A-Z Rubber Glossary of manufacturing words & terminology

Over the last 150 years or so, the rubber industry has developed many words and terms that are unique to it, and our Rubber Glossary is intended to help clarify what is meant by these words.

The longevity of the industry means that some terms are rarely used these days, or have become associated with the more recent development of theromplastic materials rather than the traditional thermoset rubbers that were discovered in the 1840’s, giving rise to the industry we have today. In our “A-Z “rubber glossary we have covered this crossover of materials, as well as looking at terms commonly used in the applications where rubber is a key constituent, such as sealing and hydraulics.

Tapping natural rubber from tree plantations

Tapping natural rubber from tree plantations, see our A-Z Rubber Glossary for an explanation of Hevea Brasiliensis

If the term you are seeking does not appear in this rubber glossary list, please contact our Technical Team, who will be pleased to help you.


Index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z



“A” Face

The cosmetically important areas or faces of a component, typically such as cabin trims or interior components that are in public view when in service.


The process of wearing down or rubbing away by means of friction.

Abrasion Resistance

The ability of a rubber compound to withstand mechanical deterioration due to mechanical wear such as rubbing, scraping or erosion, that tends to remove material from its surface. Abrasion properties of rubber are difficult to define. Many different abrasion machines have been designed but they do not always give similar results. The most common abraders are AKRON, Dunlop and DuPont. Results are recorded as volume loss per standard test piece, or as the difference in volume loss when compared with a standard material of known abrasion value. Indices of relative abrasion are also used.




The physical attraction and incorporation of a substance (liquid, gas, or vapour) into a material’s interior.

Accelerated Ageing

Heat ageing is widely used as a method of evaluating long term ageing properties. Hardness and dumbbell type tensile test pieces are placed in an air circulating oven for a specific period of time at a given temperature (e.g. 7 days at 70°C). The properties of the rubber are then tested and compared with the properties before ageing. The percentage retained for each property is recorded.

Accelerated Life Test

Test conditions designed to reproduce, in a shortened time period, the deterioration a product will obtain in its normal conditions.

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.


Chemicals which are added to natural or synthetic rubbers to accelerate the rate of vulcanisation. Rubber without accelerators takes twenty or thirty times longer to cure.

Acid acceptors

Mostly used in Chloroprenes (Neoprene) to absorb the acid produced by the chemical reactions taking place during curing. Acid acceptors are usually metal oxides.

Acid Resistant

A material that can function in an acidic environment.


Chemicals which are added to rubbers to activate curing.

Actual Size

The precise size of an o-ring or seal in decimal dimensions, inches or millimetres, including tolerances and energisation allowances. See also “Housing Size”


Clinging or sticking of two surfaces.

Adhering Flash

Extremely thin flash (usually less than .002′ thick) that adheres to the finished part during moulding or deflashing processes.


The tendency of a material to cling to a contact surface.


The family of substances used to adhere by various methods two parts together. For rubber materials, Contact, Epoxy, Cyanoacrylate, RTV or hot vulcanising adhesives are all suitable options depending on the application.

Age Protection – Active

The use of a chemical additive in a rubber compound which is preferentially attacked by oxygen or ozone thereby sacrificially protecting the rubber. See also “Anti-Ozonant”.

Age Protection – Passive

The use of a chemical additive in a rubber that will migrate to the surface of a rubber part to form a protective physical barrier.


The process of, or the results of, exposure of rubber to natural or artificial environmental conditions for a prolonged period of time, resulting in changed physical properties or appearance.

Air Checks

Surface markings resulting from the trapping of air between the material being cured and the mould surface. See also “Laking”

Air Curing

Using hot air ovens or steam to vulcanise rubber.


A composite material produced by blending polymers or copolymers with other polymers under selected conditions to achieve greater physical properties.

Ambient Temperature

The temperature of a medium or environment surrounding an object. The term is often used to denote prevailing room temperature.

Aniline Point

The lowest temperature at which equal parts of aniline and a test liquid (such as oil) will uniformly mix or blend. The aniline point of oil is a measure of aromaticity (the amount of unsaturated hydrocarbons present). The lower the aniline point, the more unsaturants are present and the higher the potential for swelling certain rubber compounds.


A process of holding a material at a temperature near, but below, its melting point, to permit stress relaxation without distortion of shape. Often used on moulded article to relieve stresses set up by flow into the mould.


The American National Standards Institute, often quoted when specifying Pipe Flange gasket dimensions, as this is the generally accepted global standard for such products.

Anti-Extrusion Ring or Device

A washer like device of a relatively hard material placed in the gland between the o-ring and groove side wall. Can also called a back up ring.


An organic compound or substance that inhibits or slows the oxidation process.


Any substance that slows the severe oxidizing effect of ozone on elastomers. Exposure to ozone typically causes surface cracking in many rubbers.

Antistatic Agents

Chemicals which impart a slight to moderate degree of electrical conductivity to elastometric or plastic compounds, thus reducing the accumulation of electrostatic charges on finished articles.


Accepted Quality Level.


A major group of unsaturated cyclic hydrocarbons containing one or more rings. A typical aromatic compound is benzene, which has a six carbon ring, containing three double bonds.

AS 568A

Aerospace standard dash numbering system used to assign o-ring sizes.

AS 9100

Aerospace Quality Systems standard.



Ash Content

The solid residue remaining after a substance has been incinerated or heated to a temperature sufficient to drive off all combustible or volatile substances.


The process of joining rubber parts by any of several methods, including glueing, hot vulcanising or shot moulding. See also “Fabrication”


Abbreviation for American Society for Testing and Materials.

Atmospheric Cracking

Cracks on the surface of an o-ring because of exposure to atmospheric conditions. See also “Perishing”


Vessel used for vulcanising rubber under heat and steam pressure.

Axial Seal

A seal, like a face seal, in which an O-ring or gasket is squeezed across the top and the bottom of its cross-section, perpendicular to its axis. See also “Face Seal”

Axial Squeeze

Compression across the top and bottom of an o-ring’s surface.



A “breaking up” of the rubber normally observed as a ragged indentation around tool split lines. It results from the rubber shrinking during cure and tearing away at points where the moulding is held.

Backup ring

A washer-like device of a relatively hard, tough material installed in the gland on the downstream side of the seal to prevent seal extrusion into the diametrical gland gap between rod and housing while under pressure. Typically used with O rings and U Seal designs. Sometimes called a Back-Up Washer.

Banbury Mixer

An internal mixer commonly used in the rubber industry. It consists of two shaped rotors which turn inside a sealed chamber, to masticate the rubber compound and fully mix all the various constituents.


That part of an extruder inside which the feed screw is located.

Base Line

The mark or datum from which all features are dimensioned.

Batch Control

A process of keeping separate a batch of a rubber compound from the time it is moulded through the entire production process up to and including the Shipping Department.

Batch Number

The unique assigned number that identifies a particular batch of rubber compound or finished parts made in any given production run, in aid of traceability.


Term used for weighed up material stock that is placed in the mould for making a cured part. See also Preload.

Blast Finishing

The process of removing flash from moulded objects and/or dulling their surfaces, by impinging media such as crushed apricot pits, walnut shells or plastic pellets upon them with sufficient force to fracture the flash.


The exudation or migration of droplets of excess plasticiser or other compound elements onto the surface of a rubber product. This happens when rubbers contain unsuitable levels of plasticisers which are not fully compatible. See also Blooming.


Anything impairing the appearance of a compound, such as a mark, deformity, or injury.


An imperfection on the surface of a rubber article caused by a pocket of air or gas beneath the surface.


A milky surface discoloration caused by the migration of certain compound components (such as antiozonants, lubricants, stabiliser pigment, plasticisers,) to the rubber’s surface after moulding or storage. The waxy or crystalline film can be deliberate and designed to serve as a protective coating shielding the part from oxidation. This discoloration DOES NOT adversely affect material performance.

Blow Moulding

A moulding process usually for thermoplastic or glass materials, whereby a tube of heated material is held vertical and clamped at the lower end in a mould, then inflated to “blow” out against the mould surface and create the required geometry. Once cooled the part can be extracted from the mould.


The mechanical or chemical force that holds an elastomer to some other object. Mechanical bonding includes component interference or a physical fixing and no molecular cross-bridging between the elastomer and substrate, whereas chemical bonding involves contact or epoxy cold bond adhesives, or full vulcanisation with heat and pressure to adhere an elastomer to a primed surface.


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

Break-out Friction

The frictional force developed at start-up, due to the tendency of seals or O rings to adhere to the micro fine grooves of surrounding surfaces.


Tendency to crack upon physical deformation.

Brittleness Temperature

The temperature at which plastics and elastomers rupture by impact under specified conditions.


British Standard, controlled and issued by BSI, now usually in line with an equivalent ISO Standard.


British Standards Institution, UK model for quality assurance in design, development, production, installation and servicing.

Building Tack

Term used to describe the ability of a rubber to stick to itself in the uncured state, so allowing pre-building of the material before curing.


