Meaning
Mechanical assembly located at the tip of an injection screw prevents the backward flow of molten plastic during the injection phase. The non-return valve is a critical component for maintaining pressure inside the barrel. It ensures that the entire volume of the shot is pushed into the mould cavity rather than leaking back toward the hopper.
Screw Assembly
Three distinct components usually form this mechanism including the retainer, the ring and the seat. The non-return valve must open quickly as the screw rotates to pick up new resin. It then closes tight as the screw moves forward to push the melt into the cavity.
Pressure Maintenance
Consistent shot weight depends on the ability of the seal to hold against massive hydraulic forces. A leaking non-return valve causes fluctuations in the cushion and leads to short shots or flashing. It is the most frequent cause of part weight variation in high precision moulding.
Part Consistency
Wear on the sliding surfaces changes the timing of the closure over months of operation. Regular inspection of the non-return valve ensures that the gap between the ring and the seat remains within the tolerance specified by the machine builder. This maintenance step avoids the hidden cost of gradual process drift.
