# Weld line

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In [manufacturing](/source/Manufacturing), the **Weld line** or **Knit line** or **Meld line** is the line where two [flow](/source/Fluid_dynamics) fronts meet when there is the inability of two or more flow fronts to "[knit](/source/Knit)" together, or "[weld](/source/Welding)", during the [molding](/source/Molding_(process)) process. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part.[1] Knit lines are considered [molding defects](/source/Injection_molding#Molding_defects), and occur when the mold or/and material [temperatures](/source/Temperature) are set too low: thus the materials will be cold when they meet, so that they do not bond perfectly. This can cause a weak area in the part which can cause breakage when the part is under stress. Weld lines therefore occur during machine start-up, when equilibrium conditions have not been met. Mouldings made in this setting-up period must be rejected.

There are many Computer Aided Engineering tools that are available that can predict where these areas could occur, but a skilled designer will be able to predict where such defects can be found by examining the tool or product.

Weld lines are not found in other manufacturing methods such as [Rotational moulding](/source/Rotational_moulding), but can exist in [extrusion](/source/Extrusion), especially where there are internal metal supports for a die. The defects are then known as [spider lines](https://en.wikipedia.org/w/index.php?title=Spider_lines&action=edit&redlink=1).

## Causes

Weld lines can be caused by several different problems:

- Low temperature of [injection moulding machine](/source/Injection_moulding_machine) barrel

- Inadequate back pressure

- Injection pressure or injection speed is too low

- Low mold temperature

- Small injection gates and/or runners

- Improper location of injection gate

- Too many gates

- Excessive gate land length

- Improper [flow rate](/source/Flow_measurement) of injected materials

- Inconsistent process cycle

- poor [product design](/source/Product_design)

## References

1. **[^](#cite_ref-1)** [KNIT LINES, Texas Plastic Technologies](http://www.plastictroubleshooter.com/ThePlasticTroubleshooter/knit_lines.htm)

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