Glossary A-M



An early stage of polymerization of thermosetting resins in which the material is still soluble in certain liquids and is fusible. See B-Stage, C-Stage.


The degradation, decomposition, and erosion of material caused by high temperature, pressure, time, percent oxidizing species, and velocity of gas flow. A controlled loss of material to protect the underlying structure.


The penetration into the mass of one substance by another. The capillary or cellular attraction of adhered surfaces to draw off the liquid adhesive film into the substrate.


A chemical additive that hastens cure or chemical reaction. A material that when mixed with a resin, will speed up the chemical reaction between the initiator and the resin (either polymerizing of resins or vulcanizations of rubbers). Also called promoter or activator.


In a GRP context, acetone is primarily useful as a cleaning solvent for removal of uncured resin from tools. It is very flammable.


Ingredients mixed into resin to improve properties. Some examples of additives are plasticizers, initiators, light stabilizers, and flame-retardants.

Advanced Composites

Strong, tough materials created by combining one or more stiff, high strength reinforcing fiber with compatible resin system. Advanced composites can be substituted for metals in many structural applications with physical properties comparable or better than aluminum.


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

Air Dry

To cure at room temperature with the addition of initiator but without the assistance of heat and pressure.


Wrinkling of the gel coat film resembling alligator hide; caused by poor cure at the time of contact with styrene from a subsequent or preceding coat.

Ambient Conditions

Prevailing environmental conditions such as the surrounding temperature, pressure and relative humidity.


Fiber directionality in which different properties respond to stresses applied along axes in different directions.


A type of highly oriented organic material derived from polyamide, (nylon), but incorporating aromatic ring structure. Used primarily as high strength, high-modulus fiber. Aromatic polyamide fibers; commonly, DuPont’s Kevlar and Akzo Nobel’s Twaron.

Areal Weight

Weight of fiber reinforcement per unit area (width times length) of tape or fabric.

Aspect Ratio

The ratio of length to diameter of a fiber, or the ratio of length to width in a structural panel.
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Also called resistal. Intermediate stage in the polymerization reaction of thermosets. After B-Stage, material softens with heat and is plastic and fusible. The resin of an uncured prepreg or premix is usually B-Stage. See A-Stage, C-Stage.


The plastic film used in sealing a laminate to be infused.


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

Bag Molding

A molding technique in which the composite material is placed in a rigid mold and covered with a flexible bag, with pressure applied by vacuum, autoclave, press, or by inflating the bag.

Balanced Laminate

A composite in which all laminates at angles other than 0 degrees and 90 degrees occur only in pairs (not necessarily adjacent pairs) and are symmetrical around the centerline.

Barcol Hardness

A hardness value obtained by measuring the resistance to penetration of a sharp steel point under spring load. The instrument, a Barber-Coleman impressor, is called a Barcol impressor. It gives a direct reading on a scale of 0 to 100. The hardness value is often used as a measure of the degree of cure of a laminate or composite.

Basket Weave

Woven reinforcement in which two or more warp threads go over and under two or more filling threads in a repeat pattern: less stable than the plain weave but produces a flatter, stronger, more pliable fabric.

Batch (or Lot)

Material made by the same process at the same time having identical characteristics throughout.


A structural member supporting a portion of a deck.


The part of a structure that bears the weight as in foundations and stowages.

Benzoyl Peroxide (BPO)

An initiator used in conjunction with aniline accelerators or where heat is used as an accelerator.

Bias Fabric

A fabric in which warp and fill fibers are at an angle to the length.


An arrangement of reinforcing fiber strands in which half of the strands are laid at right angles to the other half; a directional pattern that provides maximum product strength to those two directions.


The resin or cementing constituent (of a plastic compound) that holds the other components together. The agent applied to fiber mat or preforms to bond the fibers before laminating or molding.

Bleeder Cloth

A layer of woven or nonwoven material, not a part of the composite, which allows excess gas and resin to escape during cure.

Bleed Out

Excess liquid resin appearing at the surface, primarily occurs during filament winding or wet bag layup.


An elevation on the surface of an adherend containing air or water vapor, somewhat resembling a blister on the human skin in shape. Its boundaries may be indefinitely outlined, and it may have burst and become flattened.

Bond Strength

As measured by load/bond area, the stress required to separate a layer of material from another material to which it is bonded; the amount of adhesion between bonded surfaces.


The weaving of fibers into a tubular shape or form.


An area of a fiber layup that is designed to impede the flow of resin, usually to allow other areas to complete wet out.


