Fiber Reinforced PlasticsA

ABS: Acrylonitrile Butadiene Styrene (Thermoplastic Resin).

Ablative Plastic: A material that absorbs heat (with low material loss and char rate) through a decomposition process (pyrolysis). Absorption takes place at or near the surface exposed to the heat.

Abrasion: Wearing away by friction. Glass is highly resistant to abrasion from other materials, but can be damaged through contact with itself. Lubrication during processing and fabrication helps prevent abrasion.

Accelerated Test: Procedure in which conditions are magnified to reduce the time required to obtain a result, or to reproduce the deteriorating effects of normal service conditions in a very short time period.

Accelerator (Promoter): A highly active oxidizing agent used to speed up the chemical reaction (curing) between a catalyst and resin. Examples include diethylaniline, cobalt naphthanate and cobalt octoate.

Activator: See Accelerator.

Acoustic Emission: The amount of integrity in a material after it has been stressed, as determined by sound emission. Ideally, emissions can be correlated with defects and/or early failure.

Acrylic: Thermoplastic polymer made by the polymerization of esters of acrylic acid and its derivatives.

Actual End Count: The number of bundles or splits that are actually counted in one doff of roving. (This is less than the theoretical end count due to splitting efficiencies of less than 100%).

Addition Polymerization: Chemical reaction in which simple molecules (monomers) are added to each other to form long-chain molecules (polymers) without byproducts.

Additive: A material used to modify the properties of polymer resins. Examples include plasticizers, initiators, light stabilizers and flame-retardants.

Adhesion: The state in which two surfaces are held together by an interlocking action or force.

Adhesive: A film, liquid or paste capable of holding the surfaces of two materials together.

Admixture: Addition and homogeneous dispersion of discrete components, before cure.

Aggregate: Hard, coarse material usually of mineral origins used in composite tools. Also used in flooring or as a surface medium.

Aging: The process of exposing materials to an environment for an interval of time.

Air-Bubble: A localized, non-interconnected, spherically shaped entrapment of air within and between the plies of reinforcement.

Air Splice: The coupling made by a jet of air to entwine/snarl two roving doffs together. The air splice is used instead of a knot.

Air Vent: Small outlet to prevent entrapment of gases in a molding or tooling fixture.

Alkyd Plastics: Resin composed principally of polymeric esters, in which the recurring ester groups are an integral part of the main polymer chain or are part of the cross links present between chains.

Alligatoring: Visible cosmetic defect in exposed gel coat resembling wrinkled or alligator-like skin.

Alloy: In plastics, a blend of polymers or copolymers blended with other polymers or elastomers under select conditions.

Ambient: Surrounding environmental conditions, such as pressure, temperature, or relative humidity.

Amine Resins: A synthetic resin derived from the reaction of urea, thiourea, melamine or allied compounds with aldehydes, particularly formaldehyde.

An Isotropic: Exhibiting different properties when tested along axes in differentdirections. see An Isotropic laminate

An Isotropic Laminate: A laminate whose properties are different in different directions.

Angle-Ply Laminate : A laminate with fibers of adjacent plies oriented at alternating angles.

Antioxidant: Substance that, when added in small quantities to resin, prevents oxidation and degradation while maintaining the resin’s properties.

Antistatic Agents: Agents added to a molding material or applied to the surface of a molded object to make it more conductive and prevent the fixation of dust or buildup of electrical charge.

Arimid: A highly oriented organic material derived from polyamide with an aromatic ring structure that is used primarily as a high-strength, high-modulus fiber. Examples include Kevlar and Nomex.

Arc Resistance: Ability to withstand exposure to an electric voltage. Also, the total time in seconds that an intermittent arc may play across a plastic surface without rendering it conductive.

Ash Content: Proportion of solid residue remaining after a reinforcing substance has been incinerated (charred or intensely heated).

Aspect Ratio: Ratio of length to diameter of a fiber.

A-Stage: Early stage in the polymerization reaction of certain thermosetting resins (especially Phenolic) in which the material, after application to the reinforcement, is still soluble in certain liquids and is fusible. Also called resole. see B-Stage and C-Stage

ASTM: American Society of Test Methods.

Autoclave: Closed vessel for conducting and completing a chemical reaction or other operation under pressure and heat.

Autoclave Molding: Process in which an assembly is placed in a heated autoclave, usually at 50 to 200 psi, after lay-up, winding or wrapping. Additional pressure permits higher density and helps remove volatiles from the resin. Lay-up is usually vacuum bagged with a bleeder and release cloth.

Autosprue™: Automatic gate at the inlet sprue of the mold that eliminates the need to remove the resin supply during injection.

Axial Winding: Filaments that are wound parallel or at a small angle to the axis (0° helix angle).


Bagging: Applying an impermeable layer of film over an uncured part and sealing the edges so that a vacuum can be drawn.

Bag Molding: Process in which fluid or gas is applied through a flexible membrane to consolidate material in a mold.

Balanced Construction: Equal parts of warp and fill in fiber fabric. Construction in which reactions to tension and compression loads result in extension or compression deformations only, and in which flexural loads produce pure bending of equal magnitude in axial and lateral directions.

Balanced Laminate: Composite laminate whose 0° and 90° angles occur only in + or – pairs (not necessarily adjacent) and are symmetrical around the centerline.

Ball: See Roving

Band Width: In filament winding, the width of the reinforcement as it is applied to the mandrel.

Barcol-Shore Rockwell Hardness: A material’s ability to be indented. The Rockwell method measures the amount of penetration caused when a steel point is forced into the material. The suffix (alphabetic letter) in the Rockwell reading describes the shape of the point and units describe how much load was applied during the test. The letter and number cannot be separated. Higher numbers with the same letter indicate harder materials with greater resistance to penetration by another substance.

Bare Glass: Glass in fiber form as it flows from the bushing before a binder or sizing is applied.

Batch Oven: Large temperature-controlled oven used to heat-clean rolls of glass fiber fabric.

Batt: Felted fabrics or structures built by the interlocking action of compressing fibers, without spinning, weaving, or knitting.

Beam: A spool on which parallel ends of single or plied yarns are wound for use in weaving or similar processing operations.

Beaming: Operation in which many ends of yarn from a creel are combined on a section beam.

Bearing Strength: The maximum amount of stress that can be sustained. Also, the point on the stress-strain curve where the tangent is equal to the bearing stress divided by n% of the bearing hole diameter.

Bearing Stress: Applied load in pounds divided by the bearing area. Maximum bearing stress is the number of pounds that can be sustained, divided by the original bearing area.

Bed: The mat of chopped glass fibers deposited over a layer of resin mix on carrier film following a chopping operation.

Bias Fabric: Warp and fill fibers placed at an angle to the length of the fabric.

Biaxial Load: Loading condition in which a laminate is stressed in two different directions in its plane. Also, a loading condition of a pressure vessel under internal pressure and with unrestrained ends.

Biaxial Winding: Filament winding in which the helical band is laid in sequence, side by side, without any fibers crossing over each other.

Bi-directional: Reinforcing fibers arranged in two directions, usually at right angles.

Bi-directional Laminate (Cross Laminate): A reinforced plastic laminate whose fibers are oriented in two directions in its plane.

Binder: Coating which is applied to the surface of a chopped glass mat or preform and then cured to hold bundles or ends together in a stable form during the roving operation.

Birdnest: A large, tangled up collection of continuous glass bundles unable to run through the guide eye into roving creel. In the field, it can also be a large tangled collection of roving which does not run through the tube or guide eyes to the chopper.

Bismaleimide (BMI): A polyamide that cures through an addition rather than a condensation reaction, thus avoiding problems with volatiles forming. It is produced by a vinyl-type polymerization of a pre-polymer terminated with two maleimide groups. BMI has an Intermediate temperature capability (between epoxy and polyamide).

Bisphenol A: A condensation product formed by the reaction of two (bis) molecules of phenol with acetone (A). This polyhydric phenol is a standard resin intermediate along with epichlorohydrin in the production of epoxy resins.

Blade Packing: Glass bundles or chopper fuzz which build up and pack between the blades of a chopper. Blade packing can result in poor choppability. If it falls off, it usually does not wet-through, causing blisters or porosity.

Blanket: Fiber or fabric plies that have been laid up in a complete assembly and placed on or in the mold all at one time (flexible bag process). Also, the form of bag in which the edges are sealed against the mold.

Bleeder Cloth: Woven or non-woven layer of material used in composite parts manufacturing that allows excess gas and resin to escape during cure. The bleeder cloth is removed after the curing process and is not part of the final composite.

Blister: Flaw either between layers of laminate or between the gel coat film and laminate.

BMC: Bulk Molding Compound (Thermoset).

Bobbin: The spool or shipping package on which textile yarns are wound.

Bond Strength: Amount 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 Fiber: Fiber produced by vapor deposition of elemental boron, usually onto a tungsten filament core, to impart strength and stiffness.

Braid/Braider: A narrow tubular or flat fabric produced by intertwining a single set of yarns according to a definite pattern.

Breakup: see Dispersion

Breathing: Opening and closing a mold so that gas can escape early in the molding cycle. Also called “degassing”; sometimes called “bumping” in Phenolic molding.

