Warped Plexi

[George Elston]

This may be one of those "not enough information" questions, BUT here goes. I needed to make a jig for a pen order and I had a piece of 3/16 plexi that had been in my car trunk (Florida sun) for a couple of years. The sheet was heavily scratched and warped to the point of a wobble, but I figured it would do. I cut it to 12" x 12" and put it into a warm oven 170 degrees until it flattened out and put it into the laser and cut the jig holes. When I took the piece out it had warped concave across the piece probably from the heat of the cutting. No Problem I thought I'll just put it back in the oven and flatten it again. The problem arose when after an hour in the oven, brought it up to 170 deg. 3 times and even put the plexi between two tiles to force it flat. It still has the warp. For those that say MORE info? it was in my trunk for 3 maybe 4 years and I don't know if it's cast or extruded or even what type. BUT any info would be appreciated. I had to listen to my wife tell me you can't put plastic in the oven, you'll burn the house down. Was it really a stupid idea in the first place?

thanks

[Rodne Gold]

Spray the bottom with contact adhesive and bond it onto some plywoodor on a flat tile or something.
you need to heat to 170-180 degrees C to get the pex formable , it becomes like a sheet of elastic, if you want to go that far to flatten it. A convection oven is best

[Joe Pelonio]

I wouldn't go to the trouble for a $5 piece of material. I'd save it as a scrap to practice or experiment on, or do small items that you can cut from flat areas on it.

[jack pike]

was it clear plexi? maybe the problem is that its extruded acrylic
not manufactured to be oven formed - the molucular structures
different and causes warping impossible to get back originally flat

[Keith Outten]

According to Acrylite's "Acrylic Fab Manual" you need to heat it to a minimum of 260 degrees F for 120 seconds for thermoforming. I doubt that 170 degrees was enough to remove the residual bend caused from the exposure in your car. The ideal temperature is 310 to 320 degrees F for 210 seconds.

Although your not thermoforming the temps for straightening acrylic can't be too far off from those required to thermoform.

4 what its worth...

.

[David Epperson]

Acrylic is one of those plastics that when heated will naturally return to it's unstressed shape. But like was mentioned before you do have to get it hot enough. While it was in your car it slowly sagged (rather than warped) I would think. So heating it to the plastic stage should let it return to flat. If not then you can still heat it to that point and form it to flat. Once it cools It will stay that way.

[Eric Allen]

It's probably not in the oven long enough at the right temp. Sounds like you're doing short bursts, it will take longer now that the laser has created uneven stresses in the material. There is a formula around here for annealing plexi, that's probably what it's going to take to flatten the jig now that it's been done. With any luck, it won't alter the jig itself too much. It takes a couple of hours or so if I remember right. It depends on the thickness of the plexi. Update: http://www.sawmillcreek.org/showthread.php?t=51176&highlight=annealing

I knew I'd seen it somewhere on here. I've had some minor stress cracking issues, so I'm getting a little dedicated oven for annealing. Big Lots has a pizza oven for about $25 that looks like it can handle that and the powder coating work I want to do.

[George Elston]

Well, thanks to everyone for the info, and Rodne - yes I did cut to the chase and glue it to a piece of MDF when the pens arrived. There's a time to play and a time to get the work out. I think the temp is what caused the problem, the first time I cranked the oven up to 200 by the knob, and I know it runs hot so it probably got to 250-300 before I turned it off, and the plexi went flat as a pancake. The second time with my wife telling me "Your going to burn the house down", I used a digital thermometer with an alarm and when the oven temp got to 175 I shut it down. Thats the oven air temp, I couldn't seem to get the probe into the plexi. so all that said not long enough, not hot enough were probably right.
It was really a matter of idle hands and the devils playground. So those that said it's a lot of work for a $5 piece of plexi, were also right, but I love to play mad scientist.
But now the question is what is the temp to get plexi to the plastic stage, before it starts to vaporize and burst into flames. Rodne says 180 and Kieth quoting the "Acrylic Fab Manual" says somewhere between 260 and 320, which seems a little high, but might be what I hit the first time. I wish I had used the thermometer on the first batch. I think I have another scrap from the car trunk, I might just have to use the kitchen one more time.

Any way thanks again, I hate mysteries they keep me up at night.

[Adam Stacey]

Try using silly puddy. Cheap, conforms easily, and doens't make a mess. If you let it harder then you have a permanent jig for that particular type of pen.

[David Epperson]

But now the question is what is the temp to get plexi to the plastic stage, before it starts to vaporize and burst into flames. Rodne says 180 and Kieth quoting the "Acrylic Fab Manual" says somewhere between 260 and 320, which seems a little high, but might be what I hit the first time. I wish I had used the thermometer on the first batch. I think I have another scrap from the car trunk, I might just have to use the kitchen one more time.

Any way thanks again, I hate mysteries they keep me up at night.
I took Plastics in college (OK so it's been a while). I seem to remember that 250°F was what the school oven was set to to handle Plexi projects.

[Scott Shepherd]

But now the question is what is the temp to get plexi to the plastic stage, before it starts to vaporize and burst into flames. Rodne says 180 and Kieth quoting the "Acrylic Fab Manual" says somewhere between 260 and 320, which seems a little high, but might be what I hit the first time.

Just a guess, but Rodne's not in the USA and could have been stating Celsius, which 180C would convert to about 350F.

Just a guess on my part.

[David Epperson]

Just a guess, but Rodne's not in the USA and could have been stating Celsius, which 180C would convert to about 350F.

Just a guess on my part.
Scott seems to be correct on this. But like I said, it'd been a while. :D
http://www.acrylic-designs.com/plastic_FAQ's.htm

Acrylic plastic becomes flexible at around 280 degrees. You should set your oven at 350 to 400 degrees. It will take 2 to 8 minutes to heat depending on actual temperature, thickness of material and what you place the plastic on to heat.

[Brian Robison]

I have specs on all kinds of plastics but they are for injection molding grade plastics. We look for the melt temp not the temp and time to soften. Nominal melt temp for acrylic is 467f. It melts at 425f and max temp is 510f. I like Joe's answer of buy a new piece!

[Marc Myer]

OK, what about the other side of this?
I'm having some acrylic pieces coated by a firm who states their coating process (vacuum metalization) requires 170 degress. However, I've seen acrylic get kind of droopy at less than that. Was I just using a cheap grade of acrylic?
Plexiglas claims a constant working temp range of 180-200. Hope it's accurate.