Weld Lines / Knit Lines
Causes, Solutions & Prevention in Injection Molding
Weld lines form where two advancing melt fronts meet. If the fronts are too cool or have trapped air the weld is weak.
Below you will find general information about this defect. To get a pinpoint solution tailored to your specific machine, material, and process parameters using the world’s most comprehensive injection molding troubleshooting database — sign up and access AI-powered troubleshooting, 7 engineering calculators, and detailed project tracking.
🔍Symptoms & Visual Identification
⚠️Root Causes by Category
⚙️ Process
- Injection speed too low fronts cool before meeting
- Melt temperature too low
- Mold temperature too low
- Cold startup mold not pre-heated
🔲 Mold
- No vent AT the weld line location
- Vents crushed inspect after each lot
- Runner is half-round must be full round
- Gate positioned so weld falls in critical area
🧪 Material
- Material not dried moisture at weld makes it weak
- Metal flake pigments make weld visible
- MB carrier resin incompatible with base polymer
🏭 Machine
- Vent channels crushed by mounting mold on oversized machine
- Clamp force selected too high for projected area.
- V/P transfer point incorrect. Profile not suitable.
- Machine performance — screw position repeatability
🗂 Mold Design
- Mold vent channel depth insufficient.
- Runner design incorrect
- Flow analysis not properly done
- Mold process not properly selected
- Design errors in handle areas.
- Gate location incorrect
- Multiple gate application for large and long parts
- Multiple hot runner application for large parts
🔧 Mold Condition
- Mold vent channels crushed
- Mold temperature insufficient
📐 Part Design
- Hole design in part
- Design not suitable
📋 General
- Square holes worse than round add radii
- Wall too thin at weld location
- No sequential valve gate control
✅Solutions & Corrective Actions
⚙️ Process
- Increase injection speed
- Increase melt temperature
- Increase mold surface temperature
- Pre-heat mold before production start
🔲 Mold
- Open vent AT the weld location
- Use sequential valve gate system
- Reposition gate using flow simulation
- For visual parts use H&C system to eliminate weld lines
- Change to full-round runner cross-section
🧪 Material
- Additives and fillers may not move at the same speed as polymer where flow is interrupted. Weld line visibility increases or decreases depending on additive ratio. Appropriate carrier material must be selected for the additive.
- Plate-shaped flake types make weld lines more visible than they already are. If suitable, spherical flake types can be used instead.
- Low MFI material reaches the weld point later — increasing the mark. Higher MFI material should be selected; if not possible, switch to mold heating method.
🏭 Machine
- Each machine has max-min mold size specifications. Going below minimum increases load on the mold and crushes it. Proper machine selection should be determined by calculation.
- Excessive clamp force both reduces mold life and causes deformation. Per the "breathing mold" approach, machines with automatic pressure adjustment should be selected even if high pressure is applied.
- If V/P transfer point is not correctly set, packing pressure and time may be insufficient. Polymer arriving late at the weld point freezes early and creates a mark.
- Machine performance is determined by movement repeatability. The screw must stop at the same set point in both forward and back positions. Deviation to the back position causes short shots and increased weld marks.
🗂 Mold Design
- Vent channels must be opened per material requirements.
- If runner cross-section is not selected as full round, polymer entering the mold creates problems.
- Weld line locations should be identified in flow analysis to guide mold design. If changes cannot be made, cartridge heater application is used.
- For visual parts, H&C system should be selected to raise mold surface temperature above the material Tg point. In this case, weld lines are eliminated.
- Filter inserts are preferred for venting air in handle areas. They are pressed into the air trap location.
- In complex parts, gate position should be changed according to weld line data.
- Weld lines should be predicted with flow analysis and eliminated with cooling analysis.
- Valve gate with sequential method should be used. If valves are servo-controlled, overlap issues are also eliminated.
🔧 Mold Condition
- Mold vent channels get crushed after sustained pressure exposure. Channels should be inspected after each production lot.
- Some startup scrap is caused by weld lines — as the mold warms up during runtime, the marks decrease. Production should start after pre-heating the mold.
📐 Part Design
- If holes are square or rectangular, radii must be added to corners.
- Wall thickness at weld line location should be increased.
📋 General
- Use higher MFI grade fronts still hot when meeting
- Dry material thoroughly
- Change square holes to round with radii
🛠Step-by-Step Troubleshooting
Check Process Parameters
Increase injection speed
Inspect the Mold
Open vent AT the weld location
Verify Material Condition
Additives and fillers may not move at the same speed as polymer where flow is interrupted. Weld line visibility increases or decreases depending on additive ratio. Appropriate carrier material must be selected for the additive.
Check Machine Settings
Each machine has max-min mold size specifications. Going below minimum increases load on the mold and crushes it. Proper machine selection should be determined by calculation.
❓Frequently Asked Questions
What causes weld lines / knit lines in injection molding?
Weld Lines / Knit Lines is caused by several factors including: Injection speed too low fronts cool before meeting; Melt temperature too low; No vent AT the weld line location; Vents crushed inspect after each lot; Material not dried moisture at weld makes it weak; Metal flake pigments make weld visible.
How do you fix weld lines / knit lines in injection molding?
Start by checking key parameters: Injection speed, Melt temperature, Vent at weld location, Mold temperature. Increase injection speed
Which plastics are most affected by weld lines / knit lines?
Common materials include: PA6-GF30, ABS, PC, PP-GF30, PPO. Pay special attention to drying, shrinkage and mold temperature for each grade.
How do I prevent weld lines / knit lines in future production runs?
Prevention relies on three pillars: (1) process optimisation — verify Injection speed, Melt temperature; (2) mold maintenance — check gates, vents and cooling channels; (3) material control — ensure correct drying and virgin/regrind ratio.
What is the difference between weld lines / knit lines and similar defects?
Visible line where two flow fronts meet. Can be structural weakness. Often near holes or multiple gates. This distinguishes it from related surface defects during visual inspection.
📐Related Calculators
Run these engineering calculators to verify your process parameters and prevent this defect.
🔗Related Defects
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