Meaning
Spatial variation in the temperature of a molten polymer occurs across the flow channel due to uneven heating and viscous dissipation. Melt temperature gradient identifies the difference in thermal energy between the core of the melt stream and the material in contact with the barrel or mould walls. This variation stems from the low thermal conductivity of polymers and the heat generated by friction during screw rotation.
A high gradient leads to inconsistent part dimensions and unpredictable shrinkage.
Shear Heating
Mechanical energy converted into heat during the plasticization process creates localized hot spots within the melt. The material near the screw flights experiences higher shear rates than the material in the center of the channel. This localized heating reduces the local viscosity and causes the flow to become non-uniform.
The Thermal Profile
Variation across the melt stream directly influences the cooling rate and crystalline structure of the moulded part. Differences in temperature at the gate lead to variations in the frozen-in stress of the plastic. Thin sections may freeze before the packing phase is complete if the melt temperature gradient is too steep.
This result often produces sink marks or voids in the final product.
Measurement Strategy
Thermocouples placed at different depths provide data on the extent of thermal stratification.
