In the last article, we gave a short summary of the AFP process and how laminates are built. In this article, we will go into detail about the tools that are required to manufacture CFRP laminates from prepreg tows. We will focus on the structure of AFP layup heads and for illustration purposes we will use the fiber placement head from our colleagues at the IFW in Stade.
Note that the term “end-effector” usually refers to the last element in a kinematic chain, such as a robot system. Since this fiber placement head is supposed to be attached to a robot system, the terms “end-effector”, “tool” and “head” can be used synonymously.
Figure 1: A screenshot from the CAESA Composites TapeStation during the simulation of the layup process. The IFWs layup head is attached to a KUKA robot.
The IFW’s fiber placement head can process 4 ¼’’ (6.35mm) tows in parallel. The material storage is integrated into the layup head: 4 times 150m of material is stored on bobbins (spools) that are positioned on the top of the head. From these bobbins, the tows are guided towards an elastic compaction roller. The compaction roller presses the four parallel tows against the layup surface. When the end-effector then moves along the layup surface, a dedicated feeding unit unwinds the correct amount of material from the spools. The feeding unit reduces stress on the tow when it is laid on the mold. Without the feeding unit, the pressure applied between roller and surface and the rotation of the compaction roller would put a lot of tension onto the tows.
Since the material itself needs to be constantly kept cold to prevent the thermoset or thermoplastic matrix material from sticking, the surface it is placed upon needs to be heated. An infrared heating unit is used, which heats the area in front of the compaction roller. In this area there is either the mold surface or previously placed CFRP layers. When the surface is warm enough, the matrix of the applied material heats up on contact with the preheated layup surface and becomes sticky enough to stay in position.
In this video you can see the IFW layup head in action. The material spools are inside the cover. The heating unit starts heating the surface before material is placed. The feeding unit has fed material up to the compaction roller. Once the roller contacts the surface, the material will be fed from the feeding unit. Moving the end-effector while maintaining contact to the layup surface “pulls” the material from the feeding unit onto the surface.
Since all tows may start and end at different locations, the end-effector needs to be able to cut every tow individually. This allows for full flexibility in positioning and length of the tows. Just as the feeding unit enables separate feeding of all 4 tows, a cutting unit enables separate cutting. However, because the cutting unit cannot physically be placed in the same location as the compaction roller, it needs to be placed somewhere between the feeding unit and the point where the compaction roller makes contact to the surface. (If there is no feeding unit, the cutting unit needs to be placed between the bobbins and the contact point.) The closer it is placed toward the compaction roller, the better, as the amount of waste and the duration of the manufacturing process decreases, and the size of the tooling may be reduced. The length between the cutting unit and the compaction roller is called “minimum tow length” since no tow can physically be shorter. The IFWs layup head for instance has a short minimum tow length of 68mm.
Figure 2: A detailed description of the layup heads modules. Image taken from the article „Automated Fiber Placement Head for Manufacturing of Innovative Aerospace Stiffening Structures“ published by the IFW. [ScienceDirect]
Now that we have covered the fundamentals of the AFP process and the minimal requirements to manufacture CFRP laminates, we will go into detail about specific challenges that arise and how to prevent them.
Until then, stay safe and stay tuned.
Thanks to our colleagues at the Collaborative Research Department for Composite Technologies at CFK Nord for supporting us with their media content. Feel free to contact Dr.-Ing. Carsten Schmidt for further information or visit the LinkedIn page.