Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Weld lines (also called knit or meld lines) occur when two or more fronts of molten plastic meet and fuse during the filling stage of injection molding. This can create visible lines or weak spots in the final part, potentially compromising strength, appearance, or functionality. While weld lines are inevitable in complex geometries, they can be minimized or eliminated through design and process adjustments.

Key Strategies to Reduce Weld Lines

Here are proven methods, grouped by category for clarity. These draw from industry best practices and can be implemented iteratively during prototyping.

1. Optimize Molding Process Parameters

Increase melt and mold temperatures: Higher temperatures improve the flow and fusion of the plastic, allowing better molecular entanglement at the weld line. Aim for the upper end of the material's recommended range, but monitor for degradation.

Boost injection speed and pressure: Faster filling ensures the fronts meet with sufficient energy to bond effectively. Also, increase pack/hold pressure to push material into the weld area and reduce voids.

Adjust cooling rates: Slower cooling near weld lines can enhance fusion, but balance this to avoid overall cycle time increases.

2. Refine Mold and Gating Design

Reposition gates: Place gates strategically to direct flow fronts away from critical areas (e.g., structural or aesthetic zones). Use multiple gates with sequential valve gating to control flow timing and minimize collisions.

Incorporate hot runners: These maintain molten plastic temperature longer, reducing premature solidification and allowing flow lines to be directed to non-visible areas.

Add vents or redesign flow paths: Improve venting to release trapped air, and widen channels to promote uniform flow.

3. Modify Part Design

Simplify geometry: Reduce features like ribs, bosses, or holes that split flow. If unavoidable, orient them so weld lines align with non-critical directions.

Select appropriate materials: Use resins with good flow properties (low viscosity) and purity to enhance weld strength. Avoid fillers that hinder fusion.

4. Advanced Techniques

Simulation software: Use tools like Moldflow to predict weld line locations virtually and test adjustments before production.

Hybrid gating: Combine cold and hot runners or employ specialized hot runner systems to precisely control where lines form.