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.
Polymer Melt Flow
Meaning
Viscoelastic fluid transport inside heated barrels and runner channels defines the movement of molten macromolecular chains under applied pressure. Injection moulding and extrusion processes depend on polymer melt flow to fill mould cavities or form continuous profiles. The property governs hydraulic pressure drop, barrel heating requirements and cavity filling velocity, stopping where the material solidifies below its glass transition or melting temperature.
Standard datasheet melt flow rate values measure single-point flow under static load, whereas actual processing subjects the melt to dynamic shear rates spanning several orders of magnitude.
Shear Behaviour
Molten polymers exhibit non-Newtonian, shear-thinning characteristics where dynamic viscosity drops rapidly as shear rate increases during channel transport. Macromolecular chains untangle and align along flow streamlines inside runner channels, reducing resistance to polymer melt flow during high-speed injection phases. High shear rates at narrow gates decrease local viscosity, allowing complete filling of thin-wall part geometries without requiring excessive melt temperatures.
Regrind addition alters chain length distribution, shifting shear viscosity curves and changing pressure requirements during cavity filling.
Temperature Sensitivity
Thermal energy expands free volume between polymer chains, accelerating molecular mobility and lowering viscosity. Elevating barrel zone temperatures increases polymer melt flow rates, but excessive thermal exposure degrades molecular weight and reduces mechanical strength. Amorphous resins like polycarbonate display strong temperature dependence compared to semi-crystalline polyolefins.
Processing Boundary
Flow length limitations define the maximum part dimension attainable before melt freeze-out halts cavity filling. Excessive flow resistance causes short shots or high residual stress, whereas excessive fluidity induces parting-line flash. Mould design calculations balance wall thickness against flow length ratios to maintain stable injection pressures.