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Process Analysis and Case Studies

Collecting the axial pressure profile readings of an extruder screw running under a certain set of process conditions is valuable information when analyzing how well a particular design works. It can be used in predicting everything from the life expectancy of the screw to the temperature control of the polymer melt exiting the extruder. For instance, if a large pressure peak is observed for a particular screw design it is possible to conclude that the screw will wear out quickly in that area since the high pressure will cause the screw flights to brush against the barrel wall. Wearing of the flights is a major factor in premature screw failure.

Pressure probe along extruder

An experimental comparison study was conducted involving a Dual Spiral Systems barrier screw and a commonly available generic barrier screw design. The two different barrier screws ran the same polymer (1 MI LLDPE) under the same operating conditions. The screws were 30:1 L/D 3.5" (88.9 mm) in diameter and designed to process polyethylenes. The barrel of the extruder employed in the trials, shown above, had been drilled in eighteen separate locations for the purpose of mounting pressure transducers along the extruder axis. The axial pressure data was collected using a data acquisition system. The true melt temperature was measured with a variable depth melt thermocouple to compare the temperature gradient of the melt exiting the two screws. The measured pressure profiles are shown in the figure below.

Pressure profile comparison From the above figure it can be seen that the Dual Spiral Systems barrier screw is operating at lower axial pressures because it has a balanced barrier section. The generic barrier screw, with its unbalanced barrier section, had higher axial pressures which will cause it to wear out much sooner. The melt temperature of the Dual Spiral Systems barrier screw is also cooler by 50 °F over the generic barrier screw. In blown film this means that higher line speeds are possible because less cooling is required and the bubble is more stable. The cross channel melt temperature gradient for the Dual Spiral Systems barrier screw was maintained within 2 °F. The specific output of the Dual Spiral Systems barrier is also 17% higher than the generic barrier screw; an added benefit in most cost conscious production plants.

Solid bed breakup Solid bed intact

The data acquisition system has the capability of recording and storing the signal reading from the pressure transducers at a rate of 100 times every second (100 Hz). Recording the pressure readings at periodic time intervals in this fashion allows for an accurate analysis of how the solid bed of polymer pellets are melting. From the shape of the collected pressure data, an observer is able determine where the solid bed breaks up. This reduces the solids in contact with the barrel and hence reduces melting efficiency and output stability.

The knowledge gained from these experiments is incorporated in every Dual Spiral Systems extruder screw and passed down to the polymer processor in the form of value added performance from a barrier screw quite different from others on the market. We invite you to contact us and we will send you a more in depth description of this experimental study.

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