Sealing Industry Glossary

Welcome to our interactive online Glossary.

Because our industry’s jargon and processes are unique, we’ve dedicated this glossary to be a centralised spot that will serve as a reference to both industry veterans and newcomers alike.

Our intuitive Glossary guide helps you determine the chemical properties, temperature ranges and key uses of chemicals in O-Ring and seal production.

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  • Abrasion

    The wearing of a surface by mechanical action such as rubbing or scraping.

  • Abrasion Resistance

    The ability of a rubber compound to resist mechanical wear.

  • Absorption

    The physical mechanism by which one substance attracts and takes up another substance (liquid, gas, or vapour) into its interior.

  • Accelerated life test

    A test method designed to approximate, in a short time, the deteriorating effect of normal long-term service conditions.

  • Accelerators

    Chemical added to an elastomer to speed up the rate of vulcanisation, causing it to take place in a shorter time

  • Acid Resistant

    Withstanding the action of acids.

  • Activators

    Chemicals added to an elastomer to increase the speed of an accelerator

  • Adhere

    To stick or cling together.

  • After cure

    Continuation of the vulcanization process after the main cure has completed.

  • Age resistance

    the ability of a material to resist deterioration caused by ageing.

  • Ageing

    Change of material properties during exposure to a deteriorating environment. Can also refer to controlled exposure of rubber samples to a variety of deteriorating influences

  • Air Checks/Traps

    Surface marks or depressions caused by the trapping of air between the rubber surfaces being cured and the mould surface.

  • Aliphatic

    A major group of organic compounds with straight chains of carbon atoms. Three sub-groups that comprise aliphatic hydrocarbons are paraffins (alkanes) olefins(alkenes) and acetylene(alkynes)

  • Ambient temperature

    Temperature of the environment surrounding a given point of application.

  • AMS

    Aerospace Material Specification

  • Aniline point

    The lowest temperature at which equal parts of aniline and a test liquid (oil) will completely blend. Generally the lower the aniline point, the greater the chance of swelling of the rubber.

  • Antioxidant

    A substance which inhibits or retards oxidation.

  • Antiozonant

    A substance which inhibits or retards ozone deterioration.

  • Aromatic oil

    A process oil containing at least 35% (by mass), of aromatic hydrocarbons

  • AS568

    Aerospace Standard for O Ring numbering. All military standards use this system.

  • ASTM

    American Society for Testing and Materials

  • Atmospheric Cracking

    Cracks in surface of rubber articles caused by exposure to atmospheric conditions.

  • Axial seal

    a term usually applied to an O-Ring where the squeeze is applied on the top and bottom surfaces of the seals cross section. Also used as a term for a face seal.

  • 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.

  • Backrind

    A moulding defect showing as distortion at the parting line, usually in the form of a ragged indentation.

  • Batch

    Product of one mixing operation.

  • Bench Test

    A laboratory test approximating service conditions

  • Blank

    The rubber compound required to fill the mould cavity

  • Bleeding

    The migration to the products surface of plasticizers, waxes, or similar which form a film or beads. This is caused because of incompatibility with the base elastomer and/or compound ingredients.

  • Blemish

    A surface mark or deformity.

  • Blister

    A surface or internal imperfection, created by entrapped gases or internal void during the manufacture of rubber articles.

  • Bloom

    A white or milky coloured deposit caused by the migration of certain compound components to the rubber’s surface after moulding / storage. This does not affect the performance of the material.

  • Bond

    Chemical bonding involves contact adhesives with heat and pressure to adhere an elastomer to a primed surface.

  • Bore

    A hole in a component which permits the passage of a shaft.

  • Break

    A separation in any part of an article.

  • Break-out

    The force required to overcome initial seal to gland surface adhesion, when part movement is intermittent. This is a common term to describe one type of friction.

  • Brittleness

    Tendency to crack when deformed.

  • Brittlepoint

    The temperature at which a rubber specimen will fracture under sudden impact.

  • BSI

    British Standards Institution is a business services provider whose principal activity is the production of standards and the supply of standards-related services.

  • Buna-N

    Nitrile rubber.

  • Butyl

    A copolymer of isobutylene with small amounts of isoprene.

  • Carbon black

    Black pigment used as a reinforcing agent in elastomer compounds.

