Parameter study and development of a warp knitting yarn compensation unit as basis for the realisation of contour-accurate non-crimp fabrics: a step towards for highly material efficient non-crimp fabrics

In response to the increasing demands for high-performance fiber-reinforced composites in structural lightweight construction, this study investigates the limitations of multiaxial non-crimp fabrics (NCF) and their semi-finished products. The current manufacturing constraints of NCF, limited to a constant working width, lead to oversizing in semi-finished textile products and significant material waste throughout the value chain. This study explores the development of NCF with variable area weights and thread densities as a potential solution. The research described in this paper examines the effects of variable area weights and thread densities on textile behavior and warp knitting thread tension in the production of multiaxial NCF. The study focuses on varying key warp knitting parameters (stitch, knitting thread feed value, shape hole geometry), along with the measurement of the tensile force exerted on the warp knitting threads. Findings indicate a consistent increase in warp knitting thread tension in areas of reduced thread densities, unaffected by the fabric’s initial orientation. Higher initial yarn tension and increased yarn demand per stitch correlate with a greater tension increase in areas with lower thread density. This study proposes that refining stitching techniques and integrating adaptive yarn tension control modules could mitigate tension fluctuations and diminish fabric defects. These insights contribute to a better understanding of the material behavior of contour accurate NCF and their production. Coupled with the innovation of a warp knitting compensation unit, these findings mark a pivotal advancement toward producing contour accurate NCF in an inline and higly productive process technology, offering significant implications for the technical textile industry.


Introduction
The ever-increasing demands on structural components for lightweight applications, driven by ecological and political constraints, continuously challenge the fields of material science and engineering, especially in regard to sustainability, demand and automation [1,2].Material minimization remains a global task, affecting numerous industries [3].Particularly, textile-reinforced lightweight structures, such as fiber-reinforced composites (FRC), offer promising solutions in terms of their performance-to-mass ratio, playing a crucial role in lightweight structural engineering [4,5].
Focusing on the development of bio-inspired textiles addresses a critical research gap.By mimicking biological structures and principles, these technical textiles have the potential to revolutionize materials science, offering novel ways to enhance mechanical properties and efficiency in lightweight applications.The application of bio-inspired principles in textile design enables the creation of composite structures with optimized load-path alignment and enhanced material utilization, leading to a significant increase in performance while minimizing material use [6][7][8].
Especially multi-and biaxial non-crimp fabrics (NCF), are pivotal due to their highly oriented fiber layer in the textile manufacturing process, enabling high productivity and superior mechanical properties.Alternative methods such as multi-layer knitting or weaving cannot replicate the 2D geometric versatility necessary for the manufacturing of load-path-aligned structures and demonstrate lower productivity.2D yarn direct placement processes based on the Tailored Fiber Placement (TFP) method are primarily suitable for special components with a low number of layers, and for small to medium-sized component surfaces, requiring additional auxiliary materials such as a carrier material [9][10][11][12][13].
The integration of inline production technology facilitates the manufacturing of contour-accurate textiles directly within the production line, reducing the need for semi-preforming and enhancing automation.This method allows for precise positioning of reinforcement threads, tailored to component specifics, thereby improving material efficiency, and reducing waste.Moreover, this streamlined approach contributes to a more sustainable manufacturing process by optimizing material use and minimizing excess.Such advancements in production technology are essential for meeting the stringent demands of lightweight applications with increased efficiency and lower environmental impact.
Despite their potential, no industrially established processes exist that are both highly productive and allow for component-specific positioning of the reinforcement threads to minimize waste for contour accurate textiles.Conventionally, NCFs are produced as roll goods with homogeneous reinforcement thread positioning, leading to uniform reinforcement thread area weight across the material's breadth and length.Yet, for material efficiency, NCFs must have reinforcement thread lengths and densities adapted to component contours and loads to reduce waste and oversizing.Current technological approaches have begun adapting reinforcement thread lengths in the weft and warp direction to match component contours [14][15][16].
A key challenge with these methods is uniformly feeding warp knitting threads across the machine's full width, disregarding variations in component contours or thread densities.Due to these technological limitations, NCFs that exhibit variable densities of reinforcement threads can contain regions exclusively consisting of warp knitting threads, with an absence of reinforcement threads (figure 1).Consequently, the variable incorporation of reinforcement threads results in fluctuations in warp knitting thread tension across the working width.Surprisingly, investigations into warp knitting thread tension have shown that tension in areas without or with less reinforcement threads is higher than in areas with integrated threads, leading to structural distortions, warp knitting yarn breaking and challenges in handling during further processing into FRP components.
Addressing these challenges, this research conducts a parameter study, varying warp knitting and material parameters, alongside the development of a laboratory test stand to adjust for variations in warp knitting thread tension across the working width.This paper further analyzes the impact of the structure-fixing warp knitting technique on textile faults during the processing of NCF with variable thread density.Special attention is given to regions where the local thread density of the NCF is effectively zero, later referred to as 'shape holes.'The study systematically modulates specific warp knitting parameters, including stitch type and knitting thread feed value, to discern the intricate relationships and effects on the quality and integrity of the NCF.The objective is to provide scientifically robust and practically relevant findings for the production of contour-accurate NCFs with minimized waste.

