The Physics of Extrusion

August 8, 2022
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Last month’s Simple Science covered heat, the physics division most important to extruders. It’s the countable energy that softens the material and makes it extrudable. If you haven’t read it, it’s available here. The article will tell you that heat is one of many forms of energy and there is no such thing as cold. And for more Simple Science, I especially encourage you to read my first one in the series on chemistry and see what your plastics are made of and why people are so eager to be afraid of them.

This month: More physics that we use every day, such as gravity, which we experience as weight. Everything is attracted to everything else. I don’t know why and most of us can live without knowing why, but we do know that the Earth pulls on everything. If you drop a ball, it will fall “down” — i.e., toward the center of the Earth. An airplane wing flies because when moving forward its upper surface is curved to make the air move faster — lower pressure (changed energy form) than the air under the wing. If the push up from below is more than the pressure on top, it can overcome its gravity and not fall down. Gravity is everywhere but proportional to mass, so on our (smaller) moon the pull is less and things weigh less.

Making waves in extrusion
This pull down is why sheet and other extrudates sag or go out-of-round as they exit the die. This is resolved by controlling cooling and lip-to-coolant time/distance, and sometimes through the die design itself. Melt temperature matters, as hotter is less viscous, thus creating more sag. 

Wave motion is energy traveling in repeated spurts, rather than in a continuous manner, and depends on what generates it and what it is traveling through — air, water, or metal.

Sound that humans hear include low frequencies = spurts per second = hertz, the proper unit. Middle C on a keyboard is 256 hertz. We can hear up to 15,000. Dogs can hear higher, cats (and mice) even higher, and bats and sea mammals highest of all. Sound is seldom a single tone, but is very complex with varying loudness and overtones, which is how we distinguish individual voices and instruments. No magic, just waves.

Read more: The Physics of Extrusion

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