Part 3

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 3 of Sealing Terminology - the terms you may come across when looking for fluid sealing products.

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