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A-stage – An early stage of polymerization of thermosetting resins in which the material is still soluble in certain liquids and is fusible. See B-Stage, C-Stage.
Ablation — The degradation, decomposition, and erosion of material caused by high temperature, pressure, time, percent oxidizing species, and velocity of gas flow. A controlled loss of material to protect the underlying structure.
Absorption — The penetration into the mass of one substance by another. The capillary or cellular attraction of adherent surfaces to draw off the liquid adhesive film into the substrate.
Accelerator — Chemical additive that hastens cure or chemical reaction. A material that when mixed with a resin, will speed up the chemical reaction between the catalyst and the resin (either polymerizing of resins or vulcanizations of rubbers). Also called promoter or activator.
Acetone –In a GRP context, acetone is primarily useful as a cleaning solvent for removal of uncured resin from tools. It is very flammable.
Additives — Ingredients mixed into resin to improve properties. Some examples of additives are plasticizers, initiators, light stabilizers, and flame-retardants.
Advanced composites — Strong, tough materials created by combining one or more stiff, high strength reinforcing fiber with compatible resin system. Advanced composites can be substituted for metals in many structural applications with physical properties comparable or better than aluminum.
Aging – The process of exposing materials to an environment for an interval of time.
Air Dry – To cure at room temperature with the addition of catalyst but without the assistance of heat and pressure.
Alligatoring – Wrinkling of the gel coat film resembling alligator hide; caused by poor cure at the time of contact with styrene from a subsequent or preceding coat.
Ambient Conditions — Prevailing environmental conditions such as the surrounding temperature, pressure and relative humidity.
Anisotropic — Fiber directionality in which different properties respond to stresses applied along axes in different directions.
Aramid — A type of highly oriented organic material derived from polyamide, (nylon), but incorporating aromatic ring structure. Used primarily as high strength, high-modulus fiber. Aromatic polyamide fibers; commonly, DuPont’s Kevlar and Akzo Nobel’s Twaron.
Areal Weight –weight of a fiber reinforcement per unit area (width times length) of tape or fabric.
Aspect Ratio — The ratio of length to diameter of a fiber, or the ratio of length to width in a structural panel.
B-Stage — Also called resistal. Intermediate stage in the polymerization reaction of thermosets. After B-Stage, material softens with heat and is plastic and fusible. The resin of an uncured prepreg or premix is usually B-Stage. See A-Stage, C-Stage.
Bag — The plastic film used in sealing a laminate to be infused.
Bagging – Applying an impermeable layer of film over an uncured part and sealing the edges so that a vacuum can be drawn.
Bag Molding — A molding technique in which the composite material is placed in a rigid mold and covered with a flexible bag, with pressure applied by vacuum, autoclave, press, or by inflating the bag.
Balanced laminate — A composite in which all laminae at angles other than 0 degrees and 90 degrees occur only in pairs (not necessarily adjacent) and are symmetrical around the centerline.
Barcol Hardness – A hardness value obtained by measuring the resistance to penetration of a sharp steel point under spring load. The instrument, (the Barber-Coleman impressor) called a Barcol impressor, gives a direct reading on a scale of 0 to 100. The hardness value is often used as a measure of the degree of cure of a laminate or composite.
Basket Weave —Woven reinforcement where two or more warp threads go over and under two or more filling threads in a repeat pattern: less stable than the plain weave but produces a flatter, stronger, more pliable fabric.
Batch (or Lot) — Material made by the same process at the same time having identical characteristics throughout.
Beam — A structural member supporting a portion of a deck.
Bearer – That part of a structure that bears the weight as in foundations and stowages is called a bearer.
Benzyoyl Peroxide (BPO) — Catalyst used in conjunction with aniline accelerators or where heat is used as an accelerator.
Bias Fabric — A fabric in which warp and fill fibers are at an angle to the length.
Bi-directional – Arrangement of reinforcing fiber strands in which half of the strands are laid at right angles to the other half; a directional pattern that provides maximum product strength to those two directions.
Binder — The resin or cementing constituent (of a plastic compound) that holds the other components together. The agent applied to fiber mat or preforms to bond the fibers before laminating or molding.
Bleeder Cloth — A layer of woven or non-woven material, not a part of the composite, that allows excess gas and resin to escape during cure.
Bleedout — Excess liquid resin appearing at the surface, primarily occurs during filament winding or wet bag lay up.
Blister — An elevation on the surface of an adherent containing air or water vapor, somewhat resembling in shape a blister on the human skin. Its boundaries maybe indefinitely outlined, and it may have burst and become flattened.
