Meaning
Transient and localized increases in the temperature of the flowing resin above the setpoint of the barrel heaters can cause material degradation and moulding defects. Temperature variations can occur within a single screw rotation or during the high-speed injection phase of the cycle. These melt temperature spikes are often undetected by standard thermocouples, which react too slowly to capture rapid thermal changes in the flowing polymer.
If left unchecked, these localized heat bursts can degrade sensitive polymers and compromise the physical properties of the part.
Screw Geometry
Poorly designed screw profiles can generate high shear stresses in the metering zone of the extruder. When melt temperature spikes occur, the cause is usually excessive frictional heating from a restrictive screw configuration. This localized heating can raise the polymer temperature quickly, causing thermal scission of the polymer chains.
Melt Quality
Variations in melt temperature lead to inconsistent mold filling and variations in the weight of the moulded parts. When melt temperature spikes are present, the local viscosity drops sharply, leading to flashing at the parting line or uneven shrinkage across the part. These issues generate high scrap rates and increase production costs because the parts fail to meet dimensional tolerances.
This instability makes it difficult for a moulder to hold the tolerances specified on the technical drawings.
Process Control
Mitigating thermal instability requires the use of fast-response sensors and the optimization of screw speed and backpressure settings. Reducing the rotational speed of the screw decreases the shear energy input, which prevents the formation of localized hot spots. Implementing a more balanced screw design ensures a uniform temperature distribution throughout the melt stream.
This proactive process adjustment maintains a stable viscosity and protects the mechanical performance of the finished product.
