Skip to main content

Glossary of Terms

Abrasion – Surface wear caused by relative motion between contacting objects

Abrasion Resistance – The ability of a rubber compound to resist surface wearing by mechanical action

ABS – Acrylonitrile-butadiene-styrene

Accelerator – A chemical compound that speeds up the vulcanization of natural or synthetic rubbers

Adhering Flash – Extremely thin flash (usually less than 0.002′ thick) that adheres to the finished part during molding or deflashing processes

Age Protection – Active – The use of a chemical additive in a rubber compound which is preferentially attached by oxygen or ozone thereby sacrificially protecting the rubber

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

Aging – The change in physical and chemical characteristics of an elastomer that has been exposed to a particular environment over time

Alloy – 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 the environment surrounding a particular object

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

Annealing – 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 molded article to relieve stresses set up by flow into the mold

Antioxidant – Any organic compound that slows the process of oxidation

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

AQL – Accepted quality level

Aromatic – 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

ASA – Acrylic-styrene-acrylonitrile

ASTM – American Society for Testing and Materials, An organization for issuing standard specifications on materials, including metals and alloys

Axial – The change in physical and chemical characteristics of a rubber compound that has been exposed to a particular environment over time

Axial Seal – Squeezed, like a gasket, on both the top and bottom surfaces of the seal’s cross section. A face seal

Back Rind – Distortion of the mold parting line, usually in the form of a ragged or torn indentation

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 gap while under pressure

Backup washer – A washer made from certain material that will add strength or support when installed next to the seal. This prevents the seal from being pinched and evenly distributes the load

Base Line – The mark from which all features are dimensioned

Batch Control – A process of keeping separate a batch of a rubber compound from the time it is molded through the entire production process up to and including the Shipping Department

Batch Number – The assigned number that identifies a batch or rubber

Billet – A term used (primarily in the Rubber Mold Industry) to describe mold cavity or core inserts which can easily be replaced in production molds

Blisters – A raised spot on the seal’s surface created by an internal void, or air-filled pocket

Bloom – A milky surface discoloration caused by the migration of certain compound components (such as antiozonants) to the rubber’s surface after molding or storage. The waxy film serves as a protective coating shielding the part from oxidation. This discoloration DOES NOT adversely affect material performance

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

Bonding – The method of securing molded rubber directly to an insert. There are two types; A. Mechanical, where the insert is designed with ribs, grooves or holes which enable the adhesive action with rubber when molded around the insert. B. Chemical, where the insert is treated with a point that has an adhesive action with rubber when molded around the insert

Bore – A hole machined in a component which permits the passage of a shaft

Bore Seal – A sealing system, usually in a radial orientation, in which the primary sealing surface is between the O.D. of a seal ring and the I.D. of a bore

Boss – Protuberance on a plastic part designed to add strength, facilitate alignment, provide fastenings, etc

Break-out – The force required to overcome initial seal to gland surface adhesion, when part movement is intermittent. A common term used to describe one form of friction

Bridge Tooling – Using the SLA mold as the master for short-run injection molding using spec material

Brittleness – Tendency to crack upon physical deformation

Buff – A physical or mechanical means of removing parting line flash

CA – Cellulose acetate

CAB – Cellulose acetate-butyrate

CAP – Cellulose acetate-propionate

Carbon Black – A black material used as a filler in rubber. It gives the rubber its black color and the hardness characteristics

Cavity – The features of a mold which are directly responsible for forming the final shape of a molded part

Charge – The measurement or weight of material used to load a mold at one time or during one cycle

Chemical bonding – A method for bonding rubber to secondary parts by applying special adhesives to the part prior to molding

Chipping – A defect normally created in the after molding flash removal process where fragile part surfaces and edges are chipped

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

Clamping – In injection molding and in transfer molding, the pressure which is applied to the mold to keep it closed, in opposition to the fluid pressure of the compressed molding material

Clamping Plate – A plate fitted to a mold and used to fasten the mold to a plate

Clamping Pressure – The pressure applied to the mold to keep it closed during a cycle

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 grove and enter into the clearance space also increase, especially in applications where pressure is present. Care should be taken in radial applications to note whether clearances are being stated as radial clearances or as diametral clearances

Coating – 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

Coefficient of Thermal Expansion – Value used to determine the amount of linear dimensional change for a particular elastomer, which is temperature dependent

Cold Flexibility – Flexibility following elastomer exposure to a specified low temperature for a specified period 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 Slug – The first material to enter an injection mold; so called because in passing through a sprue orifice it is cooled below the effective molding temperature

Compound – A mixture of a polymer and associated chemical ingredients necessary to produce a finished rubber material. The term is commonly used when referring to a specific rubber formulation

Compression Molding – A molding process in which the uncured rubber compound is placed directly into the mold cavity, and compressed to its final shape by the closure of the mold

Compression Set – The permanent deformation experienced by a rubber material when compressed for a 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 molded rubber parts such as seals and gaskets, which must retain their dimensions to maintain an effective seal.

