Plastic Glossary

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A

Ablative Heat absorption through a decomposition process called pyrolysis or near the exposed surface.
Accelerator A chemical additive that hastens cure or chemical reaction.
Adhesive A substance applied to mating surfaces to bond them by surface attachment.
Adhesive Film A thin plastic film onto which premixed adhesives are cast.
Aliphatic Designates a large class of organic compounds having open-chain structures, isopropyl alcohol being one example.
Anisotropic Fiber directionality in which different properties are exhibited when tested along axes of different directions.
Aramid A high-Strength, high-stiffness aromatic polyamide fiber.
Areal Weight Weight of a fiber reinforcement per unit area (width times length) of tape or fabric.
Aspect Ratio Ratio of length to diameter of a fiber.
Autoclave Molding A molding technique in which an entire assembly (layup) is placed into an autoclave (or closed vessel with pressure/heat capability) at 50 to 100 psi pressure to consolidate the part laminate by removing entrapped air and volatiles.
Automated Tape Laying A fabrication process in which an automated machine system lays prepreg tape in a preprogrammed pattern to lay up the ply schedule on an open mold.

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Bag Molding A molding technique in which the composite material is placed in rigid mold and covered with a flexible bag, with pressure applied by vacuum, autoclave, press or by inflating the bag.
Balanced Design In filament winding, a winding pattern designed so that the stressed in all filaments are equal.
Balanced Laminate A laminate in which all laminae except those at 0 degrees / 90 degrees are placed in plus / minus pairs (not necessarily adjacent) symmetrically around the lay up centerline.
Barcol Hardness A surface hardness value obtained by measuring the penetration resistance of a given material to a sharp steel point under a spring load. The Barcol Impressor is an instrument that measures hardness on a 0-100 scale.
Basket Weave Woven reinforcement wherein two or more warp threads go over and under two or more filling threads in a repeat pattern.This weave is less stable than the plain weave but produces a flatter, stronger more pliable fabric.
Batch (or lot) Material made with the same process at the same time having identical characteristics throughout.
Bias Fabric A fabric in which warp and fill fibers are at an angle to the length.
Biaxial Winding Filament winding wherein helical bands are laid in sequence, side by side, with no haps or overlap between the fibers.
Bidirectional Laminate A laminate with fibers oriented in more than one direction on the
same plane.
Binder The agent applied to glass mat or preforms to bond the fibers prior to laminating or molding.
Bleeder Cloth A layer of woven or woven material, not a part of the composite, that allows excess has and resin to escape during cure.
Bleedout Excess liquid resin appearing at the surface, primarily occurring during filament winding.
Bond Ply A ply or fabric patch that comes in contact with a honeycomb core.
Bond Strength The degree of adhesion between bonded surfaces. The stress required to separate a layer of material from the base to which it is bonded, as measured by load/bond area.
Boron Filament A strong, lightweight fiber, with a high strength-to-weight ratio used as a reinforcement.
Boron Fiber A fiber usually of a tungsten-filament core with elemental boron vapor deposited onto it to impart strength and stiffness.
Braid A woven tubular shape used instead of a flat fabric for reinforcement.
Breakout Separation or breakage of fibers when the edges of a composite part aredrilled 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 production.
Bridging Fabric plies over a curved edge that do not come in full contact with the core material. Also, excess resin that has formed on edges during the curing process.
Broadgoods Fiber woven or stitched into fabrics that may or may not be impregnated with resin: usually furnished in rolls.
Bromine A fire retardant (halogen) used to reduce or eliminate a resin’s tendency to burn.
Buckling A failure mode usually characterized by fiber deflectionrather than breakage due to compressive action.
Bulk Molding Compound (BMC) A premixed blend of thermosetting resin, reinforcements, catalysts and fillers for use in compression-, transfer- or injection-molding processes.

