Glossary of Terms

A

• absorption – Penetration of one substance into the mass of another.
• accelerator – Accelerators are added to a polyester resin or into the epoxy resin-curing agent mixture to speed up a slow reaction.
• aging – Effect on materials of exposure to an environment for an interval of time.
• air-bubble void – Entrapment of air between plies or in a bond line.
• ambient – Surrounding environmental conditions, such as pressure, temperature or relative humidity.
• angle-ply laminate – A laminate with fibres of adjacent plies oriented at alternating angles.
• anti-static agents – Agents that, when added to a moulding material or applied to a surface make the material less conducting, hindering the attraction of dust or build-up of electrical charge.
• aramid – See Kevlar.
• areal weight – Weight of fibre fabric per unit area (width x length) of tape or fabric.
• autoclave – Pressure vessel in which assemblies are placed for curing when even pressure and temperature is required. The maximum pressure achieved while vacuum bagging is relative to the pressure on the other side of the bag. Autoclaves increase the ambient pressure & heat.

B

• bag moulding – Technique for producing moulded parts by atmospheric pressure through a flexible membrane.
• bagging – Applying an impermeable layer of film over an uncured part and sealing edges so a vacuum can be drawn.
• barrier film – Layer of film used to permit removal of air and volatiles from a composite lay-up during cure while minimizing resin loss.
• batch (or lot) – A quantity of material formed during the same process and having identical characteristics.
• binder – Bonding resin used to hold strands together in a chopped strand mat or preform during manufacture of a moulded object.
• blanket – Material plies that have been laid up in a complete assembly and placed on or in the mould all at one time. Also the form of bag in which the edges are sealed against the mould.
• bleeder cloth – Woven or nonwoven layer of material used in manufacturing composite structures to allow the escape of excess volatiles and resin during the cure cycle. The bleeder cloth is removed after the curing process and is not part of the final item.
• bleeding – Removal of excess resin from a laminate during cure.
• blow-by – Flow or leakage of internal air pressure through the part and out through the vacuum vent lines that exhaust to the atmosphere.
• bond – Adhesion of one surface to another, with or without the use of an adhesive as a bonding agent.
• breather cloth – Loosely woven material that serves as a continuous vacuum path over a part, but is not in contact with the resin.
• breathing – Opening and closing of a mould to allow gas to escape early in the moulding cycle. Also known as degassing.
• bridging – Condition where one or more plies of prepreg, bag film, breather, release film, tape, fabric, etc., span a radius, stop or chamfered edge or core without full contact.
• bundle – General term for a collection of parallel filaments or fibres.

C

• carbon fibres – Fibres made from a precursor by oxidation and carbonization and not having a graphitic structure.
• caul plate – Smooth (metal) plate free of surface defects, the same size and shape as a composite lay-up, used in immediate contact with the lay-up during the curing process to transmit normal pressure and temperature and to provide a smooth surface to the finished part.
• cavity – Depression in a mould; the space inside a mould where resin is poured; the female portion of a mould. Moulds are designated as Single Cavity or Multiple Cavity, depending on the number of depressions.
• clamping plate – Mould plate fitted to the mould and used to fasten the mould to the machine.
• clamping pressure – In injection moulding and in transfer moulding, the pressure which is applied to the mould to keep it closed; in opposition to the fluid pressure of the compressed moulding material.
• co-curing – The act of curing a composite laminate and simultaneously bonding it to some other prepared surface during the same cure cycle.
• coefficient of thermal expansion (CTE) – The change in volume per unit volume produced by a one-degree rise in temperature.
• compaction – The application of a temporary vacuum bag and vacuum to remove trapped air and compact the layup. Also see de-bulking.
• composite – A material containing two or more distinctive materials (fillers, reinforcing materials and compatible plastic resin) designed to develop specific performance properties.
• contact pressure resins – Liquid resins that thicken on heating, and when used for bonding laminates, require little or no pressure.
• continuous filament – A yarn or strand in which the individual filaments are the same length as the strand.
• coupon – Specimen for a specific test, such as tensile coupon.
• crazing – Apparent fine cracks at or under the surface of an organic matrix.
• crossply – Any filamentary laminate in which the laminas are at right angles to one another.
• cure – To change the properties of a resin by chemical reaction, which may be condensation or addition; usually by either heat or catalyst, or both, with or without pressure.
• cure cycle – The cycle of time/temperature/pressure used to cure a thermosetting resin system or prepreg.
• curing agent – Catalytic or reactive agent which, when added to a resin, causes polymerization. Also called ‘hardener.’
• curing temperature – Temperature at which a cast, moulded, or extruded product, a resin-impregnated reinforcement, or an adhesive, etc., is subjected to curing.
• curing time – Length of time a part is subjected to heat, pressure, or both, to cure the resin.
• cycle – Complete, repeating sequence of operations. In moulding, the cycle is the period, between a certain point in one cycle and the same point in the next.

