Out of Autoclave processing - A430

 

Out of Autoclave processing
Document Type	Article
Document Identifier	430
Prerequisites	Flow and consolidation management Prepreg Vacuum Bagging for Prepregs Debulking Autoclave Ensuring quality during curing of thick parts Effect of equipment in a thermal management system Oven Effect of cure on mechanical properties of a composite (Part 1 of 2) Effect of cure on mechanical properties of a composite (Part 2 of 2) Consolidation of Thermoplastics Convection Thermal behaviour

Introduction

Out-of-autoclave (OOA) prepreg processing involves curing pre-impregnated materials using a vacuum and oven only. It allows to achieve high performance composite parts by applying 0.1 MPa pressure from vacuum bag, in contrast to autoclave process, which applies 0.3 to 0.8 MPa of external consolidation pressure.

In OOA process, aerospace-grade performance is achieved with optimized prepregging process (ensuring proper fiber impregnation with resin) and controlled consolidation process conditions (ensuring no defects are formed).

OOA systems have demonstrated autoclave-equivalent porosity and mechanical standards under controlled processing. Adoption is driven by cost reduction, accessibility and scalability to very large structures.

Significance

In practice, OOA has made composite part manufacturing more accessible and initiated the development of new materials and processes. It overturned the long-standing assumption that high autoclave pressure is required for high-performance composites with the following advantages:

• Accessibility: removes capital and operating costs of autoclaves, broadening qualified composite production across the supply chain.
• Scalability: size of the part is no longer constrained by autoclave vessel limits, allowing to cure large components (fuselage barrels, wing covers, turbine blades).
• Performance credibility: despite low consolidation pressure, modern OOA systems achieve aerospace-grade porosity and mechanical strength. NASA and Boeing programs confirm comparability with autoclave baselines.
• Innovation driver: OOA process limitations (low pressure, out-time sensitivity, thermal gradients) pushed for higher quality of prepreg architecture, resin design, and predictive simulation, allowing to achieve high-performance parts under lower consolidation.

Scope

This page addresses OOA prepregs consolidated under vacuum-bag-only (VBO) conditions, with emphasis on aerospace-quality applications. The following topics are covered:

• Material design: microstructure, resin rheology, and evacuation pathways (EVaCs).
• Consolidation physics: resin infiltration, void evacuation, and sensitivity to vacuum quality, and cure cycle control.
• Geometry and tooling: bulk factor, forming behaviour, and thermal management in different tooling environments. An OOA process is compared with autoclave curing, inclusing insights from NASA and Boeing case studies.

Process Overview

OOA process manufactures composite parts by applying vacuum bag pressure, followed by curing in oven: consolidation happens under vacuum-bag-only (VBO) which provides 0.1 MPa pressure. Quality of the part depends on synchronising vacuum integrity, resin viscosity, gas evacuation, and thermal control.

Prepreg Layup

The process begins with the simple prepreg layup, the components for which are shown in Figure 1. It is essential to note that the quality of the part directly depends on the quality of the layup. The main difference in the layup with autoclave process is that OOA process includes:

• Edge breathing dams: for better resin flow. If not included, EVaCs will dry out, leaving no space for resin to escape.
• Release film: perforated to help volatiles to escape into the breather cloth.

Figure 1. A Vacuum-bag-only Layup Schematic

Consolidation

Consolidation is the next step, which progresses through three stages, illustrated in Figure 2.

Stage I: Room-Temperature Vacuum Hold (A → B)

• After layup (A), the laminate contains macro- and micro-voids.
• A high vacuum (more than 28 in Hg, 95%) is applied and held, compacting plies and initiating air andmoisture removal (B).
• Resin remains highly viscous (more than 10⁵ Pa·s), resulting in negligible flow of resin; evacuation relies on the open microstructure.