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CAD/CAM Computer-aided design / Computer-aided manufacturing.
Carbon Fiber Reinforcing fiber known for its lightweight, high strength and high stiffness. Fibers are produced by high temperature treatment of an organic precursor fiber based on PAN (polyacrylonitrile), rayon or pitch in an inert atmosphere at temperatures above 1,800 degrees F. Fibers can be pyrolized by removing still more non-carbon atoms via heat-treating above 3,000 degrees F.
Carbon A composite of carbon fiber in a carbon matrix.
Cast Polymer A nonreinforced composite (resin used without reinforcing fibers) that combines polymers, fillers and additives as composites to meet specific application requirements.
Catalyst A substance that promotes or controls curing of a compound without being consumed in the reaction.
Catalyzed Resin A resin mixture possibly still in the workable states, after it has been mixed with the catalyst or hardener.
Catenary Uniformity of strand length in a specified length of roving stretched under tension. Poor catenary means some strands in the roving length are longer than others.
Caul Sheet A plate or sheet the same size and shape used in contact with a composite layup to transmit normal pressure and temperature during cure.
Centipoise (cps) A unit of measure used to designate a fluid’s viscosity (At 70 degrees F. water is 1 cps; peanut butter is 250,000 cps).
Centrifugal Casting A processing technique for fabricating cylindrical structures, in which the composite material is positioned inside hollow mandrel designed to be heated and rotated as resin is cured.
Charge Pattern The ply schedule used in parts made from sheet molding compound (SMC); a pre-weighed number of SMC plies cut from an SMC sheet and oriented to fill the mold cavity when placed in the mold and compressed.
Chopped Strand Continuous roving that is chopped into short lengths for use in mats, spray up or molding compounds.
Circumferential Winding The process of winding filaments perpendicular to the axis during filament winding.
Co-Cured Cured and simultaneously bonded to another prepared surface.
Coefficient of Expansion A measure of the change in length or volume of an object.
Coefficient of Friction The resistance of a material to sliding forces where, the higher the number, the higher the friction.
Coefficient of Linear Thermal Expansion A measure of growth of a material in length or volume as temperature changes.
Cohesion Adherence of a single substance to itself. Also, the property holding a single substance together.
Composite A material that combines fiber and a binding matrix to maximize specific performance properties. Neither element merges completely with the other.
Compression Molding A technique for molding thermoset plastics in which a part is shaped by placing the fiber and resin into an open-mold cavity, closing the mold, and applying heat and pressure until the material has cured or achieved its final form.
Compressive Modulus Of Elasticity The ratio of force to deformation as a material is being squeezed.
Compressive Strength The capacity to resist a crushing or buckling force; the maximum compressive load a specimen sustains divided by its original cross-sectional area.
Condensation Polymerization A polymerization reaction in which simple byproducts (e.g.,water) are formed.
Consolidation A 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 materials such as mat and woven roving saturated with resin, are applied and to a mold. The cure occurs either at room temperature using a catalyst-promoter system or by heating in an oven with no additional pressure.
Contaminant An impurity or foreign substance that 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 Single or multiple strands of parallel filaments coated with sizing and wound into a cylindrical package. It may be used to provide continuous reinforcement in woven roving, filament winding, pultrusion, prepregs, or high-strength molding compounds. It may also be chopped. (See Chopped Strand.)
Core In sandwich construction, the central component to which inner and outer skins are attached. Foam, honeycomb, paper and wood are all commonly used as core material.
Core Crush Compression damage of the core.
CoreDepression A gouge or indentation in the core material.
Core Orientation The standard used on a honeycomb core to line up the ribbon direction, thickness of the cell depth, cell size and transverse direction.
Core Separation A breaking of honeycomb core cells.
Core Splicing Joining two core segments by bonding them together.
Co-woven Or Hybrid Fabric A reinforcement fabric woven with two different types of fibers in individual yarns, e.g., thermoplastic fibers woven side by side with carbon fibers.
Crazing A region of ultrafine cracks that may develop on or under a resin surface.
Creel A device for holding the required number of roving spools or other supply packages of reinforcement in the desired position for unwinding.
Creep The dimensional change in a material under physical load over time beyond instantaneous elastic deformation.
Cross Lamination Layers oriented at various angles to other layers with respect to the laminate grain. A cross-ply laminate usually has plies oriented only at 0°/90°. (See Fiber Architecture.)
Cross-Linking Polymerization reactions that branch out from the main molecular chain to form a networked pattern of chemical links.
Cross-Ply Laminate A laminate with plies oriented at 0° and 90° only.
Crystalline A molecular structure in which the atoms are arranged in an orderly, three-dimensional pattern.
CTE See coefficient of thermal expansion.
Cure To irreversibly change the molecular structure and physical properties of a thermosetting resin 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 or Hardener A catalytic or reactive agent that brings about polymerization when added to a resin.

