Forming ABS need really flat back plane

[flycast]

I am forming 0.020" thick ABS on a male tool with very little draw. I am heating the tool and find that generally, the warmer the tool the flatter the back plane of the part gets. The problem is that the back plane of the resulting part is still not completly flat. Being completly flat is important to the processes after forming.

Does anybody have any tips or ideas on getting a nice, flat back plane from 0.020" ABS?

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[rugburn]

The temp of the material must be at the same temp as the mold to alleviate any difference of temp between top and bottom of sheet.

Of course at .020" the core of the sheet is tiny.

HDTUL is about 180F.

HTH

john

[vulcom1]

Not sure if able to do that but maybe a vacuforming table. A pic would help for more info.
John

[johnd]

If the flat surface of your part is slightly wavy it is usally because of trapping a slight air film between the mold surface and the sheet. The sheet cools prior to total evacuation of the air and contact of the mold To alleviate this you can vent the top surface area that you are having a problem with a series of small vent holes say drill sizes in the mid seventy's. Another easier method is to roughen the surface up by slight sanding or actually sand blasting the mold surface.
JohnD.

[drcrash]

Flycast,

I'm not sure what you mean by "back plane." What is the shape of the part, and what are you calling the "back plane"? Is this something formed against part of the male mold, or plastic that's sucked down to the platen itself, around that?

Paul

[drcrash]

I'm no expert, and have no experience with parts that must meet fine tolerances, but I'm thinking that even if the mold is at the right temperature, you may get warping due to residual stresses if the plastic cools too fast, rather than cooling slowly, and annealing itself in the process.

How are you cooling the plastic, and how fast does it cool?

You might have less of a problem if you could use thicker plastic, which generally cools a bit more slowly. (Is that an option?) Even if the surface cooled too fast and had some residual stress, the rest might cool slowly enough and keep it flat.

BTW anybody who knows better than me about this should feel free to shoot me down; it would be educational.