Weld Line Improvement of Threaded Caps by Adjusting Gate Position in Injection Molding
Weld lines appear where two melt flows converge. Lines on thread surfaces reduce torsion strength and cause leakage. Adjusting gate position is an economical modification to transfer weld lines out of force-bearing thread zones, with matched mold and process optimization.
Risk check of original gate layout
Side single gates often make melt converge on the opposite thread surface. Weld lines on thread teeth crack easily under twisting force. For multi-cavity molds, unbalanced runner leads to filling time difference and weak fusion interface. Long flow path reduces melt temperature before convergence and worsens weld strength.
Gate position adjustment schemes
Center tip gate on the top surface lets melt spread outward evenly, shifting weld lines to outer non-thread edges. It keeps consistent circumferential flow, while gate vestige needs post processing. Symmetrical multi-point gates on flange steps shorten flow length and put weld lines on non-sealing flange zones, suitable for large thin-walled caps. Offset original side gates by rotating angle to change convergence point, which costs least modification work but has limited improvement effect.
Matched mold structure adjustment
Vents must be relocated at new melt convergence positions to eliminate trapped air and black lines. Recheck gate thickness and width to avoid material decomposition or extended cooling cycle. Balance cooling on threaded cores to keep uniform melt temperature during convergence.
Mold trial verification and acceptance criteria
Test batches after modification, check weld line position and run torsion and water leakage test. If the trace is still obvious after moving out of thread surface, adjust injection speed, mold temperature and packing pressure. Record molding cycle and gate trimming workload to balance quality and production efficiency. Gate adjustment is not universal. If product structure limits all gate plans and weld lines cannot avoid thread zones, modify wall thickness transition, add weld wells or modify plastic material formula together.
