Meaning
Backflow past the non-return valve during injection carriage forward movement reduces effective shot volume and destabilises cavity pressure during the holding phase. Excessive check ring leakage allows molten polymer to escape backward over the screw tip, which causes cycle-to-cycle part weight variation and sink mark defects. This physical phenomenon is restricted to the dynamic seal area of the reciprocating screw during transition from plasticising to injection.
Molders inspect cushion movement to confirm valve sealing.
Seal Wear
Mechanical abrasion from glass-filled resins enlarges the radial clearance between the check ring and barrel wall over repeated cycles. Increases in check ring leakage shift the cushion position forward, which forces machine controllers to extend injection stroke length to compensate for lost volume. Processors track cushion drift to identify valve degradation before parts fail dimensional inspection.
Pressure Loss
Peak cavity pressure depends on the mechanical seal formed when the ring seats against the rear thrust washer. Uncontrolled check ring leakage drops peak hydraulic pressure, which degrades part surface finish and strength. Uniform holding pressure cannot be maintained when melt slips past the valve surface.
Economic Impact
Scrapped parts and unexpected downtime for screw tip disassembly increase piece costs during production runs. Unmitigated check ring leakage inflates material expenditure by requiring wider cushion safety margins and excess shot sizing to maintain part dimensions. Replacing worn non-return components restores process capability and stabilizes cycle times.
