Weld lines (also known as "weld lines") in injection molded products are weak lines formed at junction of molten plastic during cavity filling due to reunion of split flows. Essentially, these lines are "interfacial bonding defects" that occur when melt flows reconnect after interruption. Split flows partially cool before reunion, forming a solidified layer on the surface. Upon reconnection, only a weak bond is achieved through molecular diffusion (bonding strength is only 30%-50% of that of bulk). Microscopically, these lines can be observed to disrupt molecular chain arrangement (X-ray diffraction reveals a 15%-20% reduction in crystallinity). Microcracks (less than 0.03mm in depth) often appear on the surface. In severe cases, these defects can lead to product breakage or poor appearance.

Knit lines are result of multi-dimensional imbalances in materials, molds, processes, and equipment. They can be categorized into four main, overlapping causes:
- Mold design defects (the most fundamental contributing factor):
- Improper gate placement (e.g., a single gate leading to long flow diversion, or multiple gates with excessive spacing), resulting in a long flow path and high energy loss (pressure loss > 50%) after melt diversion, and a temperature that is too low at confluence (< 80% of material melting point);
- Uneven runner/cavity layout (e.g., a main runner diameter that is too small, a sudden change in cross-section of a diverter runner), resulting in large variations in melt flow resistance (shear rate difference > 20%), and inconsistent flow velocities at confluence, leading to weak interfacial bonding;
- Poor venting (gas not being exhausted from knit line area), resulting in gas occupying interfacial space (equivalent to an "isolation layer"), hindering melt fusion.
- Out-of-control process parameters (key drivers):
- Injection speed too low (<50 mm/s), resulting in rapid cooling of melt after splitting (surface solidification layer thickness >0.1 mm), preventing effective fusion upon convergence;
- Melt temperature too low (<lower limit of material's flow temperature), high melt viscosity (e.g., ABS viscosity is 1800 Pa·s at 210℃ and 1200 Pa·s at 230℃), high flow resistance, and insufficient convergence kinetic energy;
- Insufficient holding pressure (<60% of injection pressure), preventing compaction of gas/cold material in convergence area, resulting in a loose interface.
- Material Characteristics (Hidden Risks):
- Low-flow materials (PC MI = 6-8g/10min, PP MI = 20-25g/10min) cool more quickly after diversion, making merging more difficult.
- High-shrinkage materials (PA6 shrinkage 1.2%-2.0%) shrink unevenly in merging area, leading to interfacial stress concentration (stress 1.5-2 times that of bulk).
- Materials containing volatiles (e.g., PA6 moisture content >0.1%) can vaporize to form bubbles (which occupy interfacial space and hinder fusion).
- Equipment deterioration (long-term hidden danger):
- Screw wear (clearance > 0.25mm) leads to uneven plasticization and presence of unmelted particles in melt (particles hinder melt fusion);
- Failure of check ring seal (melt backflow) causes injection pressure fluctuations (fluctuations > 10%) and unstable pressure during fusion;
- Low barrel temperature control accuracy (±5℃ or above), localized melt undercooling (temperature < 70% of melting point), preventing fusion during fusion.

 

Based on cost of repairing weld line issues, their impact on product performance, and probability of recurrence, we recommend systematically addressing them according to following priorities:

Core Logic: Mold is "channel" for melt flow and fusion. Adjusting gate position, runner layout, and venting system can directly improve melt fusion quality.

- Optimizing Gate Position and Number:
- For single-gate designs, gate should be located at the center of product (e.g., center-feeding for circular products) to shorten diversion path (flow length/wall thickness ratio <100:1);
- For multi-gate layouts, gate spacing should be controlled within 1.5-2 times wall thickness (e.g., 3-4mm spacing for a 2mm wall thickness) to ensure that diverted melt flows arrive at confluence area simultaneously (temperature difference <10℃);
- Increasing gate size (point gate diameter from 0.8mm to 1.2mm, latent gate width from 1.0mm to 1.5mm) to reduce pressure loss (increasing gate cross-sectional area by 50% reduces pressure loss by 40%).
- Improved runner and cavity layout:
- Main runner diameter was increased from φ6mm to φ8mm (50mm screw), reducing melt flow resistance (pressure drop reduced by 30%);
- Branch runner cross-section adopted a trapezoidal design (wide at the top, narrow at the bottom) to reduce velocity fluctuations during melt diversion (shear rate difference <10%);
- Venting grooves (0.03-0.05mm deep, 5-8mm wide) were added to weld line area to exhaust interface gas (reducing residual gas by 80%).
- Cooling system zoning design:
- An independent cooling circuit was added to weld line area (reducing water temperature by 10℃ and increasing cooling rate by 30%), delaying cooling in this area (keeping melt temperature >85% of melting point at confluence);
- Distance between cooling channel and cavity was controlled at 1.5-2 times channel diameter (e.g., 12-16mm for an 8mm diameter channel), ensuring uniform cooling (temperature difference <5℃).
Operational Details:
- After mold modification, Moldflow simulation verification is required (temperature at weld line > 85% of melting point, shear rate difference < 10%);
- For precision molds (such as optical components), gate size tolerances are controlled within ±0.02mm (to prevent poor merging due to machining errors).
Case Study: A mobile phone midframe (PC) had a noticeable weld line on its surface. Original gate was located at the edge of product, resulting in a long flow path (flow length/wall thickness ratio of 150:1). This was adjusted to a center gate (Ø1.2mm diameter) and a venting groove (0.04mm depth) was added to weld line area. Weld line strength increased from 30% to 65% of original product, making it invisible.

