Forming acrylic below Tg

[MedicalRandD]

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?

 

[MedicalRandD]

Another idea...does anyone have experience with using high-level ultrasound integrated into a forming press for amorphous plastics, to depress Tg?

 

[patprimmer]

I must admit this concept is new to me, so I might b talking garbage.

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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[Demon3]

Polymers don't flow 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

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Compositepro]

Generally, any reshaping of plastics is done above the Tg. But many plastics will yield below Tg. To maintain class A finish you need a polished mold or you need to not touch the surface.

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.

 

[MedicalRandD]

Thank you to all for the various inputs. Our task, new to us but a technology that was first developed in the 50s and 60s, is to "thermo"form MIL-P-25690 70%-biaxially-stretched acrylic (commercial example = Spartech Polycast 2000) into specified 3D shapes at a temperature below that which will cause it to relax the pre-stretch by 3%. That degree of relaxation is the specification boundary for the finished part to remain qualified as MIL-P-25690. We know that competitors' processes have cycle times of ~12 hours. That doesn't bother us, but we need to better understand how they're doing it, because our experiments so far are requiring us to overbend considerably in order to have the needed degree of permanent deformation, with the rest of the overbend being elastically recovered. That of course makes it very difficult to design tooling.

 

[HDS]

Much of the cycle time is probably very slow heating and cooling of the thickness. Thin acrylic is often drape formed below it's Tg but you need perfect tooling and conditions to achieve class A surfaces.

 

[MedicalRandD]

> Much of the cycle time is probably very slow heating and cooling of the thickness.

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.

 

[MedicalRandD]

Does anyone have experience in use of absorbed H2O as a reversible plasticizer for creep forming of acrylics?

 

[patprimmer]

Is this thread a troll.

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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[MikeHalloran]

The OP seems to be working with a very small amount of information that has leaked out of a closely guarded process.

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

 

[Compositepro]

Creep forming sounds like a good description. Plastics do creep under load and creep is rapidly accelerated by temperature. But the same conditions that allow creep also allow relaxation of molecular orientation. Much of this relaxation, however, would be seen as shrinkage of the sheet. This can be prevented by clamping the sheet in a frame, which is required for any forming process anyway. 20 hour, or longer, cycle times for this type of process sounds believable. That is also why this would not be a common commercial process. I would think the more commercial way to make such a part is to do the biaxial orientation and shaping in one operation.

 

[Demon3]

Just a note to remember that Tg is not a fixed number. It varies dramatically depending upon the way you measure it. If you measure it by NMR for example, you are measuring using a technique in the gigahertz range so you will get a Tg very much higher than if you measure using a slower technique.

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

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

So this could really be called thermoplastic creep forming rather than thermoforming.

Regards
Pat
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[Demon3]

Well normal thermoforming is still OK nomenclature-wise but you're right, this lower temperature version would be better called thermoplastic creep forming!

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry