Weld Lines in Injection Molding: Definition, Causes, and Solutions

I. Understanding Weld Lines

Weld lines form when multiple plastic flow fronts meet in the mold cavity after passing inserts, holes, or areas with disrupted flow. Inadequate fusion between these fronts creates linear marks, significantly reducing a product’s mechanical strength. Solutions for weld lines often overlap with those for sink marks.

II. Equipment-Related Solutions

2.1 Plasticization and Temperature Control

Issue: Poor plasticization or uneven melt temperature leads to weak weld lines.

Remedy: Extend the molding cycle for complete plasticization. Opt for a molding machine with higher plasticizing capacity if needed.

III. Mold Design Adjustments

3.1 Temperature and Flow Path Optimization

Mold Temperature Enhancement:

Raise overall mold temperature or locally heat the weld line area to improve melt fusion.

Runner and Cold Slug Well Modification:

Enlarge narrow/shallow runners and increase cold slug well volume to reduce flow resistance.

3.2 Gate and Venting Improvements

Gate Design Changes:

Adjust gate size, location, or type to avoid flow around inserts/holes. Minimize multiple gates; use buffers or relocate gates to promote uniform flow.

Venting System Upgrades:

Open, expand, or smooth venting passages, including clearances around inserts and ejector pins to release trapped air.

IV. Molding Process Tuning

4.1 Pressure, Speed, and Temperature Controls

Injection Parameters:

Increase injection pressure and extend injection time to strengthen welds.

Adjust injection speed: High speeds ensure melt reaches the meeting point before cooling; low speeds allow better air venting.

Temperature Management:

Raise barrel/nozzle temperatures to lower melt viscosity (improving flow and thinning weld lines). Balance with reduced gaseous decomposition at lower temperatures.

4.2 Other Process Adjustments

Minimize release agents (especially silicone-based types), which hinder flow front fusion.

Lower clamping force to facilitate venting.

Increase screw speed to reduce viscosity and backpressure to improve material density.

V. Raw Material Optimization

5.1 Drying and Additive Adjustments

Thoroughly dry raw materials and reduce liquid additives to prevent gas entrapment.

Add lubricants/stabilizers to low-fluidity or heat-sensitive plastics. Switch to higher-fluidity or heat-resistant resins if needed.

VI. Product Design Modifications

6.1 Wall Thickness and Insert Placement

Wall Thickness Adjustment:

Increase wall thickness in thin areas to prevent premature solidification of the melt.

Insert Positioning:

Reposition inserts that disrupt flow to minimize weld line formation.

VII. Summary of Key Strategies

Mold & Equipment: Optimize temperature, runners, gates, and venting.

Process: Tune pressure, speed, and temperature for better melt fusion.

Material & Design: Dry materials, adjust additives, and modify wall thickness/insert placement.

Addressing weld lines requires a multi-faceted approach to ensure strong mechanical properties and improved product quality.

We specialize in custom plastic injection mold production service in China and injection molding processes for 20 years, contact yoyo@hanking-mould.com for your plastic products projects.