- Out of Autoclave Composite Manufacturing
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Contents
Introduction
Out of autoclave curing composite manufacturing is an alternative to the traditional high pressure autoclave curing process commonly used by the aerospace industry. Autoclaves are utilised where the highest of material performance standards are required such as void contents of less than 2% and high Glass Transition Temperatures. Aerospace autoclaves normally operate from 120 to 230 degrees Celsius within a nitrogen environment at 7 bars of pressure. Most common materials cured in an autoclave are advanced composites such as carbon fibre and epoxy resins. Curing cycles range from 90 minutes to 12 hours.
There are many out of autoclave technologies available including Vacuum Bag Only (VBO) prepregs, Resin Transfer Molding RTM), Vacuum-Assisted Resin Transfer Molding (VARTM), prepreg compression molding, microwave curing, EBeam Curing and balanced pressure fluid molding.
Vacuum Bag Only (VBO) Prepregs
Resin Transfer Molding (RTM)
Vacuum Assisted RTM (VARTM)
Balanced Pressure Fluid Molding
Balanced Pressure molding using fluid as the heat transfer is commercially practiced as the Quickstep Process. It allows for the curing, partial curing, and joining of composite materials. The process involves a fluid-filled, pressure balanced, heated floating mould technology. The heated floating mold technology used within the process works by rapidly applying heat to the laminate which is trapped between a free floating rigid or semi-rigid mold that floats in, and is surrounded by, a Heat Transfer Fluid (HTF). The mold and laminate become separated from the circulating HTF by a flexible membrane. The part, typically under full vacuum, is subject to less than 3 psi (20 kPa) fluid pressure and can be rapidly heated to the desired cure temperature without risk of catastrophic exothermic reaction. The air is then removed under vacuum and the laminate is compacted and heated until the part is cured.
A flexible membrane beneath the mold is bonded into a pressure chamber creating the lower half of a 'clamshell' or 'chamber' like mold set. A second flexible membrane is bonded to a second pressure chamber creating the upper half of the clamshell. These pressure chambers are clamped together during processing, permitting the laminate to be compressed while reducing stress to the mold as it is floating in a balanced pressure environment within the HTF. http://www.quickstep.com.au/files/tmp/31_pressureChamber_big.jpgw350h294.jpg
The process can use thermosetting, thermoplastic prepregs (pre-impregnated composite fibers), and wet resin with dry fiber to produce superior composite parts. This out of autoclave process can achieve aerospace grade void contents of less than 2%, with extremely fast cycle times, and at significantly lower pressures and lower labor costs than many alternative autoclave production systems using many typical autoclave qualified prepregs. The Quickstep out of autoclave system is unique in that it uses fully immersed balanced pressure fluid curing and it allows the user to stop the composite cure reaction at any point in the cure cycle, and thus can halt processing on all or part of the laminate and either return to it at a later to complete cure or to co-cure, join and bond other composites to it to create larger parts.
The use of fluid to control temperature, as opposed to the gas generally used within methods such as autoclave and oven curing equates to lower energy consumption, faster cycle times and extremely accurate part temperature control.
Studies have demonstrated that the process may significantly lower overall capital costs and labor costs. Cycle times are one of the most significant differences between Quickstep and autoclave processing. The process allows for more repeatability in cure cycles, and rapid heating allows for improved interlaminar properties and improved surface quality.
Studies have also shown benefits in improved productivity levels due to lower development costs, the ability to manufacture large scale volumes, lower tooling costs, and no waiting on parts to begin cures. The use of fluid heating molding processes allows small and medium composite parts to be rapidly manufactured and cured to aerospace standards without an autoclave. It also allows the marine and automotive industries to manufacture advanced composites and traditional fiberglass compositesto standards only previously seen within the aerospace industry.
Microwave Curing
Prepreg compression molding
E-Beam Curing
References
(1) http://www.quickstep.com.au/what-is-quickstep
(2) http://www.ambercomposites.com/prepreg/out-of-autoclave-prepreg/
(4) http://www.compositesworld.com/articles/autoclave-quality-outside-the-autoclave
(5) http://www.quickstep.com.au/files/document/21_Feasibility_Study_Quickstep_SAMPE2005.pdf
(6) http://www.compositesworld.com/articles/improving-autoclave-performance
(7) http://www.quickstep.com.au/files/document/165_Out_of_Autoclave_Processing_via_Quickstep.pdf
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