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Reprocessing
55 Reprocessing Extrusion reprocessing is utilized in most operations to minimize the scrap generated and to recapture the material value. Regardless of the extrusion process, it is quite common to grind up scrap materials using a granulator or other appropriate equipment to make granules that can be fed back into the extruder. In compounding operations, the start-up material is granulated and fed back into the extruder in some predetermined level to augment the raw materials while reducing the scrap that has to be disposed. Experimental work is required to determine the percent regrind that can be added into the product without affecting the final properties. Another compounding option is to take products that do not meet customer specifications because of color or some other appearance problem, grind up the product, add black concentrate, and reextrude into a black product. Many colors can be added to the extruder with black concentrate to produce a black product that meets all the customer specifications. Regrind and off-specification materials (for color) can be mixed with black concentrate in a ribbon blender, continuous mixer, tumble blender, and so forth, to produce a uniform mixture that is extruded into a black product. Recycled or reprocessed material can come from edge trim in sheet or film operations and flash from blow molding, off-spec products, and start-up material as the extrusion process is being brought to equilibrium. Scrap can be generated as postconsumer waste (waste generated by the consumer) and postindustrial waste (waste generated by plastic companies). Regardless of the source, scrap material, if properly used, can be a cheaper raw materials source. Reusing scrap generated in an extrusion facility lowers the overall extruded product raw materials costs. However, one must realize that an operation that generates no scrap or reclaim will have the lowest raw materials cost. It costs money to recover and reprocess scrap. The size of the part or the material to be shredded or granulated determines the size and type of equipment required to recycle or reprocess material. In sheet and film extrusion, the edge trim can be removed pneumatically, if the sheet is both thin and flexible enough to be drawn away from the product. After removal, the sheet or film is either blown by air or sucked by vacuum to a shredder or chipper that cuts the film into small pieces, approximately 0.25 ✕ 0.25 inch (6.4 ✕ 6.4 mm). The chips are conveyed to the extruder feed throat, where they are mixed back into the product with the virgin material. Typical regrind concentrations range from 10–30%, with the correct ratio for a particular application determined experimentally. Regrind can affect the property profile and the processing due to viscosity and thermal stability changes. Experimentation determines
what concentration can be added to the virgin resin without losing properties while maintaining the same processing characteristics. In a coextrusion operation, the edge trim may contain 7–11 layers with 5–7 different materials. After grinding, the edge trim may be fed back as a center layer to obtain the desired product thickness. Thick sheet is fed back to a web or sheet granulator that is specifically designed to granulate the product into a chip or granular form that can be fed back into the extruder. The feed throat may be equipped with feed rollers that pull the product into the granulator at a controlled speed in a continuous process. The residual granules are pneumatically or mechanically conveyed with augers to the desired location for reprocessing. Depending on the product, the granulation may occur in a separate building where scrap is segregated and classified to be combined in a specific recipe later for feedstock into a particular product at a predefined rate. Multiple granulators or shredders may be dedicated to a particular product to prevent cross-contamination. Unwind stations for off-grade film (blown or cast), foamed sheet, or thin-gauge sheet are capable of unwinding rolls 10–20 feet wide. Once the ends of the rolls are fed into a continuous unwind operation, the film is pulled at a constant rate into a shredder, where the product is cut into small pieces (0.25 to 0.50 inch squares [6.4 to 12.8 mm]). Once again the product is either packed directly in boxes and taken to some other part of the plant for processing, or it is mechanically or pneumatically conveyed to the extruder where it is used as 100% regrind or mixed with virgin resin. The end-use requirements determine whether the product can be used as is or has to be mixed with other resin. In pipe, profile, and large-part blow, the start-up material and out-of-specification products are transported to another location in the plant where the profiles are fed manually or automatically into a large granulator to produce granules that can be fed back to the extruder. Normally the granulating is done in another building or another room to segregate the product, to minimize the noise associated with the granulation procedure on the rest of the plant, and to minimize dust and crosscontamination. Granulators are available with large feed throats and high horsepower to devour parts. If flexible tubing is being ground, a shredder may be necessary to properly cut up the part. Wire and cable provides specific challenges due to the wire or cable and the plastic. Similar to other extrusion processes, start-up resin that does not include any wire or cable can be ground up. Once the plastic is applied to the wire or cable, special equipment is necessary to strip back the covering and convey it to a shred-
EXTRUSION APPLICATIONS
524
ding process to provide the correct particle size to feed back into the extruder. In this operation, the wire or cable is more valuable than the plastic, and special efforts are used to save the wire or cable so it can be reprocessed. Laminated products present special challenges in using regrind or off-specification product. If all the components in the laminate are compatible, the laminate can be granulated and fed back into the process to produce its own special layer. Alternatively, the ground-up laminate can be fed to a compounding extruder, where the product is melted and homogenized into a useful plastic that can be sold as pellets or extruded directly into another application that meets the physical property requirements of the new compound. If the components in the laminate are not compatible, the challenge of finding a home for the product becomes greater. Compounding inhomogeneous resins or materials does not normally produce a product with a good physical property profile that can be used in other applications. One alternative is to incorporate the material in a center layer in a coextruded structure. Another alternative is to see if the product can be sold as is to a customer willing to take off-specification
or B-grade material. Reextrusion or compounding of recycle to make a new product may require adding heat stabilizers, UV stabilizers, flame retardants, impact modifiers, or other additives to improve the product performance while producing a value-added product. Munchy Ltd.[1] has a patented process to reprocess most thermoplastic products into pellets. The system can handle most waste streams, such as edge trim, flakes, film, bags, and so forth. The reprocessed plastics can be fed back into the main extrusion process at low percentage if the product meets all the customer specifications. Other applications include high melt flow resins used in nonwoven applications. The system can process nonwoven, melt blown, and spun bonded PP, feeding the products directly into the system and converting the product to pellets for reuse in the nonwoven process feed stream.
