Today’s Lesson: The Key Principles of Extrusion

The screw tries to unscrew itself backward out of the barrel. A thrust bearing takes this axial load while the opposing force pushes melt out. The screw doesn’t move axially at all. Some extruders have two screws, usually intermeshing, but the same principle applies. 

The electric motor that turns the screw needs energy to overcome viscous resistance, and is usually the main heat source to soften the plastic, except for small lines, high-heat resins, or anything preheated.

Motor speed is reduced with gears/pulleys by a ratio of 10 to 20 or more. This ratio controls top screw rpm, heat generation, and available power. A few motors don't need such reduction — know what you have and what it can and can't do.

The entering plastic is the primary coolant. The particles absorb the motor energy, and maybe some from the barrel heaters. In a few cases, there is too much heat, so barrel cooling via air or water is needed, or the formulation needs to be changed. The “conditions” are just controller settings, but most extruders show the real barrel temperatures, too. Melt temperature is not measured in the barrel, but at the end or in the head, if at all.

In the feed zone, pellets must stick to the barrel and slip on the screw for the highest feed rate; they also stick to each other, thus, rear-barrel temperature is an important controllable value. Screw surface temperature is rarely, but often usefully, controlled. The highest feed rate isn’t always best, as overbite can occur.

Materials are the biggest component of manufacturing cost. Buy wisely, label or mark everything, re-use as much scrap as possible. If mixing, keep proportions constant and hold tight tolerances.

Energy is a very small proportion of manufacturing cost. Measure it and do the calculation, if you doubt this. The hardest part is finding data on the internet, as all plastics are different and most sources deal with melt temp, not energy use. Crystalline means more energy is needed to form and break. Don’t confuse kilocalories (1000 calories = 1 Kal or Kcal) with small calories (energy to heat 1 gram of water 1°C). Or use another unit. One HP consumes 746 watts, but not all motor ratings are used all the time. Measure, don’t guess. I worry about this because extruders are efficient, and too much heat means overheated melt. Also, energy is often ignored by people trying to justify action on an environmental basis. Energy matters.

Pressure at the screw tip is the demand of the entire head and relates to safety, mixing, screen buildup, and bearing wear. Also, pressure in the barrel can be higher than at the screw tip.

Output is displacement of last flight (drag flow) minus head resistance (pressure demand) plus overbite at the feed, if any. For output calculation, you need to know melt density.

Typical shear rates: 100 reciprocal seconds (rsec) in screw channels, 100 to 1000 in most die lips, and well above 1000 in flight clearances and lips of tiny dies. Viscosity varies with shear rate.

Heating/cooling through the barrel is opposed by the motor. Heat lowers wall viscosity, screw turns easier; cooling is the reverse. Yes, there are 11 keys. There were 10 at first, but this is important and I didn't want to demote any of the others just to get to a total of 10.