Stage II: Heating and Infiltration (B → C)

• Controlled heating lowers viscosity by two to three orders of magnitude, allowing resin infiltration into tow interiors and interlaminar regions (C).
• Thickness reduces as dry regions are filled.
• This is the final effective window for volatiles to escape before channels close.

Stage III: Cure and Vitrification (C → D)

• As cure advances, resin gels (50% conversion) and vitrifies, locking fibre architecture in place (D).
• Any remaining voids are frozen into the laminate.
• A post-cure is often applied to maximise glass transition temperature (Tg) and mechanical performance.

Figure 2.The Progression of Consolidation Process in OOA Prepregs, including: Stage I – Room Temperature Vacuum Hold, Stage II – Heating and Infiltration and Stage III – Cure and Vitrification

Figure 3. Progression of Air Evacuation Through Debulk and Vacuum Hold, the Following Stages: (A) – Layup, (B) – RT Vacuum Hold, (C) Heated Cure, (D) Cured Part Throughout OOA Process

Factors Affecting OOA Process

The success of OOA prepreg processing depends on material architecture, part geometry, tooling, environmental sensitivity, and the applied process. Each of these dimensions must be carefully managed, since the absence of high consolidation pressure removes the “safety margin” normally provided by autoclave curing. The following sections expand on these considerations using the MSTEP framework.

Material

OOA prepregs use partially impregnated fibre beds with engineered vacuum channels (EVaCs). These open paths provide much higher in-plane permeability than through-thickness, enabling gases to escape under only vacuum pressure.

• Viscosity control: At room temperature, the resin is highly viscous, keeping vent paths open. Upon heating (80-120 °C), viscosity drops, allowing fibre wet-out before gelation (refer Viscosity in Composites).
• Permeability anisotropy: In-plane venting dominates over through-thickness, with much higher permeability (refer Permeability in Composites).
• Channel closure: EVaCs must remain open until resin flow begins; if they seal too early, volatiles cannot escape and porosity rises. In autoclaves, high pressure compensates for that.
• Resin content: OOA prepregs are designed as no-bleed systems, with carefully metered resin that achieves 55-60% fibre volume fraction after cure.

Success in OOA depends on keeping EVaCs open just long enough for resin viscosity to fall into the infiltration window. Without autoclave pressure, this synchronisation will result in porous defects.

Shape and Tooling

OOA prepregs typically exhibit a higher bulk factor (1.1-1.4) than autoclave systems (1.05) due to partially impregnated fibre beds and EVaCs. During cure, laminate thickness reduces significantly as dry regions are filled - beneficial for evacuation, but challenging for forming. In flat laminates, thickness reduction is uniform. However, forming defects, such as thick spots, wrinkles, bridging and local fibre distortion arise in parts with more complex tool geometries.

In OOA prepregs, tools are only necessary at initial cure stages: as cure reaches 120-130°C, resin crosslinks, becoming firm enough to hold shape. This allows to remove the tool (composite, polymeric, or even wooden) and proceed cure without it. The tool’s heat conductivity also affects part’s quality. For example, invar and aluminum offer high conductivity, allowing for better thickness control and reduce exotherm risk. On the other hand, CFRP shows low conductivity, often resulting in uneven cure and overshoot in ovens. Heated blankets can stabilise cure locally, particularly in thick or ply-drop regions.

In OOA, geometry and tooling are inseparable. Laminate loftiness interacts with tool-induced stresses, so consolidation quality depends on bulk reduction and heat transfer behaviour. Mitigation measures include ply staggering, intermediate debulks, and careful tool material selection.

Equipment

OOA prepregs are designed to cure without autoclaves, allowing processing in ovens, heated tools, or heating blankets. This shifts responsibility for laminate quality onto vacuum integrity and thermal management.

Vacuum Quality

Consolidation is limited to vacuum pressure, so a stable vacuum of larger than 28 in Hg is essential. Even minor leakage elevates void content, as there is no external overpressure to collapse bubbles. Consumables, such as edge dams and well-designed breather networks are therefore critical (refer to Vacuum Bagging for Prepregs and Debulking).

