Glossary of Rubber Terms

Abrasion: The wearing away of a surface by mechanical action, such as rubbing, scraping or erosion.

Abrasion Resistance: The ability of a rubber compound to resist mechanical wear.

Absorption: The physical mechanism by which one substance takes up another substance (liquid, gas or vapor) into its interior.

Accelerated Life Test: Any set of test conditions designed to reproduce, in a short time, the deteriorating effect obtained under normal service conditions.

Accelerated Service Test: A service or bench test in which some service condition, such as speed, temperature or continuity of operation, is exaggerated in order to obtain a result in shorter time.

Accelerator: A substance which hastens the vulcanization of an Elastomer, causing it to take place in a shorter time or at a lower temperature.

Acid Resistance: With stands the action of acids.

Acrylic: A polymer for which resistance to air and hot oil at temperatures above 30 degrees Fahrenheit are required.

Adhere: To cling or stick together.

Adhesion: Tendency of rubber to bond or cling to a contact surface.

After Cure: Continuation of vulcanization after the desired cure is effected and the heat source removed (Also Referred to as Post-Cure).

Aging: To undergo changes in physical properties with age or lapse of time.

Aging, Accelerated: Tests run on various rubbers to find out, in as short a period as possible, the destructive influence of light, oxygen, heat and ozone.

Aging, Oxygen Bomb: A means of accelerating the change in the physical properties of rubber compounds by exposing them to action of oxygen at an elevated temperature and pressure.

Air Bomb: Similar to an oxygen bomb, but used with air.

Air Checks: Surface markings or depressions due to trapping air between the material being cured and the mold surface.

Air Curing: Vulcanization of a rubber product in air, as distinguished from in a press or steam vulcanizer.

Alpha Particles: Positively charged particles composed of two protons and neutrons (often referred to simply as helium atom molecule); characterized by limited penetration.

Ambient Temperature: The surrounding temperature relative to the given point of application.

Aniline Point: The lowest temperature at which equal volume of pure, fresh aniline and oil will completely dissolve in one another is the aniline point of the oil.

Antidioxidant: An organic substance which inhibits or retards oxidation.

Antiozonant: A substance that retards or prevents the appearance of cracks from action of ozone when the Elastomer is exposed under tension, either statically or dynamically, to air containing ozone.

Antirad: A material which inhibits radiation change.

Atmospheric Aging Resistance: Loss of physical properties due to the normal action of its surroundings (weather).

Atmospheric Cracking: Cracks produced in the surface of rubber articles by exposure to atmospheric conditions.

Backrinding: Distortion at the parting line, usually in the form of a ragged indentation.

Back-Up-Ring: (Anti extrusion device) A ring of relatively hard and tough material, placed in the gland between the O-Ring and groove side walls, to prevent extrusion of the O-Ring.

Bake Out: A process whereby a vacuum system is heated for a given time at some predetermined temperature to degas all the components,i.e. gauges, fittings, valves, seals.

Banbury (TM) Mixer: A specific type of internal mixer used to blend fillers and other ingredients with an Elastomer.

Batch: The product of one mixing operation.

Bench Test: A modified service test in which the service conditions are approximated, but the equipment is conventional laboratory equipment and not necessarily identical with that in which the product will be employed.

Beta Particles: Negatively charged particles or electrons, characterized by limited penetration.

Bleeding: Migration to the surface of plasticizers, waxes or similar materials, to form a film or bead.

Blemish: A mark, deformity or injury which impairs appearance.

Blister: A raised spot in the surface, or a separation between layers, usually forming void or air-filled space in the vulcanized article.

Bloom: A dusty or milky looking deposit that sometimes appears on the surface of an molded product after molding and storage, caused by migration of a liquid or solid to the surface. Not to be confused with dust from external sources.