The action of cracking and re-closing a press during the first stage of the cure cycle to release gas and move rubber around further inside the mould cavity.


The large slab of sponge rubber which is produced as the bulk moulded blank, from which various thicknesses of sheet required are then skived off to create the semi finished materials for final conversion into finished strip and gaskets. The outer surface of the Bun is skinned, and only sheets with this outer surface will therefore have a skin, usually only on one side.

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 acrylonitrile also referred to as SBR.


Showing evidence of excessive heating during processing or use of a rubber, as evidenced by blistering, discolouration, distortion or destruction of the surface.


A copolymer of isobutylene and isoprene. Bromobutyl:



The process and machine required for turning uncured rubber into sheets or lengths by squeezing it in a multi-rollered machine to form thin sheets of uncured compound. When the rollers are heated, also used to produce cured sheeting for onward processing. Also used to coat or impregnate fabric with rubber compound.

Carbon Black

A black material used as a filler in rubber. It gives the rubber its black colour and the hardness characteristics, and adds to the physical strength of the material.


A depression, or a set of matching depressions in a mould tool which forms the outer surfaces of the moulded articles. These features are directly responsible for forming the final shape of a moulded part, and the detail on them is directly transferred to the part produced from the cavity.


The measurement or weight of material used to load a mould during one cycle. Also see “Shot Weight”

Chemical bonding

A method for bonding rubber to secondary parts by applying special adhesives or primers to the substrate prior to moulding.

Chemical Resistance

This is usually expressed as the change in properties (such as hardness, strength and elongation at break) caused by the presence of chemicals under defined conditions (including concentration, time and temperature).


The form tolerance of the surface of a moulded or ground ball in reference to a perfect sphere. Also referred to as “roundness”

Clamping Pressure

The pressure applied to the mould to keep it closed during a cycle. In injection moulding and in transfer moulding; the pressure which is applied to the mould to keep it closed, in opposition to the injection or fluid pressure of the moulding material.


Commonly used as an “extender” in rubber. Treated clays reinforce the rubber and are termed “hard” clays. Those which do not provide reinforcement are called “soft” clays.

Clearance (in a sealing system)

The space between components in a mechanical system which is present to allow for manufacturing, thermal, and dynamic variations in the size and position of the components. As measured, it is equal to the distance between the sealing surface and the entrance to the seal groove. In a radial sealing system, this will be the space between the O.D. of the shaft or piston and the I.D. of the bore. As the clearance in a system increases, the tendency of the seal to dislodge from the groove and enter into the clearance space also increases, especially in applications where pressure is present. Anti-extrusion measures should be considered in these cases. Care should be taken in radial applications to note whether clearances are being stated as radial clearances or as diametrical clearances.

Clearance, diametrical

The gap, formed by the mating metal surfaces, which a seal has to close to prevent leakage in the system.

Cloth Finish

Rubber sheeting that has a textured finish imparted by running a layer of fabric through the calendar during the curing process. Certain gasket designs work better with a cloth finish to the material.

Co extrusion

The process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling to form the finished product.


A uniform layer of chemical primers or adhesives applied to a surface 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 desired properties such as ozone protection.

Coefficient of Friction

Ratio of the force needed to move an elastomer over a given surface to the total forces pressing on the elastomer and surface.

Coefficient of thermal expansion

Average material expansion per degree of temperature over a defined range, expressed as a fraction of the initial dimension.

Cold flexibility

The flexibility still remaining in an elastomer after exposure to low temperature for a specific amount of time.

Cold Flow

A term describing the tendency of certain materials to continue to deform or “creep” under constant sealing pressure (compressive load)

Cold Resistant

The ability of a seal to function at low temperatures, influenced by both design and material.


An article or substance containing or made up of two or more different substances.


An unvulcanised mixture of a polymer and associated chemical ingredients necessary to produce a finished rubber like material. The term is commonly used when referring to a specific rubber formulation or recipe.


The step of mixing base polymer with additives such as stabilisers, fillers, pigments into a form suitable for processing into finished articles.

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 Moulding

A method of moulding in which the moulding material, sometimes preheated, is placed in an open heated mould cavity, the mould is closed with a top force, pressure is applied to force the material into contact with all mould surfaces, both heat and pressure are maintained until the moulding material has cured. This process is most often used with thermoset materials.

Compression Set

The degree to which a rubber does not recover fully to its original state after it has been compressed for a long period of time. The term is commonly used in reference to a test conducted under specific conditions wherein the permanent deformation, expressed as a percentage, is measured after a prescribed period of time. A low compression set is desirable in moulded rubber parts such as seals and gaskets, which must retain their dimensions to maintain an effective seal.

Compression Set Test

A cylindrical button of rubber of known thickness is compressed to a fixed height (typically 70% or 75% of its original height) at a defined temperature for a specific period of time. The button is then released, allowed to recover (typically for 30 minutes) and the thickness is measured. Compression Set is the height that is not recovered expressed as a percentage of the amount by which it was compressed.

Compressive Strength

The load sustained by a test specimen in a compressive test divided by the original area of the specimen.


A condition in which two or more features (cylinder, cones, spheres, hexagons, etc…) in any combination, have a common axis.


Subjecting a material to standard environmental and/or stress history prior to testing.

Conversion Celsius to Fahrenheit

C = (° F – 32) x 5.9 F = (° C x 9.5) + 32

Conversion Fahrenheit to Celsius

F= (° C x 9/5) +32 C= (° F – 32) x 5/9

Conversion Kilograms to Pounds

kg = lbs x 0.454 lbs = kg x 2.205

Conversion Millimetres to Inches

mm = in x 25.4 in = mm x 0.03937

Conversion Pounds to Kilograms

lbs = kg x 2.205 kg = lbs x 0.454

Cooling Block

Block of metal or wood used to maintain the proper shape or dimensional accuracy of a part after it is removed from the mould until it is cool enough to retain its shape without further appreciable distortion. Also known as “shrink fixture”

Cooling Channels

Channels located within the body of a mould through which a cooling medium is circulated to control the mould surface temperature.


An elastomer (polymer) resulting from the chemical combination of two dissimilar monomers. For example, Buna N from Butadiene and Acrylonitrile.


Progressive wearing away of a surface because of a chemical reaction.


Describes how capable the stable process is of meeting design specifications. The CPK measures the distance from the process average (xbar) to the nearest specification and compares it to the natural variation in the process.


Sharp breaks or fissures in rubber surfaces resulting from excessive strain or exposure to adverse environmental factors.


Fine cracks which may extend in a network on or under the surface or through a layer of rubber or plastic material.


The continued progressive relaxation of an elastomeric material under constant sealing pressure (compressive load) or stress. Also known as cold flow. Also called shrink.

Cross section

The width of a part such as 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.

Cross Section-Axial

When the part is flat (relative), the cross section that is at 90 to the linear plane.

Cross Section-Radial

When the part is flat (relative), the cross section in the linear plane.

Cross-linking Agents

A chemical or chemicals that bond the polymer chains of a rubber together during the molding process.

Cross-Sectional Compression

The deformation placed on a rubber part to affect a seal. It is expressed as a percentage of the seal’s original (undeformed) cross-section. See also Squeeze.

Cross-Sectional Reduction (of a seal)

The reduction in thickness of a seal’s cross-section, a result of material displacement caused by an applied stretch to the seal. See also Necking.

Cryogenic Deflashing

Process whereby rubber parts are chilled to low temperatures using liquid nitrogen, and tumbled to break off flash which has become brittle in the low temperature.


Rubber materials tend to crystallise on standing, resulting in an increase in hardness. This increase is usually small, but can be more significant in the case of some Neoprene (polychloroprene) grades.


Coefficient of Thermal Expansion: The change in length of a material for a unit change in temperature, per unit of length.


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


The creation of an elastomer through chemical bonding of polymers and moulded rubber, usually with the help of sulphur and an accelerator, under pressure, at elevated temperatures. See also vulcanisation.

Cure Date

The date that a part was moulded and became fully cured. A product code of 3Q04, for example, indicates a cure date of the third quarter (3Q) of 2004 (04).

Cure Time

The preset time needed to complete the curing process.

Cure Weight

The prescribed weight of stock needed to make one cure of parts, usually stated on the route card, see also Shot Weight and Charge Weight.

Curing temperature

The temperature of vulcanisation.


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 permanently changed by cross-linking into its thermoset vulcanised state. See also vulcanisation.

Custom Moulder

A firm specialising in the moulding of items or components to the specifications of another firm which handles the design or distribution of the item, or incorporates the custom moulded components in one of its own products.

Cycle Time

In a moulding operation, cycle time is the time elapsing between the start of curing a part and the same point in the next cycle.


The process of acid etching rubber in preparation for bonding with adhesives.


Chamber in which a piston is driven.



The ability of an elastomer to absorb forced vibrational energy. Different elastomers have different dampening characteristics. For most general applications the lower the material damping is, the better the Isolation; generally, together with its high mechanical properties this is why Natural rubber is the preferred choice for anti-vibration mountings.

