Variable Cutting Angles

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Variable Cutting Angles

This week’s article will be about a very specialized functionality for AFP processes that is used to reduce material waste, aka “offcut”. Instead of cutting the material tapes into rectangular pieces, an angled cut is realized by rotatable cutting units. Depending on the ply angle and underlying contour, an algorithm determines the shape of the tape in such a way that they cover parts of the laminate contour more effectively, meaning that the offcut of each respective tape is minimized. There is however one constraint to this approach. Since the material is fed from a continuous supply of the spool, cutting the material at a certain angle leaves this cut to define the angle at the end of one tape and the start of the next tape. This means that a pair of tapes must exist, which share said angle. See this example:

Figure 1

Figure 1: Top row: 2 perpendicular cutting angles lead to 3 straight tapes. Bottom row: 2 skewed angles leading to 3 skewed tapes. Note that the center tape shares a common angle with the adjacent tapes at the cut location. 

There are constellations where this is the optimal outcome – when two tapes with this angle are the optimal solution for minimizing the offcut. This is often the case for 45° plies. See the following two examples:

L Shape

© SWMS Systemtechnik Ingenieurgesellschaft mbH  

Figure 2: An optimized 45° coverage for the underlying “L”-Shape. Instead of using rectangular tapes, tapes are cut in such a way that the edge of the underlying contour is matched by the edge of the tape. When ordered correctly, this leads to an overall improvement of waste, cost, and the CO2-equivalent derived from the process.  

Grosse Schwalbe mit Nummern CAWCAW

© SWMS Systemtechnik Ingenieurgesellschaft mbH 

Figure 3: Another benefit of pick and place processes is that the order of the tapes can be altered as needed. In this example, the algorithm determined that 5 tapes need to be of narrow width (yellow). The tapes numbered from 1 to 5 can then be ordered in such a way that they can be cut from the continuous material stream of the spool. Note that tape #1 starts with a perpendicular angle, just as tape #5 ends with it. Of all possibilities, this order produces the least amount of offcut. Also note that the end of tape #4 and the start of tape #5 cannot be different, although the underlying contour would allow for it. 

Why not just cut the material as needed? Why the special ordering and matching of the cutting angles?

Due to differing end- and starting angles, little bits of offcut would need to be cut off every time. This is rarely worth the extra time and material. It is more efficient to have some offcut from two subpar tapes than to waste the costly time of the machine by cutting these small bits away, as this would halt the feeding and slow down the overall process.

Figure 4

Figure 4: An extra cut needs to be made to create a rectangular tape after a skewed one. The black triangle is 100% offcut. This behavior is not covered by the algorithm since the extra duration and offcut are generally higher in comparison to only having higher offcuts due to non-optimal tapes. 

Utilizing the aforementioned functionalities to automatically determine the material widths and cutting angles based on certain optimization parameters (cost, duration, CO2-equivalent) provides the OptiTape-toolchain with a very powerful set of functionalities and allows for an extremely efficient layup process using NMB’s tape laying machine.

The project "OptiTape - Development of a machine-independent software for mapping a virtual process chain for the economical, resource-saving and mechanically optimal production of preforms based on unidirectional thermoplastic tapes" (funding code ZF4064612 PO8) was funded by the German Federal Ministry for Economic Affairs and Energy (BMWi).

In this article, we provided insight on how variable cutting angles can be realized and how they can alleviate pick and place processes to perform even better. Next week’s article will be about the digital twin: what it looks like and how it can be used to optimize the manufacturing of a laminate. 

If your interest has been sparked, you can get more information from the websites of NMB, SWMS, and REHAU.

Until then, stay safe and stay tuned.

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