Automated profile preforming for structural components

2.1.1 Direct AFP layup

The AFP system can be implemented for the direct layup of fiber tows on dedicated preform tools along pre-specified layup paths as shown in Figure 2. As a result, this additive preform manufacturing approach provides a high level of flexibility considering mid- or large-scale components showing rather low geometric complexity. Addressing profile preforming with a significant portion of fiber reinforcement along the off-axis directions forces the AFP system to deposit the fiber tows along a high number of rather short paths, which makes this layup strategy particularly inefficient [15].

Figure 2:

Schematic of direct AFP layup of an L-shaped preform (top) and a square-type bar preform as an initial shape for L- or C-shaped profile preforms (center). Manufacturing close-up of direct AFP preforming (bottom). Source: Airbus Group Innovations.

However, certain profile geometries allow for an improved preforming efficiency by combining multiple preforms into common layup geometry. For example, C- or L-shaped profile preforms can be obtained from rectangular layup geometry as indicated in Figure 2 (center). Following the motion principle of filament winding, the rectangular preform cross-section can be processed more efficiently than the singular profile preforms in terms of the layup paths as depicted in Figure 2 (bottom). However, additional cutting is required, and a higher amount of material waste occurs.

2.1.2 Flat AFP layup, cutting, and draping

In order to overcome the drawback of the direct AFP layup strategy, a three-step procedure can be contemplated, which consists of:

• automated layup of a flat preform by means of AFP,

• cutting of the preform to sections of required dimensions, and

• draping of the preform sections onto a dedicated tool in order to obtain the final preform.

See Figure 3 (left) for a schematic visualization of this strategy. Figure 3 (right) shows a picture of a large-scale manufacturing facility for flat AFP layup following this concept, with an anthropomorphic robot mounted on an additional linear axis for motions of the robot along the length of the component. Although this approach is beneficial over the direct layup approach in terms of the AFP layup paths, the overall procedure is more complex as additional cutting and draping systems are required.

Figure 3:

Schematic of flat AFP layup (left) and anthropomorphic robot mounted on an additional linear axis with an AFP head for flat preform layup (right). Source: www.electroimpact.com

2.1.3 Selective flat AFP layup and draping

The need for an additional cutting system mentioned in the previous approach can be avoided by selective layup of flat preform sections as shown in the scheme of Figure 4 (left).

Figure 4:

Schematic of selective flat AFP layup of multiple flat preform shapes (left) and manufacturing close-up (right).

Source: www.electroimpact.com

The AFP system can again be utilized more efficiently compared to the direct AFP layup strategy described in Section 2.1.1 as the fiber tows are deposited in rather long layup paths, even for the reinforcements along the profile off-axis directions. Additionally, multiple preforms are directly manufactured in parallel when the individual fiber tows are automatically cut during the layup process as shown in Figure 4 (right). This approach avoids the cutting unit required for the layup strategy in the previous section. However, draping of the preform onto dedicated tools is still required in order to obtain the final profile preform.

2.2.1 2D braiding

The 2D braiding technology has been addressed for profile preform manufacturing in the past [20, 21] but involves some major drawbacks:

• in braidings, the fiber tows shows permanent undulations, thus, losing a significant portion of their mechanical properties,

• significant reinforcement transverse to the profile axis cannot be sufficiently fulfilled due to limitations in the achievable braiding angle (typically ranging from 10° to 85°, as, e.g. stated by Potluri et al. [18]),

• the braiding technique is limited in terms of directly covering certain cross-sectional profile shapes, particularly concave-shaped profile preforms – however, these can be formed out of circular braidings in a subsequent processing step as outlined by Uozumi et al. [22], and

• laborious mounting of roving spools on the braiding machines combined with low spool capacity.

Braided preforms initially consist of single layers of braided fiber rovings. In order to create a multi-layer preform, the braiding process needs to be repeated while reversing the axial mandrel motion after each layer. This strategy is the most common approach and referred to as over-braiding [16].