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Damage Tolerance A measure of the ability of structures to retain load-carrying capability after exposure to sudden loads (for example, ballistic impact).
Damping Diminishing the intensity of vibrations.
Debonding An unplanned separation of bonded surfaces.
Delamination The separation of ply layers due to adhesive failure or the separation of layers of fabric from the core structure. A delamination may be associated with bridging, drilling and trimming.
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 of 9,000 meters of yarn.
Design Allowable A limiting value for a material property that can be used to design a structural or mechanical system to a specified level of success with 95% statistical confidence.
Dielectric Nonconductor of electricity; the ability of a material to resist the flow of an electrical current.
Dielectric Constant The ability of a material to store an electrical charge.
Dielectric Strength The amount of volts/mil required to cause an electrical “breakthrough”. The voltage required to penetrate insulating
material. Material with high dielectric strength offers excellent electrical insulating properties.
Dimensional Stability: Change in height, width, and shape when exposed to changes in temperature.
Dissipation Factor The ability of a material to dissipate an electrical charge.
Doubler An extra layer of reinforcement for added stiffness or strength where fasteners or other abrupt load transfers occur.
Draft Angle A mandrel’s taper or angle for ease of part removal.
Draft The degree of taper allowed on the sides of a mold so the part can be removed.
Drape The ability of fabric (or prepreg) to conform to the shape of a contoured surface.
Dry Winding A filament winding operation in which resin is not used.
Durometer Hardness The ability of a non-metallic material to resist indentation using a Shore tester.

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E-Glass(Electrical Glass) Borosilicate glass fibers most often used in conventional polymer matrix composites.
Elastic Limit The greatest stress a material is capable of sustaining without permanent strain remaining after complete release of the stress.
Elasticity The property by which materials tend to recover their original size 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 percentage of elongation.
End A strand of roving consisting of a given number of filaments gathered together. The strand is considered an end or strand before twisting.
End Count An exact number of strands contained in a roving.
Engineering Plastics A general term covering all plastics, with or without fillers or reinforcements, that have mechanical, chemical and thermal properties suited for use as construction materials, machine components and chemical processing equipment components.
Epoxy Plastics Thermoset resins made by the reaction of epoxides or oxiranes with other materials such as amines and alcohols; used as a matrix resin in reinforced composites and structural adhesives.
Epoxy Resin A common thermoset material used as a bonding matrix to hold fibers together. When mixed with a catalyst, epoxy resins are resistant to chemicals and water and are unaffected by heat or cold.
Exotherm Heat released during a chemical reaction (e.g., curing).
Exothermic Characterized by a chemical reaction to heat.
Extenders Low-cost materials used to dilute or extend high-cost resins without extensive lessening of properties.

