Weld lines are a fact of life in thermoplastics; all we can do is move them out of harms way. They have two undesirable effects: they are visible and they lower the mechanical performance of a component.

Compared to many other thermoplastics, Polyamide 66 has excellent weld line strength. Unreinforced materials retain over 90% of their original strength value, whereas reinforced grades drop to about 100 Mpa, independent of the glass fibre content. To ensure maximum weld line strength, the following factors should be taken into account:

• Ensure excellent venting at the end of the weld line
• Maximize effective holding pressure; for instance make sure the weld line does not occur in a thin section far away from the injection point
• Ensure the melt is hot when the flow fronts meet

Again, with careful consideration and planning in advance the effect of weld lines can be minimized.