The manufacturing of high-quality structural composite components for aerospace applications has historically involved autoclave curing, a time-consuming, high-labor, and capital-intensive manufacturing process.

As the aerospace and defense industries look to lower costs, composite parts manufacturers are searching for low cost alternatives to traditional autoclave molding processes that can produce the same mechanical performance that autoclave curing provides. In the meantime, these fabricators are also seeking to improve manufacturability and eliminate most secondary processes, while also molding parts that have increased complexity.

Both commercial and academic research and development have been focusing on a variety of liquid molding or low-pressure consolidation processes to meet this need for lower-cost "out-of-autoclave" composite molding. Examples of processing alternatives that can achieve aerospace grade results without resort to autoclave cure include vacuum assisted resin transfer molding (VARTM) techniques, resin transfer molding (RTM) processes and novel semi-preg or prepreg systems.

One process in which there is growing interest is the Quickstep process developed by Quickstep Technologies Pty Ltd., an Australian company that holds patents on a suite of processes for the inexpensive manufacture of advanced composite components.

The Quickstep proprietary manufacturing process is a range of technologies originally developed and patented by Australian company Quickstep Technologies Pty Ltd., with the assistance of CSIRO, an Australian public sector R&D organization, which can be used in out-of-autoclave fabrication of composite components from advanced composite materials.

The balanced pressure, heated mold process promises reduced cure cycle times and product weight, as well as increased strength and improved appearance. Products produced via Quickstep processes have superior properties to products fabricated by conventional atmospheric cure techniques and have properties generally equal to or better than high pressure autoclave techniques.

Quickstep process cycle times are typically 30-60 minutes for most resin systems, a significant time saving over the 3-8 hours required in autoclave curing processes, while achieving aerospace-grade void contents of less than 2 percent.

Significant scrap rate reduction can also be expected that is otherwise incurred from interrupted cure cycles, as well as reduced capital, tooling, and lower operating costs with energy usage 70 to 90 percent less than for the equivalent autoclave process. The technology allows large composite parts to be fabricated to aerospace standards together with the added flexibility to co-cure or meld components to produce more structurally integrated complex parts.

Quickstep
Out-of-autoclave composite manufacturing mold

The Quickstep process uses a lightweight "clamshell"-like mold, equipped with one or more flexible silicone bladders on each mold side that rapidly applies heat to the enclosed uncured laminate stack. The flexible bladders, which permit the laminate to be compressed without subjecting the mold to distortion or stress, have the ability to be rapidly flooded with heat transfer fluid (HTF), so that the mold surface can be heated or cooled far more quickly than is possible with liquid piped inside solid tooling.

Three separate tanks are used to contain glycol HTF maintained at three temperatures from cold (room temperature) to hot (up to 400 F). Vibrating HTF applied at constant low pressure (1-4 psi) against the tool, together with vacuum in the tooling itself, remove air as the part is compacted and cured. Three molding cells (QS1, QS5, QS20) are available that can cure surfaces of one to 20 square meters.

The process can accommodate thermoset or thermoplastic prepregs, as well as wet resin/dry fiber composite systems. Though best suited to parts having moderate curvature such as airfoil shaped components, more complex bladders can be used to mold deeper draw parts. The largest cell, which can be moved by forklift, fits into a 40-foot container and can be installed in one day.

Quickstep
Out-of-autoclave process basics

Building on their previous collaboration, Quickstep and the Victorian Centre for Advanced Materials Manufacturing (VCAMM) have had major R&D programs that focus on composites for aerospace applications, examining particularly how Quickstep curing technique produces composites with apparent improvements in thermal, adhesion performance and other properties thought to result from a better cure at the micro and molecular levels.

Elsewhere, the National Composites Center (NCC), in collaboration with the University of Dayton Research Institute (UDRI), Ashland Performance Materials, Owens Corning, Ohio State University and WebCore Technologies Inc., have studied the Quickstep process using materials enhanced with nanoparticles with the expectation of improving the performance of components produced for the aerospace and automotive industries.

Weight reduction in aerospace and automotive market sectors is a key factor in the rapidly expanding use of lightweight adhesives to replace or complement traditional fastening methods such as welding, and the employment of bolts or rivets in industrial applications.

An additional advantage of adhesives use in these applications is that the number of potential stress concentration sites caused by the presence of rivets is also reduced. However, adhesives have well known limitations such as susceptibility to peel forces and loss of strength under high temperature and humidity conditions.

Melding, a combination of melting and welding, that joins two composites without the use of fasteners or secondary adhesives, is said to overcome the limitations of adhesives by maintaining a chemically active surface that can react to form a bonded joint. Surfaces that are left partially cured allow chemical cross-linking to occur across the joined surfaces such that a structure is formed without the inherent failure problems associated with secondary adhesive bonds.

Melding, like welding, allows the surface of one part to be melted into the surface of an adjacent part, destroying the prior existing surfaces and creating an integrated whole. However, unlike welding, this process does not form a thin weld line which concentrates the forces, but rather occurs over a large area of a part, thereby avoiding stress concentrations.

The Quickstep "Melding Process" takes advantage of Quickstep's unique ability to stop the composite cure reaction at any point in the cure cycle. With Quickstep melding, it is possible to halt, and then resume the curing cycle to co-cure, or bond one composite to another to form larger more complex parts that are free of the potential weakness of secondary bonds.

This melding requires no fasteners, adhesives, or stacking tolerances and makes it possible for example to construct in the mold, a wing from the outside in with exact aerodynamic tolerances across the wing. Quickstep's Melding works by precisely controlling the temperature at the join site. The cure reaction can be stopped at any point in the cure cycle by returning the hot HTF to the tank and using cooling HTF to quench the reaction.

As many as seven or eight bladders per tool half have been used in the Quickstep melding process to create different temperature zones. By employing hot and cold bladders at the same time, cured and uncured or partially cured areas can be left in the same part. Uncured or partially cured regions to be bonded can then be assembled in a desired more complex arrangement and the cure is completed, to form a seamless joint.

Quickstep
Co-cured spar joint

Melding is one of the most important features of Quickstep technology. With this process, multiple composite components can be effectively ‘welded’ together without the need to use adhesives, bolts or rivets. There is no physical difference or separating surface between the two joined parts and the resultant structure is homogeneous with the surrounding composite.

The process provides the manufacturer with increased design flexibility and reduced processing costs. Melding has attracted considerable international interest from a number of companies, including some of the world’s leading aircraft and aerospace companies. A patent exists covering the "melding" aspect of the Quickstep Process for composites manufacture by the Australian Patent Office. The equivalent patent was also issued in China and corresponding patents exist in Europe, the United States and Japan.