Bond: The term commonly used to denote the attachment of a given Elastomer to some other member. Bonds may be classified by types, as follows: (a) Mechanical - purely physical attachment accomplished by such means as "through" holes, interlocking fingers, envelope design, riveting, etc.; (b) Cold - adhesion of previously vulcanized Elastomer to another member through use of suitable contact cements; (c)Vulcanized - adhesion of an Elastomer to a previously primed surface using heat and pressure, thus vulcanizing the Elastomer at the same time.

Break: A separation or discontinuity in any part of an article.

Break-Out: Force to inaugurate sliding. Expressed in same terms as friction. An excessive break-out value is taken as an indication of the development of adhesion.

Brittleness: Tendency to crack when deformed.

Buna-N: See Nitrile.

Buna-S: A general term for the copolymers of butadiene and styrene. Also known as SBR and GRS.

Butaprene: See Nitrile.

Butt Joint: Joining two ends of a seal whereby the junction is perpendicular to the mold parting line.

Butyl: A copolymer of iso-butylene and isoprene.

Calender: A machine used to form a sheet of rubber between steel rollers.

Cell: A single small cavity surrounded partially or completely by walls.

Chemical Bonding: A method of bonding rubber to inserts by applying special adhesives to the insert prior to molding.

Chemigum: (TM, R.T. Vanderbilt Co.) See Nitrile.

Closure Dimension: Dimensions of a molded rubber product that are affected by flash thickness (mold closure) variation.

Coating: A uniform layer of chemical primers or adhesives to produce a chemical bond between the rubber and substrate. May also refer to special surface treatments that can be applied to rubber to achieve special properties.

Coefficient of Thermal Expansion: Average expansion per degree over a stated temperature range, expressed as a fraction of initial dimension. May be linear or volumetric.

Cold Flexibility: Flexibility following exposure to a predetermined low temperature for a predetermined time.

Cold Flow: Continued deformation under stress.

Cold Resistance: Able to withstand the effects of cold or low temperatures without loss of serviceability.

Commercially Smooth: Degree of smoothness of a surface of an article which is acceptable for use.

Compound: A term applied to a mixture of polymers and other ingredients to produce a usable rubber material.

Compression Molding: Molding process in which a preload of rubber compound is normally placed directly in the mold cavity, and compressed to shape by closure of the mold.

Compression Modulus: The ratio of the compression stress to the resulting compression strain (the latter expressed in the direction of force). Compression Modulus may be either static or dynamic.

Compression Set: The amount by which a rubber specimen fails to return to its original shape after release of compressive load.

Conductive Rubber: A rubber capable of conducting electricity. Most generally applied to rubber products used to conduct static electricity.

Copolymer: A polymer consisting of two different monomers chemically combined.

Corona Resistance: The ability of a rubber acting as insulator to withstand the effects of high voltage discharge. Indications of failure appear as surface cracks.

Corrosion (Packing): Corrosion of rigid member (usually metal) where it contacts packing. The actual corroding agent is fluid medium trapped in the interface.

Corrosive (Packing): A property of packing whereby it is assumed (often incorrectly) to promote corrosion of a rigid member of a trapped fluid.

Cracking: The sharp break or fissure in the surface. Generally due to excessive strain.

Creep: The progressive relaxation of a given rubber material while it is under stress. This relaxation eventually results in permanent deformation, or "set".

ross-Linking Agents: A chemical, or chemicals, that bonds the polymer chains together to form a thermoset rubber product.

Cross-Section: A seal as viewed if cut at right angles to the molding line, showing internal structure.

Cure: See Vulcanization.

Cure Date: Date when an O-Ring was molded, i.e. "4 Q 96" means "Fourth Quarter, 1996".

Curing Temperature: The temperature at which the rubber product is vulcanized.

Cylinder: Chamber in which piston, plunger, ram, rod or shaft is driven by, or against, the system fluid.

Damping:The quality of an Elastomer to absorb forced vibrational energy.

Deflashing: Any of various processes used to remove the waste edge from a molded rubber part.