  • Cavity

    The features of a mould which are directly responsible for forming the final shape of a moulded part. Mould cavities are formed from two or more mating components of a mould.

  • Cellular Rubber

    Generic term for materials containing many cells dispersed through the mass of material’

  • Co-polymer

    An elastomer consisting of two different monomers chemically combined.

  • Coefficient of Thermal Expansion

    Average expansion per degree over a stated temperature range expressed as a fraction of initial dimension.

  • Cold Flexibility

    Flexibility of an elastomer following exposure to a set low temperature for a specified time.

  • Cold Flow

    Describes the tendency of some materials to deform or creep under constant sealing pressure.

  • Cold Resistant

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

  • Composite Seal

    Combines the sealing performance of elastomer with the physical properties of the bonded material. (metal / plastic etc.)

  • Compound

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

  • Compression Modulus

    The ratio of the compressive stress to the resulting compressive strain (the latter expressed as a fraction of the original height or thickness in the direction of the force). Compression modulus may be either static or dynamic.

  • Compression moulding

    Process in which the uncured material is placed directly in the mould cavity and compressed to shape by closure of the mould under heat and pressure.

  • Compression Set

    The amount by which a rubber specimen does not fully return to its original shape after release from a constant compressive load.

  • Conductive rubber

    a rubber capable of conducting (generally static) electricity

  • Corrosion

    Wearing of a surface due to chemical reaction.

  • Cracking

    A sharp break or crack in the surface of the seal, generally due to excessive strain.

  • Creep

    Progressive relaxation of rubber material while under stress. Eventually results in permanent deformation.(also known as cold flow)

  • Cross-linking

    The formation of chemical bonds in the polymer chains of a rubber during the moulding process.

  • Cross-Section

    A seal as viewed if cut at right angles to the mould parting line. Also known as the width.

  • Curatives

    The chemicals involved in curing the rubber, e.g. Accelerators, vulcanising agents, activators etc.

  • Cure

    See Vulcanization.

  • Cure date

    Date when the part was moulded. For example 1Q15 indicates a cure date of first quarter 2015

  • Curing

    Also known as vulcanisation, this is the permanent change which the rubber undergoes during moulding. When mixed, rubber compounds are thermoplastic and will melt and deform easily. After curing, the rubber is heat stable.

  • Curing temperature

    Temperature at which a rubber product is vulcanised.

  • Cylinder

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

  • Deflashing

    Process used to remove the excess material (flash) from a finished moulded rubber part.

  • Degassing

    Controlled outgassing of the volatile components of an elastomer.

  • Density

    The mass per unit volume of a material. (Also known as specific gravity).

  • Diffusion

    The mixing of two or more substances (solids, liquids, gases, or a combination of them) due to the intermingling motion of their individual molecules. Gases diffuse more readily than liquids; similarly, liquids diffuse more readily than solids.

  • Dumbbell specimen

    a flat specimen of rubber having a narrow straight central portion of uniform cross section with enlarged ends. Used for testing purposes

  • Durometer

    (a) Numerical scale of hardness of a rubber compound. The numerical hardness ranges from 30-40 for softer materials to 90-95 for harder materials.
    (b) An instrument for measuring the hardness of rubber. It measures the resistance to the penetration of an indenter point into the surface of rubber

  • Dynamic

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

  • Dynamic Seal

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

  • Elasticity

    The recovery of an article which returns it to its original shape after deformation.

  • Elastomer

    A term to describe synthetic or natural materials with resilience or memory sufficient to return to its original shape after distortion.

  • Elongation

    Generally means “ultimate elongation” or percent increase in original length of a specimen when it breaks.

  • Explosive decompression

    the rupture of a rubber article caused by the rapid reversal of pressure, causing dissolved gases in the rubber to escape quickly to the surface of the vulcanizate.

  • Extrusion

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

  • Face Seal

    A seal between two flat surfaces. (Also see axial seal)

  • FDA

    Food and Drug Administration is a US Government agency that regulates ingredients in rubber compounds that are intended for use in food and medical applications.

  • Filler

    Material added to the elastomer to aid processing and improve physical properties, i.e., abrasion resistance and strength – giving it varying degrees of hardness. Typical example is carbon black.

