Polycarbonate forming

[thorsgaard]

k.. so I want to build a former... I like Doug Walsh's I like drcrash's very informative posts, along with his Instructable" and the tk560 site..

My mat'l of choice is gonna be lexan, due to availability. (Sure - I have to have the most PITA poly there is, huh?)

All I know is what I've read on 'net: Stays hotter longer, and requires drying (?) of 250°F fer dAYS.. - even then, it'll absorb moisture within seconds.

So, If I'm gonna build one, I have to use lex for formable mat'l.

anything else I should look out for?

[drcrash]

I thought that polycarbonate cooled quickly (sometimes annoyingly quickly), but required a high temperature to form.

I'd guess that either that means that polycarbonate has a high thermoforming temperature (range) but a low specific heat, or that I just misunderstood something.

(That's one of the things I was thinking about when I wrote the post about how long and hard you need vacuum for... is brief high vacuum enough for thin Lexan like for RC car bodies, because after a few seconds it's cooled too much to matter anyway? Or should you keep heating the plastic WHILE it's forming... I can do that with my over-and-under, putting the platen right up under the oven.)

[LeeWay]

I thermo form polycarbonate tube for my blade guards. I don't bother trying to remove the moisture. Not really needed since all I am doing is changing the shape without any creases or edges. I heat them up with a heat gun to around 450 degrees F. If you get it too hot without removing moisture, you will see water bubbles form in the plastic. I use a digital laser thermometer to check the heating. I do heat the whole part. Then I use a vise to change the round tube to an oblong tube on one end. I then hit it with a cold water rag. Then I blow it off with compressed air. It does take awhile to cool down and this may be because of the moisture trapped between the cells. However cooling like I do works pretty fast. Once cooled (30 seconds or so) I take the pressure off the vise and the part holds it's shape. It takes a little practice to get a feel for how it works best, but no it's really second nature and I don't need to check temps any longer. I can tell just by the amount of time I heat it.

[Geof]

I thought that polycarbonate cooled quickly (sometimes annoyingly quickly), but required a high temperature to form.

I'd guess that either that means that polycarbonate has a high thermoforming temperature (range) but a low specific heat, or that I just misunderstood something...

I have done some heat forming on polycarbonate and I think the main problem other than the water is that it has a small range of temperature over which it is easily formable. It is high compared with styrene or acrylic, for instance, and it is close to the depolymerization temperature so if you overshoot you get bubbles of monomer. Most other plastics have a wider temperature spread.

[thorsgaard]

...somewhere around here in the plethora of directories and file folders in "MY DOCUMENTS", I have a thing from G.E., that tells about thermoforming polycarbonate..

I gots to get organized some day

But it gave a specific temp of like 372°F or something like that.. (Along with the specs for drying in a kiln for days @250 or whatever)

I dunno - drcrash - I'd think thinner ~whatever~ would cool faster - but I suppose you mean 0.015 LEXAN cools faster than 0.015ABS or styrene or whatever. I have zilch experience in it, so I'll take YOUR word for troof, since you actually experience it.

THAT'S what forums are for! - sharing experience!
...and Thanks to you, too, LeeWay, for your advice. I suppose bubbles wouldn't matter for the app I'm researching...May not be good for aircraft windows, so a higher tolerance must be met in those circumstances...

Okay.. I'll keep plugging away. I plan to start the table build tomorrow... Will prolly get one of those vac pumps, and use a big shopvac for primary suck... I've got lots of tanks at my disposal for use. I'm gonna play with some mig wire and an Infinite control tonight while I'm milling out another Bowtie hitch plug for a guy in town(idea stolen from James Holwell's kiln design from another hobby of mine)

OH! The polycarbonate sheets I'm referring to (What I have an abundance of) is Pop machine fronts! I werk at Pepsi, and when machines get refurbished, or labels change, they remove the old design, and tuck it away for someone to recycle. (People use them for all sorts of stuff - just too good to throw away..) - I think they're polycarbonate, anyways.. Sure do have the same properties..

[drcrash]

.
I dunno - drcrash - I'd think thinner ~whatever~ would cool faster - but I suppose you mean 0.015 LEXAN cools faster than 0.015ABS or styrene or whatever.

Yes, that's what I meant.

I have zilch experience in it, so I'll take YOUR word for troof, since you actually experience it.

My only experience with polycarbonate---maybe---is with some 1/8" "mystery plastic" scrap that I assumed was acrylic but now think is Lexan. The results were not good. Spectacularly not good. (But my setup is better for when I try again.)

Other people here have lots more real experience than me with lots of things, which is one reason I ask so many questions.

I suppose bubbles wouldn't matter for the app I'm researching...

If your results are as bad as my first try with my mystery plastic, they might. The stuff took a long time to heat up, but then basically turned into foam---hundreds of tiny bubbles per square inch---in a big hurry, with a distinctly audible hiss as the little bubbles spread across most of the surface in 15 seconds or so. (Starting from the hottest parts.)

At first I thought it was crystallizing, and was totally mystified. It was interesting to watch, but the result was structurally weak as well as interestingly ugly.

(That happened after it got bendably soft. I could have sheet-bent it before the bubbles sprang up, but then it basically self-destructed before getting really formable.)

[chan luci]

Predry the sheet or tube to be formed at 115°C for atleast 2 hours for a 1.5 mm thick sheet.Then immediately vacuum or thermo form Lexan or any other Polycarbonate sheet. Double side heater is best for forming PC. Predrying does away with that moisture problem.
The sheet can be hung vertical in an oven in rows.

[Geof]

...somewhere around here in the plethora of directories and file folders in "MY DOCUMENTS", I have a thing from G.E., that tells about thermoforming polycarbonate......

This one?

[One of Many]

PC does have a narrow forming temp of around 40-60degF. That is why it chills of rapidly when it hits the die, yet maintains a lot of heat. Keeping the die forming temp stable gave us problems too. Too cold would produce cold flow lines and too hot would tweak out of shape if removed too early. At 3-12bucks a sq ft you cannot afford too many trials to get it tuned in.

We were forming 3/8" in large blank sizes and the part had to come off the die ASAP or it would shrink on tight.

The key to reducing bubbles is fresh stock. Big box stores or warehouse distribution can be some of the worst conditions and stale material who knows how long since its date of extrusion. We had recieved pallets that we swore was someone else surplus that took weeks in the oven to get the moisture out enough to form.

DC