Injection Mould Cavitation Selection Guidelines for Volatile Product Demand
Mitigate volatile part demand by selecting lower cavitation modular tools across standard press tonnages to avoid idle capital and startup scrap penalties.
Cushion Distance
Meaning
Linear displacement between the forwardmost mechanical stop of an injection screw and its resting position at the conclusion of hold pressure marks the remaining volume of melt ahead of the tip. This cushion distance ensures that the screw does not bottom out against the barrel nozzle adaptor during the packing and holding phases of the injection moulding cycle. Maintaining a continuous hydraulic or electric reserve transmits consistent packing pressure into the mould cavity, compensating for volumetric resin shrinkage as the polymer transitions from melt to solid.
The physical measurement loses meaning if the non-return check valve fails to seat correctly, because backwards flow over the screw flights yields an artificial forward movement.
Volumetric Compensation
Solidification of molten polymer inside an injection cavity involves significant volumetric shrinkage, ranging from under one percent in amorphous grades to over two percent in semi-crystalline grades. Pressure exerted across the cushion distance forces supplemental molten resin into the cavity throughout the gate freeze duration. If the screw bottoms out to zero displacement, pressure transfer terminates instantly, causing sink marks, internal vacuum voids and dimensional instability in thick-walled part geometries.
Holding a stable reserve of two to six millimeters provides sufficient melt mass without risking excessive residence time in the barrel barrel cap. Excessive cushion sizes expose resin to prolonged thermal degradation across multiple processing cycles.
Check Valve Integrity
Sealing behavior of the non-return valve directly governs the stability of the cushion position from shot to shot. During the transition from dynamic injection velocity to static packing pressure, the check ring must slide rearward against the thrust washer to seal the screw flights. A worn ring or scarred barrel surface allows polymer to leak backward into the screw metering channels under high pack pressure.
This leakage causes the screw to creep forward continuously during hold, resulting in inconsistent cushion measurements across consecutive cycles. Cushion variation exceeding one millimeter indicates mechanical wear or erratic polymer melt flow rates.
Cycle Control
Real-time monitoring of the screw final position acts as an immediate check on process stability and raw material consistency. Changes in regrind ratio, moisture level or virgin lot viscosity alter the pressure profile, shifting the final cushion point during continuous production runs. A declining cushion signals higher melt flow rates or incomplete non-return valve closure, producing short shots and underweight parts.
Conversely, an expanding cushion indicates lower melt temperatures, unmelted resin pellets or foreign contamination blocking gate apertures, causing high cavity packing pressures and part flash. Tracking this mechanical metric isolates process disturbances before dimensional tolerances drift out of compliance.