Extrusion Basics: Energy You Can Count On

July 31, 2020

Keeping track of the amount of energy you use during the extrusion process can help when it comes to decision making down the line.

According to Allan Griff with Plastics Today:

“People use the term energy in a non-countable way: “Look at him, full of energy” or “I don’t have the energy to go out today.” I squirm when I hear this, as to me energy is countable and indestructible. Also, it takes many forms:

  • Chemical, as in a battery or food/beverages (including “high-energy” drinks);
  • potential, as when something is high and can fall (water before a waterfall);
  • thermal (heat = how fast the molecules are moving);
  • kinetic (wind, tide, a moving car);
  • electrical (electrons running through wires); and
  • light (photocells, ultraviolet to kill viruses and give us sunburn).

“Physicists can explain magnetism (which I never quite understood) and sound (which, as an amateur musician, I do understand), but the others should be enough to make my point.

Image: Pandpstock001/Adobe Stock

“We are especially reluctant to count in connection with food, but that’s a subject worth discussing at another place and time. With regard to extrusion, the counting will help understand the costs of manufacture and justify or challenge the money-saving claims of equipment and materials.

“There is a direct correlation between your electric power consumption and money. You see it on the electric bill, but it’s not usually broken down to what is costing how much.

“For an extruder, we can get the energy used by the motor — the usual primary source — if we multiply time by the power actually used (power = motor amps times volts). Amps are read directly (see my November 2019 column) and volts are easy with a DC motor, if you know the top speed and reduction ratio. With AC, you have to know something more about the motor, but it’s not rocket science — it just isn’t always done. There is often a reluctance to quantify costs, as there is to count the Calories you eat.

“So, if you read or hear me tell you that energy is a minor cost of extrusion, you have some numbers to prove (or disprove) it. Measure production in weight per hour, divide that by average power, and you get lb. or kg per kW-hr. Since we pay by kW-hr, that should be enough to show the real cost per pound or kilogram of product. If you want calorie-based units, one kW-hr = 860 Kcal.

“Not so fast. Although most energy input is usually through the motor, barrel/die heating may also be important for small machines, high-temperature polymers like nylons and PC, extrusion coating and slow screw speeds such as twins with PVC. Preheating, often used for drying, can also be useful for energy addition and feed temperature uniformity. In those cases, if you know the wattage of heaters and the percentage of the time they are on, their contribution can be included.

“There is heat loss by radiation from hot die surfaces, fan or water-cooling of the barrel, and conduction to the hopper or extruder cover, but these won’t matter in the calculation of power costs, because we’re paying for the input. The control settings may have an influence, and desired melt temperature range may vary with application and rate. These may need consideration if a more precise analysis is wanted, but usually the power cost is so low that, once demonstrated, it suggests we look for more fruitful places to save money, such as material choice, full re-use of scrap, and thickness control.

“An important diversion regarding units: A calorie (small c) is the amount of energy needed to heat 1 gram of water 1°C. Not very much — just one breath at rest may use six to seven calories to heat the exhaled air (e-mail me if you want to know how I did this). We can use a K (kilo) to signify 1000 calories (see last month’s article about ‘Greek up, Latin down’) and get the more practical Kcal unit. However, that looks funny and is hard to pronounce so we just call them all calories; or use a capital C = Calories to signify kilocalories; or use the international Joule, which is even smaller than a calorie (4.184 Joules = 1 cal); or a variety of other units appropriate to the application. A common misunderstanding is with food, where the values on our packages and in our minds are Calories = kilocalories: 9 Kcal/gram for fats, 7 for ethyl alcohol, and 4 for proteins, sugars, and starches dry weight. Yes, all three are similar, whether animal or vegetable origin, which is yet another reason that some people avoid counting.

“And those energy drinks! There is a great variety — some with a lot of sugar (energy), some with a lot of caffeine, some with elements like sodium (the same sodium as in salt) — and although they are sold as sports drinks, most are consumed by nonprofessionals. Not all bad, but of miraculous power a ghost.

“So the next time you hear someone talk of how much or how little energy someone has, don’t say anything lest they think you’re one of us nerd engineers. Instead, think of the machines you help run and how useful it is to put energy needs in a countable perspective…”

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