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Dictionary for Dummies
Dictionary for Dummies
Plastics engineers often wince when technical terms are explained by, or for, those who aren't technically educated. But many people working in marketing, administration or product specification only need simple, or even simplistic, explanations of what's what in plastics. Here is a selection of common terms explained in everyday English.
Accumulator head. A device used on blow moulding machines to collect molten
plastic to form a parison large enough to blow parts such as drums or fuel
tanks.
Air-ring. A circular component of a blown film system that directs cool air
up the sides of a tubular blown film bubble to cool and solidify the melt.
Aircraft aluminum. Tough grade of aluminum alloy for moulds.
Alloy. Plastic made by mechanically blending two or more different polymers.
Amorphous. A polymer is said to be amorphous when its molecules are in long
random or coiled chains. Amorphous plastics tend to be transparent, and are
more soluble than crystalline plastics (qv).
Antimicrobial agent. Additive used to prevent growth of bacteria or fungi,
used especially in plastic parts for construction or medical applications.
Antioxidant. Additive that prevents oxygen from causing molecular breakdown
in polymers.
Atactic. Refers to polymers where groups of molecules are arranged randomly
along the backbone of the molecular chain.
Autoclave. Metal heating vessel that can cure thermoset materials, bond
thermoplastics and otherwise treat parts with heat and/or pressure.
Auxiliary equipment. The types of equipment used to enhance, support or add
to the precision and efficiency of injection, extrusion or other primary
processing machinery.
Back pressure. The resistance of plastic to flow when the mould is closed in
injection moulding.
Barrel. A metal tube that houses a rotating screw, or sometimes a ram, in
which plastic is heated before extrusion or moulding.
Barrier layer. A layer of material in film, sheet or a blow moulded
container that prevents the passage of moisture, flavors or certain gases.
Bead, expanded polystyrene. Polystyrene resin for foam production, supplied
as small granules with blowing agent added.
Beryllium copper. Alloy used in moulds or areas of moulds where high
conductivity is needed. Used especially in blow moulding
Bloom. A patchy coating of additives that has migrated to the surface of a
plastic part.
Blow moulding. The process of producing a hollow part by forcing air into a
soft parison inside a mould (extrusion blow moulding), or of forcing air
into a heated pre-form that has been made beforehand by injection moulding
(injection blow moulding).
Blow pin. A metal pin that injects air into a parison in the extrusion blow
moulding process.
Blowing agent. A chemical used to turn molten plastic into foam.
Blown film. Film made by extruding molten plastic through a circular die,
and forming an inflated, tubular bubble that moves through a cage as it
cools, to be sliced open, collapsed and formed into rolls.
Blush. An effect of surface irregularity or turbulence seen round the gate
area of a moulded part.
Borescoping. In extrusion, the process of aligning the centre of a barrel
discharge with the centre of rotation of the gear reducer.
Cage, blown film. A structure, usually with rollers or roller beads, through
which a bubble of blown film passes at it is extruded and cooled.
Calendering. The making of sheet or film with high-quality surface
characteristics by passing molten plastic from a kneader over a series of
rollers that flatten and/or emboss it.
Cartridge heater. Cylindrical electrical heater used to heat nozzles, hot
runners and moulds in injection and compression moulding.
Cast film. A film process, used where good cosmetics are needed, in which
molten plastic flows over a chilled roll or through a quench bath.
Cavity. 1. the hollow space of a mould; 2. The concave section of a mould,
as opposed to the core (qv).
Cellulosic. One of a family of polymers based on cellulose, but including
other monomers.
Clamp, clamping unit. The mechanism on an injection moulding machine that
holds the mould, and which closes the mould halves and holds them in place
during moulding.
Clamp tonnage. The amount of pressure an injection or blow moulding machine
can apply to close a mould and hold it shut during moulding.
Coextrusion. A process whereby two or more plastics are extruded through one
die, to produce a material combining their properties.
Coinjection. An injection moulding process whereby two or more plastics are
injected into a mould, to fill different areas or different layers of the part.
Collapsing frame. The device on which a blown film bubble is slit open and
brought to the form of flat film.
