Glossary N-Z


NCF (Non-Crimp Fabrics)

Utilize unidirectional fiber reinforcement or combinations of random fiber materials which are assembled in stacks, and lightly stitched together, instead of interlacing.

NDI (Non-Destructive Inspection)

Inspections of composites that do not cause damage or require repair of inspection area, such as ultrasonic testing

Near-Net Shape

Part fabrication resulting in final dimensions that require minimal machining or cutting.

NESHAP (National Emission Standards for Hazardous Air Pollutants)

Federal standard that will require molders to adopt what the EPA has determined is the Maximum Achievable Control Technology (MACT) for the release of HAP and/or VOC.

NFPA (National Fire Protection Association)

An organization that promotes fire protection and prevention. They publish many of the standards used by local fire officials.

NPG Gel Coat (NeoPentyl Glycol Gel Coat)

Has enhanced weatherability compared with non-NPG gel coat.

Net Shape

Part fabrication resulting in final dimensions that do not require machining or cutting.

Non-Destructive Inspection (NDI)

Determining material or part characteristics without permanently altering the test subject. Nondestructive testing (NDT) and nondestructive evaluation (NDE) are broadly considered synonymous with NDI.


Large void in the laminate.

Nonwoven Roving

A reinforcement composed of continuous rovings loosely gathered together.

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A fabrication process in which a single part is fabricated.


Used in or taken through the mouth into the body.

Oral Toxicity

A term used to denote the degree at which a substance will cause adverse health effects when taken through the mouth. Normally associated with lab-animal tests.

Orange Peel

A gel coated painted finish that is not smooth and is patterned similar to an orange’s skin.


An opening, generally referred to regarding spray tip size.

Orthophthalic (Ortho) Resin

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


Having three mutually perpendicular planes of elastic symmetry.


A machined pocket in the mold steel located just outside the shear edge. During molding, material and trapped air are vented to this area from the mold cavity, through a machined clearance in the shear edge. This results in a molded part with less porosity and reduced knit lines.


The release of solvents and moisture from composite parts under the hard vacuum of space.


That material (gel coat or resin) that is deposited off the mold during the spraying process.


A substance that yields oxygen readily to stimulate the combustion of an organic material.

Oxidizing Agent

A chemical or substance that provides the oxygen for an oxidative reaction.

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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.

Parts per Billion (ppB)

A unit of measurement for the concentration of a gas or vapor in the air, expressed in terms of number of parts per billion parts of air.

Parts per Million (ppm)

A unit of measurement for the concentration of a gas or vapor in the air, expressed in terms of parts per million parts of air.


The initial model for making fiberglass molds. See Plug.

Peel Ply

Layer of material (usually a tightly woven nylon) applied to a fiber lay-up surface while the resin is still wet. It is removed from the cured laminate prior to bonding operations, leaving a clean, resin-rich surface ready for bonding. Peel Ply is also used in vacuum bagging laminating to produce the same surface.

Peel Strength

Strength of an adhesive bond obtained by stress that is applied in a “peeling” mode.

PEL (Permissible Exposure Limit)

An exposure concentration established by OSHA that indicates the maximum concentration from which no adverse effects will follow.


A category of compounds containing an unstable o-o (or o-oh) group. Oxygen to oxygen atoms, used as initiators.


Localized areas of high thermoplastic content in the molded SMC part that may result in a non-uniform color. This results from separation of the thermoplastic shrink-control additive or B-side carrier resin from the polyester during SMC material flow.

Phenolic Resin

Thermosetting resin produced by condensation of an aromatic alcohol with an aldehyde, particularly phenol with formaldehyde.


The ingredient used to impart color, as in gel coats.

Pigment Separation

A mottled (varied colored) appearance of the gel coat surface.

Pin Holes

Small holes on the surface of a laminate or gel coat, usually caused by the contaminates on the mold used.


A residual petroleum product used in the manufacture of certain carbon fibers.


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


See Master.


One of the layers that makes up a stack or laminate. Also, the number of single yarns twisted together to form a plied yard.

Ply Schedule

Lay-up of individual plies or layers to form a laminate. Plies may be arranged in alternating fiber orientation to produce multidirectional strength in a part. Also known as a Lamination Schedule.

Polyester Resin

The term generally used for unsaturated polyesters. Formed by the reaction of dibasic organic acids and polyhydric alcohols. Also meant to include the cross-linking diluent included with the polyester molecules.


Large molecules formed by combining many smaller molecules or monomers in a regular pattern.


Chemical reaction that links monomers to form polymers.

PolyVinyl Alcohol (PVA)

A parting film applied to a mold for part releasing.


The presence of visible voids within a solid material into which either air or liquids may pass. The formation of undesirable clusters of air bubbles or other voids in the surface or body of laminates.


