Gasket Terminology Primer from Machine Design: Volume 1

Industrial Glass Types

We recently ran across a very nice primer from Stephan Mraz of Machine Design, and wanted to share it with our subscribers.  Like the guys at MD, we’ll split it into separate installments to keep the length down.  We hope you find it helpful!

Gasket Glossary, Vol. 1 (A through D):

Questions about a term that has something to do with gaskets? Get a concise definition here.
Go to a specific letter:A B C D

Image courtesy of Thinkstock

Gaskets embody a host of different technologies. There are the myriad natural and synthetic materials they can be made out of, the cutting and finishing processes used to tailor them to specific applications and conditions, and the measurements and test engineers rely on to evaluate them. Here’s a long list of words and terms used in one these aspects of gasket design, manufacturing, and application, from abrasion to zinc oxide:

Abrasion: The wearing away of a material surface by friction. Particles become detached by a combined cutting, shearing, and tearing action. Furnace carbon blacks are the best ingredients found for increasing the resistance of rubber compounds to abrasion.

Abrasion Resistance: The resistance of a material to loss of surface particles due to friction.

Accelerated Aging: These are procedures for subjecting pressure-sensitive label stock (and other materials) to special environmental conditions to predict the course of natural aging.

Acetate: A plastic synthesized from cellulose dissolved in acetic acid which exhibits rigidity, dimension stability, and ink receptivity.

Acid Resistant:Withstands the action of acids.

Acrylic Adhesive: Adhesive made from acrylic monomers that have been polymerized. They have good resistance to UV radiation, plasticizers, and extreme temperatures.

Adapters: A V-shaped ring, either male or female, to fit together with V-shaped rings to form a set of adjustable hydraulic packing.

Adhesion (a): The state in which two surfaces are held together by interfacial forces which may consist of molecular forces or interlocking action, or both.

Adhesion (b): The clinging or sticking of two material surfaces to one another. In rubber parlance, the strength of bond or union between two rubber surfaces cured or uncured. It also describes bonds between cured rubber surfaces and non-rubber surfaces such as those of glass, metal, wood, or fabric.

Adhesion Failure: The separation of two materials at the surface interface rather than within one of the materials itself.

Aging (a): (1) The irreversible change of material properties after environmental exposure for an interval of time; (2) Exposing materials to an environment for an interval of time.

Aging (b): Changes in physical and mechanical properties that occur when low carbon steel is stored for some time. Aging is also accelerated by exposure of steel to elevated temperatures.

Aging (c): A progressive change in the chemical and physical properties or rubber, especially vulcanized rubber, usually marked by deterioration. Aging may be retarded by the use of antioxidants.

Artificial Aging: Speeding up the natural aging cycle by heating the metal for a short time.

Air Curing: Vulcanizing a rubber product in air, as distinguished from vulcanizing in a press or steam vulcanizer.

Aluminum: A pliable, lightweight metal that has good electrical and thermal conductivity, high reflectivity, and resistance to oxidation.

Aluminum Seal Rings: Sealing rings for pistons made from high-grade aluminum alloy.

AMS: American Military Standards.

Annealing: A process involving high-temperature heating and cooling of the as-rolled cold rolled steel substrate to make it softer and more formable.

Anodize: The controlled oxidation of aluminum using an electro-chemical process to create a porous surface that is receptive to color dying.

Anti-Extrusion Rings: Also called back-up rings or anti-extrusion rings, used to fit behind rubber O-ring seals to prevent extrusion into the gap between the metal pieces.

Antioxidant: Usually organic and nitrogenous. A substance which inhibits or retards oxidation and other types of aging. Some antioxidants cause staining or discoloration of rubber compounds on exposure to light and are used only in black or dark-colored goods. Others (phenolic), described as non-staining, are used in white or light-colored goods.

Anti-Stick Coatings: Surface treatments that prevent gasket materials from adhering to flanges.

Anti-Vibration Mounts: Rubber molded pieces used as padding between a motor and the frame to prevent vibration transfer to the machine to which it is mounted.

