Weld lines are formed during the molding process when the melt flow front divides and then flows back together. Typically, the weld line interface is resin rich because the glass fibers tend not to cross the interface. The lack of glass fiber reinforcement across the interface results in lower mechanical strength along the weld line. Gate location and fill patterns should be planned so that weld lines will be eliminated or located in areas of minimal stress whenever possible. If weld lines must bear stress, the part design should compensate for the typical weld line strengths indicated below. Weld line strength is highly dependent on molding conditions, so the part and tool design should allow for rapid injection, a hot flow front, and thorough packing. Gas entrapment is very detrimental to weld line strength, so molds must be designed to avoid back filling and should be adequately vented in areas where weld lines form.