Degassing: The intentional, but controlled, outgassing of a rubber substance or other material.

Dielectric Properties: The ability of a material to resist puncture due to electric stress. The property is expressed in terms of "volts per MIL thickness".

Diffusion: The mixing of two or more substances (solids, liquids, gasses, or combinations thereof) due to the intermingling motion of their individual molecules. Gasses diffuse more readily than solids.

Durometer: (A) An instrument for measuring the hardness of a rubber; measures the resistance to the penetration of an indenter point into the surface of the rubber; (B) Numerical scale of rubber hardness.

Dynamic: An application in which the seal is subject to movement, or moving parts contact the seal.

Dynamic Packing: A package employed in a joint whose members are in relative motion.

Dynamic Seal: A seal required to prevent leakage past parts which are in relative motion.

Elasticity: The property of an article which tends to return to its original shape after deformation.

Elastomer: Any natural or synthetic material with resilience or memory sufficient to return to its original shape after major or minor distortion.

Electron Volt: Unit of energy in atom calculations equal to 1.602 X 1012 ergs.

Elongation: Generally means "ultimate elongation", or percent increase in original length of a specimen when it breaks.

EPDM (EPT, Nordel [TM, DuPont Co.]): Terpolymer of Ethylene-Propylene-Diene (noted for excellent ozone resistance).

Erg: Unit of energy (C.G.S.) equal to one dyne centimeter, or approximately equal to the work done by force of One (1) milligram, causing a movement of one (1) centimeter.

Evaporation: The direct conversion from liquid to vapor state of a given fluid.

Extrusion: Distortion or flow, under pressure, of a portion of a seal into clearance between mating parts.

Feather Edge: The sharp, thin edge on parts, such as wiper seals and cups. (Also called "Knife Edge").

Fixed Dimension: Dimensions on a rubber product that are not affected by flash thickness or mold closure variation.

Flame Resistance: The resistance to burning of material that will not withstand combustion under ordinary conditions.

Flash: Excess rubber left around a rubber part after molding, due to space between mating mold surfaces; removed by trimming.

Flex Cracking: A surface cracking induced by repeated bending or flexing.

Flex Resistance: The relative ability of a rubber article to withstand dynamic bending stress.

Flexural Strength: The ability of a material to flex without permanent distortion or breaking.

Flock: Fibrous filler sometimes used in rubber compounding.

Flow: Ability of heated plastic, or uncured rubber, to travel in the mold and runner system during the molding process.

Flow Cracks: Surface imperfections due to improper flow and failure of stock to knit or blend with itself during the molding operation.

Fluid: A liquid or a gas.

Fluorocarbon: (Viton) [Viton is a Registered trademark of DuPont Dow Elastomers], Fluorel [TM, 3M Co.]) A polymer designed to meet the most rigid requirements in oils, solvents, synthetic lubricants and corrosive chemicals, at elevated temperatures.

Friction: Resistance to motion due to contact of surfaces.

Friction (Break Out): Friction developed during initial or starting motion.

Friction (Running): Constant friction developed during operation of a dynamic O-Ring.

Fuel (Aromatic): Fuel which contains benzene or aromatic hydrocarbons; causes little swell of rubber.

Fuel (Non-Aromatic): Fuel which is composed of straight chain hydrocarbons; causes little swell of rubber.

Gama Radiation: Electromagnetic disturbance (photons) emanating from an atomic nucleus. This type of radiation travels in wave form, much like X-Rays or light, but has a shorter wave length (approx. 1 Ado or 107 mm). It is very penetrating.

Gasket: A device used to retain fluids under pressure, or seal out foreign matter. Normally refers to static seal.

Gas Permeability: The degree to which a substance resists permeation of gas under pressure.

Gates: The openings in an injection or transfer mold that ensure the even flow of material into the cavity.

