Reinforced plastics: glossary of terms

Activator (also: accelerator, promotor)

A chemical compound used with a catalyst to permit polymerisation at room temperature.


Specialist chemicals which are added to resins/compounds to impart specific properties, such as flame retardancy, impact improvement, UV resistance.


Used as accelerators for polyester resins.

Antimony trioxide (Sb2O3)

A commonly-used flame retardant additive for plastics, especially polyesters.

Aspect ratio

The length/diameter ratio of a fibre.

Benzoyl peroxide

Aromatic peroxide.

Blister / blistering

Undesirable raised areas in a moulded part caused by local internal pressure, usually due to trapped air, volatile reaction by-products or water entering by osmosis.


See bulk moulding compound.


An intermediate stage in the cure reaction of certain thermosetting resins, in which the material is still to some extent soluble and fusible.

Bulk moulding compound (BMC)

A polyester resin/glass fibre premix, for injection or transfer moulding. Also known as dough moulding compound (DMC).


A chemical compound (usually an organic peroxide) which initiates polymerisation of a resin. (Also known as a hardener.)


A measure of the difference in length of strands in a specified length of roving, as a result of unequal tension.


Carbon fibre reinforced plastic.

Chlorinated paraffins

Flame-retardant additives for polyester resins.

Chopped strands

Short strands cut from continuous filament strands of reinforcing fibre, not held together by any means.


Actual separation of moulded material, visible on opposite surfaces of a part and extending through the thickness (fracture).


Fine cracks which may extend in a network or under the surface of a moulded part.

Cobalt accelerator

Accelerates cure of polyester resins.


Assembly of a reinforcement yarn with a thermoplastic in yarn or thread form, in a predetermined ratio.


A composite material is defined as a combination of two or more materials that results in better properties than when the individual components are used alone. Unlike metal alloys, each material retains its separate chemical, physical and mechanical properties. The two constituents are normally a fibre and a matrix.

Compression moulding

A process for manufacturing FRP parts in which a charge of SMC or BMC of predetermined weight is placed between two dies and then heat and pressure are applied. The moulding compound flows to fill the die and cures within a few minutes. (Thermoplastic composites, usually consisting of glass fibre and polypropylene can also be compression moulded.)

Compression strength

The crushing load at failure of a material, divided by cross-sectional area of the specimen.

Contact moulding

The moulding of fibre reinforced resins without application of external pressure.


Incorporation of more than one monomer into a polymer chain.

Coupling agent

A substance which promotes or establishes a stronger bond at the resin matrix/reinforcement interface.


The process of hardening of a thermosetting resin (by cross-linking of the molecular structure), under the influence of heat and/or curing agents.

Curing agents

Chemical compounds used to cure thermosetting resins.

Curing time

The time taken for a resin to cure (polymerise) to its full extent. With a cold-curing resin, the time is measured from addition of either activator or catalyst. Curing time can be influenced by other chemical aids – retarder or accelerator.


Splitting, physical separation or loss of bond along the plane of layers of a laminated material.

Direct roving

Roving produced by winding a large and determined number of filaments direct from a bushing.

DMC / Dough moulding compound

See bulk moulding compound.


The heat given off during a polymerisation reaction.


A unit of matter of relatively short length, characterised by a high ratio of length to thickness or diameter.

Fibre placement

A hybrid between the filament winding and tape laying processes. A fibre placement machine or tow placement machine allows individual tows of prepreg to be placed.


A single textile element of small diameter and very long length, considered as continuous.

Filament winding

A process for manufacturing FRP parts in which dry fibre rovings are pulled through a resin bath prior to winding on a mandrel. Prepregged roving or tows can also be filament wound. Curing is usually carried out in an oven with or without a vacuum bag.


Material (usually low cost) added to a resin to extend it or give it special properties.


The movement of a resinous material, thermosetting or thermoplastic, under pressure, to fill all parts of a closed mould.


Fibre reinforced plastic. A composite (see separate entry) consisting of fibre reinforcement and a polymer matrix.


An accumulation of broken filaments.


The state of a resin which has set to a jelly-like consistency, solid but not yet hard.


A thin layer of unreinforced resin on the outer surface of a reinforced resin moulding (usually applied direct to the mould as the first layer), which hides the fibre pattern of the reinforcement, protects the resin/reinforcement bond, give a smooth external finish, and can also provide special properties. it is usually pigmented.

Gel time

The time taken for a resin to set to a non-fluid gel, also called setting time.


Glass reinforced plastic.


See catalyst.


A resin or reinforcement made from two or more different polymers or reinforcement materials.


Decomposition of chemical compounds by water.


Saturation of reinforcement with a liquid resin.


An additive which retards or prevents chemical reactions (such as cure).


The contact area between reinforcement and resin matrix.


A resin-impregnated reinforcement in the mould, prior to polymerisation.

Light initiator

A compound which starts a reaction by being activated by light.

Magnesium oxide

A thickening agent for polyester resins.

Maleic anhydride

An unsaturated dicarboxylic acid anhydride used primarily in the formation of unsaturated polyester resins.


