Meaning
Progressive loss of metal from the flight tips of an extruder screw and the inner surface of the barrel increases the gap through which molten polymer can backflow. Standard processing of filled resins accelerates the erosion of the tooling components. This development of screw clearance wear reduces the pumping efficiency of the screw, leading to poor melt quality.
Backflow Inefficiency
Enlargement of the gap between the screw flight and the barrel wall allows pressurized melt to escape backward during the dosing and injection phases. When screw clearance wear becomes significant, the screw can no longer maintain a constant pressure profile, which results in a loss of shot volume consistency. This backflow also subjects the polymer to uncontrolled shear histories, which can raise the melt temperature and cause localized degradation.
Regrind materials containing glass fibers often accelerate this erosion, which increases maintenance costs.
Process Fluctuation
Variabilities in the recovery time of the screw and the cushion size indicate a loss of process control. When screw clearance wear is severe, the machine struggles to hold a consistent cushion, leading to part defects such as short shots or flash. The moulder faces increased scrap rates because the process cannot maintain the tight tolerances required for high-precision components.
This degradation of the tooling forces the processor to run the machine at slower speeds, which extends the cycle time and reduces overall productivity.
Inspection Routine
Periodic measurement of the screw diameter and the barrel bore allows maintenance teams to track the rate of metal loss. Comparing these measurements against the original specifications of the manufacturer helps determine when the screw needs to be rebuilt or replaced. Applying wear-resistant coatings to the screw flights extends the service life of the tooling when processing abrasive compounds.
This preventive maintenance ensures a stable melt delivery and preserves the quality of the moulded parts by preventing the variations in process parameters that lead to dimensional drift.
