Physics is an off-putting word for many because it sounds like a science education subject. So people say, “It’s for them, we don’t need it” or “We couldn’t understand it anyway.” But physics involves things that we all deal with every day: heat, light, sound, motion and much more. Acquiring the basics does not require any special training, but does require a certain curiosity and resistance to the human need for magic and the impossible.
There is no cold
We think we know what heat is. Maybe we even know that cold doesn’t exist: things are just warmer or colder. It’s a good start. Temperature is a measure of how fast molecules move. No molecules, like in space? No temperature. All movement uses energy, and heat is a form of energy, so the more energy enters a mass of molecules, the faster they move and the hotter it is. Cooling means that energy is removed and molecules move more slowly. Energy can change form, but it does not disappear. Energy can come from sunlight, motion, breaking atom-atom bonds, electricity, condensation of steam, hot air, etc. If enough energy escapes (cooling), all molecular motion stops and we have absolute zero, which is 273° below zero Celsius or 460° below zero Fahrenheit. It is the same for all matter. It can’t get any colder than that.
Why two systems, C and F? Before about 1700, we didn’t measure temperature at all. If Shakespeare or Christopher Columbus asked him about the weather, he would get an answer like: “It’s very cold today, Will” or “Very hot, Capitán Colon.”
In the early 1700s, Daniel Gabriel Fahrenheit, a German living 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, introduced mercury inside and made the first thermometer, following a principle noted by Galileo 100 years before. He announced it in London in 1724 and the British adopted the system, using a scale from 0 to 100! Using this scale, body temperature was 90 (that’s 98.6) and ice water was 30 (that’s 32). Some believe that 100 and 0 were set as the hottest and coldest possible temperatures in Amsterdam. Or maybe his hit wasn’t perfect. Whatever the reason, the British liked it, introduced it to the colonies (that’s us), and we’re still using it 300 years later.
Back in Europe, the Swede Anders Celsius wanted a more precise scale. In 1742 he proposed freezing and boiling water at sea level between 0 and 100, which became the centigrade (Celsius) system for the rest of the non-British world. In degrees 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, at a height of a mile, because the air pressure is lower and the molecules are liquids. require less energy to escape (boil) and transform into gas (water vapor = steam). ).
The role of heat in extrusion
We need heat to melt the plastic. From this we get most of the resistance from the motor that turns the screw(s) in the barrel. Exceptions that require significant barrel heat are small machines, any machine that operates slowly, twin-screw extruders, extrusion coating, and some specialty high-temperature resins.
Plastics do not have sudden melting/boiling points like water – not all molecules are exactly the same as in water. They are melting rangesknown to transformers, and the glass transition temperatures (Tg), above which they are strong and extensible and below which they are glassy and brittle. Allg temperatures are primarily used by polymer researchers and manufacturers.
The melting range is above this transition temperature and the melt becomes less viscous (finer) as it gets hotter. We need to avoid melting too hot 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 molten material, returning more heat to it. However, if we are too hot, it can deteriorate directly. Time spent at high temperature also matters, which is why a large extruder may show degradation of the same material that performed correctly under the same conditions on a small line. Cooling capacity is important and can also be a throughput limit.
Injection molding grades have lower viscosity (higher melt index) than extrusion grades because they must flow through thin-walled molds (high strength, cold). They can be used in extrusion, but at lower melt temperatures, and may not be as strong as extrusion grades, which have longer molecules to achieve a lower melt index.
Measure temperatures and other variables
Extruders must set conditions: temperature settings on the barrel, head and die; perhaps a pressure adjustment if this is controlled; and an idea of the desired screw speed. We don’t usually run at maximum RPM, as there are many other limits. We measure motor amperage and screw speed, as well as melt pressure at or near the tip of the screw. It would also be necessary to measure melt temperature in the head, that is to say not the same as controlled metal temperatures (the conditions) but tell us when the cast iron is too hot.
In principle we could insert a mercury thermometer into the extruder head to find out the melting temperature, but in practice we use a mercury thermometer. thermocouple, as we do to control conditions. A thermocouple is a pair of wires of different metals, joined 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 to temperature units, and displayed.
I would expect processors to want maximum thermal stability from resin manufacturers and formulators. This is the result of polymerization (catalysts, reaction rate and temperature) and can be further improved 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 means AMAN-ALAP (As much as needed, as little as possible). It helps if you already add colorings.
Can we test the thermal stability of incoming materials? Yes, but it doesn’t happen often enough. A torque rheometer is also useful with PVC compounds and other plastics. Oven bleaching 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 radiator.
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 sweating (liquid water to water vapor, 539 calories/gram).
Power, heat, and energy are not the same: power is in HP or kW, heat is in degrees F or C, and energy is in Joules or KWh or calories. Cast iron also needs energy: 80 cal/g for water, less for everything else.
About the Author
Allan Griff is a seasoned extrusion engineer, who started in the technical departments of a major resin supplier and has worked on his own for many years as a consultant, an expert witness in court cases, and especially as an educator via webinars and seminars, both public. and internally, and now in its virtual version. He wrote Plastic extrusion technologythe first practical book on extrusion in the United States, as well as the Plastic Extrusion User Manual, updated almost every year and available in Spanish, French and English. Learn more on his website, www.griffex.comor send him an email at (email protected).
No live seminars are planned in the near future, or perhaps ever, because its virtual audiovisual seminar is even better than the live seminar, 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 at a single price, including the right to ask questions and get detailed answers via email. Call 301/758-7788 or email (email protected) for more information.