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An additive to polyester resin that reacts with a catalyst to speed up polymerization. This additive is required in room temperature cured resins. See Promoter.


A ketone group solvent that is used to dissolve polyester resins. Used to a large extent for clean up of tools in fiberglass operations.


Any number of materials used to modify the properties of polymer resins. Categories of additives include reagents, fillers, viscosity modifiers, pigments and others.


A visible cosmetic defect in the exposed gel coat which looks like wrinkled or alligator skin.

Antimony Trioxide

Fire retardant additive for use with resins.

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Bag Molding

An airtight film used to apply atmospheric force to a laminate." See Vacuum Bag Molding and Pressure Bag.

Barcol Hardness

A measure of surface hardness made with a Barcol Impressor instrument in accordance with ASTM D-2583. The hardness value can be used as an indication of the degree of cure of FRP laminates.

Barrier Cream

A cream used to protect the skin from contact with resins.

Benzoyl Peroxide (BPO)

An initiator for curing polyester resin. BPO is used with aniline accelerators or where heat is used to cure the resin.


Reinforcing fibers that are arranged in two directions, usually at right angles to each other.


A resin soluble adhesive that secures the random fibers in chopped strand mat or continuous strand roving.


A flaw either between layers of laminate or between the gel coat film and laminate.

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The process of pouring a mixture of resin, fillers and/or fibers into a mold as opposed to building up layers through lamination. This technique produces different physical properties from laminating.


Technically considered an initiator, catalyst is the colloquial name given to the substance added to the resin or gel coat to initiate the cure.


An elastic material used to protect joints or connections from external elements, particularly moisture.


The space between a male and female mold set in which the part is formed. Sometimes used to refer to a female mold.


A unit of measure used to describe the viscosity of a liquid. Viscosity is measured with a Brookfield Viscometer for most polyester resin applications.


A surface phenomenon indicating degradation of a cosmetic surface. Chalking is a powdery film which appears lighter than the original color.

Chopped Strand Mat

A fiberglass reinforcement consisting of short strands of fiber arranged in a random pattern and held together with a binder. Mat is generally used in rolls consisting of 3/4 oz/ft2 material to 2 oz/ft2 material.


A fiberglass reinforcement made by weaving strands of glass fiber yarns. Cloth is available in various weights measured in ounces per square yard or Kg/m2.

Color Stability

The ability of a surface coating or pigment to resist degradation due to environmental exposure.


A reinforcing fiber in a resin matrix whose cumulative properties are superior to the individual materials.

Compression Mold

A closed mold, usually of steel, used to form a composite under heat and pressure.

Compressive Modulus

A mechanical property description which measures the compression of a sample at a specified load. Described in ASTM D-695.

Compressive Strength

The stress a given material can withstand when compressed. Described in ASTM-D 695.


Where two panels are attached to each other or a panel is attached to the building.

Contact Molding

Refers to the use of a single or open mold onto which resin and reinforcement materials can be applied. Contact molding is characterized by one finished cosmetic side.

Continuous Filament Strand

A fiber bundle composed of many glass filaments. Also when referring to gun roving; a collection of string like glass fiber or yarn, which is fed through a chopper gun in the spray up process.

Continuous Strand Roving

A bundle of glass filaments which are fed through a chopper gun in the spray up process.

Continuous Laminating

An automated 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.


A low density material used between two FRP skins. Examples of core materials are end-grain balsa wood, urethane foam, PVC foam and various honeycomb materials.


Cracking of gel coat or resin due to stress.


The chemical bonding of molecules which in polymers occurs in the curing transition from a liquid to a solid.


The completion of the cross-linking process during which a composite develops its full strength.

Cure Time

Time between introduction of catalyst or initiator to a polymer and final cure.

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The separation of composite layers from each other.


A comparison of weight per volume, measured in pounds per cubic foot.

Dielectic Strength

The value of a material as an electrical insulator or the resistance to the flow of electric current.

