Pultrusion Glossary

The following terms are commonly used in the world of composites and pultrusion. Although many of these terms have various meanings and some have extended definitions, this glossary is designed as a quick reference to gain a basic understanding of a composites or pultrusion related terms.


Accelerator – An additive to resin that will quicken the curing process.

Additive – Chemicals added to thermosetting resin to increase specific properties such as UV resistance, color, or fire resistance.

Aluminum Honeycomb – Honeycomb core material made from non-corrosive hexagonal aluminum cells. When combined with composite skins, a lightweight, stiff, and strong sandwich panel can be made.

Aramid – Short for “aromatic polyamide,” aramid fibers are a class of high strength synthetic fibers in which the chain of molecules are orientated along the fiber axis, creating increased strength. Kevlar is a common aramid fiber and is trademarked by DuPont.

Aspect Ratio – Ratio of a fiber’s length to diameter.

ASTM – The “American Society for Testing and Materials” (ASTM) is an international standards organization which compiles standards and test methods for materials, products, and systems.

Autoclave Molding – Often used for advanced composite aerospace parts, autoclave molds are heated pressure vessels used for curing. Due to the uniform high pressure, temperature, and long curing times, extremely high quality parts are created with few voids.


Balsa Core – See: “End Grain Balsa”

Bi-Axial Material – Reinforcing fabric which contains fibers in both the x and y axis of directions.

Bio-based Composites – Composite material using all, or a combination of materials, derived from plants or other sustainable and renewable sources.

Blank – General term used for a raw core material as a unit or sheet.

Blister – A round raised area on a pultrusion profile which indicates improper curing of the composite.

BMC – “Bulk Molding Compound” (BMC), is a mixture of chopped strand fiber and resin, mixed into a “bulk” compound prepreg, which is used in injection or compression molding.

Breaker Bars – In pultrusion, a series of rods used to mechanically separate fibers in the resin bath to insure proper wet-out.

Bun – When manufacturing foam core material, liquid foam is poured into large reinforced containers, where the foam rises and sets. The cured foam block often resembling a bun of bread, the term “Bun” can apply to the foam block or the mold.

Bundle – Term used to describe a grouping of fiber filaments in parallel.


Carbon Fiber – An extremely lightweight textile material, when combined with a resin (most often epoxy), will produce an extremely rigid composite reinforcement. Due to the lightweight and strength features, carbon fiber is in high demand from aerospace, military, and recreational applications.

Catalyst – Chemical substance added to thermoset resin to accelerate the curing of the resin into a solid mass via an exothermic reaction.

Catenary – The natural twist, resulting from pulling a fiber tow, can cause all filaments to not be the same length, causing a catenary effect.

CNC – Computer Numerical Controls (CNC), are used in precision machinery, controlled by a predetermined computer program.

Co-Cure – The process of pultruding a composite part where in all the raw material is cured simultaneously.

Composite – Term used for material consisting of two or more materials, most often a resin with fiber reinforcement.

Compression Molding – Method of forming composites by using a shaped mold which applies pressure and often heat.

Continuous Fiber – Term used for reinforced structural fiber that is “continuous” and not broken or chopped.


Continuous Laminating – Process for forming sandwich structures continuously by applying pressure to a core and skin material laminated with adhesive. This is done on large automated machinery.

Continuous Strand Mating – Often used in pultrusion to help add cross-directional strength, continuous strand mat is fabric created with continuous fiberglass filaments laid in a spun pattern with a binder.

Core Material – General term for the center material in any sandwich structure, used to increase strength, stiffness, and insulation. Common core materials include foam, balsa, and honeycomb.

CFRP – Acronym for “Carbon Fiber Reinforced Plastic.” Term can apply to a wide range of carbon reinforced materials, but generally pertains to carbon fiber reinforced with thermoset resins.

CFRTP – Acronym for “Carbon Fiber Reinforced Thermoplastic”

Chopped Strand Fiber – “Chopped Strand Fiber” or simply, “Chopped Fiber,” is structural fiber that is cut into small lengths. Chopped fiber combined with a resin, is most often injected or sprayed into a mold.

Crazing – Term for small cracks that may develop under a pultruded surface.

Creep – Term used to describe the weakening of materials over time, when under load, the deflection of the material may slowly change.

Cross Linking – Cross linking is covalent bond linking of polymer chains to one another. Un-cross linked polymers (generally thermoplastics) can have bonds break with heat, and reform. Where as cross linked polymers (generally thermosets), cannot reform, and are set permanently once cured.

