Statistical Process Control Subgrouping Strategies for Multi Cavity Injection Molding
Rational subgrouping in multi-cavity molding must isolate between-cavity mold offsets from shot-to-shot press drift to prevent control chart desensitization.
Shear-Induced Heating
Meaning
Thermal energy generated within flowing non-Newtonian polymer melts as adjacent fluid layers slide against one another under high velocity gradients alters the temperature profile across runner systems and gate restrictions. Shear-induced heating increases melt temperature independently of machine barrel heaters or mold heating elements, concentrated predominantly in regions of high shear rate such as thin gates and narrow runner branches. The scope covers internal viscous heat dissipation within the polymer melt during injection flow, ending where conduction into mold cooling channels dominates.
Thermal Dissipation
As molten plastic forces through narrow channels at high speeds, molecular friction converts mechanical injection work into thermal energy. Shear-induced heating causes localized temperature spikes of twenty degrees Celsius or more near runner walls and gate orifices. This localized heating lowers melt viscosity, promoting uneven flow splits in secondary runner branches and altering fill patterns in multi-cavity tools.
Uncontrolled shear heating degrades sensitive polymer chains, causing cosmetic burning, splay marks, and reduced mechanical strength in finished components.
Readiness Assessment
Prototype testing using low injection speeds often fails to reveal the severe shear heating that occurs under high-speed production cycling. Pilot audits must measure melt temperature rises across gates using fast-response thermal sensors or specialized flow modeling to optimize gate dimensions before freezing tool design. Approving high-rate production parameters without compensating for shear-induced heating leads to resin degradation, excessive molded-in stress, and unexpected part cracking in downstream assembly.
Flow Balancing
Mold designers size gates and runner channels to control shear rates, ensuring uniform thermal profiles across all cavity delivery paths.