differences between polycarbonate (lexan) and PETG

I am apprentice starting in vacuum forming and would like to know the
similarities and differences in these two materials in sheet format. I a
starting to work with thicknesses of 0.020" to 0.060".
I am most interested in the "workability" during the heating process and
durability, temperature tolerances of these materials after vacuum forming.
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I don't have a whole lot of experience thermoforming either material. The main difference is that PC is an amorphous polymer while PETG is a semi-crystalline one. Since amorphous polymers only have a glass transition (Tg) and not a melting temperature, they typically soften and flow over a broader temperature range and as such have good melt strength which is ideal for thermoforming. In constrast the melting temperature of crystalline polymers is typically an order of magnitude higher than its Tg. So, when you get above its melting temperature the polymer rapidly softens and flows making it more difficult to work with. In addition, the semicrystalline polymers will change in volume as it goes through its melting temperature - expanding upon melting and retracting upon crystallizing. You'll need to take this into account when designing your tooling.
Larry Effler
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Larry Effler
Hallo chaps, I don't think your assertion is quite right, Larry.
I agree PC is amorphous and therefore pretty easy to thermoform. You just need sufficiently powerful heaters (double sided are often best) and enough time. The impact strength of the material is excellent and so it would be used for engineering parts, although the first thing I think of as an application is bodies for R/C controlled model cars!
PETG is also amorphous, not semi-crystalline, I think you may be confusing with PET, Larry. Its applications are more commonplace in packaging, it is often used for crystal clear point-of-sale goods. It is also straight-forward to thermoform.
(PET as used for fizzy-pop bottles is semi-crystalline and is very difficult to thermoform as it has a sharp melting point below which it is very stiff, and above which it is fairly fluid.)
For technical info. on PC check out the GE Plastics website, for PETG, take a look at Eastman.
Best regards, Bill
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The glycerol group lowers the crystalline but does not totally eliminate it. So, if you quench cool you'll end up with very low levels of crystallinity and hence nice clear parts. If you slow cool, its a different story.
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Larry Effler

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