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Glossary of Rubber Molding Terms

Glossary of Rubber Molding Terms

Abrasion Resistance: The ability of a rubber compound to resist mechanical wear.

Ambient Temperature: The surrounding temperature relative to the given point of application.

Backgrind: Distortion at the parting line, usually in the form of a ragged indentation.

Bake-Out: A process whereby a vacuum system is heated for a given time at some predetermined temperature to degas off the components. (Ex: Gages, Fittings, Valves, Seals, etc.)

Blisters: A raised spot or bubble on the surface or a separation between layers, usually forming a void or air-filled space in the vulcanized article.

Bloom: A dusty or milky looking deposit caused by migration of a liquid or solid to the surface occuring sometime after curring

Buna-N: Same as Nitrile.

Buna-S: Co-polymers of butadiene and styrene. Also known as SBR and GRS.

Butyl: A co-polymer of iso-butylene and isoprene.

Calender: A machine used to form sheets of rubber between steel rollers.

Compound: A mixture of polymers and other ingredients to produce a useable rubber material.

Compression Set: The amount by which a rubber specimen fails to return to its original shape after release of compression load. The numerical value is that percentage of the amount compressed that does not return, NOT a percentage of the ENTIRE speciman.

Co-Polymer: A polymer consisting of two different monomers chemically combined.

Cure Date: Date when the rubber product was molded. (Ex: 1Q01 means, 1st Quarter of the year 2001)

Durometer: An instrument for measuring the hardness of rubber.

Elasticity: The property of an article which tends to return to its original shape after deformation.

Elastomer: Any synthetic or natural material with resilience or memory sufficient to return to its original shape after major or minor distortion.

Elongation: Generally is "Ultimate Elongation", or percent increase in original length of a specimen when it breaks.

Extrusion: Distortion or flow under pressure of a portion of seal into clearance between mating metal parts.

Filler: Chemically inert, finely divided material added to the Elastomer to aid in the processing and improvement of physical properties.

Flash: Excess rubber left around a rubber part after molding due to space between mating mold surfaces.

Gasket: A device used to retain fluids under pressure or seal out foreign matter. Normally refers to a static seal.

Gate: The point of rubber entry into a mold cavity.

Hardness: Resistance to a distorting force measured by the relative resistance of the material to an indented point of a standard hardness testing instrument.

Hardness, Shore A: The rubber Durometer hardness as measured on a Shore "A" gauge. Higher numbers indicate harder material. (Ex: 30 Shore is Soft . . . 90 Shore is Hard)

HCR: High Consistency Rubber, rubber that is processed on a rubber mill having much higher viscosity than liquid silicone rubber. (Same as Millable Gum Rubber)

LIM™: Liquid Injection Molding, refers to the processing of liquid silicone rubber (LSR).

LSR: Liquid Silicone Rubber, the silicone rubber utilized in the LIM™ process.

Millable Gum Rubber: Rubber that is processed on a rubber mill having a much higher viscosity than liquid silicone rubber. (Same as High Consistency Rubber)

Mismatch: Unsymmetrical seal caused by dissimilar cavities in mating mold sections.

Modulus: Tensile stress at a specified elongation. (Usually 100% elongation for elastomers)

Mold Finish: The uninterrupted surface produced by intimate contact of rubber with the surface of the mold at vulcanization.

Monomers: A chemical compound that can undergo polymerization.

Nitrile: (Buna-N) The most commonly used Elastomer for products because of its resistance to petroleum fluids, good physical properties and useful temperature range.

Occlusion: The mechanical process by which vapors, gases, liquids or solids are entrapped within the folds of a given substance during working or solidification.

Off-Register: Misalignment of mold halves causing an out-of-round part cross section.

Oil Resistant: Ability of a vulcanized rubber to resist the swelling and deterioration effects of various types of oils.

Oil Swell: The change in volume of a rubber article due to absorption of oil or other fluids.

Out-Gassing: A vacuum phenomenon wherein a substance spontaneously releases volatile constituents in the form of vapors or gases. In rubber compounds, this may include water vapor, plasticizers, air, inhibitors, etc.

Permanent Set: The deformation remaining after a specimen has been stressed in tension for a definite period and released for a definite period.

Permeability: The rate at which a liquid or gas under pressure passes through a solid material by diffusion and solution. In rubber terminology, it is the rate of gas flow expressed in atmospheric cubic centimeters per second through an Elastomeric material one centimeter square and one centimeter thick. (atm cc/cm squared / cm/sec) 

Pigment: A colored material, usually a powder or paste, used to color rubber.

Polymer: A material formed by the joining together of many (poly) individual units (mer) of one or more monomers. Synonymous with Elastomer.

Post Cure: The final step in the vulcanization process for the more exotic elastomers. Provides stabilization of parts and drives off decomposition products resulting from the vulcanization process.

Resilient: Capable of returning to original size and shape after deformation.

RTV: Room Temperature Vulcanization, generally silicone rubber, that after mixing, will cure when exposed to air.

Rubber, Natural: Raw or crude rubber obtained from vegetable sources.

Rubber, Synthetic: Manufactured or man-made elastomers.

Runner: The distribution system within a mold connecting the sprue to the gates.

SBR: Styrene butadiene. Very good resilience & tensile strength. Poor ozone & oil resistance.

Screw: The rotating component of an injection press which delivers the rubber into the mold.

Scorching: Premature curing or setting up of raw compounding during processing.

Shrinkage: Decreased volume of seal, usually caused by extraction of soluble constituents by fluids followed by air drying.

Silicone Rubber: Elastomer that retains good properties through extra wide temperature range.

Specific Gravity: The ratio of the weight of a given substance to the weight of an equal volume of water at a specified temperature.

Sprue: The initial entry point of rubber into a mold. In a simple mold, it is the same as a gate.

Sprue Marks: Marks left on the surface of a rubber part, usually elevated, after removal of the sprue or cured compound in the gate through which the compound is injected or transferred.

Static Seal: Part designed to seal between parts having no relative motion. (See Gasket)

Tear Resistance: Resistance to growth of a cut or nick when tension is applied to the cut specimen. Commonly expressed as "pounds per inch thickness".

Tensile Strength: Force in "pounds per square inch" required to cause the rupture of a specimen of a rubber material.

Thermal Expansion: Expansion caused by increase in temperature. May be linear or volumetric.

Viscosity: The property of fluids by which they resist an instantaneous change of shape. (Ex: Resistance to flow)

Vulcanization: A thermo-setting reaction involving the use of heat and pressure, resulting in greatly increased strength and elasticity of rubber-like materials.

Last Updated on Monday, 24 September 2012 09:34

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