Listed below are many of the terms used within the seal manufacturing industry.

Term Meaning
Abrasion Resistance The ability of a rubber compound to withstand mechanical deterioration due to rubbing, scraping, or erosion.
Absorption The physical attraction and incorporation of a substance (liquid, gas, or vapour) into a material’s interior.
Accelerated Life Test Test conditions designed to determine the effect normal service conditions have on the life of the material.
Accelerated Service Test Exaggeration of service conditions such as speed, temperature, or continuity of operation, for the purpose of testing and evaluation of a material in a shorter amount of time.
Acid Resistant A material that can function in an acidic environment.
Aging Physical changes that occur due to passage of time.
Air Curing Using hot air ovens or steam to vulcanize rubber.
Ambient Temperature Environmental temperature around an object.
Antioxidant Any organic compound that slows the process of oxidation.
Aromatic A compound, such as benzene, characterized by a cyclic arrangement of unsaturated hydrocarbon molecules.
Atmospheric cracking When something in the atmosphere causes cracks in the rubber surface.
Axial Seal A seal, like a face seal, in which the O-ring is squeezed on the top and the bottom of its cross-section.
Backrind A ragged indentation at the parting line.
Back-up Ring A hard, tough ring of material installed in the gland to prevent O-ring extrusion.
Bleeding When compound components, such as plasticizers, migrate to a material’s surface and becomes a film or beads.
Blemish Anything impairing the appearance of a compound, such as a mark, deformity, or injury.
Blisters Surface bumps on the O-ring caused by an air pocket or internal void.
Bloom Migration of a liquid or solid to the surface of an O-ring resulting in a dusty or milky looking deposit. This discolouration does not adversely affect material performance.
Bond A force holding an elastomer to another material. A bond can be mechanical (e.g. screw), “cold” (e.g. cement), or “vulcanized” (e.g. heat and pressure).
Break-out Friction The frictional force developed at startup, due to the tendency of O-rings to adhere to the microfine grooves of surrounding surfaces.
Brittleness The property of a n O-ring causing cracking when it is deformed.
BSI British Standards Institute
Calender The process of turning uncured rubber into sheets by squeezing it between iron or steel rollers.
Clearance, diametrical The gap, formed by the mating metal surfaces, which the O-ring seals to prevent leakage in the system.
Coefficient of thermal expansion Average material expansion per degree of temperature over a defined range, expressed as a fraction of initial dimension.
Cold flexibility The flexibility still remaining in an elastomer after exposure to low temperature for a specific amount of time.
Cold flow Continued deformation or “creep” of a material under constant stress.
Compound A combination of ingredients, including polymers, to produce a useful elastomer.
Compression modulus The stress to strain ratio of a tested material, expressed as a fraction of the original height or thickness in the direction of the applied force.
Compression set Failure of an elastomer to return to original size after release from constant strain.
Copolymer Two monomers chemically combined to form an elastomer, like SBR formed from styrene and butadiene.
Cracking Surface breaks or fissures due to excessive strain or exposure to adverse environmental factors.
Creep Continued relaxation of a material while it is under stress.
Cross section The width of an O-ring or a seal view showing the internal structure at right angles to the mould parting line.
Cross sectional diameter (CS) One-half the difference between the ID and OD of an O-ring.
Cure The creation of an elastomer through chemical bonding of polymers and moulded rubber, usually with the help of sulfur and an accelerator, under pressure, at elevated temperatures. Also see, vulcanization.
Cure Date O-ring moulding date. A product code of 3Q04, for example, indicates a cure date of the third quarter (3Q) of 2004 (04).
Curing temperature The temperature of vulcanization.
Deflash The removal of unwanted, excess material from a finished rubber product.
Degassing The intentional but controlled outgassing of a rubber substance or other material.
Durometer An instrument for measuring the hardness of rubber or a numerical scale of rubber hardness.
Dynamic seal Any application involving reciprocating, rotating, or oscillating motion relative to the seal.
Elasticity The tendency of a material to return to its original shape after deformation.
Elastomer Any synthetic or natural material with resilience or memory sufficient to return to its original shape after major or minor distortion.
Elongation The percent increase in original length of a test material upon breakage, relative to a specified load.
Extrusion When increased system pressure forces part of the O-ring to distort or flow into the clearance between the metal parts it is sealing.