2.2.2 2D braiding, winding and UD tape integration

Overcoming some of the major disadvantages of 2D braiding, a highly complex modification of the braiding technique has been developed by Airbus Group Innovations (www.airbusgroup.com) and SGL Kümpers (www.sgl-kuempers.com), called the Braided Frames (BraF) process [23]. There, the actual braiding is combined with 90° winding layers as well as UD reinforcement tapes (see Figure 5, top-to-bottom) in a fully automated process to continuously manufacture curved frame preforms at a speed of up to 1 m/min.

Figure 5:

Processing steps of BraF, a braiding process (top) combined with winding of 90° layers (center) and UD tape integration (bottom). Source: Airbus Group Innovations.

The braiding consists of a combination of carbon fiber rovings and auxiliary thermoplastic yarns, a product from EMS-Griltech (www.emsgriltech.com) termed “Grilon^® fusible bonding yarn.” These auxiliary yarns act as supports for the reinforcement rovings during the braiding process as well as a binder material in a subsequent preform fixation step. As a result, the crimp is eliminated, and in the final preform, the carbon fiber rovings show a lower amount of undulation compared to conventional 2D braidings. Reinforcements along the profile circumference direction can be added through dry fiber winding, whereas additional reinforcement in the profile axial direction can be realized by integration of UD tape material, which is attached to the preform through over-braiding with a subsequent braiding layer.

Variations of profile curvature as well as profile height can be realized with the BraF process. However, depending on the desired final cross-sectional geometry, further offline processing steps are required such as preform fixation through thermal binder activation or cutting. These requirements disadvantageously influence the overall efficiency of this preforming technology.

2.2.3 3D braiding

3D braidings are manufactured by braiding multiple fiber rovings, which are three-dimensionally oriented and continuously intertwined to create the near-net-shape preform geometry [16, 24]. Polymer composites involving 3D braidings indicate superior mechanical properties in through-thickness direction. However, manufacturing of 3D braidings is considerably slower than processing of conventional 2D braidings because of the high level of complexity of the braiding facility, particularly in terms of the spool carrier track configuration [22]. Varying the cross-sectional geometry of 3D braidings requires laborious rearrangement of the overall track configuration, a task that contributes significantly to the overall production costs. Figure 6 shows a picture of a 3D braiding facility from Herzog Maschinenfabrik GmbH (www.herzog-online.com) operated at Institut für Textiltechnik at RWTH Aachen.

Figure 6:

3D braiding facility from Herzog Maschinenfabrik GmbH with a rectangular grid of horn gears. Source: RWTH Aachen.

2.3.1 Constant cross-sectional geometry

In a jointly accomplished project, Brötje Automation GmbH (www.broetje-automation.de), Faserinstitut Bremen GmbH (www.faserinstitut.de) and CTC GmbH (www.ctc-gmbh.com) recently developed a continuous composite preforming system (CCPS) for the automated manufacturing of curved profile preforms [25, 26]. Therein, the final preform is assembled from three sub-preforms (two Z-shaped profiles together with a C-shaped profile as shown in Figure 7), whereas each of the sub-preforms is continuously manufactured along the following sequence:

• unwinding of textile fabrics from rolls,

• stacking and pre-fixing of multiple fabric layers,

• draping of the stacked layers to the desired cross-sectional geometry,

• forming the axial curvature,

• fixing of the sub-preform, and

• cutting the sub-preform to the desired length.

Figure 7:

Processing line for an axially curved, Z-shaped preform (top) and scheme for setting up a preform with comparatively complex cross-sectional geometry from three sub-preforms (bottom). Source: Brötje Automation GmbH.