Compressive Strength – The maximum compressive stress carried by a test specimen during a compression test

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

Conversion Fahrenheit to Celsius – F=(° C x 9/5) +32 C=(° F – 32) x 5/9

Conversion Celsius to Fahrenheit – C = (° F – 32) x 5.9 F = (° C x 9.5) + 32

Conversion Factors – From To Multiply by ___________________________________ lbs/ft2 kg/m2 4.88241 lbs/in2 (psi) kg/cm2 0.070307 gallons (gal) liters (l) 3.78533 gal/min 1/sec 0.063088 miles km 1.609

Conversion Grams to Ounces – gm = oz x 28.349527 oz = gm x 0.0352739

Conversion Inches to Millimeters – in = mm x 0.03937 mm = x 25.4

Conversion Kilograms to Pounds – kg = lbs x 0.454 lbs = kg x 2.205

Conversion Millimeters to Inches – mm = in x 25.4 in = mm x 0.03937

Conversion Ounces to Grams – oz = gm x 0.0352739 gm = ox x 28.349527

Conversion Pounds to Kilograms – lbs = kg x 2.205 kg = lbs x 0.454

Cooling Channels – Channels located within the body of a mold through which a cooling medium is circulated to control the mold surface temperature

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

Core Bar – A commonly used term to describe a male portion (usually opposite of cavity) of a mold detail which usually produces a hole or recess in a molded part

Core Bar – A steel bar used to hold machined core pins in place in the mold

Core Pins – Steel pins in a cavity that forms a hole or opening in a part

CPE – Chlorinated polyethylene

CPK – 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

CPVC – Chlorinated polyvinyl chloride

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

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

Creep – The progressive relaxation of an elastomeric material under constant sealing pressure (compressive load). Also known as cold flow. also called shrink

Cross-linking Agents – A chemical or chemicals that bond the polymer chains of a rubber together during the molding process

Cross-Section (of a seal) – the axial thickness or radial width of an O-Ring or Quad-Ring® seal. For an O-Ring, this will be the circular diameter of its cross-section. For a Quad-Ring®, this will be the length of a side of a square circumscribed about the cross-section

Cross-Sectional Compression, Percent (of a seal) – 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-sectional a result of material displacement caused by an applied stretch to the seal. See also Necking

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

Cure – A. One complete cycle. B. The thermo process that causes a chemical change in the raw stock, turning it into the finished rubber part

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 load card (7-card) in ounces or grams

Curing – An informal (slang) term for the vulcanizing process that cross-links a rubber to form its characteristic elastic properties. See also Vulcanization

Curing temperature – The temperature of vulcanization

Cylinder – Chamber in which a piston is driven

Damping – The ability of an elastomer to absorb forced vibrational energy

Daylight Opening – The clearance between two platens of a clamping press in the open position

Deburr – Removal of material from a drilled or punched hole, waste from saw cuts of material or sharp or rough edges

Deep Injection – A common molding defect identified by the deep depression in the rubber part at the point of injection

Deflashing – Any of various processes used to remove extraneous rubber from a molded rubber part

Delamination – The surface of the finished part separates or appears to be composed of layers of solidified resins. Strata or fish scale-type appearance where the layers may be separated

Density – Weight per unit volume of a substance expressed in grams per cubic centimeter

Diametrical Clearance – The gap between the two mating metal surfaces forming a gland’s internal cavity. Through slight oversizing and accompanying “squeeze” the O-Ring seals this gap to prevent system leakage

Diaphragm Gate – Used in symmetrical cavity filling to reduce weld-line formation and improve filling rates

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

Direct Gate – The sprue feeds directly into the mold cavity

Dispersion – Finely divided particles of a material in suspension in another substance

Distorted – A molded rubber part that is not in proper shape. It is usually a defect and caused by a part being partially dislodged from a cavity. The part then takes a set

Double Injection – A molding defect that is 100% avoidable by following proper molding procedure. It is identified by a flat raised injection that peals away by using your thumb nail. It is caused by not completely unloading a mold of its parts and another cure is run on top of the parts that were left in the mold from the previous cure

Draft – The degree of taper of a side wall or the angle of clearance designed to facilitate removal of parts from a mold

Drooling – The extrusion or leakage of molten resin from a nozzle or nozzle sprue bushing area while filling or shooting

Durometer – Standard rubber industry test instrument which uses an indenter to measure the hardness of molded rubber. While other scales are available, the hardness of rubber is most commonly reported by a durometer using the Shore A scale

Dwell – A pause in the application of pressure to a mold, made just before the mold is completely closed, allowing any gases formed or present to escape from the molding material

Dynamic Seal – A seal used in an environment that subjects it or a mating surface to movement

EC – Ethyl cellulose

Edge Feed – A method of feeding rubber into the mold cavity. In this case, the point of injection is at the edge of the part

Ejector Housing – A “U” shaped frame which is an integral part of the tool containing the ejector retainer plate and ejector plate

Ejector Pin – (knockout Pin) A pin or plate that is driven into a mold cavity from the rear as the mold opens to force out the finished part

