Size Reduction Fundamentals
For plastics processors, the primary goal of size reduction is to convert plastic scrap back into a size that matches the new or virgin material being processed. Reprocessed scrap typically comes in the form of sprues, runners, edge trim, or nonconforming parts. By reclaiming the material, the resin reprocesses efficiently and with the proper characteristics, such as strength and optical qualities.
Common processes that lend themselves to reuse of recycled resins include injection molding, extrusion, thermoforming, blow molding and general recycling.
Common Materials include:
The size reduction equipment most commonly used by plastics processors are:
Granulators reprocess materials into a size and density similar to the virgin resin so that it can be directly into processes such as injection molding, extrusion, blow molding, and thermoforming. They typically can generate particles from 1/8” to 3/4” in size, though most granulate runs between 1/4” and 3/8”. They can reprocess process scrap, small to medium size parts, and shredded material. Granulators are most commonly found beside the press, but there are also central systems and “under the press” units primarily used in thermoforming applications.
Shredders are used to rip large parts into smaller “chunks” of about 2-4” in size. This can be a cost-effective alternative to cutting up parts manually. Shredders are often used in commercial recycling, because they are effective on a wide range of materials. They are increasingly being used by processors to reduce large parts to a size that can be fed into a granulator. Ninety percent of shredders have a single shaft configuration, but 2- and 4-shaft models are also available. Some models combine a shredder and a granulator into a single unit which is shown here.
How Granulators and Shredders Work
There are multiple ways for size reduction equipment to work:
Shearing — This action involves the actual cutting of material. As in scissors, shearing efficiency depends on the sharpness of the cutting edges working against each other and the tolerance of the space between them. Granulators primarily work by shearing material.
Tearing — Tearing involves pulling the material with such force that it comes apart. Some materials like fabric, soft metals, plastics, and tires, are more easily torn than others. Purpose-built tearing reducers are good for reducing mixed waste where small, uniform particle size is not important.
Fracturing — Some materials are brittle, such as glass, hard plastics, and certain metals, and tend to be broken or shattered in a shredder when the cutters aren't sharp or are loose. Unlike tearing, when something breaks it releases energy explosively, sometimes propelling the shards upwards into the faces of the fascinated onlookers. Always wear eye protection.
All three actions, shearing, tearing, and fracturing, are present when size reduction equipment is used. However, when the cutters are kept sharp and the tolerances tight, the dominant and most efficient reduction action should be shearing.
Each type of material is best reduced by a certain type and configuration of granulator or shredder. Different materials have their own physical characteristics which determine how they will react to the reduction process.
Materials types can be broken down into two major categories:
Ductile Materials — Ductile materials are not easily fractured but tend to tear into long strips. They are best reduced by shearing to ensure small particle size. Examples are cloth, rubber, soft plastics, paper, cardboard, or soft metals.
Friable Materials — These are materials that are easily fractured (the opposite of ductile) or broken into shards. Examples are stone, glass, cast metals, hard plastics, or wood. Shredded friable materials tend to come out as small pieces rather than the long strips.