Bond strength — As measured by load/bond area, the stress required to separate a layer of material from another material to which it is bonded; the amount of adhesion between bonded surfaces.
Braiding — Weaving of fibers into a tubular shape or form.
Brake — An area of a fiber layup that is designed to impede the flow of resin. Usually to allow other areas to complete wet out.
Breakout — Separation or breakage of fibers when the edges of a composite part are drilled or cut.
Breather — A loosely woven material that does not come in contact with the resin but serves as a continuous vacuum path over a part in a production.
Bridging — 1. A pocket of resin in the layers of glass, caused by the fiberglass not being properly applied prior to infusion, usually occurring in corners. 2. A pocket of resin on top of the laminate, usually in corners, caused by loose peel ply or a stretched bag.
Buckling — A failure mode usually characterized by fiber deflection rather than breaking under compressive action.
Butt Joint — The joint formed when two parts are placed end to end.
C-Stage — Final step in the cure of a themoset resin, resulting in irreversible hardening and insolubility.
C-scan — The back and forth scanning of a specimen with ultrasonics. A nondestructive testing technique for finding voids, delaminations, defects in fiber distribution, and so forth.
Carbon — The element that provides the backbone for all organic polymers. Graphite is a more ordered form of carbon. Diamond is the densest crystalline form of carbon.
Carbon Fiber —Reinforcing fiber known for its light weight, high strength and high stiffness produced by pyrolysis of an organic precursor fiber in an inert atmosphere at temperatures above 1,800 degrees F. Can also be graphitized by heat treating above 3,000 degrees F.
Carbon/Carbon— A composite of carbon fiber in a carbon matrix.
Catalyst — A substance that changes the rate of a chemical reaction without itself undergoing permanent change in composition or becoming a part of the molecular structure of the product. A substance that markedly speeds up the cure of a compound when added in minor quantity as compared to the amounts or primary reactants. A substance that promotes or controls curing of a compound without being consumed in the reaction.
Caul plate — Plate or sheet the same size and shape as the composite lay-up with which it will be used. The caul plate is placed in immediate contact with the lay-up during curing to transmit normal pressure and provide a smooth surface on the finished part.
Cavity – Space between matched molds (pressure molds) in which the laminate is formed. Also, a term for a female mold.
Cell — In honeycomb core, a cell is a single honeycomb unit, usually in a hexagonal shape.
Centipoise (CPS) –Unit of measure used to designate a fluid's viscosity at 70 degrees F., water is 1cps: peanut butter is 250,000 cps.
CFM — Continuous Filament Mat: Random placed continuous fibers, usually held together by a binder. Typically used in RTM and Pultrusion Processes
Chopped strand mat (CSM) – Continuous strand yarn or roving cut up into uniform lengths, usually from 1 to 2 inches long, and pressed flat to form fabric held together with a binder.
CHP — Cumyl Hydro Peroxide. A catalyst that results in slower gel times and lower exotherm reactions.
Closed cell foam — Cellular plastic in which individual cell are completely sealed off from adjacent cells.
Cobalt — Used as the accelerator for methyl ethyl ketone peroxide catalyzed polyesters.
Cocured — Cured and simultaneously bonded to another prepared surface.
Coefficient of Expansion (COE) – Measure of the change in length or volume of an object
Coefficient of Thermal Expansion (CTE) – A material’s fractional change in length for a given unit change of temperature.
Cohesion — Tendency of a single substance to adhere to itself. Also, the force holding a single substance together.
Commingled Yarn — A hybrid yarn made with two types of materials intermingled in a single yarn (for example, thermoplastic filaments intermingled with carbon filaments to form a single yarn).
Compaction– The application of temporary vacuum pressure to remove trapped air and compact the lay-up.
Composite —A material that combines fiber and a binding matrix to maximize specific performance properties. Neither element merges completely with the other. Advanced composites use only continuous, oriented fibers in polymer, metal and ceramic matrices.
Compressive Strength — Resistance to a crushing or buckling force, the maximum compressive load a specimen sustains divided by its original cross sectional area.
CONAP — Cobalt Naphtenate
Condensation — A polymerization reaction in which simple byproducts (for example, water) are formed.
Consolidation — Processing step that compresses fiber and matrix to remove excess resin, reduce voids, and achieve a particular density.
Contact Molding — A process for molding reinforced plastics in which reinforcement and resin are placed on a mold. Cure is either at room temperature or by heating in an oven at ambient pressure.
