Plastics processing

Plastics processing

Those methods used to convert plastics materials in the form of pellets, granules, powders, sheets, fluids, or preforms into formed shapes or parts. The plastics materials may contain a variety of additives which influence the properties as well as the processability of the plastics. After forming, the part may be subjected to a variety of ancillary operations such as welding, adhesive bonding, machining, and surface decorating (painting, metallizing).

Injection molding

This process consists of heating and homogenizing plastics granules in a cylinder until they are sufficiently fluid to allow for pressure injection into a relatively cold mold where they solidify and take the shape of the mold cavity. Solid particles, in the form of pellets or granules, constitute the main feed for injection moldable plastics. The major advantages of the injection-molding process are the speed of production, minimal requirements for postmolding operations, and simultaneous multipart molding. The development of reaction injection molding (RIM) allowed the rapid molding of liquid materials. This process has proven particularly effective for high-speed molding of such materials as polyurethanes, epoxies, polyesters, and nylons.

Extrusion

In this process, plastic pellets or granules are fluidized, homogenized, and continuously formed. Products made this way include tubing, pipe, sheet, wire and substrate coatings, and profile shapes. The process is used to form very long shapes or a large number of small shapes which can be cut from the long shapes. Extrusion can result in the highest output rate of any plastics processes; for example, pipe has been formed at rates of 2000 lb/h (900 kg/h). The extrusion process produces pipe and tubing by forcing the melt through a cylindrical die. See Extrusion

Blow molding

This process consists of forming a tube (called a parison) and introducing air or other gas to cause the tube to expand into a free-blown hollow object or against a mold for forming into a hollow object with a definite size and shape. The parison is traditionally made by extrusion, although injection-molded tubes have increased in use.

Thermoforming

Thermoforming is the forming of plastics sheets into parts through the application of heat and pressure. Tooling for this process is the most inexpensive compared to other plastics processes, accounting for the method's popularity. It can also accommodate very large parts as well as small parts.

Rotational molding

In this process, finely ground powders are heated in a rotating mold until melting or fusion occurs. If liquid materials are used, the process is often called slush molding. The melted or fused resin uniformly coats the inner surface of the mold. When cooled, a hollow finished part is removed.

Compression and transfer molding

Compression molding consists of charging a plastics powder or preformed plug into a mold cavity, closing a mating mold half, and applying pressure to compress, heat, and cause flow of the plastic to conform to the cavity shape. The process is primarily used for thermosets, and consequently the mold is heated to accelerate the chemical cross-linking. Transfer molding is an adaptation of compression molding in that the molding powder or preform is charged to a separate preheating chamber and, when appropriately fluidized, injected into a closed mold. It is most used for thermosets, and is somewhat faster than compression molding.

Foam processes

Foamed plastics materials have achieved a high degree of importance in the plastics industry. Foams can be made in a range from soft and flexible to hard and rigid. There are three types of cellular plastics: blown (expanded matrix, such as a natural sponge), syntactic (the encapsulation of hollow organic or inorganic microspheres in the matrix), and structural (dense outer skin surrounding a foamed core). There are seven basic processes used to generate plastics foams. They include the incorporation of a chemical blowing agent that generates gas (through thermal decomposition) in the polymer liquid or melt; gas injection into the melt which expands during pressure relief; generation of gas as a by-product of a chemical condensation reaction during cross-linking; volatilization of a low-boiling liquid (for example, Freon) through the exothermic heat of reaction; mechanical dispersion of air by mechanical means (whipped cream); incorporation of nonchemical gas-liberating agents (adsorbed gas on finely divided carbon) into the resin mix which is released by heating; and expansion of small beads of thermoplastic resin containing a blowing agent through the external application of heat.

Reinforced plastics/composites

These are plastics whose mechanical properties are significantly improved because of the inclusion of fibrous reinforcements. The wide variety of resins and reinforcements that constitute this group of materials led to the more generalized description “composites.” Composites consist of two main components, the fibrous material in various physical forms and the fluidized resin which will convert to a solid. There are fiber-reinforced thermoplastic materials, and these are typically processed in standard thermoplastic processing equipment. The first step in any composite fabrication procedure is the impregnation of the reinforcement with the resin. The impregnated reinforcement can be subjected to heat to remove impregnating solvents or advance the resin cure to a slightly tacky or dry state. The composite in this form is called a prepreg. Premixes, often called bulk molding compounds, are mixtures of resin, inert fillers, reinforcements, and other formulation additives which form a puttylike rope, sheet, or preformed shape. See Composite material

Casting and encapsulation

Casting is a low-pressure process requiring nothing more than a container in the shape of the desired part. For thermoplastics, liquid monomer is poured into the mold and, with heat, allowed to polymerize in place to a solid mass. For vinyl plastisols, the liquid is fused with heat. Thermosets are poured into a heated mold wherein the cross-linking reaction completes the conversion to a solid. Encapsulation and potting are terms for casting processes in which a unit or assembly is encased or impregnated, respectively, with a liquid plastic which is subsequently hardened by fusion or chemical reaction. These processes are predominant in the electrical and electronic industries for the insulation and protection of components.

Calendering

In the calendering process, a plastic is masticated between two rolls that squeeze it out into a film which then passes around one or more additional rolls before being stripped off as a continuous film. Fabric or paper may be fed through the latter rolls, so that they become impregnated with the plastic.

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