Rate Limitation Due to a Worn Screw

The screw was removed from the extruder and inspected to determine if the screw was in specification as defined by the schematic. The measured geometries were consistent with the schematic, but the diameter at the flights was measured at 88.15 mm, indicating that the flight clearance was 0.38 mm. The original flight 

An identical screw was ordered with the correct flight clearance. As soon as the screw was delivered, the screw was installed in the extruder. Soon after startup, the extruder was discharging at a temperature near 220 °C at the base rate of 130 kg/h. Next the rate was slowly increased to 180 kg/h. The discharge temperature increased to about 224 °C. Higher rates are likely possible. The worn screw was sent back to the screw manufacturer for refurbishment. This screw was then stored as the spare screw. 

This case study clearly shows that a worn screw will cause the discharge temperature to increase due to material flow over the flights and will decrease the heat transfer coefficient, limiting the transfer of energy through the barrel wall. With the restored flight clearance, the process was able to increase the rate by about 35%, and the quality of the product was improved. 

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