D

• debond – A deliberate separation of a bonded joint or interface, usually for repair or rework purposes.
• debulking – Compacting of a prepreg stack under moderate heat, pressure and/or vacuum to remove most of the air, to ensure seating on the tool and to prevent wrinkles.
• deformation – Change in shape of a specimen caused by the application of a load or force.
• delamination – Separation of layers of material in a laminate.
• denier – A numbering system for expressing linear density, equal to the mass in grams per 9000 metres of yarn, filament, fibre, or other textile strand. (also see TEX).
• density – The mass per unit volume. Also see Specific Gravity
• deviation – Variation from a specified dimension or requirement.
• disbond – Area within a bonded interface between two adherends in which an adhesion failure or separation has occurred.
• dry fibre area – Area of fibre not totally encapsulated by resin.
• dry lay-up – Construction of a laminate by layering pre-impregnated reinforcement (partly cured resin) in a mould, usually followed by bag-moulding or autoclave moulding. Also for dry fabric lamination before resin infusion or RTM
• ductility – The ability of a material to deform plastically before fracturing.
• dwell – A pause in the increase or decrease of temperature to a part or mould in the cure or post cure cycle.

E

• end – A single fibre, strand, roving or yarn being or already incorporated in a product.
• elongation – Deformation caused by stretching.
• extensometer – A device for measuring linear strain.

F

• fabric – A material constructed of interlaced yarns, fibres, or filaments. Nonwovens are sometimes included in this classification.
• fabric fill face – Side of the woven fabric where the greatest number of yarns are perpendicular to the selvage.
• fabricating, fabrication – Manufacture of plastic products from moulded parts, rods, tubes, sheeting, or extrusions, by punching, cutting, drilling and tapping. Fabrication includes fastening plastic parts together or to other parts by mechanical devices, adhesives, heat sealing or other means.
• felt – A fibrous material made up of interlocked fibres by mechanical chemical action, moisture or heat; made from asbestos, cotton, glass, etc.
• fibre – Term used for filament materials.
• fibre content – The amount of fibre present in a composite.
• fibre direction – Orientation or alignment of the longitudinal axis of the fibre
• fibre-reinforced plastic : FRP, GRP and CFRP – Term for composite that is reinforced with cloth, mat, strands, or any other fibre form. Glass reinforced plastic / Carbon fibre reinforced plastic
• filament – The smallest unit of a fibrous material. Filaments are usually extreme length and very small diameter.
• filament winding (continuous) – An automated process in which continuous filament strands are resin-treated and wound on a removable mandrel.
• fill – Yarn running from selvage to selvage at right angles to the warp in a woven fabric.
• filler – Inert material added to a resin mixture to reduce cost, modify mechanical properties, add color, or improve surface texture.
• filling yarn – Transverse threads or fibres in woven fabrics, running perpendicular to warp. Also known as weft.
• fish-eye – Small globular mass which has not blended completely into the surrounding material and which shows particularly in a transparent or translucent material.
• flash – Extra plastic / resin which has flowed out of the mould cavity during moulding, along the parting line which must be removed. Also adhesive flash and resin flash in vacuum / autoclave bonding.

G

• gel – The initial jelly-like solid phase that develops during the formation of a resin from a liquid. A semi-solid system consisting of solid aggregates in which liquid is held.
• gel coat – A resin applied to the surface of a mould and gelled prior to lay-up. The gel coat becomes an integral part of the finished laminate, and is usually used to improve surface appearance and bonding.
• gel point – The stage at which a liquid begins to exhibit pseudo-elastick properties.
• gel time – The time taken to achieve gel.
• graphite fibres – Fibres made from a precursor by oxidation, carbonization, or graphitization process (which provides a graphitic structure.)
• greige – Fabric that has received no finish.

