The best filler for plastic is a material that is inert, low-cost, and has a low coefficient of thermal expansion. A good choice for a filler would be glass microspheres because they meet all of these criteria. Glass microspheres are made from high-quality borosilicate glass, which is chemically inert and has a low coefficient of thermal expansion. In addition, glass microspheres are relatively low in cost and are available in a variety of sizes.