Systems for purging process lines of additives for thermoplastic materials

A system for purging and cleaning additive supplied to a thermoplastic melt from process lines, comprising:

• an additive supply tank having input and discharge ports;
• a main additive supply line for supplying additive from the discharge port of the additive supply tank to a thermoplastic melt;
• a recirculation line which fluid-connects said main additive supply line downstream of said discharge port of said additive supply tank to said input port of said additive supply tank to allow a portion of the additive in the main additive supply line to be recirculated to said additive supply tank;
• a recirculation pump disposed in said main additive supply line and having an input side fluid-connected to said output port of said additive supply tank and an output side fluid-connected to said input port of said additive supply tank through said recirculation line;
• a source of purge fluid;
• a control valve which controllably establishes fluid-communication between said source of purge fluid and said main additive supply line upstream of the thermoplastic melt during a purge cycle;
• and
• a control system which (i) operates said recirculation pump during an additive supply cycle so as to force the additive from the output side of the recirculation pump through the main additive supply line to a thermoplastic melt, while continuously recirculating a portion of the additive through the recirculation line to the input port of the additive supply tank, and thereafter (ii) stopping the operation of the recirculation pump and concurrently operating the control valve so as to establish fluid-communication between the source of purge fluid and the main additive supply line so as to back-flush the main additive supply line and the recirculation line with a purge fluid, whereby additive in the main additive supply line and the recirculation line is back-flushed into the additive supply tank through the input and discharge ports, respectively.