Weld Lines, Knit Lines and Meld Lines

A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and race track effects.

1. The Initial Weld Point		2. Section through Weld Point
The wall thickness of this example is un-even, causing the weld point to form offset from the centre of the rectangular hole.		A section through the weld point shows the melt fronts meeting at the centre point of the flow channel where the material is hottest.
3. Temperature Profile Section		4. Flow Front Temperature
Typical fountain flow results in a hot core of polymer flowing through the centre to the flow front and a frozen layer at the mould wall. Incomplete joining of the frozen layer influences weld line visibility.		Un-even wall thickness causes the flow front temperature to reduce on the left side of the aperture. A variation in temperature usually increases the visibility of the weld line.
5. Flow Direction @ Weld Point		6. Flow front angle @ 135º
The flow direction plot above, shows the change in direction of the melt fronts as the weld point forms. The weld point turns into a "meld" as the flow fronts converge and flow together.		At the initial point of contact the weld is almost always visible. Once the angle of the merging flow fronts passes 135º, the weld changes to a "meld" where the flow fronts move in parallel. see below:

Traditionally, a "meeting angle" of 135º is used to differentiate between weld lines and meld lines, as illustrated in Figure 6. For most polymers the weld line surface mark tends to reduce once the meeting angle passes through 120º to 150º. However, there is always an exception to the rule and some materials by nature tend to exhibit more visible weld lines irrespective of the angle. In addition many fillers and additives also highlight weld & meld lines. Glass fibres and metallic pigments are good examples. Typical fountain flow results in a hot core of polymer flowing through the centre to the flow front and a frozen layer at the mould wall. Incomplete joining of the frozen layer influences weld line visibility, therefore mould temperature and melt temperature are key factors influencing weld line visual quality and strength.

The exact strength of the weld line depends on the ability of the flow fronts to weld (or knit) to each other. The strength of the weld-line area can range from 10 to 90 percent as strong as the pure material. Weld lines are generally considered to be of lower quality than meld lines, since less molecular diffusion occurs across a weld line after it is formed.

Conditions that are favorable for better weld-line strength include: High injection pressure and speed; High melt and mold-wall temperature; Formation of the weld lines closer to the gate; A temperature difference of less that 10ºC between the two emerging melt fronts.

If a weld line forms before the filling phase is complete and is immediately subject to additional packing pressure, the weld line will typically be less visible and stronger. The most effective way to improve the strength of weld and meld lines is by increasing the local temperature and pressure.

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