EXTRUSION: Is Head Pressure Sapping Your Extruder’s Strength?
A redesign of all of the flow paths between the end of the barrel and die may be in order.
Every extrusion operation has head pressure, unless material is just being dumped out the end of the barrel onto the floor. Head pressure has positive and negative effects on extruder performance. In most cases the effects are negative. Most operators (and even technicians) assume it’s a condition beyond their control—something you have to deal with, so work around it.
Head pressure occurs due to the flow resistance of the die and the piping that connects the die to the end of the barrel. But it can be minimized by proper design. Using a melt pump almost entirely offsets the head-pressure effects and is the best solution in many cases. However, melt pumps may not be cost effective where there are highly corrosive or abrasive conditions and on small to very small extruders, where the pump’s cost can approach half that of the extruder. In those cases, control of head pressure is in the design of the flow piping after the end of the barrel.
Although it’s not simple to make changes in the flow piping, it certainly should not be ignored, as proper design can provide a day-in/day-out benefit that will add up to a significant cost savings over time. The die itself may not be the easiest part to redesign for pressure drop, as it has characteristics designed into it to shape the extruded part that often cannot be changed without altering the product’s shape.
All the other parts of the flow path between the end of the barrel and the die should be considered for their effects on head pressure. The calculations are relatively simple, and there is a great deal of information on the internet about piping flow for viscous fluids. Piping includes screen changers, static mixers, valves, and simple piping. I have seen some very poor designs, particularly where processors tried to cobble together flow paths out of whatever available parts they had around. Even some new equipment has not nearly optimized the flow path for pressure drop. Then the processor wonders why he/she has high pressures, polymer degradation, high melt temperatures, low output, and lack of thermal homogeneity.