The automation revolution

John Cornforth of GKN Aerospace explains where progress is being made.
To date, material deposition has been and continues to be the primary target for composite process automation but to expand into the future many other areas must be addressed. These include the successful automated manufacture of small, highly complex structures, automated non-destructive examination (NDE), robotic manipulation and assembly, effective tooling development and mould tool cleaning, out of autoclave curing, drilling and filling. Analysis of the labour hours required to manufacture a composite component carried out at GKN reveals that laying up consumes the largest proportion of time at 42% of the total, so efforts have been focused in this area. On average, hand lay-up of composites for aerospace applications typically achieves deposition rates of 0.5-1kg/hour per person. Current automated deposition equipment is capable of achieving 20kg/hour for automated tape lay-up (ATL) and up to 5kg/hour for automated fibre placement (AFP), although both values are highly dependent upon factors such as ply-book construction, fibre areal weight, ply orientation and component complexity. Other industries have achieved faster deposition rates, however, radically different processes are used for different quality standards and acceptable defect sizes. Smart from the start To accommodate a change from hand to automated processing, it is necessary to develop and mature a design rule set so components are not designed and manufactured without increasing their ‘engineered weight' due to the limitations of machinery, software or minimum ply size. Within the collaborative Next Generation Composite Wing programme, initial studies are concentrated on increasing deposition rates while tweaking the key variables of unidirectional (UD) tape and tool surface temperature, roller compaction pressure, UD tape deposition speed, UD tape tension, tackiness between tape/backing and tape/tool, environmental conditions and age of material. These trials will examine how well ATL copes with plies deposited at high speed over complex ramp geometries and what adverse features could be introduced into the laminate such as wrinkles and bridges. Research has also been directed at AFP, which has seen huge growth over the last five years with most applications being on primary structures. GKN, with its strategic partners, has manufactured bespoke equipment to address the difficulties that arise from smaller components with complex geometries. For example, the shortest length of tape needs to be much shorter than that usually accepted, which necessitates a more compact deposition head design, a high degree of articulation and a highly conformal compaction roller to avoid collisions. With complex shapes, it can be advantageous to use a mandrel mounted tool, which can present the work surface in the best orientation for the head. Rethreading a dropped tape is very time consuming so a simple material delivery system is essential in order to achieve high machine reliability and acceptable deposition rates. The ability to accelerate and decelerate quickly, adding and cutting plies at full speed is crucial to achieving satisfactory deposition rates. Today, top end rates are around 5kg/hour but much progress is being made and this figure will increase substantially in the next few years. Flexible friends In some areas there has been a move away from purpose built machinery in favour of off the shelf robots with specific end effectors, although the production rates within aerospace have not justified such investment yet. With suitably designed tape deposition heads, conventional robots can be used to manufacture components without the cost and complexity of a bespoke AFP system. Applications can include flexible systems where two or more robots are rail mounted and equipped to undertake both ATL and AFP operations. These heads can also be configured to work sequentially on different aspects of a task. This would be highly effective in the manufacture of components with both simple and complex geometries over their surface, simultaneously utilising both ATL and AFP respectively. This system could enable operations to trim the periphery and/or include some dimensional inspection within the same cell. For processes requiring autoclave curing, tooling is another area for improvement. Currently, Invar steel mould tools are predominantly used for large structures. Due to their mass and construction, these tools govern the heating and cooling periods of cure cycles. Complex backing structures can also restrict airflow and create problematic local hot and cold spots. Thus, the goal is to achieve a design that allows the tool to heat and cool at a uniform rate closely matching the component. Carbon tooling has such potential and can thus meet higher production rates. Direct surface machining and surface coatings may in the future provide the dimensional accuracy and durability associated with Invar. An additional challenge posed by automation is that larger ‘monolithic' aircraft structures are often heavy and unstable to handle if an uncured component needs to be transferred from a mandrel to a curing tool. One answer is the use of self heated tooling systems where the component does not have to be removed until after it has been cured. This is particularly applicable to tooling which is designed using out of autoclave materials and vacuum-only pressure, opening the door to cellular manufacture, thus eliminating autoclave capital costs and improving energy efficiency. Self heated composite mould tools may also prove useful for processes where controlling tool temperatures may assist in material tack and compaction. Available now As an example of these process improvements in action, in manufacturing a 10.5m spar for the ALCAS FP7 programme at GKN, automated deposition was able to lay a component in 15% of the time for hand lay-up. The spar weighed 180kg as a bare component and is manufactured from 268g/m2 standard modulus fibre pre-impregnated with MTM44-1 resin. Lay-up time was optimised by laying it as a flat developed stack onto a horizontal table with all reinforcements present. Following lay-up it was formed into its final C-section shape using hot drape forming, thereby automating approximately 47% of the overall manufacturing time. Finally it was cured on a carbon composite tool with embedded heating elements. The level of automation applied, in the form of ATL and double diaphragm forming resulted in a much higher repeatability in the final part than can be achieved by hand lay-up. Composite materials have already had a profound effect on aircraft development and through life ownership but huge opportunities still abound as a conservative, careful industry undergoes this manufacturing revolution. These techniques will be very much necessary however if we are to succeed in delivering the next generation of aircraft at reduced cost, improved performance and with greater consideration for the environment. www.gknaerospace.com