Two tubes can look identical on a datasheet and behave completely differently in service. The process chosen decides fibre volume, void content, dimensional stability and price — often more than the fibre grade does.
Every composite process is a combination of these three choices. Once you can read them, you can judge any supplier’s route.
Dry fabric + on-site mixing — cheapest and most flexible, but resin content depends on operator skill. Prepreg — factory-impregnated with an exact resin ratio, consistent, requires cold storage. Inline impregnation — fibre dipped in a resin bath during pultrusion or winding.
Vacuum bag — about 0.1 MPa, cheap but weak. Hydraulic press or autoclave — several MPa, vastly better consolidation. Mould surface or shrink tape — used in pultrusion, winding and tube rolling. Pressure is what squeezes out voids and controls fibre volume.
Ambient cure — simple but heat resistance stays low (Tg often 60–80 °C). Moderate post-cure — raises Tg towards 100–120 °C. High-temperature cure — 120–180 °C, mandatory for prepreg and the route to aerospace-grade properties.
Dry fabric is laid in a mould, resin mixed and brushed in, then vacuum bagged. Lowest tooling cost, ideal for prototypes, large one-offs and irregular shapes. Downsides: resin ratio depends on the operator, so void content and weight vary, and it is labour heavy. Usually cured at room temperature, optionally post-cured.
Dry fabric is laid dry, bagged, then low-viscosity resin is pulled through the stack by vacuum. Better fibre ratio and far less porosity than brushing by hand, and the operator never touches liquid resin. Excellent for large panels, boat hulls and wind blades. Needs careful flow design to avoid dry spots.
Factory-impregnated fabric is laid up, vacuum bagged and cured at 120–180 °C in an oven without an autoclave. Consistent resin content, high fibre volume, low void levels. Cost drivers are the prepreg price, the cold-chain storage and the oven cycle. The standard route for high-performance tubes and sporting goods.
The same prepreg stack, but cured inside an autoclave that adds positive pressure of 3–7 bar on top of vacuum. Result: void content routinely below 1% and the highest mechanical properties available. Used for aerospace and top-tier racing. Tooling and cycle cost are correspondingly high.
Stacked layers are placed in matched tooling or between press platens and consolidated under hydraulic pressure with heat. This is how most flat carbon fibre sheets and plates are produced: both faces come out smooth, thickness tolerance is tight, and short cycle times suit volume. Great dimensional control, higher tooling cost.
Fibre is pulled through a resin bath and then through a heated die that shapes and cures it continuously. No vacuum bag, unlimited length, highly repeatable, and the lowest cost per metre for a constant cross-section. Because nearly all fibres run axially, pultruded profiles are very stiff along their length but weak against crushing and torsion.
Continuous tow passes through a resin bath and is wound onto a rotating mandrel at controlled angles. Pressure comes from tow tension and the mandrel surface, with no vacuum bag in most cases. Ideal for tubes, pressure vessels and shafts, and it allows a designed balance of axial and hoop strength.
Prepreg or resin-coated fabric is rolled onto a mandrel, then wrapped with heat-shrink tape that provides consolidation pressure during oven cure. The mainstream method for round tubes: good control of wall thickness and concentricity, and the lay-up schedule can be tuned for torsion or bending.
Dry preform sits in a closed two-part tool and resin is injected under pressure. Both surfaces are finished by the mould, so parts come out clean with little secondary finishing. Higher tooling investment, best justified at medium to high volumes where cycle time pays the tooling back.
| Process | Tooling Cost | Unit Cost | Void Content | Best Volumes |
|---|---|---|---|---|
| Hand lay-up + vacuum bag | Very low | High labour | Higher | 1–50 |
| Vacuum infusion | Low | Medium | Medium | 10–500 |
| Prepreg + oven (OOA) | Low–medium | Medium–high | Low | 10–1000 |
| Prepreg + autoclave | High | Highest | Lowest | Aerospace volume |
| Compression / hot press | Medium–high | Low at volume | Low | 500+ |
| Pultrusion | High die cost | Lowest per metre | Varies by control | Continuous |
| Filament winding / roll wrap | Low–medium | Medium | Low–medium | Any tube run |
A real workshop answers immediately and explains why that route fits your tolerances and volume. Hesitation usually means the work is being resold, or that the process is chosen for the supplier’s convenience rather than your part.
Look for specified resin ratios, lay-up schedules written down, batch records and first-article inspection. These are the practical controls that keep sample No. 1 and batch No. 5 looking the same.
No — that is a common misconception. Prepreg requires heat to cure, and it also nearly always requires vacuum bagging to remove volatiles during cure; autoclave work adds positive pressure on top of vacuum. Dry fabric parts are usually cured at room temperature but are frequently post-cured in an oven to raise heat resistance. Heat, pressure source and resin format are three independent choices.
Prepreg cured in an autoclave gives the lowest void content, routinely below 1%, and the highest mechanical properties, because positive pressure of 3-7 bar consolidates the laminate on top of vacuum. Out-of-autoclave oven cure and compression moulding follow close behind at much lower cost, which is why most commercial parts use them.
Ambient cure keeps tooling simple and cost down, but the resin typically reaches a glass transition temperature of only 60-80 °C. Applying a moderate post-cure can raise heat resistance towards 100-120 °C, and prepreg systems require 120-180 °C to react properly. The higher the service temperature, the more likely a heated cure is needed.
Vacuum bagging compacts a laminate that has already been wetted by hand or supplied as prepreg, using the vacuum to press the stack and remove air. Vacuum infusion lays the fabric dry, seals it, and uses the vacuum to pull low-viscosity resin through the stack. Infusion generally gives higher fibre volume and lower void content than hand wet-out.
Prepreg is partially cured at the factory and keeps reacting slowly even at room temperature, so it must be stored around -18 °C and shipped cold, with a shelf life typically measured in months. That cold-chain cost plus ovens or autoclaves is why prepreg parts cost more, and why the same parts offer much more consistent resin content and properties.
Most round tubes are roll wrapped from prepreg and consolidated with heat-shrink tape during oven cure, which controls wall thickness and concentricity. Filament winding suits tubes needing a designed balance of axial and hoop strength, while pultrusion produces constant-section profiles continuously at the lowest cost per metre, though almost all fibres run axially.
Flat sheets are usually made by stacking layers and consolidating them in a heated press, which gives smooth faces on both sides and tight thickness control, often within ±0.05 mm on raw sheet. Thicker plates may also be vacuum infused or oven cured from prepreg depending on required properties and volume.
For a constant cross-section produced in length, yes — the process is continuous with very low labour content, so cost per metre is typically the lowest available. The trade-offs are that the fibre is almost entirely axial, giving weak crush and torsional strength, and that the die represents significant upfront tooling, so it suits repeat or long profiles.
Ask which process they will use for your specific part and why it suits your tolerances and volume, and then ask how they control void content and fibre ratio. Expect specific answers about lay-up schedules, batch records and first-article inspection. A supplier who cannot describe the route is unlikely to be producing the part themselves.
Structure decides performance — how each tube type behaves under real load.
Seven due-diligence checks including how to verify the process claims above.
Why removing air from mixed resin protects strength, surface finish and fatigue life.
