Vacuum Forming - Method of sheet forming in which the plastic sheet is clamped in a stationary frame, heated, and drawn down by a vacuum into a mold. In a loose sense, it is sometimes used to refer to all sheet forming techniques, including Drape Forming involving the use of vacuum and stationary molds.

Vacuum Metallising - Process in which surfaces are thinly coated with metal by exposing them to the vapor of metal that has been evaporated under vacuum ( one millionth of normal atmospheric pressure).

Valley Printing - Ink is applied to the high points of an embossing roll and subsequently deposited in what becomes the valleys of the embossed plastic material.

Vent - In a mold, a shallow channel or minute hole cut in the cavity to allow air to escape as the material enters.

Virgin Material - A plastic material in the form of pellets, granules, powder, flock, or liquid that has not been subjected to use or processing other than that required for its initial manufacture.

Viscosity - Internal friction or resistance to flow of a liquid. The constant ratio of shearing stress to rate of shear. In liquids for which this ratio is a function of stress, the term "apparent viscosity" is defined as the ratio.

Viscosity, Inherent - The logarithmic viscosity number determined by dividing the natural logarithm of the relative viscosity (sometimes called viscosity ratio) by the concentration in grams per 100 mls. of solution.

Viscosity, Relative (or Viscosity Ratio) - Determined by dividing the average efflux time of the solution by the average efflux time of the pure solvent.

Voids - (1) In a solid plastic, an unfilled space of such size that it scatters radiant energy such as light.(2) A cavity unintentionally formed in a cellular material and substantially larger than the individual cells.

Volatiles - That portion of a substance that is readily vaporized.

Volume Resistivity - The electrical resistance between opposite faces of a 1-cm. cube of insulating material. It is measured under prescribed conditions using a direct current potential after a specified time of electrification. It is commonly expressed in ohm-centimeters. The recommended test is ASTM D257-54T.

Vulcanization - The chemical reaction which induces extensive changes in the physical properties of a rubber and which is brought about by reacting the rubber with sulphur and/or other suitable agents. The changes in physical properties include decreased plastic flow, reduced surface tackiness, increased elasticity, much greater tensile strength, and considerably less solubility. More recently, certain thermoplastics, e.g., polyethylene, have been formulated to be vulcanisable. Cross-linking is encouraged, thereby giving resistance to deformation of flow above the melting point.