Meaning
Polymer processing mechanics define this metric as the rate at which molten material enters the mould cavity during the injection moulding cycle. Injection velocity governs the shear rate experienced by the polymer melt, which directly influences the molecular orientation and final mechanical properties of the moulded component. Excessive rates often trigger jetting or gas entrapment, whereas insufficient speeds result in premature cooling and incomplete filling of thin wall sections.
Pressure Interaction
Viscosity changes alter the required force applied by the machine as injection velocity shifts across the duration of the fill phase. High speeds lower the apparent viscosity of shear thinning materials, which reduces the resistance to flow while simultaneously increasing the cooling rate at the contact wall. Operators calibrate this balance to ensure the flow front remains stable throughout the cycle.
Precise control of the ram speed allows for the mitigation of internal stresses that arise from rapid transitions in flow geometry.
Production Readiness
Technical audits assess the capability of the hydraulic or electric drive system to maintain a consistent injection velocity during the switch from filling to packing pressure. Production engineers verify that the machine maintains performance within established tolerances because deviations at this stage alter the crystallinity and shrinkage of the plastic part. A stable rate provides the consistency needed for high volume manufacturing where dimensional uniformity determines the utility of the output.
Validation protocols usually involve monitoring the positional accuracy of the screw to confirm the actual flow matches the set profile.
Thermal Consequence
Friction between the melt and the mould surfaces generates heat as injection velocity increases through restrictive runners or gates. This localized temperature rise aids in filling complex paths but carries the risk of thermal degradation if the material exceeds its processing limit. Rapid movement creates shear heating that modifies the viscosity profile of the polymer before it settles in the mould.
Controlling the speed remains the primary mechanism for managing the thermal history of a moulded part.
