For previous reading, please refer to Daily Share: A Comprehensive Analysis and Long-Term Prevention Guide for Gas Trapping and Burning De.
Among injection molded part defects, weld lines are a classic example of a "double whammy" – they not only cause noticeable streaks on product's surface (affecting customer acceptance) but also reduce local strength (leading to assembly cracking). Essence of this defect is insufficient diffusion of molecular chains at interface when molten material flows and then recombines, or presence of air resistance/impurities, forming a weak zone of "incomplete fusion." This article, using four modules–welding mechanism, four-dimensional root cause analysis, layered solutions, and long-term prevention–combined with real-world cases from automotive and home appliance industries, will teach you how to completely eliminate this "appearance killer" and "strength hazard."

 

Core contradiction of weld lines is that separated molten material fails to form a continuous, high-strength interface at point of recombination. This requires deconstructing underlying logic from four dimensions: material fluidity, process driving force, mold flow characteristics, and environmental interference.
1. Material Characteristics: "Inherent Shortcomings" of Fluidity and Interfacial Tension
Flow state of material directly determines quality of interface fusion:
- Low-viscosity materials (e.g., PP, PE, PS): After melt separates, it tends to "act independently," resulting in high interfacial tension at point of recombination (e.g., PP interfacial tension reaches 30 mN/m), making diffusion and fusion difficult. A certain automotive instrument panel (PP material) had weld lines running through central control area because although PP melt flow index was high (25 g/10 min), high interfacial tension prevented sufficient entanglement of molecular chains during recombination.
- High-crystallinity materials (e.g., PA, POM): Crystallization rate is fast, and separated melt partially crystallizes before recombination, resulting in weak interfacial bonding force (e.g., when PA crystallinity > 40%, weld line strength decreases by 50%). A certain home appliance connector (PA66) fractured at weld line because PA crystallized too quickly, and interface solidified before complete fusion.
- Materials containing impurities/volatile gases: Impurities (such as material particle fragments) or gases (moisture/decomposition gases) form an "isolation layer" at point of recombination, hindering molecular chain contact. Medical device casing showed black spots in weld line. Testing revealed that material contained metal fragments. After cleaning material barrel, weld line became less prominent.
2. Process Parameters: "Loss of Control" in Rhythm of Driving Force and Cooling
Process settings determine "energy" and "state" of melt when it converges:
- Injection speed too slow: Divided melt cools prematurely, and interface solidifies before convergence (e.g., reducing ABS injection speed from 60% to 40% increased weld line area by 40%). A mobile phone frame showed weld lines around camera hole; increasing injection speed resulted in "hot convergence" instead of "cold splicing," and marks disappeared.
- Mold temperature too low: Low mold temperature (e.g., PC mold temperature < 80℃) leads to rapid melt cooling and insufficient interface diffusion time. A car door handle showed weld lines; increasing mold temperature from 70℃ to 90℃ increased interface molecular chain diffusion distance by 20% and improved strength by 35%.
- Insufficient holding pressure: Unable to squeeze out gases/impurities at interface, forming "air gaps" or "inclusion layers." A toy car shell showed weld lines with bubbles; increasing holding pressure from 70MPa to 90MPa squeezed out gas, resulting in complete interface fusion.
3. Mold Design: "Acquired Defects" in Flow Path and Venting
Mold structure determines "path" and "environment" of melt division and convergence:
- Improper number and location of gates: Multiple gates lead to multiple convergence points (e.g., a two-cavity mold product with 3 gates per cavity results in 6 convergence points); gates are far from convergence area (e.g., injection from both sides of product, with no material supply to central convergence point). A laptop casing showed weld lines in central area; changing from two-point injection to single-point central injection reduced convergence points by 80%, and marks disappeared.
- Unreasonable runner/convergence angle: Sudden changes in runner diameter (e.g., main runner Φ8mm → branch runner Φ4mm) cause melt jetting and splitting; convergence angle < 45° (ideal ≥ 60°) leads to violent melt collision and formation of "dead zones." A car grille exhibited a "sawtooth" weld line; adjusting convergence angle to 65° resulted in a smoother interface fusion.
- Poor venting: Gas was not expelled near convergence point (e.g., no vent groove at the bottom of a deep rib), and gas occupied interface space, hindering melt fusion. A certain electronic device casing showed burn marks at weld line; after adding a 0.02mm deep vent groove, gas was expelled, and there was no isolation layer at interface.
4. Equipment Capabilities: "Supply Shortcomings" in Injection Volume and Temperature
Injection stability of injection molding machine affects consistency of melt convergence:
- Injection volume fluctuations: Screw wear leads to unstable injection volume (e.g., gap of a Φ50mm screw increases from 0.1mm to 0.2mm, resulting in an injection volume deviation of ±10%), causing an imbalance in proportion of divided melt and fluctuations in strength of convergence interface. A certain precision gear exhibited intermittent weld lines; problem was resolved after replacing screw.
- Uneven barrel temperature: Local low temperature in barrel (e.g., nozzle area temperature is 20℃ lower than homogenization section), resulting in large temperature differences during melt splitting and difficulty in interface fusion. For a certain PP part, after reducing temperature difference between barrel sections from ±15℃ to ±5℃, uniformity of melt temperature improved, and weld marks became less pronounced.

