Meaning
Mechanical separations occurring where converging molten polymer flow fronts meet and fail to fully fuse inside a mould cavity occur at the junction of two distinct melt flows. Quality assurance engineers analyze weld line interface failure to diagnose premature structural failures in injection molded plastic parts. This defect represents a region of reduced mechanical strength and low impact resistance.
The boundary of the weakened zone depends on the temperature, pressure and flow rate of the converging melt streams.
Diffusion Dynamics
Molecular chains must diffuse across the boundary between the converged flow fronts to establish a strong bond. Preventing weld line interface failure requires high melt temperatures that increase chain mobility and allow complete entanglement before the polymer freezes. Lower temperatures restrict this molecular movement, leaving a distinct line of mechanical weakness.
Processing Influence
High injection speeds increase shear heating and maintain a hot flow front, improving molecular diffusion at the interface. Increasing the hold pressure helps force the molecules together and reduces the visible groove on the surface.
Material Impact
Reinforcing fibers rarely cross the weld line because they align parallel to the converging flow fronts instead of perpendicular to them. This alignment causes a drop in strength at the interface of short-fiber filled composites. Using regrind material, with its shorter chains and reduced chain entanglement capability, further degrades the weld line strength.
