Plastic recycling is a well-established industry that globally processes and resells several million tons of used plastic material each year. Rather than operating merely out of environmental necessity, plastic material recovery services can be both cost-efficient and productive, saving resources for a range of different manufacturing applications. The recycling and reclamation field also encompasses the industrial processes by which plastic materials are separated into their base monomers and made available for further polymerization at secondary and tertiary levels.
While the plastic recycling industry was originally focused on recovering manufacturing scraps and byproducts left over from initial plastic fabrication, present-day recycling services are capable of reclaiming heterogeneous post-consumer goods as well. The methods employed by material recovery systems tend to vary according to the type of plastic being processed, but there are some essential practices common to most recycling services. Stages such as sorting, cleaning, size reduction, separation, and pelletizing can be found in most plastic recycling operations. Likewise, the machinery used to achieve these processes generally falls into a handful of equipment categories.
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Sorting and grouping plastic materials according to resin type is an important first step in the recycling process because contamination can render a batch of material un-reusable. The most frequently recycled resins, including polyethylene terephthalate (PET), high-density polyethylene (HDPE), and polyvinyl chloride (PVC), must be carefully separated from one another in order to enable further processing. Contaminants within each type of plastic must also be removed from the base resin to ensure stock purity.
Sorting machines must rapidly identify and categorize large volumes of post-consumer plastic, often under continuous input. Although there are varying degrees of technical sophistication and capacity, an advanced sorting machine can be equipped with some or all of the following features:
• Sensors: These devices detect specific polymers within a mixed stream of plastic materials. They can be equipped with x-ray or infrared sensing that registers a polymer’s unique signature along the spectrum. Some sensors also incorporate color detection technology that sorts material according to tint and transparency.
• Ejectors: Mechanical or precision air ejection units physically group different plastic materials according to resin types. Depending on their capacity, ejectors can often handle very high rates of input.
• Computing Systems: Computer processing technology supplies the algorithms that are used to identify and sort different materials. These systems provide the controlling parameters for both sensor and ejector operations.
• User Interfaces: An operator’s interface can provide machine controls and diagnostic tools for technicians. In addition, interfaces can also offer networking abilities to help integrate a sorting machine and make rapid adjustments to its functions.
Plastic materials usually need to be cut into smaller sizes in order to allow further processing and to provide easier packaging, transportation, and distribution of recycled stock. This cutting presents certain challenges, as many plastics are abrasive to metal blades and can have wide variation in their hardness, weight, and thickness. Most standard size reduction is performed by single or multi-shaft shredders, and granulators. Multi-shaft shredders perform scissor-like cutting with a series of rotating blades that can handle moderately dirty or contaminated material, but are somewhat imprecise in the size of the cuts. Single shaft shredders perform more of a tearing motion, and have slower motors that lengthen blade lifespan. They can also handle dirty or abrasive material and usually have adjustable or replaceable blades.
Granulators are composed of a rotor attached to blades that rotate within a chamber containing a grid floor. Their capacity for processing plastic material depends on the speed of the rotor, angle of the cutting blades, spacing of the grid, and the shape of the rotor. Granulators are usually sturdy machines, capable of relatively rapid cutting rates, and the presence of the grid allows for more precise control over the size of cuts. Granulator blades typically need to be replaced regularly over the course of operations.
After the plastic has been cut into smaller pieces, or “flakes,” the stock usually needs to be washed in order to remove lingering dirt or attachments. Paper, glue, sand, and grit are some of the common elements targeted in the washing process, which can be accomplished using water baths, friction washers, or a washing line. The washing line applies a continuous hot spray over a stream of plastic material, removing some or all of the labels and dirt attached to the plastic surface. Detergents and disinfecting agents are often included in this process to improve the level of cleaning.
To reduce the potential for stock contamination most recycled plastic undergoes separation treatments, which work to remove any attachments or non-reusable materials that may be present in a batch of flakes. Most separation processes can be categorized as “wet” or “dry” methods. Float tanks are the most common wet method, separating material based on density and whether it sinks or floats, while hydrocyclones use centrifugal force to divide material according to weight.
Among dry methods, air classification differentiates between types of plastic based on the ratio of flake surface area to mass, meaning thicker materials are sifted away from thin ones. Mechanical separators usually divide flakes according to size, and sometimes shape. These machines can be designed with flat, circular, or inclined configurations. Laser spectral analyzers are most advanced machines that use spectroscopic detection to determine precise levels of contamination in a given batch. Alternative devices can employ ultraviolet or fluorescent light to separate plastic according to color or light absorption levels. Melt separation machines move plastic flakes along a conveyor or hot roller while heat is applied to separate material according to melting point.
Pelletizing reclaimed plastic is the final step in most recycling processes. Converting post-consumer plastic into pellets allows for easier distribution and remanufacturing, and ultimately benefits the speed and effectiveness of reintroducing recycled plastic into industrial manufacturing. After sorting, drawing, separating, and drying the reclaimed material, the flake stock is ready to be extruded into pellets.
Typically, single or double screw extruders are used at this stage. The ratio between the length of the extruder screw and its diameter, as well overall screw design, can vary depending on the type of resin being processed. Single screw extruders rely on pumping action and shear to shape plastic, while double screw extruders perform more of a mixing function with lower shearing force to create a compound material. Ventilation and vacuum pumps may be required to regulate the degassing effects. Once the reclaimed plastic has been pelletized, it is ready for distribution and remanufacturing.
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