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Hi there,

I am attempting to vacuum form 1/8" polypropylene and I am experiencing some symptoms in the way the plastic is forming. The polypropylene I use is glossy on both sides and I have trimmed the area to be formed over the heel in a rounded manner to prevent wrinkling around the heel cup. It is 4"x8" and my cast is about a ladies size 7 foot and is plaster. I heated the polypro for about 3min at the suppliers recommended temp of 425F. I tried a couple other sheets at 400F.

1.) After taking off the press, the surface in contact with the bladder appears to have "burns" or appears warped. I do not see any foreign objects on the bladder that would leave such an impression. I cannot explain the source of these patterns.

2.) There is a large wrinkle or fold along the arch area. It almost looks like a really good "FHL groove". This fold is not present on the opposite side.

3.) The thickness of the pressed plastic is not uniform. It appears thicker in some areas and thinner elsewhere.

Surface of my plaster cast appears smooth but it can be rather cold before the initial press. I was told by my material supplier that this may be a source of the problem due to the difference in temperature of the cast compared to the temp of the bladder. Apparently, one side of the plastic is cooling faster than the opposite leading to uneven hardening rates and thus the deformation that appear. I therefore tried heating the cast first before pressing a second time. I think the there was less wrinkling, but the "burns", warping and non-uniform thickness was still present.

Sounds like an overheating issue at first, so I lowered the heating temp to 400. It appeared to be easier to handle out of the oven but results after pressing were still mediocre. Could it be the plastic? My vacuum press? I dont know...

Any advice on this would be much appreciated.

 

Could be your oven? mayb rotating your plastic halfway through heating or testing surface temps with an Infra-red thermometer might help. I am sure you have seen cakes cook unevenly in bad ovens before. Heating at a lower temp and for longer may help heat evenly and reduce the potential for burning.

Perhaps you could raise your positive a cpl of cms to ensure a more even bladder contact.

Not sure what else it could be???

 

The way you heat it may be the problem, but it can be the material...
Some of the polypropylene I've tried to work with was simply unbearable. :craig:

From what I know, the material usually called polypropylene (eg: here in the forum) isn't really "pure" polypropylene, but Copolymer Polypropylene. On the other hand, there exists for sale both types: Copolymer Polypropylene and the "pure" polypropylene. Maybe you have the second one...
Maybe you should ask to a colleague who bought the same material if it worked fine... If it's the "hard one" I'd bet it didn't also.

Keep us updated...

Cheers,

 

Thanka for your inputs. It was actually an air pocket in the arch area that was trapped causing the plastic to fold into itself once that pocket of air got sucked out. To remedy this, I just massage out the arch area first before turning the vacuum on, no more unwanted FHL grooves! And regarding the deformation of plastic: heating issue. I lowered the temperature and allowed it to slowly heat and this lead to more uniformity in the surface texture and wall thickness.

Trial and Error are my 2 greatest teachers ;).