Ejector Plate – The part of the mold which backs up the retainer plate and keeps the knockout pins from falling out

Ejector Retainer Plate – The part of the mold which locates and retains the knockout pins

Ejector Rod – A bar that actuates the ejector assembly when the mold opens

Elastic Limit – Maximum stress that a material will stand before permanent deformation occurs

Elasticity – A rubber’s ability to return to its original size and shape after removal of the stress 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 rubber

Elastomer – Any of various polymers having the elastic properties of rubber

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

EMA – Ethylene-methyl acrylate

EMI – Electromagnetic interference

Encapsulating – Enclosing an article (usually an electronic component) in a closed envelope of plastic, by immersing the object in a casting resin to polymerize or, if hot, to cool

EP – Ethylene propylene

EPDM – Ethylene propylene diene monomer rubber

ESCR – Environmental stress crack resistance

ETFE – Ethylene-tetrafluoroethylene

EVA – Ethylene-vinyl acetate

Extending Flash – Flash that is formed at the parting line of a mold

Extrusion – When part or all of a component is forced from its groove by high continuous or pulsating pressure

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

Family Mold – A multi-cavity mold where each of the cavities forms one of the component parts of the assembled finished part

Fan Gate – This gate is used to help reduce stress concentrations in the gate area by spreading the opening over a wider area. Less warpage of parts can usually be expected by the use of this gate type

FDA – Food & Drug Administration

Feather Edge – The sharp, thin edge on parts such as wiper seals. Also called a “knife edge”

Feed – The area or orifice where rubber is injected or transferred into a mold cavity

File Transfer Protocol (FTP) – A safe and secure way to send a file from one data port to another. By using this method of data transferring, it allows both the sender and the receiver to acknowledge the transfer. It is like a cyberspace handshake

Filler – A finely divided material used to reinforce or modify elastomer physical properties, impart certain processing properties, or reduce cost. Typical examples are carbon black, clays, calcium carbonates and silicas

Fillet – A rounded filling of the internal angle between two surfaces of a part

Fin – The web of material remaining in holes or openings in a molded part which must be removed for final assembly

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

Flash – Extraneous material protruding from the surface of a molded part. Flash is generally found on a molded part at the parting line locations

Flexural strength – The ability of a material to flex without permanent distortion or breaking

Flow – The ability of heated plastic or uncured rubber to travel in the mold and runner systems during the molding process

Flow Line – A disturbance in the homogeneous structure of a molded part generally caused when material knitted or blended with itself during the molding operation

Flow Marks – Wavy surface appearance of an object molded caused by improper flow of the plastic into the mold. A mark made by the freezing-in of a visible flow pattern

Fluid – A liquid or gas

Fr – Fiber reinforced

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 microfine grooves of surrounding glandular surfaces, requiring extra initial force to break them out of these microfine grooves

FRP – Fiber=reinforced plastics

Fused Disposition Modeling Process (FDM) – Thermoplastic modeling material is fed into the temperature-controlled FDM extrusion head and heated to a liquid state. The head extrudes and deposits the material in ultra-thin layers onto a fixtureless base

Gasket – A static (stationary) sealing device used to retain fluids under pressure or to seal out foreign matter

Gate – In injection and transfer molding, 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 Edge – A gate that is cut along one edge of the mold cavity

Gate Fan – Large cross sectional area for feeding plastic into the cavity. Flow channel tapered from the runner to the gate

Gate Flash – A gate that encompasses entire periphery of part. Usually of low sectional thickness in relation to part

Gate Mark – A raised spot or small depression on the surface of an injection or transfer molded part where the gates interface the cavity. See also Sprue Mark

Gate Pin-Point – A restricted orifice, usually used in a three plate-type mold, through which molten resin flows into a mold cavity

Gate Ring – The clearance between the force plug and the vertical wall of the cavity in a positive or semi-positive mold; also the ring of excess material which escapes from the cavity into this clearance space

Gate Sprue – The type of gate in which the plastic is injected directly from the molding press into the cavity

Gate Straight-in – The rectangular area between the runner and the part. This area is tapered on three sides so the smallest area is at the point of contact with the part

Gate Underside – The orifice found between the “A” and “B” plate which carries the resin into the cavity in an overlap of the part in such a manner so as to obtain a clean break on the part. Usually found in thermoset molding

Gates – The openings in an injection or transfer mold whereby material enters the mold cavity

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

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

Gland Depth – The gland depth (pictorially depicted and described as Dimension “C” throughout this book) is the distance from the sealing surface to the seal groove surface. The gland depth-determines how much the seal is compressed and therefore how much cross-sectional compression (squeeze) is applied to the seal. In a radial sealing application, the gland depth is used to calculate the seal groove diameter by either adding or subtracting (depending on the type of seal) twice its value from the diameter of the sealing surface. In an axial (face) sealing application, the gland depth equals the groove depth when it is a zero clearance application (two directly contacting surfaces, such as a cover) or, when clearance is present, it is the distance from the sealing surface to the seal groove surface (such as in a rotary face seal application). It should be noted that the Gland depth and Clearance is separate sealing system parameters and a change in one of these parameters does not result in a change to the other. Of example, if the clearance in a system needed to be increased, the required gland depth to achieve the desired seal compression would remain the same, but the seal groove diameter would need to be adjusted to achieve the desired gland depth. Care should also be taken to distinguish between the terms gland depth and groove depth, which are separate but related concepts. See the Application section of this publication for more information on calculations involving Gland Depth