SUPPLIER 1. Munchy Ltd, Avtech House, Hithercroft Road, Wallingford, England OX10 9DA.
Review Questions 1. What is the advantage of reprocessing plastic in your facility? 2. What are some ways to recycle film? 3. Why might it be necessary to have the granulating operation in a separate room in the plant? 4. What are the different internal wastes that may be generated in an extrusion operation? 5. What is the difference between postindustrial and postconsumer waste? 6. What is the best way to recycle pipe, profile, blow molded parts, and thick sheet? 7. What is the best way to recycle coextruded parts? 8. What is the difficulty in recycling wire coating?
what concentration can be added to the virgin resin without losing properties while maintaining the same processing characteristics. In a coextrusion operation, the edge trim may contain 7–11 layers with 5–7 different materials. After grinding, the edge trim may be fed back as a center layer to obtain the desired product thickness. Thick sheet is fed back to a web or sheet granulator that is specifically designed to granulate the product into a chip or granular form that can be fed back into the extruder. The feed throat may be equipped with feed rollers that pull the product into the granulator at a controlled speed in a continuous process. The residual granules are pneumatically or mechanically conveyed with augers to the desired location for reprocessing. Depending on the product, the granulation may occur in a separate building where scrap is segregated and classified to be combined in a specific recipe later for feedstock into a particular product at a predefined rate. Multiple granulators or shredders may be dedicated to a particular product to prevent cross-contamination. Unwind stations for off-grade film (blown or cast), foamed sheet, or thin-gauge sheet are capable of unwinding rolls 10–20 feet wide. Once the ends of the rolls are fed into a continuous unwind operation, the film is pulled at a constant rate into a shredder, where the product is cut into small pieces (0.25 to 0.50 inch squares [6.4 to 12.8 mm]). Once again the product is either packed directly in boxes and taken to some other part of the plant for processing, or it is mechanically or pneumatically conveyed to the extruder where it is used as 100% regrind or mixed with virgin resin. The end-use requirements determine whether the product can be used as is or has to be mixed with other resin. In pipe, profile, and large-part blow, the start-up material and out-of-specification products are transported to another location in the plant where the profiles are fed manually or automatically into a large granulator to produce granules that can be fed back to the extruder. Normally the granulating is done in another building or another room to segregate the product, to minimize the noise associated with the granulation procedure on the rest of the plant, and to minimize dust and crosscontamination. Granulators are available with large feed throats and high horsepower to devour parts. If flexible tubing is being ground, a shredder may be necessary to properly cut up the part. Wire and cable provides specific challenges due to the wire or cable and the plastic. Similar to other extrusion processes, start-up resin that does not include any wire or cable can be ground up. Once the plastic is applied to the wire or cable, special equipment is necessary to strip back the covering and convey it to a shred-
EXTRUSION APPLICATIONS
524
ding process to provide the correct particle size to feed back into the extruder. In this operation, the wire or cable is more valuable than the plastic, and special efforts are used to save the wire or cable so it can be reprocessed. Laminated products present special challenges in using regrind or off-specification product. If all the components in the laminate are compatible, the laminate can be granulated and fed back into the process to produce its own special layer. Alternatively, the ground-up laminate can be fed to a compounding extruder, where the product is melted and homogenized into a useful plastic that can be sold as pellets or extruded directly into another application that meets the physical property requirements of the new compound. If the components in the laminate are not compatible, the challenge of finding a home for the product becomes greater. Compounding inhomogeneous resins or materials does not normally produce a product with a good physical property profile that can be used in other applications. One alternative is to incorporate the material in a center layer in a coextruded structure. Another alternative is to see if the product can be sold as is to a customer willing to take off-specification
or B-grade material. Reextrusion or compounding of recycle to make a new product may require adding heat stabilizers, UV stabilizers, flame retardants, impact modifiers, or other additives to improve the product performance while producing a value-added product. Munchy Ltd.[1] has a patented process to reprocess most thermoplastic products into pellets. The system can handle most waste streams, such as edge trim, flakes, film, bags, and so forth. The reprocessed plastics can be fed back into the main extrusion process at low percentage if the product meets all the customer specifications. Other applications include high melt flow resins used in nonwoven applications. The system can process nonwoven, melt blown, and spun bonded PP, feeding the products directly into the system and converting the product to pellets for reuse in the nonwoven process feed stream.
SUPPLIER 1. Munchy Ltd, Avtech House, Hithercroft Road, Wallingford, England OX10 9DA.
Review Questions 1. What is the advantage of reprocessing plastic in your facility? 2. What are some ways to recycle film? 3. Why might it be necessary to have the granulating operation in a separate room in the plant? 4. What are the different internal wastes that may be generated in an extrusion operation? 5. What is the difference between postindustrial and postconsumer waste? 6. What is the best way to recycle pipe, profile, blow molded parts, and thick sheet? 7. What is the best way to recycle coextruded parts? 8. What is the difficulty in recycling wire coating?