Apportionment: Referred to here as a part of Reliability Engineering. Synonymous with the term Reliability Apportionment, which is the assignment of reliability goals from system to subsystem in such a way that the whole system will have the required reliability.

Assortment Kits: A convenient package containing several sizes of the same seal, O-ring, or retainer ring.

ASTM: American Society for Testing and Materials.

Automatic U-joints: Also called u-cups, these shaped sealing rings are made from a strong pliable plastic or rubber.


Backer Coat: Usually refers to the coating on the reverse side of a prepainted sheet. The backer coating is generally not as narrowly specified with reference to its color, thickness, and composition, as is the topcoat.

Backrinding: Defect in which the rubber adjacent to the mold parting line shrinks below the level of the molded product, often leaving the parting line ragged and torn.

Backringing: Distortion at the mold parting line, usually in the form of wrinkles, folds, tears, or indentions. In severe cases may cause overall dimensional changes.

Baffle Rings: A ring used to slow the flow of fluids along a shaft.

Ball Valve Seats: A PTFE ring shaped to fit against the ball in a flow control valve.

Batch: The product of the one mixing operation in an intermittent process.

Bearings: A machined or molded plastic ring used as a guide ring or wear ring in a hydraulic cylinder.

Beater Additives: A water-based process used to make gasket material where the elastomer is chemically deposited onto the fibers and fillers.

Bearing Seals: A seal ring made to snap-fit into a ball, roller, or spherical bearing to exclude dust, dirt, or trash.

Bellows: A corrugated rubber or plastic piece which can stretch with a shaft to keep the shaft clean.

Belts: A v-belt, flat belt, or drive belt made from plastic or rubber.

Bezel: A grooved rim, which holds another covering or item. Similar to a frame.

Bias Angle: (1) Acute angle between the direction of the cut and the diameter of the wrap in the production of wrapping for hose; (2) Acute angle between the direction of the cut and the direction of the cords in the production of fabric plies.

Bill of Material: Total list of all components/materials required to manufacture the product.

Binder: The elastomer (or rubber) used in gasket material.

Bloom: An efflorescent coating creating a discoloration or visual change on the surface of a material. Sometimes caused by the migration of a substance to the surface of product, it can also be normal in some organic materials (sulfur or wax bloom).

Blister: A cavity or sac that deforms the surface of a material.

Blowing Agent: Chemicals mixed with a compounded into many formulations that form gases to create cellular structures, such as sponge rubber.

Bond: The union of materials by use of adhesives.

Bonded Seals: A seal created by a flat steel washer with a rubber sealing ring molded into the center that fits over a bolt.

Bonding Agents: Substances or mixtures of substances used for attaching rubber to metal, fabrics, or other substrates. Generally the rubber compound is vulcanized by heat in the process. Cyclized rubber or rubber isomers, halogenated rubber, rubber hydrochloride, reaction product of natural rubber and acrylonitrile, and polymers containing diisocyanates are all used.

Brittleness: Tendency to crack when subjected to deformation.

Bumpers: A rubber or plastic part used to prevent metal-to-metal contact.

Buna N: A general term for the copolymers of butadiene and acrylonitrile. Typical commercial polymers are Hycar and Paracril.

Buna S: A general term for the copolymers of butadiene and styrene.

Bushing: A rubber or plastic spacer to provide a wear surface around a shaft.

Butadiene: CH2 = CH – CH = CH2. A gaseous hydrocarbon of the diolefin series that boils at 5°C. Also known as erythrene, divinyl, pyrollylene, polymerizable, and polybutadiene. Butadiene is the chief raw material for making the synthetic rubbers today. Co-polymerized with styrene it yields SBR or GR-S; with acrylonitrile the various Buna N or nitrile synthetic rubbers are obtained.

Butyl: Isobutylene isoprene (IIR). Produced by copolymerizing isobutylene with isoprene. These rubbers exhibit excellent impermeability to gases, excellent dielectric properties, good tear-resistance, good aging at elevated temperatures, and good chemical stability.

Butt Joint: Joining two ends of material whereby the junction is perpendicular to the ID of an O-ring.

Butyl: A synthetic rubber of the polybutene type exhibiting very low permeability to gases.

