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.
Head pressure reduces the output of the screw and raises the power requirement and melt temperature, just like on any pump. Output is a pretty straightforward measurement, and the improvement possible with lower head pressure can be easily calculated. The effect of increasing melt temperature from head pressure is more subtle, but just as many extrusion operations are cooling limited as output limited. For example, on a 1-in. extruder running a 5-MFR PP, the effect of each 1000 psi of head pressure could be a 7-10 F increase in melt temperature and a reduction in output of 3-7%. On a 4.5-in. extruder the effect could be 15-20 F with a reduction in output of 5-10%, depending on the screw design.
The shorter and more direct the polymer flow is from the barrel to the die, the more easily the head pressure can be controlled. However, care must also be taken even in a straight flow path to balance the pressure loss through the pipes while maintaining sufficient velocity in the pipe to keep the walls clean. Flow piping often is been designed only in terms of pressure loss, making the flow path too large, causing stagnant flow at the walls, resulting in polymer degradation and loss of temperature homogeneity.
In coextrusion lines, piping can become very complicated, requiring various bends to allow for working space around the extruders and still mate with the feedblock. Bends do two things: They result in added head pressure and they further aggravate loss of temperature homogeneity. A short 90° elbow 1 D long can be equivalent to 20 D of straight pipe, while a 2 D long elbow reduces that to about 12 D. Two short 45° elbows are equivalent to about 16 D each—or worse than one 90° elbow.
As polymer flows around a bend it slows on the outside of the bend and accelerates on the inside, causing temperature variations to develop in the melt stream. In severe cases, stagnation can develop on the outside of the bend.
Abrupt changes in pipe size are also an issue, and the best result is usually achieved when the converging or diverging angle is approximately 60° included. Converging pipe sizes (i.e., going from larger to smaller diameter) result in large pressure drops, depending on the ratio of sizes; and diverging pipe sizes (going from smaller to larger diameter) cause stagnation at the entrance to the larger diameter.
Head-pressure control is just another aspect of extrusion technology where efficiency can be improved and costs can be squeezed out with attention to detail. Remember, flow pipes should be as short and straight as possible, with a minimum of size changes. When size changes are required, a converging or diverging angle is important. Proper design of elbows is very critical and they should be eliminated whenever possible. Don’t allow conservation of floor space to result in a complicated piping arrangement that will add cost in every minute of operation.
ABOUT THE AUTHOR
Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact firstname.lastname@example.org or (724) 651-9196.
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