Plastics processing

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

As with other materials of construction, processing of plastics is but one step in the normal design-to-finished-part sequence. The choice of process is influenced by economic considerations, number and size of finished parts, and complexity of postfinishing operations, as well as the adaptability of the plastics to the process.

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. For thermoplastics, no chemical changes occur within the plastic, and consequently the process is repeatable. Injection molding of thermoset-ting resins differs primarily in that the cylinder heating is designed to homogenize and preheat the reactive materials, and the mold is heated to complete the chemical cross-linking reaction to form an intractable solid. 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. In these processes, cold or warm, two highly reactive, low-molecular weight, low-viscosity resin systems are first injected into a mixing head and from there into a heated mold, where the reaction to a solid is completed.

Polymerization and cross-linking occur in the mold. This process has proved 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 that can be cut from the long shapes. The homogenizing capability of extruders is used for plastics blending and compounding. Pellets used for other processing methods, such as injection molding, are made by chopping long filaments of extruded plastic.

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 gained prominence because they do not require post-finishing, have better dimensional tolerances and wall thicknesses, and can be made unsym-metrical and in higher volume production.

Thermoforming

Thermoforming is the forming of plastic sheets into parts through the application of heat and pressure. The pressure can be obtained through use of pneumatics (air) or compression (tooling) or vacuum. Tooling for this process is the most inexpensive compared to other plastic processes, accounting for the popularity of the method. It can also accommodate very large parts as well as small parts, which are useful in low-cost prototype fabrication.

Rotational Molding

In this process, finely ground powders are heated in a rotating mold until melting or fusion occurs. If liquid materials, such as vinyl plasti-sols, 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. The processes require relatively inexpensive tooling, are scrap-free, and are adaptable to large, double-walled, hollow parts that are strain-free and of uniform thickness. The processes can be performed by relatively unskilled labor. On the other hand, the finely ground plastics powders are more expensive than pellets or sheet, thin-walled parts cannot be easily made, and the process is not suited for large production runs of small parts.

Compression and Transfer Molding

Compression molding is one of the oldest molding techniques and 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. The process predates, yet closely parallels, the early techniques of ram injection molding of thermoplastics. It is most used for thermosets, and is somewhat faster than compression molding. In addition, parts are more uniform and more dimensionally accurate than those made by compression molding. See Figures P.2A and P.2B showing compression molding and transfer molding.

Three types of compression molds. (a) Flash-type, (b) positive, (c) semipositive.

FIGURE P.2A Three types of compression molds. (a) Flash-type, (b) positive, (c) semipositive.

Transfer molding. (a) In the molding cycle, material is first placed in the transfer pot. (b) It is then forced through an orifice into the closed mold. (c) When the mold opens, the cull and sprue are removed as a unit, and the part is lifted out of the cavity by ejector pins.

FIGURE P.2B Transfer molding. (a) In the molding cycle, material is first placed in the transfer pot. (b) It is then forced through an orifice into the closed mold. (c) When the mold opens, the cull and sprue are removed as a unit, and the part is lifted out of the cavity by ejector pins.

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 micro-spheres 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; volatization 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.

Structural foam differs from other foams in that the part is produced with a hard integral skin on the outer surfaces and a cellular core in the interior. They are made by injection-molding liquefied resins containing chemical blowing agents. The initial high injection pressure causes the skin to solidify against the mold surface without undergoing expansion. The subsequent reduction in pressure allows the remaining material to expand and fill the mold. Coinjection (sandwich) molding permits injection molding of parts containing a thermoplastic core within an integral skin of another thermoplastic material. When the core is foam, an advanced form of structural foam is produced.

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 simplest method is to pass the reinforcement through a resin bath and use the wet impregnate directly. For easier handling and storage, 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. This B-stage condition allows the composite to be handled, yet the cross-linking reaction has not proceeded so far as to preclude final flow and conversion to a homogeneous part, when further heat or pressure is applied.

Premixes, often called bulk molding compounds, are mixtures of resin, inert fillers, reinforcements, and other formulation additives that form a puttylike rope, sheet, or preformed shape.

Converting these various forms of composite precursors to final part shape is achieved in a number of ways. Hand layup techniques entail an open mold onto which the impregnated reinforcement or prepreg is applied layer by layer until the desired thicknesses and contours are achieved; see part a of Figure P.3 depicting techniques for producing reinforced plastics and composites. The thermoset resin is then allowed to harden (cure). Often the entire configuration will be enclosed in a transparent sealed bag (vacuum bag) so that a vacuum can be applied to remove unwanted volatile ingredients and entrained air for improved densification of the composite (part b of Figure P.3). External heat may be applied to accelerate the process. Often a bagged laminate will be inserted into an autoclave so that the synergistic effects of heat, vacuum, and pressure can be obtained. At times, a specially designed spray apparatus is used that simultaneously mixes and applies a coating of resin and chopped reinforcement to a mold surface (part c). This technique is particularly useful for large structures such as boat hulls and truck cabs, covering complex shapes as readily as simple configurations.

Matched die compression molding resembles normal compression molding, although the pressures are considerably lower (part d). Pre-mix molding is essentially the same process, except that premix compounds are used. Pul-trusion is a term coined to describe the process for the continuous extrusion of reinforced plastics profiles. Strands of reinforcement are drawn (pulled) through an impregnating tank, the forming die, and finally a curing area (radio-frequency exposure). Filament winding is a process in which the continuous strands of reinforcement are drawn through an impregnating bath and then wound around a mandrel to form the part (part e). This technique is most used for the formation of hollow objects such as chemical storage tanks or chemically resistant pipe. Advanced automated processes, such as ply cutting, tape laying and contouring, and ply lamination are providing improved parts and reduced costs particularly in the aerospace industry.

 Techniques for producing reinforced plastics and composites. (a) Hand lay-up technique for reinforced thermosets; (b) vacuum bag molding method; (c) spray-up method; (d) matched metal die molding; (e) filament winding.

FIGURE P.3 Techniques for producing reinforced plastics and composites. (a) Hand lay-up technique for reinforced thermosets; (b) vacuum bag molding method; (c) spray-up method; (d) matched metal die molding; (e) filament winding.

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, usually composed of liquid resins with appropriate curatives and property-modifying additives, are poured into a heated mold in which the cross-linking reaction completes the conversion to a solid. Often a vacuum is applied to gasify the resultant part for improved homogeneity.

Encapsulation and potting are terms for casting processes in which a unit or assembly is encased or unpregnated, respectively, with a liquid plastic, which is subsequently hardened by fusion or chemical reaction; Figure P.4 depicts low-pressure plastics processes. These processes are predominant in the electrical and electronic industries for the insulation and protection of components.

Low-pressure plastics processes: (a) casting, (b) potting, (c) encapsulation, and (d) sealing.

FIGURE P.4 Low-pressure plastics processes: (a) casting, (b) potting, (c) encapsulation, and (d) sealing.

Calendering

In the calendering process, a plastic is masticated between two rolls that squeeze it out into a film that 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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