Achieving minimum tow length

In this series of blog posts, we present the manufacturing process of CFRP components. We split the many, closely linked subjects into separate posts that can be easily understood. If you are new to this series, we recommend starting here: AFP Endeffectors. In this blog post about end effectors, we give you an overview of the structure of AFP layup heads, how they are constructed, and which functions they need to fulfill. One very important factor of AFP layup heads is that the tows are limited in length by the minimum tow length, and, of course, by the amount of material left on the spool.

In this minimalistic example, you can see how this minimum tow length affects the coverage of boundaries.  

Figure 1: In this visualization, the square boundary is fully covered by tapes. More precisely: This is the optimal 45° layout for this geometry, using the 100% boundary coverage setting and ¼’’ prepreg tapes. No other coverage layout uses less material to cover the boundary.

Due to geometrical constraints of AFP layup heads, the tows laid need to be of a certain length – the minimum tow length. For almost all laminates this leads to excess material being placed where short parts of the boundary need to be covered. This might be considered one of the disadvantages of AFP processes but must be viewed in comparison to the many benefits of an automatable and efficient layup process. Alternative manufacturing processes might utilize pick-and-place technology to further reduce the waste produced. These often come with different disadvantages, such as limited part size and increased manufacturing durations.

Let us take a look at how the coverage layout changes when the minimum tow length is taken into consideration: 

Figure 2: In this visualization, the tapes have been extended to match the minimum tow length of the end-effector. In this case, the minimum tow length is 85mm. To fully cover the top left and bottom right corners, large amounts of excess material need to be placed. They are then trimmed afterward and therefore considered waste.

In this example, given the parameters chosen, the excess material cannot be reduced any further. In some scenarios, the direction in which the tows are extended might matter significantly. In the following illustrations, we show you some possible optimizations.

Figure 3: This figure shows the default case: tows are extended symmetrically to match the minimum tow length. The tows between the hole and the outside boundary need to be extended. The difference between that distance and the minimum tow length is split and added to either side of the tow.

Figure 4: This figure shows the tows when they are extended to match the minimum tow length by extending them towards their starting point on the right.

Figure 5: This figure shows the tows when they are extended to match the minimum tow length by extending them towards their endpoint on the left.

Another very useful optimization is an extension towards the center. If you compare the figures above, you will easily be able to imagine how this works. (This exercise is trivial and therefore left to the reader.) In some scenarios, this might make one additional step of trimming the excess material around the outside boundary obsolete. Conversely, always extending to the outside of the boundary might make trimming of the inside holes obsolete. This of course depends on the follow-up processes.

In this article, we showcased why AFP processes often create excess material by default. We also highlighted how the excess material can be positioned most effectively to reduce its downsides. The next article will be about the minimal gap length, which is a parameter that controls how big the gaps in the laminate are allowed to be.

Until then, stay safe and stay tuned.