Gate Mark: A raised spot or small depression on the surface of an injection or transfer molded part, where the gates interface the cavity. (Also called "Sprue Mark")

Gland: The cavity into which an O-Ring is installed. Includes the groove and mating surface of the second part, which together confine the O-Ring.


Hardness: Resistance to a disturbing force. Measured by the relative resistance of a material to an intender point of any one of a number of standard hardness testing instruments. (See Durometer).

Hardness, Shore (TM, Wilson-Shore Instruments) A: the rubber Durometer hardness as measured on a Shore (TM, Wilson-Shore Instruments) "A" Gauge. Higher numbers indicate harder materials; lower numbers, softer materials.

Heat Aging: A test for degradation of physical properties as a result of exposure to high temperature conditions.

Heat Deflection Temperature: The temperature at which a standard plastic test bar deflects 0.010 in. under a stated load of either 66 PSI or 264 PSI.

Hermetic Seal: An airtight seal having no evidence of detectable leakage.

Homogeneous: (A) General - a material of uniform composition throughout; (B) In seals, a rubber seal without fabric or metal reinforcement.

Hydrocarbon Solvents - Aromatic: Solvents having basic benzene structure, usually coat tar types such as benzene, toluene orxylene.

Hypalon: DuPont trade name for chlorosulphonated polyethylene; an Elastomer.

Identification: Colored dots or stripes on seals for identification purposes; seldom used.

Immediate Set: The deformation found by measurement immediately after removal of the load causing the deformation.

Immersion: Placing an article into fluid, generally so it is completely covered.

Impact: The single, instantaneous stroke or contact of a moving body with another, either moving or at rest, such as a large lump of material dropping on a conveyor belt.

IRHD (International Rubber Hardness Degrees): A method to characterize an Elastomer by its resistance to penetration of a known geometry indenter by a known force. The micro technique is reproduced on irregular, as well as flat, surfaces, and on cross sections as small as 1mm in thickness (.4"). The readings are similar, but not identical, to Shore (TM, Wilson-Shore Instruments) A. (See Durometer)

Injection Molding: Molding in which the rubber or plastic stock is heated and, while in the flowable state, is forced or injected into the mold cavity.

Insert: Typically, a metal or plastic component to which rubber or plastic is chemically and/or physically bonded during the molding process.

Isoprene-acrylonitrite Rubber: A low-plasticity copolymer with around 34 per cent ACN.

Knit Mark: A witness mark on a molded part, usually occurring at the midpoint between two transfer or injection sprue locations. Caused by the incomplete joining of the uncured rubber or plastic from each sprue during molding.

Leakage Rate: The rate at which a fluid (either gas or liquid) passes a barrier. Total Leakage Rate includes the amounts that diffuse or permeate the material of the barrier as well as the amount that escapes around it.

Life Test: A laboratory procedure used to determine the amount and duration of resistance of an article to specific sets of destructive forces or conditions.

Linear Expansion: Expansion in any one linear dimension, or the average of all linear dimensions.

Logy: Sluggish, low snap or recovery of a material.

Low Temperature Flexibility: The ability of a rubber product to be flexed, bent or bowed at low temperature without cracking.

MM Hg: Millimeters of Mercury: In vacuum work, a measure of absolute pressure, being the height of a column of Mercury that the air or other gas will support. Standard atmospheric pressure will support a Mercury column 760 millimeters high. Any value less than that represents some degree of vacuum.

Mechanical Bond: A method of physically bonding rubber to inserts through the use of holes, depressions or projections in the insert.

Memory: The tendency of a material to return to original shape after deformation.

Microhardness: An electronic measurement of rubber hardness for specimens below .25 inch in thickness. Micro hardness, like Shore (TM, Wilson-Shore Instruments) A Durometer, is also a measure of indentation.

Mirror Finish: A bright, polished surface.

Mismatch: Unsymmetrical seal caused by dissimilar cavities in mating mold sections.

Modulus: Tensile stress at specific elongation. (Usually 100% elongation for Elastomers.)

