Meaning
Thermal energy generation within a polymer melt arises from internal fluid friction as high viscosity material experiences rapid deformation during flow through narrow channels or tight apertures. When shear heating occurs, the process converts mechanical energy from the injection screw or nozzle directly into heat. This phenomenon governs the effective temperature of the polymer as it exits the barrel, often exceeding the heating band settings intended to control the melt state.
The boundary for this mechanism exists where the kinetic energy imparted by the screw speed equals or surpasses the ability of the material to conduct that heat into the surrounding tooling or stagnant melt zones. Such a surplus in internal energy causes degradation of the polymer chains, which alters the physical properties of the finished moulded part compared to the raw datasheet specifications.
Viscous Dissipation
Controlling these temperature spikes remains a primary concern during the injection moulding of high viscosity engineering resins. Operators adjust the screw rotation speed to modulate the velocity profile of the melt as it moves along the flight channels. If the velocity gradient remains too steep, the internal friction creates localized hot spots that travel into the mould cavity, creating surface defects or inconsistent crystallization patterns across the part geometry.
Reducing the back pressure helps minimize the work done on the material, yet this reduction requires a corresponding increase in cooling time to maintain the cycle output. The physical state of the polymer depends on the history of this energy transfer, meaning that regrind material often reacts differently than virgin pellets due to changes in molecular weight distribution and rheological response.
Tooling Geometry
Narrow gate dimensions force the polymer through a restrictive cross section, forcing the velocity of the melt front to rise abruptly. This restriction initiates high rates of deformation that trigger sudden temperature increases at the point of injection. Engineers design runners and gates to minimize this pressure drop, aiming to keep the melt within a stable thermal window that prevents burning or discolouration near the inlet.
Cavities with complex thin walls frequently suffer from this effect because the machine must apply extra force to fill the extremities before the material solidifies. Every mould design incorporates these flow constraints as a trade off between filling speed and the risk of localized thermal damage.
Process Stability
Consistent production relies on holding the melt temperature within a narrow band regardless of fluctuations in ambient factory conditions. Sensors in the barrel measure external heat, yet internal friction produces heat that the machine controller cannot directly observe or regulate through standard heating bands. Moulders establish a baseline by correlating screw torque with the resulting part dimensions to confirm that the material behaves as expected under the set parameters.
A stable process produces parts with identical mechanical properties because the energy input remains constant throughout the run. Consistent energy distribution prevents the batch to batch variability that leads to high scrap rates in precision manufacturing.
