Glossary of pultrusion terms

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a
A Glass
A soda-lime glass similar to window or bottle glass with generally poorer chemical and water resistance than E glass. Used primarily as a surface mat on pultrusions.
Accelerator
A chemical additive that hastens cure or chemical reaction.
Additive
An ingredient mixed into resin to improve properties. Examples include plasticizers, initiators, light stabilizers and flame retardants.
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.
Aspect Ratio
Ratio of length to diameter of a fiber.
b
B-Stage
An intermediate stage in the reaction of certain thermosetting resins in which the material softens when heated and is plastic and fusible but may not entirely dissolve or fuse; sometimes referred to as resistol. The resin in an uncured prepreg or premix is usually in this stage.
Balanced Laminate
A composite laminate in which all laminae at angles other than 0 degrees and 90 degrees occur only in +- pairs (not necessarily adjacent). Balance is symmetrical about the composite 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.
Batch (or Lot)
Material made with the same process at the same time having identical characteristics throughout.
Biaxial Material
Material having fibers oriented in both the warp (0 degrees) and weft (90 degrees) directions.
Binder
The agent applied to glass mat or preforms to bond the fibers prior to laminating or molding.
Blister
A rounded elevation of the pultruded surface with boundaries that may be more or less sharply defined.
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.
Bow
A conditional longitudinal curvature in pultruded parts.
Breaker Bars
A series of bars in the resin bath or impregnator which separate the planes of roving and mechanically break the roving bundles into individual filaments for thorough wetting by the resin.
Bromine
A fire retardant additive (halogen) used to reduce or eliminate a resin's tendency to burn.
Bundle
A general term for a collection of essentially parallel filaments or fibers.
Buckling
A failure mode usually characterized by fiber deflection rather than breakage due to compressive action.
c
Carbon Fiber
Reinforcing fiber known for its light weight, 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癋. Fibers can be pyrolized by removing still more non-carbon atoms via heat-treating above 3,000癋.
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 state, 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.
Cavity
The female portion of a split cavity or gun barrel drilled mold; that portion of the mold that encloses and shapes the molded article (often referred to as the die). (Depending on the number of such depressions, molds are designated as single-cavity or multiple-cavity.)
Circumferential Winding
In pultruded reinforced polymers, a winding with the filaments essentially perpendicular to the longitudinal process axis (90 degrees or level winding).
Cocure
The act of curing a composite laminate and simultaneously bonding it to some other prepared surface.
Coefficient of Expansion (COE)
A 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.
Composite
A material that combines fiber and a binding matrix to maximize specific performance properties. Neither element merges completely with the other.
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.
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.
Crack
A visual separation that occurs internally or penetrates down from the pultruded surface to the equivalent of one full ply or more of reinforcement.
Craze, Hairline
Multiple fine pultrusion surface separation cracks that exceed ? in length and do not penetrate in depth to the equivalent of a full ply of reinforcement.
Craze, Resin
Multiple fine separation cracks at the pultruded surface not penetrating into the reinforcement.
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.
Cross-Linking
Applied to polymer molecules; the setting-up of chemical links between the molecular chains. When extensive as in most thermosetting resins, cross-linking makes one infusible supermolecule of all the chains. The higher the cross-link density, the more rigid the material.
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.
Cure Time
The time needed for liquid resin to reach a solid state after the catalyst has been added, and initiation has progressed.
Curing Agent or Hardener
A catalytic or reactive agent that brings about polymerization when added to a resin.
d
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.
Die (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.
Dielectric
Nonconductor of electricity; the ability of a material to resist the flow of an electrical current.
Dielectric Curing
The curing of a synthetic thermosetting resin by the passage of an oscillating electric field produced from a high frequency generator through the resin. Commonly used is radio frequency (RF) energy.
Dielectric Strength
The voltage required to penetrate insulating material. Material with high dielectric strength offers excellent electrical insulating properties.
Die-Parting Line
A lengthwise flash or depression on the surface of a pultruded plastic part. It is only at the surface and does not weaken the part.
Dimensional Stability
Ability of a plastic part to retain the precise shape to which it was molded, cast, or otherwise fabricated.
Draft
The taper or slope of the male sections of a mold designed to facilitate shrinkage of pultruded profiles.
Dwell Time
In pultrusion, a pause in the normally continuous pulling motion which allows the material to cure under static condition. The pause allows a cleaning or "purging" action which removes tenacious material from the die wall.
e
E-Glass (Electrical Glass)
Borosilicate glass fibers most often used in conventional polymer matrix composites.
Elastomer
A material that substantially recovers its original shape and size at room temperature after removal of a deforming force.
