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Plastic Processing Methods
PROCESS DESCRIPTION
Blow An extruded parison tube of heated thermoplastic is positioned between two halves of an open split mold and expanded against the sides of the closed mold via air pressure. The mold is opened and the part ejected. Low tool and die costs, rapid production rates, and ability to mold fairly complex hollow shapes in one piece.
Calendering Dough-consistent thermoplastic mass is formed into a sheet of uniform thickness by passing it through and over a series of heated or cooled rolls. Calenders are also utilized to apply plastic covering to the backs of other materials. Low cost, and sheet materials are virtually free of molded-in stresses.
Casting Liquid plastic, which is generally thermoset except for acrylics, is poured into a mold without pressure, cured, and taken from the mold. Cast thermoplastic films are produced via building up the material (either in solution or hot-melt form) against a highly polished supporting surface. Low mold cost, capability to form large parts with thick cross sections, good surface finish, and convenient for low-volume production.
Centrifugal casting Reinforcement is placed in mold and is rotated. Resin distributed through pipe; impregnates reinforcement through centrifugal action. Utilized for round objects, particularly pipe
Coating Process methods vary. Both thermoplastics and thermosets widely used in coating of numerous materials. Roller coating similar to calendering process. Spread coating employs blade in front of roller to position resin on material. Coatings also applied via brushing, spraying, and dipping.
Cold pressure molding Similar to compression molding in that material is charged into a split mold; it differs in that it employs no heat, only pressure. Part cure takes place in an oven in a separate operation. Some thermoplastic billets and sheet material are cold formed in a process similar to drop-hammer die forming or fast cold-form stamping of metals. Low-cost matched-tool moldings exist which utilize a rapid exotherm to cure moldings on a relatively rapid cycle. Plastic or concrete tooling can be used. With process comes ability to form heavy or tough-to-mold materials; simple, inexpensive, and often has rapid production rate.
Compression molding Principally polymerized thermoset compounds, usually pre-formed, is positioned in a heated mold cavity; the mold is closed (heat and pressure are applied) and the material flows and fills the mold cavity. Heat completes polymerization and the part is ejected. The process is sometimes used for thermoplastics. Little material waste is attainable; large, bulky parts can be molded; process is adaptable to rapid automation (racetrack techniques, etc.)
Encapsulation Mixed compound is poured into open molds to surround and envelope components; cure may be at room temperature with heated postcure. Encapsulation generally includes several processes such as potting, embedding and conformal coating.
Extrusion molding Widely used for continuous production of film, sheet, tube, and other profiles; also used in conjunction with blow molding. Thermoplastic or thermoset molding compound is fed from a hopper to a screw and barrel where it is heated to plasticity then forwarded, usually via a rotating screw, through a nozzle possessing the desired cross section. Production lines require input and takeoff equipment that can be complex. Low tool cost, numerous complex profile shapes possible, very rapid production rates, can apply coatings or jacketing to core materials (such as wire).
Filament winding Excellent strength-to-weight. Continuous, reinforced filaments, usually glass, in the form of roving are saturated with resin and machine-wound onto mandrels having shape of desired finished part. Once winding is completed, part and mandrel are cured; mandrel can then be removed through porthole at end of wound part. High-strength reinforcements can be oriented precisely in direction where strength is required. Good uniformity of resin distribution in finished part; mainly circular objects such as pressure bottles, pipes, and rocket cases.
Injection molding Very widely used. High automation of manufacturing is standard practice. Thermoplastic or thermoset is heated to plasticity in cylinder at controlled temperature, then forced under pressure through a nozzle into sprues, runners, gates, and cavities of mold. The resin undergoes solidification rapidly, the mold is opened, and the part ejected. High production runs, low labor costs, high repoducibility of complex details, and excellent surface finish.
Laminating Material, usually in form of reinforcing cloth, paper, foil, metal, wood, glass fiber, plastic, etc., preimpregnated or coated with thermoset resin (sometimes a thermoplastic) is molded under pressure greater than 1000psi (7Mpa) into sheet, rod, tube or other simple shapes. Excellent dimensional stability of finished product; very economical in large production of parts.
Matched-die- molding A variation of the conventional compression molding this process employs two metal molds possessing a close-fitting, telescoping area to seal in the plastic compound being molded and to allow trim of the reinforcement. The mat or preform reinforcement is positioned in the mold and the molds is closed and heated under pressures of 150-400 psi (1-3 Mpa). The mold is then opened and the part is removed.
Pultrusion This process is similar to profile extrusion, but it does not provide flexibility and uniformity of product control, and automation. Used for continuous production of simple shapes (rods, tubes, and angles) principally incorporating fiberglass or other reinforcement. High output possible.
Rotational molding A predetermined amount of powdered or liquid thermoplastic or thermoset material is poured into mold; mold is closed, heated, and rotated in the axis of two planes until contents have fused to inner walls of mold; mold is then opened and part is removed. Low mold cost, large hollow parts in one piece can be produced, and molded parts are essentially isotropic in nature
Slush molding Powdered or liquid thermoplastic material is poured into a mold to capacity; mold is closed and heated for a predetermined time in order to achieve a specified buildup of partially cured material on mold walls; mold is opened and unpolymerized material is poured out; and semifused part is removed from mold and fully polymerized in oven. Low mold costs and economical for small production runs.
Thermoforming Heat-softened thermoplastic sheet is positioned over male or female mold; air is evacuated from between sheet and mold, forcing sheet to conform to contour of mold. Variations are vacuum snapback, plug assist, drape forming, etc. Tooling costs are generally low, large part production with thin sections possible, and often comes out economical for limited part production.
Transfer molding Related to compression and injection molding processes. Thermoset molding compound is fed from hopper into a transfer chamber where it is then heated to plasticity; it is then fed by a plunger through sprues, runners, and gates into a closed mold where it cures; mold is opened and part ejected. Good dimensional accuracy, rapid production rate, and very intricate parts can be produced.
Wet-layup or contact molding Several layers, consisting of a mixture of reinforcement (generally glass cloth) and thermosetting resin are positioned in mold and roller contoured to mold’s shape; assembly is usually oven-cured without the application of pressure. In spray molding, a modification, resin systems and chopped fiber are sprayed simultaneously from a spray gun against the mold surface. Wet-layup parts are sometimes cured under pressure, using vacuum bag, pressure bag, or autoclave, and depending on the method employed, wet-layup can be called open molding, hand layup, sprayup, vacuum bag, pressure bag, or autoclave molding. Little equipment required, efficient, low cost, and suitable for low-volume production of parts.