Materials and manufacturing method
Basic materials and setting parameters This chapter outlines the materials, machine settings and the experimental design.Graphical representations of the stitch, non-crimp fabric, shape hole geometry, and the measurement setup for assessing warp knitting thread tensile force-the most critical quantifiable parameter-are detailed in figure 4 and table 2. The materials used in these tests are listed in table 1.The warp knitting thread employed was a textured filament yarn of polypropylene (PP T, sourced from KSO Textil GmbH, Germany), with a yarn count of 11 tex.For the weft and warp threads, glass fibre (3Bthe fibreglass company, Belgium) with a yarn count of 200 tex was utilized, wound on bobbins.
The textile test samples, as listed insee table 3, were produced using the Stitch Bonding Machine Malimo 14024 equipped with a multiaxial weft laying system (figure 2) provided by KARL MAYER Textilmaschinenfabrik GmbH (Chemnitz, Germany).
The parameters for both the machine and fabric are detailed in table 2 and table 3. Machine speed, measured in revolutions per minute of the main shaft, dictates the production rate of NCF and impacts the quality of the final product.Stitch length, defined by the distance covered in one revolution of the main shaft, affects the density of the stitch; a longer stitch length results in a looser stitch, whereas a shorter stitch length produces a tighter stitch.This variation in stitch length directly influences the mechanical properties and strength of the NCF.Fabric tension and synchronous ratio play crucial roles in regulatingfabric tension during production , thereby affecting fabric shrinkage after free cutting [18][19][20][21].

