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I have experieneced polypro not taking the exact shape of the cast which I have dressed...the polypro lifts from the arch and met area...such that it is not a tight fit after the bladder is released.

ANy help here is greatly appreciated.

Sincerely,

Freeman Churchill

 

Overheating?

Or not allowing time to fully cool? Poly will shrink when cooling and needs to be held in place until pretty well cold to ensure it doesnt pull away from cast

 

Also, if the cast is too moist, then the heat of the plastic will cause gas to form, thereby causing a separation.

Just a thought :confused:

 

I agree with the first two, and sometimes a flatbed vacuum press will not pull/push the polypro down into concave curve forms (a steeper and higher arch form of foot). To help "suck" the plastic down to where you want it you can drill a 3/16 or so dia hole right through the plaster foot so that the vacuum works from the inside of your shape as well.
Regards Phill

 

I would also add that the speed with which you get the plastic from the oven and sucked down can be a factor in how easily it will take and remember your shape.

 

Doesn't this product have a greater shrinkage rate in one direction A/P verses M/L? It also has a critical temperature and the cast should be completely dry. Speed is essential. Work in a well ventilated area as this product can be toxic.
Cut a small section from the sheet and do a shrink test then mark with an arrow the direction of most shrink.

Different colours and thickness of poly shrink and mould at different temperatures and times. Black is the worst to mould. Clear, white, blue, yellow and green are easier to work with.
Once the vacuum forming has been done use a wet cloth to rapidly cool the poly maintaining vacuum suction. When cool remove from vacuum.

Good Luck

 

The polypro I am used to is grey co-polymer which is a little more rigid than homopolymer. The "safe" working temp range is pretty much 185 deg C. and use it as soon as the edges of the piece expand a little...it does not give off much in the way of fumes unless you over cook it or over heat it...but as you say...don't snort the fumes. In theory if you let it cool slower on the press it should be stronger when done....more organized arrangement of molecules in the matrix I think. The addition of dye to the polymer would also alter it behaviour...what I use comes out of Germany.
Don't seem to have noticeable trouble with shrinkage.
Regards Phill

 

Hi Free

In addition to the above suggestions (all good) check your press for any potential leaks that will cause it to lose consistent pressure over the entire surface area of the poly. Also check the rubber itself--may not be firm enough to exert even pressure over the entire surface area of the polypro on the cast.

Bob

 

Hey!
These are great and innovative helps. Thanks so much!
Freeman

 

Gday Freeman,
Just come across your thread -hope you have solved your problem!
After forming over 20,000 poly plates over the years I can say with some authority that poly is verrrry temperamental. We have always used the german poly which is great but different batches can require different heating.
The secret is definitely in the heating & having a good convection oven.

 

As EdYip suggested, positive cast moisture a big factor...as is underheated poly (Phil's suggestion).


The colour dyes tend to weaken the plaster cast; and it tends to dry much slower; so the dye should be minimal.

 

Try used gas stove oven cause sometimes electric oven doesnt have good heating because the element isnt work properly. And then press the bladder using cold towel following the shape specially arh area.