Core Principle: Improve flowability and interfacial bonding of melts during merging by increasing melt temperature, optimizing injection speed, and holding pressure.

- Optimizing Injection Speed and Melt Temperature:
- Increase injection speed to 70-90 mm/s (a 20%-40% increase compared to normal process) to reduce cooling time of split melt (surface solidification layer thickness < 0.05 mm);
- Raise melt temperature to upper limit of material's flow temperature (e.g., from 230℃ to 250℃ for ABS, from 280℃ to 300℃ for PC), reduce viscosity (from 1800 Pa·s to 1200 Pa·s), and increase confluence kinetic energy;
- For high-flow materials (e.g., PP with a MI of 25g/10min), increase injection speed by an additional 10% (to 80-90 mm/s) to avoid excessive cooling after splitting.
- Adjusting Holding Pressure and Time:
- Holding pressure = injection pressure * 70%-80% (e.g., injection pressure 100MPa, holding pressure 70-80MPa) to compact gas/cold material in confluence area (for every 10MPa increase in pressure, interfacial bonding strength increases by 15%).
- Holding time = product wall thickness * 3s/mm (e.g., 2mm wall thickness, 6s holding) to extend interfacial fusion time (for every 1s increase in time, bonding strength increases by 5%).
- Controlling Back Pressure and Cooling Time:
- Increasing back pressure to 4-6MPa (for amorphous material) or 5-7MPa (for crystalline material) increases internal friction within melt and promotes molecular chain diffusion (for every 1MPa increase in back pressure, bonding strength increases by 10%).
- Extending cooling time to product wall thickness * 1.5s/mm (e.g., 2mm wall thickness, 3s cooling) ensures sufficient solidification in confluence area (to prevent subsequent shrinkage and cracking of bond line).
Operational Details:
- For high-shrinkage materials (such as PA6), a "stepped hold" (70 MPa x 4 seconds in the first stage, 50 MPa x 2 seconds in the second stage) is used during holding phase to prevent sudden pressure drops that can cause interface separation.
- Observe product surface: If weld line is "visibly concave," this indicates insufficient holding pressure and requires increasing injection pressure to 80% and extending holding time.
Case Study: Weld line of a certain automotive interior panel (PP) was prone to fracture. Original injection speed was 60 mm/s, melt temperature was 220℃, and holding pressure was 60 MPa. Adjustments to an injection speed of 80 mm/s, a melt temperature of 240℃, and a holding pressure of 70 MPa increased weld line strength from 40% to 70% of original strength, and fracture test passed.

 

Core Logic: Material fluidity and equipment plasticization conditions are long-term risks associated with weld lines, requiring establishment of standardized control procedures.

- Material Selection and Modification:
- Prioritize high-flowability materials (e.g., MI = 30-35g/10min for PP, MI = 20-25g/10min for alternatives) to reduce split cooling rate;
- For high-shrinkage materials (e.g., PA6), add 1%-2% toughening agent (e.g., MBS) to improve interfacial toughness (elongation at break increased by 20%);
- Recycled material content ≤ 10% (to prevent molecular chain breakage and decreased interfacial bonding strength caused by repeated heating).
- Drying Process Optimization:
- Hygroscopic materials (PA6, PC) must be thoroughly dried (moisture content < 0.1%) to prevent moisture evaporation and formation of interfacial bubbles (which hinder fusion);
- Use within 4 hours of drying (re-drying is required if material is dried beyond this time) to prevent increased viscosity due to moisture absorption (PA6 viscosity increases by 15%).

