Plastic Shrinkage: Ruining a Perfect Pull
Steve Hill, CO
Oh, I hate it when this happens.
Itís in the afternoon. This solid ankle AFO needs to be done by quarter of five when the patient is expected to come in.
In a show of confidence, you turn off the oven as you remove a sheet of perfectly cooked poly-pro. With the vacuum pump chugging along in the background, you drape the cold plaster cast with the hot polyolefin, make a perfect sealóyet againóand begin blowing cool compressed air across its sweltering surface.
As the plastic starts to cool, you can see that the pull was indeed perfect: full thickness at the ankles, and not a blemish anywhere on its surface. Itís absolutely lovely.
Once itís cool enough, you begin cutting with your cast saw on the trim lines. Strangely, though, the plastic seems to be opening up, trying to regain the flat shape it had prior to being put in the oven. Finally, the brace is removed from the mold and it bears only a passing resemblance to the cast from which it was formed.
Dang! Better turn that oven on again.
So what happened? In a word, plastic shrinkage.
OK, so that was two words.
One of the biggest problems faced by any vacuum former is the tendency of plastic to shrink once it cools. Although this will occur with all plastics to one degree or another, unacceptable shrinkage can be controlled.
There are three basic indications of unacceptable shrinkage.
The first indication of plastic shrinkage is when the plastic shatters while it is being cut off the mold. Sometimes it will just form a small crack at the point where you start cutting. Other times, it will spiral from that point around the orthosis, following the grain of the plastic.
Either way, you can be certain of one thing: the orthosis will be rendered useless.
Even if the crack occurs outside of the trim lines, which almost never happens, the plastic will have been weakened and is not fit to be used on an orthosis that must support a patient.
What is the culprit?
Assuming that you are processing your plastic the same way you always have, the culprit is almost always the plastic itself. Plastic extrusions can vary wildly from manufacturer to manufacturer and from batch to batch. You might even find a few sheets of bad plastic in between many sheets of good plastic.
Sadly, you can never tell when these aberrant sheets will show up. Thereís no good way that I know of to pick these bad sheets out to prevent bad pulls. The only fool-proof way to tell is to pull it and check.
Just keep in mind that you may have to pull this cast over again, and being careful with it is of the essence. But, thatís always a good approach to dealing with patient molds.
Other faces of shrinkage
The other two indications of plastic shrinkage occupy opposite ends of the same problem. I speak, of course, of the annoying curling in and spreading out of plastic.
A common misconception about why this occurs is that youíre not letting it sit on the cast long enough. While it may be true, there are many other things that are more likely to be at fault. To understand how to fix these problems we much first understand what is causing them to happen.
Why it happens
Think of the sheet plastic as having an inside and an outside. The inside being the side that touches the tray while itís cooking and touches the cast when itís being formed. The outside is the side that is exposed to the relatively cool air of the room after forming.
In order for the finished product to retain its new molded shape, both sides of the plastic must cool at the same rateóor as close as you can get to it. As the inside of the hot plastic touches the cold plaster mold, it begins to cool right away, while the outside of the plastic remains hot, with only the room-temperature air to cool it.
Stop the spread
Thatís why itís so vitally important to cool the outside as soon as you achieve vacuum. A good vacuum former can tell when he or she has applied enough cold air to the outside to keep it from spreading out. The key here is cooling the outside at the same rate that the inside is cooling. If both sides of the plastic cool at the same rate, you shouldnít see any spreading.
Control the curl
The other problem is when the plastic starts curling in. In this situation, you are probably pulling over a foam liner, and that liner creates the opposite problem. The foam acts as an insulator, retaining heat on the inside on the plastic. In this case, you not only donít want to cool off the outside of the plastic, you might even want to wrap some insulation around it, when the surface loses its shiny appearance, to prevent it from cooling too quickly.
A number of different materials can be used for this purpose. The pink fiberglass used for home insulation works pretty well. Even wrapping it in newspaper or cardboard may be good enough.
Probably the best thing, though, is bubble wrap coated in reflective Mylarģ. It is thin enough that you can wrap it around compound curves, like at the ankle, and it is highly insulative.
†The important thing is to slow the cooling process down to match the rate at which the inside of the plastic is cooling. Therein lies the key to prevent curling and spreading.
Other factors may enter into the equation, such as extrusion, regrind and heating, but the most common factor in curling in and spreading out is how the plastic cools on the mold. You can leave it on overnight; that canít hurt. But if the outside of the plastic didnít cool at the same rate as the inside, youíre just wasting your time.
Happy vacuum forming!