  • Flash

    Excess rubber found on the rubber part after moulding. This refers particularly to thin films of rubber formed at tool split lines etc. It is removed by trimming.

  • Flex Cracking

    The tendency of some materials to crack as a result of repeated bending or stressing at the same point.

  • Flex Resistance

    The ability of a rubber article to withstand dynamic bending stresses.

  • Flow lines

    Surface imperfections on a moulded article caused by failure of the elastomer to blend with itself during the moulding operation.

  • Formulation

    A list of the various materials used in the preparation of a compound. Can also be known as describe a rubber “recipe.

  • Friction

    Resistance to motion due to the contact of surfaces.

  • Friction (Breakout)

    Friction developed during initial or starting motion.

  • Friction (Running)

    Constant friction developed during operation of a dynamic O-Ring.

  • Gasket

    A static sealing device used to retain fluids under pressure or to keep out foreign matter.

  • Gland

    Cavity into which a seal is installed.

  • Groove

    The machined recess into which an O-Ring is installed.

  • Groove Depth

    The groove depth is the distance from the sealing surface to the seal groove surface. The groove depth determines how much the seal is compressed and how much cross-sectional compression (squeeze) is applied to the seal.

  • Hardness

    Material property characterised by resistance to distortion. This is measured by resistance of a material to the indenter point of a standard hardness testing instrument.

  • Hardness Shore A

    This is the rubber durometer hardness as measured on a Shore “A” gauge. Higher numbers indicate harder material. A Shore “A” reading of 35 is considered soft and 90 is considered hard.

  • Heat Ageing

    Rubber materials are often tested for shorter times at higher temperatures to predict what will happen over longer times and at lower temperatures. This is also known as accelerated ageing.

  • Hermetic Seal

    An airtight seal.

  • Homogeneous

    (a) General – a material of uniform composition throughout.
    (b) In seals – a rubber seal without fabric or metal reinforcement.

  • I.D.

    the inside diameter of an O ring / seal.

  • I.R.H.D. (International Rubber Hardness Degrees)

    A system of defining an elastomer by its resistance to penetration of a known geometry indenter by a known force.
    The micro-technique is reproducible on irregular as well as flat surfaces and on cross-sections as small as 1mm in thickness (.04″). Measurements taken using the IRHD scale are similar, but not identical, to those obtained using the Shore A scale.

  • Immersion

    Placing an item into fluid so that it is completely covered.

  • Injection Moulding

    A moulding method in which a rubber or plastic material is heated and forced under pressure into the mould cavity.

  • Insert

    A term referring to a metal or plastic component, placed into a mould cavity prior to the start of the moulding cycle, to which rubber or plastic is chemically and/or physically bonded during the moulding process.

  • ISO

    International Organization for Standardization. ISO’s main products are international standards. ISO also publishes technical reports, technical specifications, publicly available specifications and guides

  • Joule Effect

    A characteristic of rubber where the rubber, under tension when heated, contracts rather than expands. This effect only occurs when rubber is subject to strain while being heated – unstrained rubber will expand as it is heated.

  • Leakage Rate

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

  • Life Test

    A laboratory procedure used to establish the amount and duration of resistance of an article to a specific set of destructive forces or conditions.

  • Linear Expansion

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

  • Low Temperature Flexibility

    The ability of a rubber product to be flexed or bent at low temperatures without cracking.

  • Memory

    Ability of a material to return to original shape after deformation.

  • Micro-hardness

    A measurement of rubber hardness. Micro-hardness, like Shore A duro, is also a measurement of indentation. It uses a smaller indenter and lower force than a standard instrument and allows measurements on smaller specimens and thinner sheets.


    Military Standard.

  • Mismatch

    Moulding defect which results in rings being out of line. Caused by misalignment of the mould halves i.e. the top and bottom are out of line. Also referred to as ‘off register’

  • Modulus

    Tensile stress at a specified elongation. (Usually 100% elongation).

  • Modulus of elasticity

    A measurement of stiffness or resistance to deformation.

  • Mould Cavity

    See cavity

  • Mould Finish

    The surface finish of the mould which provides the required surface quality of the finished part.

