Responses to "problems with Polypropylene"
Gabriel Beversluis
Description
Collection
Title:
Responses to "problems with Polypropylene"
Creator:
Gabriel Beversluis
Date:
6/15/2005
Text:
Thank you to all who responded!!
It appears that this issue is not yet resolved based on the variety of responses.
I'm sure the solution lies below.
Happy reading......
------------------------------------
Spreading will always be problem with fast pulls,cold molds, and cooling irregularities. I use American Plastics all the time and have no problems except that it takes forever to get me my materials. I have seen all the techie tricks that people share out there and I will always say that an accurate heating temp with appropriate cooling time will supply even results. Polypro is a 24hr type cooling time unless you can get it core temp at a faster rate. I don't do anything except pull it and get to the patient as quick as I can. If I have tech that is having polypro spreading problems,I don't blame te supplier,I blame the tech. I get a lot of boos for that but,have been doing this too long to blame the plastic for faults of the ineffiecent human equation. My best advice,check the oven and heat time. They got to be close or spreading problems. If all else fails switch plastics.Mod PE or Copoly can be a welcome change if possible. Good luck.
--------------------------------------------------------
sounds like a vaccum problem...not enough fast enough ...check your
lines..consult Gary Bedard at Becker...
-----------------------------
We shrink our plastic back to the model. Expansion is a common problem and
usually caused by cold pulls but can also be caused by the extrusion
process. Shrinking the plastic back to the model is easy and efficient.
Call if you haven't seen this technique before. I'll explain how to do it
and I think you'll like the outcome.
-----------------------------------------------------
Have you tried blowing compressed air on the plastic immediately after
achieving vacuum? The outside needs to cool faster than the inside.
-------------------------------------------
Three issues need to be dealt with. The temperature of the poly pro, the vacuum pressure, and the mold temperature.
Whatever type of oven you use, make sure the plastic is EVENLY heated to 360 degrees. buy and use an infrared thermometer (available from better hardware and tool companies). Remember poly pro goes clear before it gets to its ideal forming temp so just being clear does not indicate that it is ready to pull.
Vacuum pressure and the speed with wich you acheive it is critical, idealy you want 24-28 mercury inches of vacuum for forming poly pro. You also need to get the mold up to pressure very quickly, no more than 15 seconds after the plastic leaves the tray, so a good vacuum pump is essential!
Lastly if the molds you use are signifigantly cooler than the ambient air temp, you may want to heat them up in the oven. Remember what happens if you use a foam liner? it curls up because of the difference in cooling rates of the inside of the plastic and outside. the same thing happens if the inside cools faster than the outside only in reverse, it expands.
nail these three parameters and you should have good results, even from not so great plastics.
Good luck in you quest
-------------------------------------------------
I cannot help you as this seems inherant in plastic and that is why I
laminate things. Good luck
---------------------------------------------------
We are having these problems as well.
We stopped using American and switched to OPE, and found this made a
difference for a while, but was not consistent.
We try not to stretch the polypro at all, and if we need to, we do it very
slowly.
We also wrap in plastizote after the initial skin is formed so that the
newly vacuum formed device is safe from a draft (here in arizona the air
conditioning is always on!).
We have also started to place completely cooled casts in a sink with water
over night.
We are having fewer problems now.
Hope this helps.
-----------------------------------------------------------
The occurrence of spreading plastic can be easily mitigated but first you
must understand what it is that's causing the problem. What's happening is
that the inside surface of the plastic (touching the cold mold) and the
outside surface (exposed to ambient air) are cooling at different rates.
This causes spreading. Conversely, when you vacuum form over foam the heat
is trapped on the inner surface and it cools more slowly than the outside,
it will draw inward.
The trick is to cool the inside and outside of the plastic at the same rate.
This can be tricky to measure accurately, but an experienced vacuum former
can just tell by feel. The best thing to do is, after achieving vacuum, to
cool off the plastic until it's about 95 degrees or so (warm to the touch),
and cool it as quickly as possible. Compressed air works best. Water may
cool it too quickly and cause it to crack.
When vacuum forming over foam, you might have to slow the cooling process on
the outside with an insulator, like a cardboard box or bubble wrap.
-----------------------------------------------
Do you have a new technician pulling your AFO's now? :)
------------------------------------------------------
I am currently a Master's O&P student at Eastern Michigan University. Last semester we had a class in plastics, and they taught us that the machine direction of the plastic needs to be lined up with the length of the limb to prevent the plastic from spreading after it cools. Our instructor used some of the information from Becker Orthopeadics seminar on plastics to explain machine direction, how to determine machine direction, and how to properly place the plastic over the plaster mold to prevent it from spreading once it is cooled. You might be able to contact Becker Orthopeadics (in Michigan) to find out more about machine direction and plastics in O&P practice.
Hope this helps
----------------------------------------------------------------
The temperature we set our oven at is 380 degrees. It will take a little longer for the plastic to reach forming temperature but the extra time is worth it. leave your plastic under vaccuum for at least one hour and on the cast another 4-6 hours - over nite if possible. If you can do all your forming in the afternoon you can leave the plastic on the cast till morning. Some of what you are saying sounds like unevenly heated plastic and removed too soon from the mold.
I'm in the process of working with Bulldog Tool Company to produce injection molded versus extruded orthopedic grade polypropylene. Check back with me in a month and I should be able to give you an update. Injection molded sheets will not exhibit the problems you are experiencing with extruded plastics. 25 years ago when orthopedic grade PPG was craft extruded by small extruders the extrusion rate per minute was slow and the PPG molecules were well oriented and not stressed. As craft extruders were bought out the extrusion rate when way up and the plastic they currently label as stress relieved is not in any real sense stress relieved.
I'm hoping that a solution is on the way. Save my email address and check back.