Commodity plastics. Inexpensive plastics (usually under $1 per pound), such
as polyethylene, polypropylene and polystyrene, used for packaging and
disposable items, as well as for many durable goods.
Composite. A material made of a polymeric matrix with a fibrous
reinforcement that gives greater strength than the plastic has normally.
Compound. Plastic that has had colourants, additives or reinforcements added
in a compounding extruder or mixer.
Compounding extruder. Extruder, often with intermeshing or counter-rotating
screws, designed specifically to combine one or more polymers with additives
and colourants.
Compression moulding. Process in which moulding compound, usually thermoset,
is pre-heated and placed in a mould in which it is melted through
application of pressure.
Copolymer. A polymer produced by reacting two different monomers together to
form long chains.
Core. The convex part of a mould; in moulding a cup, the core would form the
cup's inner surface.
Crosslinking. The production of sideways links between the molecular chains
of a polymer, to produce a more rigid material.
Crystallinity. Polymers are said to be crystalline when their molecules are
in a regular, repeated lattice arrangement that makes them relatively dense;
non-transparent (because the crystals scatter light); resistant to solvents
or to chemical attack; and having a very sharp melting point. No polymer is
totally crystalline or amorphous (qv), but all tend to be one more than the
other.
Cure. To set, or become solid, either by cooling or by a chemical reaction.
Curing agent. A chemical that produces crosslinking in a plastic, making it
set.
Daylight. The total possible space between the stationary and moving mould
platens of an injection moulding machine.
Dart impact strength. A measure of a plastic's resilience, especially
film's, measured with a special dart or steel ball dropped on it.
Delamination. 1. Splitting of a moulded or formed part into irregular
layers; 2. Separation of laminated layers of sheet or film.
Die. In extrusion, a polished metal shape through which molten plastic is
continuously extruded and formed. Blown film dies are circular, and profile
dies form the plastic into the profile shape, whether that is a tube, rigid
window-frame profile, or another product.
Die-lip. In blown film, the edge of the circular die.
Dielectric strength. A measure of a plastic's ability to resist breakdown
under applied voltage stress.
Downstream equipment. In extrusion, winders, haul-offs, saws, vacuum sizing
tanks and other equipment that trims, cuts and finishes extruded film or
shapes.
Dryer. A device to absorb and/or drive off moisture from resin powder or
pellets that, if heated while still containing traces of moisture, would
make defective parts.
Ejector pin. A flat-ended rod or projection that pushes a part out of a
mould when it is formed.
Elastomer. A plastic with some of the elastic, flexible properties of
natural rubber.
Electrostatic discharge machining (EDM). A process using a spark emitted
from an electrode wire in liquid, which has the effect of eroding metal from
a mould surface; an alternative to drilling or cutting.
Engineering/engineered polymers. A broad term for plastics with higher
chemical and heat resistance than the commodity plastics that are used in
most packaging, housewares and everyday products.
Epoxy. Family of durable thermoset plastics with excellent electrical
properties and good weatherability.
Extruder. A machine that melts plastic powder, pellets or flake, possibly
with colourants and additives added. The basic machine consists of a barrel,
heater units around it, a drive system, and a fluted screw inside to move
and pressurise the plastic from the throat to the extruder die.
Extrusion. The process of melting plastic pellets, flake or powder by means
of heat and pressure, then forcing the melt through a die to produce film or
a three-dimensional profile shape.
Feedstock. Chemical source from which monomers for polymers are derived.
Feedthroat. The point at which resin or additives are added into an extruder
or injection unit.
Filler. Material such as calcium carbonate or wood flour used with plastics
to reduce costs and to impart additional mechanical properties.
Finite element analysis. Computerised method of analysing properties,
stresses and deflections of a moulded or manufactured part or mould. It
manipulates predictive equations to identify potential points of stress and
failure.
Fish-eye. An unmelted lump of plastic in a melt, especially visible in clear
films.
Flash. A thin web of plastic squeezed through the part-line where two mould
halves join, which appears as a protruding blemish on the side of a moulded
part.
Flexural modulus. A measure of a plastic's rigidity, measured as the ratio
of stress to strain within the material's proportional limit under bending
load conditions.