Contaminant — Impurity or foreign substance that negatively affects one or more properties of composite material, particularly adhesion.
Continuous Filament — An individual, small-diameter reinforcement that is flexible and indefinite in length.
Continuous Roving — Parallel filaments coated with sizing, gathered together into single or multiple strands, and wound into a cylindrical package. May be used to provide continuous reinforcement in woven roving, filament winding, pultrusion, prepregs, or high-strength molding compounds, or may be used chopped.
Core — In sandwich construction, the central component to which inner and outer skins are attached. Commonly used core materials include foam, balsa, honeycomb, and wood. Also refers to a section of a complex mold that forms undercut parts.
Corrosion resistance — the ability of a material to withstand contact with ambient natural factors or those of a particular artificially-created atmosphere, without degradation or change in properties.
Count — For fabric, number of warp and filling yarns per inch in woven cloth. For yarn, size based on relation of length and weight.
Co-woven Fabric — A reinforcement fabric woven with two different types of fibers in individual yarns (for example, thermoplastic fibers woven side by side with carbon fibers).
CPS—Centipoise — unit of measurement of viscosity. Higher centipoise indicates more viscous properties while lower centipoise indicates less viscous properties.
Crazing — Region of ultra fine cracks that may develop on or under a resin surface due to internal stress.
Creep — Dimensional change in a material under physical load over time and instantaneous elastic deformation.
Crimp — A fiber’s waviness, which determines its capacity to cohere.
Critical Length — The minimum length of a fiber necessary for matrix shear loading to develop ultimate fiber strength.
Cross-laminated — Material laminated so that some of the layers are oriented at various angles to the other layers with respect to the laminate grain. A cross-ply laminate usually has plies oriented only at 0/90º.
Crosslinking — Polymerization reactions that branch out from the main molecular chain to form a networked pattern of chemical links.
Cure — To change the physical properties of a material irreversibly by chemical reaction via heat and catalysts alone or in combination, with or without pressure.
Cure Temperature — The temperature at which a material attains final cure.
Curing Agent — A catalytic or reactive agent that brings about polymerization when added to a resin.
Damage Tolerance — A measure of the ability of structures to retain load-carrying capability after exposure to sudden loads (for example, ballistic impact).
De-bond — An unplanned non-adhered or unbonded region in an assembly.
Delaminate — The separation of ply layers due to adhesion or cohesion failure. Also includes the separation of layers of fabric from the core structure. A delamination may be associated with bridging, drilling and trimming or demolding.
Delamination — In plane separation of a laminate ply or plies due to adhesion failure, either local or covering a wide area. Can occur in the cure or subsequent life. Failure of internal bonding between layers of resin and reinforcement.
Demold — To remove a part from a tool, or a tool from an intermediate model.
Denier — A numbering system for yarn and filament in which yarn number is equal to weight in grams per 9,000 meters of yarn.
Dimensional stability — Ability of a plastic part to retain the precise shape to which it was molded, casted, otherwise fabricated.
Dimethylaniline (DMA) — Accelerator used in conjunction with BPO catalyst.
Dimples — Small sunken dots in the gel coat surface, generally caused by a foreign particle in the laminate.
Drape — The ability of fiber reinforcement to conform to the shape of a contoured surface.
Dry laminate — A laminate containing insufficient resin for complete bonding of the reinforcement.
Ductility — The amount of plastic strain that a material can withstand before fracture. Also, the ability of a material to deform plastically before fracturing.
E-Glass — Stands for “electrical glass”. A family of glasses with a calcium aluminoborosilcate composition and a maximum alkali content of 2.0%. A general-purpose fiber that is most often used in reinforced plastics, and is suitable for electrical laminates because of its high resistance.
Elasticity — The property of materials to recover their original sizes and shape after deformation.
Elastomer — A material that substantially recovers its original shape and size at room temperature after removal of a deforming force.
Elongation— The fractional increase in length of a material stressed in tension. When expressed as a percentage of the original length, it is called percent elongation.
Enamel – A gel, coat, or surface coat which cures tack free.
EPA — Environmental Protection Agency, US government agency charged with protection of the environment.
Epoxy resin — A polymerizable thermoset polymer containing one or more epoxide groups and curable by reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans.
Exotherm — Heat released during a chemical reaction. Uncontrolled exotherm can lead to violent explosion.
Fabric, Nonwoven — A material formed from fibers or yarns without interlacing.
Fabric, Woven — A material constructed of interlaced yarns or fibers.
