The range of plastic types and applications in the marketplace has led to the need for different processing methods to make plastic products. Each method is tailored to a specific end product, but resins can be used in more than one production process. Each process requires plastic workers who are highly competent in their area of expertise to design, operate, or maintain the tools and machinery safely and efficiently.
Thermoforming refers to using heat and pressure to change a sheet of plastic into any type of plastic part. The sheet of plastic is preheated, then placed in a mould and takes its shape. The method allows for tight specifications, close tolerances, and attention to detail. Thermoforming can be cost-effective and efficient, depending on the product. The most efficient products for thermoforming are parts with features on only one side.
There are three moulds most commonly used in thermoforming: machined aluminum moulds for shallow parts, cast aluminum moulds for parts with large draw ratios, and composite moulds for prototypes and short-run products.
Rotational moulding begins by placing a mould in the moulding machine. A plastic resin, in the form of a finely ground powder, is placed in the mould that is rotated, both vertically and horizontally, while being heated. The resin melts, coating the mould and taking its shape. Once cooled, it is removed from the mould, and the part is finished. The raw materials used are mostly limited to Low-Density Polyethylene (LDPE) and cross-link polyethylene, a very hard material. Some moulds are equipped to handle nylons, polycarbonates, and cryogenically ground polypropylenes for specialized applications.
This process is cost effective as it eliminates the need to create many parts and join them, and changes are easily made to the mould. Consistent wall thickness and strong outside corners are design strengths of rotational moulding. Some rotational-moulded plastics products include agricultural tanks of all kinds, fuel tanks, large outdoor toys, soft vinyl automotive armrests, spherical shapes, and toy balls.
Injection moulding is the most common plastic forming method. It uses a piston or screw to force plastic resin through a heated tube into a mould, where the plastic cools and hardens to the shape of the mould. The mould is opened and the plastic cast removed. Injection-moulding machines consist of two basic parts: an injection unit and a clamping unit. The injection unit melts the resin and injects the melt into the mould. The clamping unit holds the mould together, opens and closes it automatically, and ejects the finished part. The mechanism may have several systems: mechanical, hydraulic or hydromechanical. Some thermoplastic items made by injection moulding include toys, combs, car grills, and various containers.
The advantages of injection moulding are high production rates, design flexibility, repeatability within tolerances, the ability to process a wide range of materials, relatively low labour costs, little or no finishing of parts, and minimum scrap losses. Disadvantages of injection moulding are high initial equipment investment (some moulds run into the millions of dollars!), and high running costs (accurate cost prediction for moulding jobs is difficult).
In the extrusion process, plastic pellets are heated in a long barrel. A rotating screw then forces the heated plastic through a die with an opening of the desired shape; this is similar to a pasta-making or sausage-stuffing machine. As the continuous plastic form emerges from the die opening, it cools and solidifies, and is then cut to the desired length.
Extruded products offer a combination of strength, durability, and flexibility which, in comparison, metal components can’t match. Any number of rigid and flexible components can be combined into a single extruded plastic and with fewer parts, assembly costs are lowered. Plastic products made by extrusion include garden hoses, drinking straws, pipes, and ropes. Melted thermoplastic forced through extremely fine die holes can be cooled and woven into fabrics for clothes, curtains, and carpets.
Blow moulding is used to form containers from soft, hollow thermoplastic tubes. A mould is fitted around the outside of the softened thermoplastic tube and the tube is heated. Next, air is blown into the softened tube to force the outside of the softened tube to conform to the inside walls of the mould. Once the plastic cools, the mould is opened and the newly moulded container is removed. Blow moulding, as we know it today, was not developed until the 1940’s. Part prices are generally higher than injection moulded parts, but lower than rotationally moulded parts. Tooling costs are moderately expensive.
This process lends itself to any designs involving hollow shapes, equipment availability is good in most geographical locations and it is possible to save tooling dollars over injection moulding. The disadvantages of this process are: cycle times are slower than injection moulding and piece prices are higher than injection moulding. Blow moulding is used to make many plastic containers, including soft-drink bottles, jars, detergent bottles, and storage drums.
Compression moulding uses a technique similar to the way a waffle iron forms waffles from batter. First, thermosetting resin is placed into a steel mould. The application of heat and pressure softens the material and squeezes it into all parts of the mould to form the desired shape. This also accelerates cross-linking of the resin to form a stronger product. Once the material has cooled and hardened, the newly formed object is removed from the mould.
This was the first plastic processing method to gain large-scale commercial usage. The cost for tooling and parts is moderate. An advantage of compression moulding is the fact that parts that can be subjected to high heat since the raw material will not re-melt. The few disadvantages are the high waste or scrap costs and relatively slow cycle times. This process creates hard, heat-resistant plastic products, including dinnerware, telephones, television set frames, and electrical parts.
Reaction Injection Moulding (RIM)
In this process, liquid thermosetting resin is combined with a curing agent and injected into a mould. Most products made by reaction injection moulding are made from polyurethane. Parts can be flexible, rigid or self-skinning.
The cost of parts and tooling is relatively high. Strong, flexible, lightweight parts, which can easily be painted, are the advantages of this process; slow cycles and expensive raw materials are the clear disadvantages. Strong, sizable, and durable plastic products such as automotive body panels and spoilers, skis, and business machine housings are formed by reaction injection moulding.
Thermosetting and thermoplastic resins can be expanded by injecting gases into the plastic melt. As the resin cools, tiny bubbles of gas are trapped inside, forming a cellular plastic structure. Foam plastics can be classified according to their bubble, or cell, structure. Sponges and carpet pads are examples of open-celled foam plastics, in which the bubbles are interconnected. Flotation devices are examples of closed-celled foam plastics, in which the bubbles are sealed like tiny balloons. Foam plastics can also be classified by density, by the type of plastic resin used, and by flexibility. For example, rigid, closed-celled polyurethane plastics make excellent insulation for refrigerators and freezers. This process is used to make foam products such as cushions, pillows, sponges, egg cartons, and polystyrene cups.
The cost of this process can vary: most commonly piece prices are low but tooling costs are fairly high. The obvious advantage is lightweight parts that are particularly good when cushioning is required. The minor disadvantage is that the parts are not particularly strong or durable.