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 . thus , the impurities held by a filter are not dislodged and forced to pass through the filter due to the pressure exceeding the critical pressure point .