Thermal Management and HTC-Thickness Interaction

The heat transfer coefficient (HTC) governs how quickly and uniformly heat penetrates the laminate, affecting resin viscosity, volatile removal, impregnation, and exotherm dissipation.

• Autoclaves (3–7 bar): HTC of 60–200 W·m⁻²·K⁻¹, driven by forced convection and pressurized gas. Rapid heating but chamber gradients may exist.
• Ovens (1 bar): HTC of 10–60 W·m⁻²·K⁻¹, relying on natural convection. Slower but often more uniform.

(Refer to Effect of Equipment in Thermal Management).

Practical Design Guide

• Thin laminates (<10 mm): oven or autoclave both viable; thermal response nearly identical.
• Medium laminates (10–20 mm): ovens feasible but require tailored ramps or dwells; autoclaves more forgiving.
• Thick laminates (>25 mm): oven curing carries high exotherm risk, especially with CFRP tooling. Autoclaves with conductive tools (e.g., Invar ≥5 mm) strongly recommended.
• Tooling conductivity: Invar lowers overshoot by 5 to 10°C compared to CFRP. Tool thickness (>5 mm) further stabilizes cure.

Simulation Note (Parametric RAVEN Study)

A 1D transient thermal model (MTM45, 5–50 mm thickness) was run with variable HTC and tooling conductivity (see Figure 4):

• Oven conditions (HTC 10–60): overshoot rose sharply with thickness. CFRP tools showed more than 40°C overshoot at 40 to 50 mm; Invar reduced peaks by 5 to 10°C but did not eliminate risk.
• Autoclave conditions (HTC 60–200): higher HTC stabilised cure response. Even at 50 mm, overshoot remained les than 30°C.
• Worked example: a 40 mm laminate on a CFRP tool in an oven (HTC = 10) overshot by 45°C, far above the acceptable 20 to 25°C. The same part in an autoclave (HTC = 120) on Invar showed only 22°C overshoot, within limits. Worked examples are provided to illustrate design decision thresholds rather than precise predictive values.

Figure 4. Thermal Model Simulation Showing Changes in Exotherm With Variable HTC and Tool Material (CFRP and Invar) for Autoclave and OOA Cure. For OOA (on the left) exotherm overshoot is large but can be mitigated using a tool that acts as a heat sink, such as invar. For autoclave (on the right), exotherm is not an issue, since the high pressure is sufficient to take energy away

Process

OOA curing mirrors autoclave processes but demands stricter control due to the absence of external pressure.

• Layup & Debulking: each ply must be compacted carefully; entrapped air is the main porosity source. Intermediate debulks (5–20 min every 1–3 plies) reduce loft and release air. Automated fibre placement adds localized heat and pressure, but controlled hand layup remains common.

(Refer to Debulking).

• Room-Temperature Vacuum Hold: after layup, laminates are held under full vacuum for hours. Resin viscosity prevents cold flow, keeping evacuation channels open. This stage enables further gas and moisture removal before heating.
• Heating and Infiltration: for oven cure, an extra hold is needed to give pores enough time to escape and to ensure fibers are fully saturated in resin before gelation begins. Conversely, autoclave does not have this concern, so faster heating cycle is appropriate.

Figure 5. The Comparison of Cure Cycles for Oven (on the left) and Autoclave (on the right), Showing How Degree of Cure and Resin Viscosity Progress Through Each Heating Stage.

• Cure and Vitrification: as cure proceeds, viscosity rises, gelation occurs, and vitrification locks fibre architecture. Remaining voids are trapped. Post-cure may raise Tg.
• Process Risks (OOA vs Autoclave): in Stage I, vacuum leaks immediately raise voids (no external compensation). In Stage II, an early channel closure seals pathways prematurely. In Stage III, slow heating of thick parts in ovens can trap volatiles via exotherm overshoot.