Face seal A type of axial seal.
FDA Food and Drug Administration - A U.S. government agency that regulates the ingredients in rubber compounds that are intended for use in food and medical applications.
Filler A material, such as carbon black, clay, calcium carbonate, and silica, which is used to change the properties of an elastomer. The purpose could be for reinforcement, processing, or reduction of cost.
Flash Excess rubber left around a rubber part after moulding due to space between mating mould surfaces. Can be removed by trimming.
Flex resistance The relative ability of an elastomer to resist the stress of constant bending.
Flow cracks (or lines) Surface imperfection due to the failure of the rubber stock to properly blend with itself during moulding.
Fluid A liquid or gas.
Friction Resistance of motion caused by surface contact.
Gasket A static device used to retain fluids under pressure or seal out foreign matter.
Gland The groove and mating metal surfaces, forming the cavity in which the O-ring is installed.
Groove The machined glandular recess into which the O-ring is fitted.
Hardness Resistance of rubber to forced distortion measured by the standard hardness testing instruments.
Hardness, Shore A Durometer reading in degrees of rubber hardness as measured on a Shore A gauge. Scale is 0-100, with higher numbers indicating greater hardness.
Hermetic seal An airtight seal.
Homogenous A mixture that is uniformly dispersed. In seals - a rubber seal without fabric or metal reinforcement.
ID The inside diameter of an O-ring.
ISO International Organization for Standardization - model for quality assurance in design, development, production, installation and servicing.
Leakage rate The rate at which a fluid (either gas or liquid) passes a barrier.
Life test A comparison of the relative performance capabilities of various products. This laboratory test determines the amount and duration of product resistance to multiple destructive forces or conditions.
Low temperature flexibility The ability of an elastomeric product to be flexed, bowed, or bent at low temperatures without evidence of cracking.
Memory Tendency of an elastomeric material to return to its original size and shape after deformation.
MIL STD Abbreviation for Military Standard
Mismatch When dissimilar mating surfaces lead to an unsymmetrical seal, leading to moulded part flash.
Modulus For elastomer, this is the tensile stress (in psi) at 100% elongation.
Modulus of elasticity One of several measurements of stiffness or resistance to deformation.
Mould cavity The part of the mould that shapes the uncured rubber into its desired form, usually a hollow space or cavity.
Mould finish The part of the mould that gives the uncured rubber its finished, uninterrupted surface.
Mould lubricant A material sprayed in the mould cavity that eases the removal of the moulded rubber products.
Mould marks Surface irregularities, indentations, or ridges on the moulded product due to marks in the mould cavity, or damage to the mould.
Mould register Alignment of all the parts of the mould cavity.
Mould release See mould lubricant.
MS Abbreviation for Military Standard.
Natural Rubber A raw or crude product of certain vegetable 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 dimension Nearest fractional equivalent to actual decimal dimension, from which small dimensional (plus and minus) deviations are allowed as manufacturing tolerances.
Non-Fill An incomplete part caused by the rubber failing to completely fill the mould cavity.
O.D The outside diameter of an O-ring - a dimensional reference.
Occlusion The mechanical process by which vapours, gases, liquids, or solids are entrapped within the folds of a substance during working or solidification, or the trapped materials.
Off register See mismatch.
Oil resistant An elastomer that can be exposed to oil products without detrimental effects, such as swell and deterioration.
O-ring A packing or sealing device shaped like an “O”, which is compressed between two mating surfaces to prevent the escape of anything it is sealing.
O-ring seal The combination of a gland and an O-ring providing a fluid-tight closure. (Some designs may permit momentary or minimum leakage.) Also, see dynamic seal and static seal.
Oscillating seal A seal that moves in a restricted manner, a few turns in one direction and a few turns in the opposite direction, like a faucet. Movement is about the axis of a shaft, in an arc pattern.
Outgassing The vapourization and release of components (water vapour, plasticizers, air, inhibitors, etc.) in some rubber compounds due to pressure in a vacuum situation.
Oxidation A change in the surface of a rubber compound due to its reaction with oxygen.
Ozone resistance The ability of a rubber product to withstand the harshness of ozone, which can cause cracking and physical deterioration.
Packing A flexible device, typically used in dynamic situations to retain fluids under pressure, or seal out foreign matter.
