With the development of the dual-directional shedding mechanism, it has become possible to interlace a multi-layer warp with a set of horizontal and vertical wefts, in the fabric-width and fabric-thickness directions, respectively. The resultant fabric has a fully interlaced 3D woven structure. Khokar (2001) set out three essential requirements that must be satisfied, to successfully carry out the 3D fully interlaced preform weaving process.
i.
A multi-layer warp disposed of in a grid-like arrangement.
ii.
A dual-directional shedding operation (to form column-wise and row-wise sheds).
iii.
Two orthogonal sets of weft yarns (horizontal and vertical sets of wefts).
Fukuta et al. (1982) disclosed the construction of an apparatus which is capable of producing a fully interlaced 3D woven structure, through what is previously established as the 3D fully interlaced preform weaving process. This process complies with the three essential requirements that must be satisfied, to carry out the 3D fully interlaced preform weaving process, as set out by Khokar (2001). The 3D fully interlaced preform weaving apparatus disclosed by Fukuta et al. (1982), is illustrated in Fig. 1.
Fig. 1Plan view of the 3D fully interlaced preform weaving device by Fukuta et al. (1982). 1—frame, 2—support plate, 3—setting frames, 4—motor, 5—screw shafts, 6—weights, 7—heald device, 8—first heald bar, 9—second heald bar, 11—holes, 14—slits, 16—drive for the second heald bar, 17—frame member that defines the outside dimensions of the 3D fabric, Z—longitudinal strings
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The apparatus shown in Fig. 1 comprises two heald bars, which have a special heald eye arrangement and can reciprocate horizontally. These two heald bars are responsible for the shedding operation of the device, which forms two sets of sheds successively, one in the fabric-width direction and the other in the fabric-thickness direction. This enables the insertion of a set of wefts across each set of sheds to form a fully interlaced 3D woven fabric.
Fukuta et al. (1986) disclosed another apparatus that can be used to produce fully interlaced 3D woven structures, which is illustrated in Fig. 2. This device is similar in construction to the 3D fully interlaced preform weaving device disclosed by Fukuta et al. (1982), but it does not employ a dual-directional shedding mechanism. The mechanisms involved in producing a fabric using this device are explained in detail by Fukuta et al. (1986). However, despite its ability to produce fully interlaced 3D woven fabrics, the process does not qualify as 3D fully interlaced preform weaving, as it does not employ a shedding mechanism, which is the foremost operation of weaving.
Fig. 2Plan view of the device by Fukuta et al. (1986). 102—vertical yarn supporting plate, 103—fixing frame, 104—motor, 105—threaded shafts, 106—weights, 107,108—package stations, 109—weaving device, 110—carrier fixing plate, 111—carriers, 112—coiled yarns for lateral yarns, 113—coiled yarns for longitudinal yarns, Z—vertical yarns
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Khokar (2001) presented an experimental 3D fully interlaced preform weaving device, which incorporates a dual-directional shedding method called the linear–linear method, disclosed by Khokar (2002b). 400 warp ends can be accommodated in the grid-like warp sheet, arranged in twenty rows and twenty columns. Weft insertion is carried out using specially constructed shuttles. The shedding and weft insertion mechanisms are pneumatically controlled. Each heald frame of the device contains ten healds which allow the formation of twenty sheds simultaneously, in a given direction. Hence, twenty wefts can be inserted simultaneously in corresponding directions. Consequently, the device has a total of forty shuttles, twenty for each direction. Two pairs of shuttle banks are located for housing the shuttles, one for the vertical shuttles and the other for the horizontal shuttles. Each shuttle has a pair of boxes in the corresponding banks. A linear fabric take-up system with a profile holder is incorporated, which holds the leading ends of the warp yarns according to the profile of the cross-section of the fabric, to maintain the structure and the profile of the constructed fabric.
