Plastic processes differ greatly in both the way they form plastic products and in the shape and structural integrity of the products they manufacture. Blow molding, for example, is nearly the sole method by which plastic bottles for beverages, household cleaners and cosmetic products are made, as well as a range of low-cost toys and parts with low to medium performance. The extrusion process creates linear profiles, strips and sheets with high structural integrity useful in many construction siding, trim and furniture applications. Vacuum forming, or thermoforming, forms trays, linings and thin plastic packaging from heated plastic sheets, while dip molding forms a variety of soft molds and plastic coatings. Other plastic molding processes use casts, or dies; injection molding squeezes melted (or “plasticized”) plastic resin into a closed mold, forming a solid plastic object, while rotational molding uses a type of gyroscope to evenly coat the inside of a mold, creating hollow parts with high impact resistance.
Plastic molding processes vary greatly in cost. High end plastic molding processes, such as rotational molding and injection molding, provide precision three dimensional plastic parts with structural integrity and impact resistance few other processes or materials can provide. On the other end of the spectrum, blow molding and dip molding processes offer very affordable options for long runs and mass production of containers and household commodity items. A wide range of plastic materials are molded through these processes, although some processes are more effective with certain polymers than others.
This is one of the most common forms of plastic molding, and the process can range widely in cost, depending on the complexity of the part being molded and the materials which are used. Injection molding produces three dimensional, solid parts with mid to high strength and is unique in plastic molding processes, as it can produce relatively complex shapes. Advanced injection molding techniques include insert molding and reaction injection molding (RIM); insert molding dies contain a solid object, such as an electric coil, around which the molten plastic is injected, creating an encapsulated object. Reaction injection molding combines a liquid resin thermoset polymer (typically polyurethane) with liquid polyisocyanate, which acts as a reagent within the mold, causing the polymers to expand and form bubbles (either open or closed cell foam), filling the mold.
Injection molding machines consist of three main sections: the hopper, where plastic resin pellets are fed into the machines; the extruding barrel, containing a small screw conveyor which simultaneously “plasticizes”, or melts, the plastic resin while moving it towards the die; and the die, or closed mold, into which the melted plastic is injected under pressure. The plasticizing of the resin is often assisted by electric band heaters and cartridge heaters wrapped around the extruding barrel.
Plastic extrusions are formed similarly to the way injection molded plastics are formed, although extrusions are formed through an open die. Plastic resins such as PVC, acrylic, polypropylene or ABS are fed through a hopper into the extruding barrel, which shears and melts the resin, pushing it through the open die to form a profile or shape. This profile is immediately immersed in cold water to set the plastic; the profile is extruded continuously, passing through the die, through cold water tanks and onto a sawing table, where pre-specified lengths are cut.
Extruded plastics may be manufactured to meet the specifications for a broad range of niche applications, including but not limited to building trim, window and door sealants, vinyl siding, PVC pipe and surgical tubing. Plastic sheets and flexible plastic sheet rolls are also formed through extrusion, being pulled and stretched instead of cooled after the melted plastic exits the die; flexible plastic sheets are rolled through a series of large calenders before being rolled onto a tube.
Numerous products are made from blow molding. Any consumer item that has a three-dimensional shape and is hollow, such as tanks and CD cases, is manufactured using the blow molding process. Blow molded products are capable of holding a variety of substances such as herbicides, pesticides, cosmetics, and automotive oil. The plastic utilized for these processes are all thermoplastic resins. They include acetal, polysulfone, polyamide, polystyrene, butadiene styrene, Barex, polyvinyl chrloride (PVC), and high and low density polycarbonate.
Plastic blow molding can be categorized into three types: injection blow molding, extrusion blow molding, and stretch blow molding. All of these processes consist of two steps, which vary the most in the early stages. The ultimate shape of the blow molded plastic depends on the shape of the mold cavity. While blow molded products come in an assortment of standard shapes and sizes, there are some products that can be made using custom blow molding, and are thus used for special applications.