Design and integration of varying reinforcement yarn lengths in conventional multiaxial warp knitting machines
The developed test plan contains the parameter variations listed in table 3.These includes stitch types, reinforcing thread layers as well as contour geometries and contour spacing.Various test series investigated the impact of the parameters outlined in table 3 on the handling behaviour of textile samples and knitting thread tensile forces.The investigation focused on a specific area within the NCF structure termed a 'shape hole,' where the thread density is locally zero (refer to figure 3).The choice of the shape hole for determining the influence on warp knitting yarn tension is justified by the following aspects: • Maximum reduction in thread density: Inspecting the shape hole achieves the utmost reduction in thread density, allowing an in-depth analysis of its effects on NCF's mechanical properties and handling.• Significant changes at zero passage: Major variations in the measured variables (e.g.knitting thread tension), and consequential mechanical properties predominantly occur in the zero thread density area.Therefore, an investigation at the shape hole offers the possibility to analyse these changes and their effects on the material behaviour in detail.
• Unexplored territory: The scientific literature has not yet covered the study of materials featuring shape holes, presenting an opportunity for novel insights.
• Emphasis on waste reduction: The ultimate objective is to avoid waste completely.If textile starting materials (e.g.rovings or tapes) are to be processed into textile fabrics only in the areas where they are required for load transfer and the component geometry of the final component, shape holes will inevitably appear in the fabric.It is therefore important to investigate the behaviour of materials with shape holes, as this represents an important step towards minimising waste.
In summary, analyzing warp knitting thread tensile forces during the production of biaxial fabrics with shape holes lays the groundwork for enhanced comprehension of material behaviour during production (e.g. with regard to warp knitting faults and material recess) and fosters the development of waste reducing strategies for highly productive and automated inline textile production processes of high-performance technical textiles.
The investigation concentrated on two primary aspects: the variation in distance between shape holes in the warp direction, and the variation in the shape hole diameters.Figure 4 illustrates two distinct approaches for altering the distribution and geometry of shape holes.The left side of figure 4 depicts a decrease in distance between shape holes in the warp direction, while the right side demonstrates an increase in shape hole diameters.This study investigates how variations in shape hole distribution and geometry affect knitting thread tensile force during manufacturing.The subsequent chapter presents and extensively discusses the results of the investigation.
Additionally, this paper details the findings related to warp knitting thread force curves observed during the production of ± 45°biaxial fabrics (BAF) with varying thread densities, emphasizing the impact of shape hole geometry for production of contour accurate NCF.Table 4 lists the selected experimental parameters for this study.
Test setup for measuring the warp yarn tension Figure 5 illustrates the test setup used to measure the effective tension (force per yarn count) of warp knitting threads.Initially, measurements of warp knitting thread tension were conducted in front of the thread brake.Subsequent experiments included tension measurements behind the thread brake to quantify its influence on thread tension.With a sampling rate reaching 1000 Hz and a machine speed of 150 rpm, the setup achieves approximately 2500 measurements for each stitch formation process.This high resolution allows for detailed tracing and accurate quantification of thread tension variations throughout each stitch.

Results and discussion
Influence of discontinuity implementation on warp knitting thread tensile forces in biaxial NCF Warp knitting thread tensile force ranges, categorized as high, medium, and low, are defined according to the material requirements dictated by the stitch type, as detailed in table 5.The thread consumption varies primarily with the stitch type utilized in the knitting process.For instance, the pillar stitch consumes the least amount of yarn per stitch formation, while the cord stitch demands the most yarn length.The range warp knitting thread tension is determined by the warp knitting thread feed value measured in millimeters per rack.The Tension forces for warp knitting threads are categorized into three distinct ranges based on these criteria.The warp knitting thread feed value for the partial weft is kept constant, as shown in table 5.
The measurements of the warp knitting thread tensile forces of individual warp knitting threads, both inside and outside a shape hole, extend over at least ten stitch formations.These measurements are analyzed using both statistical averages and individual curves depicting average progressions.For illustration, figure 6 shows the course of the warp knitting thread tensile force over ten stitch formation processes, evaluated at three distinct warp knitting thread feed values (measured in mm/Rack, where 'Rack' representing to 480 stitch rows) during the production of biaxial NCFs without shape holes.Loop formation involves fluctuating tensions in the warp knitting threads, attributable to subprocesses such as the thread being hooked into the needle head or the release of the completed stitch.Consistent with expectations, these tension fluctuations are reproducible and remain uniform for each stitch formation, given the unaltered warp knitting process and fabric structure.
The introduction of shape holes intentionally creates variations in thread density across the NCF structure, leading to discontinuities in warp knitting thread tensile force curves, as depicted in figure 7.This leads to an increase in warp knitting thread tensile force within the area of the shape hole, potentially resulting in yarn breakage and defects.The analysis reveals that the tension increase in warp knitting threads occurs regardless of  the warp knitting thread feed value.However, the magnitude of this increase is directly influenced by the specific warp knitting thread feed value, as illustrated in figure 8.

