How to Avoid Weld Lines in Injection Molding: A Comprehensive Solution from Principle to Practice

1. Formation Mechanism and Key Influencing Factors of Weld Lines

1.1 Inevitability of Melt Diversion and Confluence

• Multi-Gate Design: Melt streams from multiple gates meet in the cavity, such as in three-plate molds or hot runner systems.

• Structural Obstructions: Features like holes, ribs, or bosses force the melt to divide and rejoin. For example, the spokes of a gear cause melt to flow around the hub and converge.

• Long-Flow Paths: Melt cools as it travels through complex or lengthy channels, leading to premature solidification at the confluence interface and insufficient fusion.

1.2 Core Influencing Factors

• Melt Temperature and Flowability: Low temperatures increase melt viscosity at the interface, hindering fusion. For instance, when the melt temperature of polypropylene (PP) drops below 200°C, weld line strength decreases by approximately 25%.

• Mold Temperature: A lower mold temperature accelerates melt cooling, reducing molecular diffusion time at the interface. Tests show that increasing mold temperature from 50°C to 80°C can improve impact strength at weld lines by 18% (ISO 179 standard).

• Injection Speed and Pressure: High-speed injection may cause turbulent flow and oxide film formation on the melt surface, while low speed can lead to premature solidification.

• Material Properties: High-viscosity materials (e.g., polycarbonate, PC) or reinforced materials (e.g., glass fiber-filled nylon) are more prone to weld lines due to higher flow resistance and difficult interface fusion.

2. Mold Design Optimization: Reducing Weld Line Risks at the Source

2.1 Key Principles for Gate Design

• Minimize Gate Quantity: Prioritize single-gate designs (e.g., central gates) to avoid multi-stream confluence. For example, changing a car dashboard part from four-point gates to a single hot runner gate reduced weld lines by 70%.

• Optimize Gate Location and Orientation:

  • Ensure a confluence angle ≥135°; larger angles reduce melt impact and improve fusion.

  • Use CAE tools like Moldflow to simulate melt flow and position confluence points in thick-walled areas or non-visible surfaces.

  
  

• Use Fan or Submarine Gates: Fan gates reduce melt jetting and turbulence, while submarine gates hide weld lines within the part.

2.2 Runner and Venting System Design

• Shorten Runner Length: Long runners cause significant temperature drop. Maintain the main runner length below 150mm and adjust the sub-runner diameter by material (e.g., φ6–8mm for ABS).

• Add Venting Grooves: Install venting grooves (0.02–0.05mm deep, 5–10mm wide) at melt confluence points (e.g., cavity ends) to exhaust air and prevent trapped gas from exacerbating weld lines.

2.3 Part Structure Optimization

• Avoid Sharp Corners and Sudden Wall Thickness Changes: Sharp corners (e.g., right angles) disrupt melt flow, while sudden thickness changes cause stagnation. Use rounded transitions (radius ≥0.5mm) and gradual thickness changes (transition length ≥3× thickness difference).

• Simplify Unnecessary Holes and Ribs: Reduce 分流次数 by simplifying designs in non-critical areas, such as replacing dense ribs with a single thick rib.

3. Process Parameter Adjustment: Enhancing Melt Fusion Efficiency

3.1 Temperature Control Strategies

• Increase Melt Temperature: Raising temperature within material limits (e.g., PC from 280°C to 300°C) can improve weld line strength by ~8% per 10°C increase. Avoid overheating (e.g., PVC degrades above 200°C).

• Raise Mold Temperature: Set mold temperature at 60–90°C for crystalline plastics (e.g., PE, PP) and 40–70°C for amorphous plastics (e.g., PS, ABS) to prolong melt flow and promote fusion.

3.2 Pressure and Speed Optimization

• Stage Injection:

  1. High-Speed Filling: Fill 60%–70% of the cavity at high speed (80–100mm/s) to maintain melt temperature.

  2. Low-Speed Packing: Fill the remaining cavity at low speed (30–50mm/s) with increased packing pressure (80%–90% of injection pressure) and extend packing time by 5–10 seconds to promote melt penetration at the weld interface.

  
  

• Prevent High-Pressure Turbulence: Use multi-stage speed control (e.g., fast-to-slow) for thin-walled parts to avoid air entrainment or shear heating.

3.3 Cooling System Optimization

• Ensure Uniform Mold Temperature: Design cooling channels to maintain a mold temperature difference ≤5°C and prevent local overcooling. For example, add spiral cooling channels near prone weld line areas to slow cooling.

4. Material Selection and Modification: Enhancing Melt Compatibility

4.1 Prioritize High-Flow Materials

• Choose low-viscosity grades for high-appearance parts. For example, high-flow ABS (e.g., Chi Mei PA-718) improves weld line strength by 20% compared to standard grades (PA-757).

• Avoid high-filled materials (e.g., glass fiber content >40%). If necessary, use short fibers or surface-coupled reinforced materials to improve flowability.

4.2 Blending and Compatibilizer Technology

• Add 3%–5% compatibilizers (e.g., maleic anhydride-grafted ABS) to polymer blends (e.g., PC/ABS) to reduce interfacial tension and improve weld line compatibility, increasing tensile strength recovery from 65% to 85%.

5. Defect Diagnosis and Rapid Response

5.1 Weld Line Severity Assessment

• Visual Inspection: Use a gloss meter to measure gloss differences (allowable difference ≤10GU) between weld lines and normal surfaces.

• Mechanical Testing: Compare tensile or impact strength between weld line areas and the base material, requiring a strength retention rate ≥85%.

5.2 On-Site Troubleshooting Guide

6. Case Study: Weld Line Improvement for an Appliance Panel

1. Mold Modification: Switched to a single hot runner central gate, eliminating multi-stream confluence.

2. Process Adjustment: Raised melt temperature from 230°C to 245°C, mold temperature from 50°C to 65°C, and packing pressure from 60MPa to 80MPa.
   Results: Weld lines disappeared completely, gloss uniformity met standards, and tensile strength recovered to base material levels.

Conclusion