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Composite materials glossary

Updated 15 October 2026

The terms we use when we talk about AFP, additive manufacturing and composite materials, explained in a few lines by the Spherecube team.

AFP (Automated Fibre Placement) · ATL (Automated Tape Laying) · Towpreg · Prepreg · Continuous fibre · Fibre steering · Minimum steering radius · Filament winding · Co-curing · Co-cured insert · Compression moulding · Autoclave · Vacuum-bag oven cure · Vf, fibre volume fraction · Void content · ILSS · Buy-to-fly · Thermoset matrix · Thermoplastic · Vitrimer · LCA (Life Cycle Assessment)

AFP (Automated Fibre Placement)

Automated process in which a robotic head lays several pre-impregnated tows side by side along a programmed path, compacts them with a roller and cuts them where needed. Developed for large aerostructures such as wings and fuselages, it delivers repeatability and low scrap. Spherecube's LOOP is its evolution for small and medium complex parts, on up to 6 axes.

ATL (Automated Tape Laying)

Automated laying of wide pre-impregnated tapes on flat or gently curved surfaces. Compared with AFP it uses a single, wider tape, so it is very productive on panels and wide surfaces but less suited to tight curves, double curvature and complex geometries. It is used mainly for large aircraft components with simple shapes.

Towpreg

Bundle of continuous fibres (tow) already evenly impregnated with resin, ready for automated deposition. The fibre/resin ratio is set before processing, so stiffness, strength and porosity remain predictable. Spherecube makes its own towpreg in-house on LOOM, starting from dry spools of carbon, glass, quartz, aramid, flax or hemp.

Prepreg

Fabric or tape of fibres pre-impregnated with a partially cured resin, stored cold and laid up in layers in the mould. It is the reference material for structural aerospace composites. It is laid by hand or by automated machines, then compacted and cured, often in an autoclave. With hand lay-up, ply offcuts become scrap.

Continuous fibre

Reinforcement in which fibres run without interruption along the whole length of the path, unlike short or chopped fibres dispersed in the matrix. A continuous fibre carries load as long as it is not interrupted, which is why it offers far higher stiffness and strength. In LOOP the fibre stays continuous and is interrupted only where the part requires it.

Fibre steering

Technique that curves the fibre tow in its own plane during deposition, instead of laying it only in straight lines. It allows the fibre to follow loads around holes, inserts and changes in geometry. The limit is wrinkling on the inner side of the curve: LOOP steers the tow while keeping its width and thickness constant.

Minimum steering radius

The smallest radius at which a tow can be curved in its own plane without wrinkles, folds or lifting. It depends on tow width, material and deposition head. In traditional AFP cells it limits tight curves and therefore small, complex parts. The lower the minimum radius, the more closely the fibre can follow the loads.

Filament winding

Process in which impregnated fibre is wound onto a rotating mandrel in continuous helices or hoop turns while a carriage moves along the axis. It is fast and efficient for tubes, shafts, tanks and pressure vessels. It does not allow 0° fibres on rotating parts or interruptions of the strand, and it struggles with closed ends and non-axisymmetric shapes.

Co-curing

Simultaneous curing of several elements in a single cycle, so they bond while the resin cross-links instead of being glued or fastened afterwards. It is used to join composite parts to each other or to integrate metal inserts into the laminate. It reduces assembly steps, adhesives and mechanical fasteners and makes the part a single body.

Co-cured insert

Bushing, thread or metal element placed during fibre deposition and cured together with the laminate. The continuous fibre can wrap around the insert and the part comes out with its fastening points already integrated, without later bonding. In Spherecube's bracket case study, three metal bushings are co-cured in the fastening holes.

Compression moulding

Process in which composite material is closed between heated mould and counter-mould and compacted in a press until it cures. Compared with an autoclave the cycles are fast and the surface is defined on both sides. It is one of the compaction options after deposition with LOOP, chosen according to time, cost and performance.

Autoclave

Heated pressure vessel in which composite laminates, sealed under a vacuum bag, are cured by applying temperature, pressure and vacuum together. It is the aerospace standard for low porosity and high quality, but it requires long cycles, a lot of energy and a high investment. It is the baseline for the CO₂ comparison in Andreozzi et al. 2025.

Vacuum-bag oven cure

Out-of-autoclave cure method: the laminate is sealed in an airtight bag, the air is drawn out to compact it with atmospheric pressure alone, and it is cured in an oven. It costs less than an autoclave and needs simpler equipment, but compaction is lower. It suits resins formulated for out-of-autoclave cure and less critical parts.

Vf, fibre volume fraction

Percentage of the composite volume occupied by fibres; the rest is matrix and any voids. More fibre generally means more stiffness and strength in the fibre direction, up to the limit beyond which the resin no longer wets the fibres properly. The Spherecube data sheet for 3K UD carbon/epoxy reports a Vf above 60%.

Void content

Percentage of composite volume taken up by porosity, meaning air or gas bubbles trapped during deposition and cure. Voids mainly reduce matrix-dominated properties such as interlaminar shear strength. It is measured by microscopy or by density methods. In Spherecube carbon/epoxy it is below 2%, verified by electron microscopy.

ILSS

Interlaminar Shear Strength: how well the layers of a laminate stay together when a load tends to make them slide over one another. It is usually measured with the short-beam test. It depends strongly on the matrix, fibre/resin adhesion and voids. The Spherecube data sheet for 3K UD carbon/epoxy reports 90 MPa.

Buy-to-fly

Ratio between the mass of material purchased and the mass of the finished part that flies. A part machined from solid can have a very high ratio, because almost the entire block becomes swarf. Depositing fibre only where needed brings the ratio close to one. In the bracket case study, material waste fell by 99%.

Thermoset matrix

Resin that cross-links irreversibly during cure, forming a three-dimensional network that does not become fluid again when heated. Epoxies, phenolics and cyanate esters are common examples. It offers excellent mechanical, thermal and chemical properties and is the standard for structural composites. LOOP currently works with thermoset-matrix towpreg, bio-based or flame-retardant ones included.

Thermoplastic

Polymer that softens when heated and solidifies when cooled, repeatably. In composites, consolidation happens by melting, without a cure cycle: parts are tough, weldable and recyclable. It does however require higher processing temperatures. Unidirectional tapes with structural thermoplastic matrices for LOOP are in development at Spherecube.

Vitrimer

Class of cross-linked polymers with dynamic bonds that can rearrange at high temperature. In service they behave like a thermoset, yet they can be reshaped, repaired or recycled, like a thermoplastic. In composites they are studied to make structural matrices recyclable. Their sustainability is analysed in the 2024 Procedia CIRP article by the UNIVPM group.

LCA (Life Cycle Assessment)

Life cycle assessment: a standardised method (ISO 14040/14044) that measures the environmental impacts of a product or process, from raw material extraction through production and use to end of life. According to the LCA by Andreozzi et al. (2025), the Spherecube process has −30.5% CO₂ emissions vs prepreg lay-up with autoclave curing.

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