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Extrusion basics: From Georg Ohm to Mary Jane

Extrusion basics: From Georg Ohm to Mary Jane

Ohm’s Law lets us calculate power consumed by an extrusion motor. But at this week’s co-located PLASTEC West and MD&M West show, it’s medtech OEMs that largely lay down the law for extruders. A separate event devoted to marijuana packaging showcased new opportunities for extrusion technology.

Completing my survey of electrical pioneers following profiles of James Watt, André-Marie Ampère and Count Alessandro Giuseppe Antonio Anastasio Volta, we turn to Georg Ohm (1789-1854). He was a Bavarian (there was no united Germany then) and a child prodigy, son of a locksmith, with a life of university studies and low-paying teaching appointments. He never married nor had children, and never traveled outside the German-speaking world. He is known for his discovery that in an electric circuit, the voltage and current are directly related, and the ratio of the two is the resistance. He is honored by having the unit of resistance named after him, the ohm. In equation form: Voltage E / Current I = Resistance R, E = I x R. Also, the voltage x the current (amperes) = power in watts, E x I = P.

This is Ohm’s Law, which enables us to calculate power consumed by an extrusion motor. As noted in my last articles, it explains why being near the limit of current (amps) doesn’t mean you can’t run any faster. To the contrary, with some motors if you raise the screw speed the amps may stay the same or even fall, because raising the screw rpm may mean more voltage across the motor.

And the next time someone tells you how much money you are wasting by leaving a light on, you can come right back with Ohm’s Law: A 13-watt bulb running for 3 hours is 39 watt-hours or 0.039 kW-hr, so at $0.13/kW-hr (the U.S. residential average in 2019) that is about one-half cent. Calculating the cost of an extra degree in home heating or air conditioning is more complicated, but the lesson is usually the same, and it applies to extruders as well: Don’t worry about power costs. Save it where you can—waste is still a vice—but don’t delude yourself into thinking that your anti-waste virtue is saving a lot of money.

I’m now in Anaheim at PLASTEC West, so I’ll report on what’s here in extrusion. In a word, tubing. California is med-country, and the show is co-located with a show for medical device manufacturers, Medical Design & Manufacturing (MD&M) West, and three other shows, all managed by Informa, PlasticsToday’s parent company.

Medical devices use a lot of tubing to carry various liquids and sometimes wires. They are small, some very small and some very, very small. Wall thickness uniformity is critical, as some of them go into us as catheters as well as components of machines. Actually, the precision is at least as important in the machines, as our bodily passages vary in size more than the machines. There are makers of extrusion lines and products that focus on this application, so it’s not a pool that anyone can jump into. It’s full of regulations, as needed and expected. The need for sterility has made packaging critical, too.

This application area is an exception to my 6-1-2-1 general rule for extrusion manufacturing costs: 60% material, 10% equipment (if legally amortized), 20% labor and 10% everything else. This isn’t a very exact law, but it’s a starting point for most extrusions. It reminds us that material costs more than everything else put together (reuse scrap to replace new, and control thickness); that machinery cost is minor if you run around the clock; labor must include maintenance; and power costs are negligible as part of “everything else.”          

With some medical tubing applications, material can be very expensive per pound or kilogram, but products don’t weigh very much and are not sold by weight or length, like pipe or film. Cost of sterility is a big slice of the pie, as well as testing of both product and material.

This branch of our industry also considers silicones as a separate family from plastics. Silicones are polymers based on the element silicon (Si) in chains with alternating oxygens—Si-O-Si-O-Si-O—and other atoms attached to the silicon, which, like carbon, can hold four other atoms. The two words—the element silicon and the compound family silicone—are often confused. Some of the confusion may be welcome, as the siliconers may be happy to say their products are not plastics. Their uses are often based on their rubber-like flexibility and strength, and sometimes their low barrier to oxygen. To the medfolk, silicones are reactive extrusion and plastics are just melt-extruded. Not quite true—we have reactive extrusion of plastics, too, such as crosslinking and some foaming.

A unique feature of the show was a separate section, quite far from the others, devoted to marijuana packaging, now legal in California among many other states. It’s already a big business, and extruded film (multilayer, converted) and medical-type trays are needed. Important aspects include sealing against theft and adulteration; protection against moisture, which could affect its use as well as weight (one said 10% was ideal); and odor barrier to resist sniffing dogs in places where it’s still illegal. By the way, call it cannabis, nothing else, to reflect the commercial importance it now has. Even labels are specialized and may need extruded film.

And lest you think I forgot, I didn’t. Plastics are not toxic, but are maligned by the whole world (plastics pollution) because of their connection to Big Oil, their synthetic nature and their basis on magic-busting chemistry. The medical industry is a good example of how plastics are harmless both in and outside of us, and their contribution to health via sterility and nonsupport of pathogens keeps us alive and healthy, compared, for example, with paper, which absorbs water and, thus, supports bacterial growth.

Images: Juulijs (left) and Oleksandrum/Adobe Stock

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,, or e-mail him at

Griff conducts live seminars across the country: The next one is planned in Houston on March 19. Seminars in your plant are also available. If you can’t attend his live events, he offers a Virtual Seminar, which can be seen any time, any where. E-mail Griff at the address listed above for more information.

His recent webinar, What All Extruders Should Know, is now available on demand. Watch the free webinar at your convenience by clicking here.

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