Screen classifying cutters meet productivity needs of extrusion and thermoforming lines

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Courtesy of Munson Machinery Company, Inc.

A major plastics thermoformer, whose operations include sheet extrusion and calendering, installed 20 screen classifying cutters from Munson Machinery Co. Inc., Utica, NY, to assure efficient granulation and sizing of its process scrap. It is essential that the cutters work to full capacity during production runs and deliver properly-sized granulate for processing. The company specified Munson's SCC model cutters to replace units from another supplier, which had maintenance problems caused by frequent blade changes, and which could not process trim scrap at desired production rates. 

The SCC models have a helical rotor with dozens of tungsten carbide-tipped 'teeth' attached to an array of staggered holders. Munson describes this array as 'interconnected parallelograms.' The teeth and holders create a helical pattern that downsizes scrap against twin bed knives in a steady and even cutting action with each revolution. 

Most conventional granulators have a smaller number of angled blades on their rotors that deliver a scissors-like cut. The SCC design is effective, says Munson engineering manager Dave Munsell, due to the number of teeth on the rotor and because each cutting edge makes full contact with scrap. 'The teeth on a Munson SCC screen classifying cutter nibble away at scrap, creating many small pieces, rather than slicing and re-slicing scrap into smaller strips.' The cutting action also reduces fines and other imperfections in granulate. 

The use of tungsten carbide is said to maintain the sharpness longer than blades of conventional tool steel, thus reducing maintenance and downtime. 

The blades are designed to slide onto each machined holder and be screwed into place. Changeover is fast and easy, Munsell says, and requires minimal downtime. 

The company specified Model SCC 15 screen classifying cutters, each having: 15 in (381 mm) throat width; 30, ½-in (12.7 mm) thick parallelograms each with two carbide tips (60 total); and bed knives of hardened tool steel. Scrap includes edge trim from extruders and a calender ranging in thickness from 0.008 to 0.04 in (8 to 40 mils). This is pneumatically fed to the cutter in a continuous operation that runs 24/7. Each cutter has throughput of 250 lb/h (113 kg/h). Scrap is sized in 180-deg screens and transported pneumatically to a Gaylord, which holds up to 1500 lb (680 kg). 

The screen classifying cutters enable the plant to reduce the size of process scrap in the required capacity, with the uniformity it needs to reprocess the material efficiently, making the units an integral part of the operation.

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