Cured – Term used to describe resin when it has turned into a solid state.

Curing Agent – Chemical catalyst added to resin to increase the reaction time of curing the composite, when subjected to controlled temperature environments.


Delamination – The separation of the bond between the skin and core material in a sandwich panel. Can also be applied to the separation of plies or fibers in a laminant.

Demold – The action of removing a cured composite part from a mold.

Die – Mold with a constant cross section, used in pultrusion, extrusion, and other composite manufacturing methods.

Dielectric Strength – Electrical field strength of an insulating material. Measurements and definition can be found under ASTM test procedure D149.

Divinycell – Brand name core by Diab, the material referred to as Divinycell is a rigid polymer foam commonly used in composite sandwich panels.

Doff – Common unit in which raw fiber rovings are packaged.


E-glass – Most commonly used structural reinforcement fiber, commonly known as “fiberglass.” E-glass is widely used due to its low cost, high production, lightweight, high strength, and insulative properties.

End Grain Balsa – Structural core with high shear strength. End grain balsa sheets are blocks of balsa wood, taken from a balsa tree, and aligned and adhered together to form a lightweight structural core.

Epoxy Resin- Epoxy based resin is a thermosetting matrix widely used in composites.

EPS Foam – Expanded Polystyrene (EPS) foam is common insulative foam used in sandwich structures. The density can vary, and rigid EPS foam is the most common foam used in structural insulated panels (SIPs).

Exotherm – The chemical process where in heat is released during the chemical reaction occurring during the curing of a thermosetting resin.

Extenders – Low cost additives used to dilute resins of high cost without lowering properties.

Extrusion – Method of forming plastics by means of forcing molten resin through a shaped die, producing a constant-area cross section.


Face Sheets – Face sheets is a term used to describe the skin or surface material of sandwich structures.

Fatigue Strength – Fatigue strength is the maximum cyclic stress a structure can withstand before failure.

FEA – FEA or Finite Element Analysis, is a process of using computerized modeling software to determine properties of a composite structure.

Fiber Blooming – Term used to characterize the event when fuzz is created on the surface of an FRP product. This is caused by UV oxidation of the resin.

Fiber Content – Term used to describe a composites fiber weight to resin ratio.

Fiberglass – Fiberglass is a general term for structural fiber used in reinforced composite applications. It can also be a general term for a finished fiber reinforced product.

Fiber Placement – Method of strategically placing and orientating reinforcing fiber in a composite application to maximize structural properties.

Filament Winding – Method of manufacturing composite products by winding resin impregnated structural fibers around a shaped mandrel or containment vessel. This is done using computerized CNC controls.

Filler – Fillers are inert materials added to composite resin to add volume, thus lowering the resin density. Fillers are also used to increase certain performance characteristics such as fire or crack resistance. Examples are calcium carbonate and aluminum trihydrate.

Flow – Flow is a term describing the viscosity of a resin, and its ability to “flow” of move through fiber reinforcement during wet-out.

Foam Core – By trapping air in a cellular polymer structure, foam is created. Foam is an effective core material for sandwich panels as a great deal of volume and structure can be added for very little weight. A wide range of foam cores are commercially available derived from a wide range of polymers.

Foam in Place – Foam in place, or foam in situ, is a method of manufacturing sandwich panels where raw foam in a resinous state is automatically placed in between sandwich face sheets, and is allowed to rise and cure in place.

FRP – Acronym for “Fiber Reinforced Plastic” or “Fiber Reinforced Polymer,” is a common term used to describe composite products.


Gel – Term describing polymer resin in a partial solidification state due to partial curing.

Gel Time – The amount of time for a resin to reach gel state, measured at a specific temperature after adding a catalyst.

Gel Coat – Gel coat is the top surface applied to a mold providing a superior surface finish to the final composite product.

GFRP – Acronym for “Glass Fiber Reinforced Plastic.”

Grippers – Pulling mechanism on a pultrusion machine.

GRP – Acronym for “Glass Reinforced Plastic.”


Hand-Lay-Up – Method of manufacturing composites by means of cutting fiber reinforced fabric, placing it in the correct position, and laminating resin without the assistance of machinery. It always requires human interactions.

Hardener – See Curing Agent

Honeycomb Core – Honeycomb core uses paper, aluminum, or other material to create a hexagonal cellular structure resembling the honeycomb pattern of a beehive. Honeycomb core is stiff, and extremely lightweight as the majority of the core is air.

