Physics is a repulsive word to many because it sounds like a science subject. So people say, “This is for them, we don’t need it,” or “We couldn’t figure it out anyway.” But physics includes things we all do every day – heat, light, sound, movement and more. Getting the basics does not require special training, but requires some curiosity and resistance to the human need for magic and the impossible.
There is no such thing as cold
We think we know what heat is. Maybe we even know that there is no such thing as cold – things are just hotter or cooler. This is a good start. Temperature is a measure of how fast molecules move. No molecules like in space? No temperature. Every 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 taken away and the molecules move more slowly. The energy can change its shape, but it does not disappear. Energy can come from the sun, motion, breaking atom-atom connections, electricity, condensing steam, hot air, and more. If it leaves enough energy (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 matters. It can’t get any colder than that.
Why two systems, C and F? About 1700 years ago, we didn’t measure temperature at all. If Shakespeare or Columbus had asked about the weather, he would have received an answer in the spirit of “It’s very cold today, Will” or “Very hot, Captain.án WhatFr.n”
In the early 1700s, Daniel Gabriel Fahrenheit, a German living in the Netherlands, knew that mercury was liquid at room temperature and expanded with heat. He was a professional glazier, so he made a glass tube with a very thin passage, put mercury inside, and made the first thermometer, following the principle Galileo noted 100 years ago. He declared it in London in 1724 and the British adopted the system using a scale from 0 to 100! With this scale, the body temperature was 90 (that is 98.6) and the freezing water was 30 (that is 32). Some believe that 100 and 0 are defined as the hottest and lowest possible temperatures in Amsterdam. Or his pipe wasn’t perfect. Whatever the reason, the British liked it, moved it to the colonies (that’s us) and still use it 300 years later.
Back in Europe, the Swede Anders Celsius wanted a more accurate scale. In 1742, he proposed freezing and boiling water at sea levels from 0 to 100, which became the Celsius system for the rest of the world in Britain. In Fahrenheit, sea level water boils at 212 ° and freezes at 32 °. No rock works at high altitudes – water boils at 202 ° F (94 ° C) in high Denver because air pressure is lower and liquid molecules need less energy to get out (boil). ) and become gas (water vapor = steam).
The role of heat in extrusion
We need heat to melt the plastic. We get most of the resistance of the motor that rotates 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 because they are in water. They have melting rangeswhich are known to processors, and glass transition temperatures (Tg), above which they are tough and stretchy, and below they are glassy and fragile. These Tg temperatures are used mainly 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 must avoid too hot melts, which would decompose the plastic (chain breakage, discoloration, weakening, contamination) and this is a common limitation on the speed of production. If the barrel and head / die are kept too cool, more engine power is required to transport and push the melt, which returns more heat back to the melt. If we work too hot, however, it can get worse directly. High temperature time is also important, so a large extruder may show degradation of the same material, which works well under the same conditions on a small line. Cooling capacity is important and can also be a speed limit.
Die-casting classes have a lower viscosity (higher melting index) than those of extrusion, as they must pass through thin-walled (highly resistant, cold) molds. They can be used in extrusion, but at lower melting temperatures, and may not be as robust as extruded grades, which have longer molecules to obtain a lower melting index.
Where to measure the pressure and temperature of the melt
Extruders must set conditions – settings of the temperature of the barrel, head and die; perhaps pressure adjustment if controlled; and an idea of the desired screw speed. We do not usually operate at maximum screw speed, as there are many other limitations. We measure the amperes of the engine and the speed of the screw, as well as the melting pressure at or near the top of the screw. We also need to measure melt temperature in the head, which is no 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 head of the extruder to get the temperature of the melt, but in practice we use thermocouple, as we do to control conditions. A thermocouple is a pair of wires of different metals connected at both ends to form a circuit. When one end is hotter than the other, a small current flows in the circuit and can be measured, converted to temperature units and displayed.
I expect processors to want maximum thermal stability from resin manufacturers and mixers. 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 that require less power from the engine). But additives usually cost more than the resin itself, which means AMAN-ALAP (As much as necessary, as little as possible). Helps if you are already adding colorants.
Can we test the input materials for thermal stability? Yes, but not often enough. The torque rheometer is useful with PVC joints and other plastics. Furnace decolorization is used, as well as chemical tests, but suppliers must agree on what they are responsible for.
There are three ways to transfer heat:
- Conductivity, as a hot floor burns your feet if you walk on it barefoot;
- convection, like a fan that moves fluid from one place to another;
- radiation, such as the sun or a space heater.
Often two or all of them work together. You can also change the form of energy without adding it, such as charging a battery or human sweating (liquid water in steam, 539 calories / gram). Power, heat and energy are not the same: power is HP or kW, heat is degrees F or C, and energy is joules or KW-hours or calories. Melting also needs energy: 80 cal / gm for water, less for everything else.
About the author
Alan Griff is a veteran extrusion engineer who started in the technology service as a major resin supplier and worked independently for many years as a consultant, expert witness in legal matters and especially as a lecturer through webinars and seminars, both public and internal, and now and in its virtual version. He wrote Plastic extrusion technologythe first practical book on extrusion in the United States, and Manual for plastics extrusion operation, it is updated almost every year and is available in Spanish and French, as well as in English. Learn more on his website, www.griffex.comor email him at [email protected].
There are no live seminars planned in the near future, or perhaps someday, as his virtual audiovisual seminar is even better than live, Griff said. No travel, no waiting for live dates, the same PowerPoint slides, but with audio explanations and written guidance. Watch at your own pace; group attendance is offered at a single price, including the right to ask questions and receive comprehensive answers by email. Call 301 / 758-7788 or send an email [email protected] For more information.