This invention relates to a method for controlling the introduction of additives, in the form of a plurality of strands, to a cutter. The strands are cut into pellets, and the pellets are fed to an extruder for making thermoplastic shaped articles. The feeding of the strands to the cutter and extruder involves feeding the strands through a metering device and calculating the feed rate, velocity, and delta position for each strand per feed interval in order to obtain the desired amount of additive in the resulting thermoplastic article.