Frost line. The point in a blown film bubble at which the film tube reaches
its maximum diameter, and acquires a frosted appearance as the plastic falls
below its softening temperature.
Gate. The point where molten plastic enters an injection mould from an
injection unit or a hot runner system.
Gas-assisted injection moulding. The use of inert gas to fill out thick
sections of injection moulded parts, reducing weight, warpage and
sink-marks.
Gear-pump. A system using counter-rotating gears to move melt from an
extruder barrel through to a die. Its benefits include reducing fluctuations
in melt flow.
Geomembrane. Thick extruded film or sheet used as pond liner, liner for
irrigation canals, waterproofing in landfills, etc.
Glass transition temperature. The temperature below which a given plastic
behaves like glass, being strong but brittle.
Granulator. Equipment to grind scrap plastics to a size where they can be
reprocessed.
Hand lay-up. Process for reinforced plastics in which layers of
reinforcement and thermoset plastic are applied by hand, the whole then
being cured to the formed shape.
Heat history. The amount of time a plastic has been heated in processing
equipment. Such heating affects the properties and molecular arrangement of
polymers, and each successive heat history adds to these effects.
Heater band. A band, normally of ceramic, mica or metal, that is heated, and
transfers its heat to the barrel of an extruder.
Hobbing. Method of producing mould cavities by driving a hardened steel
master (hob) into mild steel or beryllium copper.
Homopolymer. Polymer produced by joining identical monomers or by joining
two chemically different versions of the same monomer.
Hot runner. A device to convey melted, pressurised plastic to the cavity or
cavities of an injection mould in a balanced fashion, at steady, even
temperature, without the waste of forming a sprue.
Hot stamping. Decorating process for applying words or images onto plastics
by applying heat to a coated foil material.
Hybrid press. A moulding machine combining servo-electric and hydraulic or
hydromechanical drives.
Hydrophilic. Tending to absorb water.
Hydrophobic. Tending to absorb little or no water.
Hygroscopic. Tending to absorb atmospheric moisture.
Impact strength. One measure of a material's strength when dropped or hit,
usually determined by the notched Izod impact test.
Induction welding. Use of an electrical current to melt metal parts or
powder, and to cause these to transfer their heat to adjacent plastic, in
order to create a bond.
Injection blow moulding. A process whereby pre-formed plastic shapes are
heated and blown into containers or similar shapes.
Injection moulding. A process whereby plastic is heated and injected under
pressure into a tightly clamped mould, so that in cooling it takes on the
shape of the mould around it.
Injection unit. Essentially, an extruder system on an injection moulding
machine that melts the plastic before it is injected.
Insert moulding. Moulding of plastic around a pre-moulded or machined part,
such as a screwdriver blade.
Internal bubble cooling. Injection of cold air into a tubular bubble of
blown film, so that it cools and solidifies.
Ion nitriding. See Nitriding.
Isotactic. Refers to polymer structures containing asymmetrical but
regularly spaced atoms in a repeated configuration, in either a
head-to-tail, or tail-to-head pattern.
Ketone plastics. Family of engineering polymers using ketone monomers,
including polyketone and polyetherketone, and distinguished by strength and
very high heat resistance.
Kirksite. Zinc-based alloy for making prototype or short-run moulds.
Laminate. Product made by bonding two materials or two layers of one
material.
Laser sintering. A prototyping process that cures powdered resin in layers,
using a laser beam, to build up a part.
L/D ratio. The length-to-diameter ratio of a screw, which affects how it
melts a given plastic.
Liquid crystal polymers. Family of self-reinforcing plastics with densely
packed polymer chains. They have excellent flow characteristics and offer
strength at high temperatures.
Living hinge. A thin hinge that is moulded so as to use the alignment of
polymers, particularly those of polypropylene, across the bend for strength
and durability.
Lumber, plastic. Extruded and moulded plastic, usually shaped to resemble
timber, containing mixed recycled plastics, and sometimes bottle caps,
closures and labels, possibly made with an admixture of virgin resin.
Normally, polyethylene forms a matrix that contains the other waste
material.
