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Plastics - Manufacturing Methods

derived molding mold reaction

At some stage in their processing, both thermoplastics and thermosetting plastics are sufficiently fluid to be molded and formed. The manufacture of most plastics is determined by their final shape.

Many cylindrical plastic objects are made by a process called extrusion. The extrusion of thermoplastics

TABLE 3. THERMOSETTING PLASTICS
Type Chemical basis Uses
Alkyd polyesters Polyesters derived from the reaction of acids with two acid groups, and alcohols with three alcoholic groups per molecule Moldings, finishes; applications requiring high durability, excellent pigment dispersion, toughness, good adhesion, and good flowing properties
Allyls Polyesters derived form the reaction of esters of allyl alcohol with dibasic acids Electrical insulation, applications requiring high resistance to heat, humidity, and corrosive chemicals
Bismaleimides Generally prepared by the reaction of a diamine with maleic anhydride Printed wire boards; high performance structural composites
Epoxies Derived from the reaction of epichlorohydrin with hydroxylcontaining compounds Encapsulation, electrical insulations, laminates, glass-reinforced plastics, floorings, coatings adhesives
Melamines Derived from the reaction of formaldehyde and amino compounds containing NH2 groups Molded plates, dishes, and other food containers
Phenolics Derived from the reaction of phenols and formaldehydes Cements, adhesives
Polybutadienes Consist of polyethylene with a cross-link at every other carbon in the main chain Moldings, laminating resins, coatings, cast-liquid and formed-sheet products; applications requiring outstanding electrical properties and thermal stability
Polyesters (thermosetting) Derived from reactions of dicarboxylic acids with dihydroxy alcohols Moldings, laminated or reinforced structures, surface gel coatings, liquid castings, furniture products, structures
Polyurethanes Derived from reactions of polyisocyanates and polyols Rigid, semi-flexible, and flexible foams; elastomers


consists of melting and compressing plastic granules by rotating them in a screw conveyor in a long barrel, to which heat may be applied if necessary. The screw forces the plastic to the end of the barrel where it is pushed through a screen on its way to the nozzle. The nozzle determines the final shape of the extruded form. Thermosets may also be extruded if the screw in the conventional extruder is replaced with a plunger-type hydraulic pump.

Plastic powders are directly converted into finished articles by molding. Two types of molding processes are compression molding and injection molding. In compression molding, which is used with thermosetting materials, steam is first circulated through the mold to raise it to the desired temperature; then a plastic powder or tablets are introduced into the mold; and the mold is closed under high pressure and the plastic is liquefied so that it flows throughout the mold. When the mold is reopened,

TABLE 3. THERMOSETTING PLASTICS (cont'd)
Type Chemical basis Uses
Silicones Consist of alternating silicon and oxygen atoms in a polymer backbone, usually with organic side groups attached to the chain Applications requiring uniform properties over a wide temperature range; low surface tension; high degree of lubricity; excellent release properties; extreme water repellency; excellent electrical properties over a wide range of temperature and frequency; inertness and compatibility; chemical inertness; or weather resistance
Ureas Derived from the reaction of formaldehyde and amino compounds containing NH2 groups Dinnerware, interior plywood, foams, insulation


the solid molded unit is ejected. Injection molding differs from compression molding in that plastic material is rendered fluid outside the mold, and is transferred by pressure into the cooled mold. Injection molding can be used with practically every plastic material, including rubbers.

Sheets, blocks, and rods may be made in a casting process that in effect involves in situ, or in-place, polymerization. In the case of acrylics, sheets are cast in glass cells by filling cells with a polymer solution. The polymer solution solidifies and the sheet is released by separating the glass plates after chilling the assembly in cold water. Blocks can be made in the same way using a demountable container; and rods can be made by polymerizing a polymer syrup under pressure in a cylindrical metal tube.

Plastic foams are produced by compounding a polymer resin with a foaming agent or by injecting air or a volatile fluid into the liquid polymer while it is being processed into a finished product. This results in a finished product with a network of gas spaces or cells that makes it less dense than the solid polymer. Such foams are light and strong, and the rigid type can be machined.


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