How to use Automated Ply Placement to make tailored blank laminates

The key question that we ask ourselves at Airborne is: how can we convert a composite material into a tailored, optimized laminate as efficiently as possible? There are many technologies out there to do this, but there is one that is very simple conceptually, yet not used very much so far. And that is because in practice it’s a bit more complicated (well, isn’t it always?). This concept: Automated Ply Placement.

Let’s first go into what kind of laminates we want to make. To make best use of the expensive composite materials, you want to only use it where needed. This means: thickness variations, to reduce thickness in area’s where the stresses are lower. It also means an optimized shape, for example with cut-outs or free-form edges. You may want to combine materials, for example cheaper glass fibre composite with the higher performance but more expensive carbon fibre. Or have a combination of uni-directional and fabric materials.

Many composite materials come in the format of a roll of material. But there is also sheet material, for example cores to make a sandwich, or pre-consolidated sub-laminates, or recycled materials that come in patches. So ideally you want a manufacturing system that can handle all of these materials.

Available technologies versus materials

Most of the existing technologies to convert composite materials into laminates such as AFP, ATL, tape placement or continuous laminating process are based on tape. These tapes need to be slit from the wide mother rolls which requires an extra processing step and creates extra cost. Also, only a small portion of composite materials is available in slit tape form. That limits the choice for the designer and can result in sub-optimal or costly designs. Tapes are mostly UD material, so fabrics, NCF, non-wovens etc are all not available or difficult to slit.

In essence, the flexibility to create all kinds of parts, which is a great advantage of AFP, comes at the limitation that only slit UD tape is possible. The material format is standardized to allow flexibility on part level.

With Automated Ply Placement, a different route is followed. All materials are possible and the flexibility comes from cutting the required shapes, on a traditional conveyor cutter. These are then used to create any shape of the laminate. It does imply that the Automated Ply Placement system needs to be able to handle many different shapes and that is where the challenge is. For one part, one could design a bespoke end-effector and programme the pick & place robot, but what if there are many 1000’s of different shapes?  

Automated Preforming

The Airborne concept to create tailored blank laminates is Automated Ply Placement. The materials are cut into the required ply shape on a conveyor cutter. The robot then picks the plies from the cutter and places them onto the welding table to build the laminate. During placement, the ply is fixated in position by welding in the case of dry fibre or thermoplastic composites. For prepreg, the welding is not needed but a intermediate step is added to remove the foil and backing paper, after which the naked, tacky prepreg will stick by itself.

 I already said the concept is simple. It is of course the same process that a human laminator would use so for many companies this is very familiar.

Enabling technologies

To make this concept work, there were some important technology items we needed to solve.

First of all, there are many, many different shapes to pick. In principle, the variety is endless. To allow this, we created software to allow for Automated Programming. The design and ply shapes are input, together with material data. The software then creates all the required machine code and process settings. And the cool thing is that this is done on-the-fly. So the operator can upload a new design, or provide a new ply shape to the robot, and the system will adapt. Very flexible! The challenge is in handling the material. Composite materials are flexible and the plies are picked by an end-effector with an array of grippers. This results in sagging of the material between and outside of the grippers. The algorithm takes this into account and determines the best way to pick a ply to result in minimum sagging. That is why even a rectangular ply might be picked up by the robot with a rotated end-effector, so that the suction cups are positioned in the corner of the ply. Also the amount of gripper force is determined for each individual gripper and made shape dependent.  What this Automated Programming technology also does is that customers don’t need to have the expertise of robot programming; the robot is embedded in the system and the Airborne software takes care of it.

The second challenge is ‘Picking from the nest’: how to robustly separate the ply from the rest of the material (the skeleton)?  Especially for sticky prepreg, this is not easy to achieve. Also there can be uncut fibres which would result in pulling the whole skeleton from the cutter bed.

To address this, we have developed the technology to be able to pick sticky prepreg. I can’t tell you all the secrets, but you can imagine that it has to do with how you exactly pull the prepreg ply from the skeleton. We also integrated sensors that can detect uncut fibres. Of course, it’s best to set up the cutter such that always all fibres are cut. But uncut fibres can happen, for example when the cutting blades are not replaced on time. The sensors will detect the uncut fibres and put the system on hold, so that an operator can intervene.

The third challenge is accuracy of placement. If plies are placed next to each other, the gap should be within tight tolerance. For local patches, it is important to have an exact position since it you don’t want to need to trim such local patches. To improve placement accuracy, we use on-the-fly calibration based on vision, or closed-loop control. With vision, the ply is scanned when on the end-effector, and this position is compared with the expected location. The robot movement is then adjusted such that the placement is accurate. Since we scan the ply, we can also use this for quality control, for example to check the cutting quality of the edge, or to find defects in the ply.

The fourth aspect is to address the nesting efficiency. Cutting ply shapes from a wide roll can lead to considerable waste. The solution is to nest, and ideally to create a nest from a large mix of parts. The challenge is then that you need to sort the plies for the right part (a kit), and to sequence the plies within the kit. Also, if the laminate is multi-material, the cut plies need to be stored until the all the materials have been cut. For this we developed Automated Kitting, which involves a novel buffer solution which is fast and with small footprint. The robot can access the buffer and will take care of the sorting and sequencing. After creating the tailored blank preform, it can be put back in the buffer. The buffer is also accessible from the other side (the outside of the cell) so that the preform can easily be offloaded, either manual or by another robot.

One- or two-step process

The process has two steps: first a flat net-shape laminate is made, and then this is formed into shape. This allows for fast and efficient laminate build up, and also a fast forming process of the full laminate.

One could argue of course whether it is not better to make the 3D shape in just 1 step?  Well, that depends!  It depends on the material and on the shape of the final product. If the shape is very complex, it might not be possible to deform the whole laminate without wrinkles or bridging. In that case, single ply forming might be better, to form the ply during direct placement into the 3D mould. That is also what one would do in a manual process.

But for many parts we see that fast lay-up of the tailored blank, followed by forming of the whole laminate in one go is more efficient. It is also more suitable for high-rate manufacturing since the steps are divided and can be done within the available takt time. In automotive this is clearly the way to go, and also in high-rate aerospace manufacturing it’s basically the only option.

Still, 3D pick and place can be interesting, for high tolerance parts or for processes that are qualified on ply-by-ply basis. If you want to convert a qualified manual prepreg lay-up process into an automated processes, a 3D pick and place technology is the closed method to mimics the current process. At Airborne we are also developing 3D pick & place, and there are several other solutions in the market (Sarto Robotics is one example). An advantage of such a process is that the forming is ply-by-ply which provides the opportunity for inspection of each ply. 

Advantages of Automated Ply Placement

There are several important advantages of using Automated Ply Placement:

  • The materials can be used as they are made, in wide mother rolls. This saves cost of slitting and gives the opportunity to use all available composite materials
  • The technology is suitable for all three composite material categories: dry fibre, thermoplastic or prepreg
  • Multi material laminates can be made, which combine different material formats such as UD, fabric, NCF, non-woven, sheet materials, film, cores, metal layers etc
  • 100% net shape laminates can be made, with free-form edges, local patches or cut-outs. No trimming afterwards is needed
  • Since large plies can be handled per robot cycle, the output is high. Our standard end-effector is for example 1.2 x 2.5 m.  Of course, the output is dependent on ply size.
  • The laminates can easily be offloaded to the next process step by the robot

Want to know more?

 If you are interested and want to know more, feel free to reach out! We can look into your specific need, develop a business case and work with you to create the right setup. 

This blog was also published on Marcus Kremers’ LinkedIn page.

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Marcus Kremers

CTO

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