A similar device was presented by Khokar (2014), which employs the linear–linear method of dual-directional shedding. The multi-layer warp is disposed of in such a way, that the produced fabric has a ‘T’ shaped cross-section. A linear fabric take-up device with a ‘T’ shaped profile holder is incorporated and a creel that accommodates multiple warp spools is used to feed the warp yarns.
In another research, an experimental 3D weaving prototype had been developed which employs the linear–linear method of dual-directional shedding (Weerasinghe et al. 2017). The prototype is capable of accommodating a grid-like warp consisting of 100 warp ends arranged in 10 rows and 10 columns. The dual-directional shedding requires the two orthogonal sets of wefts to be inserted in two directions; horizontally and vertically, so that the wefts are inserted in both row-wise and column-wise sheds. However, in this system, both sets of wefts are inserted from one direction (i.e. horizontally), using rapiers. To accommodate this, the shedding, take-up and let-off mechanisms are rotated by 90°, for every weaving cycle, which allows the wefts to be inserted from one direction, in both column-wise and row-wise sheds. The sequence of operations of the prototype for producing a plain-weave fully interlaced 3D woven fabric was explained by Weerasinghe et al. (2017). This device employs a linear take-up system, similar to the one employed in the device developed by Khokar (2001). However, the take-up system in the device developed by Weerasinghe et al. (2017) rotates during the weaving process, whereas the take-up system of the device developed by Khokar (2001) is stationary. The let-off and take-up distances are maintained as required to regulate the warp tension and the warp crimp.
Various researches had been carried out on producing 3D woven fabric structures using circular weaving technology (Bilisik 1998, 2000, 2010a; Bilisik & Mohamed 2009). However, these techniques do not employ the dual-directional shedding mechanism and hence do not qualify as 3D fully interlaced preform weaving and the developed structures do not possess a fully interlaced 3D woven structure. Bilisik et al. (2014) proposed a method which employs the dual-directional shedding in circular weaving, which enables the production of a fully interlaced circular 3D woven preform. Details of the experimental device were not disclosed by Bilisik et al. (2014), however, the device employs a set of warp or axial yarns which are arranged in a matrix of circular rows and radial columns, a dual-directional shedding mechanism which forms two sets of sheds in the radial and circumferential directions and two orthogonal sets of weft yarns inserted in the radial and circumferential directions. Hence, the device satisfies the requirements set out by Khokar (2001) to be qualified as a 3D fully interlaced preform weaving device.
The apparatus disclosed by Fukuta et al. (1982) comprises two heald bars, which can reciprocate rectilinearly in the horizontal direction. The construction and the arrangement of the two heald bars are shown in Fig. 3. A detailed description of the sequences involved in achieving the shedding and picking mechanisms was given by Fukuta et al. (1982). Slight variations in the fabric structure can be obtained by slightly varying the shedding sequence, to realize either successive or alternate picking of the two sets of wefts.
Fig. 3Heald shafts by Fukuta et al. (1982). a Standard position, b individual heald bars. 8—first heald bar, 9—second heald bar, 11,13—holes, 12,14—slits, 15—drive for the first heald bar, 16—drive for the second heald bar
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Khokar (2002b) disclosed a dual-directional shedding mechanism, called the linear–linear method of dual-directional shedding, which can be employed in a weaving device to produce a fully interlaced 3D woven fabric structure. As illustrated in Fig. 4, the linear–linear method of dual-directional shedding incorporates two mutually perpendicular heald frames, consisting of a set of heald wires with specially designed heald eyes. One heald frame reciprocates rectilinearly in the vertical direction, while the other heald frame reciprocates rectilinearly in the horizontal direction to form row-wise and column-wise sheds respectively.