Fatigue — The failure of a material’s mechanical properties as a result of repeated stress over time.
Fatigue life — The number of cycles of deformation required to bring about failures of the test specimen under a given set of oscillating conditions (stresses or strains).
Fatigue Strength — Maximum cyclical stress withstood for a given number of cycles before a material fails.
FEA – Finite Element Analysis – A process of selecting the optimum combination of materials in a composite based on software analysis.
Feed point or feeder — a device or method of delivering resin from its source to a specific area in a close molded part.
Fiber — A general term used to refer to filamentary materials. It is a general term for a filament with a finite length that is at least 100 times its diameter, which is typically 0.004 to 0.005 inches. In most cases it is prepared by drawing from a molten bath, spinning, or deposition on a substrate. A whisker, on the other hand, is a short single-crystal fiber or filament made from a variety of materials, with diameters ranging from 40 to 1400 micro inches and aspect ratios between 100 and150. Fibers can be continuous or specific short lengths (discontinuous), normally less than 1/8th of an inch.
Fiber Architecture — The design of a fibrous preform or part in which the fibers are arranged (braided, stitched, woven, etc.) in a particular way to achieve the desired result.
Fiber Content — Amount of fiber in a composite expressed as a ratio to the matrix.
Fiber count — The number of fibers per unit width of ply present in a specified section of a composite.
Fiber Direction –The orientation or alignment of the longitudinal axis of the fiber with respect to a stated reference axis.
Fiberglass reinforcement — Major material used to reinforce plastic. Available as mat, roving, fabric, and so forth, it is incorporated into both thermosets and thermoplastics.
Fiber-reinforced plastic (FRP) — A general term for a composite that is reinforced with cloth, mat, strands or any other fiber form.
Fiberglass chopper — chopper guns, long cutters and roving cutters cut glass into strands and fibers to be used as reinforcement in plastics.
Fiber Orientation — Direction of fiber alignment in a non woven or mat laminate wherein most of the fibers are placed in the same direction to afford greater strength in that direction.
Filaments — Individual fibers of indefinite length used in tows, yarns, or roving.
Fill Threads — The crosswise fibers woven at 90º to the warp fibers; also known as the weft or woof.
Filler — Inert material added to the mixed resin to increase viscosity, improve appearance, and lower density and cost.
Filler Ply — Additional patch to fill in a depression in repair or to build up an edge.
Fillets — angles cut on the edges of core that will be in or on the finished part.
Flash point – Lowest temperature at which a substance emits enough vapors to form a flammable origin table mixture with air near the surface of the substance being tested.
Flexural Modulus — The ratio, within the elastic limit, of the applied stress on a test sample in flexure to the corresponding strain in the outermost fibers of the sample.
Flexural Strength — The strength of a material in bending, usually expressed in force per unit area, as the stress of a bent test sample at the instant of failure.
Flood — a puddle of excess resin on top of or in the laminate being infused. Usually caused by loss of vacuum or due to a siphoning effect.
Flow – the movement of resin under pressure, allowing it to fill all parts of the fiber preform. Flow can be horizontal or vertical or both.
Flow Front — The leading edge of moving resin in a fiber preform while it is being molded using closed molded.
Fracture — A rupture of the surface of a laminate due to external or internal forces; may or may not result in complete separation.
Fracture toughness — A measure of the damage tolerance of a material containing initial flaws or cracks.
Free radicals – Highly reactive molecular fragments capable of initiating chemical reactions such as polymerization of polyester or vinyl ester resins.
Fumed Silica (Aerosil, Cabosil) — A thickening agent used in resins to increase flow or sag resistance qualities.
Gel — The initial jelly-like solid phase that develops during the formation of a resin from a liquid.
Gel coat — 1.Pigmented resins applied as a coating to a mold or part to produce a smooth finish on the part. The gel coat is a protective layer for the composite. 2. — A quick setting resin applied to the surface of a mold and gelled before lay-up.
Gel time —Period of time from initial mixing of liquid reactants of the liquid material composition to the point when gelation occurs, as defined by a specific test method. Gelation is the point when the resin viscosity has increased to the point such that it barely moves when probed with a sharp object.
Graphitization — The process of pyrolization at very high temperatures (up to 5,400º F) that converts carbon to its crystalline allotropic form.
Green strength — The ability of a material, while not completely cured, set or sintered, to undergo removal from the mold and handling without distortion.
Hand lay up — A fabrication method in which reinforcement layers, pre-impregnated or coated afterwards, are placed in a mold by hand, then cured to the formed shape.
