If you look at the previous article about staggering plies, you will notice that tows often start and end outside the given boundary. Although this may be considered a waste of material, it is neither good nor bad - it depends on how the laminate will be processed and what kind of boundary was used. Oftentimes, there are multiple boundaries – an Engineering Edge Of Part (EEOP) and a Manufacturing Edge Of Part (MEOP). The EEOP denotes the dimensions of the finished part, while the MEOP denotes the boundary to be used for manufacturing the laminate. There are also differences in manufacturing tolerances between these two regions: the EEOP is explicitly used to define the dimensions of the finished part, higher tolerances to gaps, angle deviations, and steering (curvature) apply when compared to the MEOP. After manufacturing, any extra material that was placed outside the desired boundary needs to be trimmed.
Some processes use ultrasonic cutting tools to efficiently trim excess CFRP material after layup. An oscillating blade is moved along the EEOP to quickly cut out a laminate, which has the same* contour (*as close as possible) as the desired part. The boundary needs to be fully covered (and therefore overlap the boundary) by material for this process to work. If there is too little material, the blade might not properly cut the material but instead, tear little pieces apart and delaminate parts of the laminate. This means we need to make sure that for this kind of process a full coverage of the boundary is guaranteed. The same principle holds for most press systems. Because the laminate will be pressed into another shape, it is beneficial when some excess material is available, so it is ensured the EEOP is fully covered afterwards.
We will now compare three different boundary coverage types: 0%, 50%, and 100%. You will quickly see which is best suited for the ultrasonic cutting process!
Figure 1: A tow that has been created using the 0% strategy will never reach outside the boundary. This means that the start and the end of the tow are inside the boundary. This also counts for holes in geometries, which will never be covered.
Figure 2: A tow that has been created using the 50% strategy will partly overlap the boundary. The centerline of the tow is intersected with the boundary. This intersection marks the start and the end of the tow.
Figure 3: A tow that has been created using the 100% strategy will reach completely outside of the boundary. This means, that the start and the end of the tow are outside the boundary. The edges of holes in the geometry will also completely be covered. This also goes for holes on the inside of the geometry.
Depending on the kind of part that is being programmed, it might also make sense to use the boundary as a delimiter, meaning that no tow is supposed to reach outside the boundary. This might be the case for tightly nested parts or if certain holes in a boundary should not be covered. You might then use the 0% boundary coverage strategy.
In our experience, the default case of the boundary coverage parameter is 100%. However, when manufacturing CFRP laminates, the following processes need to be kept in mind, as hardly any of the processes in this complex field of manufacturing can be viewed completely independently of the others.
We hope that this article helped you learn when and why to use different boundary overlap strategies. The next article will be about how the minimal tow length inevitably leads to excess material being placed and how this excess material may be minimized.
Until then, stay safe and stay tuned.