Meaning
Continuous monitoring of part and runner mass identifies systematic variations in the amount of molten polymer delivered to injection mold cavities per cycle. Experiencing shot weight drift indicates thermal instability in the barrel, non-return valve leakage, or bulk density changes in the feed hopper. Process control systems track cycle-to-cycle weight changes to maintain tight dimensional tolerances on precision technical components.
The boundary of stable processing breaks down when weight variations exceed statistical process control thresholds, triggering automatic part rejection.
Process Instability
Wear on the screw tip non-return valve permits molten resin to backflow into the screw flights during high-pressure injection steps. Shot weight drift accelerates when non-return valve ring clearance widens beyond original manufacturer limits. Temperature fluctuations in barrel heating zones alter melt density and viscosity, changing the mass forced through the nozzle at fixed cushion settings.
Variable proportions of regrind mixed with virgin pellets also induce feed throat packing variations.
Defect Consequence
Unintended reductions in injected mass yield short shots and sink marks in thick part sections. Over-packing from upward mass drift causes parting line flash and mold damage.
Machine Limit
Modern injection moulding machines utilize closed-loop cavity pressure sensing and dynamic cushion control to counter mass variations. Automated weighing systems integrated with robotic part pickers flag downward mass trends before visual defects emerge. Preventative maintenance on barrel heaters and check valves eliminates systematic shot weight drift across multi-cavity tooling runs.
