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VACUUM FORMING. A thermoforming method of sheet forming in which the plastics sheet is clamped in a stationary frame, heated, and drawn down by vacuum into a mold.
VACUUM METALIZING. A process in which surfaces are thinly coated with metal by exposing them to the vapor of metal that has been evaporated under vacuum.
VENEER. A thin sheet or layer of wood, sliced, rotary-cut, half-round or sawn from a log, block or flitch. There is no sawkerf waste in a knife-cut veneer. Veneer is the raw material from which plywood and laminated wood are assembled. Thicknesses may range from 1/100 in. to 1/4 in. and are seldom greater.
VINYL ACETATE PLASTICS. Plastics based on resins made by the polymerization of vinyl acetate or copolymerization of vinyl acetate with no more than an equal weight of other saturated compounds.
VINYL ALCOHOL PLASTICS. Plastics based on resins made by the hydrolysis of polyvinyl esters or copolymers of vinyl esters.
VINYL CHLORIDE PLASTICS. Plastics based on resins made by the polymerization of vinyl chloride with not more than equal weights of other unsaturated compounds.
VINYLIDENE PLASTICS. (See SARAN PLASTICS.)
VINYL PLASTICS. Plastics based on resins made from vinyl monomers except those specifically covered by other classification, such as acrylic and styrene plastics. Typical vinyl plastics are polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol and polyvinyl butyral, and copolymers of vinyl monomers with unsaturated compounds.
VISCOELASTICITY. The characteristic that causes plastics materials to respond to stress as though they are a combination of elastic solids and viscous fluids. This property is exhibited in varying degrees by all plastics. This term is not applicable to elastomers.
VISCOSITY. The property of resistance to flow exhibited within the body of a material. This property can be expressed in terms of relationship between applied shearing stress and resulting rate of strain in shear. Viscosity is usually taken to mean Newtonian viscosity, in which case the ratio of shearing stress to the rate of shearing strain is constant. In Newtonian viscosity, the ratio of shearing stress to the rate of shearing strain is constant. In non-Newtonian behavior, which is the usual case with plastic materials, the ratio varies with the shearing stress. Such ratios are often called the apparent viscosities at the corresponding shearing stress. (See VISCOSITY COEFFICIENT.)
VISCOSITY COEFFICIENT. The shearing stress necessary to induce a unit velocity flow gradient in a material. In actual measurement, the viscosity coefficient of a material is obtained from the ratio of shearing stress to shearing rate. This assumes the rates to be constant and independent of the shearing stress, a condition which is satisfied only by Newtonian fluids. Consequently, in all other cases, values obtained are apparent and represent one point on the flow curve. In the metric system, the viscosity coefficient is expressed in poises. Under SI, the unit is pascal-seconds.
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