Dash Number

A three digit number preceded by a dash as specified by SAE Aerospace Standard 568A to indicate an o-ring size based on inside diameter (I.D) and cross-section (W).

Daylight Opening

The clearance between two platens of a moulding press when in the fully open position.


The removal of unwanted, excess material from a finished rubber product.


Any of various processes used to remove extraneous rubber from a moulded rubber part.


The removal of unwanted, excess gaseous material from a finished rubber product.


A deleterious change in the chemical structure, physical properties or appearance of a rubber or plastic, often caused by exposure to heat, light, oxygen or weathering.


Separation of two or more surfaces in a composite material.


(Absolute) Mass per unit volume of a substance, for example expressed in grams per cubic centimetre. See also Specific Gravity.

Diametral Clearance Gap

The difference in diameters between two mating circular surfaces.

Diametrical Clearance

The necessary gap between two mating metal surfaces that move in relation to each other, forming a gland’s internal cavity. Through slight over sizing and accompanying “squeeze” a seal is used to cover this gap and prevent system leakage.

Diaphragm Gate

Used in symmetrical cavity filling to reduce weld-line formation and improve filling rates.

Diaphragm Rubber Sheeting

Premium grade elastomer reinforced fabric sheeting intended primarily for diaphragm applications.

Diaphragm Seals

Precisely produced diaphragms and sensitive flap valves for use in gas controls, valves, actuators, regulators, pumps and metering equipment.


Tooling orifice through which uncured compound is forced in the extrusion process to form a profile.

Die Cut

The process of stamping out the I.D. or O.D. parting line flash or feed from a moulded part. The part is placed on a mechanical fixture to perform this function.

Die Cut Gaskets

Press cut and hand cut gaskets in all types of rubber and non-metallic materials. See also Rule Forme.

Dielectric Strength / Electrical Properties

The resistance of an elastomer to the conducting of an electrical current. Anti-static and conductive properties of rubbers are defined by measuring their electrical resistance. Typical measurements are obtained through the thickness of a sample by using 25mm sq. electrodes under a pressure of 45N and applying a voltage of 500V DC @ approx. 6mA. (Energy dissipated into the test piece is limited to 3W max.) The surfaces should be cleaned before measuring. Patterned surfaces should be wetted with a conductive solution or paint. See also Electrical Safety Matting.

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.

Dimensional stability

The ability of a plastic or rubber part to retain the precise shape in which it was moulded.


Finely divided particles of a material in suspension in another substance, typically fillers within rubber compounds need good, even dispersion to deliver consistent material properties.

Double Injection

A moulding defect that is fully avoidable by following proper moulding procedure. It is identified by a flat raised dome that peels away easily. It is caused by not completely unloading a mould of all its products, and running another cure cycle on top of the parts that were left in the mould from the previous cure.


The degree of taper of a side wall or the angle of clearance designed to facilitate removal of parts from a mould. This is not always necessary on elastomeric mouldings but is required on rigid materials.

Dump Groove

A groove around the periphery of a mould cavity used to an escape reservoir to accept any excess material from the cavity during moulding. Additional material beyond that which is required to fill the cavity is usually introduced into the cavity to ensure that the part is completely formed and to minimize the presence of entrapped air and voids. See also Overflow Groove.


Standard rubber industry test instrument, which uses a ball indenter to measure the hardness of an elastomeric material. Shore is one manufacturer and their Shore Hardness scale is well known, but other instruments that measure in the similar International Rubber Hardness Degree scale are also very common.


A pause in the application of pressure to a mould, made just before the mould is completely closed, allowing any gases formed or present to escape from the moulding material. See also Bumping.

Dynamic seal

Any seal used in an environment or application involving reciprocating, rotating, or oscillating motion of a shaft or rod relative to its seal. See also Static Seal.


Elastic Limit

Maximum stress that a material will stand before permanent deformation occurs.


A rubber like material’s ability to rapidly return to its original size and shape after removal of any stresses causing deformation such as stretching, compression, or torsion. It is the opposite of plasticity. The term elasticity is often loosely employed to signify the “stretchiness” of a rubber like material.


A natural or synthetic polymer material which at room temperature can be stretched repeatedly and, upon immediate release of the stress, will return with force to its approximate original length. Often, but not necessarily, a material of the rubber family.


The adjective of Elastomer; describing the general property of a material to behave in an elastic way.

Electric Strength

Describes the ability of a rubber to withstand high voltages. It is usually quoted in KV per mm of rubber thickness. See also Dielectric Strength.


Percentage increase in original length of a specimen produced by a tensile force applied to the specimen.


Generally referred to in terms of tensile (pulling apart) testing, elongation is the increase in length of a test specimen, expressed as a percentage of its original (unstretched) length. It is relative to a given load at the breakpoint.

Elongation at Break

Elongation is defined as the length at breaking point expressed as a percentage of its original length at rest. If a rubber sample reaches twice its length before breaking its elongation is 100%.


Electromagnetic interference.


Enclosing an article (usually an electronic component) in a closed envelope of plastic or rubber, by overmoulding or potting the object in a casting resin or vulcanised rubber.


The method by which a seal will engage with its housing or moving parts. This can be done either by designing an interference fit such that the elasticity of the seal provides a constant force against its housing, or, by ensuring that the application of pressure causes the seal to swell such that an increasingly tighter sealing effect is created. The material of the seal can provide energisation, or it can be assisted by the addition of rubber or spring inserts within the seal design.

Engineering Plastics

A broad term covering all plastics, with or without fillers or reinforcements, which have mechanical, chemical and thermal properties suitable for use in construction, machine components and chemical processing equipment. These materials can be moulded but products are more usually made by machining methods, taking billet stock shapes of the relevant plastic materials and cutting accurate shapes from those billets.

Explosive Decompression

When continual gas pressure is maintained on a rubber part, gas molecules can be absorbed into the rubber. This does not affect the rubber unless the pressure is suddenly released, at which point the entrapped gas tries to rapidly escape from the rubber lattice, often rupturing and exploding the rubber in the process. Careful polymer selection can compensate for this effect, typically encountered in Down Hole O&G applications. See also RGD, Rapid Gas Decompression, and our Blog on the subject.

Extending Flash

Flash that is formed at the parting line of a mould.


The product or material delivered from an extruder, for example, profiles, tubing, squares.


A machine for producing more or less continuous lengths of rubber sections such as rods, sheets, tubes, and profiles, by forcing uncured rubber compound though a suitable die, and then curing the resultant section in an autoclave or microwave oven.


Of a seal – When a part or all of a sealing component is forced from its groove from pressure, or into a clearance gap between mating mechanical components in an assembly. Of a material – The generic term given to products such as rubber or plastic lengths of material produced by an Extruder with a continuous profile.



The process of joining parts, in the case of rubber by any of several methods including glueing, hot vulcanising or shot moulding. See also “Assembly”

Fabricated Sleeves

Fabricated cylindrical or tapered tubes from a variety of rubber and composite materials, primarily used to connect pipes, finish the ends of discharge chutes or compensate for relative movement.


Front surface of a seal. See also “A” Face.

Face Seal

A sealing system in which the sealing occurs in the axial direction of the seal. If the seal ring were to be placed flat upon a desktop, the seal compression would occur between the top and bottom of the seal. See also Axial Seal and Static Seal.

Family Mould

A multi-cavity mould where each of the cavities forms one of the set of component parts of the finished assembly. This can lead to problems when unbalanced flow means that one item of the set does not mould as well as the other parts, resulting in unequal quantities of good parts.

Fatigue Strength

The maximum cyclic stress a material can withstand for a given number of cycles before failure occurs.


Abbreviation for Food and Drug Administration, the U.S. agency under the Department of Health, Education and Welfare which is concerned with the safety of products marketed for consumer use.

Feed Gate

The area or orifice where rubber is injected or transferred into a mould cavity. Various types are called Fan, Direct, Pinpoint, Tab, Tunnel, Jump or Ring.


A relatively inert substance added to a rubber compound to reduce its cost, improve processing or physical properties, particularly hardness, stiffness and tensile properties. Materials such as carbon black, clay, calcium carbonate, and silica are used to change the properties of an elastomer.


The surface texture of a finished article.

Flame Retardant

Having the ability to resist combustion. A flame retardant elastomer is considered to be one that will not continue to burn or glow after the source of ignition has been removed. Specific test standards are laid down by various industries depending on their specific requirements. Flame resistance testing requires a standard burner and fuel, which gives a flame of known characteristics. Rubber samples are then placed at a certain position in the flame and removed after an ignition period. The time taken for the rubber to self-extinguish is recorded with notes regarding after flow and any hot particles emitted by the sample at any stage.

Flame Retardants

Additives that reduce the tendency of rubbers to burn.


Excess rubber found on mouldings as a result of the moulding process, but which does not form a part of the moulding itself. This does not usually include feed gates but refers particularly to thin films of rubber formed at tool split lines or venting gaps. Can be removed by trimming. See also Cryogenic Deflashing.


Surface imperfections that occur infrequently.

Flex cracking

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

Flex Life

The time during which a material can be expected to resist cyclical bending.

