Managing Hot Runner Shear Induced Thermal Imbalance across High Density Cavity Layouts
Managing shear imbalance across high-density cavities depends on rotating channel boundary layers at runner splits to equalize melt viscosity before gating.
Flash Defect
Meaning
Thin projection of excess plastic that escapes from the mold cavity into the parting line of the tool is a common molding issue. This flash defect occurs when the pressure of the molten polymer within the cavity exceeds the clamping force of the molding machine, or when the mold steel has worn down or damaged mating surfaces. The material flows into the resulting gap, solidifying into a thin flange that must be trimmed or causes the part to be rejected.
Mold Sealing
Clamping pressure must keep the mold halves tightly compressed against the outward force of the injected polymer. When the parting line of the mold suffers from damage, debris, or deflection under high pressure, the tight seal is lost and a path opens for the resin to escape. High injection pressure or excessive melt temperature can also reduce the viscosity of the polymer to the point where it easily penetrates even the smallest gap between the steel faces.
Clamp Deficit
Insufficient machine clamping force is a frequent cause of this failure during high-pressure runs. If the injection pressure times the projected area of the molded parts exceeds the tonnage capacity of the machine, the mold halves will separate slightly during the packing phase. This separation allows the pressurized melt to escape, leaving a thin skin of plastic on the edges of the part.
Defect Remediation
Reducing the injection pressure or lowering the mold temperature increases the viscosity of the melt, making it less likely to seep into the parting line. Molders can also relocate the gates to alter the pressure profile, or they can increase the clamping tonnage of the machine to ensure the mold remains closed. These steps restore the seal and eliminate the need for manual trimming.