Bridging: Condition in which fibers do not move into or conform to radii and corners during molding, resulting in voids and dimensional control problems.

Broad Strand:see Wides or Matchsticks

Broken Strand: see Broken End

Broken End: In the roving operation, a severed strand (bundle), which causes the forming cake to stop running.

Broken Fibers: see Fuzz

B-Stage: Intermediate stage in the reaction of certain thermosetting resins in which the material softens when heated, is plastic and is fusible but may not entirely dissolve or fuse. Also called “resistol” or “resitol.” Resin in an uncured prepreg or premix is usually in this stage.

Buckling (Composite): Failure mode generally characterized by an unstable lateral material deflection due to compressive action on the structural element involved.

Buildup: Glass bundles or chopper fuzz that collect on the chopper, cot, static bars or machine frame.

Bulk Molding Composite (BMC): Thermosetting resin mixed with short strand reinforcement, filler, and other materials to form a viscous compound for compression or injection molding.

Bundle: A discrete collection of many parallel glass filaments. A collection of individual filaments; a sub-strand.

Burst Strength: (1) Hydraulic pressure required to burst a vessel of given thickness. Commonly used in testing filament-wound composite structures. (2) Pressure required to break a fabric by expanding a flexible diaphragm or pushing a smooth spherical surface against a securely held circular area of fabric. The Mullen expanding diaphragm and Scott ball burst machine are examples of equipment used for this purpose.

Bushing: Plate with holes through which molten glass is pulled to produce glass fibers.

Bushing Tip: Small tapered protrusions on the bottom of bushings, each containing an orifice through which molten glass flows and from which continuous filaments are drawn.


Cabled Yarn: Yarn that is plied more than once or made by plying two or more previously plied yarns.

Carbon: 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-Carbon: Composite material consisting of carbon or graphite fibers in a carbon or graphite matrix.

Carbon Fiber: Fiber produced by the pyrolysis of organic precursor fibers, such as rayon, polyacrylonitrile (PAN), and pitch, in an inert environment.

Carding: The process of untangling and partially straightening fibers by passing them between two closely spaced surfaces which are moving at different speeds of which one or more is covered with sharp points, thus converting a tangled mass of fibers into a filmy web.

Casting: Process of pouring resin, fillers and/or fibers into a mold vs. building up layers through lamination. Casting results in physical properties that are different than those resulting from lamination.

Catalyst (Hardener): A substance that markedly speeds up the cure of a compound by decomposing in the presence of a promoter to release an active oxygen radical. Catalyst content can vary from 0.2% to 2.0% with higher catalyst levels giving faster gel times. Examples are methyl ethyl ketone peroxide, benzoyl peroxide.

Catastrophic Failures: Totally unpredictable failures of a mechanical, thermal, or electrical nature.

Catenary: A consolidated group of individual strands which, when checked between two fixed points of length, exhibit strands of different lengths. The resulting curve between fixed points of the inextensionable material is defined as catenary. The amount of catenary is measured by the distance between the topmost and lowermost strands when stretched between two fixed points.

Caul Plates: Smooth metal plates free of surface defects with the same size and shape as a composite lay-up that contacts the lay-up during curing. Caul plates transmit normal pressure and temperature to the finished laminate while providing it with a smooth surface.

Cavity: Space inside a mold in which a resin or molding compound is poured or injected. The female portion of the mold. That portion of the mold that encloses the molded article (often referred to as the die). Depending on the number of such depressions, molds are designated as a single cavity or multiple cavity.

Cell: The vertical plane of doffs on a pallet. A pallet may be constructed for 4- or 12-end run-out depending on application.

C-Glass: Glass with a soda-lime-borosilicate composition that maintains chemical stability in corrosive environments.

Chalking: Surface phenomenon indicating degradation of a cosmetic surface. Chalking is a powdery film that appears lighter than the original color.

Chemical Size: A surface finish applied to the fiber that contains some chemical constituents other than water.

Choppability: The ease of chopping/cutting the glass fibers to a uniform length.

Clamping Pressure: In injection molding and transfer molding, the pressure that is applied to the mold to keep it closed in opposition to the fluid pressure of the compressed molding material.

Cloth: Fiberglass reinforcement made by weaving strands of glass fiber yarns.

Clump: A group of chopped bundles of glass fibers that has collected on the SMC machine and then fallen into the bed of glass. The clump produces areas of high glass content which may not wet-through.

Coefficient of Thermal Expansion (CTE): How much a material’s shape will change per each degree of temperature fluctuation.

Cold Flow: Distortion that occurs in a material under continuous load within its working temperature range and without a phase or chemical change.

Collet: A spool on which the gathered strands from the bushing are wound for further processing.

Compaction: Applying a temporary vacuum bag and vacuum to remove trapped air and compact the lay-up; also removing air in SMC machines prior to roll-up.

Compatibility: The ability of two or more substances to be combined in order to form a homogeneous composition of useful plastic properties; for example, the suitability of a sizing or finish for use with certain general resin types.

Composite: Chemical or mechanical bonding of dissimilar materials such as glass fiber and polyester resin, whose cumulative properties are superior to the individual materials.

Composite Material: A combination of two or more materials (reinforcing elements, fillers, and composite matrix binder). Although they act in concert, the constituents retain their identities; that is, they do not dissolve or merge completely into one another. Normally, the components can be physically identified and exhibit an interface between one another.

Compounder: Manufacturer who mixes a polymer, fillers, additives and glass fibers, and sells the resulting pellets for injection molding.

Compression Molding: An open molding process in which material is introduced and shaped by the pressure of closing and by heat.

Compressive Modulus: Ratio of compressive stress to compressive strain below the proportional limit. Theoretically equal to Young’s modulus determined from tensile experiments.

Compressive Strength: The amount of nonmoving load that a bar can take before it is crushed. Units are normally thousands of pounds per square inch. (103 psi) – Mega Pascals (mPa). Higher numbers indicate stronger materials that can withstand a heavier load before they break.

Condensation Polymerization: A chemical reaction in which two or more molecules combine, with the separation of water or some other simple substance.

Conductivity: Reciprocal of volume resistivity. The electrical or thermal conductance of a unit cube of any material (conductivity per unit volume).

Conformability: A mat’s ability to conform to difficult shapes without wrinkling or leaving excessively resin-rich or glass-rich radii, which may craze.

Contact Molding: Refers to the use of a single or open mold onto which layers of polymer and reinforcement materials can be applied. Contact molding is characterized by one finished cosmetic side. Cure is either at room temperature using a catalyst-promoter system or by heating in an oven, without additional pressure.

Continuous Filaments: Filaments that extend substantially throughout the length of the yarn.

Continuous Heat Resistance: Maximum temperature to which material should be subjected in a continuous application. Below this temperature, the material is acceptable. At temperatures above the maximum, the material may decompose, melt, or otherwise fail in an application. Units – degrees Fahrenheit (°F)- degrees Centigrade (°C). Higher numbers mean the material can be used continuously at higher temperatures.

Continuous Laminating: Process for forming panels and sheeting in which fabric or mat is passed through a resin bath, brought together between covering sheets, and passed through a heating zone for cure. Squeeze rolls control thickness and resin content as the various plies are brought together.

Continuous Rovings: Rovings supplied in a package that allows for continuous processing.

Continuous Strand: Fiberglass mat of very long individual fibers that have a regular crossed pattern and are loosely held together with a binder.

Copolymer: A resin produced by copolymerization. A process in which unlike molecules are arranged in alternate sequence in a chain.

Core: A low-density material used between two FRP skins. Examples are end-grain balsa wood, urethane foam, PVC foam and various honeycomb materials. The central member, usually foam or honeycomb, has a sandwich construction to which the faces of the sandwich are attached or bonded. The central member of a plywood assembly. A channel in a mold for circulation of heat transfer media. A device on which prepreg is wound.

Coronizing: Continuous heat cleaning and weave setting.

Corrosion Resistance: A material’s ability to withstand ambient natural factors or those of a particular artificially created atmosphere, without degrading or changing in properties. For metals, this could be pitting or rusting; for or organic materials, it could be crazing.

Coupling Agent: Any chemical substance designed to react with both the reinforcement and matrix phases of a composite material to form or promote a stronger bond at the interface.

Crazing: Cracking of gel coat or resin due to stress. Region of ultra-fine cracks, which may extend in a network on or under the surface of a resin or plastic material. May appear as a white band. Often found in a filament-wound pressure vessel or bottle.

Creel: Glass Fiber Manufacturing – A framework used to hold forming cakes so they can be wound or roved into roving doffs. Creels generally hold 10 to 33 forming cakes that are replaced randomly when they run out or as doffs are roved. Composite Fabrication – The area in which pallets of roving are placed and “threaded up” through metal tubes or guide eyes to a chopper.

Creep: The slow movement of a plastic material with time.

Creep, Rate of: Rate of the slope of the creep-time curve at a given time. Deflection with time under a given static load.

Cross-linking: Chemical links between molecule chains in thermosetting resins. Styrene monomer is a cross-linking agent in polyester resins.

C-Stage: Final stage in the reaction of certain thermosetting resins in which the material is practically insoluble and infusible.

CTE: see Coefficient of Thermal Expansion

Cure: Cross-linking or total polymerization of a resin’s molecules that alters a material’s properties, changing it from a liquid to a solid.