Weld lines are easily confused with trapped gas and material shortage; a four-step troubleshooting process is required, considering location, morphology, and process conditions:
Step 1: Observe location and morphology of weld line (5-minute quick assessment)
- Multi-gate convergence area: Such as middle of product or around holes, mostly due to gate design or venting problems (e.g., central control area of a car dashboard).
- Wall thickness transition area: At junction of thick and thin walls, poor convergence is caused by differences in melt flow speed (e.g., junction of reinforcing ribs and main body of a home appliance casing).
- Randomly dispersed traces: Often due to material impurities (material particle debris) or equipment injection fluctuations (screw wear).
Step 2: Material Verification and Drying (5-minute check)
- Check melt flow index and crystallinity: Low melt flow index materials (e.g., PC < 10g/10min) are prone to weld lines; increase material temperature (e.g., PC from 280℃ to 300℃, viscosity decreases by 30%); high crystallinity materials (PA crystallinity > 40%) require lower mold temperature (e.g., PA from 90℃ to 70℃) to slow down crystallization.
- Measure impurity content: Inspect weld line area; if there are black spots/foreign objects, check barrel/pellets (e.g., a PA part had metal shavings in weld line, resolved after cleaning barrel magnet).
- Material change verification: Switch to a low-impurity material of same brand; if weld lines are reduced, rule out contamination issues.
Step 3: Adjust Process Parameters (10-minute trial and error)
- Increase speed/raise mold temperature: Increase injection speed from 50% to 70% (increases melt kinetic energy, promotes interface diffusion); increase mold temperature from 70℃ to 90℃ (extends interface fusion time). For a mobile phone case with weld lines, increasing speed by 20% and mold temperature by 20℃ changed prominent streaks to subtle lines.
- Increase holding pressure/lower material temperature: Increase holding pressure from 70MPa to 90MPa (expels interface gases); lower barrel temperature from 300℃ to 280℃ (reduces low-molecular volatile gases). For a home appliance casing with weld lines and bubbles, increasing holding pressure by 20MPa and lowering material temperature by 20℃ eliminated bubbles.
- Extend injection time: Extend injection time (e.g., from 2s to 3s) to ensure split melt reaches confluence point simultaneously (avoiding "sequential confluence" which leads to interface differences). For a thin-walled part with weld lines, increasing injection time by 50% improved filling synchronicity, and lines became less noticeable.
Step 4: Check Mold Design (30 minutes - 1 hour)
- Measure gate/runner: Use a projector to measure gate diameter (e.g., ABS requires Φ6-8mm), and runner taper angle (≥60°). For a certain automotive grille with weld lines, adjusting runner diameter angle to 65° resulted in smoother flow distribution and a smoother confluence interface.
- Exhaust testing: Measure depth of exhaust groove near confluence point using a feeler gauge (e.g., 0.01-0.02mm for PC), or use a smoke test (observe smoke being discharged from exhaust groove during injection). For a certain electronic device casing with weld lines, adding exhaust grooves allowed gas to escape, resulting in no air resistance at interface.
- CAE mold flow analysis: Use Moldflow to simulate melt flow path, visually observing number of confluence points (e.g., if there are 3 melt streams converging in a certain area, reduce it to 1) and interface temperature (e.g., if confluence point temperature is < 80% of melting point, increase mold temperature).

