summary of responses- Polypro shrinkage (part 2 of 2)

summary of responses- Polypro shrinkage (part 2 of 2)

William Lifford

9/25/2003

Here's the rest of the responses:




Let me introduce myself - I am Karran Martin. I work for O&P Enterprises in sales. I've been selling plastics to the O&P industry for about 10 years. I saw your question on the O&P List & decided to contact you.
You may already know all of this, and I certainly do not know everything, but will just mention a few things to you that might could help you with your shrinkage problems.

1. Be sure you use a quality product. If it is not orthopedic (medical grade) quality material, you could have problems. Also, quality polypro will have an approximate shrinkage of 1% to 2.5%.

2. Be sure it is a stress relieved product.

3. The majority of problems with plastics can be attributed to improper heating and/or improper vaccum. If you aren't sure of the proper oven temperature to heat your plastic, consult with your supplier. It does vary with different manufacturers. Some people think that if they can put the plastic on vaccum for a short while & then leave a socket on the mold overnight, and that it will be fine the next day. But that is not true if it has not been kept under strong enough vacuum long enough. Your vacuum should have a minimum of 27 inches of mercury or 13 PSI. The hot plastic should be left on vacuum until it is completely cooled. A good rule of thumb is if the plastic is pulled over a cool/cold mold leave on vacuum at least twice as long as it took to heat it. If pulled over a warm mold leave on vacuum at least three times as long as it took to heat it.

4. Pulling any plastic over a warm mold will always increase the chances of a successful pull. When pulled over a cool/cold mold, it shocks the inside of the plastic, therefore the cool down process is not as effective. (Just be sure a mold that has been warmed isn't dried out or you'll cause another whole set of problems.)

I hope that some of this information will be helpful. If you'd like to discuss any of this with me, feel free to call me. My toll free number is: 866-313-0400. My office hours are 8-5 CST except for the noon hour.
Have a good day,
Karran Martin

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FLO-TECH manufactures the APOPPS and we use a minus 2 percent shrinkage factor in both circumference and length. Shrinkage of sockets varies based
upon plastic supply, humidity and temperature. However. I have been very happy with the overall results using this - 2% factor.

Bob


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 There are many factors that play into the shrinkage of various plastics. Virgin or re-grind material, method of vacuum forming (blister or drape), orientation of plastic as it is extruded, and temperature as well as the method used in heating are all important factors that have a definite effect on the degree of shrinkage of your final socket. If you could give me a call at 800.828.9339, I can get a better feel on what your current techniques are and can better offer suggestions that may be of benefit to your problem.

Frank Friddle, Jr., C.O.


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I know you will get a response back from Gary Bedard at Becker on this one, and if you don't call him. He has volumes of info on thermoplastic
idiosyncracies including minimizing or anticipating shrinkage due to the direction of the plastic extrusion, vac pressure, sag time, etc. Good Luck

Rick Miller, CO


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When you have time call me about the pp problems you are having.
800-666-0356 Don Biondi , O&P ENTERPRISES

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Call PDI and see what they recommend. They have a lot of experience.
Best Regards, Rick, CPO

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There a wide variety of variables involved in controlling shrinkage in thermoplastics. Too many to control realistically. That's why I use laminated frames, although I am sympathetic to your concerns about the dust.


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Polyprop as most CO will tell you can be a problem. The first rule is to buy the best quality this will limit the shrinkage problem. The other area to look at is the temp of the oven must be exactly correct and the Poly prop must be at the correct temp when pulling it if it is a little cool it will shrink back a little. Use a cooking temp gauge and place it in the middle of the oven to see how it correlates to the actual oven temp gauge. Ployprop normally shrinks more in one direction than the other take a small sample and see which way it shrinks and then apply it in the same direction so that you know which direction it will shrink. A pre heater oven also helps to limit the shrinkage the sheets should be kept at 50 degrees C and they can be kept at this temp for long periods without problems, next the sockets once draped should not be allowed to cool too quickly and placing in the 50 C oven over night will also help. I know that this sounds like a lot of work but it works very well, and once it becomes a standard method it is not so time consuming.

Best wishes

Gary Seaman
Prosthetic Consulting
Cologne, Germany
<Email Address Redacted>
<URL Redacted>

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Please post your responses. I'll bet you get a lot of responses like mine however, who have the same problem and no bankable solution.
Thanks in advance,
Jeff McDonald


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Have your technicians orient the machine direction of the plastic the same way each time. That way you can eliminate at least one of the variables
(longitudinal vs. circumferential) I would bet since you get sometimes one, then the other, that machine direction has something to do with it. Good
luck, looking forward to the replys!

sincerely,

Justin Foster
Prosthetist


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One thing you can look at is the orientation of your plastic sheets with respect to the machine direction of the sheet. Sometimes it can be hard to determine the machine direction but I know that some suppliers will put stickers on the plastic indicating the direction of extrusion if you request it.

The point is that the plastic has different properties, especially in tensile strength, in the machine and transverse directions. Therefore it is not unreasonable to think that the shrinkage will be different as well.

Controlling the orientation will at least give you some control over whether you can expect shrinkage in the socket's circumference or in it's length.

Predicting the shrinkage will be more difficult because that is more of a function of the manufacturing process of the plastic and to some extent your own manufacturing process in the lab. Increasing the cooling time after pulling your plastic may help reduce the amount of shrinkage.

I hope this helps some.

Regards,

Dave Fritz (student)


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I used to be in the plastic trade. Where are you getting your polypro? Do you happen to know who is manufacturing it? There has been a lot of consolidation in the manufacturing of plastics. Quality from a thermoforming perspective has suffered. I assume that you are allowing the material to cool overnight on the cast? I've gotten so frustrated with plastics that I've switched to epoxy laminations and urethanes.

 

Best regards,

Charles King

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end of responses