S - Polymer Processing Glossary of Terms

>> Sunday, 22 February 2009

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 Process Control. A statistical approach to controlling and improving a process.
Sprue Bushing - Bushing which connects the nozzle of the injection unit to the molds runner system. Forms the sprue of the runner system.
Standard deviation - A measure of the spread of a set of values about their average values, denoted by the letter s or sigma.
Stress Crack - External or internal crack in plastic caused by tensile stresses less than that of its short time mechanical strength. The development of such cracks are accelerated by the environment (see ESC).
Stripper Ejection - Type of ejection which stripping the molded part from the core.
Stroke - Distance moved.
Suck-Back - Also called Decompression. After the shot has accumulated, the nozzle retracts an additional amount to pull the plastic back to prevent drooling.
Surface Finish - The amount of gloss or matte on a surface.

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