UL Temperature Index: The maximum temperature below which a material maintains its electrical and mechanical integrity over a reasonable period.

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.

Undercut: A protuberance or indentation that impedes withdrawal from a two-piece rigid mold.

Underflow: The dominant flow of two confronting flows, over the other. The lesser flow reverses direction giving poor surface appearance and structural strength. Underflow should be avoided by positioning gates so that the flow fronts meet at the end of filling.

Unidirectional Flow Pattern: Plastic flowing in one direction with a straight flow front throughout filling.

Uniform Cooling Time: Cooling time the same throughout the part to avoid warping.


Tab Gate: A small removable tab about the same thickness as the molded item, but usually perpendicular to the part for easy removal.

Tackifiers: Additives used to enhance the adhesiveness or bonding ability of a material.

Tensile Modulus: (Also called modulus of elasticity). The ratio of nominal stress to the corresponding strain below the proportional limit of a material.

Tensile Strength, Break: The maximum stress that a material can withstand without breaking when subjected to a stretching load.

Tensile Strength, Yield: The maximum stress that a material can withstand without yielding when subjected to a stretching load.

Tensile Strength: The maximum tensile stress sustained by the specimen during a tension test.

Thermal Conductivity: the ability of a material to conduct heat.

Thermal Degradation: deterioration of the material by heat, characterized by molecular scission.

Thermal Shut-Off: material freezes causing a blockage.

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.

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.

Thermoplastic: material that will repeatedly soften when heated and harden when cooled.

Thermoset: A polymer that doesn’t melt when heated. Thermoset polymers “set” into a given shape when first made and afterward do not flow or melt, but rather decompose upon heating. They are often highly cross-linked polymers, with properties similar to those of network covalent solids, i.e., hard and strong.

Tie-Bar Spacing: The space between the horizontal tie-bars on an injection molding machine. Basically, this measurement limits the size of molds that can be placed between the tie-bars and into the molding machine.

Time scan: effects of mold and melt temperature and injection time changes in pressure, stress, and temperature at the end of the flow.

Toggle: A type of clamping mechanism that exerts pressure by applying force on a knee joint. A toggle is used to close and exert pressure on a mold in a press.

Tonnage: The measure by which injection molding machines are typically categorized, representing the clamping force of the injection molding machine.

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

Transfer Molding: A process of forming articles by fusing a plastic material in a chamber then forcing the whole mass into a hot mold to solidify.

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.

Tunnel Gate: See the submarine gate.


Land (Gate Area): Gate dimension parallel to the direction of melt flow.

Land Area: The area of surfaces of a mold which contact each other when the mold is closed.

Light, UV Stabilizers & Absorbers: These additives increase the ability of the material to withstand the negative effects of light and UV exposure, thus increasing the service life of the material.

Linear Mold Shrinkage: The difference between the size of the part and the size of the mold cavity. Values given are often the average of a range.

Linear Thermal Expansion: The fractional change in length of a material for a unit change in temperature.

Liquid Injection Molding (LIM): The process that involves an integrated system for proportioning, mixing, and dispensing two-component liquid resin formulations and directly injecting the resultant mix into a mold which is clamped under pressure.

Low-Temperature Flexibility: The ability of a plastic to be bent without fracture at reduced temperatures.

Lubricant: Internal lubricants, without affecting the fusion properties of a compound, promotes resin flow. External lubricants promote release from metals which aids in the smooth flow of melt over die surfaces.


Impact Modifiers: Additive used to enhance the material’s ability to withstand the force of impact.

Impact Resistance: The resistance of plastic articles to fracture under stresses applied at high speeds.

Impact Strength: The ability of a material to withstand shock loading.

Injection (also known as Fill Time): Time required to fill the cavity or mold.

Injection Blow Molding: Blow molding process by which the plastic parison to be blown is formed by injection molding.

Injection Molding Pressure: The pressure applied to the cross-sectional area of the molding cylinder.

Injection Molding: The method of forming objects from granular or powdered plastics, most often of the thermoplastic type, in which the materials are fed from a hopper to a heated chamber in which it is softened, after which a ram or screw forces the material into a mold. Pressure is maintained until the mass has hardened sufficiently for removal from the mold.

Injection Pressure: The pressure on the face of the injection screw or ram when injecting material into the mold usually expressed in PSI.

Insert Molding: Insert molding is the process of molding plastic around preformed metal inserts. This process is compatible with both thermoplastic and thermoset materials.

Insert: a removable part of the mold imparting increased resistance to wear, heat transferability, or changeable part shape to that area of the mold.

Insulated Runner (also known as Hot-Runner Mold or Runnerless Molding): A mold in which the runners are insulated from the chilled cavities and are kept hot. Hot-runner molds make parts that have no scrap.

Isobar: a line of equal pressure. Any point along an isobar sees the same cavity pressure as any other point along the same isobar.

Isochrone: a line of equal time. Any point along an isochrone is filled at the same time as any other point along the same isochrone.

Isotherm: a line of equal temperature. Any point along an isotherm is at the same temperature as any other point along the same isotherm.