Weld lines are common appearance and strength defects in injection molding, which are manifested as linear grooves or weak areas visible on the surface of parts. This article will introduce how to optimize weld line problems through mold flow analysis through core indicator analysis and actual cases.

1.Melt confluence angle
Definition: Angle when two melt fronts meet.

 

Impact: When angle is ≤75°, weld line is visible to naked eye and strength is significantly reduced (experimental data of Matsuda Company).
Analysis example: Confluence angle can be changed by adjusting wall thickness of product. For example, increasing thickness from 4mm to 4.5mm increases confluence angle from 20° to 120°, successfully eliminating appearance weld line.

2. Temperature distribution

Core indicator: Wavefront temperature difference must be ≤20℃.
Data reference: Case shows that if temperature at weld line is lower than 250℃ (PC+ABS material), it is easy to cause obvious dents.
Solution: Increase melt temperature (such as increasing mold temperature from 100℃ to 145℃), reduce thickness of condensation layer, and improve fusion strength.

 

3. Pressure gradient

Optimization goal: Ensure that pressure when weld line is formed is ≥90% of peak pressure.
Case: Pressure gradient of automobile instrument cover is improved by 40% through balance adjustment of diverter, and strength of weld mark is improved.

4. Exhaust efficiency

Standard: Residual volume of gas in mold must be <0.5%.
Improvement direction: Optimize design of full-circle exhaust groove to reduce gas wrapping (such as adding 7 exhaust grooves to shorten flow length in projector chassis case).

1.Adjust process parameters
Increase material temperature and mold temperature: Reduce melt viscosity and promote fusion.
Optimize shooting speed: Multi-gate molds adopt an accelerated filling strategy to avoid end-trapped gas (for example, injection speed of a shell part is increased from 40% to 70%, and weld mark disappears).

2. Flow diversion and flow resistance design

Application scenario: Add a "cold material well" or local thickening (flow diversion) at the end of flow to guide weld line to non-appearance area (as shown in figure, weld line moves from weak structure to reinforcing rib after diversion).
Countermeasure: By changing thickness of glue to change angle of weld line, weld line on exterior surface was successfully eliminated.

 

3.Mold exhaust optimization
Design specification: Exhaust groove depth is 0.02-0.04mm (such as PP material), and exhaust coverage rate of the entire cavity edge must be >80%.

3. Wall thickness uniformity

Principle: Thickness change of adjacent areas is ≤30% to avoid stagnation of melt front (case shows that rib thickness of a product is adjusted from 50% to 60% of bottom plate, and welding strength increases by 25%).

4. Multi-level pressure holding control

Strategy: Segmented pressure holding compensates for shrinkage, pressure holding time of welding area is extended to 1.5 times the total cycle (pressure holding time of a medical component is increased from 3s to 5s, and dent index decreases by 60%).

5. Flow channel balance adjustment

Mold flow data: Flow channel volume ratio should be controlled at 5-8% (if it is too high, cooling will be uneven). Length of main flow channel should be shortened by Runner Wizard so that filling time difference of each cavity is less than 0.04s.

Case 1: Welding mark of automobile instrument cover
Problem: Confluence of three molten materials leads to air entrapment in the middle, and reduced injection speed causes material shortage.
Countermeasures: Raise temperature of bottom hot runner nozzle by 20℃, change confluence position to form two weld lines, and gas is discharged smoothly (Figure 2-87 effect comparison).
Case 2: Mobile phone shell weld line fracture
Optimization: Use variable mold temperature technology (mold temperature increased from 100℃ to 145℃), reorient fibers in weld line area, and increase bending strength by 2 times.

Design stage: Predict weld line position through Moldflow's Weld Line analysis module, optimize gate arrangement in combination with Fill Time and Temperature results.
Mold test verification: Prioritize increasing mold temperature and injection speed, adjust pressure holding curve, verify by combining CAE with actual short shot tests.
High-risk scenarios: Materials containing glass fiber require extra attention to weld strength, and it is recommended to add structural reinforcement or change number of gates.