  • Mould marks

    Surface imperfections caused by marks on the mould

  • Mould release

    Lubricant used to assist the removal of rubber product from the mould.

  • MS

    Military Standard

  • Natural Rubber

    Raw or crude rubber obtained from plant sources.

  • Nitrile

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

  • Nominal size

    Approximate size of an O-Ring expressed in fractional dimensions.

  • Non-fill

    A defect caused by the failure of rubber material to totally fill the mould cavity.

  • O-Ring

    A torus; a circle of material with round cross-section which provides a seal through squeeze and pressure.

  • O-Ring Seal

    The combination of a gland and an O-Ring providing a fluid-tight closure.
    Moving (Dynamic) – O-Ring seal where there is relative motion between some gland parts and the O-Ring (oscillating, reciprocating, or rotary motion).
    Non-moving (Static) – O-Ring seal in where 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).

  • O.D.

    Outside diameter of an O Ring / seal.

  • Occlusion

    The mechanical entrapment of gases, liquids or solids within a substance.

  • Off register

    See mismatch.

  • Oil Resistant

    The ability of a rubber to resist swelling and deteriorating effects of various oils.

  • Oil Swell

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

  • Optimum Cure

    degree of cure giving the most desirable properties.

  • Outgassing

    Occurs in vacuums where the volatile materials in a rubber compound are vaporized and released into the surrounding environment.

  • Overcure

    An amount of cure greater than the optimum, causing some properties to be degraded, usually loss of elongation and increase of hardness.

  • Oxidation

    The reaction of oxygen on a rubber product, usually detected by a change in the appearance or feel of the surface or by a change in physical properties.

  • Ozone Resistance

    The ability to withstand the deteriorating effect of ozone (which usually causes cracking).

  • Permanent set

    The amount of deformation in a rubber part after a distorting load has been removed.

  • Permeability

    The rate at which a liquid or gas under pressure passes through a solid material by diffusion and solution.

  • Piston Seal

    A bore seal in which the seal is mounted in a groove machined into a piston. The term piston seal usually implies an application involving linear reciprocating motion.

  • Plasticisers

    Chemical agents which are added to the rubber compound to soften the elastomer and provide flexibility and improve processing.

  • Polymer

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

  • Porosity

    The presence of pores or holes in a material.

  • Post Cure

    The controlled continuation of vulcanization (usually in an oven), which completes the curing process, drives off residual products, and gives stabilized parts.

  • Potable

    A liquid that is safe or suitable for drinking

  • QS 9000

    Quality System Model establishes a set of standards for the automotive industry. This standard incorporates ISO 9001 plus the automotive requirements.

  • Radial Seal

    Compression is directed along the radius of the seal.

  • Reciprocating Seal

    Seals used in moving piston and rod applications.

  • Reinforcing Agent

    Ingredient added to a rubber formulation to improve physical properties such as tensile strength, compression set. Carbon black is a common reinforcing agent used in rubber.

  • Resilience

    The term used to describe “bounce” or “snap”. – the capability of returning to the original size and shape after deformation.

  • Resilient

    Capable of returning to original size and shape after deformation.

  • RMS

    Root Mean Square. A measure of surface roughness typically applied to the machining of metal gland and shaft surfaces.

  • Rod

    See shaft

  • Rod Seal

    A sealing system, usually in a radial orientation, in which the main sealing surface is between the I.D. of a seal ring and the O.D. of a shaft.

  • Rotary Seal

    Seals for rotating shaft, with the turning shaft protruding through the inside dia of the ring/seal.

  • Rubber

    See elastomer.

  • SAE

    Society of Automotive Engineers

  • Scorching

    Premature curing of compounded rubber during processing or storage. This may adversely affect material flow and plasticity during subsequent processes.

  • Seal

    A device used to prevent the passage of a fluid (gas or liquid).

  • Sealing Surface

    Where the seal and mating surface come in contact to form a barrier to prevent the passage of some type of medium i.e. fluid or gas.

  • Service

    Operating conditions to be met.

  • Set

    The degree to which a rubber does not fully recover to its original shape after it has been deformed for a long period of time. Also referred to as permanent set.

  • Shelf-Aging

    The change in an elastomer’s properties which can occur due to exposure to stressful environmental factors during storage. This can affect the seal performance. This problem can be avoided by using correct packaging and storage conditions.