Flex resistance

The relative ability of an elastomer to resist the stress of constant bending.

Flexural strength

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


The ability of heated plastic or uncured rubber to travel in the mould and runner systems during the moulding process. The scorchiness or viscosity of a compound will affect its ability to flow, and feed a mould effectively.

Flow Marks, cracks or lines

Surface or structural imperfection due to the failure of the rubber compound to properly blend or knit with itself during moulding. Caused by insufficient masticating of the material within the cavity during the closing of the mould, especially an issue with compression moulding, and certain materials.


A liquid or gas.

Fluid Resistance / Volume Swell

Fluid resistance is commonly measured by the effect of the fluid on the volume of the rubber exposed as a volume change (e.g. +100% indicates that the volume of the rubber has doubled as a result of exposure to the fluid, -10% indicates that the volume of rubber has decreased as a result of exposure to the fluid). Volume change is measured by determining the weight of a sample in air and water before and after exposure to the fluid under defined conditions. The sample is normally totally immersed in the fluid.

Foam Impact Protection Profiles

Impact protection foam profiles provide highly visible protection on sharp corners, edges, machinery, racking and vehicles – preventing accidents in the workplace.

Forcing frequency

The frequency at which an external force is applied to a part to sustain it at a constant “forced vibration’ i.e. with generator set running at 1500 rpm the forcing frequency is 1500CPM or 25Hz.


A general term encompassing processes in which the shape of rubber extrusions is changed to a desired configuration.


The term used to describe a rubber “recipe”.


The separation of a body usually characterised as either brittle or ductile.

Frequency ratio

The forcing frequency over natural frequency – see “transmissibility curve”. The bigger the ratio the better the isolation.


Resistance of motion caused by surface contact.

Friction (Break-out)

Friction developed in dynamic seal situations during machine start-up. When machine operation is irregular, O-rings tend to conform (adhere) to the micro fine grooves of surrounding glandular surfaces, requiring extra initial force to break them out of these micro fine grooves.


Fibre reinforced plastics.



A deformable material clamped between two surfaces so as to prevent loss of material or pressure and to create a seal. A static (stationary) sealing device used to retain fluids under pressure or to seal out foreign matter when a deformable material is sandwiched and compressed between two mating surfaces.


In injection and transfer moulding, the orifice through which the melt enters the cavity. Sometimes the gate has the same cross-section as the runner leading to it; often, it is severely restricted.

Gate Mark

A raised spot or small depression on the surface of an injection or transfer moulded part where gates interface the cavity.


Thickness of a sheet material. Or a dedicated jig to assess a limit or fit.

Geometric Dimensioning & Tolerancing

A means of specifying engineering design and drawing requirements with respect to actual function and relationship of part features. Furthermore, it is a technique which properly applied ensures the most economical and effective production of these features.


The space in a mechanical system into which a seal is installed. The gland consists of the seal groove and any additional space required for achieving the proper compression of the seal. Care should be taken to distinguish between the terms gland and groove, which are separate but related concepts.


The machined glandular recess into which an O Ring or Seal is fitted.

Groove Depth

The measured depth of a feature, frequently a slot, machined or otherwise created in a mechanical system to physically locate and constrain a seal. Care should be taken to distinguish between the terms Gland Depth and Groove.



Resistance of rubber to distortion forces. Measured with a Hardness Tester in degrees and based on the resistance to penetration into a standard sized cured rubber sample of a defined indenter under a set load. Three scales are commonly used: IRHD (International Rubber Hardness Degrees), Shore A for standard solid elastomers and Shore D for hard materials over 90° Shore A. IRHD is preferred for most specifications but Shore A is also in widespread use. High values indicate harder materials while low values indicate softer materials. The Shore A and IRHD scales are similar but do not directly correlate.


Heat-deflection test.


One part or set of parts produced by one cycle of a moulding press. (Also see “lift”)

Heat Aging

Loss of physical properties as a result of exposure to heat over time. Also known as “accelerated ageing”. Rubber materials are often tested for shorter times at higher temperatures to predict what will happen over longer times and at lower temperatures.

Heat deflection temperature

The temperature at which a standard plastic test bar deflects 0.010 in. (.254mm) under a stated load of either 66 or 264 psi (4.55 or 18.2 Bar)

Heat Resistance

Rubber compounds capacity to undergo exposure to some specified level of elevated temperature and retain a high level of its original properties.

Heat Sealing

A method of joining plastic parts or films by simultaneous application of heat and pressure to areas in contact. Heat may be supplied by conduction or dielectrically.

Hermetic seal

An airtight seal.


polymer composed of differing monomers.

Hevea Brasiliensis

The commonly known “Rubber Tree”  is actually correctly known as Hevea brasiliensis, or, in south america, the Pará rubber tree. It is the main source of latex, and from its origins in south america was transplanted by the British to India, Malaya and Africa in the 1870’s to establish an alternative source of latex for natural rubber production. Natural rubber is still sometimes called Para rubber due to its original source tree. Latex can also be harvested from other species of plant, such as the dandelion family.




High-strength moulding compound.


High molecular weight – typically of an engineering plastic.


A mixture that is uniformly dispersed. In finished seals a rubber seal without fabric or metal reinforcement.


A machining process that sharpens, enlarges, and smoothes material through the use of a fine-grit stone, typically used on piston bores to create the optimum sealing surface conditions.

Housing Size

The size of the metalwork containing a seal, being the space in which the seal is designed to operate.


The difference between the amounts of energy absorbed when a rubber is stretched and the amount of energy released when the rubber is relaxed. High hysteresis indicates a high loss of energy (and so is good for energy absorbing applications). Low hysteresis rubbers are more resilient. Also refers to the build up of heat in an elastomer sample caused by rapid deformation.


I.D. (Inner Diameter)

The innermost (smallest diameter) surface of a circular object, such as a bore or a round seal, often an O Ring. The term I.D. is frequently used to indicate both the circumferential surface itself as well as the measured diameter of that surface.

Impact Resistance

Relative susceptibility of plastic products to fracture by shock or stresses from high speed impact. Indicated by the energy expended by a standard pendulum type impact machine in breaking a standard specimen in one blow


A foreign object or particle found to be present in a moulded part. Such defects can compromise the strength of a moulded part.

Inflatable Seals

Specially fabricated elastomer seals designed to inflate with air to form a tight barrier between two surfaces.

Injection Feeder

The cone shaped or tapered holes that feed down to the cavity area of a transfer or injection type mould. Elastomeric material is forced through these holes when the mould is under pressure. The rubber then fills the cavity area and forms the rubber parts.

Injection Moulding

The method of forming objects from granular or powdered plastics, most often of the thermoplastic type, in which the materials is fed from a hopper to a heated chamber in which it is softened, after which a ram or screw forces the material into a mould. Pressure is maintained until the mass has hardened sufficiently for removal from the mould. In the case of Rubber, the unvulcanised solid material is preheated and then injected through a series of runners into the closed cavity, where pressure and temperature is maintained until cure is achieved.

Injection Pressure

The pressure on the face of the injecting ram when injecting material into the mould. It is usually expressed as P.S.I.

Injection Ram

The ram or screw that applies pressure to the heated plastic or rubber material to force it into the mould cavities.


An article of metal, plastic or other material which is incorporated into a rubber moulded part either by pressing the insert into the finished moulded part, or by placing the insert in the cavity prior to moulding so that it becomes an integral part of the moulded part, either by simple overmoulding, or by chemical bonding during the curing process.

Insertion Rubber

A sheet rubber material with fabric “inserted” into the rubber to provide tear resistance. Used principally as a gasket or diaphragm material, “Insertion” can have various combinations of the number and type of layers of fabric and the type of rubber binding them together, depending on the application. Conveyor Belting is a specific example of this technique, where the longitudinal stiffness of the belt is provided by the fabric layers, whilst remaining flexible enough to pass around the drive rollers of the belt system.

Interference Fit

Where mating components, often metallic, are intended to have no clearance between them when fitted together, locking the parts together.

Internally Lubricated Rubber

A rubber formulation containing lubricating materials. An internally lubricated rubber is designed to reduce the coefficient of friction of the rubber material.

IRHD (International Rubber hardness Degrees)

A system of characterising an elastomer by its resistance to penetration of a known geometry indenter by a known force. This 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.

Ishikawa Diagram

Also known as the Herringbone diagram, a tool for Root Cause Analysis.


Abbreviation for Isocyanate, a major constituent used in the production of Polyurethane solid and foam products in conjunction with Polyol.


Abbreviation for the International Organization for Standardisation – model for quality assurance in design, development, production, installation and servicing.


International Trade in Arms Regulation, the protocol applying to US controlled military products.



A tool for holding parts of an assembly during the manufacturing process.


Just in Time manufacture, a lean manufacturing technique developed by the Japanese automotive industry in the 1950’s to reduce inventory as far as possible.



Developed by the Japanese automotive industry in the 1950’s, designed to “pull” parts onto a production line only when needed, whether by cards or physical bins containing stock.