Additional exposure to elevated temperature, often occurring without tooling or pressure, which improves finished mechanical properties. In certain resins, complete cure and ultimate mechanical properties are attained only by exposure of the cured resin to higher temperatures than those of curing.

Pot Life

The length of time in which an initiated thermosetting resin retains sufficiently low viscosity for processing.


1. A fibrous reinforcement preshaped on a mandrel or mock-up to approximate the contour and thickness desired in the finished part.
2. The same fibrous reinforcement as in 1, but formed on a core material, such as foam, to form a near net-shaped component, such as a stiffener.
3. The fiber lay up inside (or on) a mold, that is formed in or on the mold.


A condition caused by SMC starting to cure prior to completion of flow in the mold cavity. This results in localized areas of dull discoloration on the part surface that may be rough and contain porosity.


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


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


Premature release of the gel coat or laminate from the mold.

Pressure Bag Molding

A process for molding reinforced plastics in which a tailored, flexible bag is placed over the contact lay-up on the mold, sealed, and clamped in place. Fluid pressure, usually provided by compressed air or water, is placed against the bag, and the part is cured.

Primary Laminate

Bulk or second laminate, laminate applied after the skin coat has cured. Primary laminate consists of the fiber reinforcement ply(s) which supply most of the strength of the laminate. Generally thicker than the skin coat.

Print Through

A distortion in the surface of a part that allows the pattern of the core or fiberglass enforcement to be visible through the surface. Also known as Print Out, Telegraphing, or Read Through.

Promoted Resin

Resin with accelerator added but not initiator. Primarily applicable to room temperature cure resins.


Additives that accelerate the decomposition of peroxides. Some additives work synergistically with others in the decomposition of peroxides.


The initial fabrication of a product that is used to develop and refine product specifications, design, and process methods.


A continuous molding process for manufacturing composite profile shapes, such as rods, tubes, and structural shapes having a constant cross section. Roving and other reinforcements are saturated with resin and continuously pulled through a heated die, where the part is formed and cured.


A break in the composite skin of a sandwich structure, which may or may not go through to the core material or completely through the part.


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

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Approximating isotropy by orienting plies in several directions.

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Race Tracking

In closed molding, the resin finding an unplanned channel to speed through, bypassing the lamination, and going directly into the vacuum source.


The term that describes the tendency of a substance to undergo a chemical change with the release of energy, often heat.

Reducing Agent

In an oxidation reaction, this is the material that combines with oxygen or provides the oxygen.

Reinforced Molding Compound

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


The key element added to the matrix to provide the required properties (primarily strength); ranges from short fibers though complex textile forms.

Release Agent

Used to prevent cured matrix material from bonding to tooling; usually sprayed or painted on mold.

Release Film

A non-stick film layer that does not bond to the composite during cure.


Polymer with indefinite and often high molecular weight and softening or melting range that exhibits a tendency to flow when subjected to stress. As composite matrices, resins bind together reinforcement fibers.

Resin Content

The amount of resin in a laminate expressed as a percentage of either total weight or total volume.

Resin Film Infusion (RFI)

Method of closed molding in which a film of viscous resin is added to a dry preform, and heated to flow the resin through the preform.


Filled with excess resin as compared to consistent resin/fiber ratio.


Lacking sufficient resin for fiber wet out.

Resin Tearing

Separation of pigments in a gel coat affecting cosmetic appearance.

Resin Transfer Molding (RTM)

A molding process in which initiated resin is pumped into a two-sided, matched mold where a fibrous reinforcement has been placed. The mold and/or resin may or may not be heated. RTM offers the ability to consolidate structural parts. Its major drawback is the high cost of the initial, two-sided mold.

Resin-Rich Area

Space which is filled with resin and lacking reinforcing material.

Resin Viscosity

Viscous property of a resin system or solid to liquid transition resistance to flow, which can be altered by temperature and pressure as necessary to achieve desired flow characteristics.

Rib Read-Out (Sink Mark)

Surface depression caused by SMC maternal shrinkage during curing and located over ribs, bosses, or thick sections of the part shape. In addition to the localized surface deformation, it may appear as a lighter color than the surrounding substrate


A collection of bundles of continuous filaments either as untwisted strands or as twisted yarn.

RTM (Resin Transfer Molding)

Closed mold process using liquid resin under pressure to saturate a dry fiber preform in mold.

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Sandwich Structure

Composite composed of lightweight core material (usually honeycomb or foam) to which two relatively thin, dense, high strength, functional or decorative skins are adhered.

Scarf Joint

A lapped joint made by beveling off, notching, or otherwise cutting away the edges of two adjoining surfaces, so that they may be joined together, usually without increasing the original thickness.


