RESPONSES: AFO Fabrication Question, PART TWO

RESPONSES: AFO Fabrication Question, PART TWO

Paul Rieth

10/9/2001

PART TWO
Initial Inquiry:

Problem: Pulling thermoplastics over an AFO lined with aliplast or
> plastizote causes the plastic to curl in, regardless of how long it is
> left on the model. I end up using a blow torch to reheat the plastic and
> torque it back out into proper alignment. There has got to be a better
> way. I would appreciate any suggestions.

PART TWO RESPONSES:

Put a layer of nylon stocking on top of the liner and it will vacuum down very nicely.
If you use one layer it will allow for vacuum, but the liner will still adhere to the
plastic nicely.

we made a lot of night splints out of
Polyethylene over aliplast without the edges curling
in. However, as a resident, I mistakenly used copoly
and resulted in the same problems you describe.
Perhaps not all thermoplastics cause this problem.

Lower the vacum used during forming that will help a bit

First you can orient your plastic 90 degrees to what you have been
cutting it and that is supposed to help reduce the curling. Next, I
would suggest you use an electric heat gun for all of your plastic
heating when adjusting or modifying as needed.

The problem you have is that the outside of the plastic is cooling at a much
faster rate than the side that is against the aliplast. It is the opposite
of what happens when you pull plastic directly over a cool plaster mold
without foam. The foam is acting as an insulator and keeping the inside of
the plastic hotter than the outside. It is creating internal stress in your
plastic. I can give you some tips/or suggestions.
1) If you are using compressed air to cool the plastic once vacuum is
established....STOP. Also do not submerse the pulled afo in water to
expedite the cooling process. You want to prolong this process.
2) Try pulling over a warm mold. I know that this seems like it would add to
the problem but it will help to slow the cooling process of the entire
thickness of plastic.
3) Keep a heat gun (or torch if used properly) hot and ready. After you have
established vacuum and trimmed your excess plastic, lightly heat the entire
surface of the plastic. No need to overdue it just try to prolong the
cooling of the outside surface.
4) As a general rule you should leave the mold under vacuum for twice as
long as it took to cook the plastic. I know that we don't always have that
luxury but it is a good practice when possible.

Try using insulation for heating ducts on your thermoplastics, the reason
that it is curling is because when you line the plastic with plastizote or
aliplast it acts as a insulator itself and keeps the inside of the
thermoplastic hot longer and the outside cools faster. That causes your
plastic to cool.
    Make your self a foil wraped bubble plastic boot and put it over your
vacuum formed model soon as it skins over so you don't scar up your afo
pulling the boot over. That should help cut down the curling of your pafo's.

The problem you are having is that the plastic is
cooling to fast. The foam liner holds heat in side
next to the liner and the plastic cools faster on the
outside away from the liner. this causes the plastic
to cup. Next time try allowing the plastic to cool
much slower do not blow air to cool it down. you can
even rap the plastic in bubble rap or old lab coat to
hold heat in this must be done after the plastic has
cooled just enough so no imprint can be left on it.
You also need to keep under vacuum a little longer
until you can touch the plastic with your bear hands
it may still be hot but if you can hold your hand
there it ok to shut off vacuum. hope this helps
              it is a good idea to not cut off mold

We have had the same problems in the past. Not only with lined
AFO's but also lined TLSO's. The cause is the heat retention internally
due to foam causing the inner surface of the plastic to cool much slower
than the outer surface. Once the outer plastic has cooled just enough
that touching it does not leave a finger print, wrap the AFO with a
towel, blanket or a sheet of foil insulation wrap (pick this up at any
of the home builder stores). This will allow for more even cooling and
it has taken care of these problems for us.

Two ways I known put vaucum hose on cast then plastizote then
hose.steel brush on plastizote.

I've thought about molding in an aluminum calf band at the superior trim line. This ought to keep the shape. I haven't done it yet. This shouldn't increase weight appreciably. Playing devil's advocate, can you see why this wouldn't work?

IMHO, there are two main factors for shape changing, and they go together:
#1 cheap thermoplastic material - almost always rolled to spec thickness instead of being ejected from a proper thickness tool. This causes stretching of the molecules, which tend to contract again once the plastic sheet is heated to correct temperature. If the piece you take out is smaller and thicker than the one you put in, you will know that this is the case. This leads to;
#2 stretching the plastic since it has shrunk in the heater. This once again causes molecule elongation and inherent tension in the material. If the roll direction is radial to your model, the effects will be more noticeable (change of circumference) than if it is in the axial direction.

The problem of post curl-in in a thermoplastic AFO is due to the recrystallization of the polymer. The molecular layer in contact with the foam liner cools at a slower rate than the outside. Therefore you have more shrinkage and higher tensile strength along the inner surface, which pulls the plastic inward. The outside layer
is more amorphous while the inner surface is able to recrystallize at a higher rate.

We wrap our AFOs with a double bubble pack insulation that has a Mylar covered surface. It is used in the HVAC industry and usually can be found at a good hardware store. We wrap the models as soon as the plastic can take take the pressure of the wrap without marring the surface.

I will be teaching a thermoforming course at the NJ-AAOP meeting in the first week of November. I have taught a thermoforming and thermoplastic composite course for the Academy for the past 9 years. The course covers many of the common problems encountered with our thermoforming process.
Gary G. Bedard, CO, FAAOP
Clinical Marketing Manager
Becker Orthopedic Company