Practical Skills for Eliminating Injection Molding Weld Lines

In injection molding production, weld lines are one of the most common defects affecting product appearance and strength, especially obvious in transparent parts, appearance parts and structural parts. Essentially, weld lines are thin marks formed when two or more molten plastic fronts meet in the mold cavity and fail to fully fuse. To fundamentally eliminate or weaken them, systematic optimization from material, mold, process and structure is required. Below are practical, on-site applicable skills that are simple and efficient.

Optimize Injection Molding Process Parameters

Process adjustment is the fastest way to solve weld lines, with the core of improving melt fluidity and fusion strength. Appropriately increase barrel and nozzle temperatures to keep the melt active, making molecular fusion easier—especially effective for PC, ABS, PA and PET. Increase injection speed to accelerate filling and convergence, reducing front cooling, but avoid trapped gas and burning from excessive speed.

Raise injection pressure to enhance melt compaction, making molecules at the convergence point tighter. Increase mold temperature to slow cooling and extend fusion time, which is particularly useful for appearance and transparent parts. Adopt segmented injection, increasing speed/pressure near weld lines to densify convergence points. Extend holding pressure time and increase pressure to compensate the weld area, reducing depressions and thin lines.

Optimize Mold Design and Venting

Many weld lines result from poor venting and unreasonable gate positions. Trapped gas when melt meets forms obvious black/white lines, so ensure unobstructed venting at weld lines—add venting grooves/pins or deepen existing ones, especially for transparent and precision parts.

The gate position determines weld line location and visibility. Guide weld lines to non-appearance surfaces, edges or thicker areas, avoiding plane centers and stress-bearing parts. Avoid multiple gate 对冲 to reduce melt intersections. Increase runner size, reduce bending and polish inner walls to reduce shear and temperature drop, ensuring better melt convergence. For uneven wall thickness products, fill from thick to thin walls to prevent premature front cooling.

Optimize Product Structure and Materials

Reasonable product structure reduces weld line probability. Ensure uniform wall thickness to avoid flow velocity differences and unsynchronized melt fronts. For holes, columns and inserts (prone to weld lines), add guide ribs, adjust hole positions or thicken edges to stabilize convergence.

Add process ribs and guide grooves to change weld line positions or increase convergence angles, improving strength. Cover slight weld lines on appearance parts with texture, frosted surfaces or texturing.

Choose high-flow raw materials (on the premise of meeting strength) to reduce weld line visibility. Avoid excessive recycled materials, which reduce fluidity and fusion strength. Add appropriate lubricating additives, but note their impact on demolding and spraying. For glass fiber reinforced materials, reduce fiber content or use long-fiber materials, and increase material/mold temperatures.

On-Site Inspection and Conclusion

On-site, solve problems in this order: check weld line position to judge trapped gas, excessive cooling or unreasonable gates; prioritize venting for black/yellow lines; increase temperature/speed for white thin lines; strengthen holding pressure/temperature for poor strength; polish runners, increase mold temperature and optimize venting for foggy lines on transparent parts.

Weld lines, though small, affect qualification rates, appearance and strength. The effective solution is coordinated optimization of process, mold, structure and material. Mastering these skills solves on-site problems, avoids defects from design, stabilizes production, improves yield and reduces costs, making it indispensable for injection molding practitioners.