Manifold, hot runner. The part of a hot runner system that divides and
branches the flow of molten plastic, so that it is evenly distributed to the
different cavities of an injection mould.
Melt channel. The channel or tunnel in a mould or hot runner that carries
molten plastic to the mould cavity.
Melt. Term to describe molten plastic.
Melt flow. The manner and speed with which molten plastic moves under
pressure through a hot runner or nozzle, and into a mould.
Melt strength. A plastic's strength while molten, particularly as it applies
to a parison or the material at the base of a blown film bubble.
Melt zone. The main section of a screw, which handles plasticising by heat
and pressure.
Metallocene catalyst. A category of Ziegler-Natta catalysts for
polymerising polyolefins. The structure of its own molecules orients the
olefin monomers into polyolefin polymers with tightly repeatable shapes and
properties.
Metering zone. The relatively shallow discharge end of a screw.
Mica. Natural, plate-like silicate mineral used as a reinforcement or
mineral filler.
Modular die. Film or sheet extrusion die that has modules which can be added
or removed to change the number of layers of material and/or the product
configuration.
Mould. A hollow shape of metal, epoxy or sometimes rubber or polyurethane,
used for forming plastic parts by means of pressure and then by cooling from
the initial molten state.
Mould cell. An arrangement of a moulding machine with auxiliary equipment
electronically linked to it for automated manufacturing.
Mould flow. The behaviour of molten plastic as it moves into and fills a
mould, and also the study of this process.
Monomers. Single molecules, in gaseous or liquid form, that can be joined in
chains to form polymers, the building blocks of plastics.
Net shape. Term used in computerized design of moulds and parts, referring
to digitally defined components of their shape and surface.
Nickel Vapour Deposition. Very precise gas process for producing moulds by
building up nickel deposits on a pre-formed mandrel.
Nip rollers. A pair of rolls at the top of a blown film tower that close the
film bubble, and also, by their action, regulate the rate at which molten
plastic is pulled from the extrusion die.
Nitriding. Hardening process for steel screws and barrels, also referred to
as ion nitriding.
Nonwoven fabric. Fabric made by blowing fibres against a screen or another
mechanical production process that does not involve regular weaving.
Nozzle. The last section of an injection system, through which the plastic
is injected into the mould. It is sometimes heated or cooled to achieve
specific effects in the material.
Nylon. Term for the family of polyamide polymers, first synthesized by
DuPont scientists in 1938. These crystalline plastics are tough and offer
good heat and chemical resistance.
Oscillating nip rollers. Collapsing system on top of a blown film tower that
rotates back and forth to randomise, or distribute, gauge variations in the
film being blown.
Orientation. The direction in which polymer chains lie, or are made to flow,
in plastic film, sheet or parts.
Pack out. To fill an injection mould in its entirety.
Parison. An extruded length of molten plastic that is enclosed in a mould
and has air injected in the blow moulding process.
Part-line. 1. The point where two mould halves meet; 2. The line that forms
on a moulded part where the two halves of a mould join together.
Photopolymer. A plastic that cures in response to ultra-violet or another
form of light.
Plastic. A material of organic origin, of high molecular weight, that can be
shaped by being made to flow through the application of heat and/or
pressure, and that will set solid after the forming process. The word
derives from a Greek root, plassein, which means to mould.
Plasticiser. A chemical additive that makes a plastic material softer and
more pliable.
Plastisol. A liquid plastic material, in the form of a fine dispersion of
particles of polymer that fuses on heating.
Plastomer: A soft, low-density thermoplastic similar to an elastomer.
Plate-out. The appearance on the surface of a plastic part of a coating of
pigment, lubricant or other additive.
Platen. A metal plate, often drilled for the passage of air, water or oil,
onto which one half of an injection mould is mounted.
Poly-. Prefix denoting a substance made up of chains of organic molecules.
It is used for naturally occurring substances (e.g., polypeptides) or
synthetic ones (e.g., polyethylene).
Polyester. A thermoset or thermoplastic material, made by polymerising any
of a range of ester monomers, usually with properties of toughness,
stiffness and/or a high melting point.
Polymer. Compound forming a chain of chemically linked monomers, or single
molecules; a chemist's description for a basic plastic.
