Extrusion basics: To twin or not to twin?

My usual hello: There are no toxic plastics. Worldwide concern is due to our appearance as corporate, humanipulated (not natural or predictable) and science-based (chemical) and, thus, denying magic and miracles, which we need. Our critics are not stupid nor uneducated—they are scared. Questions or comments? Please call or write.

Gemini thinkersNow, to extrusion. I have lived with the dilemma of being an only child (typical in the depression years) and being a Gemini (the twins). I’ve had a lot of duality in my life: Two educations, two spouses and residence on two coasts. The symmetry is not perfect—somehow I had three children and three important dogs to teach me honesty and fidelity (Did you ever see a sneaky dog?). So, the difference between single-screw and twin-screw extrusion is an attractive topic, and is sometimes misunderstood.

First of all, there are many “breeds” of each. It isn’t only twin or single, as extruders also differ in diameter, length, speed range, power limits and screw design. If you’re buying an extruder, you should know what you want first, based on the product you want to make and the expected production rate in pounds or kilograms per hour, or sometimes feet or meters per minute (which can convert to weight/hour, if dimensions and materials are known). If I am asked, “What do I need to make x product?", I reply with, “How much do you want to make?” In many cases, the specification of the die and downstream equipment is at least as important as the extruder itself. If I’m asked, “Who makes the best extruders?”, I see someone who trusts brand over all, and may be denying his own responsibility for knowing what he needs. Some suppliers may, indeed, be helpful, but you have to invest some time to find the best match.

Most extruders are single-screw—about 90% is my best estimate. But that is misleading, because the twins are concentrated in two market areas—rigid PVC and compounding—and the twins for one application are very different from the other. The PVC lines have one-piece screws, which intermesh in a figure-eight barrel. They are positive-conveying devices, which means their volumetric output is closely controlled and less dependent on what is happening in the barrel. In fact, they are usually run with the option of varying the portion of the screw that is filled with melt, which is done by keeping the feed rate constant and changing the actual screw speed. A longer full section will give more mixing but also add more heat (more motor energy needed to turn in more resistance).

Most PVC twins have parallel screws today, but conicals were developed in the 1970s to solve the thrust bearing problem. In a conical, the shafts diverge in the back, allowing larger bearings to take more head pressure (thrust backward). There are a few old conical machines still in service, and more are being made today for small- to middle-output lines, where the bearing problem still makes them preferable.

Most twins are powder-fed, which is easy for their positive-feed ability, and relates to the overall economics of such extrusion: The twins are more expensive at similar output and quality, but save money because powder blends are cheaper than pre-pelletized compounds. Also, they have one less heat history, which means less stabilizer and, thus, even better economics. Do the math, and deal with machine cost responsibly—payback in x years, depreciation, 24-hour operation (if you don’t run 24, your competitors will). There are special cases with internal sales or very specialized products (space, medical), but that’s a small segment of our industry.

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