PMMA Annealing

[gkan]

Hi everyone,

I have made some laser cut parts (4 mm thick) from PMMA which have bent significantly after the cutting process.Sadly, I do have a flatness tolerance for these parts so I need to find a way to correct them. I checked about annealing processes and found this:

http://www.plexiglas.com/acrylicresin/technicaldata/partdesign/annealingparts

So, what I did is, I put the part between 2 thick aluminum flat surfaces to make it flat and then I inserted the assembly in a heat press. I applied a small load (the part was already straight from the weight of the aluminum plate anyway) and I followed the annealing procedure. I wasn't sure which type of PMMA I had so I used the highest temperature procedure on the list. The results were disappointing. The bending was not removed at all. I tried higher and higher soaking temperatures, at some point a couple of thinner features melted. Even then, the bending was still present. At first I thought it had something to do with the cooling rates, maybe one side of the part was hotter than the other or it was cooling faster than the other, but then I noticed that I had put 2 parts upside down and they both retained their original bending. So I am not sure how to correct this..

Does anyone have any ideas?

Best,

George

 

[MikeHalloran]

I'm guessing your cycle is way too short.
For 1/4" thick acrylic, with lapped flat platens, to get a few hours at the recommended annealing temperature, we had to use an oven with a fancy control, lock the door, and ramp up and down very slowly, so the whole cycle took ~40 hours. All attempts to speed the process resulted in warped parts.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

I agree with Mike. Most people have no real concept of how low the thermal conductivity of polymers really is. They also have a much higher heat capacity than metals. This results in thermal diffusivity (essentially the speed of heat flow) that is thousands of times lower than metal. The whole point of annealing is to get the entire part thickness to a uniform temperature where the material will creep to relieve stress, and to maintain this uniformity while cooling. An well controlled oven will likely work better than a press.

 

[APDplastmold]

George,

Since the laser cutting introduces significant heat at the cutting lines(surfaces) that leads to a new energy equilibrium of the polymer causing warpage, you may want to consider less intrusive cutting methods like CNC with low feed rates or water jet cutting.

George

G. Markus

http://www.robustmolding.com