H

• hand lay-up – The process of placing successive plies of reinforcing material or resin-impregnated reinforcement in position on a mould by hand.
• hardness – Resistance to deformation; usually measured by Rockwell.
• heat cleaned – Glass or other fibres which have been exposed to elevated temperatures to remove preliminary sizings or binders which are not compatible with the resin system to be applied.
• heat sealing – Joining plastic films by applying heat and pressure simultaneously.
• heterogeneous – Term for a material consisting of dissimilar materials.
• homogeneous – Term for a material of uniform composition throughout.
• hot wet lay-up – A method of fabricating a reinforced product by applying a hot resin system as a liquid when the reinforcement is in place. Hot setting adhesive/resin requires a temperature at or above 100′C to set.
• humidity, relative – The ratio of the pressure of water vapour present to the pressure of saturated water vapour at the same temperature.
• hybrid – A composite laminate comprised of fibres of two or more composite material systems (eg. carbon & Kevlar)
• hydraulic press – Press in which moulding force is created by the pressure exerted by a fluid.
• hydroclave – A pressure vessel that uses water as the pressure medium. A hydroclave can be pressurized to 206 bar. The risk of explosion or fire is considerably less in a hydroclave.
• hydrophobic – Capable of repelling water.
• hygroscopic – Capable of adsorbing and retaining atmospheric moisture.

I

• Impregnate – In reinforced plastics, the saturation of the reinforcement with a resin.
• Impregnated fabric – A fabric Impregnated with a synthetic resin. (See prepreg.)
• Inclusion – A physical and mechanical discontinuity occurring within a material or part, usually consisting of solid, encapsulated foreign material.
• insulator – Material which conducts minimal electric current or heat.
• integrally heated – Tooling which is self-heating, through electrical heaters or hot liquid.
• lnterlaminar – Term pertaining to the area existing between two or more layers of a laminate.

K

• Kevlar (aramid) – Organic polymer composed of aromatic polyamides.
• knitted fabrics – Fabrics produced by inter-looping chains of yarn.

L

• laminate – To unite sheets of material by a bonding material or a resin system, usually with pressure and heat, commonly referring to flat sheets. A product made so by bonding.
• lay-up – As used in reinforced plastics, the reinforcing material placed in position in the mould; the process of placing reinforcing material in position in the mould with resin. A description of the component materials of a laminate.

M

• mandrel – A fixture or male mould used for the base in the production of a part by hand layup or filament winding
• mat – A fibrous material consisting of randomly oriented chopped filaments or swirled filaments with a binder; available in blankets of various widths, weights, and lengths.
• mat binder – Resin applied to the fibre and cured during the manufacture of mat, to hold the fibres in place and maintain the shape of the mat.
• matrix – The essentially homogeneous material (resin system) in which the fibre system of a composite is embedded.
• moisture absorption – The pick-up of water vapour from air by a material. It relates only to vapor withdrawn from the air by a material and must be distinguished from water absorption, which is the gain in weight because of the take-up of water by immersion.
• mould – Cavity into which the plastic composition is placed, and from which it takes form; to shape plastic parts by heat and pressure.
• mould-release agent – A liquid, powder or wax used to prevent sticking of moulded articles in the cavity. (See parting agent.)
• mould surface – Side of laminate that faced the tool.
• moulding – Shaping of a plastic composition in or on a mould, usually under heat and pressure.
• moulding cycle – Period of time for the complete sequence of operations to take place on a moulding press.
• moulding pressure – Pressure applied to the ram of an injection machine or press to force softened plastic to fill the mould cavities completely.
• monomer – A relatively simple compound which can react to form a polymer (Styrene).

N

• nesting – Planning the use of material so that a minimum of raw material is used (cloth / foam core). Placing of plies of fabric so that yarns of one ply lie in the valleys between the yarns of the adjacent ply (nested cloth).
• nonwoven fabric – Material produced by compressing together (or needling) yarns, fibres, rovings, with or without a scrim carrier.
• nylon – Generic name for all synthetic polyamides.

O

• out-time – The time a prepreg is exposed to ambient temperature, i.e., the total amount of time the prepreg is out of the freezer. The primary effects of out time are to decrease the drape and lack of the prepreg while also allowing it to absorb moisture from the air.
• overcuring – The beginning of thermal decomposition because of too high a temperature or too long a moulding time.
• oven dry – The condition of a material that has been heated under prescribed conditions of temperature and humidity until there is no further significant change in its mass.
• overlay-sheet – A non-woven fibrous mat (in glass, synthetic fibre, etc.) used as the top layer in a cloth or mat lay-up to give a smoother finish, or minimize the appearance of the fibrous pattern.