The separation or breakage of fibers when the edges of a composite part are drilled or cut.


A loosely woven material that does not come in contact with the resin but serves as a continuous vacuum path over a part in a production.


1. A pocket of resin in the layers of glass, caused by the fiberglass not being properly applied prior to infusion, usually occurring in corners.
2. A pocket of resin on top of the laminate, usually in corners, caused by loose peel ply or a stretched bag.


A failure mode usually characterized by fiber deflection rather than breaking under compressive action.

Butt Joint

The joint formed when two parts are placed end to end.

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The final step in the cure of a thermoset resin, resulting in irreversible hardening and insolubility.


The back and forth scanning of a specimen with ultrasonics. A nondestructive testing technique for finding voids, delamination, and defects in fiber distribution.


The element that provides the backbone for all organic polymers. Graphite is a more ordered form of carbon. Diamond is the densest crystalline form of carbon.

Carbon Fiber

A reinforcing fiber known for its light weight, high strength and high stiffness produced by pyrolysis of an organic precursor fiber in an inert atmosphere at temperatures above 1,800 degrees F. Can also be graphitized by heat treating above 3,000 degrees F.


A composite of carbon fiber in a carbon matrix.


A chemical substance that causes a chemical reaction, but does not become part of the reaction. Catalyst is the inaccurate colloquial term sometimes used for the substance added to the resin or gel coat to initiate the cure. (See Initiator.)

Caul Plate

A plate or sheet the same size and shape as the composite lay-up with which it will be used. The caul plate is placed in immediate contact with the lay-up during curing to transmit normal pressure and provide a smooth surface on the finished part.


The space between matched molds (pressure molds) in which the laminate is formed. Also, a term for a female mold.


In honeycomb core, a cell is a single honeycomb unit, usually in a hexagonal shape.

Centipoise (CPS)

A unit of measurement of viscosity. Higher centipoise indicates more viscous properties while lower centipoise indicates less viscous properties. Water is 1cps; peanut butter is 250,000 cps.

Chopped Strand Mat (CSM)

Continuous strand yarn or roving cut up into uniform lengths, usually from 1 to 2 inches long, and pressed flat to form fabric held together with a binder.

Closed Cell Foam

Cellular plastic in which individual cells are completely sealed off from adjacent cells.


Used as the accelerator for methyl ethyl ketone peroxide initiated polyesters.


Cured and simultaneously bonded to another prepared surface.

Coefficient of Expansion (COE)

A measure of the change in length or volume of an object.

Coefficient of Thermal Expansion (CTE)

A material’s fractional change in length for a given unit change of temperature.


The tendency of a single substance to adhere to itself. Also, the force holding a single substance together.

Commingled Yarn

A hybrid yarn made with two types of materials intermingled in a single yarn (for example, thermoplastic filaments intermingled with carbon filaments to form a single yarn).


The application of temporary vacuum pressure to remove trapped air and compact the lay-up.


A material that combines fiber and a binding matrix to maximize specific performance properties. Neither element merges completely with the other. Advanced composites use only continuous, oriented fibers in polymer, metal and ceramic matrices.

Compressive Strength

Resistance to a crushing or buckling force, the maximum compressive load a specimen sustains divided by its original cross sectional area.


A polymerization reaction in which simple byproducts (for example, water) are formed.


The processing step that compresses fiber and matrix to remove excess resin, reduce voids, and achieve a particular density.

Contact Molding

A process for molding reinforced plastics in which reinforcement and resin are placed on a mold. Cure is either at room temperature or by heating in an oven at ambient pressure.


An impurity or foreign substance that negatively affects one or more properties of composite material, particularly adhesion.

Continuous Filament

An individual, small-diameter reinforcement that is flexible and indefinite in length.

Continuous Filament Mat (CFM)

Random placed continuous fibers, usually held together by a binder. Typically used in RTM and Pultrusion Processes

Continuous Roving

Parallel filaments coated with sizing, gathered together into single or multiple strands, and wound into a cylindrical package. May be used to provide continuous reinforcement in woven roving, filament winding, pultrusion, prepregs, or high-strength molding compounds, or may be used chopped.


In sandwich construction, the central component to which inner and outer skins are attached. Commonly used core materials include foam, balsa, honeycomb, and wood. Also refers to a section of a complex mold that forms undercut parts.

Corrosion Resistance

The ability of a material to withstand contact with ambient natural factors or those of a particular artificially-created atmosphere, without degradation or change in properties.