HAP — Hazardous air pollutants
Hard tool — A tool made of metallics or and ”hard” material that is generally impervious to damage during normal use.
Hardener — Substance used to promote or control curing action by taking part in it, as opposed to a catalyst.
Hazardous air pollutants: pollutants
HDT — heat distortion temperature
Heat — Term used colloquially to indicate any temperature above ambient (room) temperature to which a part or material is or will be subjected.
Heat-Distortion Temperature (HDT) — Temperature at which deflection occurs under specified temperature and stated load.
Heat resistance — The property or ability of plastics and elastomers to resist the deteriorating effects of elevating temperatures.
Honeycomb — Resin-impregnated material, most commonly manufactured in hexagonal cells, that serves as a core in sandwich structure. May also be metallic or polymer in rigid, open-cell structure.
Hot pot — catalyst is mixed with the gel coat or resin in the material container prior to spraying, as opposed to internal or external gun mixing.
Humidity — Moisture content of the air.
Hybrid Composite — A composite with two or more types of reinforcing fibers. A combination of two or more different fibers, such as carbon and glass or carbon and aramid, into a structure. Tapes, fabrics and other forms maybe combined; usually only the fibers differ.
Hybrid resin — composite with two or more types of chemistries combined.
Hydrophobic – Moisture resistant capability, moisture repelling.
Hygroscopic – Moisture absorbing capability.
Impact Strength — a material’s ability to withstand shock loading as measured by fracturing a specimen.
Impregnate — To saturate the voids and interstices of a reinforcement with a resin.
Inhibitor — A material added to a resin to retard polymerization. Lengthens the gel time.
Interlaminar — Existing or occurring between two or more adjacent laminae.
Interlaminar shear — Shearing force that produces displacement between two laminae along the plane of their interface.
Isotropic — Fiber directionality with uniform properties in all directions, independent of the direction of applied load.
Jackstrawing — Prominence of fiberglass pattern having turned white in the laminate because the glass has separated from the resin due to excessive exothermic heat, usually associated with thick resin rich laminates.
Kevlar — Trademark of Du Pont for aramid fibers used as a reinforcement fiber. An organic polymer composed of aromatic polymide having a para-type orientation (parallel chain extending bonds from each aromatic nucleus).
Knitted fabrics — Fabrics produced by interlooping chains of yarn, rather than weaving, allowing a more drapable fabric, but typically yielding a lower fiber content.
Lag — in closed molding, the difference between the flow of resin on the surface of the fabric lay up and the flow on the bottom of the lay up.
Lamina – a single ply or layer in a laminate made up of a series of layers (organic composite). A flat or curved surface containing unidirectional fibers or woven fibers embedded in a matrix.
Laminae – plural of lamina
Laminate –(noun) the product evolved by bonding resin-impregnated reinforcement and sandwich materials. The laminate generally contains multiple plies (layers) of mat, roving, chopped strand saturated with resin.
Laminate (verb) — The process of preparing a laminate (noun) by uniting plies of reinforcement or sandwich materials or chopped glass with resin.
Laminate Ply — One fabric/resin or fiber/resin layer that is bonded to adjacent layers in the curing process.
Layup — Placement of layers of reinforcement in a mold.
Longitudinal — Running in the direction of forward and aft.
Low profile — Rein compounds formulated for low, zero, or negative shrinkage during molding.
MACT — maximum achievable control technology- as specified by the EPA for limits or VOC emissions, such a styrene.
Mandrel — Elongated mold around which resin-impregnated fiber, tape or filaments are wound to form structural shapes or tubes.
Mass — The quantity of matter contained in a specific body.
Master (plug) –The permanent tool used to build molds for the manufacture of fiberglass parts.
Mat — A fibrous reinforcing material composed of chopped filaments (for Chopped Stand Mat) or swirled filaments (for continuous-strand mat) with a binder applied to maintain form; available in blankets of various widths, weights, thicknesses and lengths.
Matrix — Material in which reinforcing fiber of a composite system is embedded; polymer, metal, ceramic.
MEK Peroxide (MEKP) — abbreviation for methyl ethyl ketone peroxide; a free radical source commonly used as the initiator for polyesters in the FRP industry.
MEK (solvent) – abbreviation for methyl ethyl ketone; a colorless flammable liquid commonly used in clean up procedures.
Microcracking — Microscopic cracks formed in composites when thermal stresses locally exceed the strength of the matrix.
MIL — the unit used in measuring film thickness and the diameter of fiber strands, glass, wire, etc., (one mil = .001 inch).