Cure Cycle: The time/temperature/pressure cycle used to cure a thermosetting resin system or prepreg

Cure Temperature: Temperature at which a cast, molded, or extruded product, resin-impregnated reinforcement, adhesive or other material is subjected to curing.

Cure Time: The time required for liquid resin to reach a cured or fully polymerized state after catalyst has been added.

Curing Agent: A catalytic or reactive agent that, when added to a resin, causes polymerization. Also called hardener.

Cut Ends on Doff: Severed ends generally caused by abrasion during shipping or by careless use of a knife when the package is removed from the pallet.

Cycle: The complete, repeating sequence of operations in a process or part of a process. In molding, cycle time is the period (or elapsed time) between a certain point in one cycle and the same point in the next.


DAP: Diallyl Phthalate (Thermoset Resin).

Damping: The decay with time of the amplitude of free vibrations of a specimen.

Daylight: The distance, in the open position, between the moving and fixed tables or the platens of a hydraulic press. In a multi-platen press, daylight is the distance between adjacent platens. Daylight provides space so a molded part can be removed from the mold.

Deep-Draw Mold: A mold whose core is long in relation to its wall thickness.

Deflashing: A finishing technique used to remove excess, unwanted material (flashing) on a plastic molding.

Deflection Temperature Under Load: Temperature at which a simple beam has deflected a given amount under load (formerly called heat distortion temperature).

Deformation Under Load: Dimensional change of a material under load for a specific time following the instantaneous elastic deformation caused by the initial application of the load. (Also, ‘cold flow’ or ‘creep’.)

Delamination: Separation of composite layers, either local or covering a wide area. Can occur in the cure or subsequent life.

Denier: A direct numbering system for expressing linear density, equal to mass in grams per 9000m of yarn, filament, fiber, or other textile strand.

Density: A material’s weight per unit volume. Units refer to pounds per cubic inch (lb/cu in) grams/cubic centimeter. Higher numbers indicate heavier materials. Note: Density in lbs/cu in, and Specific Gravity are conveniently related by the following ratio:

Density (LB/cu in) Density (gr/cu cm)

——————– = 0.0361 ——————– = 0.9975

Specific Gravity Specific Gravity

Density, Fiber:

Mass per unit volume of the solid matter of which a fiber is composed, measured under specified conditions.

Dielectric: A nonconductor of electricity. A material’s ability to resist the flow of electrical current.

Dielectric Constant: An assembly’s ratio of capacitance when its two electrodes are separated solely by a plastic insulating material to its capacitance when the electrodes are separated by air.

Dielectric Heating: Heating materials by dielectric loss in a high-frequency electrostatic field.

Dielectric Strength: An electrical property indicating how well a material acts as an electrical insulator. It describes how much of an electrical voltage can be built up on one side of the material before it is communicated to the other side. Units are measured in volts per mil of thickness (volts/mil). Higher numbers indicate materials with better insulation properties. C means that the material conducts electricity and therefore has no dielectric strength.

Dimensional Stability: A plastic part’s ability to retain the precise shape to which it was molded, cast, or otherwise fabricated.

Direct-Sized Yarn: Specially formulated sizing on textile yarns that allows them to be resin compatible.

Dispersion: Degree to which roving separates into discrete bundles after being chopped. Good dispersion is characterized by a bed of bundles uniform in width. Poor dispersion is characterized by a wide distribution in the widths of various bundles in the bed. Poor dispersion can cause poor wet-through and wet-out.

Distortion: Change in shape from that which is intended. Symptomatic of laminating difficulties, curing problems, tooling problems or resin shrinkage.

Doctor Blade or Bar: A straight piece of material used to spread resin, as applying a thin film of resin for use in hot melt prepreg or as an adhesive film. Also called paste metering blade.

Doff:see Roving Doff

Doff Collapse: The failure of the roving doff to maintain its shape and stability during run-out or storage. Doff collapse generally occurs when there is only a 1/2″ to 1/4″ ring of roving left from the original doff.

Draft: The taper or slope of a mold’s vertical surface allowing molded parts to be removed.

Drape: The ability of pre-impregnated broad goods to conform to an irregular shape; textile conformity.

Dry Loft: Height of the bed of chopped fibers.

Dry Spot: A rea of incomplete surface film on laminated plastics; in laminated glass, an area over which the interlayer and the glass have not become bonded.

Dwell: A pause in the application of pressure or temperature to a mold, made just before it is completely closed, allowing gas to escape from the molding material.


E Glass: A family of calcia-alumina-silicate glasses with a certified chemical composition used for general purpose and most electrical applications. (ASTM D578-90.)

Elasticity: A material’s ability to recover its original size and shape after the force deforming it has been removed.

Ejection (Demolding): Removing a molded part from the mold by hand, mechanical means or use of compressed air.

Ejection Plate: A metal plate used to operate ejector pins; designed to apply a uniform pressure to them in the process of ejection.

Elastic Limit: The greatest stress a material can sustain without permanent strain after the stress has been completely released. A material is said to have passed its elastic limit when the load is sufficient to initiate plastic, or non-recoverable, deformation.

Elastomer: A material that substantially recovers its original shape and size at room temperature after a deforming force is removed.

Elongation: As mentioned under tensile modulus, when a bar is pulled it gets longer. The elongation tells how much longer it gets before it breaks.

Encapsulating: Completely surrounding an object with resin or a fiber resin composite. Sometimes used specifically in reference to the enclosure of capacitors or circuit board modules.

End: A single bundle of filaments.

End count: An exact number of ends supplied on a ball of roving.

Environment: The aggregate of all conditions (such as contamination, temperature, humidity, radiation, magnetic and electric fields, shock, and vibration) that externally influence the performance of an item.

Epoxy: A polymerizable thermoset polymer containing one or more epoxide groups cured by its reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans. An important matrix resin in composites and structural adhesive. Epoxies generally have higher physical properties than polyester resins. They are also more costly and difficult to process, and less able to withstand sunlight.

Even Tension: Applying the same amount of tension to each end of roving in a ball.

Exotherm: The liberation or evolution of heat during the curing of a plastic product.

Exothermic Heat: Heat given off during polymerization by chemical ingredients as they react and the resin cures.

Extend: To add fillers or low-cost materials in an economy producing endeavor. To add inert materials to improve void-filling characteristics and reduce crazing.

Extenders: Low-cost materials used to dilute or extend high-cost resins without extensive lessening of properties.

Extruder: Machine that pushes molten plastic through small holes to form fibers.


Fabrication: The process of glass fiber production during which forming cakes are put into creels and “roved” or fabricated onto doffs.

Fabricator: Manufacturer of reinforced plastic products.

Fall: Shape or pattern of chopped fibers as they drop from the chopper to the bed.

Fan or Curtain: Chopped bundles that fall or are thrown off the chopper and cot.

Fatigue: Failure or decay of mechanical properties after repeated applications of stress. Fatigue tests indicate a material’s ability to resist cracking, which eventually causes failure due to a large number of cycles.

Fatigue Life: How many cycles of deformation it takes before a test specimen will fail under a given set of oscillating conditions (stresses and strains).

Fatigue Limit: The maximum level under which a material can be stressed cyclically for an infinite number of times before it fails.

Fatigue Strength: The maximum cyclical stress a material can withstand for a given number of cycles before failure occurs. The residual strength after being subjected to fatigue.

Fiber: The major reinforcement material component in a composite matrix. Often, fiber is used synonymously with filament.

Fiber Content: The amount of fiber present in a composite. This is usually expressed as a percentage volume fraction or weight fraction of the composite.

Fiber Diameter Letter Designation: Fibers are generally classified in hundred thousandths, i.e., a “K” fiber has a mean average diameter of 50+ to 55 height. See chart below,

Fiber Direction: The orientation or alignment of the longitudinal axis of the fiber with respect to a stated reference axis.

Fiber Glass: Primarily means glass in fiber form. Also used to describe composite processing and applications, for example, fiber glass molding plant, fiber glass car.

Fiber Glass Reinforcement: Major material used to reinforce plastic. Available in mat, roving, fabric and other forms, it is incorporated into both thermosets and thermoplastics.

Fiber Orientation: Fiber alignment in a non-woven or a mat laminate where the majority of fibers are in the same direction, resulting in greater strength in that direction.

Fiber Pattern: Visible fibers on the surface laminate or molding. The thread size and weave of glass cloth.

Fiber Reinforced Plastic (FRP): A general term for a composite that is reinforced with cloth, mat, strands, or any other fiber form.

Filament: A single, threadlike fiber of glass.

Filament Winding: Process in which a resin-saturated strand of glass filament is wound around a rotating mandrel.

Filament Yarn: A yarn composed of continuous filaments assembled with or without twist.

Fill: The system of yarns running crosswise in a fabric (short for filling). Also known as weft. see Warp

Filler: An inexpensive substance that is added to plastic resins to extend volume, improve properties, and lower cost.

Fillet: A rounded filing of the internal angle between two surfaces of a plastic molding.

Fines: Bundles that have been split apart into smaller bundles composed of only a few or single filaments. Fuzz is usually made of fines.

Finish:see Size

Fire Retardants: Chemicals that reduce a resin’s tendency to burn.

