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

Cleanroom: A cleanroom is a controlled environment which uses filtration to remove airborne contaminants, creating a consistent and repeatable manufacturing environment.

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