For the sealing expert, the world of sealing terminology comes naturally.
But for those who don't live and breathe seals every day, the list of terms can quickly become quite daunting.
Thanks to our partners at Minnesota Rubber, this blog post will examine Part 2 of Sealing Terminology - the terms you may come across when looking for fluid sealing products.
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 durometer, IRHD, 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’s protopying 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’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
Pressure – 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