GP – General Polymers

Groove – The machined glandular recess into which the 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 Depth, which are separate but related concepts

Hardness – A measurement of the resistance to penetration of a rubber sample by an indenter. High values indicate harder materials while low values indicate softer materials. (See also durometerIRHD, and Shore A

HDPE – High-density polyethylene

HDT – Heat-deflection test

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 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

HIP – High-impact polystyrene

HLMI – High-load melt index

HM – High-modulus

HMC – High-strength molding compound

HME – High-vinyl modified epoxy

HMW – High molecular weight

Hob – A master model in hardened steel. The hob is used to sink the shape of a mold into a soft metal block

Honing – A machining process that sharpens, enlarges, and smooths material through the use of a fine-grit stone

Hot Manifold – Thermoplastic molding method that directs the melted plastic internally within the mold base directly to the cavity thus eliminating the runner. Also known as “Runnerless molding”

Hot Rubber – See Hot Manifold

Hot Runner Mold – A mold in which the runners are insulated from the chilled cavities and are kept hot

Hot Stamping – Engraving operation for marking plastics in which roll leaf is stamped with heated metal dies onto the face of the plastics. by means of felt rolls, ink is applied to type or by means of heat and pressure, type is impressed into the material, leaving the marking compound in the indentation

Impact Resistance – Relative susceptibility of plastics to fracture by shock as indicated by the energy expended by a standard pendulum type impact machine in breaking a standard specimen in one blow

Injection – The cone shaped holes that taper down to the cavity area normally in the top plate of a transfer type mold. The stock is forced through these holes when the mold is under pressure. The rubber then fills the cavity area and forms a rubber part

Injection Molding – A molding method in which a rubber or plastic material is heated and forced under pressure into the mold cavity

Injection Pressure – The pressure on the face of the injecting ram when injecting material into the mold

Injection Ram – The ram or screw that applies pressure to the molten plastic material to force it into the mold cavities

Injection Tearout – A molding defect identified by a hole in the rubber part at varying widths and depths located at the point where the rubber is forced into the cavity area

Insert – A term referring to a metal or plastic component, placed (“inserted”) into a mold cavity prior to the start of the molding cycle, to which rubber or plastic is chemically and/or physically bonded during the molding process

Internally Lubricated Rubber – A rubber formulation containing lubricating materials. An internally lubricated rubber is designed to reduce the materials coefficient of friction

IRHD (International Rubber hardness Degrees) – A system of characterizing an elastomer by its resistance to penetration of a known geometry indenter by a known force. the microtechnique 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

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

Jetting – A turbulent flow in the resin melt caused by an undersized gate or where a thin section becomes thicker

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

Knit Line – A molding defect resembling a flow-line found at the point where the rubber meets between the injections. This defect is caused from excessive lube or oily stock

Knit Mark – A witness mark on a molded part, often occurring at the midpoint between two transfer or injection sprue locations. It is caused by the incomplete joining of the uncured rubber or plastic from each sprue during the molding process

Knock outs – Normally pins or blades that, when activated internally to the mold, eject the part from the mold (also called “ejector pins”)

Knockout Plate – A pin or thin plate that is driven into a mold cavity from the rear as the mold opens, forcing out the finished piece. Also called “knockout pin”

Knockout Rod – Bar that actuates the ejector assembly when mold is opened. Also called “knockout Bar”

Kotokast – Minnesota Rubber & Plastic’s prototyping system for rubber parts

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

Laminated Object Manufacturing (LOM) – A laser cuts the outline of a cross section, then a new layer of material is indexed, bonded to the previous layers, and cut. This process is repeated using very thin layers of material

Land – 1) The horizontal bearing surface of a semi-positive or flash mold by which excess material escapes; 2) the bearing surface along the top of the flights of a screw in a screw extruder; 3) the surface of an extrusion die parallel to the direction of melt flow; 4) length of the gate from the runner to the cavity

LCP – Liquid crystal polymer

LDPE – Low-density polyethylene

LIM – Liquid injection molding

LIM – “Liquid injection molding” is a closed manufacturing process using LSR in an injection molding machine

LLDPE – Linear low-density polyethylene

LMC – Low-pressure molding compound

LMW – Low molecular weight

Low temperature Flexibility – The ability of an elastomeric product to be flexed or bent at low temperatures without cracking

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

L/D Ratio – The term used to define an extrusion screw. This is the screw length to screw diameter ratio

MA – Maleic Anhydride

Machine Shot Capacity – Generally based on polystyrene. This is the maximum weight of plastic which can be displaced or injected by a single stroke