Butyl Rubber: A copolymer of isobutylene and isoprene, polymerized almost instantaneously in methyl chloride with aluminum chloride at about ≤140°F. Butyl is resistant to ozone and the action of many other corrosive chemicals. Butyl rubber is resistant to permeation by gases.


Caliper: The thickness of a sheet material. The thickness is usually expressed in one thousands of an inch and in millimeters (e.g., 0.050 is expressed as 50 mils).

Camber: The deviation of a side edge from a straight line, the measurement being taken on the concave side with a straight edge.

Carbon black (a): Elemental carbon in finely divided form used to reinforce elastomeric compounds.

Carbon Black (b): Finely divided carbon formed by the incomplete combustion of natural gas or petroleum in large, closed furnaces.

Carbon Steel: Steel which owes its properties chiefly to carbon without substantial amounts of other alloying elements; also known as straight carbon steel or plain carbon steel.

Catalyst: A chemical in small quantities which accelerates a chemical reaction without itself necessarily becoming part of the final product.

Cavity: The area on a die where blades are formed to cut. A die with one or more cutouts that are the same size for each label cut.

Cellular Rubber: Rubber products which contain cells or small hollow receptacles. The cells may either be open or interconnecting or closed and not interconnecting.

Characteristics Matrix: An analytical technique for displaying the relationship between process parameters and manufacturing stations.

Checking: Short, shallow cracks on the surface of a rubber product that are created by damage from environmental conditions.

Checking Sunlight: The development of minute surface fissures as a result of exposing rubber articles to sunlight, generally accelerated by bending or stretching.

Chemical Resistance: The resistance offered by elastomer products to physical or chemical reactions as a result of contact with or immersion in various solvents, acids, alkalis, salts, etc.

Chemical Treatment: An aqueous solution of corrosion-inhibiting chemicals, typically chromates or chromate/phosphate.

Chevron Packings: Also called v-packing, vee packing, Chevron packing, parachute packing, or v-set packing. A complete vee packing set contains multiple v-shaped sealing rings stacked and nested together with a male adapter on one end and a female adapter on the other end.

Chloroprene: 2-Chloro-l, 3-butadiene, a volatile, colorless liquid which boils at 59°C, synthesized from acetylene. It is used in the manufacture of neoprene, which is obtained by polymerizing chloroprene under suitable conditions.

Chevrons: See Chevron Packing above.

C.I.: The abbreviation for cloth-inserted, indicating a sheet of rubber containing one or more plies of fabric covered with rubber.

Closed Cell: A cell totally enclosed by its walls and hence not interconnecting with other cells.

Coil Breaks: Creases or ridges in sheet that appear as parallel lines across the direction of rolling, and that generally extend the full width of the sheet or strip.

Cold Flow: Continued deformation under stress.

Cold Rolled Products: Flat rolled products for which the required final thickness has been obtained by rolling at room temperature.

Cold Working: Applying a mechanical force (such as deep drawing) to metal at room temperature at such a rate that strain-hardening occurs.

Color Standard: A painted sheet panel with a prescribed color of paint representing the precise color it is intended to produce in the pre-painted sheet. The color standard will preferably also be expressed in terms of physical attributes of hue, lightness, and saturation called tristimulus values or derivatives of these values. A complete color standard definition will usually include painted panels representative of the limits of acceptable deviation from the precise standard color, as well.

Coefficient of Expansion: The coefficient of linear expansion is the ratio of the change in length per degree to the length at 0°C. The coefficient of surface expansion is two times the linear coefficient. The coefficient of volume expansion (for solids) is three times the linear coefficient. The coefficient of volume expansion for liquids is the ratio of the change in volume per degree to the volume at 0°C.

Commercial Steel (CS): Sheet of this quality is for simple bending or moderate forming. Commercial Steel sheet can be bent flat upon itself in any direction at room temperature.

Compact Seals: Multi-piece seal sets which are generally used as piston seals in a hydraulic cylinder. Made to fit in a limit space, compact piston seals contain a primary sealing component, guide rings, and back-up rings in one convenient set.