The prototype facility as shown in Figure 7 (top) was implemented for manufacturing of profiles from a maximum of eight layers of 60 mm to 300 mm wide raw material. The resulting profile preforms showed a maximum thickness of 12 mm and a minimum radius of curvature of 1500 mm. They were processed at a maximum speed of 2 m/min [26]. The raw material used for the tests was a carbon fiber non-crimped fabric, equipped with a thermoplastic powder binder. The fixation steps were realized inline by thermal activation of the binder through IR radiant heating together with compression between rollers. Finally, the curved sub-preforms are cut to the desired length by means of a rotating cutting disc unit.

Following manufacturing and cutting of the sub-preforms along three parallel preforming lines, a material handling unit – actuated by an anthropomorphic robot – places them into a compacting-and-mounting tool, where the final preform is established as indicated in Figure 7 (bottom).

At the Institut für Verbundwerkstoffe GmbH (www.ivw.uni-kl.de), a similar concept was followed in order to develop a Continuous Profile Preforming System (CPPS) as shown in Figure 8. Therein, T- and I-shaped profile preforms with a height of 150 mm can be continuously manufactured at a production rate of up to 6 m/min as reported by Grieser et al. [27]. The preforming technique follows the subsequent scheme [28]:

• unwinding of textile fabrics from rolls,

• folding of the fabrics by continuously guiding the material over metal folding sheets,

• fixing of the profile cross-section, and

• cutting of the preform to the desired length.

Figure 8:

Schematic (left) and processing sequence (right) of the Continuous Profile Preforming System (CPPS). Source: Institut für Verbundwerkstoffe GmbH.

The fixation of the final profile preform shape is accomplished by means of stitching technology. This leads to a higher degree of flexibility of the profile preform compared to the approach involving binder-based fabrics. However, stitching technology involves limitations in the maximum preform thickness. Typical industrial flat stitching machines can process preform fabrics at a thickness of up to 7 mm, the limiting parameter being the maximum needle-lifting distance [24].

Cinquin et al. [29] presented another technique for continuous profile preforming developed in the project MOJO. There, carbon fiber-based layers of NCF material is continuously formed into an H-shaped preform, which finally acts as a joining element for sandwich structures. The NCF layers are fixed into the preform shape by activating the binder material through inductive heating, a technology patented by a group of authors from this project [30]. The heating technology is based on applying high frequency current to a coil, causing an electromagnetic field of equivalent frequency. The field acts on the carbon fibers and induces electric current. As a result of the electric resistance of the carbon fibers, they heat up and, thus, activate the binder material. Shaping the geometry of the coil according to the specific application allows for highly efficient heating.

Within project MOJO, this technology has been successfully tested for manufacturing preforms for RTM applications as well as for processing in a pultrusion line [29]. The H-shaped cross-section of the preform showed a width of 70 mm and a height of 7 mm, while the preform thickness varied from 3 mm along the flanges to 1.5 mm on the web. The continuous preforming process was driven by a pulling device typically known from pultrusion lines.

All of the technologies reviewed in this subsection allow for fully automated fabrication of near-net-shape preforms of profiles with constant cross-sectional geometry. The modular setup of the preforming lines enables flexibility in material, layup, and profile geometry. While rather simple cross-sectional geometries can be fabricated directly, more complex profile geometries require separate processing of sub-preforms and subsequent assembly tasks. In any case, the maximum length of the preforms is limited by the available amount of NCF material on the rolls.

2.3.2 Variable cross-sectional geometry

Advancing the CCPS system described in the previous section, IDVA GmbH (www.idva.de) and Airbus subsidiary enterprise CTC GmbH (www.ctc-gmbh.com) recently developed a prototype facility for the continuous manufacturing of a Z-shaped profile (see Figure 9, bottom) preform with variable cross-sectional geometry [31]. The development is based on an analysis of the draping forces occurring when forming the planar fabric into the desired Z-shape. A set of actuators, individually controllable for positioning pairs of guiding rollers, is used to continuously adjust the base length of the profile as shown in Figure 9 (top), a technology that has recently been patented [32].

Figure 9:

Facility for continuously adjusting the cross-sectional geometry of profile preforms (top) and Z-shaped profile preform, axially curved and with variable profile base length (bottom). Source: IDVA GmbH.