Dimensional Stability

A description of the change in size of an object during the molding process or in varying temperature conditions or under various loads.


A change in shape form that which is intended.


The angle of the vertical components of a mold which allow removal of the part.


Originally formulated for use in electric circuitry, E-glass is the most common glass formulation used in fiberglass reinforcements.

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Standard measure for the amount a sample can stretch as a percentage of original length before it fails or breaks.


Completely surrounding an object with resin or a fiber resin composite.

Epoxy Resin

A polymer resin characterized by epoxide molecule groups.

Exothermic Heat

Internally developed heat accompanying a chemical reaction, such as might be created when curing a thermosetting resin.

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Manufacturer of reinforced plastic products.

Female Mold

A concave mold used to precisely define the convex surface of a molded part.


Reinforcement material which is a major component in a composite matrix.


Glass which has been extruded into extremely fine filaments. These filaments vary in diameter, and are measured in microns. Glass filaments are treated with special binders and processed similar to textile fibers. These fibers come in many forms such as roving, woven roving, mat and continuous strands.


A single thread-like fiber of extruded glass. Typically microns in diameter.

Filament Winding

A process which involves winding a resin-saturated strand of glass filament around a rotating mandrel.


Usually inert organic or inorganic materials which are added to plastics, resins or gel coats to vary the properties, extend volume, or lower the cost of the article being produced.

Fire Retardants

Compounds mixed with the resin to reduce flammability.

Fish Eye

The effect of surface contamination which causes a circular separation of a paint or gel coat.

Flame Retardant Resin

A polyester resin which has been specifically formulated to reduce the flame spread and/or smoke generation characteristics.


A measure of how fast a material will burn under controlled conditions. ASTM D-635/UL E-84 tests.


An extension around the perimeter of a mold or part for the purpose of demolding, stiffening or connecting two components.

Flash Point

The lowest temperature at which a substance gives off enough vapors to form a flammable mixture.

Flexural Modulus

ASTM D-790. An engineering measurement which determines how much a sample will bend when a given load is applied.


A lightweight, cellular plastic material containing gas-filled voids. Typical foams include urethane, PVC and polyester.


The process of creating a foam by the combination of two liquid polymers. See In-situ.


Fiber Reinforced Polymer, a matrix of polymeric material that is reinforced by fibers or other reinforcing material. Historically, also known GFRP (Glass Fiber Reinforced Polymer), CFRP (Carbon Fiber Reinforced Polymer), AFRP (Aramid Fiber Reinforced Polymer), FRP (Fiber Reinforced Plastics, GRP (Glass Reinforced Plastics) and RP (Reinforced Plastics).

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The irreversible point at which a polymer changes from a liquid to a semi-solid. Sometimes called the "B" stage.

Gel Coat

A surface coat of a specialized polyester resin, either colored or clear, providing a cosmetic enhancement and weatherability to a fiberglass laminate.

Gel Time

The length of time from catalyzation to gel or "B" stage.


The formation of a gel.

Good Side

The side of a molding in contact with a mold surface.


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


Glass reinforced plastics. Generally based on polyester resin. See Fiberglass.

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Hand Lay Up

The process of manually building up layers of fiberglass and resin using hand rollers, brushes and spray equipment.

Heat Distortion Point

The temperature at which the strength of a material begins to degrade.

HET-Acid Resin

Polyester resin with exceptional fire qualities.

Honeycomb Core

Strips of paper, plastic, metal, etc., joined together to form a honeycomb pattern. Used as a lightweight core in sandwich moldings.

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To saturate with resin. The most common application is saturating fiberglass with a catalyzed resin.


An additive to polyester resin or styrene used to slow the chemical reaction which leads to curing.


A piece of material put into a laminate during or before molding to serve a definite purpose.


A fire-retardant technology that causes an otherwise flammable material to foam, forming an insulating barrier when exposed to heat.


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


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


The description of equal strength properties in all orientation. Isoptropic composites are usually achieved by random fiber orientation.