For fabric, number of warp and filling yarns per inch in woven cloth. For yarn, size based on relation of length and weight.

Co-Woven Fabric

A reinforcement fabric woven with two different types of fibers in individual yarns (for example, thermoplastic fibers woven side by side with carbon fibers).


Region of ultra fine cracks that may develop on or under a resin surface due to internal stress.


Dimensional change in a material under physical load over time and instantaneous elastic deformation.


A fiber’s waviness, which determines its capacity to cohere.

Critical Length

The minimum length of a fiber necessary for matrix shear loading to develop ultimate fiber strength.


Material laminated so that some of the layers are oriented at various angles to the other layers with respect to the laminate grain. A cross-ply laminate usually has plies oriented only at 0/90º.


Polymerization reactions that branch out from the main molecular chain to form a networked pattern of chemical links.

Cumyl Hydro Peroxide (CHP)

An initiator that results in slower gel times and lower exotherm reactions.


To change the physical properties of a material irreversibly by chemical reaction via heat and initiator alone or in combination, with or without pressure.

Cure Temperature

The temperature at which a material attains final cure.

Curing Agent

An initiating or reactive agent that brings about polymerization when added to a resin.

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Damage Tolerance

A measure of the ability of structures to retain load-carrying capability after exposure to sudden loads (for example, ballistic impact).


To remove entrapped air from a matrix by subjecting it to vibration or a vacuum, as in cast polymer marble


The breakdown of materials or substances into other substances or parts of compounds; usually caused by heat or chemical reactions.


An unplanned non-adhered or unbonded region in an assembly.

Degree of Cure

The extent to which curing or hardening of a thermosetting resin has progressed. Can be measured with a Shore D or Barcol hardness meter in the plant and by more advanced equipment in the laboratory


The separation of ply layers due to adhesion or cohesion failure. Also includes the separation of layers of fabric from the core structure. A delamination may be associated with bridging, drilling and trimming or demolding.


In-plane separation of a laminate ply or plies due to adhesion failure, either local or covering a wide area. Can occur in the cure or subsequent life. Failure of internal bonding between layers of resin and reinforcement.


To remove a part from a tool, or a tool from an intermediate model.


A numbering system for yarn and filament in which yarn number is equal to weight in grams per 9,000 meters of yarn.


A comparison of weight per volume, measured in pounds per cubic foot, grams per liter, or pounds per gallon.

Dibutyl Phthalate

A lubricant for spray equipment

Dielectic Strength

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

Diethylaniline (DEA)

A promoter used in conjunction with BPO initiator or as an accelerator for cobalt/MEKP systems.


A reducing or thinning agent, usually used with epoxy resins.


To reduce the concentration level (thinning)

Dimensional stability

Ability of a plastic part to retain the precise shape to which it was molded, casted otherwise fabricated.

Dimethylaniline (DMA)

An accelerator used in conjunction with BPO initiator.


Small sunken dots in the gel coat surface, generally caused by a foreign particle in the laminate.

Dispersing Agent

Materials added to a suspending medium to promote and maintain the separation of discrete, fine particles of solids or liquids.


A change in shape from that which is intended.


The angle of the vertical components of a mold that allows removal of the part.


The ability of fiber reinforcement to conform to the shape of a contoured surface.

Dry Laminate

A laminate containing insufficient resin for complete bonding of the reinforcement.


The amount of plastic strain that a material can withstand before fracture. Also, the ability of a material to deform plastically before fracturing.

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E-Glass (Electrical Glass)

A family of glasses with a calcium aluminoborosilicate composition and a maximum alkali content of 2.0%. A general-purpose fiber that is most often used in reinforced plastics, and is suitable for electrical laminates because of its high resistivity.


The property of materials to recover their original sizes and shape after deformation.


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


The fractional increase in length of a material stressed in tension. When expressed as a percentage of the original length, it is called percent elongation.


A gel coat or surface coat which cures tack free.


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


A strand of roving consisting of a given number of filaments is considered an end before twisting.

End Count

The number of filament strands contained in a roving.

Environmental Protection Agency (EPA)

The U.S. Government Agency charged with protection of the environment.

Epoxy Resin

A polymerizable thermoset polymer containing one or more epoxide groups and curable by reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans.

Evaporation Rate

The rate at which a liquid loses mass at a given temperature, pressure, and humidity. The faster a material will evaporate, the sooner it will become concentrated in the sir, creating either an explosive/combustionable mixture or toxic concentration, or both.


