JEC COMPOSITES MAGAZINE - Issue #142 - September/October 2021 - 55

Holistic optimization of the high-speed tape lay-up process using a digital twin
Fig. 3: Cross-section of a PP-GF laminate with a 3-mm original gap (dashed box) after consolidation; the highlighted area of influence shows collapsed layers
according to his needs. The optimization can focus on minimizing the
process costs, offcut or CO2
footprint.
One step to a close-contoured preform, and thus to reduced offcut, is
to implement an angle cutting system. To avoid unnecessary offcut, the
cutting angles are selected considering that the subsequent strip exhibits
the same angle at its beginning as the previous strip at its end. Another
approach to component optimization is to determine optimum material
widths. The width is calculated based on the features of the TLM used,
e.g. the number and specifications of tape role lines. This information
must be transferred to the CAESA software and can be adjusted easily.
Once these are known, specific widths can be determined according to
the desired laminate structure. To ensure that the algorithm results are
always transparent to the user, a digital twin of the system, which enables
a simulative optimization of the laminate, is necessary. The process can
be simulated and tested before actually producing the component.
The possibility of using different material widths in one stacking presented
the project team with complex problems. The analysis of the part
geometries is the first essential step to achieve the best starting point for
planning the lay-up. The resulting parameters are the basis for further
optimization. The interaction of the different input values and the many
possible combinations of tape and gap widths, as well as a specific optimization
goal, required new, intelligent algorithms. Therefore, the different
optimization targets for cost reduction, off-cut reduction, lay-up time
and CO2
minimization were considered separately. For each target, the
main influencing factors were determined and integrated into the calculation
algorithms. Since production time is another important aspect of
the manufacturing process, all the influencing parameters are considered
by the software. This also includes welding energy and time, depending
on the selected tape material. The focus of the optimization strategy influencing
the lay-up can be selected by the user. When reducing production
time, wider strips are used. On the other hand, the narrower strips
are preferred if the offcut is minimized. Cost- and CO2
-optimized strategies
are a combination of both. Machine utilization, hourly rate and CO2
footprint of the tapes used, as well as energy consumption and emissions
of the processing equipment, are considered. The user can choose
the optimization strategy depending on what is most important to the
product. But there is further potential in the 2D lay-up procedure. The
question: " How can it be ensured that a change in the process parameters
does not have a negative effect on the mechanical properties? " must
be raised. The answer is given by material analyses regarding the lay-up
process and their consideration by the CAESA software.
A detailed look at properties
The influence of important process parameters and their effects on the
laminate properties are part of the holistic optimization approach. To
enable CAESA to conduct these optimizations, the unidirectional tape
used and its processing conditions had to be examined. Two of the investigated
factors presented as examples here are the gaps between tape
strips and the ultrasonic (US) welding spots. The gaps result from tape
coverage tolerances and can be exploited to save individual strips by optimizing
the lay-up. The US welding spots are required to fix the UDreinforced
strips together for later handling of the preform.
The inter-tape gaps were explored using PP-GF and PA6-CF unidirectional
laminates, each with a 45% fibre volume content (FVC). 1 to
6-mm gaps were introduced between two strips (Figure 3 shows a 3-mm
gap). The structural analysis after end consolidation in the FORCE process
revealed that all the gaps could be filled entirely with matrix and fibres
during consolidation. Pores, however, could not be detected in the
cross section. Yet, there is an influenced area left, where fibre deflections,
locally-reduced fibre volume fractions and collapsed layers are present
(see Figure 3). For PP-GF, the area of influence could be reduced to a
minimum of 32% of the original gap width and for PA6-CF, a maximum
reduction to 62% of the original gap width could be achieved. Based on
the results, an acceptable gap width between tapes can be set depending
on the user's requirements. The number of strips used, and therefore material
and laying time, can be minimized by the software based on these
results.
The TLM considered in this work uses ultrasonic welding for fixing
the individual tapes to one another. Different weld energy levels for a
fixed ultrasonic generator power output influence the welding time and
therefore the processing time. The analysis and optimization of the ultrasonic
welding process revealed that the tensile strength of end-consolidated
unidirectional PP-GF laminates reduces significantly with
increasing weld energy (by increasing welding time and/or amplitude),
up to -72% in the welding area (see Figure 4). The microscopical analysis
of the fibre-matrix interface revealed that increased weld energies
deteriorate the fibre matrix bond in the laminate, restricting the introduction
and transfer of loads within the composite. The non-polar nature
of the PP matrix supports this behaviour. The PA6-CF-laminates,
on the other hand, did not show such correlations of weld energy and
tensile strength. Equally, the fibre-matrix bond was not affected by the
weld energy. Recommended weld energies (PP-GF: 35-70 J; PA6-CF:
20-60 J) for the two materials were derived from these results, guaranN°142
September - October 2021 / jec composites magazine
55

JEC COMPOSITES MAGAZINE - Issue #142 - September/October 2021

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