Extrusion basics: Why it’s important to know how fast your screw can turn

Top screw speed is the top motor speed divided by the reduction ratio, which is determined by the gear box and (sometimes) pulley sets. In most extruders it is somewhere around 100 rpm.      

One exception to this is the twin-screw extruder used for rigid PVC pipe and profiles, and lately for PET sheet and other resins, as well. These screws run more slowly—30 rpm is high! They cost more than singles per unit output at equivalent quality, but this cost is justified as they allow powder feed and generate less heat for equivalent mixing. This saves material cost, as PVC powder blends are cheaper than pellets, and the lower process temperatures as well as the saving of a heat history mean less stabilizer is needed.

Another exception in the opposite direction: Twin-screw extruders, especially big ones, used for compounding. These are usually segmented, with many components arranged along a keyed shaft. The major makers have promoted their products by increasing torque capacity as well as screw speed, and the extruders are fitted with intense water cooling to deal with the overheating. Although it may sound good to be able to run at 1000 rpm and up, the real measure of value is the cost-per-unit-output  of non-degraded salable material, including delivery, installation and training.

Single-screw extruders have been built to run very fast, too—I remember HPM showing a machine around 30 years ago that could run at 400 rpm, or was it 800? Makes no difference, as this type of machine has pretty well standardized at around 100 rpm tops, and most lines don’t even push it that far. In Europe there are a few lines running well above that, especially for in-line thermoforming, and the claim is made that they save power and space. The power savings may be more important there, but I’d want to see the differential in euros before I can go along with this reasoning. 

Other exceptions back on the slower side include very small lines, where high speed would result in an insufficient residence time to get acceptable mixing and a thermally uniform melt. Also, very large diameter single-screw lines don’t always run fast, either, as increased circumferential speed in the channels (which varies with diameter squared) will increase frictional heat generation and thus risk overheating. 

So how fast can your screw turn? The simplest way is to run the speed control up to the max and count rpm. I say “count,” because I don’t trust displays until I’ve had experience with them (somewhat like people). However, it isn’t a good idea to run empty for a long time, and running it full may waste material and time. If you can do this just before a scheduled shutdown, you’ll minimize the waste. 

Look at the motor nameplate (see photo). If it’s a DC motor, you will see maximum voltage. It may not say “max,” but it’s usually 500 V for middle-large lines, maybe less for smaller ones. You now can tell the voltage actually used, as it is in proportion to the speed. For example, if actual screw speed is 60 rpm and max is 120, then the voltage is 250 V, and the available power (volts x amps) is half what it is at max voltage. That protects the system against excess torque, which can twist or break shafts—that happens when too much power is delivered at low speeds. 

With the AC motors that have been preferred for the last 20 to 25 years, the principles are the same but the calculations aren’t as simple. There are several different types, so talk with the motor maker, if needed, to learn how to calculate torque and actual power in use.

If you look at the max screw speed on the nameplate, you’ll see a number usually around 1750 rpm. That’s the max motor speed, so divide that by the max screw speed and you have the reduction ratio. If you see two numbers—1750/2100, for example—it’s a field-weakening DC motor, which may be able to get higher screw speeds but no more than rated power; that is useful if you are already at top speed but not drawing full power. And if you are running close to or at max amps (also visible on the motor nameplate) you may want to reduce top speed with pulleys or some other means, so that top speed is less but more power is available at any one speed.

One final word on screw speed, which echoes the worries about running too fast that I’ve expressed in this column before. Before blindly thinking that faster is better, remember that faster may be worse if you don’t sell the increased production. Warehousing the unsold production has a price, and speed may reduce thickness control and may risk mechanical problems. That’s why optimum, rather than maximum, is desirable: Optimum takes into account other aspects (sales, thickness control, overheating and so forth), whereas maximum is just a mathematical concept and doesn’t consider such factors.

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at [email protected].

In the fall, Griff will present his one-day practical seminar, "Introduction to Extrusion," in Chicago, Los Angeles and Houston. Topics include the ten (11) key principles of extrusion, plastics chemistry for non-chemists, a review of extrusion hardware, the limits to production rate, quality control of raw materials, simplified rheology, start-up and shut-down procedures and troubleshooting common extrusion problems. E-mail him at the address listed above for more information.