Glossary Of Plastic Tooling Terms 


The state in which two surfaces are held together by interfacial forces which may consist of valence forces or interlocking action or both.

Coefficient Of Thermal Expansion

The fractional change in length of a material for a unit of change in temperature. Normally expressed in “inch per inch per degree Fahrenheit”.

Compressive Strength

1/2” x 1/2” x 1” specimen is mounted in a compression-type tester between two heads that exert pressure at a constant rate of movement until the specimen fails by rupturing or deforming to a % of its’ height. Normally expressed in “pounds per square inch”, this test provides the maximum load sustained, divided by the original area of the specimen.


To change the physical properties of a plastic or resin by chemical reaction, which may be condensation, polymerization, or addition: usually accomplished by the action of either heat or catalyst or both, and with or without pressure.

Deflection Temperature

The temperature at which a 5” x 1/2” x 1/8”–1/2” specimen deflects .010 inches under a stated load of 66 or 264 pounds per square inch.


Mass per unit volume of a substance, expressed in units such as grams per cubic centimeter, pounds per cubic foot or pounds per gallon.


Pertaining to a reaction which is accompanied by the absorption of heat.


Pertaining to a reaction which is accompanied by the evolution (giving off) of heat.

Flexural Modulus

The ratio, within the elastic limit, of the applied stress on a test specimen in flexure to the corresponding strain in the outermost fibers of the specimen.

Flexural Strength

5” x 1/2” x 1/8” specimen is placed on supports 4” apart and a standard load is applied to the center of the specimen at a specified rate. The maximum stress in the outer fiber at the moment of crack or break is expressed in “pounds per square inch”.

Gel Time

With reference to thermosetting resins, the interval of time between introduction of the catalyst and the formation of a semi-solid system consisting of a network of solid aggregates in which a liquid is held. The initial jelly-like solid phase that develops during the formation of a resin from a liquid, usually 4 fluid ounce mass.

Glass Transition Temperature (Tg)

A reversible change that occurs when plastic is heated to a certain temperature range, characterized by a rather sudden transition from a hard, glassy, or brittle condition to a flexible or elastomeric condition.

Percentage Of Elongation

Increase in length of a specimen at the instant before rupture occurs. Normally expressed as a percentage.


The process of forming an uncured thermosetting resin article, then completing the curing after the article has been removed from its forming mold or mandrel.

Pot Life (Working Time)

The period during which a compound, after mixing with a catalyst, solvent or other compounding ingredients, remains suitable for its intended use.

Shore Hardness (Indention Hardness)

See Guidelines
The hardness of a material as determined by either the size of an indention made by an indenting tool under a fixed load, or the load necessary to produce penetration of the indenter to a pre-determined depth. To measure the Shore hardness of a material a Shore testing instrument is used which is comprised of spring-loaded indentor point projecting through a hole in a presser foot and a device to indicate the distance the point projects beyond the face of the foot. The scale readings range from 0 (for 0.100 penetration) to 100 (for zero penetration). A Shore “A” instrument employs a “sharp” indentor point with a load of 822 grams. The Shore “D” instrument employs a “blunt” point and the load is 10 pounds.

Taber Wear Index

The ability of a material to withstand mechanical action such as rubbing, scraping or erosion, that tends to progressively remove material from its surface. Usually expressed in milligrams loss per number of cycles per a given load.

Tensile Modulus

The ratio of stress to corresponding strain below the proportional limit of the material. Expressed normally in pounds per square inch.

Tensile Strength

1/8” thick x 3/4” to 1/2” wide x 8-1/2” long specimen is inserted in Instron Tester and is pulled apart at specified rates until the specimen fails by separating. Usually expressed in pounds per square inch.


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


Resins or plastic compounds which in their final state as finished articles are capable of being repeatedly softened by increased temperature and hardened by decrease of temperature by means of physical change.

Thermosetting Plastics (thermosets)

Resins or plastic compounds which in their final state as finished articles are substantially infusible in insoluble. Thermosetting resins are often liquids at some stage in their manufacture or processing, which are cured by heat, catalysis or other chemical means. After being fully cured, thermosets cannot be resoftened by heat.


A flow characteristic evidenced by a decrease in viscosity of a fluid when it is stirred at a constant or increasing rate of shear. When the stirring or shearing is discontinued, the apparent viscosity of the fluid gradually increases back to the original value. Changes in both directions are dependent on time as well as shear.


See Guidelines
The thickness of a substance and its resistance to flow. The higher the viscosity number, the thicker the substance.


Source Of Terms: Whittington’s Dictionary Of Plastics, Lloyd R. Whittington