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A visual effect of glass fiber turning white in a cured laminate. This usually does not affect the strength of a laminate, but could be an indication of materials incompatibility.


Any fixture for holding parts in position while joining them together or to maintain their shape.


A line or distinction formed when two panels are connected. Also referred to as a seam.

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The product of lamination. A composite consisting of a layer or layers of thermoset polymer and fiber reinforcement.


To place into a mold a series of layers of polymer and reinforcement. The process of applying FRP materials to a mold. To lay up.


Applying layers of glass and resin to a mold. Also used to describe a single ply of laminate.


A single ply of lay up or laminate.

Lay Up

The 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.

Low-pressure Laminates

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

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Male Mold

A convex mold where the concave surface of the part is precisely defined by the mold surface.

Master (plug)

A full scale representation of the intended part; usually retained as a reference and the part from which production molds are made.


See Chopped Strand Mat.

Matched Die Molding

Technique for producing long runs of identical parts with two finished sides.

Matched Molds

Two or more tools arranged in a set as a male and female mold. Normally used in a press.


The liquid component of a composite or laminate.

MEK Peroxide (MEKP)

Methyl Ethyl Ketone Peroxide. An initiator often referred to as catalyst and used to initiate polymerization of a resin.

MEK Solvent

Methyl Ethyl Ketone. A colorless, flammable liquid sometimes used in clean up procedures.


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

Mil (Mil Thickness)

The unit used in measuring film thickness. One mil equals one thousandth of an inch. (l mil = .001 ").

Milled Fibers

Glass fiber processed by a hammer mill into lengths of 1/32" to 1/8". Commonly used as a reinforcement in polyester putty.

Modulus of Elasticity

An engineering term used to describe a material's ability to bend without losing its ability to return to its original physical properties.


The tool used to fabricate the desired part shape. Also used to describe the process of making a part in a mold.


The process of using a mold to form a part.

Mold Release

A wax or polymer compound that is applied to the mold surface which acts as a barrier between the mold and the part, thus preventing the part from bonding to the mold.


One of the constituents of polyester resin.

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NPG Gel Coat

Neopentyl glycol gel coat has enhanced weatherability compared to non-NPG gel coat.

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Orange Peel

A gel coated or painted finish which is not smooth and is patterned similar to an orange's skin.

Orthophthalic or Ortho Resin

A polyester resin based on orthophthalic acid, also known as a general purpose resin (GP).

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Parting Agent

See Mold Release and PVA.

Parting Line

The location on a molded product between different segments of the mold used to produce the product.


The initial model for making fiberglass molds. See Plug.


A colorant added to gel coat or resin.

Pigment Separation

Occurs when the pigment is not thoroughly mixed into the gel coat during formulation or the gel coat is improperly mixed prior to use. It is characterized by a nonhomogeneous surface color.


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


Organic chemical compounds called polymers which can be formulated to produce a wide range of properties.


A composite industry term for a pattern or model.

Polyester Resin (Unsaturated)

The 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.


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


The chemical bonding of polymer molecules during the curing reaction.

Polyvinyl Alcohol (PVA)

A parting film applied to a mold for part releasing.


Entrapped gas bubbles or voids in a gel coat film.


To cure by application of heat after the chemical exothermic reaction has subsided.

Pot Life

The time during which the catalyzed resin remains liquid or "workable." See Gel Time.


Reinforcing material mixed with resin and, usually, with pigment, filler and catalyst, before placing in the mold.


Reinforcing material impregnated with resin prior to the molding process and cured by the application of heat.

Pressure Bag

A membrane which conforms to the inside of a laminate laid up on a mold. The membrane or bag is then inflated applying pressure which consolidates and densifies the laminate.

Print Through

A distortion in the surface of a part which allows the pattern of the core or fiberglass reinforcement to be visible through the surface. Also known as print out, telegraphing or read through.


A reagent which speeds resin cure. See Accelerator.


A thickened mixture of resin made by adding fillers, thixotrophs and reinforcing fibers.