  • Shore A Hardness

    See Hardness and Durometer.

  • Shrinkage

    a) Decreased volume of seal, usually caused by extraction of soluble constituents by fluids followed by air drying.
    b) Difference between finished part dimensions and mold cavity used to make the part.

  • Silicone Rubber

    Elastomer that retains good properties through extra wide temperature range.

  • Size Number

    Number assigned to indicate inside and cross section diameters of an O-Ring. For example, sizes in standard AS 568A were adopted by military and industry

  • Size, Actual

    Actual dimensions of the O-Ring or other seal, including tolerance limits.

  • Size, Nominal

    the approximate size of an O-Ring expressed in fractional dimensions.

  • Specific Gravity

    The ratio of the weight of a given substance to the weight of an equal volume of water at a specified temperature. It is used as a comparison tool to determine the relative density of seal materials, helping to identify base polymers and compounds.

  • Spiral failure

    Spiral failure occurs when certain parts of the O ring slide while other parts simultaneously roll. The ring gets caught on an eccentric component or against the cylinder wall, rolls and is twisted in the application. This results in cuts or marks which spiral around its circumference

  • Sprue Marks

    Marks, usually raised, left on the surface of a rubber part, after removal of the sprue (pouring nozzle) or cured compound in the gate through which the compound is injected or transfer molded

  • Squeeze

    The compression of O-ring between surface of the groove bottom and mating metal part in the assembly.

  • Static Seal

    Part used to seal between parts having no relative motion.

  • Strain

    Deformation of rubber material due to an applied force (stress).

  • Stress

    An applied force or load.

  • Stress Relaxation

    Decreasing stress with constant strain over a given time.

  • Surface Finish

    A term used to refer to the roughness of a surface’s texture.

  • Swell

    Increased volume of a specimen caused by immersion, usually in a liquid (oils).

  • Synthetic Rubber

    Manufactured or man-made elastomers.

  • Tear Resistance

    Resistance to the growth of a cut or nick when tension is applied to the specimen. Commonly expressed as pounds per inch (psi) thickness.

  • Temperature Range

    Maximum and minimum temperature limits within which a seal compound will function effectively in a given application.

  • Tensile Strength

    The extension force per cross-sectional area required to fracture a material specimen.

  • Tensile Strength

    Force in pounds per square inch required to cause the rubber specimen to reach breaking point.

  • Terpolymer

    A polymer consisting of three different, chemically combined monomers.

  • Thermal Degradation

    Deterioration of the material properties due to exposure to heat / high temperatures.

  • Thermal Expansion

    Expansion caused by increase in temperature.

  • Thermoplastics

    Polymeric materials that soften and can be re-formed when heated, returning to original properties when cooled.

  • Thermoset

    Elastomers that undergo a permanent chemical crosslinking of molecules when processed, heated and moulded and cannot be reprocessed.

  • Torque

    Turning or twisting force, generally associated with the rotation of a shaft.

  • Torsional strength

    Ability of a seal to withstand damage due to twisting.

  • TPE

    Thermoplastic Elastomer combines the rubberlike performance of elastomers with the processing advantages of plastic. Scrap material can be recycled without significant loss of physical properties.

  • Trim

    Removal of excess material (flash) from a moulded rubber article.

  • Trim cut

    Damage to the moulded article by trimming too close.

  • Ultimate Elongation

    See Elongation.

  • Undercure

    Removing rubber from the mould before the vulcanisation process has been completed. Often results in porosity in the rubber or unusually poor compression set.

  • Viscosity

    The resistance of a material to flow under stress.

  • Voids

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

  • Volume Change

    A change in the volume of a seal as a result of immersion into a fluid, expressed as a percentage of the original volume.

  • Volume Swell

    Increase in physical size caused by the swelling action of a liquid.

  • Vulcanization

    A process involving the use of heat and pressure, resulting in a permanent chemical change, giving greatly increased strength and elasticity of rubber materials.

  • Vulcanizing Agent

    A material which assists vulcanization of an elastomer.

  • Weathering

    The detrimental effect upon a material after outdoor exposure. For example surface cracks on an O ring after exposure to ozone.

  • Width

    Seal cross-section or thickness


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