Milled fibers –carbon or glass used for making fiber-filled putty or BMC strands milled into short fiber lengths of 1/32, 1/16, 1/8 and 1/4 inch.
Modulus — Measure of the ratio of applied load (stress) to the resultant deformation of a material, such as elasticity or shear. Can be low, intermediate, high or ultrahigh.
Moisture Absorption — Pickup of water vapor from air by a material. Refers to vapor withdrawn from the air only as distinguished from water absorption, which is weight gain due to absorption of water by immersion.
Mold — 1 (verb) to shape plastics parts by heat and pressure. 2 (noun) the cavity or matrix into/onto which the plastics composition is placed and from which it takes its form. Female – made into. Male – made onto. 3. the assembly of all parts that function collectively in the molding process.
Mold coat — a coat of resin over the bare mold. Used to seal the mold and make a smooth surface on which to fabricate parts.
Mold release — a substance used on the mold or in the compound to prevent sticking and for ease of part releases.
Monomer — A single molecule that reacts with like or unlike molecules to form a polymer. The smallest repeating structure of a polymer; for addition polymers, this represents the original unpolymerized compound. Styrene is the predominant monomer used for reinforced polyester material.
Monomers – Reactive diluents blended with the polymer to yield liquid resins.
Multifilament — A yarn consisting of many continuous filaments.
NCF — Non crimp fabrics, utilize unidirectional fiber reinforcement or combinations of random fiber materials which are assembled in stacks, and lightly stitched together, instead of interlacing.
NDI — Non Destructive Inspection, inspections of composites that do not cause damage or require repair of inspection area, such as ultrasonic testing
Near-Net Shape — Part fabrication resulting in final dimensions that require minimal machining or cutting.
NESHAP — National Emission Standards for Hazardous Air Pollutants. Federal standard that will require molders to adopt what the EPA has determined is the Maximum Achievable Control Technology (MACT) for the release of Hazardous Air Pollutants and/ or Volatile Organic Compounds.
Net Shape — Part fabrication resulting in final dimensions that do not require machining or cutting.
Nondestructive Inspection (NDI) — Determining material or part characteristics without permanently altering the test subject. Nondestructive testing (NDT) and nondestructive evaluation (NDE) are broadly considered synonymous with NDI.
Nonwoven Roving — A reinforcement composed of continuous rovings loosely gathered together.
One-off — A fabrication process in which a single part is fabricated.
Orthotropic – Having three mutually perpendicular planes of elastic symmetry.
Outgassing — Release of solvents and moisture from composite parts under the hard vacuum of space.
Peel Ply — Layer of material (usually a tightly woven nylon) applied to a fiber lay-up surface while the resin is still wet. It is removed from the cured laminate prior to bonding operations, leaving a clean, resin rich surface ready for bonding. Peel Ply is also used in vacuum bagging laminating to produce the same surface.
Peel strength — Strength of an adhesive bond obtained by stress that is applied in a “peeling” mode.
Peroxides — category of compounds containing an unstable o-o (or o-oh) group. Oxygen to oxygen atoms, used as initiators.
Pigments — The ingredient used to impart color, as in gel coats.
Pigment separation — a mottled (varied colored) appearance of the gel coat surface.
Pin holes — Small holes on the surface of a laminate or gel coat, usually caused by the contaminates on the mold used.
Pitch — Residual petroleum product used in the manufacture of certain carbon fibers.
Plug — see master.
Ply — One of the layers that makes up a stack or laminate: also, the number of single yarns twisted together to form a plied yard.
Ply schedule — Lay-up of individual plies or layers to form a laminate. Plies may be arranged in alternating fiber orientation to produce multidirectional strength in a part. Also known as a lamination schedule.
Polyester resin – the term generally used for unsaturated polyesters. Formed by the reaction of dibasic organic acids and polyhydric alcohols. Also meant to include the cross-linking diluent included with the polyester molecules.
Polymer — large molecules formed by combining many smaller molecules or monomers in a regular pattern.
Polymerization — Chemical reaction that links monomers to form polymers.
Porosity — The presence of visible voids within a solid material into which either air or liquids may pass. The formation of undesirable clusters of air bubbles or other voids in the surface or body of laminates.
Postcure — Additional exposure to elevated temperature, often occurring without tooling or pressure, that improves the mechanical properties. In certain resins, complete cure and ultimate mechanical properties are attained only by exposure of the cured resin to higher temperatures than those of curing.
Pot Life — Length of time in which a catalyzed thermosetting resin retains sufficiently low viscosity for processing.
