if you can heat it to just above the temperature it last cured at, the resin will soften slightly, again enabling you to manipulate it. This may also be what is causing your distress now, if the parts reached a higher temperature in storage than they were made at, then they will creep if there was a constant load on them . One thing that may be a bother, is, if the foam was not preformed by gentle heat before, the part was molded, and was just pulled into the mold, by the vacuum.
If this is the case, the application of heat on the glass alone may allow the foam to relax, flattening your part, on the other hand the foam may have had enough time to creep to it's formed shape by now. Be careful !
B.E.

You are judged not by what you know, but by what you can do.

 

[dereklola]

Got you on the slow heating. Just give me some guidance on the temp to aim for 80? 100? 120? - will use a temp gun to check both surfaces. Many thanks - Derek

 

[berkshire]

I cannot help you on the temp to aim for, because I do not know the highest temp the layup has achieved.
Take a guess at the highest temp the storage facility has seen, and start 10 degrees higher than that. As soon as you get to a temp higher than the resin has cured to, it will soften. If you over do it, you will get print through with the weave of the fabric showing.
B.E.

You are judged not by what you know, but by what you can do.

 

[dereklola]

Thx