Hello and welcome to another Rule of Thumb post. Today we discuss how to handle weld lines in polymer injection moulding.
Check out my other posts on this topic here.
In a lot of plastic part designs, welding lines are unavoidable, especially if your design has holes. The biggest downside of weld lines is that they impact the aesthetics and strength of a plastic part. However, we can minimize them, move them, and increase the strength by following the recommendations presented here:
- Mould and Material Adjustments: Increasing mould or material temperature can improve flow and supports better entanglement of the poylmers at the weld position.
- Gate Placement: Strategic positioning of gates can influence the location and appearance of weld lines. Changing the gate position can results in moving the weldline to a more hidden position. filling simulations such as MoldFlow can support you to find the optimum gate location even before making the tool.
- Part Design: Modifying wall thickness or creating a single flow path can help to eliminate weld lines. Changing the wall thickness will shorten or elongate filling time and thus moving the weld line.
- Runner System Optimization: Reducing runner dimensions allows you to increase melt temperature and prevent weld line formation due to better polymer entanglement (see number 1).
- Injection Speed: Increasing injection speed will lower the filling time, prevent a too early cooling of the melt and potentially reduce as well as move weld lines.
- Material Selection: If possible, considering a lower viscosity or lower melting point polymer can enhance flow properties.
- Post-Moulding Machining: For critical areas, machining of the desired part features such as a hole after moulding, can be used to remove weld lines entirely.
Figure 1 summarizes the aforementioned points.
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| Figure 1: Overview of the considerations for minimizing, and/ or moving weld lines, as well as increase weld line strength. |
While these methods can be effective, it is essential to consider the potential impact on other product characteristics and manufacturing costs.
Also, before implementing any of these techniques, consulting with a plastics expert helps to avoid unintended consequences.
Check out my other rule of thumb posts here.
Thanks for reading and #findoutaboutplastics
Greetings,
Herwig
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