Monday 13 September 2021

Rule of Thumb for Plastics Injection Moulding: Weld Line vs Meld Line

 In this rule of thumb post, we discuss the difference between a weld line and a meld line in polymer injection moulding as well as how we can apply effective troubleshooting.

In short: the angle at which the converging polymer flow front meets in the tool again determines the difference between weld line and meld line.

A weld line is formed if the angle is less than 135°.  A meld line is formed if the angle is greater than 135° and the polymer molecules are more uniform compared to the orientation formed after a weld line. This is demonstrated in the picture below.

It is important to understand the flow fronts and the flow angle for achieving proper weld lines. Such an understanding allows the use of multiple submarine gates rather than fan gates to be used. Prediction of weld lines prior to the tool being cut allows the optimization of the part design and gate location which in turn reduces tool development time. Consideration of material, filler and pigmentation is important and this is why discussion with the technical team at your material supplier at the early stages are useful.

Rule of Thumb: Weld Line vs Meld Line

Troubleshooting of weld and meld lines

Weld and meld lines can cause a decrease in mechanical properties and are often clearly visible on the surface. An effective way of troubleshooting them is to move them in a non-functional / non-visible area. This can be achieved by changing the gate position or part thickness. Furthermore the allover quality can be improved by increasing the melt and mould temperature. This facilitates a better interfusion of the flow fronts. Also injection speed can help as well as an optimization of the runner system design.

Thank you for reading and #findoutaoutplastics


Herwig Juster

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