Modulus of Elasticity: One of several measurements of stiffness or resistance to deformation, but often incorrectly used to indicate specifically static tension modulus.

Mold Cavity: Hollow space, or cavity, in the mold, which is used to impart the desired form to the product being molded.

Mold Finish: The uninterrupted surface produced by intimate contact of rubber with mold surface at vulcanization.

Mold Lubricant: A material usually sprayed onto the mold cavity surface prior to the introduction of the uncured rubber, to facilitate the easy removal of the molded part.

Mold Marks: Indentations or ridges embossed into the skin of the molded product by irregularities in the mold cavity.

Mold Register: Accuracy of alignment/fit of mold sections.

Molding Solutions: In the field of custom molded rubber or plastic, the solution to your problems.

Mooney Scorch: The measurement of the rate at which a rubber compound will cure or set up by means of the Mooney Viscometer test instrument.

Mooney Viscosity: Measurement of the plasticity or viscosity of an uncompounded, or compounded vulcanized, Elastomer seal material by means of the Mooney Shearing Disk Viscometer.

Neoprene: (TM, DuPont) (GR-M) A polymer of chloroprene which is prepared from coal, salt and limestone.

Nitrile: (see also Buna-N) The most commonly used Elastomer for O-Rings because of its resistance to petroleum fluids, its good physical properties, and its useful temperature range.

Nominal Dimension: Nearest fractional equivalent to actual decimal dimension.

Non-aromatic: Straight chain organic structures, such as petroleum type solvents.

Non-blooming: The absence of "bloom".

Occlusion: (A) The mechanical process by which vapors, gases, liquids or solids are entrapped within folds of a given substance during working or solidification; (B) the materials so trapped.

Off-register: Misalignment of mold halves causing out-of-round O-Ring cross section.

Oil Resistant: Ability to vulcanize rubber to resist the swelling and deteriorating effects of various types of oils.

Oil Swell: The change in volume of a rubber article due to absorption of oil or other fluid.

O-Ring: A torus; a circle of material with round cross section which effects a seal through squeeze or pressure.

O-Ring Seal: The combination of a gland and O-Ring providing a fluid tight closure. (Some designs permit minimum leakage.)

O-Ring Seal / Moving (Dynamic): O-Ring seal in which there is relative motion between some gland parts and the O-Ring; oscillating, reciprocating or rotary motion.

O-Ring Seal / Non-moving (Static): O-Ring seal in which there is no relative motion between any part of the gland and the O-Ring. (Distortion from fluid pressure or swell from fluid immersion is excluded.)

Optimum Cure: State of vulcanization at which the most desirable combination of properties is attained.

Outgassing: A vacuum phenomenon wherein a substance spontaneously releases volatile constituents in the form of vapors or gases. In rubber compounds, these constituents may include water vapor, plasticizers, air, inhibitors, etc.

Over-Cure: A degree of cure greater than the optimum, causing some desirable properties to be degraded.

Overflow Groove: A groove around the mold cavity used to accept excess material from the cavity and to create a better "pack" for the part.

Oxidation: The reaction of oxygen on a compound, usually detected by a change in the appearance or feel of the surface, or by a change in the physical properties, or both.

Oxygen Bomb: A chamber capable of holding oxygen at an elevated pressure which can be heated to an elevated temperature. Used for an accelerated aging test.

Ozone Resistance: Ability to withstand the deteriorating effect of ozone (which generally causes cracking.)

Packing: A flexible device used to retain fluid under pressure, or seal out foreign matter. Normally refers to a dynamic seal.

Parting Line: The line on the surface of a molded part where the mold plates meet.

Permanent Set: The deformation remaining after a specimen has been stressed in tension for a definite period, and released for a definite period.

Permeability: The rate at which liquid or gas, under pressure, passes through a solid material by diffusion and solution. In rubber terminology: the rate of gas flow, expressed in atmospheric cubic centimeters per second, through an elastomeric material, one centimeter square and one centimeter thick.