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.
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 with little lessening of properties.
f
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.
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 Blooming
A pultrusion surface condition exhibiting a fiber prominence or fiber show that usually has a white or bleached color and a sparkling appearance. This condition usually is the result of surface degradation by 'UV' on a pultruded part where a synthetic surfacing veil was not used.
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.
Fiber Prominence
A visible and measurable pattern of the reinforcing material on the surface of a pultruded part.
Fiber Reinforced Polymer (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 polymers; the term "advanced composite" usually denotes high-performance aramid or carbon fiber-reinforced plastics.
Fiberglass
1.    An individual filament made by attenuating molten glass. A continuous filament is a glass fiber of great or indefinite length; a staple fiber is a glass fiber of relatively short length (generally less than 17 inches).
2.    General description of polymetric (plastic) matrix reinforced with fibrous glass.
Filaments
Individual fibers of indefinite length used in tows, yarns or roving.
Filler
Material added to the mixed resin to increase viscosity, improve appearance and/or lower density and cost.
Finite Element Analysis (FEA)
A process of selecting the optimum combination of materials in a composite based on software analysis.
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 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.
g
Gel
A partial cure of polymer resins; a semi-solid, jelly-like state similar to gelatin in consistency.
Gel Time
The period of time from initial mixing of liquid reactants to the point when gelation occurs, as defined by a specific test method.
Glass Fiber
See Fiberglass.
Glass Transition
The reversible change in an amorphous polymer between a viscous, rubbery condition and a hard, relatively brittle one.
Glass-Transition Temperature (Tg)
The approximate temperature above which increased molecular mobility causes a material to become rubbery, rather than brittle. The measured value of Tg can vary, depending on the test method.
Grooving
Long, narrow grooves or depressions in a surface of a pultrusion parallel to its length.
h
Halogenated Resin
A resin combined with chlorine or bromine to increase fire retardancy.
Hardener (or Curing Agent)
A substance that reacts with resin to promote or control 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-Distortion Temperature (HDT)
The temperature at which a test bar deflects a certain amount under specified temperature and stated load.
Hybrid Composite
A composite made with two or more types of reinforcing fibers.
Hygroscopy
A material's readiness to absorb or retain moisture.
i
Ignition Loss
The difference in weight before and after burning. For glass reinforcements, the burning of the binder or size. For composites, the burning off all organic constituents.
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
See Prepreg.
Inclusion
Any foreign matter of particles that are either encapsulated or imbedded in the pultrusion.
Initiator
A material used as a source of free radicals. Initiators are used in free radical polymerizations, curing thermosetting resins, as cross-linking agents for elastomers and polyethylene, and for polymer modification. Frequently referred to as "catalyst".
Insufficient Cure
A pultrusion abnormality created by lack of, or incomplete, cross-linking of the resin.
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.)
Interlaminar
Existing or occurring between two or more adjacent laminae.
Interlaminar Shear
A shearing force that produces displacement between two laminae along the plane of their interface.
Isophthalic Polyester
An unsaturated polyester resin prepared with isophthalic acid as the starting acid constituent.
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.
j
k
Knitted Fabrics
Fabrics produced by interlooping strands of yarn, roving, and so forth.
l
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.
Loss of Ignition
Weight loss, usually expressed as percent of total, after burning off an organic sizing from glass fibers, or an organic resin from a glass fiber laminate.
Low Profile
Resin compounds formulated for low or zero shrinkage during molding.
m
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.
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 strength of the matrix.
Mil
The unit used in measuring the diameter of glass fiber strands, wire and so forth (1 mil = 0.001 inch).
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 a stiff 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 it takes form.
Mold Release (External)
A substance used to coat the mold to prevent sticking of the resin that will be used to make the part.
Mold Release (Internal)
A lubricant, liquid or powder, used to prevent sticking of molded articles in the cavity by incorporation into the resin formulation. Referred to as an internal rather than external mold release.
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 shape and size.
Monomer
A single molecule which can react with like or unlike molecules to form a polymer; the smallest repeating structure of a polymer(s); for addition polymers, this represents the original unpolymerized compound. Styrene is the predominant monomer used for reinforced polyester material.
Multifilament
A yarn consisting of many continuous filaments.
n
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.
o
Orthophthalic Resin
An unsaturated polyester resin of which phthalic anhydride was the starting point.
Outgassing
The release of solvents and moisture from composite parts under a vacuum.
p
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 Line
A continuous mark on a pultruded piece where the sections of a mold have met in closing. Tight mold tolerances can minimize this objectionable mark.
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 an aromatic 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.
