Heating Plastic

By Steve Hill, BOCO, CO

Uneven Heating of Plastic

Have you ever noticed how, when you put a sheet of plastic in the oven, it never heats up evenly? Either the center gets clear before the edges or the edges become clear before everywhere else. If you have the latest and greatest oven for heating plastic, then why is it so difficult to get your plastic to heat evenly? Once again, the answer is disturbingly simple yet its cure elusive.

Material Warping

All things expand and contract when exposed to heat. Everyone knows this yet we’re surprised to see its dramatic effects when we put in a sheet of polyolefin into a four hundred degree oven.

The first thing that happens is that the cool plastic sheet hits the hot oven tray and immediately begins to warp. Depending on which way the molecular grain is running, it’ll either bow with the center reaching skyward or with the center the only part touching the hot tray. Either way, you can depend on one or the other reaction happening, every single time. If the plastic would simply remain flat and touch the tray at all points, you might have a shot at evenly heating the plastic. Even then it wouldn’t be perfectly even due to other mitigating forces.

Instead what happens is the plastic is being subjected to heating conditions that exceed just that of the heat source. There are essentially three sources of heat that are affecting the plastic surface.

The oven tray is metal (whether it’s aluminum or steel) and may be at any number of varying temperatures depending on the heating cycle and how long you’ve had it out of the oven. Let’s assume that the tray is at or near the oven temperature. It holds and imparts heat differently than does the surrounding hot air, which is another source of heat. Add to those two the ovens own heat source – whether it’s convection or infrared – and you have a third heat source.

Each of these affects the plastic in different ways. When the plastic bows, as it inevitably will, parts of the plastic will be touching the tray and become heated very quickly. The metal of the tray is very efficient at heat transfer and will start the cooking process immediately. These areas of the plastic will be the first to turn clear.

The rest of the plastic surface has raised up and is not in direct contact with the hot tray, but is now at a different distance from the heat source. This phenomenon is most noticeable in infrared ovens where the distance to the IR source is important and part of the cooking equation. Now areas of the plastic nearest to the IR emitters will absorb more heat.

So here we have three areas of the same sheet of plastic that are absorbing heat through three different sources at many different levels. No wonder it doesn’t all just turn clear at the same time. It’s been subjected to heat three ways to Sunday.

Burp Your Plastic

After a little while, the plastic will begin to straighten out. The thermodynamic and kinetic forces that have been assailing it have leveled out and the real heating begins as more of the plastic touches the tray and less if it is too close to the heat source. It is at this point that another problem may rear its ugly head.

The plastic was warped but has now straightened out. The result was that now air bubbles have been trapped underneath the now partially cooked plastic. Its signature is unmistakable. The plastic all around the edges has made contact with the tray, heated clear, and sealed itself to the tray; trapping an air bubble inside that is still not heated thoroughly. If that air bubble isn’t dealt with, it’ll wreak havoc on your pull.

There are two ways to deal with this inevitability. The first is to carefully lift up on one side of the plastic halfway through cooking and “burp your plastic”. Care must be taken to a) avoid stretching and thinning the plastic and b) don’t wait too long to “relieve the gas”.

If you over-heat the edges before giving the center a chance to cook, you run the risk of creating an unwanted bubble mark on the surface of the plastic. If you do notice a bubble mark, you can usually remove it by cooking the plastic a little longer. It’s just a cosmetic consideration, but every little bit helps.

The other way to avoid this bubble from forming is to use perforated Teflon sheeting. Tiny holes in the material allow air to escape, reducing or eliminating the amount of air that becomes trapped underneath the plastic. Although I like this method very much, I’ve heard others complain about the perforations imparting an undesirable texture to the under surface of the plastic. I don’t know about that, but I guess it depends on how big those perforations are. Yeah, a few hundred quarter inch holes are going to make an impression, but a few thousand tiny holes ought not.

Truly Evenly Heated Plastic Sheeting

If you want to assure your plastic is heated thoroughly and evenly, there is one sure method, but it will take an oven capable of maintaining a constant (relatively) temperature for a long time and a little patience on your part.

This method takes more time to achieve results, but if evenness of temperature is your ultimate goal, it’s your only shot. Critical to this method though, is an accurate oven. A standard convection type oven can only maintain a temperature variation of + or – 50o of its set temperature. That’s a one hundred degree temperature variation total and can easily result in plastic scorching or burning if left in too long.

In an oven that can maintain a temperature within reasonable limitations, you should be able to bring your plastic up to its recommended forming temperature and leave it there until you’re ready to form. The longer the plastic can maintain one temperature, the more evenly that temperature can spread throughout the sheet.

Here’s a good example. When cooking polypro and copoly, set your oven temperature around 375^o. This temperature will allow the plastic to eventually get to its peak cook temp, but if the oven is accurate, it won’t exceed the cook temperature which might result in the plastic burning.

At its peak cook temperature, the plastic can be held, ready to cook, almost indefinitely. At some point in time, the molecular structure will begin to break down, but you’ll probably want to form your AFO before then. The patient is waiting after all…

Is Even Heating Important?

Although every area of the plastic didn’t cook at exactly the same rate, the important thing is that it’s all roughly the same temperature at the time of vacuum forming. The cooking process is affected by many different things and a person can’t expect the entire length of the plastic to turn clear at exactly the same time. What a person can expect is that the plastic is cooked thoroughly when thermoforming occurs.

One exception to this rule is when you find yourself bubble forming. Because the plastic isn’t in contact with a hot tray to start off with and because it’s usually much thicker than sheet plastic, it’s not as prone to nearly as much warping. And, due to the design of bubble forming frames, the material is being heated from both sides, more equally.

You still won’t find the plastic turning clear at exactly the same rate, but it’s subject to fewer of the outside forces plaguing the thinner sheet plastic. Normally, bubble forming doesn’t pose the most of the same heating problems associated with flat plastic.

Cooling The Tray

One possible thing you might want to try is cooling the tray off prior to inserting the sheet plastic. It’s the only way to properly cook polyethylene – LDPE tends to heat very quickly - and it may help to alleviate the “hot spot” in the center. It adds to the cooking times, but you might find some benefit.