Hybrid Composite – A composite material consisting of two or more different fiber reinforcements, such as fiberglass and carbon.

Hydraulic Press – Is a method of manufacturing sandwich panels by placing adhesive between the skin and core, and applying pressure using hydraulic machinery, thus creating an air tight bond.


I-Beam – An I-beam is an efficient method of creating a structural beam while using a minimal amount of material. An I-beam profile accomplishes similar structural results as sandwich panels.

Impact Strength – Impact strength is the ability of a material or sandwich panel to resist fracture during shock or impact.

Impregnate – The process of wetting-out and filling all voids of a fiber material with a resin matrix.

Inhibitor – Inhibitors are chemicals added to thermoset resins to slow down the chemical reaction causing the resin to cure.

Injection Molding – Method of manufacturing plastics (most commonly thermoplastics) by injecting molten resin into a heated mold under high pressure.

Isotropic – Is a description of composite material having equal structural properties in the three coordinate directions of X-Direction, Y-Direction, and Z-Direction.


Joint – Term used to describe the connection of sandwich panels.


Kevlar – Trademarked material manufactured by DuPont. Kevlar is an aramid fabric, combining light weight and high strength. Kevlar is noticeable by its unique yellow color, and was first made popular for use in bullet-proof vests.

Knitted Fabrics – See Stitched Fabrics.


Laminant – Term used to describe a fiber reinforced composite structure, or composite sandwich panel skin sheet.

Laminate – Term used to describe the application of resin to fiber reinforced fabric, can also be used to describe the process of adhering skin to a core material.

Lap Joint – A joint made by bonding two overlapping materials or parts.


Mandrel – Positive internal shape used in manufacturing hollow composite products.

Mat – See continuous strand matting.

Matrix – Term used to describe the structure of resin molecules.

Modulus of Elasticity – See: “Young’s Modulus”

Mold – Shape, most often made of steel, used to maintain the shape of the desired composite product.

Mold Release – Chemical used in composite manufacturing to prevent the laminant from sticking to the mold or die.

MEKP – Acronym for Methyl Ethyl Ketone Peroxide, a common catalyst used in resins.


Nomex – Trademarked fire-resistant material by DuPont. Nomex paper is used in making a honeycomb core material.


Non-Woven Fabric – See: “Stitched Fabric”


Outgassing – Vapors released from a cured composite into the atmosphere.


Parting Line – A parting line is a small raised mark or line on a molded composite part where a small amount of resin was allowed to seep into the crevice where the two mold sections meet.

Peel Ply – A lightweight synthetic fabric applied to the surface of a composite during lamination or molding. Peel ply provides a chemically clean surface and allows molded parts to be easily removed from the molds.

Phenolic Resin – “Fire-resistant” resin used in many different industries beyond composites.

Pigment – Chemicals added to resin to create coloring of the composite.

Pin Holes – Tiny holes in the surface of a composite caused by improper catalyzation.

Platens – Mounting plates where the pultrusion die is placed. Used to heat and hold the die in place.

Ply – Term used to describe a single layer of fiber reinforced fabric.

Ply-Schedule – The determined layering of various reinforcement plies in a laminant.

Poisson’s Ratio – Is the ratio to describe the tendency of material to contract in one direction when stretched from other directions. Poisson’s ratio is the transverse strain divided by the axial strain.

Polyester Resin – Common thermoset resin used in composite applications.

Polymer – Is a commonly used term to describe plastic. Polymers are structural molecular units connected by covalent bonds.

Polyurethane Resin – Thermosetting resin that has increased impact properties, but shorter pot life.

Post Cure – Is a method for strengthening composite laminates, after the resin is technically cured, by applying heat for an extended amount of time thus imparting increased cross-linking of the resin.

Pot Life – Pot Life is a term used to describe the period of time a thermosetting resin will stay in useable gel form before curing.

Pultrusion – Method of manufacturing composites, by “pulling” raw fiber wetted with resin, through a heated die. Pultrusion is used to efficiently produce large amounts of a continuous profile.

Pre-form – Term used to describe a unit or stack of fiber reinforcements prior to adding resin or molding.

Prepreg – Fiber reinforcement that is impregnated with a resin matrix prior to being used; prepreg material is cured by adding heat and pressure.


Reinforcement – Term used to describe the fibrous structural material that, when added with resin, can provide a strong solid laminant.

Resin – Chemical matrix to bond and hold structural fiber in place. Most resins are either thermosetting resins, or thermoplastic resins.

