In injection moulding, weld lines (knit lines) form when two melt fronts meet. If the melt fronts do not coalesce completely, at best there will be a cosmetic flaw. At worst there will be a mechanical weak-spot, with strengths of the order of 10 – 90% of the material potential.
Intuitively moulders can improve the situation by increasing melt temperature, injection speed, pressure hold time and mould temperature. However, the problem should have been anticipated and addressed at an earlier stage. At the part design/mould design stage, diverging melt fronts should be avoided but, if unavoidable, recombination should occur as close to the gate as possible. If a weld line problem is anticipated, thin sections should be avoided and gates should have generous dimensions. This gives best filling conditions. Mould venting, to allow air and volatiles between the two melt fronts to escape, can also help.
Using moulding simulation software, part design and gate positions can be modified to ensure the inevitable weld line is in a less obvious and less stressed position. The big breakthrough for Moldflow in the very early days was in successfully predicting how adjustment of wall thickness in a food mixer casing would move the weld line to a less obvious position.
Although accepted wisdom is to increase the temperature of the melt at the weld, the real problem is not so much the absolute temperature but the temperature differential between the two converging melt fronts, as a result of different cooling patterns in the two path lengths. Simulation guidelines suggest the differential should be less than 10 degrees Celsius for acceptable weld lines. It is also worth remembering that the mechanical weakness is associated with the lack of cohesion where the two ‘frozen layers’ meet. As a last resort, moulders are now applying localised induction coil heating to improve weld lines cosmetically and for strength.
Amorphous thermoplastics tend to give stronger weld lines than semi-crystalline. However, in fibre filled grades, orientation of glass fibres in the weld area can be a further source of weakness.