Dynamic Viscosity Curve Generation and Transfer Point Calibration in Technical Molding
Calibrating transfer points via press-side viscosity curves optimizes fill pressure, cuts part mass variation, and locks in DIN 16742 tolerance bands.
Injection Velocity Scan
Meaning
Empirical evaluation procedures for thermoplastic moulding establish the relationship between injection speed, cavity fill time and melt viscosity by tracking fill dynamics across stepped ram speeds. An injection velocity scan converts raw machine hydraulic pressure and position measurements into an apparent viscosity curve at the mould gate. The resulting plot identifies the point of shear thinning where incremental increases in injection speed yield minimal reductions in melt viscosity.
Data derived from this trial isolates the stable filling region from zones prone to jetting or excessive frictional heating. The measurement applies only during the unpressurised velocity-controlled phase of mould filling, losing validity once cavity pack-out begins or pressure transfer occurs.
Rheological Profiling
Machine settings during initial mould sampling require empirical viscosity data to account for non-Newtonian flow through sprues, runners and restrictive gate orifices. Conducting an injection velocity scan entails filling the part to roughly ninety-five percent capacity at varying screw speeds with holding pressure disabled. Higher injection speeds increase the shear rate within the polymer melt, which untangles macromolecular chains and drops apparent viscosity.
Plotting relative viscosity against fill time produces a characteristic curve displaying steep shear-thinning at slow speeds and an asymptotic plateau at elevated speeds. Selecting a process speed along this plateau ensures minor machine speed fluctuations do not create massive pressure spikes.
Viscosity Transition
Viscosity measurements taken on capillary rheometers rarely match the effective dynamic conditions inside an active production tool. Virgin resins respond predictably to shear, but regrind blends introduce variable chain lengths that shift the shear-thinning knee toward longer fill times. Using an injection velocity scan identifies whether a material lot requires higher injection velocity to achieve uniform mold filling without generating sink marks.
Excessive velocity past the flat portion of the curve risks thermal degradation through shear burning, gate blush and material splay. Operating below the transition knee leads to short shots, pronounced weld lines and high cavity-to-cavity weight variation.
Window Definition
Tool qualification protocols rely on dynamic viscosity curves to separate robust processing conditions from unstable operating points. The optimum operating velocity identified by an injection velocity scan provides a baseline fill speed that minimises gate freeze sensitivity and balances runner delivery. Process windows established without this scan often suffer from intermittent hesitation defects when ambient plant temperatures shift hydraulic oil viscosity.
Tight dimensional tolerances on thin-walled parts depend directly on maintaining injection speeds inside the shear-independent zone identified during tool sampling. Setting transfer points based on this data prevents over-pressurisation and protects tooling steel from early fatigue.