MBS – Methyacrylate-buadiene-styrene

MDPE – Medium-density polyethylene

Mechanical bond – A method of creating a molded part where the rubber is mechanically attached to an overmolded insert through the use of holes, depressions or projections on the insert

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

Memory – Ability of an elastomeric material to return to its original size and shape after deformation

Metering Screw – An extrusion screw that has a shallow constant depth and pitch section usually over the last three or four flights

Metric Units – Milli Centi Deci Meters Liters kilo Deka Grams Hecto

Microhardness – A measurement of rubber hardness for specimen’s below 0.25 inches (6.35mm) in thickness. Microhardness, like Shore A durometer, is also a measurement of indentation

Mismatch – Unequal O-ring cross-sectional radii caused by dimensional differences in the mold cavity

MMA – Methyl methacrylate monomer

Modulus – A measure of resistance of a material to deformation. It is measured by the force required to reach a predetermined compression or extension

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., obey Hooke’s Law throughout the test. The ratio of stress to strain within the perfectly elastic range

Moisture Marks or Welds – Marks on the finished part that are caused from moisture in the resin and look like weld lines except they may be in the wrong areas for weld lines

Mold – Typically made from steel. Product is formed within machined cavity

Mold Base – Refer to mold frame

Mold Cavity – Hollow space of the mold within which the uncured rubber compound is shaped and cured to the desired finished product form

Mold Finish – The surface finish of the mold which imparts the desired surface quality to the finished molded product

Mold Hot Runner – A mold in which the runners are insulated from the chilled cavities and are kept hot. Parting line is at gate of cavity and runners are in separate plate(s), so they are not ejected with he piece. Also called “insulated runner mold”

Mold Injection – A mold into which a plasticized material is introduced from an exterior heating cylinder

Mold Marks – Slight irregularities in the surface of molded articles caused by mold machining marks, or damage to the mold itself

Mold Pick Out Block – A mold in which a split cavity block is assembled in a chase to permit the forming of undercuts in a molded piece. These parts are ejected from the mold and then separated from the piece

Mold Release – A lubricant, often wax, used to assist in the removal of rubber products from the mold also called “release agent”

Mold Shrinkage – The difference in dimension, expressed in inches per inch, between a part and the cavity in which it was molded, when both are measured while at room temperature

Mold Three Plate – Integral moving plates in a conventional mold, frequently containing the cavities; allows the use of pin point gating and a shredder plate

MSDS – Material safety data sheet

Mud Set – A purchased universal mold base which usually can be finished at a significant labor savings

MW – Molecular weight

Mold – A mold which is open when the material is introduced and shapes the material by heat and by the pressure of closing

Natural – A natural product of the juices of certain tropical plants (latex), improved through heat treating with sulfur (vulcanization)

Necking – An informal (slang) term for seal cross-sectional reduction

Nest Plate – A retainer plate with a depressed area for cavity blocks

Nominal Dimension – The mean dimension of a molded article, from which small dimensional (plus and minus) deviations are allowed as manufacturing tolerances

Non-fill – An unintentional void or absence of material in the rubber structure of a part

Nozzle – The hollow cored metal nose screwed into the extrusion end of the heating cylinder of an injection machine or a 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 mold. The front end of nozzle may be either flat or spherical in shape

O-Ring – A doughnut-shaped object, or torus, that functions as a seal, blocking the passage of liquids or gases, by being compressed between the two mating surfaces comprising the walls of the cavity (gland) into which the ring is installed

Off register – Eccentric O-ring cross-sectional radii caused by lateral shift of one mold cavity relative to the other

Oil Resistant – Ability of vulcanized rubber to resist swelling and other detrimental effects of exposure to various oils

Orange Peel – A surface finish on a molded part that is rough and splotchy. Usually caused by moisture in the mold cavity

Oscillating Seal – Most commonly used in faucet valves, in this application the inner or outer member of the gland moves in an arc around the axis of a shaft. Movement is limited to a few turns in one direction and a few turns in the return direction (i.e. faucet on, faucet off)

Outgassing – Primarily occurring in vacuum situations, the volatile (evaporative) components of some rubber compounds may become vaporized in the vacuum and released (outgassed) by the compound into the surrounding environment

Overflow – A designed escape reservoir for rubber compound used to insure mold cavity fill and also allowance for excess compound

Overflow Groove – A groove around the periphery of a mold cavity used to accept any excess material from the cavity during molding. 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

Oxidation – The reaction of oxygen with a rubber compound, typically 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 oxidizing agent

O.D. (Outer 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

PA – Polyamide (nylon)

Packing – The filling of the mold cavity or cavities as full as possible without causing undue stress on the molds or causing flash to appear on the finished parts

PAI – Polyamide-imide

PAN – Polyacrylonitrile

Parting line – The witness line on the surface of a molded part corresponding to the location where the mold plates were in contact