Compound (a): A term applied to either vulcanized or unvulcanized mixtures of elastomers and other ingredients necessary to make a useful rubber-like material.

Compound (b): In chemistry, it is the material resulting from the chemical union of two or more elements in definite proportions and in which the properties of the individual elements have disappeared. In rubber manufacture, it is the composition or formula of stock, the ingredients of which, however, may not all be chemically combined and is therefore more of a physical mixture.

Compression Deflection Characteristics: The tests for compression-deflection characteristics constitute methods of compression stiffness measurement. One compression test involves the determination of a load required to case a specified deflection, and another is a compression test in which a specified weight or compressive force is placed on the specimen, and the resulting deflection is measured and recorded.

Compression Set (a): The residual decrease in thickness of a test specimen measured 30 minutes after removal from a suitable loading device in which the specimen has been subjected for a definite time to compressive deformation under specified conditions of load application and temperature. Method a measures compression set of vulcanized rubber under constant load. Method B employs constant deflection.

Compression set (b): The residual deformation of a material after removal of the compressive stress.

Compressibility: The percent of loss of thickness when subjected to a given load applied by a disc of a given diameter for a specified short time and at a specified temperature. Defined by ASTM F-36 test procedures.

Conductivity: To conduct or transmit heat or electricity.

Conductive Adhesive: An adhesive that incorporates conductive fibers. These fibers have the ability to conduct electricity through the thickness of the adhesive and/or in the plane of the adhesive. Ideal for EMI/RFI shielding and EMI/RFI gasket attachments.

Conformability: The ability of an adhesive tape to mold itself to the shape of an object without wrinkling or creasing.

Converting: The process of taking a material or adhesive and altering it from one form to another.

Contact Stain: Discoloration of a product by another material or by a rubber article in the area directly touching it.

Copolymer (a): A polymer consisting of two different monomers chemically combined.

Copolymer (b): A copolymer is a polymer consisting of molecules containing large numbers of units of two or more chemically different types in irregular sequence. Butadiene and styrene form a copolymer known as GR-S.

Copper Seal Rings: Rings made from thin copper formed over fibrous filler to seal in high temperature.

Corrosion: Gradual chemical or electrochemical attack on a metal by atmospheric moisture or other agents.

Crazing: A surface effect on rubber parts characterized by many small cracks.

Creep: The deformation, in either cured or uncured rubber under stress, which occurs with lapse of time after the immediate deformation.

Creep Relaxation: In a flange gasket, loss of stress accompanied by constantly decreasing compressed thickness. This type of relaxation is encountered in bolted flange joints.

Cross Linked: The establishment of a chemical bond between the molecular chains of a given polymer, thereby enhancing physical properties.

Critical Surface: Intended for material applied to critical exposed/painted applications where cosmetic surface imperfections are objectionable. The prime side surface will be free of repetitive type imperfections, gouges, scratches, scale, and slivers. This surface can only be furnished as a pickled product.

Cross Section: An O-ring as viewed if cut at right angles to the axis showing internal structure.

Crush Washers: A washer made to be crushed to form a seal.

Crown: A contour on a sheet where the thickness increases from some edge measurement to the center.

Cup Packing: Sealing devices made in the shape of a cup with outer lips curved upward usually made from rubber, fabric reinforced rubber or polyurethane.

Cure: The act of vulcanization. See Vulcanization.

Cushioning Seals: Sealing rings mounted into a cylinder to cushion the stroke or prevent metal-to-metal contact.

Custom Molded Products: Special shaped parts molded from rubber or plastic made to fit the machine or device it is used in.

Cut: The distance between cuts or parallel faces of articles produced by repetitive slicing or cutting of long pre-shaped rods or tubes such as lathe cut washers.

Cut Edge: Removal of the as-rolled hot mill edge. Coil ends are cropped back to gauge when cut edge is ordered.

Cut Outs: The spaces or holes designated in the label. This material is punched and removed during the manufacturing process.

Cystalinity: Stretched natural rubber forms a highly oriented state and shows X-ray diffraction patterns and other properties common to truly crystalline materials. The amorphous and crystalline regions are not mechanically separable phases, but the same molecule may at the same time have part of its length in a crystalline, and the remainder in an amorphous region.