Polymerise. To join monomers into a chain, or polymer, in a reactor.
Polyolefin. A crystalline or semi-crystalline plastic derived from
petrochemical feedstock.
Polyurethane. One of a family of thermoset plastics made by conjoining a
polyol and polyester-based materials.
Pre-form. An injection-moulded hollow shape that can be made into a
container or larger hollow part by the injection blow or injection
stretch-blow process.
Pre-preg. A reinforcement, such as glass fibre, which is combined with a
thermoset liquid plastic that can be stored until needed for reinforced
plastics moulding, then moulded by means of heat and pressure.
Press. A moulding machine, usually for injection or compression moulding.
Pressure forming. A forming process whereby sheet is pressed against a
heated mould surface until it takes on the mould's contours. Part of the
thermoforming/vacuum forming family of processes.
Primary processing equipment. Machinery that actually moulds, extrudes or
initially forms plastic parts and products.
Processor. Manufacturer who takes plastics raw materials, or semi-finished
materials, and moulds, extrudes or otherwise forms finished parts from them.
Pultrusion. A process whereby reinforcing fibre, such as glass roving
(strand) is drawn through liquid plastic, then formed by passing through a
die, to produce strong, rigid parts such as ski poles, industrial decking,
girder beams, etc.
Radius. The degree of curvature at the angle or corner of a moulded part.
Reaction injection moulding. Injection moulding using plastics such as
polyurea or polyurethane, that cure when two components are combined inside
a mould.
Recovery rate. The amount of plastic that the screw in an injection unit can
discharge in a set amount of time, when operating at 50 per cent of
capacity.
Reinforcement. A material such as glass fibre or bead, ceramic or polymeric
fibre, used to increase the rigidity and strength of plastics.
Reprocessing. Recycling of plastics back into processible material, usually
through pulverising or re-extrusion.
Resin. Term for plastics raw materials in powder, pellet or flake form.
Rheology. The scientific study of the deformation and flow of matter,
particularly as it moves through a processing line.
Rotational moulding. Process in which powdered resin is heated inside a
rotating mould, so that it coats the inside and takes on the required shape.
Screen. A mesh plate that eliminates impurities from a melt stream.
Screw. The rotating, flighted part of an extruder or injection unit, which,
in turning rapidly helps to melt, and convey, the plastic from the hopper
throat to the die or nozzle.
Shear. An effect whereby parts of a melt separate and move against each
other. This is desired and essential in the heating stage, but excessive
shear produces breakdown of a plastic's properties.
Sheet moulding compound. Sheet form of reinforced thermoset plastic
material, usually polyester reinforced with randomly oriented glass fibres.
Shooting pot. A chamber, used with an additional extruder on a two-stage
injection moulding machine, where melt for a large part is accumulated
before being shot into the mould.
Short shot. An injection moulding cycle where insufficient plastic is
injected into the mould, resulting in a distorted part.
Shot. The amount of plastic used in one injection moulding cycle.
Shrinkage. The tendency of parts, especially thermoplastics, to shrink as
they cool after moulding, and which must be allowed for in mould design.
Silicone. Polymeric material related to conventional plastics, but with
silicon-based molecules forming a crucial part of its chemical structure.
Sink-mark. Depression that forms in the surface of a moulded part,
particularly over a rib or thick section, as the plastic cools and shrinks.
Spider. Assembly of supporting pins that holds a shape such as a torpedo in
a profile extrusion die.
Spin welding. Process for rotating circular plastic components against each
other at high speed, to form a friction bond.
Spiral die. A die used for blown film that brings the molten plastic through
spiral channels from an extruder to the die-lip, ensuring a homogeneous melt
when the melt emerges to form the bubble.
Splay. A fan-shaped surface blemish near the gate-mark of an injection
moulded part.
Sprue. 1. Tapered channel through which plastic travels from an injection
moulding machine nozzle to the main mould cavity; 2. The plastic that forms
and sets in this channel.
Stabiliser, heat. A chemical additive that prevents deterioration of plastic
during heating.
Stabiliser, light. A chemical additive that helps screen plastic from the
deleterious effects of ultra-violet light.
