To achieve optimum weld strength, it is important to ensure that the parent sheet or pipe and filler rod are the same material and colour and preferably come from the same manufacturer. Otherwise, the melt flow index of the base plastic and the welding rod may be different and they may melt at different temperatures. The melt flow index of the polymer is dependent on the mix, which will vary from manufacturer to manufacturer. Colourants, fillers, plastisizers etc, will vary in their respective quantities. This has a direct bearing on the melt flow index, therefore, wherever possible, like for like should be used.
In terms of rod profile, a triangular section weld rod will give a weld of acceptable strength with reasonable appearance. Round section should always be used where joint strength is the important factor; either using several runs or, if more applicable, use an extrusion weld rather than a hot gas welded joint.
See further information about plastic welding and testing.