Preform — 1. a fibrous reinforcement preshaped on a mandrel or mock-up to approximate contour and thickness desired in the finished part. 2. The same fibrous reinforcement as in 1, but formed on a core material, such a foam, to form a near net shaped component, such as a stiffener. 3. The fiber lay up inside (or on) a mold that is formed in or on the mold.
Pre-release — Premature release of the gel coat or laminate from the mold.
Pressure Bag Molding – A process for molding reinforced plastics in which a tailored, flexible bag is placed over the contact lay-up on the mold, sealed, and clamped in place. Fluid pressure, usually provided by compressed air or water, is placed against the bag, and the part is cured.
Primary laminate — bulk or second laminate, laminate applied after the skin coat has cured. Primary laminate consists of the fiber reinforcement ply(s) which supply most of the strength of the laminate. Generally thicker than the skin coat.
Promoted Resin – Resin with accelerator added but not catalyst. Primarily applicable to room temperature cure resins.
Promoters – Additives that accelerate the decomposition of peroxides. Some additives work synergistically with others in the decomposition of peroxides.
Puncture — Break in composite skin in sandwich structures that may or may not go through to the core material or completely through the part.
Quasi-isotropic — Approximating isotropy by orienting plies in several directions.
Race tracking — In closed molding, the resin finding an unplanned channel to speed through, bypassing the lamination, and going directly into the vacuum source.
RARTM — rubber assisted resin transfer molding.
Reinforcement — Key element added to the matrix to provide the required properties (primarily strength); ranges from short fibers though complex textile forms.
Release agent — used to prevent cured matrix material from bonding to tooling; usually sprayed or painted on mold.
Resin — Polymer with indefinite and often high molecular weight and softening or melting range that exhibits a tendency to flow when subjected to stress. As composite matrices, resins bind together reinforcement fibers.
Resin content – the amount of resin in a laminate expressed as a percentage of either total weight or total volume.
Resin-Rich — filled with excess resin as compared to consistent resin/fiber ratio.
Resin-Starved — Lacking sufficient resin for fiber wet out.
Resin Transfer Molding (RTM) — a molding process in which catalyzed resin is pumped into a two-sided, matched mold where a fibrous reinforcement has been placed. The mold and/or resin may or may not be heated. RTM offers the ability to consolidate structural parts. Its major drawback is the high cost of the initial, two-sided mold.
Resin-rich area – Space which is filled with resin and lacking reinforcing material.
Resin viscosity — Viscous property of a resin system or solid to liquid transition resistance to flow, which can be altered by temperature and pressure as necessary to achieve desired flow characteristics.
RFI — resin film infusion, method of closed molding in which a film of viscous resin is added to a dry preform, and heated to flow the resin through the preform.
RIM reaction injection molding
RRIM — reinforced reaction injection molding
Roving — A collection of bundles of continuous filaments either as untwisted strands or as twisted yarn.
RTM — resin transfer molding. Closed mold process using liquid resin under pressure to saturate a dry fiber preform in mold.
Sandwich structure — Composite composed of lightweight core material (usually honeycomb or foam) to which two relatively thin, dense, high strength, functional or decorative skins are adhered.
Scarf Joint — A lapped joint made by beveling off, notching, or otherwise cutting away the edges of two adjoining surfaces, so that they may be joined together, usually without increasing the original thickness.
Sealant — A paste or liquid applied to a point that hardens in place to form a seal.
Sealant tape — a thick double-sided sticky tape used to seal bag film to a mold.
Set up – to harden, as in curing of a polymer resin.
Shear — An action or stress resulting from applied forces that causes or tends to cause two contiguous parts of a body to slide relative to each other.
Shear Rate – With regard to viscous fluids, the relative rate of flow or movement.
Shelf Life — Length of time in which a material can be stored and continue to meet specification requirements, remaining suitable for its intended use.
Sizing — Compound that binds together and stiffens warp yarn, providing abrasion resistance during weaving. Sizing is normally removed and replaced with finish before matrix application.
Skin — A layer of relatively dense material used on the surface of the core of a sandwich structure.
Skin coat – A layer of resin and chopped strand mat applied just under the gel coat to prevent blistering, and provide protection to the primary laminate.
Soft tool — A tool made of composites or similar “soft” material that is vulnerable to damage during use, storage or transportation.
Solvent — A liquid used to dissolve, dilute, and clean materials.
Specific Gravity — Density (mass per unit volume) of a material divided by that of water at a standard temperature.