Stack mould. A concertina-like, two-level injection mould, used where high
volume is needed.
Stereolithography. A process for making prototype parts by subjecting a
light-sensitive liquid polymer to a tightly controlled laser beam in a
closed chamber, thus curing the polymer in segments until a part is formed.
Stretch blow moulding. A process in which a pin stretches a pre-form, or
grippers stretch a parison, to aid in blow moulding.
Stripper plate. In an injection mould, a plate that is pushed by ejector
rods to move a part or parts out of the mould.
Structural foam moulding. An injection moulding process whereby nitrogen or
another gas is injected into the melt to induce foaming, and to produce a
light, rigid finished part.
Structural reaction injection moulding. Reaction injection moulding (qv)
using a glass mat or other pre-inserted reinforcement to strengthen it.
Styrenic. One of a family of materials using styrene monomer as a building
block for their polymers, and including ABS and SAN as well as polystyrene.
Terpolymer. Plastic that combines three different monomers in one polymer,
such as acrylonitrile-butadiene-styrene (ABS).
Thermocouple. A monitoring device that measures temperature through
electrically responsive wires, welded together at one end, that generate a
small current in direct proportion to the heat they sense.
Thermoforming. A process for heating and former previously extruded plastic
sheet into items such as refrigerator doors, panels for inside buses, trays
for cookie packages, etc.
Thermoplastic. A polymer that can be heated and reformed any number of
times.
Thermoset. A polymer that, once heated and formed, cannot be re-melted
without fundamental degradation, because its polymer chains are heavily
crosslinked.
Three-D blow moulding. Blow moulding of geometrically complex or
asymmetrical parts, where a parison is accurately positioned in the mould to
avoid flash and to achieve precise formation.
Tiebars. Bars, also called tie-rods, on an injection or blow moulding
machine, on which the mould platens ride and are aligned as the mould opens
and closes. Usually four in number, sometimes two or three, rods are used to
provide rigidity to the clamping mechanism.
Tiebarless. Used to describe a blow moulding or injection moulding press
that has a C-clamp as opposed to tiebars to align the platens.
Toggle. A mechanical knee-joint system that closes and pressurises an
injection mould.
Transducer. An input/output electronic device that can measure load or
pressure when placed in a sensor cell adjacent to the flow path of a melt.
Transfer moulding. Process similar to compression moulding, except that the
moulding compound, usually thermoset, is heated to plasticity in a pot
before being forced into the closed mould.
Ultrasonic welding. Use of high-frequency sound vibrations, emitted through
a metal block called a horn, that melt two plastic parts at the point where
they meet, forming a bond.
Undercuts. Inset areas of a formed or moulded part that require removable
inserts, slides or swing-out sections to be formed.
Vacuum forming. A forming process whereby sheet is pulled by a vacuum
against a heated mould surface until it takes on the mould contours; a close
relative of thermoforming.
Vacuum sizing tank. Downstream device used in extrusion for sizing profiles
without the distorting effects of air, and employing water for lubrication
and for sealing the vacuum.
Venting. Providing means for exhaust gas to escape from a mould, to prevent
part distortion.
Vibration welding. Use of high-speed vibrations to cause friction in
plastics parts, forcing them to bond.
Vinyl. Common abbreviation for polyvinyl chloride, a plastic used for some
packaging and housewares but particularly for construction items such as
pipe, window frames, doors and siding for houses. The term is also sometimes
applied to other plastics in the range of vinyl copolymers.
Virgin resin. Plastic resin that has never been previously heated or formed.
Warpage. Distortion or curving of a plastic part that occurs after
processing, either because of stresses induced during processing, or
subsequent exposure to heat.
Wollastonite. Natural calcium metasilicate, a fibrous mineral used as a
reinforcement.
Wood flour. Finely ground, dried wood, used as a filler, especially in
thermosets or imitation wood.
Ziegler-Natta catalysts. Large family of polymer catalysts discovered by
Karl Ziegler, and developed by Giulio Natta, with whom he shared the Nobel
Prize for their work in 1963.
Zippering. An effect seen in blown film where a puncture or defect results
in a visible tear line appearing in the film surface.