Permanent set The deformation remaining in a rubber specimen after a definite time of stress and relaxation.
Permeability Rate of gas flow through a solid material, under pressure. Expressed in atmospheric cubic centimetres per second through an elastomeric material one centimetre square and one centimetre thick (atm cc/cm2/cm/sec).
Pit or pock mark A small circular depression.
Plasticiser A chemical added during elastomer processing to decrease stiffness, improve low temperature properties, improve processing, increase elongation, etc.
Polymer The chemical combination of many (poly), similar molecules (mers). See elastomer.
Post cure A process, following initial vulcanization, used to stabilize parts and clear the elastomer of residual decomposition products.
Reciprocating seal Seals used in moving piston and rod situations.
Resilience See elasticity.
RMS Root mean square. A measure of surface roughness or deviation from true flat.
Rotary seal A seal in which the shaft, turns about its axis in relation to the fixed element in one direction only.
Rough trim The removal of remaining flash or sprue through picking or pulling.
Rubber A common name for both naturally occurring and synthetically made elastomer.
Running Friction The frictional force developed during continuous operation of an O-ring.
Runout (shaft) Gyration, or twice the radial distance between the shaft axis and axis of rotation. May be accompanied by the abbreviation “TIR” which is total indicator reading, or expressed in inches.
SAE Society of Automotive Engineers.
Scorching Prematurely curing or setting up a raw compound during processing.
Seal Any device used to prevent the passage of fluid (gas or liquid) or fine particles.
Service Seal operating conditions.
Shaft Reciprocating or rotating rod within a cylinder.
Shelf-aging The change in a seal’s performance and properties due to time in storage, alleviated by proper packaging and storing conditions.
Shelf life The life remaining on stock that is in storage usually according to a defined standard such as BS3574.
Shore A hardness See hardness, Shore A.
Shrinkage Decreased seal volume due to adverse environmental factors, or plasticizer extraction from systemic fluids.
Specific gravity The ratio of the weight of a given substance to the weight of an equal volume of water at a specified temperature.
Spiral failure Generally found on long stroke, hydraulic piston seals when the surface of the O-ring exhibits a series of deep, spiral 45 degree angle cuts. Caused when some segments of the O-ring slide while other segments simultaneously roll.
Squeeze O-ring compression between the two mating surfaces comprising the groove. Can be axial, or squeezed on the top and bottom surfaces, or radial, squeezed on the inner and outer surfaces.
Static seal A gasket type application in which the O-ring is contained within two non-moving gland walls. Also, see axial seal.
Stress A force applied to a material.
Swell Increased seal volume due to immersion in a fluid, such as oil.
Synthetic Rubber Manufactured or man-made elastomer
Tear resistance The ability of a material to withstand tension without growth of a cut or nick in the seal.
Temperature range Minimum and maximum operating temperatures of a compound.
Tensile strength The strength of a material when stretched to breaking point. Measured in psi.
Thermal expansion Linear or volumetric expansion caused by temperature increases.
Thermoplastics Polymeric materials that soften and can be re-formed when heated, returning to original properties when cooled.
Thermoset Materials that undergo a chemical cross linking of molecules when processed, heated, and moulded, and therefore cannot be re-shaped.
Trim Removal of excess material (flash) on a moulded rubber article.
Trim cut Damage to the moulded article by trimming too close.
Ultimate elongation The % of specimen stretching at the point of breaking.
Under-cure Shortened cure time leading to signs of tackiness or poor physical properties.
Vacuum Negative atmospheric pressure.
Vapour The gaseous state of a solid or liquid.
Viscosity Resistance to flow.
Voids Empty pockets, or an absence of material, where not intended.
Volume change A change in the volume of a seal as a result of immersion in a fluid. Expressed as a percentage of the original volume.
Volume swell Increase in physical size caused by the swelling action of a liquid.
Vulcanisation The chemical conversion of basic polymer chains into moulded rubber products, or elastomer, resulting in increased strength and elasticity.
Vulcanising agent A material that produces vulcanization of an elastomer.
Weathering The tendency of some O-ring seals to surface crack upon exposure to atmospheres containing ozone and other pollutants.
Width The cross sectional diameter of an O-ring or one half the difference between the ID and OD.