Permeation: The diffusion of a media (generally a gas) through a rubber or plastic component.

PH: Determines the concentration of either an acid or a base.

Pit (or Pock) Mark: A circular depression, usually small.

Plasticity: When subject to sufficient shearing stress, any given body will be deformed. After stress is removed, if there is no recovery, the body is completely plastic. If recovery is complete and instantaneous, the body is completely elastic. A balance between the two is required.

Plasticizer: A substance, usually a heavy liquid, added to an Elastomer to decrease stiffness, improve low temperature properties, and improve processing.

Plastometer: An instrument for measuring the plasticity of a raw or unvulcanized compounded rubber.

Polymer: A material formed by joining together many (poly) individual units (mer) of one or more monomers; synonymous with elastomer.

Polymerization: Chemical reaction whereby simple materials, either one or more, are converted to complex material which possesses properties entirely different from the original materials used to start the reaction.

Polyurethane: An organic material noted for its high abrasion, ozone, corona and radiation characteristics.

Porosity: Quality or state of being porous.

Post Cure: The second step in the vulcanization process for some specialized Elastomers. Provides stabilization of parts and drives off decomposition products resulting from the vulcanization process.

Pot: The chamber, in the transfer or injection mold, where raw material is placed before it is transferred into the cavity.

Pure Gum State: A non pigmented, translucent basic polymer.

Radiation: An emission, of varying content, from a disturbed atom undergoing internal change. There are two broad classifications: (A) Corpuscular, comprising streams of particles, either neutral or charged, e.g. protons, neutrons, electrons; and (B) Electromagnetic, comprising wave-like emissions as gamma, ultraviolet, etc.

Radiation Damage: A measure of the loss in certain physical properties of organic substance, such as Elastomers, due principally to ionization of the long chain of molecules; believed to result in redundant cross-linking and possible scission of the molecules. The effect is cumulative.

Radiation Dosage: The total amount of radiation energy absorbed by a substance. This value is usually expressed in "ergs per gram", and is denoted by the following units: (A) Roentgen - a quantity of gamma or X-Ray radiation equal to approximately 83 ergs of absorbed energy per gram of air; (B) REP (Roentgen Equivalent - Physical) - a quantity of ionization that causes an energy absorption of approximately 83 to 93 ergs per gram of tissue; (C) REM (Roentgen Equivalent - Man) - similar to REP, except used to denote biological effects; and; (D) RAD - the unit of dosage related to Elastomers, independent of type of radiation specimen, and denoting energy absorption level of 100 ergs per gram of Elastomer: approximately equal to 1.2 Roentgens.

Rebound: A measure of the resilience, usually as a percentage of vertical return of a body which has fallen and bounced.

Reciprocating Seal: Seal used in linear motion application.

Register: The accurate matching of the plates in a mold.

Reinforcement Agent: Material dispersed in an Elastomer to improve compression, shear or other stress properties.

Relative Humidity: The ratio of the quantity of water vapor actually present in the atmosphere, to the greatest amount possible at a given temperature.

Resilience: Ability of an Elastomer to return to original size and shape after deforming forces are removed; generally expressed in per cent of the ratio of energy removed, to the energy used in compressing. (Resilient: having that capability.)

Rotary Seal: A seal, such as an O-Ring or a Quad-Ring seal, exposed on either the ID or OD sealing surface to a rotating component,e.g. shaft seals.

Rough Trim: Removal of superfluous material by pulling or picking; usually the removal of a small portion of the flash or sprue which remains attached to the product.

Rubber: See Elastomer.

Rubber Natural: Raw or crude rubber obtained from vegetable sources.

Rubber, Synthetic: Manufactured or man-made Elastomers.

Runner: The system for leading rubber and plastic materials into the gate of an injection mold.

Runout (Shaft): Expressed in inches and/or TIR (Total Indicator Reading); refers to twice the radial distance between shaft axis and axis of rotation.