The facility is designed to work with four layers of tri-axial carbon non-crimped fabrics, unrolled from reels of constant material width. Owing to varying the base length of the Z-shaped profile and tightly guiding one of the two preform flanges, the length of the second flange is changing as well. In order to obtain a constant flange length in the final preform, its edges need to be cut. The desired final preform geometry is fixed by thermally activating a thermoplastic binder finely dispersed on the fiber tows of the reinforcing fabric layers.

Borgwardt and Hühne [33, 34] describe a continuous manufacturing line for I-beam preforms with variable web height. In this line, NCF tape material is unwound from rolls and first formed into L-shapes by means of sheet metal guides. These are equipped with heating elements for activating the thermoplastic binder in the corner regions. The binder is subsequently consolidated by means of specifically contoured guiding rollers. Then, the two pre-shaped material strips are directed against each other such that they form a reversed T-shape. This preform is further formed into an I-shape, where the web sections are connected through compaction rollers and thermal activation of the binder. Finally, the preform is guided through a split-roller stand with a vertically moveable upper roller, actuated through a linear axis. This way, the web height can be continuously adjusted between 9 mm and 100 mm. As the NCF stripes show constant width, the upper flange needs to be cut in order to obtain near-net-shape preform quality. This is accomplished by means of an integrated cutting unit. The prototype line was successfully run at a maximum speed of 1.2 m/min for a 25.7-mm wide I-beam showing a web height between 10 mm and 35 mm. A quadri-axial carbon NCF material with 512 g/m² areal weight and tricot stitching was used for the tests.

The processing speed is predominantly limited by the heat transfer from the heating units into the NCF material for activating the thermoplastic binder. Moreover, geometric restrictions are given in terms of the minimum radius between web and flange as a result of the limited drapability of the reinforcing material.

In a continuation of this work, a prototype facility for continuously preforming of C-, Z-, L-, or Omega-shaped profile preforms with variable axial curvature was developed at DLR as published by Stahl and Borgwardt [35]. The process is closely related to roll-forming, particularly known for manufacturing of curved metal profiles, and was recently patented [36]. Guiding the profile along its flanges by pairs of drive rollers, the curvature of the resulting profile is adjustable through the ratio of driving speed of the inner and the outer profile flange. The operation principle was verified at the prototype facility by processing bi-axial carbon NCF stripes (50 mm wide and 1500 mm long) with an areal weight of 415 g/m² into a curved C-shaped preform at a maximum speed of 0.3 m/min.

2.4.1 Preform rollforming

The basic principle of preform rollforming involves a processing head guided by an automated positioning system for draping layers of the reinforcing fabric onto a stationary preforming tool or core. The preform is manufactured in an additive procedure, i.e. layer-by-layer. The actual draping task is accomplished by means of guiding rollers mounted to the processing head. Fixation of the reinforcing fabric is realized by thermal activation of a thermoplastic binder. After draping a single layer of fabric, a cutting unit crops the material. Alternatively, pre-cut material batches can be used as well in order to avoid the need for inline cutting. However, feeding of the material batches to the processing head needs to be addressed separately in this case.

In a joint project, Premium Aerotec (www.premium-aerotec.com) and DLR (www.dlr.de) implemented a prototype preform rollforming system for U-shaped profiles as depicted in Figure 10 [37, 38]. There, the processing head is guided by means of an anthropomorphic robot, allowing for a high level of flexibility in terms of layup paths. The non-crimped fabric is unreeled from a roll mounted to the processing head, which contains an ultrasonic cutting unit for cropping the material after deposition of the individual layers.

Figure 10:

Robot-based rollforming setup for an axially curved profile preform with U-shaped cross-section. Source: Premium Aerotec GmbH.

