Extrusion Basics: Hot Can Be Good, but It’s a Matter of Degree

It’s hot outside now and, in some places, very rainy. But people still find it hard to believe that water is a chemical compound, and natural, too (see last month’s column). Chemistry is just a code to help people from different countries talk to one another. For all of us, a water molecule is two hydrogen atoms and one oxygen, or H₂O. Same for steam and ice (although ice gets more complicated, but it’s still H₂O). 

These translations started way back when printing was invented (Gutenberg, 1455), soon used by Martin Luther to translate the Bible from Latin to German, and Paracelsus, a Swiss doctor who did the same for medical knowledge of the time. He is remembered by med students to this day for his emphasis on how much matters, a lesson he applied to medicine but that is still ignored today by people who accuse plastics of toxicity yet deny the material’s benign chemistry, indigestibility, and inability to pass through our tiny capillaries and circulate in blood. They are today’s example of: “If it sounds chemical, be afraid!”

If you want to understand plastics and additives, leave your fears of impersonal science at the door and learn the code for the most important elements: H = hydrogen, O = oxygen, N= nitrogen, C = carbon, and Cl = chlorine. It’s different if you speak another birth language, but still quite simple. For more details, see my Simple Science columns. For more elements and their combinations (compounds), use your phone to get symbols for all 92 elements.

The cold truth

Back to heat. There’s no such thing as cold. Heat is a measure of how fast atoms are moving. When they don’t move at all, we’re at absolute zero. That turns out to be -273°C (-460°F), and you can’t get any colder than that.

To extrude, we need to get the plastic up to somewhere between 300° and 600°F (150° and 315°C) so it can flow through the system. The actual numbers depend on the plastic — how long and branched its chains are, how fast it’s moving — and sometimes on the additives. Some waxes act as lubricants and the lower viscosity allows lower temperature and, thus, less stabilizer/antioxidant is needed. Some mineral powdered solids and cross-linking agents can raise the viscosity, so they have to run hotter. Additives change cost and other properties, too, so we can’t assume an ideal melt temperature without experience or testing or both.

That’s a problem, too. Even if we measure melt temperature in the extruder — at the screw tip, in the head or die, for example — the temperature at other points won’t be the same. The material is moving, so it’s subject to heat from friction, plus heating or cooling depending on barrel-wall settings. Where cooling is wanted, it's usually done with plant water, but some extruders don't have these circuits. Today’s math and computer technology can suggest settings when the viscosity-temperature relation is known, but this will be affected by flow rates, too, even if all other dimensions stay the same. Further, every lot of material will not have the exact same viscosity-temperature relation, and this may be inconsistent even within a lot.

The wearing of the screw

It's tempting to believe that wear of the screw matters, but it’s not all that common. Rebuilding is expensive and stops production, unless you have a spare, and getting a new one is even more expensive and requires coming to a decision on its dimensions and metal makeup. Frequent measuring of screws is still a good idea, and screw suppliers can be asked for recommendations, as pulling and handling a big screw needs planning and appropriate equipment. Are we measuring the entire length, or just part of it? If a part, which part? Are we measuring hot, or otherwise considering thermal expansion? Where do we put it when pulled? Do we clean it when it’s out, and note where it looks overheated? If there is wear, it’s critical to know where it is to understand remedies. Are we considering the natural sag of the screw when it’s still in the extruder supported at one end only? Take pictures — easy to do today, but someone still has to plan to do it.

Don’t forget the barrel(s), either. Usually their metals are less likely to wear, but that depends on what you’re extruding. Glass fibers, for example, will wear down most anything, and that may be preferred to higher-cost metals.

The feed temperature effect

Finally, feed temperature will affect the heating process, as will particle shape and size, which will affect the in-push rate and frictional heat that develops as the particles are pushed into and through the screw(s). Feed temperature is often neglected, except for preheating, and even then its control may be imprecise. Outdoor and railcar storage is relevant, as is timing of transfer from these containers. In some climates the difference between night and day can be seen and traced to feed temperature.

I’ve made a good case for saying all this is too much for an operator, even with digital help from various measurements after leaving the die. But too-thin product may fail in service, and product that is too thick wastes material (cost) and may not work where precise dimensions are needed, such as pipe fittings or some medical tubing. So I can go along with having a fixed set of barrel conditions, to be changed only with reason.

You need conditions to preheat before starting (should be a lockout to prevent the danger of starting with too-cold material). Don’t think that these are the only right conditions to run the product. Another set may work well, too, but change them as little as possible, and know why you are changing and what to expect. Another preventable danger is a change of one or more conditions when a new shift comes on. 

By conditions, I mean setting of temperature controllers on the barrel, head, and die. For some dies, the controllers are a major source of heat and flow control. And in some lines, there are pressure controls associated with screening and gear pumps, which may be affected by changes in melt temperature or resin variation. These can be alarmed to flash or make noise to tell operators that something’s wrong.  

When the heat around the extruder gets too high, reading gauges becomes a burden but can't safely be avoided. Go in an air-conditioned office and scroll through the PlasticsToday website.

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house, and now in his virtual version. He wrote Plastics Extrusion Technology, the first practical extrusion book in the United States, as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at [email protected].

No live seminars planned in the near future, or maybe ever, as his virtual audiovisual seminar is even better than live, says Griff. No travel, no waiting for live dates, same PowerPoint slides but with audio explanations and a written guide. Watch at your own pace; group attendance is offered for a single price, including the right to ask questions and get thorough answers by e-mail. Call 301/758-7788 or e-mail [email protected] for more info.