Resin Rich – Area of a cured composite containing excess resin. The generally means the ply-schedule was incorrect and under value.

Rib – Solid material connecting the skins in a composite sandwich structures to provide increased stiffness and strength.

Roving – Term for structural fiber in raw unit form, much like a thread of yarn. Generally available in multiple strands of filaments.

RTM – Acronym for “Resin Transfer Molding”, RTM is a method of forming a composite laminate by injecting resin into a closed mold, or by “pulling” resin through a mold using a vacuum.

RTP – Acronym for “Reinforced Thermoplastic”


Sandwich Panel – Structural panel consisting of a lightweight core, with skin material on either side. Sandwich panels are used to add structure, stiffness, and insulation to applications without adding considerable weight.

S-glass – S-glass is a fiberglass reinforcement similar to E-glass, but with a slightly different chemistry providing lighter and stronger properties.

Sizing – Chemicals added to reinforcing fiber to help create a more uniform and complete bond with the resin matrix.

Shear Strength – In sandwich panels, this term is often applied to the core properties and the core shear modulus, G. The higher the core shear modulus, the better the resistance to bending of the sandwich panel. Core shear modulus is measured through ASTM tests, such as 273.

Skin – Surface material used on a sandwich panel; skins provide the tension and compression strength, as well as providing impact resistance. Skins are often made of OSB for structural insulated panels, and FRP for composite sandwich panels.

Solvent – Chemical used in cleaning composite resins.

Splice – The joining of filaments or fabrics from end to end.

Steel Belted Press – A large machine used in the manufacturing of laminated products, and sandwich panels in a continuous fashion. Large steel belts apply pressure and temperature to the top and bottom of a panel, while moving the product forward through the production line.

Sterling Board – UK trade name for orientated strand board

Stringer – See “Rib”

Stitched Fabric – Unidirectional fiber reinforcement which is layered and orientated on top of one another and stitched together to form a fabric. The fibers are orientated at off-axis angles, most commonly + or – 90 degrees, and + or – 45 degrees. This provides increased strength to the end FRP product.

Structural Insulated Panels (SIPs) – Commonly known as SIPs, structural insulated panels are a new method of constructing walls, roofs, and floors of buildings. Most commonly, EPS foam is sandwiched between OSB panels. These panels are cut to pre-engineered sizes at a factory; then erected together at the job site. Advantages include, faster construction time, less scrap material, and improved insulation.


Tape – Tape is a term to describe unidirectional fiber reinforcement held together in a prepreg matrix, either thermoplastic or thermoset. Tape is cut, laid, and cured using a combination of heat and pressure.

Tensile Strength – The stress required to pull a material or structure from opposite directions to the point of failure.

Thermocouple – Temperature sensing instrument to help control the heating of a pultrusion.

Thermoplastic – Is a polymer resin that is in a solid state at room temperature, but turns liquid with heat, allowing for forming and molding. Thermoplastics are generally able to be remolded or reformed, allowing them to be easily recyclable.

Thermoset – Is a polymer resin that begins in a liquid state, and is cured with the addition of heat, radiation, or a through a chemical reaction. Thermosetting resins are more often stronger then thermoplastic, but due to the chemical structure, the molecules cannot be reformed or remolded, and are often difficult to recycle.

Tool – See Die


Uni-Directional – Is a term used to describe structural fiber reinforcement that runs along only a single axis of direction.

Under-Value – Generally, a pultrusion with low fiber content.


Vacuum Bag Molding – Is a method of laminating composites and sandwich structures by placing the preform composite in a sealed plastic bag; then applying a vacuum thus removing all air prior to dispensing resin into the sealed mold.

Veil – Veil is a cloth material laid on the surface of a composite lamanant prior to cure. Veils can provide increased surface finish to composite products, or be printed with logos or designs. They are often used to assist in the prevention of fiber blooming, caused primarily by UV.

Viscosity – Term used to describe the fluidity of a liquid resin, and its ability to flow, or not flow, with ease.

Void – Term used to describe gaps or air pockets within a cured laminate. Voids are considered flaws, and can diminish desired properties of a composite.

VARTM or VRTM – Acronym for “Vacuum Assisted Resin Transfer Molding,” see “Vacuum Bag Molding”.

Vinylester Resin – Commonly used thermosetting resin in composites manufacturing. Used due to low costs, excellent corrosion-resistant properties, and good structural properties.


Wet-Out – Term used to describe the resin impregnation on a composite lamanant




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