Forming acrylic below Tg
Forming acrylic below Tg
(OP)
We need to precisely "thermo"form a ~half inch thick aerospace acrylic sheet with Class A surfaces at no hotter than 218F, which is well below its Tg. We're willing to build custom equipment of any complexity to accomplish this.
Has anyone got experience with using say 300psi of isostatic pressure to depress the Tg of acrylic to say 205-210F ? We're thinking of hot-oil pressing between elastomer films, but it's hard to find elastomeric films with good oil resistance and a Class A surface. How about direct contact of a heating liquid with the acrylic so as to avoid re-texturing Class A surfaces...what would you use so as to avoid chemical interaction, absorption, plasticization and property change?
Has anyone got experience with using say 300psi of isostatic pressure to depress the Tg of acrylic to say 205-210F ? We're thinking of hot-oil pressing between elastomer films, but it's hard to find elastomeric films with good oil resistance and a Class A surface. How about direct contact of a heating liquid with the acrylic so as to avoid re-texturing Class A surfaces...what would you use so as to avoid chemical interaction, absorption, plasticization and property change?





RE: Forming acrylic below Tg
RE: Forming acrylic below Tg
Will thermoforming temperature even if TG is depressed cause a degrade in surface quality. Exceeding TG does break certain bonds by definition no matter what the temperature.
If it is outgassing that damages the finish, then depressing TG might work.
I would have thought ultrasonics would depress TG by exciting the molecules in a similar fashion to heat and therefore create the same problems.
Regards
Pat
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RE: Forming acrylic below Tg
You can polymerize the monomer to make the part in the final shape, or rather, a specialist could probably do that for you.
Why not above 218F?
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: Forming acrylic below Tg
Pressure and ultrasound has no effect on Tg.
Your type of processing is usually done by vacuum forming a pre-heated sheet into a cold, polished mold. As the sheet comes close to touching the mold it cools and hardens so the surface stays optically smooth and only those areas not yet close to the mold continue to move. I do not know if your temperature is sufficient to get the movement you require.
RE: Forming acrylic below Tg
RE: Forming acrylic below Tg
RE: Forming acrylic below Tg
While heat/cool time is relevant, in this case the forming is done at 218F to stay (just) below the 3% relaxation point, so the time required is obviously not as great as it would be at 330F.
A majority of the time...we're told...is in very slow deformation of the material, so as to allow time for the polymer chains to unwind as needed and slip into the desired rearrangement.
RE: Forming acrylic below Tg
RE: Forming acrylic below Tg
So far the OP repeatedly makes statements that defy all rules of polymer behaviour re processing and temperature.
Initially I was prepared to accept that this was an area outside my experience (probably because of the big words used) but I am now very sceptical.
In my experience Acrylic is always thermoformed at about 150 to 170C
Degradation starts to kick in at about 160C and accelerates to normally unacceptable levels at about 180C when out gassing and surface defect soon becomes a problem.
TG varies a bit from grade to grade but is about 114C It is brittle and has low elongation below this temperature.
The data I have mostly mentions Vicat Softening point rather than TG, but they seem to have the same value.
You cannot form PMMA below these temperatures due to its low elongation at break.
I never heard of suppressing TG with pressure or ultrasonics before as I always believed TG was related to inter molecular bond strength, bond distance and kinetic energy in the molecule. Kinetic energy in the molecule is directly related to temperature due to the relationship between the speed of electrons in orbit and absolute temperature.
The use of water as a plasticiser is useful for nylons, but is useless for PMMA partly due to the low water uptake levels. At 114c the water would be outgassing and degrading the surface.
I have seen acrylic sheet thermoformed at 150C with excellent clear surface finish, including aircraft canopies.
Regards
Pat
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RE: Forming acrylic below Tg
There exists the possibility that the leak was detected, and misleading data was inserted in the information stream, for commercial reasons.
Not necessarily a troll.
Mike Halloran
Pembroke Pines, FL, USA
RE: Forming acrylic below Tg
RE: Forming acrylic below Tg
Why am I mentioning this? Polymers don't move much below Tg so you may not be forming below Tg after all. You are just forming so slowly that the Tg is much lower than normal.
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: Forming acrylic below Tg
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: Forming acrylic below Tg
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry