Abstract: A waste recovery method for producing essentially pure thermoplastic material from waste thermoplastic material containing impurities is disclosed to be comprised of an extruder, primary and secondary filtes, and a pelletizer. In addition, a plurality of pressure sensors are provided at several points within the system for measuring the pressure of a flow of molten thermoplastic material therethrough. A microcomputer is coupled to the pressure sensors for determining the pressure sensed by each, the microcomputer further being coupled to a plotter for plotting the pressures. An operator, by observing a rise in the plotted pressures, is enabled to activate a filter changing system in order to replace a clogged filter with a clean filter when the pressure equals a predetermined critical pressure point indicative of a filter being substantially clogged with impurities.