A term sometimes used for thermal insulation value or coefficient of thermal conductivity.


Deformation caused by bending or twisting a tube or profile so that it returns upon itself.

Knit Lines

Marks visible on a finished part made by the meeting of two flow fronts during moulding. When stressed, may result in a peel failure. See also Splits and Voids.


L/D Ratio

The term used to define an extrusion screw. This is the screw length to screw diameter ratio.


Surface markings resulting from the trapping of air between the material being cured and the mould surface. See also “Air Checking”

Laminar Flow

The flow of a resin in a mould is accompanied by solidification of the layer in contact with the mould surface that then acts as an insulator through which material flows to fill the remainder of the cavity.

Laser Etching

Physically making a component with a laser, often to uniquely identify each individual component with a unique serial number. Rubber products can be Laser Etched satisfactorily.


Natural colloid solution harvested by “tapping” from the Para tree or crushing other vegetable sources such as Dandelion and Guayule. It is then refined into the organic compound Isoprene, commonly known as “Natural Rubber”.


Liquid crystal polymer.

Leakage rate

The rate at which a fluid (either gas or liquid) passes a barrier.

Life Group

Shelf Life of rubber products can be established and controlled by “Life Group”, where packaging and storage conditions are laid down by polymer type. See BS 2230 for the various conditions and polymers covered by such a standard.

Life test

A comparison of the relative performance capabilities of various products or materials. This laboratory test determines the amount and duration of product resistance to multiple destructive forces or conditions. Samples may be standardised for universal comparison, or specific for bespoke testing and comparison.


The set of rubber parts resulting from a single mould cycle or pressing, created by the number of cavities in the mould tool. Also called a “heat”.


“Liquid injection moulding” is a closed manufacturing process using LSR in an injection moulding machine.

Load / Deflection testing

measuring the amount of load required to deflect a part by a given amount. Often a bespoke test of parts to determine whether the finished part or assembly is within design parameters.

Low temperature flexibility

The ability of an elastomeric product to be flexed, bowed, or bent at low temperatures without evidence of cracking.

Low Temperature Flexibility

The ability of an elastomeric product to be flexed at low temperatures without cracking.


“Liquid silicone rubber” is composed of a two-component, low viscosity, heat-curable rubber system.


Material Safety Data Sheets (MSDS)

Documentation regarding the toxicity or hazards associated with contact with some substances. These are prepared by the manufacturer of the substance.

Mating Surfaces

Points where different parts of an assembly meet.

Mechanical bond

A method of creating a moulded part where the rubber is mechanically attached to an overmolded insert through the use of holes, depressions or projections on the insert, thus requiring no bonding agent to create a chemical bond.

Mechanical Property

Properties of rubber and plastics which can be classified as mechanical include abrasion resistance, creep, ductility, friction resistance, elasticity hardness, impact resistance, stiffness and tensile properties, etc.

Melt Index

A measure in grams of plastic material that can be forced through an orifice of 0.0825 inches when subjected to a force of 2160 grams in 10 minutes at 190°C


The tendency of a plastic or rubber article to revert in dimension to a relaxed size previously existing at some stage in its manufacture, after deformation.

Metric Units

Milli Centi Deci Meters Liters kilo Deka Grams Hecto.

Micro hardness

A measurement of rubber hardness for specimen’s below .25 inches (6.35mm) in thickness. Micro hardness, like Shore A durometer, is also a measurement of indentation.


Abbreviation for Military Standard.

Mineral Reinforcements

Inorganic substances used as filler for rubber and plastics. Some common examples are: clay, mica, talc.

Minimum Specifications

The minimum values, usually of mechanical properties that a compound must meet by Quality Assurance prior to shipment.


Unequal or offset split lines leading to an asymmetric product, typically O-ring cross-sectional problems can be caused by this effect due to lateral errors in the mould cavity, also creating excess flash on parts.


Derived from the Latin word meaning “small measure”, modulus is the ratio of stress to strain in the linear region of the s-e logistic curve. It is measured as the force per unit area required to extend a rubber to a stated percentage of its original length e.g. to 100%, 200% or 300%. It is often written as M100 = 3.0 MPa (i.e. modulus at 100% = 3.0 Mpa).

Modulus in Compression

The ratio of compressive stress to strain within elastic limits of the material.

Modulus in Flexure

The ratio of the flexure stress to strain, within elastic limits of the material.

Modulus in Shear

The ratio of shear stress to strain within elastic limits of the material.

Modulus of Elasticity

The ratio of stress to corresponding strain below the proportional limit of a material in tensile testing, as measured on a rubber specimen.

Modulus of Elasticity (tension)

Force which would be required to stretch a substance to double its normal length, on the assumption that it would remain perfectly elastic, i.e., obeys Hooke’s Law throughout the test. The ratio of stress to strain within the perfectly elastic range.

Modulus of Resilience

The energy that can be absorbed per unit volume without creating a permanent distortion.

Moisture Absorption

The pick-up of moisture from the environment by a material.

Moisture Marks or Welds

Marks on the finished part that are caused from moisture in plastics or resins and look like weld lines, but can appear in places not typical of weld lines.

Molecular Weight

The sum of the atomic weights of all atoms in a molecule.


The smallest unit quantity of matter which can exist by itself and retain all of the properties of the original substance.


A relatively simple compound, usually containing carbon and of low molecular weight, which can react to form a polymer by combination with itself or with other similar molecules or compounds.

Mould (n) or Mold

A hollow form or matrix having the features & detail required of the final component into which a plastic or rubber compound is placed and which imparts to the material its final shape as a finished article.

Mould (v) or Mold

To impart shape to a plastic or rubber compound by means of a confining cavity or matrix.

Mould Bolster

A standard frame or plate set that allows for interchangeable cavity inserts of standard pattern to be mounted in it. Each Bolster insert can however have a different cavity cut in it to produce a range of different parts from the same basic moulding press setup.

Mould Cavity

Hollow space in the mould tool within which plastic or uncured rubber compound is shaped and cured to the desired shape of the product.

Mould Finish

The surface finish of the mould which imparts the desired surface quality to the finished moulded product. Special finishes such as leather grain, spark eroded or patterned can be defined from various standards, and applied by various techniques, such as acid etch.

Mould lubricant

A material sprayed in the mould cavity that eases the removal of the moulded rubber products. See also Mould Release.

Mould marks

Surface irregularities, indentations, or ridges on the moulded product due to marks in the mould cavity, or damage to the mould. Can also be caused by an accumulation of release agent or wax deposits from previous moulding cycles, all of which detracts from the design intent of the cavity.

Mould register

Alignment of all the parts of the mould cavity, various methods including dowel pins, taper locks etc.

Mould Release

A lubricant or baked on coating used to coat the surface of the mould to enhance ejection of the moulded article or prevent it from sticking to the tool.

Mould Shrinkage

Dimensional loss in a moulded rubber product that occurs during cooling after it has been removed from the mould. See our blog on Shrinkage.

Moulding Cycle

The period of time occupied by the complete sequence of operations on a moulding press requisite for the production of one set of moulded articles.

Moulding Pressure

The pressure applied to the ram of an injection machine or compression press to force softened plastic or rubber to completely to fill the mould cavities.


Abbreviation for Military Standard.


Material safety data sheet.


Natural frequency

The frequency or number of cycles per minute (CPM) or cycles per second (Hz) that a part will oscillate at if it is excited and then left to vibrate at its own speed until it comes to rest. It is dependent on the AV mountings spring stiffness, & hence the AV mountings deflection under load. As a rule of thumb, the greater the mounting deflection, the lower the natural frequency & the better the isolation

Natural Rubber

A raw or crude product of certain vegetable sources, usually the “Para” rubber tree Hevea Brasiliensis, although other plants such as Dandelion and the desert shrub Guayule also create Latex that can be harvested.

Neo-Hookian Solid

A mathematical model predicting the hyperelastic deformation behaviour under non-linear stress-strain of elastomeric materials undergoing large deformations.


Progressive mode of seal failure that occurs when excessive pressure forces a portion of the o-ring into a clearance gap, also known as Extrusion.

Nitrile (Buna-N)

Copolymer of butadiene and acrylonitrile, good oil resistance and widely used for commercial O rings and seals because of its resistance to petroleum fluids, good physical properties and useful temperature range.

Nominal Size/Dimension

The mean dimension of a moulded article, from which small dimensional (plus and minus) deviations are allowed as manufacturing tolerances.


A defect in a finished rubber product caused by the rubber failing to completely fill the mould cavity.


The hollow cored metal nose screwed into the extrusion end of the heating cylinder of an injection moulding machine, or transfer chamber where there is a separate structure. A nozzle is designed to form a seal under pressure between the heating cylinder or the transfer chamber and the mould. The front end of the nozzle will be shaped to engage directly with the mould tool into which rubber will be injected as the first part of the moulding cycle.


O.D. (Outer or Outside Diameter)

The outermost (largest) diameter surface of a circular object, such as a shaft or a round seal. The term O.D. is frequently used to indicate both the circumferential surface itself as well as the measured diameter of that surface.


