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Home / How to Troubleshoot Weld Line Failures on Plastic Injection Moulding Machine？

How to Troubleshoot Weld Line Failures on Plastic Injection Moulding Machine？

2025/07/25 By Topstar

Weld Line Defects in Injection Molding

Weld line defects in injection moulding account for 25% of cosmetic and structural part failures and are the most common problem encountered by customers. Weld lines are formed when two melt fronts meet, and poor melt conditions can result in poor bonding or visible flaws. Troubleshooting this type of failure begins with determining the exact location and type of defect. In addition, record processing parameters, material grade, and mould temperature. By collecting detailed data in advance, you can lay the foundation for targeted corrective actions on the plastic injection moulding machine, reduce trial and error, and improve first-time pass rates.

Analyze plastic injection moulding machine material melt and temperature settings

The number one cause of plastic injection moulding machine weld line failure is often inconsistent melt temperature. If the melt temperature fluctuates by ±5°C around the set point, viscosity changes prevent the two flow fronts from merging properly. Therefore, the barrel area should be maintained within a tolerance of ±2°C. Active voice: I recommend using calibrated thermocouples in each barrel area and verifying the readings on a weekly basis. Additionally, verify that the melt temperature meets the resin manufacturer’s specifications. If the temperature is too low, the melt cannot flow. If the temperature is too high, the material will degrade and weaken the weld line. By stabilising the temperature of the injection moulding machine, you can improve melt uniformity and enhance weld line strength. Additionally, consider using a uniform screw design and regular barrel cleaning to maintain stable heat transfer and prevent degradation of high-performance engineering resins.

Analyze plastic injection molding machine material melt and temperature settings

Optimize the injection speed and pressure curves of the plastic injection moulding machine

Injection speed and pressure also play an essential role in the formation of weld lines. First,ensuree the injection speed is sufficient to push the melt front together before cooling,typicallyy 50-80% of the maximum machine speed. Additionally, you must maintain the holding pressure for at least 3–5 seconds to compress the bonding area. By fine-tuning the injection curve to include a steep speed ramp and a subsequent pressure holding plateau. In addition, reduce the switching delay between speed and pressure control to prevent flow hesitation at the weld line. By optimising these curves on the plastic injection moulding machine, you can promote stronger fusion and minimise visible lines. Additionally, evaluate the shear rate to prevent polymer chain scission, and consider applying moderate back pressure during plasticization to enhance melt uniformity and mitigate molecular orientation imbalance at the weld line.

plastic injection moulding machine 1-2

Improve mould design and gate location

Mould design and gate location also have an impact on the quality of weld lines in plastic injection moulding machines. For example, you should position the gate so that the flow fronts intersect in an area that is not visible or structurally significant. This way, any small weld lines will not affect the part’s aesthetics or strength. Therefore, it is necessary to work with the mould designer to adjust the gate size and angle to ensure a balanced filling pattern. In addition, add flow guides or side gates to delay the convergence of the weld line front. Optimising the mould geometry and gate layout can minimise problematic weld lines and improve part integrity with your plastic injection moulding machine settings. Of course, you can use moulumoulsedmould simulation software to predict the weld location, optimise the geometry, and verify the venting location to eliminate air pockets that can cause line defects.

Improve mold design and gate location

Ensure proper venting and cooling balance

Inadequate venting or uneven cooling can exacerbate weld line defects on plastic injection moulding machines. In addition to melt and pressure settings, confirm that the vent is unobstructed and provides a gap of at least 50 µm to allow trapped air to escape. Therefore, the formation of air pockets that hinder fusion can be prevented. You need to check the mould vents and add venting channels if necessary. Additionally, balance the cooling circuit to maintain a uniform mould temperature (±3 °C across the cavity) to prevent premature solidification of the flow front. Proper venting and cooling balance on the injection moulding machine ensures optimal melt convergence and strong weld lines. In addition, implement a regular mould maintenance program, including removing flash from the vent line and verifying coolant flow rates to maintain consistent thermal conditions during high-volume production.

Reduce the occurrence of weld line failures.

Troubleshooting weld line failures on plastic injection moulding machines requires stabilising melt temperature, optimising injection speed and pressure, optimising mould design and gating, improving venting and cooling, and monitoring the process. I recommend adjusting each parameter systematically and using real-time data to confirm the improvement. By applying these best practices, you will achieve stronger weld lines, more aesthetically pleasing parts, and higher overall production quality on the injection moulding machine.

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