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Machine Design

Laminated Plastics

Laminated plastics are a special form of polymer-matrix composite consisting of layers of reinforcing materials that have been impregnated with thermosetting resins, bonded together, and cured under heat and pressure. The cured laminates, called high-pressure laminates, are produced in more than 70 standard grades, based on National Electrical Manufacturers Association (NEMA) specifications.

Laminated plastics are available in sheet, tube, and rod shapes that are cut and/or machined for various end uses. The same base materials are also used in molded-laminated and molded-macerated parts. The molded-laminated method is used to produce shapes that would be uneconomical to machine from flat laminates, where production quantities are sufficient to warrant mold costs.

Strength of a molded shape is higher than that of a machined shape because the reinforcing plies are not cut, as they are in a machined part. The molded-macerated method is used for similar parts that require uniform strength properties in all directions.

Other common forms of laminated plastics are composite sheet laminates that incorporate a third material bonded to one or both surfaces of the laminate. Metals most often used in composites are copper, aluminum, nickel, and steel. Copper-clad sheets (one or both sides) for printed-circuit and multilayer boards comprise the largest volume of metal composite sheet laminates. Nonmetallics include elastomers, vulcanized fiber, and cork. Composite metal/plastic materials are also produced in rods and tubes.

Vulcanized fiber is another product often classified with the laminated plastics because end uses are similar. Vulcanized fiber is made from regenerated cotton cellulose and paper, processed to form a dense material (usually in sheet form) that retains the fibrous structure. The material is tough and has good resistance to abrasion, flame, and impact.

Resins: Phenolics are the most widely used resin in laminated plastics. These low-cost resins have good mechanical and electrical properties and resistance to heat, flame, moisture, mild acids, and alkalies. Most paper and cloth-reinforced laminates are made with phenolics.

Polyesters are used for both electrical and mechanical service requiring moderate heat resistance. The resins are usually mineral filled to improve dimensional stability and flame retardancy and to reduce cost.

Malemine resins are used primarily in electrical-grade laminates because of their excellent resistance to arcing and tracking, high mechanical strength, and good resistance to alkalies.

Epoxies are recommended for applications requiring resistance to chemicals and humid environments. They have low moisture absorption and good dimensional stability, mechanical strength, bond strength, and fungus resistance.

Silicones, used primarily with glass-cloth reinforcement, have very high heat resistance (to 550 °F). Laminates based on silicones have low moisture absorption, and they maintain their electrical properties over a wide range of service conditions.

Polyimide binders extend the use-temperature range of glass laminates upward. These resins can also withstand lengthy solder-bath exposure without blistering.

Reinforcements: Papers are the lowest-cost reinforcing materials used in making laminates. Types include kraft, alpha, cotton linter, and combinations of these. Papers provide excellent electrical properties, good dimensional stability, moderate strength, and uniform appearance.

Cotton cloth is used for applications requiring good mechanical strength. The lighter-weight fabrics are not as strong but have excellent machinability.

Asbestos, in the form of paper, mat, or woven fabric, provides excellent resistance to heat, flame, chemicals, and wear.

Glass-fiber reinforcements, in woven fabric or mat, form the strongest laminates. These laminates also have low moisture absorption and excellent heat resistance and electrical properties.

Nylon fabrics provide excellent electrical and mechanical properties and chemical resistance, but laminates reinforced with these materials lack dimensional stability at elevated temperatures.

TAGS: Materials
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