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

Ability of a vulcanised rubber to resist swelling and other detrimental effects of exposure to various oils. The specific oil or hydrocarbon encountered may dictate the best choice of polymer to deliver the required service performance.


Not able to transmit light.

Open Steam Cure

Vulcanisation process typically in an autoclave during which the material to be cured is directly exposed to steam, such as large extrusions. On many compounds this leaves a distinctive surface signature.

Optimum Cure

The point at which the rubber compound has achieved its best possible physical properties.

Orange Peel

A surface finish on a moulded part that is rough and splotchy. Usually caused by moisture in the mould cavity or material.


Solid elastomer ring seal of circular cross-section or torus, that functions as a seal, blocking the passage of liquids or gases, by being compressed between two mating surfaces comprising the walls of the cavity (gland) into which the ring is installed. Some designs may permit momentary or minimum leakage. Also, see dynamic seal and static seal.

Oscillating Seal

In such designs the inner or outer element of the gland moves in an arc around the axis of a shaft, most commonly used in faucets.


Primarily occurring in vacuum situations, the volatile (evaporative) components of some rubber compounds such as moisture, plasticiser, air, inhibitors etc may become vaporised in the vacuum and released by the compound into the surrounding environment. See also Explosive Decompression and RGD.

Overflow Groove

A groove around the periphery of a mould cavity used to an escape reservoir to accept any excess material from the cavity during moulding. Additional material beyond that which is required to fill the cavity is usually introduced into the cavity to ensure that the part is completely formed and to minimize the presence of entrapped air and voids. See also Dump Groove.


Encapsulating an insert in rubber or plastic, such that the insert is securely fixed within the rubber or plastic, with or without additional chemical bonding.


The reaction of rubber with oxygen, often resulting in surface cracking of the rubber material. As oxidation involves the transfer of electrons, reduction in the physical strength of elastomers may also occur from exposure to the oxidising agent.

Oxygen Index

A flammability test based on the principle that a certain volumetric concentration of oxygen is necessary to maintain combustion of a specimen after it has been ignited.

Ozone Resistance

The ability of rubber to withstand exposure to ozone without cracking or breaking. Ozone can be naturally present, or produced by adjacent processes. Test pieces are usually placed under a small degree of tension e.g. by bending round a mandrel or stretching by 5%. The sample is exposed under static conditions to a controlled atmosphere containing ozone (typically 50 pphm). The cracks are graded by standard photographs, by measurement or by description, e.g. ‘visible under 10 x magnification’ ‘visible to the unaided eye’ etc. Results may be recorded as the time taken to reach a particular grade of cracking, or by the grade of cracking apparent after a fixed period of time.



A flexible sealing device, typically a collar, gasket or lipped seal used in dynamic applications to retain fluid under pressure, or seal out foreign matter.


The filling of the mould cavity or cavities as full as possible without causing undue stress on the moulds or causing excess flash to appear on the finished parts.


A hollow tube of heated plastic or glass that is clamped at its lower end in a mould and then blown to expand against the inside of the mould to create finished parts when cooled. See also Blow Moulding.

Parting Line

The raised witness mark on a moulded article caused by flow of material into the crevices between adjoining parts of a mould tool.


The surface cracking of a rubber, typically when exposed to general weathering or ozone attack. Unmodified natural rubbers are particularly prone to this effect, which can be compensated for in a number of ways.

Permanent Set

The deformation remaining in a rubber specimen following both stress and relaxation over a period of time. See also Set and Compression Set.


Rate of gas or liquid 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).


The diffusion of a liquid or gas through a rubber or plastic material.

Pinch Off

A raised edge around the mould cavity that seals off the part and separates excess material as the mould closes, typically used around the parison in a blow-molding operation.

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.

Pit mark

A small circular depression.


A material that contains as an essential ingredient one or more organic polymeric substances of large molecular weight, is solid in its finished state, and, at some stage in its manufacture or processing into finished articles, can be shaped by melt and flow, before cooling to take a final form.

Plastic Deformation

A change in dimensions of an object under load that is not recovered when the load is removed.


To render a plastic or elastomeric material softer, more flexible and/or more mouldable by the addition of a plasticiser or heat.


A liquid or solid material incorporated into a plastic or an elastomeric material to increase its flexibility, workability or extensibility. The chemicals added during elastomer processing can decrease stiffness, improve low temperature properties, improve processing, increase elongation, improve tack, etc.


The ability of a material to withstand continuous and permanent deformation by stresses exceeding the yield value of the material without rupture.

Plate Finish

Rubber sheeting that has a smooth surface finish, due to the polished surfaces of the calendar that sets the thickness and cures the rubber.

Pock Mark

A small circular depression.

Poisson’s Ratio

The constant relating the changes in dimensions which occur when a material is stretched. It is obtained by dividing the change in width per unit length by the change in length per unit length.


Abbreviation for Polyol, the major liquid constituent of Polyurethane solid and foam products in conjunction with Isocyanates.


 The chemical term used to describe all organic materials which are formed from chains of repeated chemical units. Consisting of a long molecular chain formed by the chemical combination of many similarly structured, small molecular units.


(Synthetic) The product of a polymerisation reaction. The product of polymerisation of one monomer is called a homopolymer, monopolymer or simply a polymer. When two monomers are polymerised simultaneously the product is called a copolymer. The term Terpolymer is sometimes used for polymerisation products of three monomers.

Polymer Structure

A general term referring to th relative positions, arrangement in space, and freedom of motion of atoms in a polymer molecule.


A chemical reaction in which the molecules of a simple substance (monomer) are linked together to form large molecules whose molecular weight is a multiple of that of the monomer.


The major liquid constituent of Polyurethane solid and foam products in conjunction with Isocyanates.

Post cure

The process of baking or autoclaving parts after moulding. This process is used to improve the heat and compression set resistance of specific elastomers (e.g. silicones and fluorocarbons) by driving off volatile curatives and other decomposition products, fully stabilising the molecular structure of the material.


The chamber in a transfer or injection mould where raw material is placed before it is transferred into the cavity.


Term used for weighed up material stock that is placed in the mould for making a cured part. See also Blank.

Pressure Rollers

Rollers of various types covered in elastomeric coatings to provide a compliant surface. They can be very small such as tape recorder capstan wheels with a fine ground surface, or large items for cable hauls etc.

Primary Sealing Surface

The primary location in a sealing system where a seal and a mating surface come in contact with the intention of forming a barrier to prevent the passage of some type of medium, such as a fluid or a gas. The Primary Sealing Surface is usually distinguished from other sealing surfaces by the presence of relative motion in the case of a dynamic seal, or by the interface of assembled components in the case of a static seal. This term is often used interchangeably with the more generic term Sealing Surface.

Proportional Limit

The greatest stress which a material is capable of sustaining without deviation from proportionality of stress and strain. (Hooke’s Law).

Prototype Tool

A preliminary mould built upon which the final mould will be based.

Pure Gum

Natural Rubber compound devoid of fillers, containing only curing agents. Also known as Para Rubber.


In extrusion or injection moulding, the cleaning of one colour or type of material from the machine by forcing it out with the new colour or material to be used in subsequent production, or with another compatible purging material.


Cleaning one colour or type of material from the cylinder of an injection moulding machine by forcing it out with the new colour of material to be used in subsequent production. Dedicated purging materials are available, that have additives to help clean the internal parts of the injection chamber and check valves.


QS 9000

Quality System model, used in conjunction with the ISO 9000 standard, for the automotive industry.

Quad Ring

Solid elastomeric ring seal with a four lobed cross-section, intended for similar applications to standard O Rings, but have better seal efficiency and lower friction than conventional O Rings.


Radial Seal

Compression is applied to a seal perpendicular to the seal centre-line across the Outside Diameter (O.D.) and Inside Diameter (I.D.)

Radial Squeeze

Compression factor on an O-ring or seal outside diameter (O.D.) and inside diameter (I.D.).

Rapid Prototyping

Fabrication of a physical, three-dimensional part of arbitrary shape directly from a numerical description (typically a CAD model) by a quick, highly automated and totally flexible process. Methods include SLS (Selective Laser Sintering) of a powder or SLA (Sterolithography Apparatus) of an ultraviolet cureable photopolymer. Products are built layer by layer, and as such shapes can be generated that are not manufacturable by any other method.

Rebound Resilience

The measure of the rebound (bounce) of a metal ball dropped at a given height onto an elastomer sample.

Reciprocating Seal

Dynamic seal used to seal piston or rods which operate in a linear movement along the axis of the seals. The opposite to a Rotary Seal.


The degree to which a material will resume its shape after deformation under stress.


Waste thermoplastic material such as sprues, runners, excess parison material and reject parts from injection moulding, blow moulding and extrusion, which has been reclaimed by shredding or granulating. Regrind is usually mixed with virgin compound at a predetermined percentage for remoulding.

Reinforced Plastic

A plastic composition in which fibrous reinforcements are embedded, with strength properties greatly superior to those of the base resin.


