Re: RESPONSES: AFO Fabrication Question, PART TWO
Gary G. Bedard
Description
Collection
Title:
Re: RESPONSES: AFO Fabrication Question, PART TWO
Creator:
Gary G. Bedard
Date:
10/11/2001
Text:
Dear List Members,
The problem of “curl-in” as described in Paul Rieth's posting is only one manifestation of a non-uniform or stratified polymer chain structure. It is a common problem and one that is quite obvious to the naked eye, but there are also molecular issues of this phenomenon.
Ironically, the Fabrication Sciences Society is currently taking a closer look at this problem. Last month, the Academy board approved the use of society funds to undertake a research project that will measure the rate of recrystallization in polypropylene samples that have been cooled through a series of techniques. We are using Differential Scanning Calorimetry (DSC) to measure
the original crystallization rate of the polymer and the obtained rated following a thermoforming event with polypropylene. This is a proof of concept study. The results, which should provide definitive numbers, could lead to other analytical tools that will demonstrate that there should be a definitive service life to clinical products that are fabricated from homogenous
thermoplastic materials, especially the polyolefins.
The use of Mylar covered twin layer bubble pack has been in use for several years. It is a simple technique to slow the cooling of the polymer, which allows the polymer chains to more into a crystalline formation. Anthony Donnelly of Toronto Orthopedic first suggested the technique to me at a British Columbia O&P association meeting in July of 1996. He is one of the finest plastic
technologist that I know. He confirmed the effectiveness of the material through a consult with a professor of thermodynamics. The technique has been part of my thermoforming course since. If you or your staff is interested in a course on thermoforming, there will one offered at the NJ AAOP meeting in November.
Regards,
Gary G. Bedard, CO, FAAOP
Clinical Marketing Manager
Becker Orthopedic
Chairman
AAOP Fabrication Sciences Society
Paul Rieth wrote:
> 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
>
>
The problem of “curl-in” as described in Paul Rieth's posting is only one manifestation of a non-uniform or stratified polymer chain structure. It is a common problem and one that is quite obvious to the naked eye, but there are also molecular issues of this phenomenon.
Ironically, the Fabrication Sciences Society is currently taking a closer look at this problem. Last month, the Academy board approved the use of society funds to undertake a research project that will measure the rate of recrystallization in polypropylene samples that have been cooled through a series of techniques. We are using Differential Scanning Calorimetry (DSC) to measure
the original crystallization rate of the polymer and the obtained rated following a thermoforming event with polypropylene. This is a proof of concept study. The results, which should provide definitive numbers, could lead to other analytical tools that will demonstrate that there should be a definitive service life to clinical products that are fabricated from homogenous
thermoplastic materials, especially the polyolefins.
The use of Mylar covered twin layer bubble pack has been in use for several years. It is a simple technique to slow the cooling of the polymer, which allows the polymer chains to more into a crystalline formation. Anthony Donnelly of Toronto Orthopedic first suggested the technique to me at a British Columbia O&P association meeting in July of 1996. He is one of the finest plastic
technologist that I know. He confirmed the effectiveness of the material through a consult with a professor of thermodynamics. The technique has been part of my thermoforming course since. If you or your staff is interested in a course on thermoforming, there will one offered at the NJ AAOP meeting in November.
Regards,
Gary G. Bedard, CO, FAAOP
Clinical Marketing Manager
Becker Orthopedic
Chairman
AAOP Fabrication Sciences Society
Paul Rieth wrote:
> 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
>
>
Citation
Gary G. Bedard, “Re: RESPONSES: AFO Fabrication Question, PART TWO,” Digital Resource Foundation for Orthotics and Prosthetics, accessed November 8, 2024, https://library.drfop.org/items/show/217596.
