20/05/2025
Weld lines are common defects in injection-molded plastic parts that occur when two or more molten polymer flows converge within the mold cavity. This phenomenon arises due to variations in flow paths, obstructions (e.g., inserts or core pins), or complex part geometries that split the molten material into separate streams.
As the flows meet, insufficient fusion can occur at the interface, resulting in a visible line and structural weakness. Key issues associated with weld lines include:
β1. Reduced Mechanical Strength: The incomplete bonding of polymer chains at the weld line often leads to diminished tensile, impact, or fatigue resistance, making the part prone to failure under stress.
β2. Aesthetic Defects: Weld lines may appear as surface imperfections (e.g., grooves, discoloration), compromising visual quality, especially in consumer-facing products.
β3. Material and Process Sensitivity: Factors like melt temperature, injection speed, material viscosity, and filler content (e.g., glass fibers) can exacerbate weld line severity. For instance, fibers tend to align parallel to the weld line, further weakening the interface.
To mitigate weld lines, engineers often optimize mold design (e.g., adjusting gate locations, adding flow leaders), fine-tune processing parameters (e.g., higher melt temperatures or injection pressures), or select materials with enhanced flow compatibility.
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