A strong, inert fibrous material incorporated in a plastic or rubber mass to improve its physical properties. Can be either a woven fabric of chopped fibre.

Reinforcing Agent

Fillers, such as Carbon Black, added to the elastomeric batch mix to improve such physical properties as tensile strength.


The capability of returning to original size and shape after deformation Resilience: The technical term used to describe “bounce” or “snap”. Increasing resilience means an increasing “bounciness”. Measured on standard test equipment (commonly examples being Dunlop, Tripsometer, Lupke, and Rebound). Standard test pieces are struck by a standard ‘hammer’ and the ‘bounce back’ of the hammer measured. This is expressed as a percentage of the flight path of the hammer.


Where the forcing frequency and the natural frequency of a suspended mass are the same i.e.; when the frequency ratio is equal to one, then the phenomenon known as resonance will occur. This is because the energy in the forcing motion is in phase with the natural frequency and energy is stored and builds up in the system. A good example of this is a playground swing where pushing the swing in phase the swing ark increases and the swing gets higher & higher. Rubber mounts must be designed to avoid amplification of the natural frequency of an installation.


Degradation of a rubber compound characterised by tackiness due to overcure in manufacture or excessive heat in service. Natural rubber is prone to this effect, as the crosslinks start to deteriorate under longer term heat.


Radio-Frequency Interfence, see also EMI.


Rapid Gas Decompression, see also Explosive Decompression.


Reaction injection moulding of Polyurethanes.


Rubber Manufactures Association.


Root Mean Square, a measure of variation of statistical values, can be applied to surface roughness, or deviation from true flat.

Rockwell Hardness

A standard method for measuring the hardness of metals. The hardness is expressed as a number related to the depth of residual penetration of a steel ball or diamond cone (“brale”) after a minor load of 10 kilograms has been applied to hold the penetrator in position. This residual penetration is automatically registered on a dial when the major load s removed from the penetrator. Various dial readings combined with different major loads, give “scales” designated by letters varying from “A” to “H” the “B” and “C” scales are most commonly in use.

Rotary Seal

Sealing designs for rotating shafts, with the turning shaft protruding through the I.D. of the Seal, and containing pressure or liquid on one side of the Seal.

Rough trim

The removal of remaining flash or sprue through picking or pulling, which may leave a degree of flash still present on the split line, to be removed by other methods if necessary. See also Tear Trim.


Reinforced plastics.

RTV Moulding

A procedure where a part is moulded in a Room Temperature Vulcanising silicone cast tool, or the process of casting RTV material into a metal or Rapid Prototyped tool to produce a cast silicone part.


The common name for both naturally occurring and synthetically made elastomers.

Rubber – Natural

 A natural product refined from the sap of various suitable trees or plants. Properties can be improved by combining with sulphur in the process called Vulcanisation.

Rubber – Synthetic

Manmade elastomers derived from chemical feedstocks producing polymers such as Nitrile, EPDM, Butyl, Flurocarbon, Silicone etc.

Rubber Bellows

Moulded or fabricated protective covers for hydraulic rams and sensitive equipment. Standard parts or bespoke solutions are available.

Rubber Extrusions

Rubber profiles, sections and seals produced by extrusion technique.

Rubber Mouldings

Rubber components produced by compression, transfer or injection methods.

Rule Forme

The steel knife blade that can be bent into shape the inserted into plywood boards to create a gasket cutting tool suitable for stamping through sheet material. Such tools are known as “Rule Forme Cutters”.

Run out (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.

Running Friction

The frictional force developed during continuous operation of an O-ring or seal.



Society of Automotive Engineers.


The premature onset of curing of compounded rubber stock during the milling process or storage, with the potential for adversely affecting material flow and plasticity during subsequent shaping and curing processes. It can also occur during the moulding process. Usually caused by the generation of excess heat in processing, adjustments to the compound mix or process conditions can tailor the effect. Scorched rubber is no longer processable. See also “Setting Up”


In extrusion, the shaft provided with helical grooves which conveys the material from the hopper outlet or strip feed slot through the barrel of an extruder or injection press, and forces it out through the die or into the shot chamber.


Any device used to prevent the passage of a fluid (gas or liquid) or fine particles past a designed barrier.

Sealing Surface

The primary location in a sealing system where a seal and a mating surface come in contact with the intention of forming a barrier to prevent the passage of a medium such as a fluid or a gas. The design and specification of the relative elements of the components that act on the surface, and the material and finish of the surface will dictate the performance and reliability of the seal.

Sealing System

The components and attributes which compose the sealing environment, including the seal, the components being sealed, housing designs, dynamic parts, the medium or media being sealed, and the environmental conditions such as temperature, pressure, and motion.


A feature on an insert which is used to control which surfaces of the part will be covered by rubber during the moulding process. A seal-off usually takes the form of a flat surface or rib which seats against a corresponding mould surface to restrict the flow of rubber.


Operating conditions of a seal or component.


The degree to which a rubber part does not fully recover to its original shape after it has been deformed for a long period of time. See also Permanent Set and Compression Set.

Setting Up

The onset of premature cure during a milling operation, due to excessive heat being generated in the mix. See also “Scorching”.


A load-bearing and/or power-transmitting component of a mechanical system which is generally cylindrical in shape and frequently rotates or reciprocates, usually with a locating bearing or cylinder housing.

Shear Strength

The maximum load required to shear a specimen in such a manner that the moving portion has completely cleared the stationary portion.


Normally thin gauge precisely calendared elastomer sheet made from polymers such as Natural, Neoprene, Nitrile & EPDM. Fabric can be “Inserted” in one or more layers inside the sheeting during calendaring, to reinforce the rubber. A smooth or “Plate” finish is generated by polished finishes on the calendar rollers, and textures can be applied by running a cloth through the process on top of the rubber. Cloth finishes are preferred certain gasket applications. Sheets are distinguished from films in the plastics industry only according to their thickness. In general, sheets have thicknesses greater than .040”.

Shelf Aging

The potential degradation of product performance capabilities due to exposure of elastomers to stressful environmental factors during storage such as sunlight, heat or deformation.


Length of time a moulded component or uncured compound can be stored without suffering significant loss of physical properties or processability, Can be alleviated by optimum packing and storage conditions, see BS 3574 for typical values.

Shore A

A hardness scale invented by the Alfred Shore in the 1920’s and used to measure the hardness of moulded rubber. The Shore A scale is most effectively used to measure rubber with hardness from 10 to 95 Shore A. For materials harder than 90 to 95 Shores A, the Shore D scale is recommended. See also Durometer and IRHD.


One complete cycle of a moulding machine.

Shot Capacity

The maximum volume of material that can be delivered to an injection mould by one stroke of the ram.

Shot Weight

The weight or charge of material required to fill a mould tool and produce a complete set of parts from the tool in one moulding cycle.


The linear contraction upon cooling of a moulded rubber or thermoplastic part. All rubber shrinks to some degree following de-moulding. Decreased component or seal volume due to adverse environmental factors, such as plasticiser extraction by system fluids.


Abbreviation for Le Systeme International d’Unites.

Single Cavity Mould

A mould having only one cavity and therefore producing only one finished part per cycle.

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.


Sterolithography Additive manufacturing, where a vat of material is cured by a laser one layer at a time, gradually building up a series of thin layers to form a 3D shape in solid material. Depending on the resin in the vat, various material types and properties can be delivered for finished parts.


Selective Laser Sintering is an additive manufacturing process that uses a laser to selectively sinter a powder into a fused solid, gradually building up a series of thin layers to form a full 3D shape in solid material. Depending on the material being sintered, the finished parts can have  a variety of properties.


Substances with the ability to dissolve other substances.


Society of Plastics Engineers.

Specific Density

See Specific Gravity.

Specific Gravity

The ratio of the density of a material as compared to the density of water at standard atmospheric pressure (1 ATM) and room temperature (20°C). Soft Natural rubber typically has an SG of 1.0, but for harder grades this will increase to 1.3, and synthetics such as Viton may vary from 1.8 to 2.5. Measured by weighing a sample in air and then water.

Specific Volume

The volume of a unit of weight of a material; the reciprocal of density.

Spiral Failure

Generally found on long stroke, hydraulic piston seals, spiral failure results when certain segments of the O-ring slide, while other segments simultaneously roll. At a single point on its periphery, the O-ring gets caught on an eccentric component, or against the cylinder wall, causing twisting and development of 45 degree angle, surface cuts.

Spiral Twist

A type of seal failure resulting from a twisting action that strains or ruptures the rubber.


A common moulding defect. It is found when the rubber part is stretched and the part will start to tear, normally at the knit point where material flows have come together. Excessive mould release is often the cause, or sometimes too hot a cure temperature resulting in surface scorch is the problem. See also Knit Lines, Voids and Weld Lines.


The feeder through which rubber passes into a mould, which may lead directly into the mould cavity, or in turn feed a series of sub-gates.