SBR: Copolymer of Butadiene and Styrene; an all purpose type synthetic, similar to natural rubber. (Butadiene is a gaseous material of petroleum; Styrene, a reaction product of ethylene and benzene.)

Scorching: Premature curing or setting up of a raw compound during processing.

Seal: Any device used to prevent the passage of a fluid, gas or liquid.

Seal-Off: A minimum .032 inch (.813 mm) step on an insert that minimizes flash on the OD, and prevents flash from forming across the insert.

Service: Operating conditions to be met.

Shaft: Reciprocating or rotating member, usually within a cylinder; not in contact with the walls.

Shelf Aging: The change in a material's properties which occur in storage with time.

Shore (TM, Wilson-Shore Instruments) A: See Durometer.

Shore (TM, Wilson-Shore Instruments) M: A Durometer hardness instrument, using a micro-indenter designed for the purpose of measuring O-Ring hardness.

Shrinkage: 1) The ratio between a mold cavity size and the size of a product molded in that cavity, 2) Decreased volume of a seal, usually caused by extraction of soluble constituents by fluids followed by air drying.

Silicone Rubber: Elastomer that retains good properties through extra wide temperature ranges.

Size, Actual: Actual dimension of the product, including tolerance units.

Size, Number: Number assigned to indicate inside and cross section diameters of an O-Ring; established in the SAE standard AS 568, adopted by the military and industry.

Sliding Core: A pin on a mold that automatically retracts when the mold opens.

Sorption: A term used to denote the combination of adsorption and absorption processes in the same substance.

Specific Gravity: Ratio of the weight of a given substance, to the weight of an equal volume of water, at any specific temperature. Sphericity: The measure of a tolerance of a molded ball, or ground ball, in reference to a perfect sphere; also described as "roundness".

Spiral Twist: A type of seal failure in reciprocating application that results from the twisting action that strains or ruptures the rubber.

Sprue: The primary feed channel that runs from the outer face of an injection or transfer mold, to the mold gate in a single cavity mold, or to the runners in a multiple cavity mold.

Sprue Marks: Marks left on the surface of a rubber part, generally elevated, after removal of the sprue or cured compound in the gate, through which the compound is injected or transfer molded.

Squeeze: Cross section diametrical compression of O-Ring between surface of the groove bottom and surface of the other mating metal part in the gland assembly.

Static Seal: Part designed to seal between parts having relative motion. (See Gasket)

Statistical Process Control (SPC): The use of statistical techniques on processes and their output, to establish process stability and increase capabilities.

Strain: Deflection due to force.

Stress: Force per unit of original cross section area.

Stress Relaxation: Decreasing stress with constant strain over a given time interval. (Viscoelastic response.)

Sublimation: The direct conversion of a substance from solid state to vapor state, passing through a transitory liquid state. The vapor, upon recondensing, reforms into the solid state with no intervening liquid phase.

Surface Finish: A numerically averaged value of surface roughness, generally in units of microinches or micrometers.

Sun Checking: Surface cracks, checks or grazing caused by exposure to direct or indirect sunlight.

Swell: Increased volume of a specimen, caused by immersion in a fluid (usually liquid).

Tack: The degree of adhesion of materials of identical nature to each other.

Tear Strength: The force required to rupture a sample of stated geometry.

Tear Resistance: Resistance to growth of a cut or nick when tension is applied to the cut specimen. Commonly expressed as pounds per square inch thickness.

Temperature Range: Maximum and minimum temperature limits in which a seal compound will function in a given application.

Tensile Strength: Force, in pounds per square inch, required to cause the rupture of a specimen of rubber material.

Tension Modulus: Resistance to being stressed; defined as the force, in pounds, necessary to stretch a piece of rubber, one square inch in cross section, a specified amount. Normally expressed as a percentage of original length, and the stress as pounds per square inch at the fixed elongation.