See Polyvinyl Alcohol.

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Reinforced Molding Compound

Compound consisting of a polymer and a reinforcement fiber or filler supplied by raw material producer in the form of ready-to-use materials.


A fiber that when encapsulated in a polymer resin matrix forms a composite or fiberglass laminate. Also refers to a structural member designed to stiffen a molded part.

Release Agent

A compound used to reduce surface tension or adhesion between a mold and a part.


A liquid polymer which when catalyzed cures to a solid state.

Resin Tearing

Separation of pigments in a gel coat affecting cosmetic appearance.


A collection of bundles of continuous filaments in untwisted strands. Used in the spray-up (chopping) process.

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Sandwich Construction

A laminate with two composite skins separated by, but bonded to, a structural core material. Used to create stiff, lightweight structures.


See Joint.

Self Extinguishing

Ceases to burn when the source of flame is removed.

Self-tapping Screws

Hardened screws that cut their own thread as they are set.


An engineering term referring to forces applied normal to the surface of a given material. The movement between plies of a laminate is referred to as interlaminate shear.

Shelf Life

The allowable storage time before a product must be used.

Ship Lap

Method of joining two panels together by means of one panel having a recessed shelf to receive the other panel on top of it leaving a flush surface.

Skin Coat

The first layer of laminate next to the gel coat, generally one ply of chopped strand mat.

Specific Gravity

The ratio between the density of a given substance and the density of water.

Split Mold

An open mold made in two or more pieces.

Spray Up

The process of spraying glass fibers, resin and catalyst simultaneously into a mold using a chopper gun.

Styrene Monomer

A component of polyester resin that provides crosslinking sites and reduces the polyester to a workable viscosity.

Surfacing Mat

A lightweight tissue (10-30 mils thick) of glass or synthetic fiber used to provide a resin-rich surface. See Veil.


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.

Syntactic Foam

A foam made by mixing microspheres with a resin.

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Surface stickiness.

Tack Free

A surface which is not sticky after cure.


A narrow width reinforcing fabric or mat.

Tensile Load

A dulling load applied to opposite ends of a given sample.

Tensile Elongation

An engineering term referring to the amount of stretch a sample experiences during tensile strain. ASTM D-638.

Tensile Strength

A measurement of the tensile load a sample can withstand. ASTM D-638.

Thermal Coefficient of Expansion

Measures dimensional change of a material when heated or cooled. Measured in inches per inch per degree.

Thermal Conductivity

Measures the transfer of heat through a material.


A group of plastic materials that become elastic or melt when heated and return to their rigid state at room temperature. Examples are PVC, ABS, polystrene, polycarbonates, nylon, etc.


Materials that undergo a chemical crosslinking reaction going from liquid to solid or semi-solid. This reaction is irreversible. Typical thermosets are polyesters, acrylics, epoxies and phenolics.


A term describing the rehology (or flow characteristics) of a liquid that resists flowing or drainage during application.

Thixotropic Index (T.I.)

A measure of thixotropy using a Brookfield Viscometer. The low speed viscosity divided by the high speed viscosity.

Tooling Gel Coat

A gel coat formulated for mold surfaces.


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

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An area of a part or mold that has an acute angle between two surfaces. If a part has an undercut, a split mold is necessary.


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

UV Stabilizer

A chemical compound which improves resistance to degradation from ultraviolet radiation.

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Vacuum Bag Molding

Process for eliminating voids and forcing out entrapped air and excess resin from lay ups by drawing a vacuum from a plastic film which blankets a laminate.


The liquid properties of a material. Resistance to flow.

Void Free

A molding containing no entrapped air cavities, blisters or voids.

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Water Absorption

The amount of water which a laminate will absorb.


A compound used as a release agent. See Release Agent.


The action of saturating a glass fabric with resin. Also a measure of the speed with which a fabric soaks up resin.

Woven Roving Fabric

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

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Twisted strands of roving, used to weave textile reinforcements.