Dull streaks, spots, or areas on the molded SMC part that transfer residue to the tool surface. This residue may build up on the tool surface and affect appearance quality and the release of the part. Scumming may result from a problem with the internal mold release additive at molding temperature or from incompatibility of resin additives.

Secondary Bonding

Bonding to a previously cured laminate surface.


A paste or liquid applied to a point that hardens in place to form a seal.

Sealant Tape

A thick, double-sided, sticky tape used to seal bag film to a mold.


Ceases to burn when the source of flame is removed.


A substance that on first exposure causes little or no reaction; however, with repeated exposure, will induce a marked response not necessarily limited to the exposure site. Usually associated with skin sensitization.

Set Up

To harden, as in curing of a polymer resin.


High-strength glass fiber, commonly used in high-performance parts. S-Glass has high compression strength.


An action or stress resulting from applied forces that causes or tends to cause two contiguous parts of a body to slide relative to each other.

Shear Rate

With regard to viscous fluids, the relative rate of flow or movement.

Shelf Life

Length of time in which a material can be stored and continue to meet specification requirements, remaining suitable for its intended use.

Sheet Molding Compound (SMC)

A molding compound consisting of a B-staged resin and chopped glass fiber used in compression molding.

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.

Slave Pump

An initiator pump that is driven by the resin pump through a pair of level arms.


Specification of properties, characteristics, or requirements that a particular material or part must have to be acceptable to a potential user.

Solid Surface

A blend of unsaturated polyester, acrylic or blend, alumina trihydrate, mineral fillers, pigment and initiator.


Compound that binds together and stiffens warp yarn, providing abrasion resistance during weaving. Sizing is normally removed and replaced with finish before matrix application.


A layer of relatively dense material used on the surface of the core of a sandwich structure.

Skin Coat

A layer of resin and chopped strand mat applied just under the gel coat to prevent blistering, and provide protection to the primary laminate.

Soft Tool

A tool made of composites or similar “soft” material that is vulnerable to damage during use, storage or transportation.


A liquid used to dissolve, dilute, and clean materials.

Specific Gravity

The density (mass per unit volume) of a material divided by that of water at a standard temperature.

Split Mold

An open mold made in two or more pieces.

Spray Gun

A fluid handling device that converts a stream of fluid into a useful, shaped spray pattern. There are a number of types of guns, including: Air-Assisted Airless (AAA), Conventional-Air-Atomizing, High-Pressure Airless, and High-Volume Low-Pressure (HVLP)


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


An additive for polymers that aids maintenance of certain performance properties.


Anything taken by general consent as a basis of comparison. An approved model.

STEL (Short Term Exposure Limit)

The maximum allowable concentration of a substance that one can be exposed to for less than 15 minutes and not produce adverse health effects.


Physical adhesion of the part to the mold. This results in difficulty in removing the part from the mold and may lead to cracking.


Relationship of load to deformation for a particular material.

Storage Life

Amount of time a material can be stored and retain specific properties.


Elastic deformation resulting from stress.


A primary bundle of continuous filaments combined in a single compact unit without twist.

Streaking (Abrasion)

Dark directional patterns in the line of f low of the SMC. Streaking is most often found in pigmented molding compounds and is generally located over reinforcement strands.


Internal resistance change in size or shape, expressed in force per unit area.

Stress Concentration

The magnification of applied stress in the region of a notch, void, hole, or inclusion.

Stress Corrosion

Preferential attack of areas under stress in a corrosive environment, where such an environment alone would not have caused corrosion.

Stress Crack

External or internal cracks in a composite caused by tensile stresses; cracking may be present internally, externally, or in combination.

Structural Adhesive

An adhesive used to transfer loads between adherents.

Structural Bond

A bond joining load-bearing components of an assembly.

Styrene Monomer

A component of polyester resin that provides crosslinking sites and reduces the polyester to a workable viscosity commonly referred to as a reactive diluent.


A material upon the surface of which an adhesive-containing substance is spread for any purpose, such as bonding or coating.

Surface Waviness (Ripples)

Surface irregularities usually seen at the termination of flow on flat molded SMC surfaces.

Surfacing Mat

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

Surfacing Veil

Used with other reinforcing mats and fabrics to enhance the quality of the surface finish. Designed to block out the fiber patterns of the underlying reinforcements and provide a resin rich layer for chemical resistance; also called surfacing mat.


Chemical used to modify or change the surface tension properties of a liquid.

Syntactic Foam

A foam made by mixing microspheres with a resin.

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Surface stickiness.

Tack Free

A surface that is not sticky after cure.


A narrow-width reinforcing fabric or mat.

Tensile Load

A pulling 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

Maximum tensile stress sustained by a composite specimen before it fails in a tension test.