Terpolymer: A polymer consisting of three different monomers, chemically combined.

Thermal Expansion: Expansion caused by increase in temperature; may be linear or volumetric.

Thermoplastic: A plastic capable of being repeatedly softened by increase of temperature, and hardened by decrease of temperature.

Thermoplastic Rubber: Rubber that does not require chemical vulcanization and will repeatedly soften when heated and stiffen when cooled; and which will exhibit only slight loss of original characteristics.

Thermoset: An Elastomer or plastic cured under application of heat or chemical means, to make a product substantially infusible or insoluble.

Thiokol: An organic polysufide.

Threshold: The maximum tolerance of an Elastomer to radiation dosage, expressed as a total number of ergs per gram (or rads) beyond which physical properties are significantly degraded. This is generally an arbitrary value, depending on function and environment.

Torque: The turning force of a shaft.

Torr: The unit of pressure used in vacuum measurement; equal to 1/760 of a standard atmosphere, and for practical purposes, is equivalent to one millimeter of Mercury (mmHg).

Torsion Strength: Ability of rubber to withstand twisting.

Total Indication Reading (TIR): System that measures the roundness of a part in relationship to a center line.

Transfer Molding: A method of molding in which material is placed in a pot, located between the top plate and plunger, and squeezed from the pot through gates (or sprues) into the mold cavity.

Trapped Air: Air which is trapped in a product or a mold during cure. Usually causing a loose ply or cover, or a surface mark, depression or void.

Trim: The process involving removal of mold flash.

Trim Out: Damage to mold skin or finish by close trimming.

Under Cure: Degree of cure less the optimum; may be evidenced by tackiness, loginess or inferior physical properties.

Ultimate Elongation: A measure of how far a material will stretch before breaking; expressed as a percentage of its original length.

Undercut: A groove on either the outside or the inside of a molded part.

Vacuum: The term denoting a given space that is occupied by a gas at less then atmospheric pressure. For degrees of vacuum, see Vacuum Level.

Vacuum Level: The term used to denote the degree of vacuum evidenced by its pressure in torr (or mmHg):(a) Rough vacuum = 760 torr to 1 torr, (b) Medium vacuum = 1 torr to 10-3 torr, (c) High vacuum = 10-3 torr to 10-6 torr, (d) Very High vacuum = 10-6 torr to 10-9 torr, (e) Ultra High vacuum = below 10-9 torr.

Vapor: The gaseous state of a fluid that normally exists as a liquid under atmospheric conditions, i.e. a gas whose temperature is below its critical temperature.

Vapor Pressure: The maximum pressure exerted by a liquid or a solid, heated to a given temperature in a closed container.

Vibration Dampening: The ability of an Elastomer to absorb vibrational or shock energy.

Viscosity: The property of fluids and plastic solids by which they resist an instantaneous change of shape, i.e. resistance to flow.

Void: The absence of material, or an area devoid of materials where not intended.

Volatilization: The transition of either a liquid or a solid directly into vapor state. In the case of a liquid, this transition is called evaporation, whereas in the case of a solid, it is termed sublimation.

Volume Change: A change in the volume of a seal as result of immersion in a fluid; expressed as a percentage of the original volume.

Volume Swell: An increase in the physical size caused by the swelling action of a liquid.

Vulcanization: A thermosetting reaction involving the use of heat and pressure, resulting in greatly increased strength and elasticity of rubber-like materials.

Vulcanizing Agent: A material which produces vulcanization of an Elastomer.

Weather Resistance: The ability to withstand weathering factors, such as: oxygen, ozone, atmospheric pollutants, erosion, temperature cycling and ultraviolet radiation.

Weathering: The detrimental effect upon an Elastomer or plastic after outdoor exposure.

Width: Seal cross-section or thickness.

Wiper Ring: A ring employed to remove excess fluid, mud, etc., from a reciprocating member before it reaches the packings.


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