Polyamide Dimensional Shrinkage Differential under Varied Cavity Pressure Conditions
Cavity pressure directly controls polyamide crystallization and volumetric shrink, requiring precise transducer monitoring to hold DIN 16742 tolerances.
Check Ring Wear
Meaning
Progressive mechanical erosion of the non-return valve assembly on an injection screw leads to inconsistent melt volume control. Undetected check ring wear allows polymer melt to flow backward over the screw during the injection and holding phases of the cycle. This leakage reduces the effective pressure transmitted to the cavity, causing shot size variation.
The condition develops gradually over thousands of cycles and requires regular monitoring of the screw recovery time and cushion size.
Wear Mechanism
Abrasive glass fibres and corrosive additives accelerate the physical loss of metal from the contact surfaces of the non-return valve. As the screw moves forward to inject the plastic melt, the check ring wear prevents a secure seal against the thrust washer. High injection pressures force the molten polymer through these eroded gaps.
This backflow worsens as the viscosity of the resin decreases or the injection speed increases. Over time, the continuous flow of abrasive material creates deep channels in the metal, which increases the clearance and reduces the maximum pressure the screw can build.
Process Deviation
Moulders track this mechanical degradation by observing changes in the screw cushion at the end of the holding stage. When check ring wear becomes significant, the screw continues to creep forward instead of stopping at a consistent cushion position. This instability causes weight variations in the moulded parts and leads to sink marks or short shots.
Cycle-to-cycle repeatability degrades, forcing operators to adjust packing parameters to compensate. This process instability indicates that the physical boundaries of the process window are shifting.
Maintenance Action
Regular injection moulding audits include checking the non-return valve for clearance and physical damage. Technicians measure the outer diameter of the ring and the mating face of the seat. If the dimensions fall below the wear limits specified by the equipment manufacturer, the assembly must be replaced.
Preventive replacement avoids costly scrap runs and maintains process capability.