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Fabric, Nonwoven A material formed from fibers or yarns without interlacing (e.g., stitched nonwoven broadgoods).
Fabric, Woven A material constructed of interlaced yarns, fibers or filaments.
Fabrication The process of making a composite part or tool.
Fatigue The failure of a material’s mechanical properties caused by repeated stress over time.
Fatigue Strength The maximum cyclical stress a material can withstand for a given number of cycles before it fails.
Fiber Filamentary material.
Fiber Architecture The design of a fibrous part in which the fibers are arranged in a particular way to achieve the desired result. This may include braided, stitched or woven fabrics, mats, rovings or carbon tows.
Fiber Bridging Reinforcing fiber material bridging an inside radius of a pultruded product. The condition is caused by shrinkage stresses around such a radius during cure.
Fiber Content The amount of fiber in a composite expressed as a ratio to the matrix. (The most desirable fiber content is a 60:40 ratio, or 60 percent fiber and 40 percent matrix resin.)
Fiber Reinforced Plastics (FRP) A composite material or part that consists of a resin matrix containing reinforcing fibers such as glass or carbon having greater strength or stiffness than the resin. The term FRP is most often used to denote glass fiber-reinforced plastics; the term “advance composite” usually denotes high-performance aramid or carbon fiber-reinforced plastics.
Fiber Wash Processing distortion wherein resin flow may unevenly bunch or spread fibers in an area of the part.
Fiberglass Reinforcing fiber made by drawing molten glass through bushings. The predominant reinforcement for polymer matrix composites, it is known for its good strength, processability and low cost.
Filament Winding An automated process for fabricating composites in which continuous roving, either preimpregnated with resin or drawn through a resin bath, is wound around a rotating mandrel.
Filaments Individual fibers of indefinite length used in tows, yarns or roving.
Fill Threads (Or Weft or Woof) The crosswise fibers woven at 90° to the warp fibers.
Filler Material added to the mixed resin to increase viscosity, improve appearance and/or lower density and cost.
Filler Ply An additional patch used to fill in a depression in repair or to build up an edge.
Film Sheeting having a nominal thickness not greater than 0.25 mm (0.010 inches).
Fabric, Nonwoven A material formed from fibers or yarns without interlacing (e.g., stitched nonwoven broadgoods).
Fabric, Woven A material constructed of interlaced yarns, fibers or filaments.
Fabrication The process of making a composite part or tool.
Fatigue The failure of a material’s mechanical properties caused by repeated stress over time.
Fatigue Strength The maximum cyclical stress a material can withstand for a given number of cycles before it fails.
Fiber Filamentary material.
Fiber Architecture The design of a fibrous part in which the fibers are arranged in a particular way to achieve the desired result. This may include braided, stitched or woven fabrics, mats, rovings or carbon tows.
Fiber Bridging Reinforcing fiber material bridging an inside radius of a pultruded product. The condition is caused by shrinkage stresses around such a radius during cure.
Fiber Content The amount of fiber in a composite expressed as a ratio to the matrix. (The most desirable fiber content is a 60:40 ratio, or 60 percent fiber and 40 percent matrix resin.)
Fiber Reinforced Plastics (FRP) A composite material or part that consists of a resin matrix containing reinforcing fibers such as glass or carbon having greater strength or stiffness than the resin. The term FRP is most often used to denote glass fiber-reinforced plastics; the term “advance composite” usually denotes high-performance aramid or carbon fiber-reinforced plastics.
Fiber Wash Processing distortion wherein resin flow may unevenly bunch or spread fibers in an area of the part.
Fiberglass Reinforcing fiber made by drawing molten glass through bushings. The predominant reinforcement for polymer matrix composites, it is known for its good strength, processability and low cost.
Filament Winding An automated process for fabricating composites in which continuous roving, either preimpregnated with resin or drawn through a resin bath, is wound around a rotating mandrel.
Filaments Individual fibers of indefinite length used in tows, yarns or roving.
Fill Threads (Or Weft or Woof) The crosswise fibers woven at 90° to the warp fibers.
Filler Material added to the mixed resin to increase viscosity, improve appearance and/or lower density and cost.
Filler Ply An additional patch used to fill in a depression in repair or to build up an edge.
Film Sheeting having a nominal thickness not greater than 0.25 mm (0.010 inches).
Film Adhesive An adhesive in the form of a thin, dry resin film with or without a carrier. Commonly used for adhesion between laminate layers.
Finish Material applied to fibers (after sizing is removed) to improve bonding between resin and fiber.
Finite Element Analysis (FEA) A process of selecting the optimum combination of materials in a composite based on software analysis.
Flammability Rating The ability of a material to harbor a flame.
Flexural Modulus Of Elasticity The ratio of force to deformation when a material is being flexed or bent. 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.
FlexuralStrength The resistance of a material to being bent or flexed. The strength of a material in bending expressed usually in terms of force per unit area, as the stress of a bent test sample at the instant of failure.
Fracture A rupture of the surface of a laminate, due to external or internal forces; may or may not result in complete separation.
Film Adhesive An adhesive in the form of a thin, dry resin film with or without a carrier. Commonly used for adhesion between laminate layers.
Finish Material applied to fibers (after sizing is removed) to improve bonding between resin and fiber.
Finite Element Analysis (FEA) A process of selecting the optimum combination of materials in a composite based on software analysis.
Flammability Rating The ability of a material to harbor a flame.
Flexural Modulus Of Elasticity The ratio of force to deformation when a material is being flexed or bent. 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.
FlexuralStrength The resistance of a material to being bent or flexed. The strength of a material in bending expressed usually in terms of force per unit area, as the stress of a bent test sample at the instant of failure.
Fracture A rupture of the surface of a laminate, due to external or internal forces; may or may not result in complete separation.

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Halogenated Resin A resin combined with chlorine or bromine to increase fire retardancy.
Hand Layup A fabrication method in which reinforcement layers are placed in a Mold by hand, saturated with resin, then cured to the formed shape.
HAPs Hazardous Air Pollutants, as determined by the Environmental Protection Agency (EPA).
Hardener A substance that reacts with resin to promote or control (Or Curing Agent) curing action.
Heat A term used colloquially to indicate any temperature above ambient (Room) temperature to which a part or material is or will be subjected.
Heat Deflection Temperature The temperature at which a material will bend a given distance when a given load is applied.
Heat-Distortion The temperature at which a test bar deflects a certain amount under specified temperature and stated load.
Helical Describing ply laid onto a mandrel at an angle, often a 45° angle.
Helix Angle The angle at which continuous filaments are wound in relation to the longitudinal mandrel axis in the filament-winding process.
Honeycomb A lightweight cellular structure made from either metallic sheet materials or nonmetallic materials (e.g., resin-impregnated paper or woven fabric) and formed into hexagonal nested cells.
Hoop Ply laid onto a mandrel at a 90° angle.
Hoop Stress Circumferential stress in a cylindrically shaped part as a result of internal or external pressure.
Hybrid Composite A composite made with two or more types of reinforcing fibers.
Hygroscopy A material’s readiness to absorb or retain moisture.

