Meaning
Hydraulic resistance encountered by a viscous molten polymer flowing through restricted channels generates a loss of energy measured as a gradient between two spatial points. The melt pressure drop governs the required screw torque and barrel pressure during extrusion and injection moulding. Higher pressure losses occur across narrow runners and complex die geometries.
The metric applies strictly to fluid flow within the molten state and ceases to yield processing data once thermal vitrification begins.
Rheological Resistance
Viscosity changes driven by shear rate and temperature directly alter the magnitude of pressure loss along the flow length. A high melt pressure drop indicates increased resistance, forcing processing equipment to consume more electrical energy to maintain a target volumetric flow rate. Virgin resins with narrow molecular weight distribution display predictable pressure profiles, whereas regrind batches exhibit fluctuating resistance due to thermal degradation.
Channel Restriction
Geometrical transitions within runner systems or breaker plates create localized stress concentrations in moving polymer streams. Abrupt channel contractions produce extensional flow that elevates processing head pressure, generating localized frictional heating. Process engineers adjust nozzle orifice diameters to control this energy loss.
Defect Generation
Excessive fluid resistance yields incomplete cavity filling and dimensional warping in moulded components. When the melt pressure drop exceeds machine capacity, injection hesitates, introducing structural voids in thin-walled sections. Stable pressure management stabilizes part weight across continuous production shifts.