PB – Polybutylene

PBT – Polybutylene terephthalate

PBTP – Polybutylene terephthalate

PC – Polycarbonate

PCTFE – Polychlorotrifluoroethylene

PE – Polyethylene

PEC – Polyphenylene ether copolymer

PEEK – Polyetherether ketone

PEH – Polyphenylene ether homopolymer

PEI – Polyetherimide

PEO – Polyethylene oxide

Permanent Set – The deformation remaining in a rubber specimen following both stress and relaxation over a period of time

Permeation – The diffusion of a medium (generally a gas) through a rubber or plastic material

PES – Polyethersulfone

PET – Polyethylene terephthalate

PETG – Glycol-modified PET

PETP – Polyethylene terephthalate

PF – Phenyl-formaldehyde

PFA – Perfluoroalkoxy (resin)

PI – Polyimide

PIB – Polyisobutylene

Pinch Off – A raised edge around the mold cavity that seals off the part and separates excess material as the mold closes around the parison in the flow-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

PLAS-TEC – Minnesota Rubber & Plastic’s prototyping system for thermoplastic parts

Plasticize – To soften a material and make it plastic or moldable, either by means of a plasticizer or the application of heat

Plasticizer – A chemical agent added to the rubber compound batch mix to soften the elastomer for processing, as well as to improve physical properties of the compound product (i.e., increase elongation, reduce harness, improve tack)

Plate Retainer – The plate on which demountable pieces, such as mold cavities, ejector pins, guide pins, and bushings are mounted during molding

Plate Stripper – A plate that strips a molded piece from core pins or force plugs. The stripper-plate is set into operation by the opening of the mold

Platen Movable – The large back platen of an injection molding machine to which the back half of the mold is secured during operation. This platen is moved either by a hydraulic ram or a toggle mechanism. May be moving platen of a vertical press, to which one half of mold is secured

Platen Stationary – The large front plate of an injection molding machine to which the front plate of the mold is secured during operation. This platen does not move during normal operation. Also can be fixed platen of vertical press

Platens – The mounting plates of a press to which the entire mold assembly is bolted

Plunger – The ram which applies pressure in the process of injection or transfer molding, forcing the material into the mold cavities. Also called an injection ram

PMMA – Polymethyl methacrylate

PMS – Paramethylstyrene

PMT – Polymethylpentene

Polymer – A long molecular chain material formed by the chemical combination of many similarly structured, small molecular units

Post-curing – The process of baking or autoclaving parts after molding. This process is used to improve the heat and compression set resistance of specific elastomers (e.g. silicones and fluorocarbons)

Pot – The chamber in a transfer or injection mold where raw material is placed before it is transferred into the cavity

PP – Polypropylene

PPO – Polyphenylene oxide

PPS – Polyphenylene sulfide

Preload – The term used for the weighed up stock that is placed in the mold for making a cure

Pressure Injection – The pressure on the face of the injection ram at which molding material is injected into a mold. It is usually expressed as P.S.I.

Primary Sealing Interface – See Primary Sealing Surface

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

PS – Polystyrene

PTFE – Polytetrafluoroethylene

PU – Polyurethane

Purging – Cleaning one color or type of material from the cylinder of an injection molding machine by forcing it out with the new color of material to be used in subsequent production. Purging materials are also available

PVC – Polyvinyl chloride

PVDF – Polyvinylidene Fluoride

PVF – Polyvinyl Fluoride

Plate – A retainer plate with a depressed area for cavity blocks used in injection molding

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

Quench Bath – The cooling medium used to quench molter thermoplastic materials to the solid state

Quick Cast – An SLA build style that produces a thin shell with a honeycombed interior. This method is used mostly for investment castings when the SLA model will be used as a sacrificial mold

Radial Seal – Compression is applied perpendicular to the seal center-line

Ram – The portion of an injection machine that moves backward and forward to deliver the shot of resin to the nozzle and mold. the screw in a reciprocating screw machine

Ram travel – The distance the injection ram (or screw) moves in filling the mold

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.

Reciprocating Seal – Seals used in moving piston and rod situations

Regrind – A process that finely grinds the rejects and flash pad, which would normally be thrown away as waste, by use of what’s called a regrind mill. After this breakdown process, it is added to 30% of the same but virgin compound. This process is common with some silicone and viton compounds. The combination retains the same desirable properties but at a cost savings

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

Resilience – The capability of returning to original size and shape after deformation

RF – Radio-Frequency

RH – Rockwell hardness

RH – Relative Humidity

RIM – Reaction injection molding

RMS – “Root Mean Square”. A measure of surface roughness typically applied to the machining of metal gland and shaft surfaces. RMS stands for the square root of the sum of the squares of micro-inch 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 – Seals for rotating shafts, with the turning shaft protruding through the I.D. (hole) of the O-ring

RP – Reinforced plastics

RTM – Resin-transfer molding

RTV Molding – A procedure where a part is molded in a silicone surrounding to reproduce up to twenty urethane parts

Rubber – A common name for both naturally occurring and synthetically made elastomers

Runner – The channel that connects the sprue with the gate for transferring the molten plastic to the cavities

Running Friction – A force which resists objects already in motion

Runout (shaft) – Same as Gyration. When expressed in inches along, or accompanied by abbreviation “TIR” (total indicator reading) it refers to twice the radical distance between shaft axis and axis of rotation

Synthetic – Man-made elastomers such as Nitrile, Fluorocarbon, Silicone, etc.

