Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Weld lines (also known as knit lines) occur when two or more fronts of molten plastic converge in the mold cavity but fail to fully fuse, creating a visible line or weak structural point. This often happens around obstacles like cores, pins, or inserts that split the flow. While they can't always be completely eliminated (especially in complex parts), they can be minimized or strategically placed in non-critical areas through design, material, and process adjustments.

Key Strategies to Avoid or Minimize Weld Lines

Here are proven methods, grouped by category, based on industry best practices:

1. Optimize Part Design

Uniform wall thickness: Maintain consistent thickness throughout the part (ideally avoiding ratios greater than 2:1) to promote even flow and reduce premature cooling. Thinner sections can also make weld lines less visible.

Relocate features: Position holes, ribs, bosses, or other flow-splitting elements away from visible or high-stress areas. If possible, machine features like holes post-molding to eliminate splits altogether (though this increases cost).

Minimize obstructions: Reduce the number of splits by simplifying geometry where feasible.

2. Improve Mold Design

Strategic gate placement and type: Use multiple gates positioned to direct flow fronts away from critical surfaces, or opt for sequential valve gating where gates open in sequence behind the advancing front to prevent convergence. Larger gates and runners improve flow.

Hot runner systems: Switch from cold to hot runners to keep plastic molten longer, reducing cooling at convergence points and allowing better fusion.

Proper venting: Add vents at potential weld line locations to release trapped air, preventing flow restrictions and weak spots.

Overall mold layout: Eliminate sharp angles, corners, or thick sections that slow flow; ensure gates are near thickest areas for balanced filling.

3. Select Appropriate Materials

High-quality resin: Choose impurity-free plastics with good flow properties and weld strength (e.g., avoid filled or reinforced materials prone to weak bonds). Test resins for compatibility.

Color and additives: Use non-metallic or light colors to hide lines visually; avoid flake-filled pigments that accentuate them. Painting post-molding can also mask appearance.

4. Adjust Processing Parameters

Increase temperatures: Raise melt temperature (within material limits to avoid degradation) and mold temperature to keep fronts hotter and more fluid for better merging.

Boost pressure and speed: Higher injection speed fills the mold faster, keeping material hot; increase injection and hold/pack pressure to force better fusion and eliminate low-pressure zones.

Control flow rate: Avoid excessive material volume that slows later flow; ensure uniform cooling by balancing speed.