If an extrusion die is properly designed to meet a specific condition of output and polymer properties, then it should need very little adjustment to shape an extrudate to the desired form when operated under those conditions.
Die design is basically a procedure where the internal die configuration is shaped to deliver polymer from the entrance to the exit with the same pressure drop. That provides a balanced flow where all of the polymer is exiting at the same velocity and closely duplicates the shape of the die orifice. That task is complicated by the fact that polymers exhibit different degrees of non-Newtonian (shear-thinning) behavior when exposed to different shear rates in the die’s varying cross-section.
Additionally, temperature has a strong effect on die flow. With polymers having a strong non-Newtonian nature, just changing the output can unbalance the die flow. This makes for a complicated analysis that can be simplified somewhat by assuming that the process in the die is isothermal, so that only shear thinning must be considered. This leads to a potential variation in die flow for many designs, since the conditions in the die are seldom truly isothermal. As a result, a die is a surprisingly specific apparatus that works best at one set of conditions.
In order to give the die a range of operating conditions, mechanical adjustments are often present to adjust the flow. In practice, these mechanical adjustments are often used to correct for a thermal imbalance. Consequently, extrusion dies can often be adjusted thermally with better results than with the mechanical adjustments. Poor understanding of the need for thermal uniformity leads to use of the mechanical adjustments to correct for thermal imbalance, resulting in even more distorted die flow. In actual operation, part thickness control—not equal velocity—is the criteria for die-flow adjustment. The weight distribution of the final part may be corrected with mechanical adjustments, but the resulting unbalanced flow can affect part performance.
For example, having to make an annular die non-concentric to equalize the desired distribution suggests there is a flow issue. Correcting the weight by adjusting off-center will cause the flow to vary in velocity around the circumference as it exits the orifice. It is obvious that when you are attempting to extrude the same volume through a smaller opening of the die the velocity has to increase. This results in an uneven extrusion that may have ripples where the highest velocity occurs.
Even if these issues are corrected by some downstream tooling or mold, the higher velocity portion will have more orientation in the extrusion direction and less in the cross-machine direction. With rapid cooling, this orientation will be temporarily “locked” into the part as stress. As the polymer seeks its natural molecular orientation after processing, this stress is relieved and causes higher shrinkage in the areas with higher orientation. This can cause distortion or even cracking of the part, as well as weakness once the relaxation is completed. By using heat to balance the flow, the original die geometry is maintained. This corrects differences in pressure drop that lead to varying velocity exiting the die.
When the exit velocity is balanced, the subsequent stress and shrinkage are more uniform. Crystalline polymers can shrink as much as 20% as they cool and seek their natural molecular orientation. Amorphous polymers generally shrink substantially less primarily because there is little molecular reorientation. Consequently, thermal die balancing provides more benefits to the properties of the final part (sheet, pipe, tube, film) with crystalline polymers, but the flow improvements are just as pronounced with amorphous polymers.
One of the problems in pursuing thermal die-flow balancing is that many dies are not properly set up for temperature balancing in the first place, leaving mechanical balancing as the only option. Inherent thermal variation can be caused by simple choice of die-temperature settings, insufficient number of temperature zones, or poor distribution of the heating in the die body.
For example, I have seen large-diameter dies with heater bands around the entire outer circumference. This permits no thermal adjustment around the circumference, forcing the operator into using the mechanical adjustments to hold gauge. Wrinkling of a section of the tube-like extrudate due to the velocity difference when exiting the die may result because of a non-concentric gap. The same is true in a slit die. Here, center flow often predominates because of heat losses at the ends of the die. Often, there is no separate heat control at the die ends, or the thermocouples are located too far from the ends.
A suggested procedure for initial die setup is to set the die gaps even and/or concentric, and use temperature to get as close as possible to correct part-weight distribution. Only then use mechanical adjustments to make the final tweaking, rather than the other way around. You will be using the die design as intended and will have a much more sensitive adjustment to make mechanically. If the die cannot be thermally adjusted to very close to the desired slit or annulus shape, either the heating is not designed properly or the die is unsuited to the polymer.
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 email@example.com or (724) 651-9196.