P

• PAN fibres – Polyacrylonitrile spun and stabilized fibres.
• parting agent – (Mould release agent) Lubricant or release agent used to coat a mould or cavity to prevent the moulded piece from sticking to it, making it easier to remove.
• peel ply – Outside layer of a laminate which is removed to achieve improved bonding of additional plies. The peel ply can be left on temporarily for greater protection during handling.
• peel strength – Adhesive bond strength, as in pounds per inch of width, tested by stress applied in a peeling mode.
• pH – A measure of acidity or alkalinity of a solution, with the value of 7 representing neutrality, and increasing acidity corresponding to lower values, and increasing alkalinity corresponding to higher values.
• pick – An individual filling yarn or roving in a fabric. (Weft)
• pick count – The number of filling yarns per inch of woven fabric.
• pin holes – Small cavities penetrating the surface of a cured part, or holes in bag film.
• pitch fibre – Fibres derived from a special petroleum pitch.
• plasticizer – A material of lower molecular weight added to a polymer to separate the molecular chains.
• pleats – In placing the vacuum bag over the part, pleats in the bag provide extra material over the part to eliminate bridging and possible voids in the laminate. Pleats are at least 50 to 100mm in height, every 500mm, as necessary, to allow room for unusual shapes or for protrusions in the lay-up without danger of puncturing the bag. (See drawing on page 6.)
• polymer – An organic material composed of molecules characterized by the repetition of one or more types of monomeric units.
• polymerization – A chemical reaction in which the molecules of monomers are linked together to form polymers.
• porosity – A condition of trapped pockets of air, gas, or vacuum within a solid material.
• positive mould – A mould designed to apply pressure to a piece being moulded with no escape of material.
• post-cure – Additional elevated temperature cure, usually without pressure, to complete the cure. In certain resins, complete cure is attained only by exposing the cured resin to higher temperatures after the original cure cycle.
• pot-life – The length of time that a catalyzed resin system retains a viscosity low enough to be used in processing.
• precursor – Either the PAN or pitch fibres from which carbon and graphite fibres are derived.
• prefit – Method for checking the fit of mating parts before bonding.
• Preimpregnation – See prepreg.
• prepreg – Ready-to-mould material in sheet form which may be cloth, mat, or paper impregnated with resin and stored for use, The resin is partially cured to a “B’ stage and supplied to the fabricator who lays up the finished shape and completes the cure with heat and pressure.
• pressure – There are various kinds of pressure: dead weight, vacuum, super-heated steam, fluid, water pressure, compressed air, and inert gas pressure. Dead-weight pressure is preferred for room temperature curing of parts. Vacuum pressure requires bagging to evacuate the air in a part. Vacuum produces a maximum of 1bar pressure. Super-heated steam pressure has become obsolete because of plumbing corrosion, and the expense of replacing valves, pipes, etc. Fluid or water pressure, (hydraulic pressure) is common in hydroclave operations. The main requirement is to have sufficient pressure on the part in all directions to obtain a void-free assembly.
• pressure bag moulding – A process for moulding reinforced plastics, in which a tailored flexible bag is placed over the contact lay-up in the mould, sealed and clamped in place. Fluid pressure, usually compressed air, is placed against the bag, and the part is cured.

R

• reinforced plastic – A plastic with relatively high stiffness or very high strength fibres imbedded in the composition.
• release agent – A material applied in a thin film to the surface of a mould to keep the resin from sticking to the mould.
• resin – Organic material which flows when subjected to stress. Most resins are polymers.
• resin content – The amount of matrix present in a composite either by percent weight or percent volume.
• resin-starved area – Area of composite part where the resin has a non-continuous smooth coverage of the fibre.
• RTM – Abbreviation for “resin transfer moulding”. This could be by pressure or vacuum or both VARTM: vacuum assisted RTM.

