polymer processing

Weld Line - The point on a part where two plastic flow fronts meet during injection. Witness Line - A visible line on part due to two molding surfaces coming together. Witness lines always occur at parting lines.

Variation - A change in the value of a measured characteristic. Vent - A shallow channel in the mold to allow air to escape from the cavity during injection. Virgin Material - Plastic material that has not been subject to use or processing other than that required for its initial manufacture. Viscosity - Internal friction or resistance to flow of a melt or liquid. The constant ratio of shearing stress to shearing rate. It is a measure of the materials resistance to flow.

Ultimate strength - Term used to describe the maximum unit stress a material can withstand before breaking when subjected to a load in compression, tension or shear. Undercut - An indentation or protuberance on a part that impedes the removal of core and/or cavity.

Tensile strength - The resistance of a material to longitudinal tension stress. Thermal conductivity - Ability of a material to conduct heat. Thermal degradation - Deterioration of material by heat. Thermal diffusivity - The ability of a material to transfer heat per unit time and area. Thermoplastic - Capable of being repeatedly softened by heat and hardened by cooling. Thermoset - A material that will undergo or has undergone a chemical reaction by the action of heat, catalysts, ultraviolet light,etc. Three-plate mold - Mold with two parting lines, one for part and the other to free trapped cold runner. Two-plate mold - Mold with one parting line used to free both part and runner.

Shear - An action resulting from applied forces which cause or tends to cause two contiguous parts of a body to slide relative to each other in a direction parallel to their place of contact. Shear heating - Frictional heat generated within a plastic due to shear forces either in the extruder or in the runner system during injection. Shot - The yield of parts from a mold after one complete molding cycles, including scrap. Shot capacity - The maximum amount of material that the injection units ram or plunger can push out in one forward stroke. Shot size - The amount of plastic required to fill the mold. Shrinkage - The decrease in dimensions a plastic undergoes from molten state to a solid state. Shrinkage is compensated for by packing pressure, but some material shrinkage occurs after gate is frozen and for as much as 24 hours after it is molded. Sink Mark - An imperfection or depression in the surface of a part, generally cause by non-uniform shrinkage. SPC - Statistical Proce...