Meaning
Moulding instrumentation measures the compressive force per unit area exerted by molten polymer within the machine barrel, nozzle, runner system, or tool cavity during the forming process. This melt pressure governs the velocity of the advancing flow front, the compaction of resin during packing, and the mechanical clamping force required to keep mould halves closed without flashing. Sensors installed directly behind ejector pins or flush with cavity walls record this variable throughout the injection and holding phases.
The measurement tracks dynamic polymer viscosity, flow resistance, and thermal state under active processing conditions. The metric ceases to represent bulk material flow behavior after the gate solidifies, converting into a localized measure of residual locked-in cavity stress.
Cavity Transduction
Piezoelectric transducers mounted flush inside the tool cavity record the physical force applied by molten polymer in real time. Monitoring melt pressure enables automated switchover from velocity control to pressure hold based on true cavity filling rather than screw position. Consistent cavity pressure curves ensure uniform part weight.
Sensor data detects short shots immediately.
Hydraulic Translation
Machine hydraulic pressure generates forward screw thrust, which intensifies into melt pressure according to the machine intensification ratio determined by cylinder and screw surface areas. A drift in melt pressure at the nozzle indicates viscosity fluctuations caused by temperature variations, moisture content, or lot-to-lot polymer inconsistencies. Regrind additions exacerbate pressure volatility because variable bulk density in the feed hopper and inconsistent flake sizing disrupt screw recovery efficiency.
Virgin resins melt predictably, maintaining tight nozzle and cavity pressure curves across multi-shift production runs. Excessive pressure spikes cause tool separation, generating parting line flash that damages mould parting surfaces over time. Insufficient pressure delivery fails to compress semicrystalline resins adequately against mould walls, leading to uneven part shrinkage and warpage.
Decoupled moulding strategies isolate cavity pressure management from machine hydraulics to deliver repeatable part dimensions despite incoming resin variations.
Viscous Attenuation
Friction against cold cavity walls rapidly degrades plastic pressure as the melt front advances away from the injection gate. Substantial melt pressure loss along extended runner channels requires higher nozzle pressures to achieve proper compaction in remote part features. Thin-wall mouldings suffer extreme pressure attenuation, demanding high-speed injection to induce shear thinning before freeze-off occurs.
Datasheet viscosity curves provide guidelines, yet actual pressure attenuation depends heavily on mould cooling layout. Maintaining sufficient terminal pressure ensures uniform crystalline density across every moulded corner.