Heat released during a chemical reaction. Uncontrolled exotherm can lead to violent explosion.


Low-cost materials used to dilute or extend higher-cost resins without excessive reduction in properties.

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Fabric, Nonwoven

A material formed from fibers or yarns without interlacing.

Fabric, Woven

A material constructed of interlaced yarns or fibers.


A manufacturer of reinforced plastic products.


Degradation of color in gel coat or coatings.


The failure of a material’s mechanical properties as a result of repeated stress over time.

Fatigue Life

The number of cycles of deformation required to bring failures of the test specimen under a given set of oscillating conditions (stresses or strains).

Fatigue Strength

The maximum cyclical stress withstood for a given number of cycles before a material fails.

FEA (Finite Element Analysis)

A process of selecting the optimum combination of materials in a composite based on software analysis.

Feed Point or Feeder

A device or method of delivering resin from its source to a specific area in a close molded part.

Female Mold

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


A general term used to refer to filamentary materials. A filament with a finite length that is at least 100 times its diameter, which is typically 0.004 to 0.005 inches. In most cases it is prepared by drawing from a molten bath, spinning, or deposition on a substrate. Fibers can be continuous or specific short lengths, normally less than 1/8th of an inch. A whisker, on the other hand, is a short single-crystal fiber or filament made from a variety of materials, with diameters ranging from 40 to 1400 micro inches and aspect ratios between 100 and150.

Fiber Architecture

The design of a fibrous preform or part in which the fibers are arranged (braided, stitched, woven, etc.) in a particular way to achieve the desired result.

Fiber Content

Amount of fiber in a composite, expressed as a ratio to the matrix.

Fiber Count

The number of fibers per unit width of ply present in a specified section of a composite.

Fiber Direction

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

Fiberglass Reinforcement

Major material used to reinforce plastic. Available as mat, roving, fabric, and so forth, it is incorporated into both thermosets and thermoplastics.

Fiber-Reinforced Plastic (FRP)

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

Fiberglass Chopper

Chopper guns, long cutters and roving cutters cut glass into strands and fibers to be used as reinforcement in plastics.

Fiber Orientation

Direction of fiber alignment in a non woven or mat laminate wherein most of the fibers are placed in the same direction to afford greater strength in that direction.


Individual fibers of indefinite length used in tows, yarns, or roving.

Filament Winding

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

Fill Threads

The crosswise fibers woven at 90º to the warp fibers; also known as the weft or woof.


Inert material added to the mixed resin to increase viscosity, improve appearance, and lower density and cost.

Filler Ply

Additional patch to fill in a depression in repair or to build up an edge.


Angles cut on the edges of core that will be in or on the finished part.


In the classification of powdered or granular materials according to particle size, fines are the portion of the material that is smaller than a specified size, usually under 100 mesh.

Fire Retardants

Compounds mixed with the resin to reduce flammability.

Fish Eye

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

Flame-Retardant Resin

A polyester resin that 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.


Any liquid that has a flash point of 100 degrees Fahrenheit or below. Also, any solid that can sustain fire and ignite readily.


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

Flash Point

The lowest temperature at which a substance emits enough vapors to form a flammable origin table mixture with air near the surface of the substance being tested.

Flexural Modulus

The ratio, within the elastic limit, of the applied stress on a test sample in flexure to the corresponding strain in the outermost fibers of the sample.

Flexural Strength

The strength of a material in bending, usually expressed in force per unit area, as the stress of a bent test sample at the instant of failure.


A puddle of excess resin on top of or in the laminate being infused. Usually caused by loss of vacuum or due to a siphoning effect.


The movement of resin under pressure, allowing it to fill all parts of the fiber preform. Flow can be horizontal or vertical or both.

Flow Front

The leading edge of moving resin in a fiber preform while it is being molded using closed molded.


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


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


A rupture of the surface of a laminate due to external or internal forces; may or may not result in complete separation.

Fracture Toughness

A measure of the damage tolerance of a material containing initial flaws or cracks.

Free Radicals

Highly reactive molecular fragments capable of initiating chemical reactions such as polymerization of polyester or vinyl ester resins.

FRP (Fiber Reinforced Plastics)

A thickening agent used in resins to increase flow or sag resistance qualities. Also known as GFRP (Glass Fiber Reinforced Plastic), GRP (Glass Reinforced Plastic), RP (Reinforced Plastic), and Composites. Fumed Silica (Aerosil, Cabosil)

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The initial jelly-like solid phase that develops during the formation of a resin from a liquid.


