weld design for vibration welding must include a relatively large flat surface, although some out of plane curvature can be accommodated for. the most common type of joint is a butt joint, where two flat pieces with the same cross section are welded together. variations on this joint can include u-flanges, tongue and groove joints, and even double tongue and groove joints. when appearances are important, flash traps can be used. flash traps refer to  hollow areas in the cross section next to the weld area that collect the flash and hide it from view.