Characterization of warp knitting thread tensile forces in non-crimp fabrics with discontinuities
After identifying the increase in warp knitting thread tension as the primary cause of the textile defects, this study characterized it through systematic parameter variation.To elucidate the impact of thread density discontinuities on warp knitting thread tension, measurements were taken at various positions and changing conditions.
Test results reveal that the tension increase associated with each shape hole is reproducible and reverts to the baseline value when the distance between the shape holes is sufficiently large.
This effect likely results from the draw-off force, generated during fabric release, spreading evenly across the NCF's entire width.At points where reinforcement threads previously facilitated the transfer of force, in a shape hole, the force must be transfered only by the warp knitting threads.This leads to an increase in warp knitting thread tension.When reinforcement threads re-enter the warp knitting zone and resume force transfer, the tension in the warp knitting threads gradually normalizes to its initial value.Future research will further investigate this phenomenon, aiming for detailed quantification and validation using simulation models.
Moreover, the initial tension in warp knitting threads, determined by the feed value, directly influences the magnitude of tension increase due to the presence of shape holes, as illustrated in figure 8.The data show a direct correlation between increased warp knitting thread tension and the yarn feed value, as depicted in figure 9. Higher yarn tension leads to a more significant rise in tensile force specifically within in the area of shape holes.To prevent warp knitting defects such as warp knitting thread breakage, it is crucial to account for the tension increase within shape holes.
Furthermore, the degree of tension increase in warp knitting threads varies with the type of stitch as shown in figure 10.A higher yarn requirement per stitch causes a higher increase in tensile force in shape holes.From this it is deduced that an adjustment of the type of stitch could effectively reduce the increase in tensile force within shape holes and thus the occurrence of warp knitting defects.
The results in figure 11 also show that the increase in warp knitting thread tension varies according to the measurment location, affecting both the tension's magnitude and its rate of increase.Warp knitting thread tension is observed to be higher in front of thread tension regulating devices, like thread brakes, compared to positions closer to the knitting unit.Consequently, incorporating flexible thread tension regulating modules may significantly contribute to controlling and reducing tension spikes associated with shape holes.Furthermore, the increase in warp knitting thread tension is confined to the shape hole area, implying that the textile's mechanical properties remain predominantly unaffected in regions outside the shape holes.This observation aligns with visual assessments.
Analysis reveals that tension increase at the warp knitting unit, occurring behind the thread brake, does not depend on the size of the contour hole.This flexibility in handling shape hole sizes is crucial for designing contour-accurate NCFs.
Additionally, figure 12 demonstrates that a similar increase in warp knitting thread tensile force is observed in lattice-shaped +−45°biaxial NCFs.This suggests the phenomenon's independence from scrim orientation.Therefore, it is imperative to consider this factor in the development and design of textiles incorporating shape holes.

Development and validation of single-warp knitting yarn compensation module
To attribute the occurring lay-up defects unequivocally to the local increase in knitting yarn tension as the cause, a specialized individual yarn compensation system was designed.This system adjusts the warp knitting yarn tension for each thread individually.Thus, tension compensation is now applied on a yarn-by-yarn or group basis, rather than uniformly across the entire width of the fabric.Figure 13 presents a schematic alongside the actual implemention of the individual yarn compensation mechanism.This completely prevented any increase in warp knitting yarn tension, as evidenced in figure 14.Employing the developed individual warp knitting yarn compensation significantly reduced lay-up defects.No yarn breaks occured.This evidence confirms that the increases in warp knitting yarn tension contribute to a number of lay-up defects.Consequently, regulating this tension enhances the quality of both contour-accurate multi-axial and biaxial NCF lay-ups featuring shape holes.