Platens
The mounting plates of a pultrusion machine in which the entire mold is contained for the dual purposes of restraint and heating.
Ply
One of the layers that makes up a stack or laminate. 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 the ratio of the change in width per unit width to the change in length per unit length.
Polyester Resin
a.    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.
b.    The resin 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 Alloy (or Polymer Blend)
A blend of polymers, copolymers or elastomers.
Polymer
A high-molecular weight organic compound, natural or synthetic, whose structure can be represented by a repeated small unit, the "mer"; for example, polyethylene, rubber, polyester, cellulose. Synthetic polymers are formed by addition or condensation polymerization of monomers. Some polymers are elastomers and some are plastics. When two or more different monomers are involved, the product is called a copolymer.
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.
Post Cure
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.
Postforming
The forming, bending, or shaping of fully cured, C-stage thermoset laminates that have been heated to make them flexible. On cooling, the formed laminate retains the contours and shape of the mold over which it has been formed.
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 preshaped on a mandrel or mock-up to the approximate contour and thickness desired in the finished part.
Preheating
The heating of an impregnated material prior to molding, to facilitate the operation or to reduce the molding cycle.
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.
q
Quasi-isotropic
Approximating isotropy by orienting plies in several directions.
r
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 tendency to 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 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.
Roving
A collection of bundles of continuous glass fiber filaments, either as untwisted strands or as twisted yarn.
s
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.
Saw Burn
Blackening or carbonization of a cut surface of a pultruded section. Fiberglass parts do not dissipate heat as quickly as metal, so the speed of cutting fiberglass must be controlled to prevent saw burns.
Scale
A condition wherein unreinforced, cured resin particles exit the die on the surface of the part.
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.
Shelf Life
The length of time a material, substance, product, or reagent can be stored under specified environmental conditions and continue to meet all applicable specification requirements and/or remain suitable for its intended processing function.
Shrinkage
The relative change in dimension between a dimension measured on the mold when it is cold and the dimension of the molded object 24 hours after it has been molded.
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.
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.
Splice
The joining of two ends of glass fiber roving or mat.
Spun Roving
A bulky, low-cost glass fiber strand consisting of filaments that are continuous but doubled back on each other providing some level of off-axis reinforcement.
Stiffness
A material's ability to resist bending; relationship of load to deformation for a particular material.
Starved Area
An area in a plastic part that has an insufficient amount of resin to completely wet out the reinforcement.
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.
Styrene
A water-thin liquid used to thin polyester resins prior to cure and which serves as the chemical which links polyester chains together during the curing process.
Substrate
A material on which an adhesive-containing substance is spread for any purpose, such as bonding or coating.
Surfacing Agent
An oily or waxy material which rises to the surface of a polyester resin during cure.
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 of materials other than glass or carbon, such as polyester.
t
Tensile Strength
The maximum stress sustained by a composite specimen before it fails in a tension test.
Terephthalic Resin
An unsaturated polyester resin prepared with terephthalic acid as the starting acid constituent.
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.
Thickeners
Materials added to the resin to thicken it or raise the viscosity index of the resin so that it will not flow as readily. Used most frequently to produce sheet molding compound (SMC).
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).
Twist
A condition of longitudinal progressive rotation found in pultruded parts.
u
Ultraviolet Stabilizer
Any chemical compound which, when admixed with a thermoset or thermoplastic plastic resin, selectively absorbs UV rays.
Undercure
A condition or reverse draft on the mold resulting from the allowance of too little time and/or temperature for adequate hardening of the mold.
Unidirectional (UD) Laminate
A reinforced polymer laminate in which substantially all of the fibers are oriented in the same direction. In pultrusion, unidirectional products are produced with all continuous rovings.
v
Veil
An ultrathin mat often composed of organic fibers as well as glass fibers and used primarily as a corrosion barrier.
Vinyl Ester Resin
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.
Viscosity
The tendency of a material to resist flow. As temperature increases, the viscosity of most materials decreases.
Voids
Pockets of entrapped gas 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 evaporate 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 an impregnated reinforcement during cure.
w
Warp
The yarn running lengthwise in a woven fabric; a group of yarns in long lengths and approximately parallel.
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
The transverse threads or fibers in a woven fabric; those fibers running perpendicular to the warp. Also called fill or filling yarn.
Wet-out
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.
Woven Fabrics
Fabrics produced by interfacing strands at more or less right angles.
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.
x
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.
y
Yarn
Continuously twisted fibers or strands suitable for weaving into fabrics.
Yield
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.
z
Zero Bleed
A laminate fabrication procedure that prohibits loss of resin during cure.