{{More citations needed|date=July 2023}} In [[manufacturing]], the '''Weld line''' or '''Knit line''' or '''Meld line''' is the line where two [[Fluid dynamics|flow]] fronts meet when there is the inability of two or more flow fronts to "[[knit]]" together, or "[[welding|weld]]", during the [[molding (process)|molding]] process. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part.<ref>[http://www.plastictroubleshooter.com/ThePlasticTroubleshooter/knit_lines.htm KNIT LINES, Texas Plastic Technologies]</ref> Knit lines are considered [[Injection molding#Molding defects|molding defects]], and occur when the mold or/and material [[temperature]]s 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]], but can exist in [[extrusion]], especially where there are internal metal supports for a die. The defects are then known as [[spider lines]]. == Causes == Weld lines can be caused by several different problems:
* Low temperature of [[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 measurement|flow rate]] of injected materials * Inconsistent process cycle * poor [[product design]]
== References == {{Reflist}}
[[Category:Injection molding]]
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