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The ability of a material to resist fracture at a machined “V” notch when hit with a swinging weight.
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.
Impregnated Fabric SeePrepreg.
In Situ (in the original position) — In filament winding, designates a mandrel that remains in place after winding, as opposed to a mandrel that is removed after winding.
Inhibitor A chemical additive that slows or delays cure cycle.
Injection Molding A method of forming a plastic to the desired shape by forcibly injecting the polymer into a mold.
Integral Heating A system in which heating elements are built into a tool, forming part of the tool and usually eliminating the need for an oven or autoclave as a heat source.
Interface The surface between two materials (in glass fibers, for instance, the area at which the glass and sizing meet; in a laminate, the area at which the reinforcement and laminating resin meet.)
Interior Components Finished internal aircraft components usually made of glass fiber composites, including overhead stowbins, sidewalls, floor panels, ceiling panels, laboratories and galleys.
Interlaminar Existing or occurring between two or more adjacent laminae.
Interlaminar Shear A shearingforce that produces displacement between two laminae along the plane of their interface.
Isocyanate A highly reactive monomer used in reaction injection molding (RIM).
Isotropic Fiber directionality with uniform properties in all directions, independent of the direction of applied load.
Isotropic Laminate A laminate in which the strength properties are equal in all directions, such as contact-molded laminates or metals.

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Kevlar A strong, lightweight aramid fiber trademarked by DuPont and used as a reinforcement fiber.

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Laminate To unite layers with a bonding material, usually via pressure and heat.
Laminate Ply A fabric/resin or fiber/resin layer that is bonded to adjacent layers in the curing process.
Lap Joint A joint made by overlapping two parts and bonding them together.
Layup Placement of layers of reinforcement in a mold.
Liner The continuous, usually flexible, reinforced resin barrier on the inside surface of a plastic or thermoset laminate used to protect the laminate from chemical attack or to prevent leakage under stress.
Low Profile Resin compounds formulated for low or zero shrinkage during molding.

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Melting Point The temperature at which a materials change from a solid to a liquid.
MACT Maximum achievable control technology. A technology-based air pollution control standard developed by the Environmental Protection Agency (EPA) aimed at reducing emissions of HAPs during manufacturing processes.
Mandrel An elongated mold around which resin-impregnated fiber, tape or filaments are wound to form structural shapes or tubes.
Mat A fibrous reinforcing material composed of chopped filaments (for chopped-strand 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.
Matched Metal Molding See Compression Molding.
Matrix The material in which the fiber reinforcements of a composite system are imbedded. Thermoplastic and thermoset resin systems, as well as metal and ceramic, can be used.
Microcracking Cracking in composites at points where thermal stresses exceed the strengthof the matrix.
Mil The unit used in measuring the diameter of glass fiber strands, wire and so forth (1 mil = 0.001 inch).
Milled Fiber Continuous glass or carbon strands hammer-milled intovery short fibers.
Modulus The physical measurement of stiffness in a material, equaling the ratio of applied load (stress) to the resultant deformation of the material, such as elasticity or shear. (A high modulus indicates astiff material.)
Moisture Absorption A material assimilation of water vapor from air, as distinguished from water absorption by immersion, which results in weight gain.
Mold The cavity or matrix into or on which the resin/fiber material is placed and from which ittakes form.
Mold Release Agent A lubricant used to prevent a part from sticking to a mold.
Molding The forming of a resin/fiber material into a solid mass of prescribed shapeand size.
Monomer A single molecule that can react with like or unlike molecules to form a polymer.
Multifilament A yarn consisting of many continuous filaments.

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Naphtha Solvent naphtha is a petroleum distillate commonly used as a solvent for natural resins and rubber.
Near-Net Shape A part fabrication with final dimensions that require minimal machining or cutting.
Net Shape A part fabrication with final dimensions that do not require machining or cutting.
Nondestructive Inspection (NDI) Determination of material or part characteristics without permanent alteration of the test subject. (Nondestructive testing (NDT) and nondestructive evaluation (NDE) are generally considered synonymous with NDI.)
Nonwoven Roving A reinforcement composed of continuous fiber strands loosely gathered together.
Nylon The generic name, by common usage, for all synthetic polyamides.

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One-Off A single part that is individually fabricated.
One-Part Resin System A resin system (often used in resin transfer molding) in which the neat resin and catalyst are mixed together by the material supplier as part of the resin production operation.
Original Equipment Manufacturer(OEM) A company that designs and builds products bearing its name, such as Boeing 777.
Out-Time The period of time in which a prepreg retains acceptable handling and other properties outside a specified storage environment (a freezer, in the case of thermoset prepregs).
Outgassing The release of solvents and moisture from composite parts under a vacuum.