A sheet-type reinforcement made up of filaments, fibres or strands, cut or uncut, oriented or random, lightly bonded together. (Also see Needled mat.)

Methacrylic acid

Unsaturated monocarboxylic acid, used in the manufacture of resins and plastics.

Methyl ethyl ketone peroxide (MEKP)

An aliphatic peroxide, widely used for curing unsaturated polyester resins.

Methyl methacrylate

Unsaturated carboxylic acid ester used in the manufacture of resins and plastics.


A compound containing a reactive double bond, capable or polymerising or copolymerising.

Needled mat

A mat formed of strands cut to short length and felted together in a needle loom, with or without a carrier.


Multi-functional phenol.

Open moulding

Open moulding is the simplest and most widely used process to manufacture FRP parts. The mould is generally fabricated from FRP and is one sided. It can be male (part is moulded onto) or female (part is moulded into). The cosmetic surface of the part is fabricated next to the mould. The back of the mould is open.

Parting agent

See Release agent.


Oxygen-rich compounds used to cure polyester resins.


Basic feedstock for plastics.

Phthalic anhydride

Aromatic dicarboxylic acid anhydride, a chemical intermediate in the plastics industry.


A small hole in the surface of a moulded product, usually occurring in multiples.


A chemical compound added to some plastics to render them softer or more flexible.


Preparation of polyesters with liberation of water.


Usual term for unsaturated polyester resin.


A long-chain molecule consisting of many repeating units.


The chemical reaction (linking of monomers) to produce a polymer.


Air pockets or voids in a moulded product. Porosity can also be expressed as the ratio of void volume to total product volume.


Application of external heat to bring a resin to a stable state of cure in the shortest possible time.


The time during which a liquid resin remains useable, after addition of catalyst and accelerator. Also known as working life. Typically pot-life refers to neat resins (unreinforced) and working life refers to prepregs (reinforced.)


Reinforcement pre-shaped to the general geometry of the intended moulded part, usually by light pressing or by distribution of chopped fibres over a perforated former. It is used on more complex and deep-draw mouldings, to optimise distribution and orientation of fibres.


A moulding compound prepared prior to, and part from, the moulding operation, containing all components necessary for moulding.


A factory-made combination of reactive resins and reinforcing fibres, plus other necessary additive chemicals, ready to be moulded. To process prepregs, individual layers of prepreg are laid up on a tool and a vacuum bag is placed over the lay-up and the air evacuated. The bagged part is placed in an oven or an autoclave and cured for the specified time, temperature and pressure.


See Accelerator.


A process for making continuous profiles of composite materials. Fibres that are impregnated with a thermosetting resin and pulled through a heated die where curing takes place.


A variation of pultrusion where fibre is wound in the crosswise direction during the pultrusion process.

Reactive resins

Liquid resins which can be cured by catalysts and hardeners to form solid materials.

Release agent

A substance which prevents a moulding from sticking to the mould surface, thus facilitating its release from the mould after curing. It may be chemical compound or a solid material such as a plastic film. Also know as a Parting agent.

Resin transfer moulding (RTM)

A moulding process in which catalysed resin is injected into a closed mould already containing the preformed reinforcement.


Endless glass fibre bundles. A collection of parallel strands (assembled roving) or parallel filaments (direct roving) asembled without intentional twist.

Separating compound

See Release agent.

Setting time

See Gel time.

Shelf life

Storage life. The time during which a liquid resin can be stored under specified conditions, while remaining suitable for use.

Sink mark

A depression on the surface of a moulding, usually caused by local internal shrinkage associated with variation in thickness.


A coating applied to glass fibres or filaments during manufacture, to improve handling and protect from abrasion.


See Sheet moulding compound.


A process for manufacturing FRP parts in which continuous glass roving is fed into a special 'gun' which chops the fibres into short lengths and simultaneously mixes them with a resin (usually polyester) that is then sprayed onto the tool. Manual compaction with rollers is used to compact the lay-up.


An assembly of parallel filaments simultaneously produced and lightly bonded, without intentional twist.


An unsaturated monomer, widely used with polyester resins.


A plastic which softens each time it is heated.


A plastic which flows and then sets permanently on first heating, as a result of setting up a three-dimensional crosslinked molecular structure, and subsequently will not soften or dissolve.

Thick moulding compound (TMC)

Similar to BMC (see separate entry) but continuously produced in sheet form with a thickness of 25 mm or more.


The ability of resins to change their viscosity, liquefying on being shaken or stirred.

Unsaturated polyester resin

A condensation product made by reacting an unsaturated dicarboxylic acid with a diol.

Vacuum moulding (infusion moulding, resin infusion, vacuum infusion)

Reinforcements and resins are laminated under a vacuum bag operation. 

Vinyl ester resin

A reaction product of methacrylic acid and epoxy resin.


The resistance of a liquid to flow (and therefore to mixing/processing). High viscosity is thick, low viscosity is thin.


A pocket of air or gas trapped in a laminate or moulding.


Long- or short-term undulation of the surface of a moulding.


Complete wetting/saturation of a fibrous surface with a liquid resin.

Working life

See Pot life.