SAN – Styrene-acrylonitrile

SBR – Styrene-butadiene (rubber)

Scorching – Premature curing of compounded rubber stock during processing or storage, with the potential for adversely affecting material flow and plasticity during subsequent shaping and curing processes

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

Seal – Any device used to prevent the passage of a fluid (gas or liquid) or fine particles

Sealing System – The components and attributes which compose the sealing environment, including the seal, the components being sealed, the medium or media being sealed, and the environmental conditions such as temperature, pressure, and motion

Selective Laser Sintering (SLS) – A laser traces the shape of the part to be modeled in a thin layer of powder. The laser sinters (softens and bonds) the powder particles together. This process is repeated over layers of powder

Separation Blister – A molding defect. It, unlike a normal blister, is one that appears to be vein like and takes on different shapes and sizes. It is generally believed to be caused from wet stock or stock containing too much moisture

Shaft – A load-bearing and/or power-transmitting member o fa mechanical system which is generally cylindrical in shape and frequently rotates or reciprocates

Shore A – A hardness scale used to measure the hardness of molded rubber. The Shore A scale is most effectively used to measure rubber with a hardness from 10 to 95 Shore A. For materials harder than 90 to 95 Shore A, the Shore D scale is recommended (see also durometer)

Shore M – A durometer hardness instrument using a microindicator, designed for the purpose of measuring o-ring hardness

Short shot – A condition where there is insufficient material introduced into a mold cavity to completely fill the cavity, resulting in a partially formed part

Shot – The complete resin from a molding cycle, including scrap

Shot Capacity – The maximum weight of material which an injection machine can inject with one forward motion of the ram

Shrink Fixture – (cooling fixture) A jig or block to hold the shape of a molded part after it is removed from the mold until it is cool enough to retain its shape

Shrinkage – The linear contraction upon cooling of a molded rubber or thermoplastic part

Single Cavity Mold – A mold having only one cavity and producing only one finished part per cycle

Sink Marks – A shallow depression or dimple on the surface of a finished part due to shrinkage or low fill of the cavity

Sliding Core – A pin on a mold that automatically retracts when the mold opens

SMA – Styrene maleic anhydride

SMC – Sheet molding compounds

SP – Softening point

SPC – (statistical Process Control) Methods of controlling our processes to prevent defects from being made

SPE – Society of Plastics Engineers

Specific Gravity – The ratio of the mass o f a unit volume of a material to that of the same volume of water at a specified temperature

Sphericity – Term formerly used to denote Circularity

Spin Welding – A process of fusing two objects together by forcing them together while one of the pair is spinning until frictional heat melts the interface. Spinning is then stopped and pressure held until they are frozen together

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

Splay Marks – Marks or droplet-type imperfections found on the surface of the finished parts that may be caused by the spraying of the hot melt through the gates and into the cool cavity where they set up

Splits – A common molding defect. It is found when the rubber part is stretched and the part will tear normally at the knit point. Excessive lube or sometimes too hot a cure temperature is the cause

Sprue – The primary feed channel that runs from the outer face of an injection or transfer mold to the mold gate in a single cavity mold or to runners in a multiple-cavity mold

Sprue Bushing – A hardened steel insert in the mold that accepts the extrusion nozzle and provides an opening for transferring the molten plastic

Sprue Gate – A passageway through which resin flows from the nozzle to the mold cavity

Sprue Lock – The portion of resin retained in the cold slug well by an undercut; used to pull the sprue out of the bushing as the mold opens. The sprue lock itself is pushed out of the mold by an ejector pin

Sprue Mark – A small raised spot or depression left on the surface of an injection or transfer molded part. The sprues are the locations at which the elastomer enters into the mold cavity. Also called “gate mark”

Sprue Puller – In injection molding, a portion of the plastic composition which is held in the cold slug well by an undercut; used to pull the sprue out of the bushing as the mold is opened. The puller itself is pushed out of the mold by an ejector pin. When the undercut occurs on the cavity block retainer plate, this pin is called the “sprue ejector pin”

Squeeze – An informal (slang) term for the deformation placed on a rubber part to affect a seal. Although it is usually expressed as a percentage of a seal’s original (undeformed) cross-section, it is also occasionally expressed as an absolute value of the deformation. See also Cross-sectional Compression, Percent

Stack Up Tolerance – The summation of sealing system tolerance’s

Staking – The insertion of a cylindrical metallic insert into a cavity formed in a part. May be inserted with heat

Static Seal – A seal that, except for pulsations caused by cycle pressure, does not move in its environment

Statistical Process Control (SPC) – The use of statistical techniques on processes and their output to establish process stability and increase capabilities

Stereolithography Apparatus (SLA) – Selective curing of a photopolymer resin by an ultraviolet laser, built on a descending platform, in a vat