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PV or Limiting PV The ratio of simultaneous load and speed in relation to wear.
PAN See Polyacrylonitrile.
Part Consolidation A process of composites fabrication in which multiple discrete parts are designed and fabricated together into a single part, thus reducing the number of fabricated parts and the need to join those parts together.
Parting Film A layer of thin plastic to prevent bagging materials from sticking to a part It may be perforated to vent excess resin. It is removed after cure.
Peel Ply Layer of material applied to a lay up surface that is removed from the cured laminate prior to bonding operations,leaving a clean, resin-rich surface ready for bonding.
Peel Strength Strength of an adhesive bond obtained by stress that is applied in a peeling mode.
Phenolic Resin Thermosetting resin produced by condensation of anaromatic alcohol with an aldehyde, particularly phenol with formaldehyde.
Pin Holes Small holes caused by the mold used.
Pitch Residual petroleum product used in the manufacture of certain carbon fibers.
Planar Winding Filament winding in which the filament pathlies on a plane that intersects the winding surface.
Plastic A high molecular weight thermoplastic or thermosetting polymer that can be molded, cast, extruded or laminated into objects. A major advantage of plastics is that they can deform significantly without rupturing.
Ply One of the layers that makes up a stack orlaminate. Also, the number of single yarns twisted together to form a plied yarn.
Ply Schedule Lay up of individual plies or layers to build an FRP part laminate. Plies may be arranged (scheduled) in alternating fiber orientation to produce a multi-directional strength part. (See Fiber Architecture).
Poisson’s Ratio When a material is stretched, its cross sectional area changes as well as its length. Poisson’s ratio is the constant relating these changes in dimensions, and is defined as theratio of the change in width per unit width to the change in length per unit length.
Polar Winding Filament winding in which the filament path passes tangent to the polar opening at one end of the chamber and tangent to the opposite side of the polar opening at the other end of the chamber.
Polyacrylonitrile (PAN) A polymer, which is spun into fiber that is used as a precursor material in the manufacture of certain carbon fibers.
Polyester Thermosetting resins produced by dissolving unsaturated, generally liner, alkyd resins in a vinyl-type active monomer such as styrene. The resins are usually furnished in solution form, but powdered solids are also available.
Polymer Alloy (or Polymer Blend) A blend of polymers, copolymers or elastomers.
Polymer A large molecule formed by combining many smaller molecules or monomers in a regular pattern.
Polymerization A 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.
Postcure Additional elevated temperature cure, usually without pressure, to improve final properties and/or complete the cure. 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 The length of time in which a catalyzed thermosetting resin retains sufficiently low viscosity for processing.
Precursor For carbon fibers, the rayon, PAN or pitch fibers from which carbon fibers are made.
Preform A fibrous reinforcement pre-shaped on a mandrel or mock-up to the approximate contour and thickness desired in the finished part.
Prepreg Resin-impregnated fibers, fabric or mat in flat form that is preimpregnated with resin before being stored for later use in molds and in hand layup.
Promoter (or Accelerator) A chemical, which hastens the reaction between a catalyst and a resin.
Prototype The process of creating a test article not intended for commercial release that establishes design, material and fabrication parameters for a new product. May entail multiple iterations to arrive at final/ commercial part design.
Pultrusion An automated, continuous process for manufacturing composite rods, tubes and structural shapes having a constant cross-section. Roving and other reinforcements are saturated with resin and continuously pulled through a heated die, where the part is formed and cured. The cured part is then cut to length.
Puncture A break in composite skin in sandwich structure that may or may not go through to the core material or completely through the part.
Pyrolysis The decomposition or transformation of a compound caused by heat.

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Quasi-isotropic Approximating isotropy by orienting plies in several directions.

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Ramping A gradual programmed increase/decrease in temperature or pressure to control cure or cooling of composite parts.
Reagent A substance used in a chemical reaction to produce other substances.
Regrind Scrap composites (both thermoset and thermoplastic) collected in-plant or from post-consumer sources that are reground into pellets or fine powder for use in new parts, either as new base material or in
combination with virgin materials.
Reinforcement The key element added to matrix to provide required properties (primarily, strength and stiffness); ranges from short fibers and continuous fibers through complex textile forms.
Release Agent A substance usually sprayed or painted on mold to prevent cured matrix material from bonding to tooling.
Release Film An impermeable film layer that does not bond to the composite during cure.
Resin A solid or pseudosolid material with indefinite and often high molecular weight and a softening or melting range that exhibits a tendencyto flow when subjected to stress. (As composite matrices, resins bind together reinforcement fibers.)
Resin Rich Localized area filled with excess resin, as compared to consistency of resin/fiber ratio.
Resin Starved Characterizing a localized area lacking sufficient resin for fiber wetout.
Resin System A mixture of resin and ingredients required for an intended processing method and final product.
Resin Transfer Molding (RTM) A molding process in whichcatalyzed resin is pumped into a two-sided, matched mold, which a fibrous reinforcement has been placed. The mold and/or resin may or may not be heated.
Resin Viscosity The viscous property of a resin system, or solid-to-liquid transition resistance to flow, which can be altered by temperature and pressure to achieve desired flow characteristics.
Ribbon Direction On a honeycomb core, the way the honeycomb can be separated; the direction of one continuous ribbon.
ReactionInjection Molding (RIM) A process involving high pressure mixing of two components, primarily to mold polyurethane.
Reinforced Reaction (RRIM) A molding process that mixes two highly reactive resin Injection Molding:components for cure. To one of the resin components is added a reinforcement, usually consisting of flake glass or milled fibers, to stiffen the part and reduce thermal expansion.
Rockwell Hardness The ability of a material to resist indentation using a Rockwell Tester.
Roving A collection of bundles of continuous glass fiber filaments, either as untwisted strands or as twisted yarn.

