plaster mold vs. thermoplastic temp variance - responses

plaster mold vs. thermoplastic temp variance - responses

Skewes, Ed

3/7/2008

Yea, it's Friday ! Have a great weekend.

 We were curious about this discussion because when vacuum forming a
thermoplastic afo (on a moist cold cast), we occasionally get a wrinkle
on the apex of the heel and thought it might be due to the temperature
difference between the plaster mold and the heated thermoplastic. We
realize it may also be technique, but were not sure.

  Thanks for all the responses, you too Joel. :-)

      Ed Skewes

 

    Here was the original post and below are the responses.


  We have measured temperatures of the positive plaster molds and the
heated thermoplastic prior to vacuum forming ankle foot orthoses.
 The AFO positive plaster mold temperature is approximately 65 - 75 F,
and the heated thermoplastic temperature just prior to vacuum form is
approximately 350 F.
   Do you ever preheat the plaster model to decrease this variance in
temperature?
If so, what method do you use to increase the temperature of the plaster
molds?
Heating blanket, oven at low temperature, etc?


 

  RESPONSES - thank you to all

 

1) Be careful heating up the mold. The pipe will expend and explode
the mold if it gets hot enough. Better to use an insulating blanket
over the molding until it is cool.

 

2) At Connecticut Childrens (formerly Newington) we used to stick
molds in oven, and don't recall lowering temp, fairly certain it was
same temp for plastic. Sure you've heard Gary Bedard of Becker would
suggest best thing is to slow cooling of plastic after formed (heat
blanket, etcm). He suggests problem is not with cold model but outside
cools quicker than inside. He'd be the guy to consult without doubt.
Hope alls well.

 

3) The best finish you will receive is when the mold is heated to
within 10 degrees of the set temperature of the material being formed.
You will then need to leave the vacuum on as long as possible after
forming. Vacuum time of at least twice the materials heating time is
required for cold or cool casts. Vacuum times of three times the
materials heating time is required for warm or hot casts. You may just
heat your cast in the oven.
Just remember the material needs to be at least cooled to its set
temperature before removed from vacuum to reduce the risk of shrinkage.
If material is not left on vacuum long enough , it won't matter how long
you leave it on the mold before cutting it off, you could still have
problems.

 

4) At freedom fabrication we usually preheat our molds. Generally we
put the mold in the oven at the same time as the plastic and remove the
mold just before the plastic is ready. Naturally molds that require more
setup will come out earlier and molds that require a foam liner don't
get heated at all.

I hope this helps!

 

5)

    Since you did not define what problem you are trying to solve, I
will assume that you are trying to decrease the amount of spread or
warpage that you are seeing in your orthoses. Heating you positive
models will help to reduce the warpage. It is standard practice to
maintain hot mold temperatures in industrial thermoforming.

The typical temp for PP is 170F, which is 20 degrees below the set
temperature of PP. Of course, the industrial folks are cooling their
aluminum molds to that temperature, not heating. The cycle rate of
production requires the cooling. The other task you should incorporate
into your process is to align the machine direction of your sheet with
the long axis of your AFO. In this manner, the maximum shrinkage or
orientation will occur from top to bottom instead of in the transverse
plane.

I had a 4 hour clinical thermoforming DVD available through our 800
number. You might want to take a look at the section that covers your
spreading problem.

            This course is used by all the O&P schools in the US

             Regards,

                  Gary G. Bedard, CO

 

6) We keep our dry models at room temperature. Use two layers of
treated nylon. Heat the PPG to 370 degrees and then try not to stretch
the plastic when draping and sealing. Use vacuum at 20 mm/hg. We no
longer use curing blankets (we're currently using a craft extruded PPG
from O&P Systems which is extruded slowly to ensure uniform cross
linking). The craft extruded PPG is the key. Large extruders extrude as
quickly as they can (I'll spare you the lengthy chemical and mechanical
explanation) which leads to very unstable PPG. We leave vacuum-formed
models in the vacuum stands for about 30 minutes under full vacuum and
then let them cure for 24 hours at room temperature before removing from
the plaster model. Our orthoses are very stable. Our oven is set to 420
degrees F.

 

7) It's not necessary. Use good nylons and heat your plastic hotter.
We like to get our plastic to at least 375. We use a laser temp. gun to
check heat. Polypro does very well at 400 and the Friddles transfers
work better at hotter temps. Try heating plastic hotter. You may see a
difference.

 

8) In reality, there would be far fewer failures & problems with
thermoforming if all molds were heated before the hot plastic was pulled
over it.

I'm not familiar with the heating blanket you mentioned, but warming in
an oven certainly can be done. The most important thing is to just get
the mold warmer - get the chill off the surface - it doesn't have to be
heated to the same temperature as the plastic. Others that respond may
have suggestions for this.
Also be aware that warming a mold will increase vacuum time since it's
not only cooling the plastic but the mold. But that is also a positive
for more successful pulls.

 

9) Before switching to CAD CAM I used to warm the cast in the oven about
10 minutes to increase the temperature of the cast.

    I do not need to do this now with the foam blanks carved to the
shape as the foam is not a heat sink.

     

10) Yes, we always heat the plaster mold. We found ten minutes in the
same oven & same temp. as the plastic is sufficient. Make it routine, at
the same time you put your plastic in the oven, place your mold in there
too. Set a timer for ten minutes, when it goes off remove the cast and
prepare it as usual then pull the plastic when it's ready. If we are
using Oklahoma or Tamarack joints we just lightly wrap a heat resistant
glove around the joint section and they are fine. You will find you get
a much better resulting pull if you do this. If you have a high enough
oven interior it may help to boost the tray up by setting it on some
fire bricks, that way you can place the cast under the tray and save
room for plastic.

 

11) My practice is to place the cast (wrapped with cling wrap and thin
tubular

sleeve) into the over at the same time as the plastic is put in. (PTS
infa red oven). This seems to bring the cast temp up sufficiently and
has dealt with most of my problems of spread.

Regards

Shane Jansen

Orthotist

New Zealand

 

12) The heating of the positive model prior to thermoforming is
something that has been debated by P&O practitioners and plastic
manufacturers. The most common reason for pre heating is if the molds
were not completely dry or the surface temperature is cooler than room
temperature.The preheating in prosthetics is so that the plastic does
not cure or cool too quickly especially with components that are on the
cast. In most cases if your cast is dry and at room temperature you
usually will not need to pre heat the mold. Some manufacturers preheat
to as high as 200 degree F but I have never found that necessary with
AFO's or foot orthotics when metal parts are not involved. When metal
parts are involved preheating sometimes helps so that the plastic forms
and cools uniformly. If you are molding and leaving the Polypro to sit
over night then you should not have any problems.As far as spreading or
rebounding of the plastic due to not preheating that is usually not a
problem if you do not remove the plastic from the cast too quickly.

I hope this answers your question. I also suggest that you speak to G.

Bedard CO at Becker Orthopedic, he's an excellent resource for plastics
in lower extremity orthotics.

 

Thanks again !
Edward A. Skewes CPO - Director Orthotic and Prosthetic Department -
Shriners Hospitals for Children - 950 W. Faris Road - Greenville, SC.
29605
 


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