Strain – The deformation caused by an applied stress

Stress – Force per original cross section that is applied to a specimen

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

Stripper Plate – A plate that strips a molded piece from core pins or force plugs. The stripper plate is set into operation by the opening of the mold

Submarine Gate – (tunnel gate) A type of edge gate where the opening from the runner into the mold is located below the mold parting line

Suck Back – When the pressure on the sprue is not held long enough for the resin to cool before the screw returns, some of the hot resin in the cavities or runner system may expand back into the nozzle and cause sink marks on the finished part

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 mold plates and components to a specified dimension and to provide a high accuracy and smooth surface finish

Swell – The linear or volumetric change of a material resulting from immersion in a particular liquid for a specified period of time. Swell is a general indicator of the compatibility of a material for use in a particular environment

Tab Gate – A small removable tab about the same thickness as the molded item but usually perpendicular to it for easy removal

Tack – The degree of adhesion of materials of identical nature to each other

Tear Resistance – Resistance to the growth of a cut in the seal when tension is applied

Tear Strength – The force required to rupture a sample of stated geometry

Tear Trim – A designed in feature on a mold which allows easy and precise tear off of overflow bead

Temperature Range – The working range marked by the limits of minimum and maximum operating temperatures for effective seal performance

Tensile Strength – The extension force per cross-sectional area required to fracture a material specimen

Terpolymer – A polymer resulting from the chemical combination of three monomers

TFE – Polytetrafluoroethylene

Thermal Expansion – Linear or volumetric expansion caused by temperature increases

Thermoplastic – A material which when thermally processes undergoes a reversible phase change to become plastic and capable of being molded to a desired shape. Upon cooling, the material reverts to its original properties

Thermoplastic Elastomer (TPE) – A material which combines the processing characteristics of a plastic but displays rubber-like properties upon completion of processing

Thermoset – A material, either an elastomer or plastic, which when thermally processed undergoes an irreversible chemical reaction to achieve its final material state

Threaded Core – A part of a mold that shapes an internal thread and must be unscrewed from the finished piece

TIR – A measurement of roundness with relationship to a center-line and expressed in total diametric deviation

TMC – Thick molding compound

Toggle Action – A mechanism which exerts pressure developed by the application of force on a knee joint. It is used as a method of closing presses and also serves to apply pressure at the same time

Torque – 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 center-line, should be interpreted

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

TPE – “Thermoplastic Elastomer” combines the rubber-like performance of elastomers with the processing advantages of plastic. Scrap material can be recycled without significant loss in physical properties, unlike thermoset materials

TPE – Thermoplastic elastomers

TPR – Thermoplastic rubber

TPU – Thermoplastic polyurethane

TPX – Polymethylpentene

Transfer Molding – A method of molding in which material is placed in a pot located between the top plate and plunger and squeezed from the pot through sprues into the mold cavity

Trim – Removal of excess material (flash) from a molded rubber article

Tunnel Gate – (submarine gate) A type of edge gate where the opening from the runner into the mold is located below the mold parting line

Two Shot Molding – A means of producing two-color parts in thermoplastic materials by successive molding operations. May also be used to produce parts of two different thermoplastic materials

UHM – Ultra-high-modulus

UHMW – Ultra-high molecular weight

UL – Underwriters’ Laboratories

Ultimate Elongation – The % of specimen stretching at the point of breaking. Generally referred to in tensile testing

Ultimate Elongation – Expressed as a percentage of its original length, a measure of how far a material will stretch before breaking

Undercut – A feature on a part which has a corresponding feature in the part’s mold which is perpendicular to the direction of mold movement. An undercut complicates the manufacture of a molded part by making it difficult to remove the finished part from the mold

UV – Ultraviolet

VAE – Vinyl Acetate-ethylene

Valve Gating – This type of gate is where a pin is held in the gate or channel by spring tension and as the plunger comes forward, it compresses the plastic in the runner. When this pressure build-up is sufficient to overcome the spring tension, the pin is then pushed back (pulled) and the fast decompression of the plastic fills the cavity at extremely high speed

Vent – A shallow channel or opening cut in the cavity to allow air or gases to escape as the material fills the cavity

Viscosity – The measurement of the resistance of a material to flow under stress

Volume Change – The measure of the swell or shrinkage of a material resulting from immersion in a particular media for a specified period of time at a specified temperature

Volume Swell – A term generally used to describe the increase in physical size of a specimen immersed in a particular chemical agent

Vulcanization – The thermally initiated, irreversible process whereby polymer chains are cross-linked to form the final physical and chemical state of a rubber

Warpage – Dimensional distortion in a molded object

Weathering – The detrimental effect upon an elastomer or plastic after outdoor exposure

Weld Lines – (knit lines) The marks visible on a finished part made by the meeting of two flow fonts during molding

Width – The cross-sectional diameter of an O-ring. one half the distance between the I.D. and O. D. of the ring

ZST – Zero strength time. A test for molecular weight

Need help solving a tough engineering problem?