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S-Glass Magnesia/alumina/silicate glass reinforcement designed to provide very high tensile strength. (Commonly used in high-performance parts.)
Sandwich Structure A composite composed of lightweight core material to which two relatively thin, dense, high-strength, functional or decorative laminate skins are adhered.
Sealant A paste or liquid applied to a joint that hardens in place to form a seal.
Secondary Bonding The joining by adhesive of two or more already cured composite parts.
Separator A permeable layer that separates and also acts as a release film (e.g., porous Teflon-coated fiberglass). Often placed between lay up and bleeder to facilitate bleeder systems’ removal from laminate after cure.
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 Strength The maximum shear stress that a material is capable of sustaining.
Shear Strength The resistance of a material to being punched or sheared.
Sheet Molding Compound (SMC) A ready-to-mold glass fiber-reinforced polyester material primarily
Shelf Life The length of time for which a material can be stored and continue to meet specification requirements, remaining suitable for its intended use.
Shot One complete cycle on an injection-molding machine. Shot weight is the measured compound delivered to completely fill the mold in injection or transfer molding.
Sizing A solution of chemical additives used to coat filaments. The additives protect the filaments from water absorption and abrasion; they also lubricate the filaments and reduce static electricity.
Skin A relatively dense laminate comprising the outer surfaces (layers) of the core in a sandwich structure.
Soft Tool A tool made of composites or a similar soft material that is vulnerable to damage during use, storage or transportation.
Solvent A liquid used to dissolve and clean materials.
Spec Specification of the properties, characteristics or requirements a particular material or part must have to be acceptable to a potential user of the material or part.
Specific Gravity The density (mass per unit volume) of a material divided by that of water at a standard temperature.
Sprayup A technique in which continuous strand roving is fed into a chopper gun, which chops the roving into predetermined lengths and sprays the chopped fiber, along with a measured amount of resin and catalyst, onto an open mold.
Stiffness A material’s ability to resist bending; relationship of load to deformation for a particular material.
Structural Reaction Injection Molding (SRIM) A process that uses a fiber reinforced preform or mat in clamped molds to inject reactive resin that impregnates fibers and cures quickly.
Starved Area An area in a plastic part that has an insufficient amount of resin to completely wet out the reinforcement.
Storage Life The amount of time a material can be stored and retain specific properties.
Strain Elastic deformation resulting from stress.
Strand A collection or bundle of continuous glass filaments.
Stress Internal resistance to 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, that alone would not have caused corrosion.
Stress Crack External or internal crack in a composite caused by tensile stresses; cracking may be present internally, externally or in combination.
Substrate A material on which an adhesive-containing substance is spread for any purpose, such as bonding or coating.
Surfacing Veil Accompanying other reinforcing mats and fabrics to enhance the quality of the surface finish. Designed to block out the fiber patterns of the underlying reinforcements, it often adds ultraviolet protection to the structure.
Synthetic Fiber Fiber made ofmaterials other than glass or carbon, such as polyester.
Specific Gravity The ratio of the weight of an object, to the weight of the same volume of water.
Surface Resistivity The ability of a material to impede the flow of electricity across its surface.