Fig. 4Linear–linear method (Khokar 2002b). a General construction of heald wires, b modified heald wires. 1,2—heald frames, 3—heald wires, 4ne—heald eye, 4se—superimposed heald eyes, 5—openings, 6a—active warp ends, 6p—passive warp ends, 6ps—additional axial warp ends, 10—clearance
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Figure 4a illustrates the construction and arrangement of the heald frames, while Fig. 4b illustrates a slight modification done to the heald frames to accommodate additional non-interlacing stuffer warp yarns into the fabric structure. A small clearance is provided at the ‘corners’ of the superimposed heald wires of the two heald frames. Inclusion of such non-interlacing, crimp-less stuffer warp yarns is expected to improve the mechanical properties of the fabric. Figure 5 illustrates the structure of a fully interlaced 3D woven fabric produced from a 3D fully interlaced preform weaving process, which employs the linear–linear method of dual-directional shedding. A slight variation in the interlacing pattern of the 3D woven structure can be obtained by simply altering the order of shedding to realize successive picking of the wefts in the ‘to and fro’ directions (Fig. 5a) or alternate picking of the wefts in the ‘to and fro’ directions (Fig. 5b). Both these constructions have a fully interlaced network-like structure. The resulting structure has hollow pockets, which can be filled with longitudinally oriented non-interlacing additional stuffer warp yarns, by employing slightly modified heald frames as shown in Fig. 4b.
Fig. 5Fully interlaced 3D woven structure (Khokar 2002b). a Successive picking, b alternate picking. 6a—active warp ends, 6p—passive warp ends, 7,8—orthogonal sets of wefts, 11—hollow pockets
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Figure 6 illustrates the elements which are employed in effecting the linear–angular method of dual-directional shedding, which was disclosed by Khokar (2002c). The arrangement of the multi-layer warp is shown in Fig. 7. In this system, cylindrical heald shafts are used, unlike the mutually perpendicular heald frames used in the linear–linear method. These assemblies are constructed such that they can be reciprocated in two directions; along the shaft axis and about the shaft axis. These two reciprocating movements constitute the linear and the angular motions of the healds respectively, to form the sheds in the width and thickness directions of the fabric, hence the name linear–angular method of dual-directional shedding.
Fig. 6Heald shaft arrangement in linear–angular method of dual-directional shedding (Khokar 2002c). 2—heald shafts, 3—flat healds, 4—heald eye, 5—heald guide, S—supports
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Fig. 7Arrangement of the multi-layer warp (Khokar 2002c). a Cross sectional view, b longitudinal view. 2—heald shafts, 3—flat healds, 6—multi-layer warp, 7—active warp yarns, 8—passive warp yarns, A,C,E,G,I—columns of active warp yarns, B,D,F,H—columns of passive warp yarns, a,c,e,g,i—rows of active warp yarns, b,d,f,h—rows of passive warp yarns
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A fully interlaced (plain-weave) 3D woven fabric as shown in Fig. 8 can be obtained through the alternate column-wise and row-wise shed formation and the corresponding weft insertion. Similar to the linear–linear method, slight variations in the fabric construction can be obtained by changing the order of shedding so that the wefts of a given set occur successively or alternatively. Furthermore, non-interlacing stuffer yarns can be integrated into the fabric structure in the fabric-width, thickness and the two diagonal directions, by slightly changing the shedding and picking order.
Fig. 8Fully interlaced 3D woven structure produced by the linear–angular method (Khokar 2002c). 7-active warp yarns, 8-passive warp yarns, 12c,12r-wefts, 101–104, 111–114, 121–124, 131–134, 141–144, 151–154, 161–164, 171–174 and 181–184—occurrence of active warp yarns in a square helix in the fabric interiors, A–D, J–M, P–S and W–Z—occurrence of active warp yarns in a triangular helix at the fabric edges and surfaces
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A recent invention describes a newly developed dual-directional shedding mechanism, which involves an array of rotating disks (Kale 2015). The array of rotating disks is arranged horizontally on the weaving machine surface. The number of disks in the array is determined by the required width and the thickness of the fabric structure to be produced. As shown in Fig. 9a, each disk is equipped with four perpendicular guides, through which the warp ends are threaded. As shown in Fig. 9b an optional guide is provided in the middle of each disk, through which non-interlacing warp ends can be introduced to the fabric structure, if required.