It appears that this issue is not yet resolved based on the variety of responses.
I'm sure the solution lies below.
Happy reading......
------------------------------------
Spreading will always be problem with fast pulls,cold molds, and cooling irregularities. I use American Plastics all the time and have no problems except that it takes forever to get me my materials. I have seen all the techie tricks that people share out there and I will always say that an accurate heating temp with appropriate cooling time will supply even results. Polypro is a 24hr type cooling time unless you can get it core temp at a faster rate. I don't do anything except pull it and get to the patient as quick as I can. If I have tech that is having polypro spreading problems,I don't blame te supplier,I blame the tech. I get a lot of boos for that but,have been doing this too long to blame the plastic for faults of the ineffiecent human equation. My best advice,check the oven and heat time. They got to be close or spreading problems. If all else fails switch plastics.Mod PE or Copoly can be a welcome change if possible. Good luck.
--------------------------------------------------------
sounds like a vaccum problem...not enough fast enough ...check your
lines..consult Gary Bedard at Becker...
-----------------------------
We shrink our plastic back to the model. Expansion is a common problem and
usually caused by cold pulls but can also be caused by the extrusion
process. Shrinking the plastic back to the model is easy and efficient.
Call if you haven't seen this technique before. I'll explain how to do it
and I think you'll like the outcome.
-----------------------------------------------------
Have you tried blowing compressed air on the plastic immediately after
achieving vacuum? The outside needs to cool faster than the inside.
-------------------------------------------
Three issues need to be dealt with. The temperature of the poly pro, the vacuum pressure, and the mold temperature.
Whatever type of oven you use, make sure the plastic is EVENLY heated to 360 degrees. buy and use an infrared thermometer (available from better hardware and tool companies). Remember poly pro goes clear before it gets to its ideal forming temp so just being clear does not indicate that it is ready to pull.
Vacuum pressure and the speed with wich you acheive it is critical, idealy you want 24-28 mercury inches of vacuum for forming poly pro. You also need to get the mold up to pressure very quickly, no more than 15 seconds after the plastic leaves the tray, so a good vacuum pump is essential!
Lastly if the molds you use are signifigantly cooler than the ambient air temp, you may want to heat them up in the oven. Remember what happens if you use a foam liner? it curls up because of the difference in cooling rates of the inside of the plastic and outside. the same thing happens if the inside cools faster than the outside only in reverse, it expands.
nail these three parameters and you should have good results, even from not so great plastics.
Good luck in you quest
-------------------------------------------------
I cannot help you as this seems inherant in plastic and that is why I
laminate things. Good luck
---------------------------------------------------
We are having these problems as well.
We stopped using American and switched to OPE, and found this made a
difference for a while, but was not consistent.
We try not to stretch the polypro at all, and if we need to, we do it very
slowly.
We also wrap in plastizote after the initial skin is formed so that the
newly vacuum formed device is safe from a draft (here in arizona the air
conditioning is always on!).
We have also started to place completely cooled casts in a sink with water
over night.
We are having fewer problems now.
Hope this helps.
-----------------------------------------------------------
The occurrence of spreading plastic can be easily mitigated but first you
must understand what it is that's causing the problem. What's happening is
that the inside surface of the plastic (touching the cold mold) and the
outside surface (exposed to ambient air) are cooling at different rates.
This causes spreading. Conversely, when you vacuum form over foam the heat
is trapped on the inner surface and it cools more slowly than the outside,
it will draw inward.
The trick is to cool the inside and outside of the plastic at the same rate.
This can be tricky to measure accurately, but an experienced vacuum former
can just tell by feel. The best thing to do is, after achieving vacuum, to
cool off the plastic until it's about 95 degrees or so (warm to the touch),
and cool it as quickly as possible. Compressed air works best. Water may
cool it too quickly and cause it to crack.
When vacuum forming over foam, you might have to slow the cooling process on
the outside with an insulator, like a cardboard box or bubble wrap.
-----------------------------------------------
Do you have a new technician pulling your AFO's now? :)
------------------------------------------------------
I am currently a Master's O&P student at Eastern Michigan University. Last semester we had a class in plastics, and they taught us that the machine direction of the plastic needs to be lined up with the length of the limb to prevent the plastic from spreading after it cools. Our instructor used some of the information from Becker Orthopeadics seminar on plastics to explain machine direction, how to determine machine direction, and how to properly place the plastic over the plaster mold to prevent it from spreading once it is cooled. You might be able to contact Becker Orthopeadics (in Michigan) to find out more about machine direction and plastics in O&P practice.
Hope this helps
----------------------------------------------------------------
The temperature we set our oven at is 380 degrees. It will take a little longer for the plastic to reach forming temperature but the extra time is worth it. leave your plastic under vaccuum for at least one hour and on the cast another 4-6 hours - over nite if possible. If you can do all your forming in the afternoon you can leave the plastic on the cast till morning. Some of what you are saying sounds like unevenly heated plastic and removed too soon from the mold.
I'm in the process of working with Bulldog Tool Company to produce injection molded versus extruded orthopedic grade polypropylene. Check back with me in a month and I should be able to give you an update. Injection molded sheets will not exhibit the problems you are experiencing with extruded plastics. 25 years ago when orthopedic grade PPG was craft extruded by small extruders the extrusion rate per minute was slow and the PPG molecules were well oriented and not stressed. As craft extruders were bought out the extrusion rate when way up and the plastic they currently label as stress relieved is not in any real sense stress relieved.
I'm hoping that a solution is on the way. Save my email address and check back.
Citation
Gabriel Beversluis, “Responses to "problems with Polypropylene",” Digital Resource Foundation for Orthotics and Prosthetics, accessed November 5, 2024, https://library.drfop.org/items/show/225041.
