PLASTICS
Overview:
"Plastic" is a generic term for a multitude of materials. Now one of the most-used materials in the world, the earliest forms of plastic date back to the mid-19th century. But it wasn't until the 1940's and World War II when shortages of products such as rubber and iron spurred the rapid development of plastic as a substitute for these natural resources. The first decade after the war saw the development of polypropylene and high density polyethylene and the growth of new plastics in many applications.1
Generally, plastic is engineered to have enhanced mechanical properties and often greater durability than other materials. For example, polycarbonate plastic is used to resist impact. Polyamides like nylon resist abrasion. Plastic fibers are plastic that has been spun into filament used to make fabrics, string, ropes, cables and optical fibers. Some of the most recognizable plastic fibers are polyester, nylon and rayon. There are also plastic coatings and plastic adhesives.
Plastic Types and Processing Methods
Plastics tend to be grouped into two general classes: thermoplastics and thermosets.
Thermoplastics are polymers that melt when heated, can be formed or shaped when in that state, and then solidify when cooled. Thermoplastics can be re-melted and essentially returned to their original state. Thermoplastics usually are produced first in a separate process to create small pellets; these pellets then are heated and formed to make other products. Thermoplastics include polyethylene, polypropylene, polyvinyl chloride, polystyrene, nylon, polycarbonate, and others.
Industrial fabricators of plastic products tend to think of plastics as either "commodity" resins or "specialty" resins. Commodity resins are plastics that are produced at high volume and low cost for the most common disposable items and durable goods. They are represented chiefly by polyethylene terephthalate (PET), polypropylene, polyvinyl chloride (PVC), and polystyrene.
Specialty resins are plastics whose properties are tailored to specific applications and that are produced at low volume and higher cost. Among this group are the so-called engineering plastics, which are plastics that can compete with die-cast metals in plumbing, hardware, and automotive applications. Important engineering plastics, less familiar to consumers than the commodity plastics listed above, are polyacetal, polyamide (aka trade name nylon), polytetrafluoroethylene (trademark Teflon), polycarbonate, polyphenylene sulfide, epoxy, and polyetheretherketone.2
Thermosets are a separate class of polymers that can be formed when heated, but unlike thermoplastics, thermosets remain in a permanent state once molded. Thermosets include vulcanized synthetic rubber, acrylics, polyurethanes, melamine, silicone, epoxies, and others. The reaction used to produce
thermosetting plastics is not always heat, sometimes a chemical reaction between specialized materials is sufficient to produce the desired result.3
Handling Characteristics and Challenges:
The conversion from granules, flakes or powders (aka resins) into various shapes typically involves sourcing them into an extruder (thermoplastics) or a mold (thermosets), where they are melted and formed into a continuous profile or a shape. These resins can also have other varying properties, including density and flow characteristics.
If the resin arrives at the processing facility in bulk bags, the frames used to discharge the bags should be equipped with additional accessories designed to facilitate the transfer of the material from the bulk bag to either a conveyor or directly into an inlet on the extruder or mold. These devices can elongate and stretch the bags, which promotes a better flow and removes any pockets of resin cornered in the bags. Some of these bag activating devices offer a dust tight seal between the bulk bag spout and the receiving vessel.
If the resin is being discharged from a bulk bag into conveying equipment before reaching an extruder or a mold, it will first flow into a receiving hopper. As the resin fills the hopper, the air inside that vessel is forced out. Unless this air passes through a filter, airborne dust particles can escape into the surrounding atmosphere. A dust collector mounted on the discharger frame will contain the dust inside the conveyance system, lowering the risk of potentially dangerous dusting and reducing the time necessary to perform the routine cleaning typically required in most processing environments.
Feed hoppers should be designed with proper geometry and, unless the resin is free flowing, they should incorporate devices such as vibrators to promote flow.
If the resin is packaged in smaller bags, a bag dumping station with a dust hood and filtration devices may be sufficient to support the manual unloading of the material in hoppers. A hopper screen above the receiving vessel will help to prevent the introduction of foreign objects and protect the operators from potentially dangerous conveying equipment. Some plastics processing includes the blending of various powders with the resin prior to extrusion or molding. These powders may add an entirely different dimension of characteristics, including dusting or hygroscopic agglomeration that will need attention.
If a flexible screw is being used to convey the resin, it is important to use a screw that matches the plastic's characteristics and other application requirements. Generally a round screw is necessary for moving resin up an incline.
If the resin is being pneumatically conveyed into a processing system, the blower used to move resin through the air line must be sized to meet the demands of system.
Due to the abrasive nature of some plastic pellets, such as mineral- or glass-filled pellets, care must be taken to avoid excessive use of bends or sweeps in the convey line, as these are prone to wear and failure. The motion of the pelletized resins skidding along the convey line often creates friction and the subsequent heat can melt the pellet surfaces, creating streamers or "angel hair," that can cause downstream quality problems. Deflection elbows can be employed to mitigate the problems of excessive elbow wear and the formation of "snakeskins," or streamers.
Flexicon Applications:
Consultation with a Flexicon specialist will help you decide if a flexible screw or pneumatic solution best fits your plastics application. Flexicon also offers three separate bulk bag discharging options. Flexicon's expert design and engineering staff will consider each parameter and recommend the best material-handling solution for you. Upon request, Flexicon's test lab will simulate your plastic processing application before the system is installed in your plant.
With over 25,000 installations worldwide, Flexicon has a depth and breadth of bulk handling experience, including plastic resins, unequalled by any comparable manufacturer in the world. A Flexicon bag dump system helped contain Titanium Oxide dust during PVC blending. One of the largest plastics processors in Mexico City improved safety and efficiency with a Flexicon bulk bag unloading system with pneumatic and flexible screw conveyors. A Dutch company running eight compounding lines cut labor and improved productivity with Flexicon bulk bag dischargers and flexible screw conveyors. Plant geography issues in a repurposed Rhode Island, USA textile mill was overcome with two Flexicon bulk bag dischargers and flexible screw conveyors feeding an extruder.
Related Articles on Plastics:
1 SPI Plastics Industry Trade Association
2 http://www.britannica.com/science/plastic
3 Plastics Europe
Sources: Where noted. All other information courtesy of Flexicon Corporation.