Although the production speed is lower compared to continuous profile preforming methods, this technology comes with a higher degree of flexibility in terms of layup changes, number of layers, as well as curvature along the major profile axis. A drawback of this approach is the constrained complexity of the folding unit in the processing head, limiting this technique to processing of geometrically rather simple profile cross-sections. Moreover, the material throughput is rather low compared to other profile preforming techniques.

2.4.2 Pick and drape

Within the context of the INSTRUKT research project, Airbus Helicopters (www.airbushelicopters.com, formerly Eurocopter) and the Institut für Werkzeugmaschinen und Betriebswissenschaften (www.iwb.tum.de) at Technical University Munich developed a processing head for (a) picking up pre-cut batches of fabrics by means of an anthropomorphic-robot and (b) draping and fixing these material batches onto a mold to additively build Z-shaped profile preforms (see Figure 11). The triangular-shaped processing head contains arrays of rectangular manipulation elements for picking up the batches by means of vacuum technology. Draping and fixing is accomplished using IR heaters located inside the processing head.

Figure 11:

Processing head for the Pick&Drape technology. Source: Airbus Helicopters.

3.1.1 T-shaped stringer for aeronautic applications

The setup of structural components in aeronautic applications, such as sections of the main wing structures or components of the tail unit is a flat or slightly curved skin reinforced with stringers as depicted exemplarily in Figure 12 (top). The stringers are incorporated for establishing the required flexural stiffness of these large-scale structures while maintaining low component weight. This type of setup is advantageous in terms of lightweight design considerations, particularly when using carbon fiber-based reinforcements and is typically used in modern aircrafts.

Figure 12:

Example of a structural aeronautic component composed of stringer-reinforced panels (top) and geometric parameters of the T-shaped stringers (bottom).

Up to now, stringer-stiffened panels are manufactured by processing skin and stringers along separate lines followed by an assembly process [39], which is expensive, labor-intensive, and does not fully exploit lightweight potentials. Alternatively, co-curing or co-bonding of prepreg materials can be considered [40–43]; however, these techniques require autoclave processing and, thus, cause high manufacturing costs as well.

Novel composite manufacturing strategies for aeronautic applications focus on out-of-autoclave technologies. Assuming an adequate resin infusion strategy, the stringer-stiffened panels can be manufactured by vacuum-assisted resin infusion (VARI). For this purpose, efficient preforming techniques for both, the skin as well as the T-shaped stringers, are required. While the skin preform represents a geometrically rather simple and large-scale structure, which can easily be addressed by means of AFP layup, manufacturing of the stringer preforms is a more challenging task due to the combination of increased geometrical complexity, the aspect ratio of the component dimensions, and the requirement for a predominant fiber orientation along off-axis directions of the stringer. For this work, an application scenario for automated manufacturing of T-shaped stringer preforms is designed on the basis of the following parameters:

As listed in Table 1, the T-shaped stringer preform is designed without curvature and 5 m in length, 50 mm in height, 30 mm in width and at a constant preform thickness of 5 mm. The geometric stringer preform parameters are visualized in Figure 12 (bottom).

The production scenario is outlined with six stringers per panel, six panels per airplane, and 80 airplanes per year, resulting in a total production rate of 2880 stringer preforms per year.

3.1.2 C-shaped spar for wind turbine rotor blades

Today, rotor blades for traditional wind turbines predominantly consist of glass fiber-reinforced polymer composite materials and are manufactured in series production at lengths of up to 75 m. They are composed of four major components as depicted in Figure 13 (top):

• the upper and lower shell structures, particularly designed with respect to aerodynamic properties,

• tension booms, i.e. rather thick laminates of primarily unidirectional reinforcements along the blade axis bearing the tensile loads acting on the blade, and

• spars connecting the shell elements and ensuring the flexural stiffness of the blade.

Figure 13:

Inner setup of a wind turbine rotor blade (top) and geometric parameters of the spar preform (bottom).

