Meaning
Process duration values quantify the period during which molten polymer or a formed moulding remains under elevated temperature, mechanical pressure, or tooling restraint before release. In injection moulding and thermoforming, dwell time determines both the extent of thermal exposure within a heated cylinder and the duration of compressive packing inside the mould cavity. The parameter governs molecular chain relaxation, heat transfer from the part into cooling channels, and the progression of semi-crystalline solidification.
Its measurement ceases at the point of clamp opening, mechanical demoulding, or web advancement into ambient cooling.
Cycle Phase
Moulding cycles divide this temporal boundary into heated barrel residency and cavity pressurisation intervals. Holding time under screw forward position represents the primary high-pressure dwelling phase where additional melt packs into cooling part geometry. Barrel residence duration represents the unpressurised phase where granules soften and homogenise under barrel heater bands.
Extending the cavity dwell interval allows thick wall sections to solidify below the resin heat deflection threshold. Short durations cut overall cycle time but permit unrelaxed internal stresses to freeze within thin part membranes.
Defect Generation
Inadequate hold time causes incomplete packing, producing sink marks over ribs alongside uncontrolled volumetric shrinkage. Excessive holding dwell elevates residual stress concentrations near the gate, inducing part warpage after ejection. When dwell time inside the heated injection barrel extends past resin stability thresholds, thermal degradation cleaves long polymer backbones.
Moulders observe flash, jetting, or molecular splay when degraded low-viscosity polymer floods the cavity under normal injection pressures.
Specification Boundary
Datasheet cycle recommendations establish baseline target windows derived from simple plaque geometries running under pristine laboratory conditions. Production moulders must adjust these durations upward when running thick industrial parts, high-cavitation tooling, or regrind blends carrying variable thermal conductivities. Reclaimed resin tolerates substantially shorter barrel dwell intervals than virgin material before undergoing thermal discolouration and embrittlement.
Real production scrap rates climb whenever plant floor cycle timing ignores the longer cooling dwell demanded by regrind-induced crystallisation shifts.