Damper: The use of a variety of materials to deaden or damp a vibration.

Deep Drawing: The process of working metal blanks in dies on a press into shapes which are usually more or less cup-like in character.

Deep Drawing Steel (DDS): Sheets of this designation should be used when drawn steel will not provide enough ductility for fabricating parts to stringent drawing requirements, or applications that require the sheet be free from aging. DDS is made by special steelmaking and finishing practices.

Density: The weight per unit volume of a material: usually expressed in PCF (pounds per cubic foot).

Design Failure Mode and Effects Analysis (DFMEA): An analytical technique used by a design responsible engineer/team as a means to assure, to the extent possible, that potential failure modes and their associated causes/mechanisms have been considered and addressed.

Design for Manufacturability and Assembly: A simultaneous engineering process designed to optimize the relationship between design function, manufacturability, and ease of assembly.

Design Information Checklist: A mistake proofing checklist designed to assure that all important items are considered in establishing design requirements.

Design Reviews: A proactive process to prevent problems and misunderstandings.

Design Validation: Testing to ensure that product conforms to defined user needs and/or requirements. Design validation follows successful design verification and is normally performed on the final product under defined operating conditions. Multiple validations may be performed if there are different intended uses.

Design Verification: Testing to ensure that all design outputs meet design input requirements. Design verification may include activities such as:

Dielectric Strength: The measure of a product’s ability to resist passage of a disruptive discharge produced by an electric stress; the voltage that an insulating material can withstand before breakdown occurs.

Die Cutting: When parts are cut into individual pieces using a steel rule die. A sharp steel rule die is formed to the desired shape in a wooden carrier for cutting labels. A die may be one or more up (one cavity or more).

Die Guide: A guide around a label that assists with positioning of die and/or keeping art to edge tolerances.

Die Impression: A piece of material that has been cut with a die, but not cut all the way through.

Discs: Flat, round, saucer-shaped pieces made from rubber or plastic.

Disperse: To cause particles or molecules of matter to separate and become uniformly scattered throughout a medium. In a rubber compound, the particles of compounding ingredients are dispersed in the rubber. In latex, rubber globules are dispersed in an aqueous medium.

Distributor Seals: Sealing rings used to seal in oil and seal out dust, dirt, or trash on an automobile engine electric spark distributor.

Double Acting Seals: Seal rings which seal in two directions, on the push and the pull stroke of a hydraulic or pneumatic cylinder.

Double-Coated: Tape with adhesive on both sides.

Drawing Steel (DS): Sheets of this quality have more ductility and is more consistent in performance than commercial steel due to higher standards in production, selection, and melting of the steel.

Duck: A firm, compact, heavy, plain weave fabric made from cotton, synthetic fibers, or a combination of both. Duck is also known as canvas, army duck, belt duck, harvester duck, hose duck, and shoe duck.

Ductility: The ability to permit change of shape without fracture. In flat rolled steel, ductility is usually measured by hardness or mechanical properties in a tensile test.

Dumb-Bell (Test-Piece): In the physical testing of rubber, a test-piece is used that is shaped like a dumb-bell, i.e., constricted in the middle and flaring out at the ends, as distinguished from circular or ring test-piece. The dumb-bell is the most commonly used form or test-piece. Dimensions are set by ASTM standards.

Duocone Seals: A special cone-shaped sealing ring.

Durability: The probability that an item will continue to function at customer expectation levels at the useful life without requiring overhaul or rebuild due to wearing out.

Durometer: The most common Durometer, Type A or A-2, is an instrument for determining the hardness or rubber by measuring its resistance to the penetration (without puncturing) of a blunt indentor point impressed on the rubber surface against the action of a spring; a hand and special scale indicate the resistance to penetration. The scale reads from zero to 100, zero (0) being very soft and 100 being very hard. The Type D durometer has a sharp indentor point and is used to measure varying degrees of hard rubber up to ebonite.

Dust Seals: Seals used to keep dust out of machines or devices.