Conclusion and outlook
As a basis for the development of a defect-free processing procedure to produce contour-accurate multiaxial non-crimp fabrics, the research work carried out, focused on the analysis of the warp knitting thread tensile forces during the fixation of contour-variable yarn layers by means of warp knitting.For this purpose, relevant warp knitting parameters of the stitch formation process were highlighted and evaluated with regard to the occurring increase of the warp knitting thread tensile force.Our findings indicate that controlling the tension across variable contour yarn layers is critical in reducing textile faults, thereby enhancing the stitch-bonding process for the creation of multiaxial fabrics with variable thread densities.
Based on the knowledge gained in this work, the relevant interdependencies for the processing of contourvariable reinforcing yarn layers could be analysed and quantified.The results of the investigation prove the high influence of the warp knitting thread brake on the increase of the warp knitting thread tension.If it is possible to keep the tension of the warp knitting threads at the same level, either individually or in segmental groups of warp knitting threads, the probability of occurrence and severity of textile faults could be reduced.Significantly, the ability to maintain consistent warp knitting thread tension can lead to a 50% reduction in material waste and usage, particularly in the production of contoured CFRP components.This not only bolsters the economic efficiency of medium-sized lightweight construction companies but also contributes to the ecological sustainability of the manufacturing process.The adaptation of the warp knitting thread feed, e.g. by integrating a segmented yarn brake, and of the warp knitting process, e.g. by local adaptation of the stitch using the Jaquard warp knitting technique, can be decisive basic prerequisites for the production of cut-free NCF.
The research results presented in this paper form a basis for further investigations and development work in the field of processing contour-accurate reinforcing yarn layers with a highly productive inline process technology for waste-free high-performance technical textiles.Future research efforts should focus on validating and quantifying the discovered cause-effect relationships to enable practical modelling and implementation.In addition, sensor technologies and automated monitoring and control systems within the multiaxial warp knitting machine could be explored to increase process reliability and efficiency when processing contouraccurate yarn plies.

Figure 1 .
Figure 1.Comparison of NCF with and without component contour-adapted reinforcement thread layouts.

Figure 3 .
Figure 3. Left: car sidewall with waste-intensive shape holes; right: shape hole of a contour-altered biaxial NCF with occurring defect patterns.

Figure 4 .
Figure 4. Variation of shape hole distribution and geometry within the fabric structure (left: Variation of shape hole spacing in warp direction; right: enlarged diameter), dimensions in mm.

Figure 5 .
Figure 5. Position and measuring set-up for determining the knitting thread tensile force on a Karl Mayer Malimo 14024 with a DTX-500 yarn tension measuring device from Hans Schmidt & Co GmbH (Waldkraiburg, Germany).

Figure 6 .
Figure 6.Warp knitting thread tension force over 10 stitch formation processes of a biaxial NCF under variation of the warp knitting thread feed value (stitch: tricot).

Figure 7 .
Figure 7. Diagram of knitting thread tensile force curve for contour-changed biaxial NCF over four shape holes (visualised by red circles); stitch: tricot; knitting thread feed value: 5500 mm/rack.

Figure 8 .
Figure 8.Average warp knitting thread tensile force for contour-changed biaxial fabrics with variation of the warp knitting thread feed value.

Figure 9 .
Figure 9. Quantification of the influence of the warp knitting thread feed values on the increase of the warp knitting thread tensile force (ccBAF, stitch: tricot).

Figure 10 .
Figure 10.Influence of the stitch on the increase of the warp knitting thread tensile force in the area of the shape hole (ccBAF).

Figure 11 .
Figure 11.Influence of the measuring point on the course of the warp knitting thread tensile force (shape hole diameter 200 mm) of a contour-changed biaxial fabric (stitch: tricot; warp knitting thread feed value: 5600 mm r −1 a −1 ck −1 ).

Figure 12 .
Figure 12.Diagram showing the occurrence of the warp knitting thread tensile force increase with open +−45°NCF (stitch: tricot; warp knitting thread feed value: 5600 mm r −1 a −1 ck −1 ); red rhombuses indicate the locations of the shape holes.

Figure 13 .
Figure 13.Schematic representation and technical implementation of the developed individual warp knitting yarn compensation.

Table 1 .
Overview of the thread materials used.

Table 3 .
Overview of the selected parameter variations for test sample production.

Table 4 .
Test parameters for ± 45°BAF with variable thread densities.