Cool-Clave: an energy- and cost-efficient autoclave

The goal of this on-going project is to achieve autoclave consolidation of fibre-reinforced composites with individual, or combinations of, change to enhance the energy- and cost- efficiency:

a) without heating parasitic materials (hence saving energy),
b) with shorter cycle times (increased production rate), and
c) without prepreg, i.e. using infused laminates (for lower cost).

Out-of-autoclave (OOA) processing of composites inevitably results in laminate fibre volume fractions being limited by the maximum ~1000 mbar pressure given the power-law compressibility characteristics of the reinforcement. The lower fibre volume fraction produces a higher resin volume fraction which becomes resin-rich volumes (RRV). Increasing clustering of both the fibre reinforcements and the matrix (RRV) is implicated in reductions in the laminate strength at constant fibre volume fraction [1]. The inevitable increase in resin content of OOA composites thus compromises composite performance, and results directly in parasitic weight, which will lead to higher fuel consumption in transport applications. Retention of autoclave processing is therefore recommended for highest performance when compression press moulding is not appropriate (e.g. for complex 3D components).

The traditional autoclave heats not only the component to be cured but also parasitic air (or inert gasses) and the vessel insulation. Energy savings can result from decoupling the heat and pressure in the autoclave. Subject to minor modifications to the pressure vessel, electrically-heated tooling and cool air pressurisation could be implemented [2]. The use of (electrically) heated mould tools to bring the laminate to temperature eliminates the heating of the parasitic systems (pressure vessel walls, refractory insulation and heat transfer gasses). This approach would need to balance the insulation of the heated tool surface (and any heater blanket on the counter-face) against the quenching effect during introduction of the pressurised cool air. Further to the potential for significant reductions in energy consumption, the laminate on the heated tool could be taken to the end of the dwell period before loading the autoclave leading to significant reductions in cycle times. Autoclave loading efficiency could be improved by curing different composite systems simultaneously with the composites brought to their respective curing temperatures before loading the autoclave provided there are sufficient power and control circuits in the autoclave, which would further enhance process efficiency.

Snow [3] undertook a number of thermal energy simulations. The first validation model for transient convective heat transfer used the Heisler Chart method with Solidwoks Simulation Flow software. The second validation case followed the cooling of the Plymouth autoclave from steady state at 180°C. Simulations were then run for heated tooilng only or vessel heating only. These initial models suggest there could be a 95% energy reduction when using heated tooling.

While autoclave processing has usually involved vacuum-bagged pre-impregnated (or wet-laid) reinforcements, there is scope for using the pressure vessel to cure vacuum-infused composites. Lewin et al [4], Wilkinson et al [5] and Harrod [6] have undertaken preliminary experiments towards optimisation of the process methodology for high-quality resin-infused laminates cured in the autoclave. The use of resin infusion, rather than preimpregnated reinforcements, removes the need for the separate impregnation stage and consequent costs. It also eliminates the requirements for freezer storage of prepreg materials, and hence saves energy.

Summaries of this on-going project have been published [7-8].

References:

1. DM Basford, PR Griffin, SM Grove and J Summerscales, Research report: the relationship between mechanical performance and microstructure in composites fabricated with flow-enhancing fabrics, Composites, September 1995, 26(9), 675-679.

2. Gary Prudham and John Summerscales, Cost reduction in autoclave processing I: heated tooling and cool air pressurisation, 13th International Conference on Flow Processes in Composite Materials (FPCM 13), Kyoto – JAPAN, 6-9 July 2016, Paper 37, oral presentation.

3. Jacqueline C Snow, Thermal anlysis of composites manufacture in the autoclave with heated tooling and cool air pressurisation, BEng stage 4 project report, University of Plymouth, May 2018 (not in the public domain).

4. Grant Lewin, Richard Cullen and John Summerscales Cost reduction in autoclave processing II: curing resin infused composites, 13th International Conference on Flow Processes in Composite Materials (FPCM 13), Kyoto – JAPAN, 6-9 July 2016

5. Robert Wilkinson, Richard Cullen and John Summerscales, Curing resin infused composites in the autoclave, 21st International Conference on Composite Materials (ICCM-21), Xi’an – CHINA, 20-25 August 2017, oral 2043. PEARL.(Track 6.2 5215-3 Friday 1215-1235 Room 15 (206) AT99).

6. Joe Harrod, optimisation of the post-flow reservoir in autoclave cure of resin infused composites, BEng stage 4 project report, University of Plymouth, May 2019 (not in the public domain).

7. John Summerscales, Taking the high energy demand Out of Autoclave processing of composites, Out of Autoclave Processing of Composites - Challenges and Opportunities, Belfast, 15 March 2018.

8. John Summerscales, Taking the high energy demand Out of Autoclave Processing of Composites, SAMPE Europe 2018, Southampton, 11-13 September 2018, oral presentation. ISBN 978-90-821727-9-9.