Glossary: Technical Terms

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R

Reagent Resistance

The ability of a plastic to withstand exposure to chemicals.

Regrind

Waste material such as sprues, runners, excess parison material and reject parts from injection molding, blow molding and extrusion, which has been reclaimed by shredding or granulating. Regrind is usually mixed with virgin compound at a predetermined percentage for remolding.

Reinforced Plastic

A plastic composition in which fibrous reinforcements are imbedded, with strength properties greatly superior to those of the base resin.

Reinforcement

A strong, inert fibrous material incorporated in a plastic mass to improve its physical properties.

Resin

(Synthetic) The term is use to designate any polymer that is a basic material for plastics.

Runner

In an injection mold, the feed channel, usually of circular cross section, that connects the sprue with the cavity gate. The term is also used for the plastic piece formed in this channel.

Runner System

This term is sometimes used for the entire resin feeding system, including sprues, runners and gates, in injection molding.

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S

Screw

In extrusion, the shaft provided with helical ridges which conveys the material from the hopper outlet through the barrel and forces it out through the die.

Shear Strength

The maximum load required to shear the specimen in such a manner that the moving portion has completely cleared the stationary portion.

Short Shot

In injection molding, failure to fill the mold completely.

Shot

One complete cycle of a molding machine.

Shot Capacity

The maximum weight of material that can be delivered to an injection mold by one stroke of the ram.

Shrinkage Allowance

The dimensional allowance which must be made in molds to compensate for shrinkage of the plastic compound on cooling.

SI

Abbreviation for Le Systeme International d'Unites.

Solvents

Substances with the ability to dissolve other substances.

SPE

Abbreviation for the Society of Plastics Engineers.

Specific Gravity

The ratio of the density of a material as compared to the density of water at standard atmospheric pressure (1 ATM) and room temperature (73F).

Specific Volume

The volume of a unit of weight of a material; the reciprocal of density.

SPI

Abbreviation for the Society of the Plastics Industry.

Splay Marks

Silver streaks like marks caused by moisture in material, material degradation, or wiping of the entrapped air on surface of mold.

Sprue

In an injection mold, the main feed channel that connects the mold filling orifice with the runners leading to each cavity gate.

Stabilizer

An agent used in compounding some plastics to assist in maintaining the physical and chemical properties of the compounded materials at suitable values throughout the processing and service life of the material and/or the parts made therefrom.

Stiffness

The capacity of a material to resist elastic displacement under stress.

Strain

In tensile testing, the ratio of the elongation to the gage length of the test specimen, that is, the change in length per unit of original length.

Stress

The force producing or tending to produce deformation in a body measured by the force applied per unit area.

Stress-Crack

External or internal cracks in a plastic caused by tensile stresses less than that of its short-time mechanical strength. Note: The development of such cracks is frequently accelerated by the environment to which the plastic is exposed.

Stress Relaxation

The decay of stress at a constant strain.

Stress-Strain Curve

The curve plotting the applied stress on a test specimen versus the corresponding strain. Stress can be applied through shear, compression, flexure, or tension.

Structural Foam

A term originally used for cellular thermoplastic articles with integral solid skins having high strength-to-weight ratios, but now sometimes also used for high density cellular plastics which are strong enough for structural applications.

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T

Tensile Strength

The maximum tensile stress sustained by the specimen during a tension test

Thermoelasticity

Rubber-like elasticity exhibited by a rigid plastic resulting from an increase in temperature.

Thermoforming

The process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to tender it soft and flowable. Then applying differential pressure to make the sheet conform to the shape of a mold or die positioned below the frame.

Thermoplastics

Materials that become soft when heated and solid when cooled to room temperature. This softening and setting may be repeated many times.

Thermoplastic Elastomers

The family of polymers that resemble elastomers in that they can be repeatedly stretched without distortion of the unstressed part shape, but are true thermoplastics and thus do not require curing.

Thermosets

Materials that may not be reheated and softened again. Once the structural framework is set, these plastics cannot be reformed, characterized by molecular cross-linking.

Tool

In injection molding, the term sometimes used to describe the mold.

Transition Temperature

The temperature at which a polymer changes from (or to) a viscous or rubbery condition (or from) a hard and relatively brittle one.

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U

UL

Abbreviation for Underwriters Laboratories, the non-profit safety testing organization.

Ultimate Elongation

In a tensile test the elongation at rupture.

Ultimate Strength

Term used to describe the maximum unit stress a material will withstand when subjected to an applied load in a compression, tension, flexural, or shear test.

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V

Vacuum Forming

A method of forming plastic sheets or films into three-dimensional shapes, in which the plastic sheet is clamped in a frame suspended above a mold, heated until it becomes softened, drawn down into contact with the mold by means of a vacuum, and cooled while in contact with the mold. Often used interchangeably with thermoforming.

Vicat Softening Point

The temperature at which a flat-ended needle will penetrate a thermoplastic specimen under specific conditions. (ASTM D-15225)

Virgin Material

Any plastic compound or resin that has not been subjected to use or processing other than that required for its original manufacture.

Viscoelasticity

This property, possessed by all plastics to some degree, dictates that while plastics have solid-like characteristics such as elasticity, strength and form-stability, they also have liquid-like characteristics such as flow depending on time, temperature, rate and amount of loading.

Viscosity

A measure of the resistance to flow due to internal friction when one layer of fluid is caused to move in relationship to another layer.

Void

In a solid plastic, an unfilled space.

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W

Weld Line

The intersection of two confronting flow fronts creating areas of local weakness. Weld lines should be positioned in the least sensitive areas.

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