Meaning
Thermal distribution across the wall thickness of a thermoplastic preform dictates the material flow during stretch blow moulding. This parameter, known as core temperature gradient, represents the temperature difference between the outer skin and the inner core of the preform. When this distribution is uneven, it leads to non-uniform material stretching and localized thin spots in the final container.
Process Impact
Differential cooling or heating cycles determine the thermal profiles of the preform before the stretch rod descends. Heat profiles must remain symmetric to achieve balanced wall thickness in the blow mould. When the core temperature gradient becomes too steep, the outer layers of the polymer may stretch faster than the cooler core.
This disparity often causes pearlescence or micro-voiding in the blown part. It can also lead to premature gate off-centering during the mechanical stretching phase.
Measurement Method
Surface temperature sensors often fail to capture the internal thermal state of the thick-walled polymer. Accurate measurement relies on specialized infrared sensors or thermal imaging cameras immediately after the preform exits the heating ovens. Numerical simulations of the heating cycle also help predict the internal temperature curve.
Thickness Variation
Adjusting the power of individual heating zones in the oven offers a direct way to manage the thermal slope. Proper ventilation and soak time reduce the temperature gap between the inner and outer skin. This control ensures consistent material distribution and optimal top load strength in the finished bottles.
