Meaning
Frictional resistance within a flowing polymer melt generates internal thermal energy that increases the temperature of the material. The occurrence of shear induced heating is most pronounced in high-velocity regions such as gates, runners, and thin-wall sections of the mould. This temperature rise can be beneficial by lowering viscosity, but it can also cause material degradation.
Viscosity Influence
As the polymer melt is sheared, the alignment of the molecular chains reduces the resistance to flow. The localized temperature increase from shear induced heating further reduces the viscosity, allowing the material to fill thin sections. This behavior must be managed carefully to avoid localized burning or outgassing.
Process Control
Injection molding machines must regulate the injection speed to control the amount of frictional heat generated. When shear induced heating is excessive, the polymer chains degrade, reducing the mechanical properties of the finished part. Engineers adjust gate sizes and runner profiles to minimize the shear rate.
This geometric adjustment keeps the melt temperature within the safe processing window.
Defect Prevention
Moulders monitor the melt temperature close to the gate to detect thermal spikes. High shear can cause discoloration, splay, or brittleness in the molded parts. These defects are avoided by optimizing the fill speed.
