Rubber Terminology
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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".
Cross-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.
GRS: See SBR.
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.