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Tensile Elongation The % a material will stretch as it is pulled in a slow stretch until it breaks.
Tensile Modulus Of Elasticity The ratio of force to deformation when a material is being slowly pulled apart.
Tensile Strength The resistance of a material to being slowly pulled apart.
Tack Stickiness of an uncured prepreg.
Tape Thin unidirectional prepreg in widths up to 12 inches.
Tape Laying An automated fabrication process in which preimpregnated tape is laid side by side or overlapped to form a structure.
Tensile Strength The maximum stress sustained by a composite specimen before it fails in a tension test.
Thermal Conductivity The ability to transfer heat.
Thermal Stress Cracking Crazing and cracking of some thermoplastic resins from overexposure to elevated temperatures.
Thermocouple Wire assembly used with a control device to sense temperature.
Thermoplastic A composite matrix capable of being repeatedly softened by an increase in temperature and hardened by a decrease in temperature.
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.
Thixotropic A consistency that is gel-like at rest, but fluid when agitated. Having high static shear strength and low dynamic shear strength simultaneously. Losing viscosity under stress.
Tool The mold, either one- or two-sided and either open or closed, in or upon which composite material is placed to make a part.
Toughness The ability of a material to absorb energy.
Tow An untwisted bundle of continuous filaments (usually carbon), typically designated by a number followed by K, indicating multiplication by 1,000 (for example, 12K tow has 12,000 filaments).
Tracer A fiber, tow, or yarn added to a prepreg to verify fiber alignment or to distinguish warp fibers from fill fibers.
Thermal Conductivity The ability of a material to insulate changes in temperature.

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Unidirectional (UD) Orientation of fibers in the same direction, as in unidirectional fabric, tape or laminate.

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Volume Resistivity The ability of a material to impede the flow of electricity through itself.
Vacuum Bag Molding A molding technique wherein the part is cured inside a layer of film from which entrapped air is removed by vacuum.
Vacuum-Assisted Resin Transfer Molding(VARTM) An infusion process by which a vacuum draws resin into a one-sided mold; a cover, either rigid or flexible, is placed over the top to form a vacuum-tight seal.
Veil An ultra-thin mat often composed of organic fibers as well as glass fibers and used primarily as a corrosion barrier.
Vinyl Esters A class of thermosetting resins containing ester of acrylic and/or methacrylic acids.
Viscosity The tendency of a material to resist flow. As temperature increases, the viscosity of most materials decreases.
VOCs Volatile organic compounds. Chemical substances, such as solvents, that readily evaporate or volatilize into the air. Many VOCs are considered hazardous air pollutants (HAPs) because of potential health concerns.
Voids Pockets of entrapped gases that have been cured into a laminate. (In a composite that has been cured properly, void content is usually less than 1 percent.)
Volatiles Materials in a sizing or resin that can be vaporized at room or slightly elevated temperatures.
Volatile Organic Compounds (VOC) Carbon-containing chemical compounds (e.g., solvents and styrene) that evaporates readily at ambient temperatures Environmental, safety and health regulations often limit exposure to these compounds; so low VOC content is preferable.
Volatile Content The percent of volatiles that are driven off as a vapor from a plastic or animpregnated reinforcement during cure.

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Water Absorption The ability of a material to absorb moisture.
Warp Yarns running lengthwise and perpendicular to the narrow edge of woven fabric.
Warpage Dimensional distortion in a composite part.
Water
Absorption
The ratio of weight of water absorbed by a material to weight of dry material.
Water Jet High-pressure water stream used for cutting polymer composite parts.
Weave The 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, the pattern produces a satin appearance with the warp roving crossing over several fill rovings and under the following one. (For example, eight-harness satin would have warp roving over seven fill rovings and under the eighth.)
Weeping A slow passage of fluid through an FRP laminate that can occur when a leak path is established by extensive cracking.
Weft Threads See Fill Threads.
Wet Layup Application of a resin to a dry reinforcement in the mold.
Wet Winding Filament winding wherein fiber strands are impregnated with resin immediately before they contact the mandrel.
Wetout Saturation with resin of all voids between strands and filaments.
Wetting Agent A surface-active agent that promotes wetting by decreasing the cohesion within a liquid.
Whisker A short single crystal fiber or filament used as a reinforcement in a matrix.
Wind Angle The measure in degrees between the direction parallel to filaments and an established reference.
Winding Pattern The regularly recurring pattern of the filament path in a filament winding after a certain number of mandrel revolutions.
Wire Mesh A fine wire screen used to increase electrical conductivity; typically used to dissipate the electrical charge from lightning.
Woof Threads See Fill Threads.
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. This occurs in vacuum-bag molding when the bag is improperly placed.
Wear or K Factor The ability to resist wear against steel.

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X-, Y-, Z-Axis The axis in the plane of a laminate used as 0° reference. The y-axis is the axis in the plane of a laminate perpendicular to the x-axis. The z-axis is the through-the-plane thickness.

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Yarn Continuously twisted fibers or strands suitable for weaving into fabrics.
Yield Point The first stress in a material, less than the maximum rate attainable stress, at which the strain increases at a higher rate than the stress. The point at which permanent deformation of a stressed specimen begins to take place.
Young’s Modulus The ratio of normal stress to the corresponding strain for tensile or compressive stresses less than the proportional limit of the material.

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Zero Bleed A laminate fabrication procedure that prohibits loss of resin during cure.