SHORTLY AFTER TKOF; THE FLT AND CABIN CREW MEMBERS OF AN MLG SMELLED BURNING PLASTIC AND THE FLT RETURNED TO LAND. PRECEDING THE ACFT'S DEP; A RAMP WORKER HAD NOTIFIED A MECH THAT A SMALL BAG HAD BEEN SUCKED INTO THE APU; BUT THE MECH WAS UNABLE TO FIND ANYTHING DURING AN EXTERNAL INSPECTION. EXAMINATION OF THE APU INLET SCREEN AFTER THE ACFT RETURNED; REVEALED THAT 3 PLASTIC BAGS HAD BEEN SUCKED INTO THE INLET.

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PRIOR TO PUSHBACK THE A&P MECH SAID THAT A RAMP WORKER TOLD HIM THAT SHE THOUGHT A SMALL PLASTIC BAG WAS SUCKED INTO THE APU. WE LOOKED AT THE GAUGES AND ALL APU OPERATING INDICATIONS WERE NORMAL. THE MECH MADE AN EXTERNAL INSPECTION OF THE APU WITH HIS FLASHLIGHT AND SAW NOTHING. AFTER PUSHBACK AND START; I TURNED OFF THE APU. SO AFTER THE 1 MIN AUTO DELAY; IT SHUT DOWN. THIS WAS 30 SECONDS PRIOR TO TKOF. APPROX 1 MIN AFTER STARTING TKOF AND DURING CLEANUP; I TURNED TO THE FO AND SAID; 'DOES THAT SMELL LIKE SOMETHING BURNING?' HE SAID; 'OH YES; IT DOES.' IMMEDIATELY THE INTERPHONE RANG; THE FLT ATTENDANTS SMELLED THE STRONG ODOR OF BURNING PLASTIC IN THE CABIN. THE FO AND I DECIDED TO MAKE AN IMMEDIATE RETURN FOR LNDG. I HAD THE FO COORDINATE WITH COMPANY; RUN CHKLISTS AND INFORM FLT ATTENDANTS. I FLEW THE AIRPLANE ON AUTOPLT AND TALKED TO DEP CTL AND THE TWR. AFTER A NORMAL APCH AND LNDG WE TAXIED IN WITHOUT STARTING APU. WE KEPT THE R ENG RUNNING FOR ELECTRICAL AND VENTILATION; AS AMBIENT WAS 86 DEGS F WITH A FAIRLY FULL PAX LOAD. THE MECH CONNECTED ME WITH MAINT CTL WHO HAD US TRY EACH AIR CONDITIONING SYS SEPARATELY. THE AIR WAS CLEAN THROUGH EACH SYS. THE MECH ACCESSED THE APU INLET AND FOUND 3 LARGE PLASTIC BAGS STUCK IN AGAINST THE APU INLET SCREEN. DETERMINATION: WHEN THE APU SHUT DOWN; THE LACK OF INLET SUCTION ALLOWED THE PLASTIC BAGS TO RELAX AWAY FROM THE INLET SCREEN; BRUSH AGAINST SOME HOT AREAS; AND THE 'HOT PLASTIC AIR' WAS SUCKED INTO THE 2 AIR CONDITIONING SYS; ALL HAPPENING RIGHT AT TKOF. WITH NO IMMINENT FIRE DANGER; THE FLT WAS REDISPATCHED. LESSON: SOMETHING SUCKS INTO THE APU; MAKE AN INSPECTION. DISCUSSION; POSTFLT; WITH MAINT CTLR REVEALED THAT THINGS FREQUENTLY GET SUCKED INTO APU INLET; LIKE RAMPERS' CAPS; ETC. THIS IS THE VERY FIRST I'VE HEARD OF IT.