Fish Eye: Effect of surface contamination causing a circular separation of paint or gel coat.

Flammability: How fast a plastic material will burn when subjected to a particular ASTM test. In this test, a flame is applied to one end of a strip of material. When the material starts burning the flame is removed and the time to consume a given amount of material is measured. Units are measured in inches per minute (in/min.). Higher numbers indicate that the material will burn faster under this particular test’s conditions. S.E. means self extinguishing; S.E. classified materials stop burning when the flame is taken away.

Flash: Portion of the charge that flows or is extruded from the mold cavity during the molding. Also refers to extra plastic attached to a molding along the parting line that must be removed before the part is considered finished.

Flash Point: Lowest temperature at which a substance gives off enough vapors to form a flammable mixture.

Flexible Molds: Rubber or elastomeric plastic molds used for casting plastics. They can be stretched to remove cured pieces with undercuts.

Flexural Modulus: A number referring to a material’s stiffness. It is used to calculate how far a bar will bend when a bending load is applied. Units are normally millions of pounds per square inch. (106 psi) – Giga Pascals (gPa). In two materials of equal thickness, the one with a higher number is more resistant to deflection.

Flexural Strength: Also known as bending strength. Describes how much nonmoving load can be applied to a bar before it yields or breaks. Units are normally thousands of pounds per square inch. (103 psi) – Mega Pascals (mPa). Higher numbers indicate stronger materials that can withstand a heavier load.

Flow: The movement of resin under pressure, allowing it to fill all parts of a mold. The gradual but continuous distortion of a material under continued load, usually at high temperatures; also called creep.

Flow Line: A mark on a molded piece made by the meeting of two flow fronts during molding. Also called striae, weld mark, or weld line.

Flow Marks: Wavy surface appearance of an object molded from thermoplastic resins, cased by improper flow of resin into the mold.

Fly: Fibers that fly out into the atmosphere during handling and processing.

Foam: Lightweight, cellular plastic material containing glass-filled voids. Typical foams include urethane, PVC, and polyester.

Force: The male half of the mold that enters the cavity, exerting pressure on the resin and causing it to flow. Also called punch.

Forming: Process of glass fiber production during which fibers are drawn, attenuated from molten glass and collected in forming cakes.

Forming Cakes or “Cakes”: Package of glass fibers produced during forming. This package is generally found on a tube placed on a forming carrier and sent through a drying/curing oven. “Cakes” are subsequently put into a roving creel and collected together into a roving doff.

Fracture: When a surface ruptures without the laminate completely separating, or where there is complete separation of a body because of external or internal forces.

Fracture Stress: The true, normal stress on the minimum cross-sectional area at the beginning of fracture.

Fracture Toughness: The damage tolerance of a material containing initial flaws or cracks. Used in aircraft structural design and analysis.

FRP: Acronym for fiber glass-reinforced or fiber-reinforced plastic, polymer or polyester.

Fuzz: Creel Fuzz – In glass fiber manufacturing, the broken filaments found around and on a roving creel. Chopper Fuzz – In Composite Fabrication, the broken filaments found around the glasscutter or chopper. see Fines. In the field, the broken filaments found around a roving pallet.

Fuzz Plug: Small, broken, compacted filaments of glass that collect inside the guide eye tubes feeding the chopper, preventing glass from running through.


Gate: Point at which molten thermoplastic enters the injection molding tool cavity.

Gel: A partial cure of plastic resins; a semisolid, jelly-like state similar to gelatin in consistency.

Gel: The initial jelly-like solid phase that develops during a resin’s formation from a liquid state. A semisolid network of solid aggregates in which liquid is held.

Gelation: The point during resin cure when viscosity has increased so much that resin barely moves when probed with a sharp instrument.

Gel Coat: Surface coat of a specialized, quick-setting polyester resin, either colored or clear, providing cosmetic enhancement and weather ability to a fiberglass laminate. Gel coat is an integral part of the finished laminate.

Gel Point: When a liquid begins to exhibit pseudo-elastic properties. This stage may be conveniently observed from the inflection point on a viscosity time plot.

Gel Time: Time required to change a flowable liquid resin into a non-flowing gel.

GFRP: Glass fiber-reinforced plastic, polymer or polyester.

Glass Blends: When several different fiber types, i.e. different lengths and diameters, are blended in the fiber slurry.

Glass Content: Percentage of glass in the compound.

Glass Fiber Wet-Process: Process of forming a glass mat on modified papermaking equipment.

Glass Transition: Reversible change in an amorphous polymer or amorphous regions of a partially crystalline polymer from or to a viscous, rubbery, or hard and relatively brittle condition.

Glass Transition Temperature (Tg): The appropriate midpoint of the temperature range over which glass transition takes place.

Good Side: Side of molding in contact with a mold surface.

Graphite Fiber: Fiber made from a precursor by oxidation, carbonization and graphitization process (which provides a graphitic structure).

Green: Resin, which has not completely cured and is still rather soft and rubbery.

Green Strength: That ability of the material, while not completely cured, to undergo removal from the mold and handling without tearing or permanent distortion.

GRP: A derivation commonly used in Europe referring to glass-reinforced plastic, polymer or polyester.

Glint: A visual defect in a fiberglass reinforced cured organic (usually corrosion- resistant resin) panel. The defect appears as many small visible unwet or foreign substances similar to a salt-and-pepper effect. The defect is not visible before cure but appears at the panel’s exotherm.

Guide Pin: A pin which guides mold halves into alignment on closing.


Hand: Fabric softness as determined by touch (individual judgment).

Hand Lay-up: The process of placing (and working) successive plies of reinforcing material or resin-impregnated reinforcement in position on a mold by hand. Method of molding room temperature curing thermosetting polymers, mainly epoxies and polyesters, in association with glass, mineral, or fiber reinforcements. Catalyzed resin mixtures are sprayed, brushed, or spatulated on a mold. A precut reinforcing layer is laid on the wet resin. After the resin soaks into the reinforcement, subsequent layers are built up to the required thickness and are cured, removed from the mold and trimmed. Some variations of hand lay-up techniques are bag molding, drape molding, vacuum molding and spray-up molding. Typical parts are custom auto bodies and boat hulls.

Hardener: A substance or mixture added to a plastic composition to promote or control curing by taking part in it.

Hard Glass: A roving product that is not very soluble in acetone or styrene, causing individual bundles to retain their integrity (hardness) and not filamentize in the matrix resin. Hard glass is often stiff and brash. The size is generally less than 40% soluble in acetone. see Soft Glass

Hardness: Resistance to surface indentation usually measured by the depth of penetration (or arbitrary units related to the depth of penetration) of a blunt point under a given load using a particular instrument according to a prescribed procedure.

Heat Cleaning: Batch and continuous processes in which organic yarn binder is removed from glass fabrics.

Heat Distortion Temperature: The temperature at which a material will bend under a given load. It was developed for thermoplastic materials, which soften considerably when heated. It has relatively little value as a design figure for thermosetting reinforced plastics. During this test, a load is applied in bending to cause 264 psi stress in the material. The temperature of the material is then raised until the material bends one-tenth of an inch at the center. Units are measured in degrees Fahrenheit (°F) minus degrees Centigrade (°C). Higher numbers mean that the material can be heated to a higher temperature before it deflects one-tenth of an inch under the arbitrary load of 264 psi.

Heat Distortion Point: Temperature at which a material’s strength begins to degrade. Now called deflection temperature.

Heat Resistance: The ability of plastics and elastomers to resist deterioration due to elevated temperatures.

Heat Sink: A contrivance for the absorption or transfer of heat away from a critical element or part. Bulk graphite is often used as a heat sink.

Helical Winding: In filament wound items, a winding in which a filament band advances along a helical path, but not necessarily at a constant angle (except on cylinders).

Het-Acid Resin: Polyester resin with exceptional fire qualities.

High-Pressure Laminates: Laminates molded and cured at pressures not lower than 6.9 MP (1.0 ksi), and more commonly between 8.3 to 13.9 Mpa (1.2 to 2.0 ksi).

Homogeneous: Describes a material with a uniform composition.

Homopolymer: A compound produced by polymerization.

Honeycomb: Manufactured product of resin-impregnated sheet material (paper, glass, fabric) or metal foil, formed into hexagonal-shaped cells. Used as a core material in sandwich construction.

Hoop Stress: The circumferential stress in a material of cylindrical form subjected to internal or external pressure.

Hybrid: A composite laminate consisting of two or more composite material systems. Two or more different fibers, such as carbon and glass or carbon and aramid, combined into a structure.

Hydraulic Press: A press in which molding force is created from pressure exerted by a fluid.

Hygroscopic: Material that absorbs moisture from the air.

Hysteresis: The energy absorbed in a complete cycle of loading and unloading. Mechanical energy is converted into friction energy (heat).


Ignition Loss: With glass, the difference in weight before and after binder or size has been burned off.

Impact Strength-Izod: A moving load is one that is moving when it strikes a bar. The effect of such a load is denoted by the work “impact”. The impact strength of a material is a measure of how much energy is absorbed by the bar when it is broken by a moving weight. Izod is one of many different test methods for measuring impact. Units are measured in foot pounds per inch of width; sometimes given as foot pounds per inch of notch. Joules/Meter (J/M). Higher numbers mean that the material will absorb more energy before it is broken by a moving weight.

