In this article, we want to outline the AFP process and showcase some of its advantages and limitations.
Narrow stripes of prepreg material (tows) need to be placed in such a way that a certain surface is covered by multiple layers of CFRP material. There should be no areas left uncovered. The result of the layup is a layer structure called laminate. In its simplest form, the laminate is made up of flat layers that are placed on top of each other. Every layer of the laminate is called a ply, and every ply needs to be laid up step-by-step since the layup tools can only place a limited number of tows at a time. These steps are called courses. In order to lay a ply, the ply is split into many courses. A course is a group of tows, that the layup head can handle at the same time. These courses are then laid one after another until the ply has been finished. The next article will cover what these layup heads look like and how they work.
Laminates can be manufactured by placing the plies on top of the layup surface: flat laminates are stacked on a flat table and complex laminates are placed either on top of or inside of a layup mold. The geometry of this layup mold is derived from the part that will be manufactured. Common materials for molds are polyurethane foams, wood, and steel.
This simple laminate may be placed on any flat layup table. For example:
This curved laminate needs to be manufactured on a curved surface, as seen below:
Figure 2: The same stacking as in Figure 1 but here a curved surface is used as tooling. The distances between the layers have been exaggerated. Depending on the curvature of the tooling and the requirements of the part, the laminate can become very complex.
Figure 3: In this case, the mold is relatively simple, as the geometry only curves in one direction.
As soon as a layup mold is curved in more than 1 direction, like the mold above, geometrical problems arise. Think of trying to drape a piece of paper around a tennis ball. Where full size mats and twills would stretch, wrinkle, or tear, placing multiple tows in parallel can alleviate most of these problems. By disentangling the CFRP mats into many tows that can strategically be placed and modified, laminates can be manufactured even in complex curved molds.
How exactly these laminates need to be oriented and ordered is defined in the so-called design requirements. These are derived from the usage of the part and have been laid out by the design and engineering processes. They are often the result of FEM analyses. They define the different orientations of the layers, the boundaries that need to be covered, many specific details about how the tows must be placed relative to the others, and also how much they are allowed to differ in orientation and curvature. The requirements must be transferred to a CAM programming software to create the layup program of the machine that is to be used. CAM software like the CAESA® Composites TapeStation eases the programming of the laminate manufacturing process and simulates the layup process and check for any collisions.
To place multiple tows, specialized machinery is needed. In the next article, we will give an in-depth insight into the structure of these AFP end-effectors. Afterward, we will go into detail about how a laminate is structured and how layers can be configured.
Until then, stay safe and stay tuned.