Case Study of Using Out of Autoclave Processing - C131

 

Case Study of Using Out of Autoclave Processing
Case study
Document Type	Case study
Document Identifier	131
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
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

Inroduction

OOA prepregs are preferred against autoclave in multiple programs. There are two well-documented examples of that: coupon-level (NASA) and structural component (Boeing), which illustrate both the equivalence potential and the sensitivities of OOA processing.

NASA Comparative Study

NASA compared four unidirectional systems processed as quasi-isotropic laminates: autoclave-qualified IM7/8552-1 and IM7/977-3, as well as OOA-qualified IM7/MTM45-1 and T40-800B/5320 (see Figure 1). Panels were produced in “fresh” condition and in “out-life” condition (materials stored beyond vendor recommendations).

• Laminate quality: fresh OOA panels achieved void contents less than 0.1%, equivalent to autoclave systems.
• Out-of-life MTM45-1 panels rose to 8% voids, with reduced fiber volume fraction from 58% to 53%).
• Mechanical performance: under fresh conditions, OOA and autoclave laminates had comparable open-hole compression (OHC) and strength of 335-374 MPa. Out-of-life OOA laminates lost more than 50% of OHC strength, while autoclave systems showed only modest drops.
• Handling & tack: OOA systems displayed shorter tack life of 21 days, while it was 60 days for autoclave resins.
• Implications: OOA can equal autoclave standards when fresh, but it is far more vulnerable to out-time excursions. Processing control is therefore critical (see Porosity in Composites and Prepreg Storage & Handling).

Figure 1. NASA Study: Comparison of Autoclave and OOA Composites for Fresh and Out-life Prepregs

Boeing Structural C-Channel: Configured Part

Boeing fabricated a structural C-channel using IM7/8552 (autoclave) and CYCOM® 5320/T40-800B (OOA), both cured under representative conditions.

• Laminate quality: cross-sectional inspection showed no voids, wrinkles, or pooling. Resin content was consistent (27-32 wt%), and Tg met target (154°C).
• Mechanical properties: across load cases, SBS, CLC, OHC, tension, and flange bending, the OOA channel was statistically equivalent to its autoclave counterpart. Small deviations, such as flange bending, were within expected scatter.
• Implications: at the component level, OOA prepregs can deliver structural performance equivalent to autoclave systems, provided cure and layup discipline are maintained.

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. 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.

References

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

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.

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.

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.