Gravimetrics aid fluff/pellet metering.

An in-line scrap recycling system for film extrusion lines that uses a "fluff" feeder to control the scrap ratio gravimetrically has been developed by Process Control Corp., Atlanta. When combined with the company's Gravitrol system, which gravimetrically controls the virgin feed rate, the new Gravifluff system is said to provide accurate control of the scrap-to-virgin ratio in both monoextrusion and coextrusion lines.

Conventional in-line film-scrap recyling systems grind various forms of scrap and then proportion the scrap feed volumetrically at the extruder throat to control the scrap-to-virgin ratio. These systems have been an accepted way of recycling edge trim, bleed trim, roll scrap, and loose scrap back into the extruder without repelletizing.

One limitation of putting fluff back into an extruder with conventional in-line recylcing systems is that the fluff is an unknown quantity and cannot be calculated precisely. Therefore, accurate gravitmetric control of the extruder screw speed to maintain desired output cannot be achieved, particularly with variable quantities of fluff from roll scrap or loose film. By using loss-in-weight monitoring to control and feed fluff to the extruder throat in the desired scrap-to-virgin ratio, the Gravifluff combines in-line recylcing of fluff with gravimetric extrusion control. Following are the first complete details on the machine, which was quietly introduced at NPE in June (PT, Aug. '91, p. 88).

HOW IT WORKS

The Gravifluff system is composed of a fluff surge hopper, gravimetric fluff metering hopper, RMT refeed machine, and virgin weight hopper. A trim-removal system removes edge and bleed trims from the line and conveys them in ribbon form to the grinder. At the grinder, roll scrap may also be introduced through a roll feeder. Next, the fluss is conveyed by blower to the fluff surge hopper through a screen that determines maximum particle size.

The fluff is blown into a cyclone mounted atop the fluff surge hopper, which agitates and feeds the ground scrap into the fluff metering hopper below it. The fluff surge hopper is mounted on a load cell that uses weight setpoints to automatically turn the roll feeder on and off, thus maintaining an adequate supply of ground scrap to the metering hopper. An auger at the bottom of the surge hopper delivers a specified amount of scrap as required by the fluff metering hopper.

The fluff metering hopper, which also is continuously agitated, uses the weight-loss principle to meter ground scrap in correct proportion to the refeed machine. For example, if the total extruder rate is 1000 lb/hr and the desired scrap-to-virgin ratio is 20%, then the gravimetric fluff metering hopper will control the fluff feed rate of 200 lb/hr. When the weight in the metering hopper drops below a specified "load weight," it signals the surge hopper above to supply a fresh load of scrap.

Fluff and virgin pellets are fed into an RMT refeed machine mounted directly on top of the extruder throat. The refeed machine uses a dual-hopper design to segregate pellets and scrap. Fluff is contained in an inner compartment surrounded by an outer one containing virgin pellets. Both materials are kept separate until just above the extruder throat.

A starve-fed fluff auger in the refeed machine meters the scrap from the inner hopper into the extruder throat. Virgin material is fed into the outer compartment of the refeed machine from a virgin weight hopper that continuously monitors the extruder's use of virgin material. Just above the extruder throat, pellets surround the outlet of the inner (fluff) hopper, so that the weight of the pellets is directed inward, slightly compacing the scrap. A "sausage" of virgin pellets surrounding compacted fluff reportedly feeds in a consistent manner.

Gravifluff controls that regulate the scrap metering rate also monitor the actual extruder rate by receiving signals from the virgin weigh hopper. Actual rate is compared with the required virgin rate, and extruder screw speed is adjusted by the controller to maintain the desired virgin rate. This way, both virgin and scrap rates are controlled. Accuracy is said to be better than +/-1% for controlling fluff and +\-0.5% for virgin pellets.

So far, Process Control has supplied three Gravifluff systems for both monoextrusion and coextrusion lines. Price can vary widely with the application, but starts at about $35,000 for the fluff-handling components. (CIRCLE 3)

At least two other supplier offer gravimetric fluff/pellet proportioning systems: HydReclaim Corp., Fenton, Mich., offers its Model 470 Integrated Fluff/Pellet Weight/Loss Blending system; and Brampton Engineering Inc., Brampton, Ontario (represented by Polynetics Inc., Atlanta), sells a system that gravimetrically blends pellets and separately weighs and meters fluff before feeding back into the extruder. (CIRCLE 4)