A hot topic, literally, in extrusion technology

Physics is a stop word for many, as it sounds like the science education subject it is. So people say, ‘It’s for them, we don’t need it’ or ‘Anyway, we couldn’t understand it’. But physics involves things that we all deal with every day – heat, light, sound, motion, etc. Getting the basics done doesn’t require special training, but does require a certain curiosity and resistance to the human need for magic and the impossible.

Nothing like the cold

We think we know what heat is. Maybe we even know that cold doesn’t exist – things are just hotter or colder. It’s a good start. Temperature is a measure of the speed at which molecules move. No molecules, like in space? No temperature. All movement uses energy, and heat is a form of energy, so the more energy there is in a mass of molecules, the faster they move and the hotter it gets. Cooling means energy is removed and molecules move more slowly. Energy can change form, but it does not disappear. Energy can come from the sun, from motion, from the breaking of atom-atom bonds, from electricity, from the condensation of steam, from hot air, etc. If enough energy leaves (cooling), all molecular motion stops and we have absolute zero, which is 273° below zero Celsius or 460° below zero Fahrenheit. It’s the same for any material. It can’t get colder than that.

Why two systems, C and F? Before about 1700, we didn’t measure temperature at all. If Shakespeare or Columbus asked him about the weather, he would get a response like “It’s very cold today, Will” or “Muy caliente, Capitan collarwherenot.”

In the early 1700s, Daniel Gabriel Fahrenheit, a German who lived in Holland, knew that mercury was liquid at room temperature and expanded with heat. He was a professional glassblower, so he made a glass tube with a very fine passage, put the mercury inside and made the first thermometer, according to a principle noted by Galileo 100 years before. He announced it in London in 1724 and the British adopted the system, using a scale of 0 to 100! With this scale, body temperature was 90 (that’s 98.6) and ice water was 30 (that’s 32). Some believe that 100 and 0 have been defined as the hottest and coldest possible temperatures in Amsterdam. Or his tube wasn’t perfect. Whatever the reason, the Brits loved it, brought it to the colonies (that’s us), and we’re still using it 300 years later.

Back in Europe, Anders Celsius, a Swede, wanted a more accurate scale. In 1742 he proposed the freezing and boiling of water at sea level from 0 to 100, which became the centigrade (Celsius) system for the rest of the non-British world. In Fahrenheit, water at sea level boils at 212° and freezes at 32°. Neither scale works at high altitudes – water boils at 202°F (94°C) in Denver, a mile high, because atmospheric pressure is lower and liquid molecules need less energy. energy to escape (boil) and become gaseous (water vapor = steam).

The role of heat in extrusion

We need heat to melt the plastic. We get most of the resistance to the motor by turning the screw(s) in the barrel. Exceptions that require significant barrel heat are small machines, any machine that runs slowly, twin-screw extruders, extrusion coating, and some special high-temperature resins. Plastics do not have sharp melting/boiling points like water – not all molecules are exactly the same as they are in water. They have cast iron rangesknown to transformers, and the glass transition temperatures (Tg), above which they are hard and stretchy and below which they are glassy and brittle. Allg temperatures are mainly used by researchers and polymer manufacturers.

The melting range is above this transition temperature and the melt becomes less viscous (thinner) as it gets hotter. We need to avoid overly hot melting which will degrade the plastic (break chains, discolor, weaken, contaminate) and this is a common limit to production speed. If the barrel and head/die are kept too cold, more engine power is required to transport and expel the melt, putting more heat back into the melt. If we run too hot, however, it can degrade directly. High temperature time also matters, so a large extruder may show degradation of the same material that ran well under the same conditions on a small line. Cooling capacity is important and can also be a flow limit.

Injection molding grades have lower viscosity (higher melt index) than extrusion grades because they must flow through thin-walled (high strength, cold) molds. They can be used in extrusion, but at lower melting temperatures, and may not be as strong as extrusion grades, which have longer molecules to achieve the lower melt index.

Where to measure melting pressure and temperature

Extruders need to set conditions – temperature settings on barrel, head and die; possibly a pressure setting if controlled; and an idea of ​​the desired screw speed. We generally don’t operate at the maximum screw speed, as there are many other limitations. We measure motor amps and screw speed, as well as melt pressure at or near the tip of the screw. We should also measure to melt Temperature in the head, which is not same as controlled metal temperatures (conditions) but tells us when the melt is too hot.

In principle, we could put a mercury thermometer in the extruder head to get the melting temperature, but in practice we use a thermocouple, as we do to control the conditions. A thermocouple is a pair of wires of dissimilar metals, connected at both ends to form a circuit. When one end is hotter than the other, a small current flows through the circuit and can be measured, converted into temperature units and displayed.

I would expect processors to want maximum thermal stability from resin makers and mixers. This is the result of polymerization – catalysts, reaction rate and temperature – and can be further enhanced by additives (stabilizers, antioxidants), as well as processing aids (viscosity reducers which require less engine power). But additives usually cost more than the resin itself, which stands for AMAN-ALAP (As Much As Necessary, As Little As Possible). It helps if you are already adding dyes.

Can we test the thermal stability of incoming materials? Yes, but it’s not done often enough. A torque rheometer is also useful with PVC compounds and other plastics. Oven discoloration is used, as well as chemical testing, but suppliers must agree on what they are responsible for.

There are three ways to transfer heat:

  • Conduction, because a hot floor burns your feet if you walk barefoot on it;
  • convection, like a fan moving fluid from one place to another;
  • radiation, such as the sun or a heater.

Often two or three of them work together. You can also change the form of energy without adding any, such as charging a battery or human perspiration (liquid water to water vapor, 539 calories/gram). Power, heat and energy are not the same: power is HP or kW, heat is in degrees F or C and energy is in joules or KW-hour or calories. Melting also requires energy: 80 cal/g for water, less for the rest.

Allan Griff About the Author

Allan Griff is a seasoned extrusion engineer, starting out in technical service for a major resin supplier, and working on his own for many years as a consultant, expert witness in legal cases, and especially as an educator through webinars and seminars, both public and in-house, and now in its virtual version. He wrote Plastics extrusion technologythe first practical book on extrusion in the United States, as well as the Plastics Extrusion User Manual, updated almost every year and available in Spanish and French as well as English. Find out more on his website, www.griffex.comor email him at [email protected].

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

Jessica C. Bell