Impact Test: Measure of the energy necessary to fracture a standard sample by an impulse load.

Impregnate: In reinforced plastics, to saturate a reinforcement, especially fiberglass, with a resin.

Inhibitor: A substance that retards polymerization, thus extending shelf life of a monomer. Also used to influence gel time and exotherm.

Initiator: Peroxides used as sources of free radicals. They are used in free-radical polymerization, for curing thermosetting resins, as cross-linking agents for elastomers and polyethylene, and for polymer modification.

Injection Molding: Method of forming plastic to the desired shape by forcing a heat-softened thermoplastic polymer into a relatively cool cavity under pressure or thermosetting polymer into a heated mold.

Inorganic Pigments: Natural or synthetic metallic oxides, sulfides, and other salts that impart heat and light stability, weathering resistance, color, or migration resistance to plastics.

Insert: An integral part of plastic molding consisting of metal or other material that may be molded into the part or pressed into position after the molding is completed.

In-Situ: In place. In the position which it will finally occupy, e.g. molding or forming foam.

Instron: Instrument for determining the tensile and compressive properties of materials.

Interface: A surface that lies between two different materials.

Interlaminar: Descriptive term pertaining to an object (for example, voids), event (for example, fracture), or potential field (for example, shear stress) referenced as existing or occurring between two or more adjacent laminae.

Interlaminar Shear: Shearing force that produces a relative displacement between two laminae in a laminate along the plane of the interface.

Intumescent: Fire-retardant technology causing an otherwise flammable material to foam, forming an insulating barrier when exposed to heat.

Irreversible: Not capable of re-dissolving or re-melting. Chemical reactions that proceed in a single direction and are not capable of reversal (as applied to thermosetting resins).

Isocyanate Plastics: Plastics based on resins made by the reaction or organic isocyanates with other compounds.

Isophthalic: Polyester resin based on isophthalic acid, generally higher in properties than a general purpose or orthothatic polyester resin.

Isotropic: Having uniform properties in all directions.

Izod Impact Test: A test for shock loading in which a notched specimen bar is held at one end and broken by striking, and the energy absorbed is measured.


Jackstrawing: Visual effect of glass fiber turning white in a cured laminate. It may not affect the strength of a laminate, but could indicate air entrapment or water contamination.


Kevlar: An organic polymer composed of aromatic polyamides having a para-type orientation (parallel chain extending bonds from each aromatic nucleus).

Knitted Fabric: A textile structure produced by interlooping one or more ends of yarn or comparable material.

Knot: The means of joining strands of two doffs of roving. The knot is generally a reduced triple loop surgeon’s knot, square knot or overhand knot.


Laminate: Primarily means a composite material system made with layers of fiber reinforcement in a resin. Sometimes used as a general reference for composites, regardless of how made. Examples of usage: laminate consumption by market, compression-molded laminate.

Land: Portion of a mold which provides the separation or cutoff of the flash from the molded article; in the screw of an extruder, the bearing surface along the top of the flights; in an extrusion die, the surface parallel to the flow of material; in a semi-positive or flash mold, the horizontal bearing surface; in a two-piece mold, a platform build up to the split line.

Lay: In glass fiber, the spacing of the roving bands on the roving package expressed in the number of bands per inch; in filament winding, the orientation of the ribbon with some reference, usually the axis of rotation.

Laydown: The degree of uniformity in thickness that a bed of chopped glass fibers exhibits across the width of the chopped pattern.

Layer: The horizontal plane of doffs on a pallet. A pallet usually has four layers of twelve doffs each. also see Cell

Lay-up: Act of building up successive layers of polymer and reinforcement. Layers of catalyzed resin and fiberglass or other reinforcements are applied to a mold in order to make a part. Also refers to the reinforcing material placed in position in the mold, the process of placing reinforcing material in position in the mold, or the resin-impregnated reinforcement.

L/D Ratio: A term used to define an extrusion screw, which denotes the ratio of the screw length to the screw diameter.

Lengthwise Direction: Refers to the cutting of specimens and application of loads. For rods and tubes, lengthwise is the direction of the long axis. For other shapes of materials that are stronger in one direction than in the other, lengthwise is the direction that is stronger. For materials that are equally strong in both directions, lengthwise is an arbitrarily designated direction that may be with the grain, direction of flow in manufacture, longer direction, etc.

Liquid-Crystal Polymer: A newer thermoplastic polymer that is melt process capable and develops high orientation in molding, resulting in tensile strength and high-temperature capability.

Load-Deflection Curve: A curve in which the increasing tension or compression of flexural load are plotted on the ordinate axis and the deflections caused by those loads are plotted on an abscissa axis.

Loom: A mechanical device that interlaces fibers at right angles with varying degrees of weave construction (weight, thickness and design). More modern looms are air jet but rapier and more traditional shuttle equipment is still in use.

Loom Beam: A large, flanged cylinder onto which all warp yarns are wound and from which yarns enter the loom.

Loop: Small open place in the strands due to excessive length of one or more strands.

Loss on Ignition: Weight loss, usually expressed as a percent of the total, after burning off an organic sizing from glass fibers, or an organic resin from a glass fiber laminate.

Low-Pressure Laminates: Laminated, molded, and cured using pressures from 400 psi down to and including the pressure obtained by mere contact of the plies.

Low-Pressure Molding: The distribution of relatively uniform low pressure (200 psi or less) over a resin-bearing fibrous assembly of cellulose, glass, asbestos, or other material, with or without application of heat from an external source, to form a structure possessing definite physical properties.

Lubricant: A material added to most sizing to improve the handling and processing properties of textile strands.


Mandrel: The core tool around which resin-impregnated paper, fabric or fiber is wound to form pipes, tubes or structural shell shapes.

Manipulator: Highly efficient system eliminating the need for a press screw driven frame structure that opens and closes the mold in a controlled line of draw to prevent tool damage.

Mat: A fibrous material for reinforced plastic consisting of randomly oriented chopped filaments, short fibers (with or without a carrier fabric) or swirled filaments loosely held together with a binder.

Mat Binder: Resin applied to glass fiber and cured during the manufacture of mat that holds fibers in place and maintains the mat’s shape.

Matched Metal Molding: A reinforced plastics manufacturing process in which matching male and female metal molds are used (also called compression molding) to form the part with time, pressure and heat.

Matrix: The resin component of a polymer composite. Both thermoplastic and thermoset resins may be used, as well as metals, ceramic and glasses. (Plural: matrices.)

Mat Strength: The mat’s ability to resist being pulled apart under tension during impregnation and molding.

Matchsticks: Strand-to-strand adhesion. A matchstick is a wide bundle that has 3 to 4 times as many filaments than the majority of bundles in the bed.

Mechanical Properties: A material’s properties, such as compressive and tensile strength and modulus, that are associated with elastic and inelastic reaction when force is applied. The individual relationship between stress and strain.

Melamine: Thermoset resin.

Metallic Fiber: Manufactured fiber composed of metal, plastic-coated metal, metal-coated plastic or core completely covered by metal.

Microballoons: Microscopic bubbles of glass, ceramic or Phenolic, used as a filler or to create syntactic foam or putty mixtures.

Microcracking: Crack formed in composites when thermal stresses locally exceed the strength of the matrix.

Micron: One micron = .001 millimeter = .00003937 inch.

Mil: Unit used to measure the diameter of glass fiber strands (1 mil = 0.001 in.).

Milled Fiber: Continuous glass strands hammer milled into very short glass fibers. Useful as inexpensive filler or anti-crazing reinforcing filler for adhesives.

M.I.T™: see Multiple Insert Tooling

MPG: see Mold Protection Guard

Modulus, Initial: The slope of the initial straight portion of a stress strain or load-elongation curve.

Modulus of Elasticity: How much a material can bend without losing its ability to return to its original physical properties.

Moisture: The amount of volatiles on the glass expressed as a percentage of the total weight.

Moisture Content: The amount of moisture in a material determined under prescribed conditions, expressed as a percentage of the mass of the moist specimen; that is, the mass of the dry substance plus the moisture present.

Mold: The cavity or matrix into or on which the plastic composition is placed and from which it takes form. The tool used to fabricate the desired part shape.

Molded Edge: An edge that is not physically altered after molding for use in final form, and particularly one that does not have fiber ends along its length.

Molding: The forming of a polymer or composite into a solid mass of prescribed shape and size.

Molding Compounds: Plastics in a wide range of forms meeting specific processing requirements. Granules or pellets are popular forms.

Molding Cycle: The period of time required for the complete sequence of operations on a molding press to produce one set of moldings.

Molding Pressure: Pressure applied to the ram of an injection machine or compression or transfer press forcing softened plastic to completely fill mold cavities.

Molding Pressure, Compression: Unit pressure applied to material in the mold. The area is calculated from the projected area taken at right angles to the direction of applied force and includes all areas under pressure during complete closing of the mold. Unit pressure is calculated by dividing the total force applied by the projected area, expressed in pounds per square inch.

Mold Protection Guard: Prevents over pressurization of the RTM mold during the injection process.

Mold-Release Agent: Lubricant, liquid or powder (often silicone oils and waxes) that prevents molded articles from sticking in the cavity.

