A hot topic, literally, in Extrusion Technology

Physics is a word for extinction for many because it seems like a science-education subject. So people say, “It’s for them, we don’t need it” or “We can’t understand it anyway.” But physics involves the things we do every day: heat, light, sound, motion, and so on. No special training is required to get the basics, but it does require a bit of curiosity and the need for human magic and the impossible.

Nothing like cold

We think we know what heat is. We may know that there is no cold; things are hotter or cooler. That’s a good start. Temperature measures how fast molecules are. No molecules, like in space? No temperature. All movements use energy, and heat is a form of energy, so the more energy into a mass of molecules, the faster they move and the hotter it gets. Cooling means that energy is removed and the molecules move more slowly. Energy can change shape, but it does not disappear. Energy can come from the sun, motion, breaking atomic-atom bonds, electricity, condensation vapor, hot air, and so on. If enough energy is allowed (cooling), all molecular motion stops and we have absolute zero, which is 273 ° zero below Celsius or 460 ° zero below Fahrenheit. It’s the same in all things. It can’t get any colder than that.

Why two systems, C and F? Before 1700, we did not measure temperature at all. If Shakespeare or Columbus were to ask about the weather, they would receive an answer: “It’s very cold today, Will.”Very hot, Capitán Col.ón”.

In the early 1700s, Daniel Gabriel Fahrenheit, a German living in the Netherlands, knew that mercury was liquid at room temperature and spread by heat. As a professional glassblower, he made a very thin glass tube, inserted mercury inside it, and made the first thermometer, following a principle stated by Galileo 100 years earlier. It was announced in London in 1724 and the British approved the system, using a scale from 0 to 100! With this scale, the body temperature was 90 (98.6) and the frozen water was 30 (32). Some believe that 100 and 0 were set in Amsterdam as hot and cold as possible. Or his tube was not perfect. Whatever the reason, the British liked it, took it to the colonies (that’s who we are), and we’re still using it 300 years later.

Returning to Europe, the Swede Anders Celsius wanted a more accurate scale. In 1742, he proposed freezing and boiling water at sea level between 0 and 100, which became a centigrade (Celsius) system for the rest of the non-British world. In Fahrenheit, the sea level boils at 212 ° and freezes at 32 °. The scale is not valid at high altitude – water boils at 202 ° F (94 ° C) at Denver mile high because the air pressure is lower and the liquid molecules need less energy to escape (boil) and turn into gas (water vapor = steam). ).

The role of heat in extrusion

We need heat to melt the plastic. We get the most out of the engine resistance by rotating the cap screw (s). Exceptions that require significant heat barrels include small machines, any slow-moving machine, 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 because they are in water. They have melting rangesknown by processors, and glass transition temperatures (Tg), above which they are hard and elongated, and below them crystalline and brittle. These Tg it is mainly used by temperature researchers and polymer makers.

The melting range is above this transition temperature, and the melting becomes more viscous (thinner) as it gets hotter. We need to avoid overheating that would damage the plastic (breaking, coloring, weakening, contaminating the chains) and this is the usual limit of production speed. If the barrel and head / trowel are kept too cold, more engine power is needed to transport and expel the melt, which in turn returns more heat to the melt. If we do it too hot, however, it can degrade directly. High temperature time is also important, which is why a large extruder can show degradation of the same material that worked well under the same conditions in a small line. Cooling capacity is important, and there may even be a rate limit.

Injection molding grades have a lower viscosity (higher melting index) than extrusion grades because they must pass 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 levels, which have longer molecules to achieve a lower melting rate.

Where to measure melting pressure and temperature

Extruders must set conditions – temperature settings in barrel, head and month; perhaps if a pressure setting is controlled; and the idea of ​​the desired screw speed. We don’t usually ride at the top speed of the screw because there are so many other limitations. We measure the amperes of the motor and the speed of the screw, and we melt the pressure at or near the tip of the screw. We should also measure melt temperature in the head, that is no the same as the temperature (conditions) of controlled metals, but tells us when melting is too hot.

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

I would expect processors to want maximum thermal stability from resin makers and composers. This is the result of polymerization — catalysts, reaction rate, and temperature — and can also be improved with additives (stabilizers, antioxidants) and processing aids (viscosity reducers that require less power from the engine). But additives usually cost more than the resin itself, which means AMAN-ALAP (As little as possible, as little as possible). It’s helpful if you’re already adding colorants.

Can we test the input materials for thermal stability? Yes, but it is not often done. The momentum rheometer is also useful with PVC composites and other plastics. It is used to leave the color of the oven, as well as chemical tests, but suppliers should agree on what they are responsible for.

There are three ways to transfer heat:

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

Often, two or three are working together. You can also change the shape of the energy without adding anything, such as battery charging or human sweating (liquid water water vapor, 539 calories / gram). Power, heat and energy are not the same: Power is HP or kW, heat is F or C degrees and energy is Joules or KW hours or calories. Melting also requires energy: 80 cal / gm for water, less for anything else.

About the author

Allan Griff is a veteran extrusion engineer who started in the technology service of a leading resin supplier and has been working on his own for many years now as a consultant, legal expert witness and primarily as an educator through webinars and seminars, both public and private. and at home, and now in its virtual version. He wrote Plastic Extrusion TechnologyThe first practical extrusion book in the United States, as well Plastic Extrusion Manual, It is updated almost every year and is available in Spanish, French and English. Get more information on their website, www.griffex.comor send an email to [email protected].

No live seminars are planned for the near future, or perhaps never, because his virtual audiovisual seminar is even better than live, Griff says. No travel, no waiting for live dates, the same PowerPoint slides but with audio explanations and a written guide. See at your own pace; group assistance is provided at a single price, including the right to ask questions and receive specific answers by email. Call 301 / 758-7788 or email [email protected] for more information.

Leave a Comment