Dip molding plastic is one of the simplest means of molding plastic and, like blow molding, is capable of producing a large number of parts or products at low cost. The dip molding process serves in one of two manufacturing capacities: to create whole flexible or rigid products such as rubber gloves, condoms and plastic caps; or to coat pre-manufactured products such as wire racks, wire cable and plastic coated handles.
Both manufacturing methods use the same basic process: a large volume of glass or metal molds are surface prepped, so that the final product may be easily stripped from the mold when dry. Objects which are to be coated in plastic are buffed and prepped so that the plastic coating stays firmly on. The molds are then dipped into a vat of molten polymer or elastomer resin; depending on the desired thickness and the consistency of the polymer material being used, molds are often dipped multiple times, sometimes alternating between cold water or another solidifying reagent and the molten polymer vat. Flexible products with complex layers, such as kitchen gloves, may have other layers, such as a fabric layer, applied in between polymer coats. Once the molten plastic has set, the plastic product is ready to be removed from the mold, trimmed, cleaned and finished.
Polyurethane is a material that is valued for its uses in “memory foam” products due to its flexibility and rigidity. Polyurethane is also a valuable material for products such as solid plastic forms, polyurethane rods, urethane wheels, urethane brushings, and urethane sheets. Polyurethane moldings have an excellent reputation for their high performance. Their longevity is greater than that of plastic, and are more impact-resistant than rubber. It also has elastic memory, reduces noise, and is resistant to heat and chemicals. It possesses many of the good qualities of metal, rubber, and plastics, and is capable of forming strong adhesive bonds with most plastics and metals.
Polyurethane molded parts require little to no additional finishing. This process is used by manufacturers to fabricate a wide array of moldings, parts, bowling balls, urethane bumpers, polyurethane belts, conveyor bushings, electrical potting compounds, press tool blocks, and pneumatic seals. Polyurethane products are used extensively in industries such as athletic equipment, engineering, manufacturing, industrial, food processing, automotive, and construction.
Rotational plastic molding is capable of achieving plastic parts with more strength and structural complexity than any other plastic molding processes. Unlike other plastic molding methods, rotational molding produces a relatively low volume of parts in what are typically short runs, due to the amount of time and equipment required for rotational molding. Rotationally molded plastics may not be mass produced like blow molded, dip molded or thermoformed parts, and this process is typically reserved for complex or high-performance parts such as plastic figurines and military-spec rackmount carrying cases.
The rotational molding process is unique among plastic molding techniques. A molten polymer – often polyethylene or polycarbonate – is poured into a closed three dimensional mold; the mold is then placed within a gyroscope-like structure which slowly spins, evenly coating the entire interior of the mold with the polymer to form a hollow shape. This process is not only more effective at creating complex three dimensional structures, but it also creates parts with uniform material consistency and, consequently, very high strength.
Vacuum formed plastics are used as faceplates and semi-flat components in a wide range of industries, particularly in electronic equipment such as fax machines, keyboards, phones and home appliances. Also known as thermoforming, or pressure forming, the vacuum forming process begins with stock plastic sheets rather than polymer resin pellets; these sheets are heated until the polymer reaches a flexible temperature, then they are vacuumed into an open mold, causing the heated sheets to “thermoform” to the exact shape of the die mold beneath. Twin sheet thermoforming is commonly used to create large and precision application parts such as hot tubs and interior wall panels for aerospace, but thermoforming is also a highly cost-effective means of producing three-dimensional plastic packaging. Blister packs, clamshells, plastic covers, plastic trays and other plastic packaging can be produced for low costs at high runs by vacuum forming.
In the creation of fiberglass-reinforced plastic products, fiberglass molding is the most frequently used process. Fiberglass is made when molten glass is extruded through very fine openings in a tool. This extrusion process produces threadlike formations in the glass that are later put through heat treatment or pressing and mixed with plastic resin.
Fiberglass molding is used to fabricate a multitude of products such as machinery support products, electric circuit boards, shells of racing cars, and panels. This process is used extensively because fiberglass does not shrink or expand with changes in temperature. Furthermore, it does not absorb water, is resistant to chemicals, has high strength-to-weight ratio, is not flammable, and can function as an insulator from electricity.