Extrusion Basics: Labor and Problem, and Plastics Processing
Halloween is still two weeks away, but stores are full of pumpkins, real and plastic, and other oranges and blacks (Princeton colors). Here is our chance to face death while having fun. Or so it seems. I spent a Halloween night in a graveyard with dozens of serious mind seekers who weren’t around for fun, and I’ll never forget it.
Connection to extrusion? In the illustration, plastic bristles on the broom and plastic tube for the broom handle. Plus, lots of orange-colored concentrate for plastic pumpkins, which won’t spoil or attract critters, and can be extruded blown. And if you turn on anything, there is plastic insulation on the wires.
Deeper Connection: People believe what they want to be true, and while this may help us cope with death, it also supports the common but mistaken belief in the toxicity of plastics.
Just as the real world has to follow the imperfect but undeniable natural laws of chemistry and biology, extrusion has its own laws. Here are my “rules,” the “10 (11) Key Principles,” adapted from my Extrusion User Manual, and presented during Informa Markets Virtual Engineering Days last July.
- A single screw usually turns counterclockwise when viewed from the back, unscrewing from the back, but the thrust bearing holds it in place, while the opposite force pushes the plastic forward. and outward. The twins can turn in one or both directions, but the principle is the same
- We need energy (heat) to soften the plastic enough to push it through and out. We then need to make the formed product solid again, cooling it with water, a metal surface and / or air.
- The speed of the motor is usually much higher than the speed of the screw, so we need a reduction – gears, pulleys or a combination. Know your motor and reduction ratio to know the speed limits of the screw. Not all power is available at all speeds, which protects the system.
- Most of the heat needed to soften the plastic comes from the motor to overcome the viscous friction in the barrel. This can limit the speed of the screw, which in turn can control the production rate. The exceptions are small extruders, some twin screws, paper coating, and some high temperature polymers.
- The pellets must adhere to the barrel wall and slide on the screw base, in a single screw line, and also stick to each other, for maximum thrust (cc / rev).
- Materials are the most important part of manufacturing in almost all cases, often more than everything else put together. Use scrap and trim as a replacement for new purchases and pay attention to thickness accuracy to achieve the lowest goal without product failure.
- The energy to be extruded represents a very small proportion of the manufacturing cost. It sounds counterintuitive, as we all have very valid concerns about saving energy, but do the numbers if you’re not sure. If the melt gets too hot, it will not extrude to the desired shape, degrade, or both.
- Measure and understand process data – head pressure (relative to screening, mixing, bearing life), melt temperature (where?) And motor current (amps).
- The egress rate is the displacement of the last flight (drag flow) less head resistance (pressure demand) more any overbite to the power supply. Keep in mind that the output is melted, so the melt density applies and can be up to 20% lower than the solid density.
- Typical shear rates are 100 reciprocal seconds (rsec) in screw channels, 100-1000 in most die lips, and well above 1000 in barrel and tiny die sets. The viscosity depends on it: Faster = lower, less resistance to flow. Refers to flow in matrices, melt index tests.
- Heating / cooling through the barrel is countered by the engine. The heat lowers the viscosity of the wall, so that the screw turns more easily, generates less friction and heat. Cooling is the reverse.
Comments and questions, please write [email protected]. For Halloween, if you are an opera fan, listen to / see the end of Mozart Don Giovanni, who has a ghost singing in a cemetery. And for All Saints’ Day, November 1st, see my column on the “Patron saint of extrusion. “
About the Author
Allan Griff is a seasoned extrusion engineer, who started out in the technical department of a large resin supplier and has worked alone for many years as a consultant, expert witness in legal matters, and most importantly as a educator through webinars and seminars, both public and in-house, and now in its new audiovisual version. He wrote Plastic extrusion technology, the first practical extrusion book in the United States, as well as the Plastic Extrusion User Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on its website, www.griffex.com, or send him an e-mail at [email protected].
No live seminars are planned for the near future, or maybe 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. Watch at your own pace; participation in groups is offered for one price, including the right to ask questions and get detailed answers by e-mail. Call 301 / 758-7788 or email [email protected] for more information.