The formation of a gel.

Gel Coat

1. Pigmented resins applied as a coating to a mold or part to produce a smooth finish on the part. The gel coat is a protective layer for the composite.
2. A quick setting resin applied to the surface of a mold and gelled before lay-up.

Gel Coat Sagging

Relates to gel coat resin that has been applied excessively thick or that had insufficient resin thixotropy to be retained in position on the mold. It also could relate to gel coat film having excessive thickness.

Gel Coat Surface Lift

A gel coat that lifts from the mold surface; may result from over-initiation or high cure temperature. Also known as pre-release.

Gel Time

The period of time from initial mixing of liquid reactants to the point when gelation occurs, as defined by a specific test method. Gelation is the point when the resin viscosity has increased to the point such that it barely moves when probed with a sharp object.

Glass Transition

Reversible change in an amorphous polymer between a viscous or rubbery condition and a hard, relatively brittle one.

Glass Transition Temperature (Tg)

The approximate temperature above which increased molecular mobility causes a material to become rubbery rather than brittle.


The process of pyrolization at very high temperatures (up to 5,400º F) that converts carbon to its crystalline allotropic form.

Green Strength

The ability of a material, while not completely cured, set or sintered, to undergo removal from the mold and handling without distortion.

GRP (Glass Reinforced Plastics)

Generally based on polyester resin. (See Fiberglass or FRP.)

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Hand Lay Up

A fabrication method in which reinforcement layers, pre-impregnated or coated afterwards, are placed in a mold by hand, then cured to the formed shape.

Hard Tool

A tool made of metallic or any “hard” material that is generally impervious to damage during normal use.


A substance used to promote or control curing action by taking part in it, as opposed to an initiator.

Hazardous Chemical

Any substance that is a physical or health hazard, or both.


The term used colloquially to indicate any temperature above ambient (room) temperature to which a part or material is or will be subjected.

Heat-Distortion Temperature (HDT)

The temperature at which deflection occurs under specified temperature and stated load, sometimes referred to as Heat Deflection Temperature.

Heat Resistance

The property or ability of plastics and elastomers to resist the deteriorating effects of elevating temperatures.

HET-Acid Resin

Polyester resin with exceptional fire qualities based on chlorendic acid.


A resin-impregnated material, most commonly manufactured in hexagonal cells, that serves as a core in sandwich structure. May also be metallic or polymer in a rigid, open-cell structure.

Hot Pot

Initiator is mixed with the gel coat or resin in the material container prior to spraying, as opposed to internal or external gun mixing.


Moisture content of the air.

Hybrid Composite

1. A composite with two or more types of reinforcing fibers.
2. A combination of two or more different fibers, such as carbon and glass or carbon and aramid, into a structure. Tapes, fabrics and other forms may be combined; usually only the fibers differ.

Hybrid Resin

Resin with two or more types of chemistries combined.


Moisture resistant capability, moisture repelling.


Moisture absorbing capability.

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Impact Strength

A material’s ability to withstand shock loading, as measured by fracturing a specimen.


To saturate the voids and interstices of a reinforcement with a resin.


Materials that could cause unwanted results or dangerous reaction from direct contact with one another. This term is also used in regard to some liquids that are not soluble in one another.


Taking a substance in through the mouth.


The breathing of a substance in the form of a gas, liquid, vapor, dust, mist, or fume.


A material added to a resin to retard polymerization. Lengthens the gel time.


Existing or occurring between two or more adjacent laminae.

Interlaminar Shear

Shearing force that produces displacement between two laminae along the plane of their interface.


For unsaturated polyesters, a substance added to start the polymerization process. It is decomposed by the promoter or by heat to create free radicals, which in turn initiate polymerization.


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


A coating technology that causes the material to foam when exposed to heat, forming an insulating barrier to resist fire.


In the position that 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 orthophthalic polyester resin.


Fiber directionality with uniform properties in all directions, independent of the direction of applied load.

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Prominence of fiberglass pattern having turned white in the laminate because the glass has separated from the resin due to excessive exothermic heat, usually associated with thick resin rich laminates.


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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Trademark of DuPont for aramid fibers used as a reinforcement fiber. An organic polymer composed of aromatic polyamide having a para-type orientation (parallel chain extending bonds from each aromatic nucleus).

Knitted Fabrics

Fabrics produced by interlooping chains of yarn, rather than weaving, allowing a more drapeable fabric, but typically yielding a lower fiber content.

