Materials that are too soft to ultrasonically weld can be made stiffer by the addition of mineral (such as talc) or reinforcement (such as glass). Ten percent seems to be a good starting point for experimentation if this seems to be a possible solution to a problem. Generally, up to about twenty-five to thirty percent reinforcement or filler will imrpove welding results by increasing the stiffness of the material and therefore its sound transmission capability, as well as improving its dimensional stability part-to-part. Beyond thirty percent, the filler or reinforcement will be displacing weldable polymer and probably interfere with welding to a greater degree. Often the surfaces will be enriched, that is to say they will have a greater concentration of filler or reinforcement right where welding needs to occur, creating poorer than expected joint strength. Reinforcement tends to orient parallel to mold steel, and therefore parallel to the joint line in molded or extruded parts. There is almost no hope of getting any of this reinforcement to cross over the joint line and improve the strength of the joint, so in the most perfect of situations the best one could hope for is that the joint strength approaches that of unreinforced base resin. Since manufacturing generally does not occur in the best of all possible situations, the rule of thumb is to expect stregth to be eighty percent or less of the strength of the unreinforced base resin.