- Screw and Check Ring Maintenance:
- Regularly inspect screw wear (clearance ≤ 0.15mm for a 50mm diameter screw). Exceeding tolerance requires replacement (excessive clearance can result in unmelted particles in melt, hindering fusion).
- Check ring tightness test (red lead powder contact area > 85%). Replacement is required if clearance falls below this value (to prevent pressure fluctuations caused by melt backflow).
- Barrel Temperature and Pressure Calibration:
- Calibrate barrel temperature control accuracy (within ±2℃) to avoid localized overcooling (e.g., a PP barrel temperature of <220℃ in a certain section).
- Calibrate hydraulic system pressure stability (pressure fluctuation during injection <5%) and replace aging seals (e.g., O-rings every 2000 hours).
Case: A precision gear (PA66+GF30) experienced frequent breakage at joint line. Investigation revealed insufficient material drying (moisture content 0.15%) and a screw clearance of 0.3mm (exceeding tolerance). After replacing screw (with a 0.18mm clearance) and increasing drying temperature to 125℃ for 4 hours (moisture content reduced to 0.05%), weld line breakage rate dropped from 30% to 2%.

Core Logic: Ambient temperature and humidity, along with auxiliary measures, can further suppress weld lines, especially for high-flow or precision parts.

- Workshop Environmental Control: For high-flow materials (such as PP), workshop temperature is controlled at 25±2℃ (to prevent ambient heat radiation from overheating melt) and humidity is maintained at 40%-50% RH (to prevent moisture absorption).
- Mold Preheating: For cold-start molds, preheat to process temperature 30 minutes in advance (e.g., PP molds are heated from room temperature to 60℃) to prevent undercooling of melt during initial filling, which can lead to poor fusion.
- Post-Processing Aids: For products with high-stress weld lines (such as optical lens holders), an annealing process (80℃ for 2 hours) is added to promote molecular chain diffusion (increasing bond strength by 20%).
Case Study: Weld line strength of a PMMA optical lens holder was low in a high-humidity environment (65% RH). After workshop humidity was reduced to 45% RH and mold preheating time was extended to 45 minutes, added annealing process increased weld line strength from 50% to 75% of original material.

Case 1: Obvious Surface Weld Line on Mobile Phone Case (PC+ABS)
- Symptom: A penetrating weld line (5-8mm in length) appeared on A side of product, resulting in poor appearance (customer rejection).
- Troubleshooting Process:
1. Mold Inspection: Single gate located at the edge, long flow path (flow length/wall thickness ratio 140:1), and no venting groove in weld line area;
2. Process Inspection: Injection speed 60 mm/s (too low), melt temperature 230℃ (too low), holding pressure 60 MPa (inadequate);
3. Material Verification: PC+ABS moisture content 0.08% (meets standard), but gas remains at weld line (bubbles hindering fusion).
- Solution:
- Mold: Adjusted to a central gate (Ø1.2mm), and added a venting groove (0.04mm depth) in weld line area;
- Process: Increased injection speed to 80mm/s, melt temperature to 250℃, and holding pressure to 70MPa;
- Material: Added 0.5% MBS toughening agent;
- Result: Weld line became invisible, and appearance acceptance rate increased from 70% to 98%.
Case 2: Fracture at weld line of a PP automotive door panel
- Symptom: Impact fracture occurred at weld line of B-side of product (elongation at break <5%), resulting in assembly failure.
- Troubleshooting Process:
1. Mold Inspection: Multiple gate spacing was too large (5mm > 2 times wall thickness of 4mm), and split melt was not synchronized (temperature difference of 15℃);
2. Process Inspection: Holding time was 5s (wall thickness of 2mm x 3s/mm = 6s, insufficient), and back pressure was 4MPa (too low);
3. Equipment Inspection: Screw clearance was 0.28mm (out of tolerance), and uneven plasticization resulted in unmelted particles in melt.
- Solution:
- Mold: Adjust gate spacing to 8mm (2 times wall thickness of 4mm), increase back pressure to 6MPa;
- Process: Extend holding time to 6s (2 x 3s/mm = 6s);
- Equipment: Replace screw (clearance 0.16mm) to ensure uniform plasticization;
- Result: Weld line fracture rate dropped from 25% to 1%, and assembly reliability met standard.

 

Managing product weld lines requires a systematic approach: mold design is foundation, process control is the key, and materials and equipment are guarantee. Frontline engineers must master core skills such as gate position optimization (matching material flow), runner zoning (balancing diversion pressure), and pressure-holding segmented control (extending interface fusion time). This allows them to shift from passive mold repair to proactive prevention, ultimately achieving a long-term solution to weld line issues.
For further reading, please refer to Solutions and Practical Guide to Product Strain Problems on Injection Molding Machines Under 800T.