S

• sample – A small portion of a material or product intended to be representative of the whole.
• scrim – A low cost, nonwoven open-weave reinforcing fabric made from continuous filament yarn in an open mesh. Often a carrier of other material.
• sealant – Material in paste or liquid form that hardens or cures in place, forming a seal.
• sealant tape – Material used to seal a vacuum bag to a mould or to the bag itself.
• selvage – Outer woven-edge of a fabric parallel to the warp or length.
• shelf life – Length of time a material can be stored and retain all its original characteristics.
• shrinkage – Relative change in dimension between the length measured on the tool when it is cold and the length measured 24 hours after it has been removed.
• solute – The dissolved material.
• specific gravity – The density or mass per unit volume of a material divided by the density of water.
• splice – Joining of two ends of glass fibre yarn or strands usually by means of an air-drying adhesive.
• spray-up – Techniques of using a spray (chopper) gun as the processing tool. In reinforced plastics, fibrous glass and resin can be simultaneously deposited in a mould.
• staging – An intermediate stage of a thermosetting resin that is between monomer stage and complete cure.
• starved area – An area in a plastic part which has too little resin to wet out the reinforcement completely. This may be caused by improper wetting or excessive moulding pressure.
• starved joint – An adhesive joint which does not have enough film thickness of adhesive due to insufficient adhesive spreading or because of excessive pressure during lamination.
• storage life – The period of time during which a liquid resin or packaged adhesive can be stored under specified temperature conditions and remain suitable for use. (Also called shelf life.)
• stress crack – External or internal crack caused by mechanical stresses
• surfacing mat / surface tissue – A very thin mat, usually 7 to 20 mils thick, of highly filamentized fibre glass used to produce a smooth surface on a reinforced plastic laminate.

T

• tack – Degree of stickiness when referring to a resin prepreg material.
• tape – Unidirectional prepreg material.
• tex – Unit of measure of fibre mass per
• thermocouple – Thermocouples are used to measure and record temperature differential in the part being cured. Thermocouple wires are usually placed in the flash or trim area. These wires can be iron Constantine, type J, or equivalent and chrome alumel Type K. The chrome alumel TC wires are usually used where the cure temperature is very high, such as 8000F+. Wires must not be bent or crimped, as they will become inoperable.
• thermoplastic – A plastic material that is capable of being repeatedly softened by application of heat and repeatedly hardened by cooling.
• thermoset – Plastic material that changes, during cure, into an infusible and insoluble material.
• time – The interval between events, i.e., start and completion of a cure cycle.
• Tow – Untwisted bundle of continuous filaments or fibres.
• tracer – A fibre added to a prepreg to verify fibre alignment . On peelply it shows that it is still on the laminate.

U

• unbond – An area within a bonded interface between two adherends in which the intended bonding action failed to take place. (See debond.)
• unidirectional laminate – A laminate with nonwoven reinforcements and all layers layed up in the same direction.

V

• vacuum – A state of being sealed off from external/environmental influences; state of emptiness. A near-perfect vacuum is 1 Bar or 29.5 inches (749.3mm) of Hg. NOTE: Approx 75mm mercury (Hg) is comparable to 0.1 Bar.
• vacuum bag moulding – A process in which a sheet of ‘flexible material, plus a bleeder cloth and release film are placed over the lay-up, sealed at the edges of the mould, and a vacuum is applied between the bag and the lay-up. The entrapped air is removed by vacuum and the part is cured with temperature, pressure, and time. The part is bagged to remove excess air and volatiles, and also to apply an even distribution of pressure during the cure cycle.
• vent cloth – Layer or layers of open-weave- cloth to provide a path for vacuum to reach the area over a laminate being cured.
• venting – In curing a part in an autoclave, turning off vacuum source and venting vacuum bag to the atmosphere.
• voids – Air or gas that has been trapped and cured into a laminate.
• volatiles – Materials in a resin formulation that are driven off as vapour during cure cycle.

W

• warp – Yarn running lengthwise in a woven fabric. (See ‘fabric fill face”).
• water absorption – Ratio of the weight of water absorbed by a material to the weight of the dry materials.
• weave – Particular manner in which the fabric is formed by interweaving yarns.
• wet lay-up – A method of making a reinforced product by applying the resin system as a liquid while the reinforcement is put into place.
• whisker – A short single crystal fibre or filament. Whisker diameters range from I to 25 microns, with aspect ratios between 100 and 15,000.
• working life – The period of time during which a liquid resin or adhesive, after mixing with catalyst, solvent, or other compounding ingredients, remains workable. (see also pot life)
• wrinkle – A surface imperfection in laminated plastics in one or more outer sheets of paper, fabric or other base which has been pressed in or out.

X

• X-axis – In composite laminates, an axis in the plane of the laminate which is used as the 0 degree reference for designating the angle of a lamina.

Y

• yarn – A generic term for strands or bundles of continuous filaments or fibres usually twisted and suitable for making textile fabric.
• yarn, plied – Yarns made by collecting two or more single yarns together. Normally the yarns are twisted together, though sometimes they are collected without twist.

Z

• zero bleed – A laminate fabrication procedure that does not allow loss of resin during cure. Also called ‘net resin systems’.