SRIM — structural reaction injection molding
Stiffness — Relationship of load to deformation for a particular material.
Storage life — Amount of time a material can be stored and retain specific properties.
Strain — Elastic deformation resulting from stress.
Stress — Internal resistance change in size or shape, expressed in force per unit area.
Stress Concentration — The magnification of applied stress in the region of a notch, void, hole, or inclusion.
Stress Corrosion — Preferential attack of areas under stress in a corrosive environment, where such an environment alone would not have caused corrosion.
Stress Crack — External or internal cracks in a composite caused by tensile stresses; cracking may be present internally, externally, or in combination.
Structural Adhesive — an adhesive used to transfer loads between adherents.
Structural Bond — a bond joining load-bearing components of an assembly.
Substrate — A material upon the surface of which an adhesive-containing substance is spread for any purpose, such as bonding or coating.
Tensile strength — Maximum tensile stress sustained by a composite specimen before it fails in a tension test.
Thermal conductivity — Ability to conduct heat.
Thermal Stress Cracking — Crazing and cracking of some thermoplastic resins from overexposure to elevated temperatures.
Thermoplastic — Composite matrix in advanced composites formed by heat and cooling. Can be reshaped more than once.
Thermoset — Composite matrix cured by heat and pressure or with a catalyst into an infusible and insoluble material. Once cured, a thermoset cannot be returned to the uncured state.
Thermosetting polyesters – a class of resins produced by dissolving unsaturated, generally linear, alkyd resins in a vinyl-type active monomer such as styrene, methyl styrene, or diallyl phthalate.
Thixotropic — materials that are gel-like at rest, but fluid when agitated; having high static shear strength at the same time; losing viscosity under stress.
Tooling resins — plastic resins, chiefly epoxy and silicone that are used as tooling aids.
Toughness — a measure of the ability of a material to absorb energy.
Tow — an untwisted bundle of continuous filaments, usually designated by a number followed by K, indicating multiplication by 1,000 (for example, 12k tow has 12,000 filaments).
Uni-directionals — Referring to fibers that are oriented in the same direction, such as unidirectional fabric, tape, or laminate. Often called UD or uni.
UV resin — resin system designed to cure only with the application of ultraviolet light
VARTM — vacuum assisted resin transfer molding. Uses vacuum to assist in filling fiber preform, also may use pressure to inject resin into mold.
Vacuum bag molding — Molding technique where a resin impregnated laminate is cured inside a sealed bag film which entrapped air is removed by vacuum, and fiber reinforcement is compressed by vacuum pressure.
Vinyl ester – a family of thermosetting resins having no ester linkages along the polymer chain but present only at the ends of the molecule providing improved corrosion resistance.
VIP — Vacuum infusion process. Generic term to include any infusion process which uses vacuum as the sole force to saturate fiber preform.
Viscosity — Tendency of a material to resist flow. The ratio of shear stress to shear strain. Measured in centipoise at 77 F.
VOC — volatile organic compounds
Voids — Pockets of entrapped gas that have been cured into a laminate.
Volatiles — Materials in a sizing or resin that can be vaporized at room or slightly elevated temperatures.
Warp —Yarns running lengthwise and parallel to the narrow edge of woven fabric.
Warpage — Dimensional distortion of a composite part.
Water Absorption — Ratio of weight of water absorbed by a material to the weight of dry material.
Weave — Pattern by which a fabric is formed from interlacing yarns. In plain weave, warp and fill fibers alternate to make both fabric faces identical. In satin weave, pattern produces a satin appearance with the warp tow over several fill tows and under the next one (for example, eight harness satin would have warp tow over seven fill tows and under the eighth).
Weft — Yarns running perpendicular to the warp in a woven fabric. Also called woof.
Wet Layup — Application of a resin to a dry reinforcement in the mold.
Wet out — Saturation with resin of all voids between strands and filaments.
Woof — Yarns running perpendicular to the warp in a woven fabric. Also called weft.
Woven roving — Heavy, coarse fabric produced by weaving continuous roving bundles.
Wrinkle — Imperfection in the surface of a laminate that looks like a crease in one of the outer layers.
X-Axis — The axis in the plane of the laminate used as 0º reference.
Y-Axis — The axis in a laminate that is perpendicular to the x-axis.
Yarn — Continuously twisted fibers or strands suitable for weaving into fabrics.
Z-axis — the reference axis normal to the laminate plane in composite laminates.
Zero bleed — Laminate fabrication procedure that does not allow loss of resin during cure.