O-ring compression between two mating surfaces comprising the groove or gland. Can be axial, or squeezed on the top and bottom surfaces, or radial, squeezed on the inner and outer surfaces. Referring to the deformation placed on a rubber part to affect a seal and usually expressed as a percentage of the relaxed original cross-section, it is also occasionally expressed as an absolute value of the deformation.


Synthetic Bonded Resin Paper. Phenolic Resin bonded paper, which forms a rigid highly insulating board, trade names being Paxolin, Formica and Tufnol. Flame Retardent versions known as FR2.

Stack Up Tolerance

The summation of system tolerances, in an assembly or sealing system.


Describes an application in which there is no relative motion between mating surfaces.

Static seal

A gasket or O Ring type application in which the seal is contained within two non-moving gland walls. Except for pulsates caused by cycle pressure, does not move in its environment and there is no relative movement between the housing surfaces.

Statistical Process Control (SPC)

The use of statistical analysis techniques on processes and their outputs to establish process stability, reduce rejects and increase capability.


The capacity of a material to resist elastic displacement under stress.


Uncured rubber compound with given composition from which a specific item is to be manufactured. Billet form raw materials, or unconverted sheet material.


The deformation caused by an applied stress. In tensile testing, the ratio of the elongation to the gauge length of the test specimen, that is, the change in length per unit of original length. A rubber stretched to double its original length would be at 100% strain.


The force producing or tending to produce deformation in a body measured by the force applied per unit area. Measured like pressure in Pascals, ie Newtons per square meter.

Stress Relaxation

Decreasing stress with constant strain over a given time interval (viscoelastic response)

Stress-Strain Curve

The curve plotting the applied stress on a test specimen versus the corresponding strain. Stress can be applied through shear, compression, flexure, or tension.

Surface Finish

A term usually used in reference to the roughness parameter of a surface’s texture, general expressed in units of microinches (µin) or micrometers (µm)

Surface Grinder

A commonly used machine tool which is used to grind mould plates and components to a specified dimension and to provide high flatness, accuracy and surface finish.


Increased seal volume caused by exposure to adverse operating conditions, such as exposure to oils, fluids, heat, gas etc, resulting in absorbtion of foreign elements within the rubber, causing it to gain size or length. Indicates an unsuitable material for the application in question.

Synthetic Rubber

Manufactured or man-made elastomer derived from hydrocarbon base feedstocks, first developed in the 1920’s. Examples are Nitrile, Fluorocarbon, Silicone, etc.



Stickiness. The degree of adhesion of materials of identical nature to each other. Tack agents can be added to help make rubber fabrications easier to handle prior to vulcanising.

Tear Resistance

The ability of a material to withstand tension without growth of a cut or nick in a seal or part when tension is applied.

Tear Strength

The force required to rupture a sample of stated geometry. Measured as the force required tearing a standard piece. The standard test pieces are specifically designed to produce controlled weak points where a tear is initiated.

Tear Trim

The removal of remaining flash or sprue through picking or pulling, which may leave a degree of flash still present on the split line, to be removed by other methods if necessary. See also Rough Trim.

Temperature Range

The working range of a material or product marked by the limits of minimum and maximum operating temperatures for effective performance. Operations outside these ranges may be permissible, stated as Intermittent performance.

Tensile strength

The strength or tensile stress of a material when stretched to breaking point. This is measured in various units eg Psi and MPa and is expressed as a force per unit area. A standard dumbbell type test piece of known cross sectional area is used which is stretched until it breaks. The force required to do so is then recorded and expressed as force per unit area.

Tension Set

Increase in the length of an elastomeric specimen following initial stretching and release. Can be caused by de-moulding a part before it is fully cured.


A polymer resulting from the chemical combination of three monomers.

Thermal Conductivity

The ability of an elastomer to conduct heat.

Thermal Expansion

Linear or volumetric expansion caused by temperature increase.


The process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to render it soft and flowable. A differential pressure is then applied to make the sheet conform to the shape of a mould or die positioned below the frame.

Thermoplastic Elastomers (TPE)

The family of polymers that resemble rubber-like elastomer performance in that they can be repeatedly stretched without distortion of the unstressed part shape, but are true thermoplastics and in that they do not require curing, and can be reshaped.


Materials that become soft when heated, shaped and then become solid when cooled to room temperature. This softening, shaping and re-freezing may be repeated many times.


Plastic materials that undergo a permanent chemical cross linking of molecules when processed, heated and moulded, and therefore cannot be softened and reshaped. Further heating outside its temperature range will degrade the material rather than soften it, causing reversion or charring.


The generic term used to describe an extrusion die, mould, rule forme, template, or cutter.


A turning or twisting force generally associated with the rotation of a shaft.

Total Indicator Reading (TIR)

A term used to indicate how the measurement of the roundness of a part, as rotated about its centre-line, should be interpreted. Expressed in total diametric deviation.


Property of resisting fracture or distortion. Usually measured by impact test, high impact values indicating high toughness.


Thermoplastic rubber, see also TPE.


Thermoplastic polyurethane.

Transfer Moulding

 Method of moulding thermoset materials. The elastomeric compound is placed in a transfer chamber or pot which is part of the mould, heated, then squeezed by a piston in the top of the pot down through a sprue runner, feeding into a closed mould cavity. A useful method for close tolerance control, bonding inserts in place, or making high integrity parts where injection mould tools are not cost effective.


Removal of excess material (feed gates, flash or sprue) on a moulded rubber article, by knife, scissor, buffing, punching or cryogenic deflashing.

Trim cut

Damage to a moulded article by trimming too close and removing part of the intended geometry. Finish cut of a moulded seal to create the final dimension.





Ultra-high molecular weight.


Underwriters’ Laboratories.

Ultimate Elongation

In a tensile test, expressed as a percentage of its original length, a measure of how far a material will stretch before breaking, the elongation at rupture.

Ultimate Strength

Term used to describe the maximum unit stress a material will withstand when subjected to an applied load in a compression, tension, flexural, or shear test.

Under cure

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


A feature on a part which requires a corresponding feature in the mould which is perpendicular to the direction of extraction. An undercut complicates the manufacture of a moulded part by making it difficult to remove the finished part from the mould, and often requires the mould tool to be designed to have a removable insert that releases the undercut face prior to demoulding the part.



Negative atmospheric pressure.


The gaseous state of a solid or liquid.

Virgin Material

Any compound or resin that has not been subjected to use or processing other than that required for its original manufacture.


A material that exhibits deformation properties that are a combination of both viscous and elastic materials, with strain that changes over time.


A measure of the resistance to flow, often under stress due to internal friction when one layer of fluid is caused to move in relationship to another layer, or a containing wall.

Viton Adhesive

A flexible multipurpose adhesive made from Viton fluoroelastomer with good adhesion at high temperatures.

Viton Sealant

Special caulk sealant made from Viton fluoroelastomer – ideal as a gap sealer in corrosive and hazardous environments.

Viton Sheeting

High performance rubber sheet made from Viton fluoroelastomer.


In a solid rubber, an unfilled space, empty pocket, or an absence of material, where not intended, caused by process conditions. See also Splits, Knit Lines and Weld Lines.

Volume Change or Swell

The measure of the swell or shrinkage of a material, seal or component resulting from immersion in a particular media for a specified period of time at a specified temperature. Expressed as a percentage of the original volume. See also Swell.


Heat and pressure induced process whereby the long chains of rubber molecules become crossed linked by an additional chemical agent to form three dimensional elastic structures. First discovered by Charles Goodyear, the term was actually patented by Joseph Mackintosh. The reaction turns weak materials into strong ones, chemicals such as sulphur which accomplish this change are called vulcanising agents and crosslink existing molecular rubber chains. The chemical change is permanent, creating a Thermoset material. See our Blog on Vulcanisation

Vulcanised Joined Rings

Sealing rings normally made from extruded sections or O Ring cord, with fully vulcanised joints.

Vulcanising agent

A material that produces vulcanisation of an elastomer.


Weather Resistance

Weather resistance is a fairly subjective test and it is necessary to state clearly the conditions under which the exposure took place i.e. dates, geographic location, angle and direction of exposure relative to the sun etc. The properties of test pieces are measured before and after exposure and expressed as percentage changes (with exception of hardness changes which are usually recorded in degrees). The change in any property may be measured but the most common are hardness, tensile strength, elongation at break and modulus at 100% and 300%.


A broad term encompassing exposure of plastic and rubber materials to solar or ultraviolet light, temperature, oxygen, salt water, biological attack, humidity, rain, snow, wind, and air-borne biological and chemical agents. Resistance to weathering is not easy to define, but typically glazing seals for architectural installations will demand a 25 year life. Accelerated testing can measure the change in typical physical properties, but devising fully representative testing is difficult. See also Perishing.

Weld Lines

Small defects visible on a finished part made by the meeting of two flow fronts during moulding. May result in a peel failure if twisted or bent. See also Splits Knit lines and Voids.


Dimension of an o-ring or washer cut at a 90 degree angle to the mould parting line. The cross sectional diameter of an o-ring, being one half the distances between the I.D. and O. D. of the ring. O rings are usually specified by width/cross section and inside diameter.