Moldflow (or Moldex) is the ideal tool to predict the position(s) of the weld line(s) so that we can anticipate potential mechanical issue and today, unless using non reliable material data (or severe design), the predictions are rather good. if the gate position and design are frozen and if a weld line is critical (with potentially a failure), we might try using a spillover or overflow tab (but only for some specific designs) which will lead to an increase of the weld line strength. A study made some years ago illustrated the technique (and highlight the limitations).
When performing anisotropic mechanical calculations on reinforced polymers, taking explicitly the weld line weakness is a way to better predict the ultimate strength of the structure. In that case,