Mold Shrinkage: Immediate shrinkage that occurs when a part is removed from a mold and cooled to room temperature; the difference in dimensions, expressed in inches between a molding and the mold cavity in which it was molded (at normal temperature measurement); the incremental difference between the dimensions of the molding and the mold from which it was made, expressed as a percentage of the dimensions of the mold.

Monomer: A simple molecule capable of reacting with like or unlike molecules to form a polymer; the smallest repeating structure of a polymer (mers); for addition polymers, this represents the original unpolymerized compound.

Multiple-Cavity Mold: A mold with two or more mold impressions; that is, a mold producing more than one molding per molding cycle.

Multiple Insert Tooling: Low-risk, high-output composite tooling breakthrough. MIT tooling technology offers the composite molder quick-change multiple mold surfaces without multiple tooling costs. Each surface is an exact clone of its counterpart, guaranteeing mold cavity accuracy and replication of molded parts.


Nesting: In reinforced plastics, placing of plies of fabric so that the yarns of one ply lie in the valleys between the yarns of the adjacent ply (nested cloth).

NOL Ring: Parallel filament wound test specimen used in whole or in part to measure a material’s mechanical strength properties.

Non-Air-Inhibited Resin: Resin in which the surface cure will not be inhibited or stopped by the presence of air.

Non-Woven Fabric: A textile structure produced by bonding or interlocking fibers, or both, accomplished by mechanical, chemical, thermal and/or solvent means.

Notch Sensitivity: The extent to which a material’s sensitivity to fracture is increased by a surface in homogeneity such as a notch, a sudden change in section, a crack or a scratch. Low notch sensitivity is usually associated with ductile materials and high notch sensitivity with brittle materials.


Orange Peel: Gel coated or painted finish that is not smooth and is patterned similar to an orange’s skin.

Organic: Matter originating in plant or animal life, or composed of chemicals of hydrocarbon origin, either natural or synthetic.

Orientation: Position with relation to flow of polymer in the mold.

Orthophthalic Resin: Polyester resin based on Orthophthalic acid, also known as a general purpose resin (GP).

Overlay Sheet: Non-woven fibrous mat (of glass, synthetic fiber or other material) used as the top layer in a cloth or mat lay-up to provide a smoother finish, minimize the appearance of a fibrous pattern, or permit machining or grinding to a precise dimension. Also called surfacing mat.

Overspray: A specially formulated binder applied to texturized yarn that helps retain the yarn bulk after texturizing.


Parting Line: A mark on a molded piece where mold sections have met in closing.

PBT: Polybuthlene Therephthalate (Thermoplastic Polyester Resin).

PET: Polyethylene Terephthalate (Thermoplastic Polyester Resin).

Phenolic Resin: Thermosetting resin produced by the condensation of an aromatic alcohol with an aldehyde, particularly of phenol with formaldehyde. Used in high-temperature applications with various fillers and reinforcements.

Pigment: Colorant added to gel coat or resin.

Pinholes: Small holes on the exposed gel coated surface that are about the diameter of common pins and may be easily counted.

Pit: Small regular or irregular crater in the surface of a plastic, usually of equal width and depth.

Plastic: Material of which an essential ingredient is an organic polymer of large molecular weight and also contains hardeners, fillers and reinforcements; is solid in its finished state; and has been shaped by flow during some stage of its manufacture or processing. Made of plastic. A plastic may be either thermoplastic or thermoset.

Plastic Deformation: Change in dimensions of an object under load that is not recovered when the load is removed; opposed to elastic deformation.

Plasticizers: Material added to increase a plastic’s workability and flexibility. Normally used in thermoplastics. Also a lower molecular weight material added to epoxy to reduce stiffness and brittleness, thereby resulting in a lower glass transition temperature for the polymer.

Plastic Tooling: Tools (mostly for metal forming trades) constructed of plastics, generally laminates or casting materials.

Platens: Mounting plates of a press to which the entire mold assembly is bolted.

Plied Yarn: Yarn formed by twisting two or more single yarns together in one operation. (Synonyms: folded yarn, formed yarn)

Plug: A composite industry term for a pattern or model.

Ply: The number of single yarns twisted together to form a plied yarn; also the number of plied yarns twisted together to form a cord. The individual yarn in a plied yarn or in a cord. One of several layers of fabric.

Poisson’s Ratio: The ratio of transverse strain to axial strain during axial load.

Polyamide: A polymer in which the structural units are linked by amide or thioamide groupings. Many polyamides are fiber-forming.

Polyester Combination Yarn: A polyester/fiber glass hybrid yarn.

Polyester (Unsaturated): Product of an acid-glycol reaction commonly blended with a monomer to create a polymer resin. In its thermosetting form it is the most common resin used in the FRP industry.

Polymer: Chain molecule composed of many identical groups, commonly found in plastics.

Polymerization: Chemical bonding of polymer molecules during the curing reaction.

Porosity: Entrapped gas bubbles or voids in a gel coat film.

Positive Mold: Mold that applies pressure to the piece being molded without material escaping.

Post Bake: see Post Cure

Post Cure: Heat cycle that a roving doff goes through after fabrication. Generally used to help “set” the ribbon on the outside of the doff, improving the doff’s stability and resistance to package collapse.

Pot Life: The length of time that catalyzed resin retains a viscosity low enough to be used in processing.

PPO: Polyphenylene Oxide (Thermoplastic Resin).

PPS: Polyphenylene Sulfide (Thermoplastic Resin).

Preform: Preshaped fibrous reinforcement formed when chopped fibers are distributed by air, water flotation or vacuum over the surface of a perforated screen to the approximate contour and thickness desired in the finished part. Also, a preshaped fibrous reinforcement of mat or cloth formed to the desired shape on a mandrel or mock-up prior to being placed in a mold press. Also, a compact “pill” formed by compressing premixed material to facilitate handling and control the uniformity of charges for mold loading.

Pregel: An unintentional extra layer of cured resin on part of a reinforced plastic’s surface. (not related to gel coat)

Premix: A compound prepared prior to and apart from the molding operation containing all components required for molding: resin, reinforcement, fillers, catalysts, release agents and other compounds.

Prepreg: Either ready-to-mold material in sheet form or ready-to-wind material in roving form, which may be cloth, mat, unidirectional fiber or paper impregnated with resin and stored for use. The resin is partially cured to a B-stage and supplied to the fabricator, who lays up the finished shape and completes the cure with heat and pressure. The two distinct types of prepreg available are (1) commercial prepregs, where the roving is coated with a hot melt or solvent system to produce a part meeting specific customer requirements, and (2) wet prepreg, where the basic resin is installed without solvents or preservatives but has limited room-temperature shelf life.

Pressure Bag: A membrane conforming to the inside of a laminate laid up on a mold. The membrane or bag is then inflated, applying pressure that consolidates and densifies the laminate.

Print Through: Distortion in a part’s surface through which the pattern of the core or fiberglass reinforcement is visible. Also known as print out, telegraphing or read through.

Pultrusion: Continuous process for manufacturing composites with a constant cross-sectional shape. The process consists of pulling a fiber-reinforcing material through a resin impregnation bath and shaping die, where the resin is subsequently cured.

Putty: Thickened mixture of resin made by adding fillers and reinforcing fibers.




Reaction Injection Molding (RIM): Process for molding polyurethane, epoxy and other liquid chemical systems. Combining two to four components in the proper chemical ratio is accomplished by a high-pressure impingement-type mixing head, from which mixed material is delivered into the mold at low pressure, where it reacts (cures).

Re-Chop: Bundles that have clung to the chopper or cot and are chopped again into shorter lengths. Re-chop causes excessive chopper fuzz as the strands are cut and mashed into smaller bundles.

Reinforced Molding Compound: Compound consisting of a polymer and reinforcement fiber or filler, supplied by a raw material producer as ready-to-use materials.

Reinforced Plastics: Molded, formed, filament-wound, tape-wrapped or shaped plastic parts consisting of resins to which reinforcing fibers, mats and fabrics have been added before the forming operation to provide strength properties greatly superior to those of the base resin.

Reinforced Reaction Injection Molding (RRIM): A reaction injection molding with reinforcement added. see Reaction Injection Molding

Reinforcement: Strong material bonded into a matrix to improve its mechanical properties. Reinforcements are usually long fibers, chopped fibers, whiskers and particulates. The term should not be used synonymously with filler.

Release Agent: Compound used to reduce surface tension or adhesion between a mold and a part.

Resin: Solid or pseudo-solid organic material, usually of high molecular weight, that tends to flow when subjected to stress. Most resins are polymers. In reinforced plastics, the material used to bind together the reinforcement material. see Matrix and Polymer

Resin Content: The amount of resin in a laminate, expressed either as a percent of total weight or total volume.

Resin-Rich Area: Localized area filled with resin and lacking reinforcing material.

Resin-Starved Area: Localized area of insufficient resin, usually identified by low gloss, dry spots or fiber showing on the surface.

Resin Tearing: Separation of pigments in a gel coat affecting cosmetic appearance.

Resin Transfer Molding (RTM): A process in which catalyzed resin is transferred or injected into an enclosed mold where fiberglass reinforcement has been placed.

