specialty glossary

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The glossary is designed to provide you with a complete list of unfamiliar terms and their definitions. To move within the page, select the beginning letter of the word you are searching. If you need further assistance, please contact our technical support team.


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.


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.


See Accelerator.


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%.)


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


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


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


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


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.


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.

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.


American Society of Test Methods.


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

Axial Winding

Filaments that are wound parallel or at a small angle to the axis (0° helix angle).Ball See Roving

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.

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.


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


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.

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.


Bulk Molding Compound (Thermoset).

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.


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.

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.


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.


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.

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.


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


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.


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


Fiberglass reinforcement made by weaving strands of glass fiber yarns.

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.


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.


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.


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.


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

Continuous Filaments

Filaments that extend substantially throughout the length of the yarn.

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.


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

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.


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.


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


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


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.


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.

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.')


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


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


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.


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.


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.

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.


See Roving Doff


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


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


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


A single bundle of filaments.

End count

An exact number of ends supplied on a ball of roving.


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.


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.


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 Reinforced Plastic (FRP)

A general term for a composite that is reinforced with cloth, mat, strands, or any other fiber form.


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.


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.


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


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.


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


A partial cure of plastic resins; a semi-solid, jelly-like state similar to gelatin in consistency.


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


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.


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.

Green Strength

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


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.


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


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.

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).

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).


Describes a material with a uniform composition.


A compound produced by polymerization.


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.


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.


Material that absorbs moisture from the air.


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.


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


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


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.


Instrument for determining the tensile and compressive properties of materials.


A surface that lies between two different materials.


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


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.


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.


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.


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.


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.


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


The horizontal plane of doffs on a pallet. A pallet usually has four layers of twelve doffs each. See Cell


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.

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.


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


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


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.


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


Mat Strength

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


Strand-to-strand adhesion. A matchstick is a wide bundle that has three to four 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.


Thermoset resin.


One micron = .001 millimeter = .00003937 inch.


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.

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.


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.


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.


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.


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.

Non-Woven Fabric

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

Orange Peel

Gel coated or painted finish that is not smooth and is patterned similar to an orange's skin.


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


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.


Polybuthlene Therephthalate (Thermoplastic Polyester Resin).


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.


Colorant added to gel coat or resin.


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


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


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.


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.


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)


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.


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.


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


Chemical bonding of polymer molecules during the curing reaction.


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.


Polyphenylene Oxide (Thermoplastic Resin).


Polyphenylene Sulfide (Thermoplastic Resin).


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.


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.


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.


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.


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).


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


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.


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.


Reinforcing member of a fabricated or molded part.


The propensity of glass bundles to "stick" together and act as a strand or end.


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.


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).


Reinforced plastic, polymer or polyester.


See Reinforced Reaction Injection Molding


See Resin Transfer Molding

RTM Light

See Vacuum Molding


Sometimes used to distinguish reinforced thermoplastic from reinforced thermosetting plastic.


The process of pulling glass from the doff to the chopper.


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.


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


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.


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.


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.


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

Skin Coat

First layer of laminate next to the gel coat.


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


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.


See Doff Collapse


See Sheet Molding Compound


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.


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.


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.

Splitting Efficiency

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


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.


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.


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.


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.


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.


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 might be.


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.


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.

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.


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.


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 affected 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" four ways in forming will have 64 theoretical ends.


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


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


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


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.

Ultimate Tensile Strength

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


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 three to four times as many moldings as that produced through open mold, with acceptable repeatability (but not equal to that of RTM).


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.


Measure of a liquid’s resistance to flow.

Void Content

The percentage of voids in a laminate.


See Vacuum Molding.


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.


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


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.


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.


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.


Term describing bundles of roving that are wider than most of the other bundles in a bed of chopped glass fibers. They usually contain three to four 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.


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.


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 180 yards in one pound of glass.


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.


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.