For this work, an application scenario is designed for the automated manufacturing of C-shaped spar preforms with tapered cross-sectional dimensions on the basis of the following set of parameters:

As listed in Table 2, the spar preform height reduces from 1000 mm to 100 mm along a length of 50 m, whereas the width reduces from 200 mm to 30 mm. The preform thickness is assumed to be constant at 10 mm over the preform length. See Figure 13 (bottom) for a visualization of the geometric spar preform parameters. The production scenario is outlined with two spars per blade, three blades per wind turbine and 80 wind turbines per year, resulting in a total production rate of 480 spar preforms per year.

3.2.1 T-shaped stringer for aeronautic applications

The cost estimation is based on two key parameters as derived in Table 1: the total mass of reinforcement material for the T-shaped stringer preform (1.84 kg) and the production rate of 2.880 stringer preforms per year. Based on these parameters, the four major cost contributors are calculated for the application scenario sketched in Section 3.1.1 for five different automated preforming techniques. The results of this cost estimation are shown in Table 3.

Note that the T-shaped profile preform is designed to be set up from two L-shaped subpreforms for the two AFP-based as well as the rollforming techniques, whereas the continuous preforming technique is designed to directly cover the T-shaped cross-section of the profile. Finally, the 2D braiding approach is based on over-braiding of multiple layers followed by draping this tubular preform into the desired T-shaped geometry.

The least manufacturing costs are estimated for the continuous preforming technique, with braiding being worse by just about 8% in terms of relative costs. The manufacturing costs estimated for the three other techniques are significantly higher.

In general, the total preforming costs are driven by the material costs. Considering the cost structure in more detail, the cost drivers for the two AFP-based technologies are also found in the high deduction costs, as a result of the relatively high facility investment costs, as well as in the labor costs, which are dominated by the rather high production time (the T-shaped preform is set up from two L-shaped subpreforms) and an increased need for working personnel. The costs for the rollforming technology are primarily driven by the labor costs, which in turn result from the rather high production time. The continuous profile preforming technique comes along with the lowest preforming costs as a result of the highest level of estimated production rate, which in turn result in comparatively low operational and labor costs.

3.2.2 C-shaped spar for wind turbine rotor blades

The cost estimation is based on (a) a total mass of glass reinforcement of 558.60 kg and (b) a production rate of 480 spar preforms per year as derived in Table 2. Based on these parameters, the four major cost contributors are calculated for the application scenario sketched in Section 3.1.2 for five different automated preforming techniques. The results of this cost estimation are shown in Table 4. Note that the AFP-based preforming facilities considered here are designed with an anthropomorphic robot on an additional linear axis for motions along the preform as outlined by Dell’Anno et al. [14]. Based on information collected from AFP manufacturing companies, this is the best concept in terms of economic considerations.

The manufacturing costs are generally driven by labor and material costs as a result of the large dimensions of the C-shaped spar preform and the correspondingly high manufacturing time. The least manufacturing costs are estimated for the AFP selective flat layup approach. The AFP direct layup approach and the continuous preforming technique result in moderately higher costs (about 28% and 33%, respectively), the preforming costs estimated for rollforming and braiding are significantly higher compared to the AFP selective flat layup technique.

The higher preforming costs estimated for the AFP direct layup technique are caused by the comparatively low production rate and, thus, high labor and facility operational costs. This obviously results from the need for a large amount of rather short layup paths making the technique inefficient for this application scenario. Similarly, the continuous profile preforming technique shows a lower production rate as well, which again results in higher labor costs. Furthermore, material costs (binder-based NCF materials) are considerably higher compared to the AFP-based techniques. The rollforming and braiding techniques show the highest preforming costs, primarily driven by the labors costs as a result of comparatively low production rates and, thus, high manufacturing times.

3.3.1 T-shaped stringers for aeronautic applications

The results of the technology comparison for automated preforming of T-shaped stringer profiles as described in Section 3.1.1 are summarized in Table 5. The five preforming technologies are compared with respect to a set of seven evaluation criteria, six of them related to technical aspects, and the last one representing the preforming costs as estimated in the previous section.