Rib: Reinforcing member of a fabricated or molded part.

Ribbon: The propensity of glass bundles to “stick” together and act as a strand or end.

RIM: see Reaction Injection Molding.

Room Temperature Curing Adhesives: Adhesives that set to handling strength within an hour at 68 to 86 degrees F, and later reach full strength without heating.

Roving: A collection of untwisted strands wound together into a doff (ball). Also, another name for the fabrication process step.

Roving Ball: A term used to describe the supply package offered to the winder. It consists of a number of ends or strands wound to a given outside diameter onto a length of cardboard tube.

Roving Doff or “Doff”: The final product sold to the customer. Made by roving or pulling together a group of forming cakes. (the number of which depends upon the product being made)

RP: Reinforced plastic, polymer or polyester.

RRIM: see Reinforced Reaction Injection Molding

RTM: see Resin Transfer Molding

RTM Light: see Vacuum Molding

RTP: Sometimes used to distinguish reinforced thermoplastic from reinforced thermosetting plastic.

Rule-of-Mixtures: A composite’s properties are the combination of properties in its two constituent materials. The composite property equals the amount of fiber property multiplied by the volume percentage of fiber, plus the amount of matrix property multiplied by the volume percentage of matrix.

Run-Out: The process of pulling glass from the doff to the chopper.

Runner: The channel through which thermoplastic material moves through a mold.


SAN: Styrene Acrylonitrile (Thermoplastic Resin).

S Glass: A family of magnesium-alumina-silicate glasses with a certified chemical composition that conforms to an applicable material specification and produces high mechanical strength.

Sandwich Constructions: Panels composed of a lightweight core material, such as honeycomb or foamed plastic, to which two relatively thin, dense, high-strength or high-stiffness faces or skins are adhered.

Scrim: Light woven or non-woven fabric with relatively large openings between the yarns, used to reinforce paper and other products.

Secton Beam: Flanged cylinder onto which yarn is drawn and accumulated from yarn bobbins or packages.

Serving: Wrapping of yarn around a product in one or more layers to form a protective covering.

Set-Up: To harden, as in curing.

Sewing Thread: Flexible small diameter yarn or strand, usually treated with a surface coating and/or lubricant, used to stitch one or more pieces of material together or stitch an object to a material.

Shear: Engineering term referring to force normally applied to the surface of a given material. The movement between plies of a laminate is referred to as interlaminate shear.

Shear Edge: Cut-off edge of the mold.

Sheet Molding Compound (SMC): Composite of fibers, usually polyester resin, pigments, fillers and other additives, that have been compounded and processed into sheet form to facilitate handling in the molding operation.

Shelf Life: Allowable storage time before a product must be used.

Shore Hardness: A material’s resistance to indentation from a spring-loaded indenter. Higher numbers indicate materials with greater resistance.

Shrinkage: The relative change in dimension between the length measured on the mold when it is cold and the length of the molded object 24 hours after it has been taken out of the mold.

Single Yarn: The simplest strand of textile material suitable for weaving, knitting and other operations.

Sink Mark: A shallow depression or dimple on the surface of an injection molded part due to its surface collapsing from local internal shrinkage after the gate seals; an incipient short shot.

Size: Treatment applied to glass fiber that allows resin and glass to adhere to one another. Also allows glass fiber to be conveniently handled.

Skein: A loose coil of roving or strands generally used as a Quality Control sample.

Skin Coat: First layer of laminate next to the gel coat.

Slashing: Applying size to a width of warp yarns on a continuous basis.

Slink: Glass bundles thrown off the chopper or cot during chopping.

Sliver: A term describing the geometry of fibrous glass reinforcement in the forming operation. For example, 2K37 S/2 means a configuration that makes a nominal fiber diameter in the “K” range (3700 yards to a pound) and is split into two discrete bundles in the forming cake.

Sluffing: see Doff Collapse

SMC: see Sheet Molding Compound

Snarl:see Birdnest

Soft Glass: A roving product whose sizing is moderately soluble in acetone or styrene, resulting in bundles that tend to open readily or filamentize the matrix resin. The size is generally between 50% and 80% soluble in acetone.

Solid: The amount of sizing on glass expressed as a percentage of the total weight.

Solvent Resistance: The non-swelling of a material; also, a material’s ability to resist being dissolved by a particular solvent.

Specific Gravity: A material’s weight in relation to the weight of an equal volume of water. For example, a material with a Specific Gravity of 2.0 weighs twice as much as an equal volume of water. Because specific gravity is a ratio of values for two materials, there are no units. Higher numbers indicate heavier materials.

Specific Heat (Thermal Capacity): The amount of heat needed to raise the temperature of one pound of material one degree Fahrenheit (F). Units are measured in BTUs per pound per degree (BTA/LB/°F) – Joules/Kilogram Kelvin (J/KgK). Higher numbers indicate more input heat energy is needed to raise the temperature of a material.

Specimen: An individual piece or portion of a sample used for a specific test; also, of specific shape and dimensions.

Splice : Joining two ends of yarn by intertwining, knotting, overlapping or adhering them together.

Split: One bundle.

Split Mold: A mold whose cavity is formed of two or more components held together by an outer chase. The components are known as splits.

Spray-Up: Technique in which a spray gun is used as an applicator tool. In reinforced plastics, for example, fibrous glass and resin can be simultaneously deposited in a mold.

Sprue: Connector between the runner and nozzle in an injection-molding machine.

Splitting Efficiency: Ratio of the actual number of ends divided by the theoretical number of ends in a roving doff, expressed as a percentage.

Staple: Filaments produced in short lengths from the bushing (usually less than 17 inches) gathered into strands or sliver. see Continuous Filament

Starved Area: Portion of a plastic part without sufficient resin to completely wet out the reinforcement. Usually due to improper wetting, impregnation or excessive molding pressure.

Static: Buildup of an electrical charge causing the chopper roving to “cling” or stick to the chopper, line and/or people. The static level is quantified by measuring the electrical field strength in kilovolts per inch.

Sticker: see Trapped End

Stiffness: The relationship of load to deformation; a term often used when the relationship of stress to strain does not conform to the definition of Young’s modulus.

Strand: In the roving process or shop, a primary group of bundles gathered together in a creel. A strand is that which is pulled out of a doff; also a plurality of drawn and elongated individual filaments combined together to form an individual strand. Strands are held together and protected by sizing.

Strand Count: According to the U.S. Yardage System, the length in hundreds of yards of a single strand having a mass of one pound. In the European TEX System, the mass in grams of a strand 1000 meters in length.

Strand Integrity: The size’s ability to keep all filaments in a bundle stuck together during chopping. Good strand integrity is required for good flow in or wet-through and wet-out on the mold.

Strength, Flexural: Maximum stress that can be borne by surface fibers in a beam in bending. Flexural strength is the unit resistance to maximum load prior to failure by bending, usually expressed in pounds per square inch.

Stress-Strain: Stiffness, expressed in pounds per square inch or kilograms per square centimeter, at a given strain.

Stress-Strain Curve: Simultaneous readings of load and deformation, converted to stress and strain, plotted as ordinates and abscissa to obtain a stress-strain diagram.

Structural Reaction Injection Molding (S-RIM): Evolution of two other plastic molding processes, RIM and RTM. S-RIM uses the fast polymerization reactions of RIM-type polymers, its intensive resin mixing procedures and its rapid resin injection rates. S-RIM also employs preforms like RTM to obtain composite mechanical properties.

Styrene/Acetone Solubility: Percentage of sizing on glass fiber dissolved off the strand in styrene or acetone after soaking in the solvent.

Styrene Monomer: A water-thin liquid monomer used to thin polyester resins and act as the cross-linking agent.

Surfacing Mat: Very thin mat, usually 180 to 510 mm (7 to 20 mil) thick, used primarily to produce a smooth, resin-rich surface on a reinforced plastic laminate, or for precise machining or grinding. see Veil

Surfactant: Chemicals used to modify or change the surface of a layer of resin or polymer. Usually used to form a film on a curing resin, producing a tack-free surface.


Tack: Surface stickiness.

Tack Free: Surface which is not sticky after cure.

Tangent Modulus: Slope of the line at a predefined point on a static stress-strain curve, expressed in force per unit area per unit strain. This is the tangent modulus at that point in shear, tension or compression, as the case may be.

Tape: A narrow fabric whose mass per unit area is less than 0.5 kg/m 2 (0.1 LB/ft2) for each 25.4 mm (1 in.) of width; used primarily for utilitarian purposes.

Tenacity: Term used in yarn manufacture and textile engineering to denote the strength of a yarn or filament of a given size. Numerically, it is expressed as grams of breaking force per denier unit of yarn or filament size; grams per denier, gpd. The yarn is usually pulled at the rate of 12 inches per minute. Tenacity equals breaking strength (grams) divided by denier.

Tensile Elongation: Engineering term referring to the amount of stretch a sample experiences during tensile strain.

Tensile Load: Lo ad applied away from and to opposite ends of a given sample.

Tensile Modulus: When a bar is pulled in tension, it gets longer. Tensile modulus calculates how much longer it will get when a certain load is applied. Units are normally millions of pounds per square inch. (10 6 psi) – Giga Pascals (gPa). Higher numbers indicate materials that do not elongate as much as others under equal tensile loading conditions.