Applications

OOA prepreg processing has progressed from laboratory trials to serialised production in multiple sectors. Its appeal lies in eliminating autoclaves while still achieving aerospace-grade laminate quality.

General Aviation - Serialised Production

The Cirrus SR-22 is a benchmark program: its fuselage and wing structures are manufactured entirely from OOA prepregs, at production rates over 500 aircrafts per year. The program demonstrates that OOA laminates can meet FAA certification standards for safety-critical structures.

Marine & Renewable Energy - Large Structures

The SeaGen tidal turbine (1.2 MW, 2008-2019) incorporated OOA blades fabricated at full scale. Achieving void contents below 2% in thick laminates validated OOA for harsh marine environments. Similar approaches are now used in yacht construction and wind turbine blades, where autoclave size and cost are prohibitive.

Space & Defense - Advanced Demonstration

NASA’s Composite Cryotank Technology Demonstration produced large OOA cryogenic tanks that passed full-scale ground testing under cryogenic conditions. Although not yet in serialised service, the program confirmed that OOA can achieve aerospace-grade performance at very large scales where autoclaves are impractical.

Consumer Products - Commercial Adoption

We Are One Composites manufactures high-performance bicycle frames and components using OOA prepregs. These are sold commercially, proving that OOA can meet both structural and economic demands in consumer goods.

In practice, OOA is strongest for thin-to-moderate laminates and large structures, while autoclaves remain preferred for very thick or safety-critical parts.

Conclusion

OOA prepreg processing is a viable alternative to autoclave curing when its unique constraints are managed. It lowers cost and removes vessel size limits, enabling broader adoption, but demands strict control of vacuum integrity, cure cycles, and thermal management. In practice, OOA is best suited for thin-to-moderate laminates, while autoclaves remain preferred for thick or highly critical structures.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Introduction to Out of Autoclave Prepreg Processing - A393

References

About	Help

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

The use of multiphysics models to predict the outcome of real-world scenarios. May be analytical (closed form, "hand calculations") or computational (implementation on computers is required due to the large number of calculations involved. e.g. finite element method, finite difference method)

Most often in composite materials engineering, simulation refers to either:

• Process simulation (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance simulation (predicting the stiffness/strength/suitability of a structure, or the inverse problem of the material properties and dimensional requirements to achieve a desired stiffness/strength/suitability of a structure)

(same as "Modelling")

A central processing theme in the manufacturing cycle. This theme is concerned with managing the thermal response of materials during storage and handling or parts/tools when they are subsequently heated.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

A tow is a bundle or yarn of individual fibres. The tow size is inherent to the fibre manufacturing process (i.e. a tow is manufactured in one process, rather than each fibre individually then bundled together after).

Typically, smaller tows are better because they result in a more homogeneous material.

The larger the tow:

• The faster it is to deposit material
• The easier is it for resin to flow between tows
• Harder for resin to saturate

Typical tow sizes:

• 1k (thousand)
• 3k
• 6k
• 12k
• 24k
• 50k

Fibre architecture refers to how the fibre is assembled.

Most common architectures:

• Unidirectional
• Chopped strand mat (CSM) and chopped fibre (chopper gun)
• Continuous filament mat (CFM)
• Woven fabric
• Non crimp fabric (NCF)
• Braided
• Prepreg

The glass transition temperature (T_g) is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It is one of the most important properties of any amorphous polymer.

Material and process (M), Shape (S), Tooling and consumables (T) & Equipment (E) - all have an interlinked effect on the Process step (P). (See MSTE factory ontology)

Permeability refers to the resistance to fluid flow through a porous material.

• Resin flow through fibre
• Gas flow through prepreg

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

With regards to manufacturing, risk is the combination of the probability and consequences of undesirable manufacturing outcomes. Manufacturing risk can lead to technical issues, program/schedule delays, and cost overruns.