When a CFRP part is integrated into an existing assembly, the CAD definition of the laminate may contain holes for screws or other fasteners. The laminate, however, should not contain any holes as these would lessen the structural strength of the part. Inserting fasteners later will almost always be a better choice than leaving holes in the laminate, as such holes are then not covered by fibers in the process at all. In such cases, CAM software is necessary to provide the programmer with the possibility of controlling how large a gap must be to be left out when placing prepreg material.
Two more factors directly relate to the minimum gap length; the first being manufacturing time. The manufacturing time decreases when small gaps can just be covered by material since the end-effector does not need to slow down to cut, therefore decreasing the overall duration of the process. Secondly, some end-effector systems also require a minimum gap length since they cannot possibly cut and feed very short material lengths reliably.
The following figure illustrates the functionality of the parameter as it is implemented in the CAESA® Composites TapeStation:
Figure 1: The blue circles denote a gap of 25mm in length. The gray rectangles represent CFRP prepreg material. In 1) the gap is larger than the minimum gap length and therefore will not be covered with material. In 2) The gap is shorter than the minimum gap length and therefore covered.
Figure 2: A minimum gap length of 0mm: All gaps shorter than 0mm are covered. Therefore, all gaps are in the ply are left as is.
Figure 3: A minimum gap length of 75mm: All gaps shorter than 75mm are covered with material. All circular holes are filled with material, while the triangular hole in the bottom center is not. Note, that the contour was scaled up to 500% - usually values of 25mm and less apply.
While the minimum gap length is a rather simple parameter, its usefulness is not to be underestimated. In combination with the minimum tow length and the boundary overlap, flat plies can be programmed very effectively and holistically. Next up is an article about how users can individually adapt all these parameters and achieve the optimal coverage for a ply.
Until then, stay safe and stay tuned.