Tensile Strength: The amount of nonmoving load a bar can withstand before it breaks due to elongation. Units are normally thousands of pounds per square inch. (103 psi) – Mega Pascals (mPa). Higher numbers indicate materials that can withstand a stronger pull before breaking.

Tensile Stress: Normal stress caused by forces directed away from the plane on which they act.

Tension Device: A mechanical or magnetic device that controls tension.

TEX: A unit for expressing linear density, equal to the mass in grams of 1 km of yarn, filament, fiber or other textile strand.

Texturized Glass Yarn: Yarn processed from continuous filaments that have been disoriented, adding bulk.

Thermal Coefficient of Expansion: Measures how much the length of a material will change when it is heated or cooled. The value given is based on the inch as a unit. The number given shows how much one-inch of material will increase if its temperature is raised one degree Fahrenheit. Units are expressed in inches per inch per degree Fahrenheit (in/in/°F) – Meters/Meter/°C. Higher numbers mean that the material will expand or lengthen more for each degree that its temperature increases. Smaller numbers indicate relative stability to changes in temperature.

Thermal Conductivity (K factor): The amount of heat transferred by conduction; i.e., how much heat is transferred from one side of a plate to the other. It is measured as BTUs (units of heat in the English system) per hour per unit area (square feet) for a thickness of one inch and a temperature difference of one degree Fahrenheit between both sides of the plate. Units are expressed as BTU/hr/sq/°F/inch. – Watt/(Meter Deg Kelven) W/MK. Higher numbers mean that the material will absorb more energy before it is broken by a moving weight.

Thermoplastic: Capable of being repeatedly softened by an increase of temperature and hardened by a decrease in temperature. Applicable to those materials whose change upon heating is substantially physical rather than chemical, and that in the softened stage, they can be shaped by flow into articles by molding or extrusion.

Thermoplastic Polyesters: Class of thermoplastic polymers in which the repeating units are joined by ester groups. The two important types are (1) polyethylene terephthalate (PET), which is widely used as film, fiber and soda bottles; and (2) polybutylene terephthalate (PBT), primarily a molding compound.

Thermoset: A material that undergoes a chemical reaction caused by heat, catalyst or other condition, which results in the formation of a solid. Once it becomes a solid, it cannot be reformed.

Thermosetting Polyesters: 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. Cure is effected through vinyl polymerization using peroside catalysts and promoters or heat to accelerate the reaction. The two important commercial types are (1) liquid resins that are cross-linked with styrene and used either as impregnants for glass or carbon fiber reinforcements in laminates, filament-wound structures and other built-up constructions, or as binders for chopped-fiber reinforcements in molding compounds, such as sheet molding compound (SMC), bulk molding compound (BMC) and thick molding compound (TMC); and (2) liquid or solid resins cross-linked with other esters in chopped-fiber and mineral-filled molding compounds (for example, alkyd and diallyl phthalate).

Theoretical End Count: The maximum number of bundles in a roving doff; for example, a roving doff made with 18 forming cakes in the creel that were “split out” 4 ways in forming will have 64 theoretical ends.

Thickeners: Material added to resin to thicken it or raise its viscosity index so that it will not flow as readily.

Thixotropic: The property of becoming a gel at rest, but liquefying again on agitation.

Tooling Gel Coat: Gel coat formulated for mold surfaces.

Transfer: The smooth and successful transition from one roving doff to another during processing.

Translucent: Permits a percentage of light to pass but not optically clear like window glass.

Trapped End: A loop embedded into a doff during the roving process that gets stuck during run-out with such tenacity that it prevents the entire strand from running freely to the chopper.

Tube Stoppage or Plug: The failure of glass to run through metal tubes or guide-eyes from the creel to the chopper. Usually caused by a large knot or small birdnest becoming stuck inside the tube or guide-eye.

Turn: One 360° revolution of the components around the axis of the strand.

Turnaround: The portion of the roving doff where the roving changes direction when it is pulled out of the doff.

Twist and Ply Frames: Machines used to twist and ply glass yarns.


Ultimate Tensile Strength: The ultimate or final stress sustained by a specimen in a tension test; the stress at moment of rupture.

Undercut: Having a protuberance or indentation that impedes withdrawal from a rigid two-piece mold; any such protuberance or indentation, depending on the design of the mold (tilting a model in the mold may eliminate an apparent “undercut”).

Unidirectional: Strength lying mainly in one direction. A glass reinforcement in which the fiber is oriented in one direction.

Untied Ends: The lack of a knot or splice between two doffs, one on top of the other, which prevents successful transfer from the top doff to the bottom doff in a creel par.

Untreated: A descriptive term for glass fiber yarns having no applied chemicals or coatings other than minimal lubricant or binder for controlling intra-fiber abrasion.

UV Stabilizer: Chemical compound which improves resistance to degradation from ultraviolet radiation.


Vacuum Bag Molding: Process in which a sheet of flexible transparent material, bleeder cloth and release film are placed over the lay-up in the mold and sealed at the edges. A vacuum is applied between the sheet and the lay-up. Entrapped air is mechanically worked out of the lay-up and removed by the vacuum. The part is cured with temperature, pressure and time. Also called bag molding.

Vacuum Molding: Low cost entry method into RTM which uses a rigid cavity mold half and a semi-rigid upper mold half, both made of FRP. Capable of producing 3-4 times as many moldings as that produced through open mold, with acceptable repeatability (but not equal to that of RTM.)

Veil: Ultra thin mat similar to a surface mat.

Vinyl-Coated Glass Yarn: Continuous glass filament yarn coated with pigment and plasticized vinyl chloride resin.

Vinyl Esters: Thermosetting resins containing esters of acrylic and/or methacrylic acids, many of which have been made from epoxy resin. Cure is accomplished as with unsaturated polyesters by CO-polymerization with other vinyl monomers, such as styrene.

Viscosity: Measure of a liquid’s resistance to flow.

Void Content: The percentage of voids in a laminate.

VM: See Vacuum Molding.


Warp: Yarn running lengthwise in a woven fabric. A group of yarns in long lengths and approximately parallel, put on beams or warp reels for further textile processing including weaving, knitting, twisting, dyeing, etc.

Warp Size: Chemicals applied to the warp yarn to improve strand integrity, strength and smoothness in order to withstand rigors of weaving.

Weave: Particular manner in which a fabric is formed by interlacing yarns; usually assigned a style number.

Weft: The system of yarns running crosswise in a fabric. Also known as fill.

Wet Flexural Strength (WFS): Flexural strength after water immersion, usually after boiling the test specimen for two hours in water.

Wet Lay-Up: Reinforced plastic with liquid resin applied after the reinforcement is laid up. Opposite of “dry lay-up”, “prepreg”.

Wet-Out Rate: Time required for a plastic to fill the interstices of a reinforcement material and wet the surface of the reinforcement fibers; usually determined by optical or light transmission means.

Wet-Process: A process for forming a non-woven web from a water slurry on “papermaking’ equipment. Also known as “wet-laid” or “wet-formed”.

Wet Strength: Strength of paper when saturated with water, especially in discussions of processes whereby the strength of paper is increased by the addition, in manufacture, or plastic resins; the strength of an adhesive joint determined immediately after removal from a liquid in which it has been immersed under specified conditions of time, temperature and pressure.

Wet-Through: The degree and/or rate of encapsulation of sized glass fiber bundles in a laminate. Also, the rate and/or degree of which the polymer matrix resin system can flow through a bed of sized glass bundles or strands and encapsulate each bundle of filaments.

Wides: Term describing bundles of roving that are wider than most of the other bundles in a bed of chopped glass fibers. They usually contain 3 to 4 times more filaments than most of the other bundles in the roving. (See Matchstick).

Woven Roving Fabric: Heavy fabrics woven from continuous filament in roving form. Usually in weights between 18-30 oz. per square yard.

Wrinkle: Surface imperfection pressed into laminated plastics similar to a crease or fold in paper, fabric or other base. Also occurs in vacuum bag molding when the bag is improperly placed, causing a crease.




Yardage: Similar to Yield, but used to describe the linear density of “bare glass” or an unsized product. Yardage specifies the number of yards of glass required to weigh one pound, measured in hundreds. For example, K18 is a K fiber diameter that has 180yards in one pound of glass.

Yarn: Generic term for a continuous strand of textile fibers, filaments or material in a form suitable for knitting, weaving or intertwining to form a textile fabric.

Yield: Linear density of a roving or yarn, measured by the number of yards per pound.

Yield Point: First stress in a material, less than the maximum attainable stress, at which strain increases at a higher rate than stress. The point at which permanent deformation of a stressed specimen begins to take place. Only materials that exhibit yielding have a yield point.

Yield Strength: Stress at the yield point. Stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain. The lowest stress at which a material undergoes plastic deformation. Below this stress, material is elastic; above it, material is viscous. Often defined as the stress needed to produce a specified amount of plastic deformation (usually a 0.2% change in length).

Young’s Modulus: Ratio of normal stress to corresponding strain for tensile or compressive stresses less than the proportional limit of the material.

Core Materials Prepregs Polyester/Vinyl Ester Resins Advanced Composites Advanced Reinforcements Advanced Resins