Fig. 9Rotating disks (Kale 2015). a Isometric view, b top view. 1—disk, 2–5—thread guides, 6—warp threads, 7—extra non-interlacing warp threads
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Weft insertion is carried out using a set of rapiers, which carry the two orthogonal sets of weft yarns. One set of rapiers are used to insert the weft yarns across the fabric-width direction, while another set of rapiers are used to insert the weft yarns in the fabric-thickness direction. The weft threads are inserted in the gaps between the thread guides of the disks (Fig. 10a). The hairpin-like wefts are locked by a locking needle and a locking thread and a crowbar is used to hold them in position. The inserted weft is beaten up to the fell of the fabric after which, the disks in the array are rotated by 90° about their axes (Fig. 10b), to form a new shed. The rotation of the disks, changes the position of the warp threads in such a way, that the wefts inserted during the previous shed, interlace with the warp threads (Fig. 10c). The wefts are again inserted in both orthogonal directions and beaten up to the fell of the fabric (Fig. 10d). The above sequence of operations combined with the other primary and secondary motions of the weaving machine results in a highly integrated 3D woven fabric of plain weave construction.
Fig. 10a–d Interlacement mechanism (Kale 2015). 1—disk, 2–5—thread guides, TX1,TX2,TY1,TY2—wefts, TX3,TX4,TY3,TY4—next set of wefts
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The dual-directional shedding mechanism employed in the experimental device presented by Bilisik et al. (2014) is explained here. To produce a fully interlaced 3D woven fabric structure using the circular weaving technology, the multi-layer warp must be arranged in circular rows and radial columns as shown in Fig. 11a. First, multiple sheds are formed by the sequential movement of the warp yarns in the radial column direction as shown in Fig. 11b. Subsequently, the circumferential weft is inserted between each layer of the multi-layer warp, in the circular row direction as shown in Fig. 11c. Next, another set of sheds are formed by the sequential movement of the warp yarns in the circular row direction as shown in Fig. 11d. Finally, the radial weft is inserted in the radial column direction and a fully interlaced circular 3D woven fabric structure is obtained as shown in Fig. 11e.
Fig. 11a–e Shedding sequence in circular 3D fully interlaced preform weaving (Bilisik et al. 2014)
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When comparing the different types of shedding mechanisms discussed above, several important differences can be identified. Due to the manner in which the heald eyes are distributed on the heald frames disclosed by Fukuta et al. (1982), the weft insertion should be carried out in the two diagonal directions of the heald frames. In contrast, the linear–linear (Khokar 2002b) and linear–angular (Khokar 2002c) methods of dual-directional shedding as well as the rotating disk shedding mechanism (Kale 2015), require the weft insertion to be carried out horizontally and vertically. The dual-directional shedding mechanism used in circular weaving, disclosed by Bilisik et al. (2014), requires the weft insertion to be carried out in the circumferential and radial directions. However, it should be noted that in 3D fully interlaced preform weaving, despite the direction of weft insertion, it must be carried out in such a way that the weft carrier is positively guided across the multiple sheds, as each weft carrier should precisely move in a linear path inside the corresponding column or row of the multi-layer warp, without disturbing the neighboring weft carriers. Furthermore, the linear–linear, linear–angular and rotating disk methods of dual-directional shedding enable the incorporation of additional non-interlacing stuffer warp ends in the longitudinal direction and it is not possible with the construction of the heald frames disclosed by Fukuta et al. (1982). It will be further clarified in a later section of this paper, that the inclusion of non-interlacing stuffer warp yarns is important for improving the mechanical properties of the resultant 3D woven fabric structure. The rotating disk shedding mechanism offers the advantage of carrying out the weft insertion in both orthogonal directions in one shed formation, compared to the other methods of dual-directional shedding and this enhances the efficiency of the weaving process. Furthermore, the rotating disk shedding mechanism enables the production of 4-end leno weaves. The dual-directional shedding mechanism employed in circular weaving is capable of producing fully interlaced circular 3D woven preforms, which is not possible using the dual-directional shedding mechanisms employed in flat weaving. The device disclosed by Fukuta et al. (1986) is capable of producing preforms with circular cross-sections, but the device does not employ any shedding mechanism, and hence it was not considered as a 3D fully interlaced preform weaving device.