Knit Line / Flow Front

An area in a molded SMC part resulting from the convergence of flow fronts. This condition creates weak spots in the laminate where the reinforcement orientation and minimal bridging across knit lines can significantly reduce laminate strength. Knit lines occur where two flow fronts meet or where a single flow front is forced to flow around an obstruction, such as a core pin.

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In closed molding, the difference between the flow of resin on the surface of the fabric lay up and the flow on the bottom of the lay up.

Laking (Lake-ing)

Isolated dull effects on the surface of the molded SMC part that have a lake-like shape.


A single ply or layer in a laminate made up of a series of layers (organic composite). A flat or curved surface containing unidirectional fibers or woven fibers embedded in a matrix.


Plural of lamina.


1. (noun) the product evolved by bonding resin-impregnated reinforcement and sandwich materials. The laminate generally contains multiple plies (layers) of mat, roving, chopped strand saturated with resin.

2. (verb) -The process of uniting plies of reinforcement or sandwich materials or chopped glass with resin.

Laminate Ply

One fabric/resin or fiber/resin layer that is bonded to adjacent layers in the curing process.


A single ply of lay-up or laminate.


Placement of layers of reinforcement in a mold.


Running in the direction of forward and aft.

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.

Low-Profile Resins

Low shrink resins that reduce surface distortion.

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MACT (Maximum Achievable Control Technology)

As specified by the EPA for limits or VOC emissions, such as styrene.

Male Mold

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


An elongated mold around which resin-impregnated fiber, tape or filaments are wound to form structural shapes or tubes.


The quantity of matter contained in a specific body.

Master (Plug)

The permanent tool used to build molds for the manufacture of fiberglass parts.

Master Batch

A large volume of mix that can be tapped on demand to make smaller-quantity batches for molding. This relates to a large uninitiated batch of cast polymer matrix.


A fibrous reinforcing material composed of chopped filaments (for Chopped Stand Mat) or swirled filaments (for Continuous-Strand Mat) with a binder applied to maintain form; available in blankets of various widths, weights, thicknesses and lengths

Matched Molds

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

Matched-Metal Molding

A high-volume molding method that uses two machined metal molds in a hydraulic press. Generally, the process uses sheet molding compound or bulk molding compound.


The material in which reinforcing fiber of a composite system is embedded; polymer, metal, ceramic.

MEK P (Methyl Ethyl Ketone Peroxide)

A free radical source commonly used as the initiator for polyesters in the FRP industry.

MEK (Methyl Ethyl Ketone)

A colorless flammable liquid solvent commonly used in clean up procedures.


Microscopic cracks formed in composites when thermal stresses locally exceed the strength of the matrix.


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


The unit used in measuring film thickness and the diameter of fiber strands, glass, wire, etc., (one mil = .001 inch)

Milligram (mg)

A unit of measurement of weight. There are 1,000 milligrams in one gram.

Milligrams per Cubic Meter (mg/M³)

A unit of measurement usually associated with concentrations of dusts, gases, or mists in the air.

Milled Fibers

Carbon or glass used for making fiber-filled putty or BMC strands milled into short fiber lengths of 1/32, 1/16, 1/8 and 1/4 inch.

Million Particles per Cubic Foot (mppcf)

A unit of measure usually used to describe airborne particles of a substance suspended in air.


Measure of the ratio of applied load (stress) to the resultant deformation of a material, such as elasticity or shear. Can be low, intermediate, high or ultrahigh.

Moisture Absorption

Pickup of water vapor from air by a material. Refers to vapor withdrawn from the air only as distinguished from water absorption, which is weight gain due to absorption of water by immersion.


1. (verb) To shape plastics parts by heat and pressure.
2. (noun) The cavity or matrix into/onto which the plastics composition is placed and from which it takes its form. Female-made into. Male-made onto.
3. (noun) The assembly of all parts that function collectively in the molding process.


The process of using a mold to form a part.

Mold Coat

A coat of resin over the bare mold. Used to seal the mold and make a smooth surface on which to fabricate parts.

Mold Release

A substance used on the mold or in the compound to prevent sticking and for ease of part releases.


Chemical units composed of one or more atoms.


A single molecule that reacts with like or unlike molecules to form a polymer. The smallest repeating structure of a polymer; for addition polymers, this represents the original unpolymerized compound. Styrene is the predominant monomer used for reinforced polyester material.


Reactive diluents blended with the polymer to yield liquid resins.


A yarn consisting of many continuous filaments.

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