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B
Back-Up Ring: A washer like device of a relatively hard material placed in the gland between the o-ring and groove side wall, also called a back up ring.
Blemish: Deformity on the surface of a molded product.
Brittleness: The tendency to crack when deformed.
BUNA N: A general term for the copolymer butadiene and acrylonitrile also referred to as NBR or Nitrile.
BUNA S: The copolymer of butadiene and acrylonitrie also referred to as SBR.
BUTYL: A copolymer of isobutylene and isoprene.
C
Coefficient of Thermal Expansion: Average material expansion per one degree change in temperature, expressed as a fraction of the initial dimension.
Cold Flexibility: O-Ring flexibility (resistance to cracking and breaking) at low temperatures.
Cold Resistant: The ability of an o-ring to function at low temperatures.
Compound: A mixture of an elastomer and other ingredients to produce a rubber like material.
Compression Molding: Thermoset molding technique in which the uncured rubber compound is put in a heated open mold cavity and closed under pressure. The material then flows completely filling the cavity.
Compression Set: The amount, expressed as a percentage of deflection, a rubber specimen fails to return to is original shape after being released from a constant compressive load.
Copolymer: An elastomer composed of two dissimilar monomers. For example, SBR from Styrene and Butadiene.
Corrosion: Progressive wearing away of a surface because of a chemical reaction.
Cross Section: An O-Ring cut at right angles to the mold parting lines, exposing the seals internal structure.
Cure: Another term for “vulcanization”. A heat induced process resulting in the cross linking of polymer chains.
D
Dash Number: A three digit number preceded by a dash as specified by SAE Aerospace Standard 568A to indicate the -o-ring size based on inside diameter (I.D) and cross-section (W).
Deflashing: Any of various processes used to remove the waste edge from a molded rubber part.
Diametral Clearance Gap: The difference in diameters between two mating surfaces.
Differential Pressure: The difference in the amount of force being exerted on the high-pressure side of a seal relative to the low pressure side
Durometer: Instrument that measures the hardness of rubber.
Dynamic Seal: A seal used in an environment that subjects it to movement.
E
Elasticity: The tendency of a material to return to its original shape after deformation.
Elastomer: A general term used to describe both natural and synthetic polymers possessing the resilience to return to ---its original shape after deformation.
Elongation: Percentage increase in original length of a specimen produced by a tensile force applied to the specimen.
Extrusion: When a part or all of a component is forced from its groove from pressure.
F
Face: Front surface of a seal.
Filler: Relatively inexpensive and inert material added to an elastomer to reinforce or modify properties.
Flash: The excess material protruding from the surface of a molded part at the mold junctions.
Flaws: Surface imperfections that occur infrequently.
Flexural Strength: Ability of a material to flex without permanent distortion or breaking.
G
Gasket: Static seal effected when a deformable material is sandwiched and compressed between two mating surfaces.
Gates: The openings in an injection or transfer mold that ensure even flow into the cavity.
Gate Mark: A raised spot or small depression on the surface of an injection or transfer molded part where gates interface the cavity.
Gland: Complete cavity into which the o-ring is installed.
Groove: The machined glandular recess into which the o-ring is fitted.
H
Hardness: Measurement of the resistance to penetration of an indenture into a cured rubber sample.
Heat Aging: Loss of physical properties as a result of exposure to heat.
Heat Resistance: Rubber compounds capacity to undergo exposure to some specified level of elevated temperature and retain a high level of its original properties.
Heteropolymer: polymer composed of differing monomers.
I
I.D.: The inside diameter of an o-ring
Injection Molding: Process where preheated rubber is injected under pressure through a series of runners and into a closed mold cavity.
L
Leakage Rate: The rate at which a fluid passes through a barrier.
Life Test: A laboratory test of the amount and duration of a products resistance to destructive forces.
Low Temperature Flexibility: The ability of an elastomeric product to be flexed at low temperatures without cracking.
M
Mating Surfaces: Points where different parts of an assembly meet.
Memory: Ability of an elastomeric material to return to its original shape after deformation.
Modulus of Elasticity: Ratio of the stress to the strain as measured on a rubber specimen.
Mold Cavity: Hollow space of the mold within which the uncured rubber compound is shaped and cured to the desired shape of the product.
Mold Shrinkage: Dimensional loss in a molded rubber product that occurs during cooling after it has been removed from the mold.
