Problem taped

GKN has developed an automated system it claims will produce carbon fibre aerospace components up to 40 times faster than existing methods.

The automated tape lay-up (ATL) machinery developed by GKN can lay composite tapes 150mm or 300mm wide at the rate of about 18.2kg/hour. This compares with hand lay-up rates of 0.45kg to 1.45kg/hour.

'If carbon fibre wing spars were made by hand, the man hours required would be extremely high,' said John Cornforth, head of technology at GKN Aerospace. 'GKN felt that this product would only be commercially successful if an automated method was developed.'

The Airbus Military A400M primary wing spar — the largest all-composite wing spar produced — is the first to be made using the technology. Future parts could include skin panels or flaps.

The process begins with a bobbin or roll of material that sits on a multi- axis robotic head, which moves to the required position and the compaction roller touches the part. As the head moves along the surface the material is fed to the compaction roller, which presses the material firmly on the surface.

When the head has travelled the distance required, an ultrasonic knife cuts the carbon tape to the required shape. The excess material is then automatically put on a scrap pile. This process is repeated hundreds of times until a full part it created.

'The process is rather like wallpapering, covering a surface with strips of material in different orientations,' said Cornforth.

The ATL then lays the material on to a flat tool. A separate, drape-forming process is used to form the part into the desired shape.

While ATL is excellent in the production of large, flat structures, it can cause composite fibre buckling in more highly shaped components, such as nacelle cowlings, doors and engine ducts.

So GKN is also investigating a complementary technique — automated fibre placement (AFP) — to speed up the production of more complex shapes.

Cornforth explained that a fibre placement machine is similar to an ATL except that instead of using one very wide band of material it uses multiple thinner bands. The reason for using small tapes is that they have more steer before buckling, which means they can be driven into a curve.

'For example, if you tried to lay a large piece of paper around a football The paper would start to fold and tear,' said Cornforth. 'But if very small strips of paper were used no buckling or tearing would occur.'  

By using small tapes, very complex shapes can be manufactured directly without hot drape-forming. Due to the small tape width in AFP, no waste material is created, so the ultrasonic knife operation is not required.

For years automation has been used with caution in aerospace production, with concerns that robots are not as accurate as manual work.

Yet GKN has claimed parts made with ATL machinery are even more accurate than those produced by hand. The 14m wing spars the company makes now are accurate to within 0.5mm, compared with about 300mm for the corresponding metal components they are replacing.

The company said automated production is also increasing material quality. During production bubbles can form inside composites and when the materials harden it leaves a void, or imperfection, which can cause weak spots.

While for most products four per cent of void is acceptable, GKN claims to be achieving up to 1.5 per cent.

With these increases in quality and consistency being achieved automatically, GKN said there could be a time when it will be unnecessary to inspect every component post-production.

Cornforth said AFP and ATL machines are effectively ready to go. He said manufacturing tool provider MAG Cincinnati and machine tool manufacturer MTorres are both making machines to be delivered for other applications within the aerospace sector.