The factors for weighing the individual evaluation criteria were chosen virtually uniform at a level of 0.15. The ability of the preforming technologies to cover fibers along directions transverse to the preform axis was weighted slightly higher as this is of particular importance for the stringers acting as stiffening elements in the aeronautic application scenario. By contrast, flexibility toward axial curvature as well as the number of reinforcing layers was weighted slightly lower as these aspects are considered less relevant for this application scenario.

The highest overall rating was found for the continuous preforming technique, indicating this to be the best suitable approach for the application scenario. The second highest rating was obtained for the rollforming technique, tightly followed by the braiding approach. The two AFP-based techniques show the lowest ratings.

The AFP-based techniques are well suited in terms of covering the required fiber orientation along the profile transverse directions as well as for their flexibility in realizing multiple reinforcement layers and for realizing a certain degree of precompaction of the material. However, their ability to directly realize the T-shaped cross-sectional geometry is limited, and they result in comparatively high preforming costs. The AFP direct layup approach is particularly disadvantageous due to the need for running the system along a high number of short paths, making the technique inefficient.

The braiding approach comes along with a number of suboptimal properties, particularly in terms of the process-based limitations with respect to the fiber orientation, reinforcement precompaction and overall line speed. The rollforming technique is disadvantageous as it is incapable of directly realizing the T-shaped preform geometry together with the layer-by-layer additive manufacturing approach causing high manufacturing times.

The continuous profile preforming approach shows the highest degree of efficiency of all the approaches compared in this work as it represents a clearly continuous way of manufacturing the profile preforms in a direct way. Moreover, the approach is advantageous in terms of relatively low preforming costs. However, limitations are seen in the number of guidable reinforcement layers. An increased number of layers results in higher frictional forces when guiding the material through the line. This can result in fiber misalignment or even fiber breakage.

3.3.2 C-shaped spar for wind turbine rotor blades

The results of the technology comparison for automated preforming of C-shaped spars as described in Section 3.1.2 are summarized in Table 6. Five preforming technologies are compared with respect to a set of seven evaluation criteria, six of them related to technical aspects and the final criterion representing the preforming costs as estimated in the previous section.

The factors for weighing the individual evaluation criteria were chosen virtually uniform at a level of 0.15. The ability of the preforming technologies to cover fibers along directions transverse to the preform axis was weighted slightly higher due to its relevance for the wind turbine application scenario. On the other hand, flexibility toward axial curvature was weighted lower as this is of minor importance for this application scenario.

The highest overall rating was found for the direct AFP layup technique, indicating this to be the best suitable approach for this application scenario. The second highest rating was obtained for the rollforming approach, followed by the continuous preforming technique and AFP selective flat layup. The lowest overall rating was found for the braiding approach as it comes along with process-based limitations with respect to the fiber orientation, reinforcement precompaction and overall line speed combined with the highest preforming costs.

The continuous profile preforming approach is advantageous in terms of the choice of fabrics as well as the low preforming costs. However, the large cross-sectional dimensions and the resulting high frictional forces are disadvantageous. Moreover, the strongly varying profile dimensions ask for a batch-wise operation principle, which impedes the idea of continuous preforming.

The rollforming technique is advantageous as it is capable of directly realizing the C-shaped preform geometry together with its flexibility in terms of fiber orientation as well as the number of reinforcing layers. However, the preforming costs are clearly unfavorable for this approach.

The AFP-based techniques are well suited for this application scenario in terms of covering the required fiber orientation along the profile transverse directions as well as for their flexibility in realizing multiple reinforcement layers and for realizing a certain degree of precompaction of the material.

The AFP selective flat layup is sub-optimal as a result of the need for draping the large-scaled flat preform into the final C-shaped spar preform geometry. By contrast, the AFP direct layup approach is particularly interesting as it allows for a direct manufacturing of the C-shaped cross-sectional geometry.