N
Nibbling: Progressive mode of seal failure that occurs when excessive pressure forces a portion of the o-ring into a clearance gap.
Nitrile (Buna-N): Copolymer of butadiene and acrylonitrile presently the industry’s most common elastomer.
Nominal Size: Approximate size of an o-ring or seal in fractional dimensions.
Non-Fill: A defect in a finished rubber product caused by the rubber failing to completely fill the mold.
O
Occlusion: The mechanical trapping of gasses, liquids or solids within the folds of a substance during working or solidification.
O.D.: The outside diameter of an o-ring.
O-Ring: Solid elastomer ring seal of circular cross-section.
Oscillating Seal: In this application the inner or outer member if the gland moves in an arc around the axis of a shaft, most commonly used in faucets.
Oxidation: The reaction of rubber with oxygen.
Ozone Resistance: The ability of rubber to withstand exposure to ozone without cracking or breaking.
P
Packing: A collar or gasket used to seal mechanical devices to retain fluids under pressure or seal out foreign matter.
Permanent Set: Amount of deformation in a rubber part after a distorting load has been removed.
Permeability: Measure of the ease with which a liquid or gas can pass through a material.
Polymer: A long molecular chain material formed by the chemical combination of many similarly structured, small molecular units.
Q
Quad Ring: Solid elastomeric ring seal with a four lobed cross-section
R
Radial Seal: O-ring or seal having compression applied to its outside diameter (O.D.) and inside diameter (I.D.).
Radial Squeeze: Compression on an O-ring outside diameter (O.D.) and inside diameter (I.D.).
Reciprocating Seal: Dynamic seal used to seal piston or seals.
RMA: Rubber Manufactures Association
RMS: Root Mean Square, a measure of surface roughness.
Rotary Seal: Seals for rotating shafts, with the turning shaft protruding through the I.D. of the o-ring.
Rubber Natural: A natural product fro the juices of tropical plants.
Rubber Synthetic: Man made elastomers such as Nitrile.
S
SAE: Society of Automotive Engineers.
Scorching: Premature curing of rubber during storage or processing, usually caused by excess heat.
Shaft: Rotating or reciprocating component that operates within a cylinder or housing.
Shelf Aging: The potential degradation of seal performance capabilities due to exposure of o-ring elastomers to stressful environmental factors during storage
Shelf-Life: Length of time a molded compound can be stored without suffering significant loss of physical properties.
Shrinkage: Decreased seal volume due to exposure to adverse environmental factors. All rubber material shrinks to some level during molding.
Size Nominal: Basic dimensions of a part from which plus and minus tolerances are developed to account for the range of actual dimensions expected during manufacturing.
Squeeze: Compression of the o-ring between the two mating surfaces comprising the walls of the cavity or gland in which it is installed.
Static: Describes an application in which there is no relative motion between mating surfaces.
Static Seal: Seal functioning in an environment in which there is no relative motion between the mating surfaces.
Swell: Increased seal volume caused by exposure to adverse operating conditions, such as exposure to oils, fluids, heat and the like.
T
Tear Resistance: Resistance to the growth of a nick or cut in a rubber specimen when tension is applied.
Tensile Strength: Force in pounds per square inch (psi) required to break the rubber specimen.
Tension Set: Increase in the length of an elastometric specimen following initial stretching and release.
Thermal Expansion: Linear or volumetric expansion caused by temperature increase.
Thermoset: Materials that undergo a chemical cross linking of molecules when processed, heated and molded, and therefore can not be reshaped.
Tool: Alternative name for a mold
Torque: A turning or twisting force generally associated with the rotation of the shaft.
Transfer Molding: Method of molding thermosetting materials. The elastomeric compound is placed in a transfer chamber which is part of the mold, heated, then squeezed down through a sprue, a runner, and agate leading into a closed mold cavity.
U
Under-cure: A condition where rubber has not been cured enough, exhibiting poor physical properties.
Ultimate Elongation: The % of specimen stretching at the point of breaking.
V
Viscosity: Resistance to flow
Vulcanization: Heat induced process whereby the long chains of the rubber molecules become crossed linked by vulcanizing agent to form three dimensional elastic structures. This reaction turns weak materials into strong ones.
W
Weathering: The tendency of some o-ring seals to surface crack upon exposure to atmospheres containing ozone another pollutants.
Width: An o-ring cut at a 90 degree angle to the mold parting line. The cross sectional diameter of an o-ring.
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