For those of us who don't have the equipment to post cure efficiently.

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#1

Many of us don’t have ovens or autoclaves. Lots of us just infuse or wet lay parts, make them look pretty, and hand them over to the customer. For us, if we can’t post cure to keep our parts from warping in the sun. What else can we do to prevent warping?

I spoke to a fellow down here in Florida who’s a rep for Composites One a while back and he suggested I put my parts in the sun after infusing to auto post cure… I asked him if that was really ok and he said everybody in Florida does it. As fun and easy as it sounds I never tried it until today. I demoled a twizzler.

What other methods can we do to help if we can’t effectively post cure? Are there certain weaves and thicknesses of fabrics that can help? Certain resins? Is there a way to safely post cure in nature’s oven?

#2

I would guess that the main factor would be how the part is supported, how many mount point is has to support the structure? Things with a wide span and used in the sun/outdoors all day would have to be the worst culprit? Do you have any photos? I’m interested to know what type of parts are more susceptible to warping.

#3

I’ll post a few photos of my parts tonight.

#4

Depending on the intensity of a heat soaked part in direct sunlight, you could try to manage a post cure with heat to some extent. But…it will only be as good an outcome as the consistency of the holding temp and ramp up you resin of choice requires.

For example, if it was only as easy as plopping the mold and product out in the sun for half a day…how stellar would THAT be?

But reality would be clouds dropping temps or perhaps an overcast day or even too much heat.

The next reality would be one side getting heat soaked and the opposite side receiving a much different exposure.

Not good.

Think about how inexpensive a indoor box oven could be made using 2"thick aluminum skinned insulation foamboard and a PID controller with ramp and soak. It could be put together with sheetrock screws,aluminum tape and some plywood or better sheet metal scrap.

I think Herman was working on a write-up for working PID control??

#5

I’ve been doing research on some of the powdercoating forums for DIY PC oven builds. With the controller taken care of, it looks like that would be the way to go, not to mention I have also been looking into the element heated reusable silicone bags Alan Harper has been showing off.

#6

I just had a thought - there is a (fairly) local business here that makes caravans entirely from sandwich panels (fibreglass, CSM only I think, with a polyurethane foam core and gelcoat on one side). They lay it all up on very large tables and they create the curved front sections by just bending the panel (after some relief cuts on the back side I think). Anyway, the important part is that upwards of the chassis the entire structure is composite.

I was thinking that would have to be a prime candidate for warped surfaces - large panels that stay in the sun for long periods. I was asking some questions when I picked up some foam sheets from them, they gave me a quick tour and I didn’t see any ovens in there. If they’re still in business I’ve gotta guess that they never have any problems with warped panels.

So maybe the answer is just to use some kind of core to help support the shape? Would I be right in guessing that it’s mainly cosmetic parts that have this problem?

Also if the Tg is reached, that doesn’t mean the whole part is just going to melt. Wouldn’t it require some kind of force to be applied before any really bad warping would occur? For instance if the part was installed/used with some kind of tension applied?

Anyway maybe I’ll answer my own questions after seeing some photos.

#7

There is another way of heating that may be useful (don´t know Price): Heating blankets / pads. Perfect for panes.

Making an oven is simple and not expensive with isulating panels (I,ve used 50 mm polystyrene up to 120º Centigrades)

If a laminate without stress reaches it´s Tg, nothing will hapen to it. Tg is the temp at which its mechanical properties start to decrease. Progressive. So, it depends on temp and stresses.

#8

The reason why you could be warping is because of thermal mismatch from sun-side to back-side of the part!!! heating one side will stress the part and cause it to warp.

You can try doing a black painted hot-box with a circulation fan on the inside. To make sure it doesn’t get too hot, maybe PID an exhaust fan to control the inside temp.

Of course, many resins will react differently, along with weaves and thickness, and shape of the part. A flat panel will warp easier than a complex shape.

#9

For those who cannot postcure:

Get a box, and a couple of lamps. Place lamps in box, place part in box. Close box, turn on lamps. Presto! Your postcure box.

#10

I will stitch a heating tent together. I have sourced cheap high temp fans and havent decided on the heating source (very large area to be heated)

#11

I use a large infra red heat lamp, place it facing the part and gauge the temp using a infra red heat gun, can adjust the temperature quite easily by moving the lamp closer or further away. obviously not great for large objects but works well with smaller ones.

#12

Build a tent from black poly ethlyene building plastic - you know, the stuff you used to make waterslides with as a kid! Put your parts under the tent in direct sunlight and leave them to post cure. Youd be surprised how hot it gets in there…And no energy bill… but i guess ive got it easy living in the tropics :slight_smile: