Supporting UK Composites

Glossary of Terms

Terms used in the composites industry

There are many terms used in the composites industry. The following are some of the more popular terms and abbreviations.

Accelerator: accelerates cure of a resin

Additives: the term used for a large number of specialist chemicals which are added to resins/compounds to impart specific properties, for example, flame retardancy, and UV resistance

Adhesive: substance applied to mating surfaces to bond them together by surface attachment. An adhesive can be in liquid, film or paste form.

Aramid: high-strength, high-stiffness aromatic polyamide fibres

Blister, blistering: undesirable raised areas in a moulded part caused by local internal pressure, due usually to rapped air, volatile reaction by-products or water entering by osmosis.

Bulk Moulding Compound (BMC): polyester resin/glass fibre premix, for injection or transfer moulding, also known as dough moulding compound (DMC)

Carbon fibre: reinforcing fibre known for its light weight, high strength and high stiffness.

Catalyst (also called hardener): a chemical compound (usually an organic peroxide) which initiates polymerisation of a resin

Chopped strands: short strands cut from continuous filament strands of reinforcing fibre, not held together by any means

Composite: a material made up of resin and reinforcement (usually fibre)

Compression strength: the crushing load at failure of a material, divided by cross-sectional area of the specimen

Core: in sandwich construction, the central component to which inner and outer skins are attached. Foam, honeycomb and wood are all commonly used core materials.

Corrosion resistance: the ability of a material to withstand contact with ambient natural factors without degradation or change in properties. For composites, corrosion can cause crazing.

Coupling agent: a substance, which promotes or establishes a stronger bond at the resin matrix/reinforcement interface

Cracking: actual separation of moulded material, visible on opposite surfaces of a part and extending through the thickness (fracture)

Cure: the process of hardening of a thermosetting resin (by cross-linking of the molecular structure), under the influence of heat

Curing agents: chemical compounds used to cure thermosetting resins

Curing time: the time taken for a resin to cure to its full extent

Delamination: 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

Dough moulding compound (DMC): polyester/resin fibre premix, for injection or transfer moulding, also known as bulk moulding compound (BMC)

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

Filament: a single textile element of small diameter and very long length considered as continuous

Filler: material (usually low cost) added to a resin to extend it, or give special properties

Finishing: application of coupling agent to textile reinforcements to improve the fibre/resin bond

Flexural strength: the strength of a material in bending expressed as the stress if a bent test sample at the instant of failure.

Flow: the movement of a resinous material, thermosetting or thermoplastic, under pressure, to fill all parts of a closed mould

Fracture: cracks, crazing or delamination resulting from physical damage.

Gel: the state of a resin, which has set to a jelly-like consistency

Gelcoat: a thin layer of unreinforced resin on the outer surface of a reinforced resin moulding; it hides the fibre pattern of the reinforcement, protects the resin/reinforcement bond, gives smooth external finish and can also provide special properties; it is usually pigmented

Glass fibre: reinforcing fibre made by drawing molten glass through bushings. The predominant reinforcement for polymer composites, it is known for its good strength, processability and low cost.

Hardener: see catalyst

Honeycomb: light weight cellular structure made from either metallic sheet materials or non-metallic materials and formed into hexagonal nested cells, similar in appearance to the cross-section of a beehive

Hybrid: a resin or reinforcement made from two or more different polymers or reinforcement materials

Impact strength: a material’s ability to withstand shock loading as measured by fracturing a specimen

Impregnation: saturation of reinforcement with liquid resin

Laminate: the structure resulting from bonding multiple plies of reinforcing fibre or fabric

Lay-up: a resin-impregnated reinforcement in the mould, prior to polymerisation

Mat: a widely used sheet-type reinforcement made up of filaments, staple fibres or strands, cut or uncut, oriented or random, lightly bonded together

Monomer: a compound containing a reactive double bond, capable of polymerising

Polyester: usual term for an unsaturated polyester resin

Polymer: a long-chain molecule, consisting of many repeat units

Porosity: numerous air pockets or voids in a moulded product

Post-cure: application of external heat to bring a resin to a stable state of cure in the shortest possible time

Preform: reinforcement pre-shaped to the general geometry of the intended moulded part; it is used on more complex and deep-draw mouldings, to optimise distribution and orientation of fibres

Pre-preg: a factory-made combination of reactive resins and reinforcing fibres, plus other necessary additive chemicals, ready to be moulded

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; it may be a chemical compound or a solid material such as a cellulose or plastics film

Reinforcement: key element added to resin (matrix) to provide the required properties; ranges from short fibres and continuous fibres through complex textile forms

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

Resin transfer moulding (RTM): a moulding process in which catalysed resin is injected into a closed mould already containing the pre-formed reinforcement

Roving: endless glass fibre bundles; a collection of parallel strands (assembled roving) or parallel filaments (direct roving) assembled without intentional twist

Sandwich structure: composite composed of lightweight core material to which two relatively thin, dense, high strength, functional or decorative skins are adhered

Sheet moulding compound (SMC): a flat pre-preg material, comprising thickened resin, glass fibre and fillers, covered on both sides with polyethylene or nylon film, ready for press-moulding

Strand: an assembly of parallel filaments simultaneously produced and lightly bonded

Thermoplastic: a plastic, which softens each time it is heated

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

Wet-out: complete wetting/saturation of a fibrous surface with a liquid resin



ACM: advanced composite material

AFRP: aramid fibre reinforced polymer

BMC: bulk moulding compound

CFRP: carbon fibre reinforced polymer

DMC: dough moulding compound

GFRP: glass fibre reinforced polymer

FRP: fibre reinforced polymer

SMC: sheet moulding compound

TMC: thick moulding compound


Exclusive information, support & event offers


Need more information on composites or membership?