 

If main cause is process parameters:
- Optimize injection curve: Use "fast at the beginning, slow at the end" segmented injection (e.g., 80% speed during flow distribution stage → 60% speed during confluence stage) to ensure melt reaches confluence point simultaneously.
- Increase interface energy: For every 10℃ increase in mold temperature, diffusion speed of interface molecular chains increases by 15%; for every 10MPa increase in holding pressure, interface gas extrusion rate increases by 20%.
If main cause is mold problems:
- Reduce confluence points: Change multiple gates to a single gate (e.g., a laptop casing from 3 gates → 1 central gate), or adjust gate position to allow unidirectional melt flow (avoiding cross-flow).
- Add venting/flow guides: Embed porous steel (e.g., PM-35) near confluence point, or add "flow guides" at the end of runner (to guide melt to converge smoothly and reduce collisions).
If main cause is equipment capability:
- Replace/repair screw: When screw wear causes fluctuations in injection volume, replace screw with a new one (gap ≤ 0.1mm), or perform nitriding treatment on barrel (to improve wear resistance).
- Calibrate barrel temperature: Use an infrared thermometer to calibrate temperature of each section (temperature difference controlled within ±5℃) to ensure uniform melt temperature.

1. Product Design Stage: Avoiding Weld Line Risks
- Reduce flow division points: Prioritize single-point gating (e.g., center gating for flat products) to avoid multiple gate intersections and flow division.
- Optimize wall thickness transitions: Use rounded transitions (R≥1.5mm) at abrupt changes in wall thickness to reduce differences in melt flow velocity (e.g., for a certain home appliance casing, increasing radius of reinforcing rib root from 0.5mm to 1.5mm reduced weld lines by 60%).
2. Mold Development Stage: Optimizing Flow Convergence Environment from Source
- High-precision machining of runners: Runner diameter change angle ≥60°, convergence angle ≥65°, reducing melt collision.
- Reserve venting space: Add venting grooves downstream of convergence point (depth = material viscosity * 0.005mm, e.g., 0.01mm for high-viscosity PC, 0.02mm for low-viscosity PP).
3. Mold Trial and Verification Stage: Establishing a "Weld Line Database"
- Record critical parameters: For each product, record parameters such as "minimum mold temperature without obvious weld lines" and "shortest injection time," for direct use in subsequent production.
- Weld line strength testing: Test strength at weld line using a tensile testing machine (target ≥ 80% of base material strength). If it fails to meet standard, optimization is required (e.g., a certain automotive part had a weld line strength of only 60%, which was increased to 85% by increasing mold temperature and adjusting the gate).
4. Production Management Stage: Dynamic Monitoring and Regular Maintenance
- Real-time monitoring of mold temperature/injection volume: Install mold temperature sensors and injection volume counters; an alarm is triggered when mold temperature fluctuation > 5℃ or injection volume deviation > 5%.
- Regularly clean venting grooves: Clean venting grooves near convergence point weekly with a copper brush (to prevent mold release agent/plastic residue from clogging, leading to gas accumulation).

Essence of weld lines is "incomplete fusion at interface when molten material flows and converges." Troubleshooting requires a four-dimensional approach, considering materials (flowability/impurities), process (speed/mold temperature), mold (gate/venting), and equipment (injection volume/temperature). Prioritize adjusting process (low cost, quick results), then optimize mold (to solve structural problems), and finally upgrade equipment (to address fundamental limitations). The key to long-term error prevention is "design prediction + parameter standardization + dynamic monitoring," ultimately achieving "flawless appearance and meeting strength requirements."

 

Core Mantra:  First check convergence point for weld lines, speed and mold temperature are crucial; try reducing gate size and increasing venting; ensure clean materials without impurities; design to avoid multiple gates and use single-point injection; archive data to prevent recurrence.