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.

Thermal Stress Cracking

Crazing and cracking of some thermoplastic resins from overexposure to elevated temperatures.


Composite matrix in advanced composites formed by heat and cooling. Can be reshaped more than once.


Composite matrix cured by heat and pressure or with an initiator into an infusible and insoluble material. Once cured, a thermoset cannot be returned to the uncured state.

Thermosetting Polyesters

A 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.


A term describing the rheology (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.

TLV (Threshold Limit Value)

A term used by OSHA to describe the airborne concentration of a material to which nearly all persons can be exposed to day in and day out and not develop adverse health effects.

Tooling Resins

Plastic resins, chiefly epoxy and silicone, that are used as tooling aids.


A measure of the ability of a material to absorb energy.


An untwisted bundle of continuous filaments, usually designated by a number followed by K, indicating multiplication by 1,000 (for example, 12k tow has 12,000 filaments).

TWA (Time Weighted Average)

The airborne concentration of a material to which a person can be exposed over an eight-hour workday (an average).

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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.


The incomplete polymerization of the molding compound; usually accompanied by a dull surface, styrene odor, initiator odor, blisters, blown bosses, and delamination of the SMC part.


Referring to fibers that are oriented in the same direction, such as unidirectional fabric, tape, or laminate. Often called UD or uni.

UV Resin

A resin system designed to cure only with the application of ultraviolet light

UV Stabilizer

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

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VARTM (Vacuum Assisted Resin Transfer Molding)

Uses vacuum to assist in filling fiber preform, also may use pressure to inject resin into mold.

Vacuum Bag Molding

Molding technique where a resin impregnated laminate is cured inside a sealed bag film which entrapped air is removed by vacuum, and fiber reinforcement is compressed by vacuum pressure.

Vapor Density

A term used to define the weight of a vapor or gas as compared with the weight of an equal volume of air. Materials lighter than the equal volume of air have a vapor density of less than 1.0; whereas materials heavier than air have a vapor density greater than 1.0.

Vinyl Ester

A family of thermosetting resins having no ester linkages along the polymer chain but present only at the ends of the molecule providing improved corrosion resistance.

VIP (Vacuum Infusion Process)

Generic term to include any infusion process which uses vacuum as the sole force to saturate fiber preform.


Tendency of a material to resist flow. The ratio of shear stress to shear strain. Measured in centipoise at 77 F.

Vapor Pressure

A number used to describe the pressure that a saturated vapor will exert on top of its own liquid in a closed container; usually, the higher the vapor pressure, the lower the boiling point; therefore, the more dangerous the material can be if it is flammable.

Volatile Organic Compounds (VOC)

Carbon-containing chemical compounds (e.g., solvents or liquids) that evaporate readily at ambient or process temperatures. Environmental, safety, and health regulations often limit exposure to these compounds, making low VOC content preferable.

Volatile Material

Material vaporizing under specific conditions short of decomposition. Non-volatile materials remain as a liquid.


Pockets of entrapped gas that have been cured into a laminate.

Void Free

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

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Yarns running lengthwise and parallel to the narrow edge of woven fabric.


Dimensional distortion of a composite part.

Water Absorption

Ratio of weight of water absorbed by a material to the weight of dry material.


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


Pattern by which a fabric is formed from interlacing yarns. In plain weave, warp and fill fibers alternate to make both fabric faces identical. In satin weave, pattern produces a satin appearance with the warp tow over several fill tows and under the next one.


Yarns running perpendicular to the warp in a woven fabric. Also called woof.

Wet Layup

Application of a resin to a dry reinforcement in the mold.

Wet Out

Saturation with resin of all voids between strands and filaments.

Wetting Agent

Surface-active agent that promotes wetting by decreasing the cohesion within a liquid.


Short single crystal fiber or filament used as a reinforcement in a matrix.


General light or whitened surface appearance in pigmented SMC molded parts. Whitening is more prevalent in dark-colored parts that do not use a polystyrene thermoplastic additive.


Yarns running perpendicular to the warp in a woven fabric. Also called weft.

Working Life (Pot Life)

The period during which a compound, after mixing with an initiator or other compounding ingredients, remains suitable for its intended use.

Woven Roving

Heavy, coarse fabric produced by weaving continuous roving bundles.


An imperfection in the surface of a laminate that looks like a crease in one of the outer layers.

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The axis in the plane of the laminate used as 0º reference.

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The axis in a laminate that is perpendicular to the x-axis.


Continuously twisted fibers or strands suitable for weaving into fabrics.

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The reference axis normal to the laminate plane in composite laminates.

Zero Bleed

Laminate fabrication procedure that does not allow loss of resin during cure.

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