In the existing literature, either shuttles or rapiers had been used in carrying out weft insertion in the 3D fully interlaced preform weaving devices. The 3D fully interlaced preform weaving process demands a weft insertion mechanism, which is more advanced than what is available on conventional 2D weaving machines. In case of multi-phase weaving, 2D weaving machines form multiple sheds, but in only one direction and several weft carriers are required. However, the 3D fully interlaced preform weaving devices form two sets of sheds in two directions, hence several weft carriers equal to the total number of sheds formed are required, which can move across the sheds in a highly precise manner. Several drawbacks of the shuttle weft insertion mechanism employed in the experimental device developed by Khokar (2001) can be identified. The use of shuttles as the weft carrier requires large shed openings to be formed, which develops more strain on the warp, affects the packing density of the fabric produced and it adversely affects the efficiency of the weaving process. The large shed openings create large converging angles of warp to the fell of the fabric, which creates more stress for the beat-up mechanism, leading to tension variations in the warp. The weft cannot be laid very close to the fabric fell, leading to larger weft movements during beat up, causing more abrasion between the warp and the weft. During the weft insertion, the shuttles come in contact with the warp, leading to more abrasion in the warp. Two pairs of shuttle banks are employed in the device, which contains individual shuttle boxes for each shuttle. This consumes a large space, more energy, requires the pirn winding process and generates noise and vibration. Despite these drawbacks, the shuttle weft insertion mechanism allows the direct formation of selvedges without any additional process.
The inventor of the rotating disk shedding mechanism claimed that the linear–linear and the linear–angular methods of dual-directional shedding, form large sheds, to facilitate the passage of the weft carriers such as shuttles or rapiers, and identified this as a disadvantage of those shedding mechanisms (Kale 2015). However, this claim may not be completely accurate. The height of the shed formed is usually determined by the size of the weft carrier, where shuttles, due to their larger size, require larger shed openings than rapiers. The experimental 3D weaving device developed by Khokar (2001), which employs the linear–linear method of dual-directional shedding, uses shuttles of specific construction as the weft carrier. However, it is also possible to use other types of weft carriers such as rapiers, in conjunction with the linear–linear method of dual-directional shedding, as evident from Weerasinghe et al. (2017). The prototype 3D weaving device developed by Weerasinghe et al. (2017) employs rapiers as the weft carriers and the linear–linear method as the shedding mechanism and the authors claimed that the shed height is about 5 mm. The rapiers employed in this device have an outer diameter of 2 mm, which allows them to pass through the shed comfortably without abrading against the warp. The small shed opening reduces the strain on the warp, allows the warp ends to be closely packed and reduces the time required for shed formation resulting in higher efficiency. It also reduces the converging angle of the warp to the fell of the fabric, to a value as small as 2.89° (Weerasinghe et al. 2017), which consequently eases the beat-up mechanism and reduces the tension variation in the warp. The rapiers lay the weft about 16 mm towards the fabric fell away from the shedding zone (Weerasinghe et al. 2017), reducing the abrasion between the warp and the weft during beat-up.
The use of rapiers for weft insertion, instead of shuttles, in the device described by Weerasinghe et al. (2017), enables a simpler and more compact design of a 3D weaving machine, compared to the experimental device developed by Khokar (2001). This is due to the elimination of the need for shuttle banks, which take up a large amount of space. Furthermore, the ability of the prototype to insert the wefts from only one direction (i.e. horizontally) into both the column-wise and row-wise sheds has also contributed to the compact design of the prototype. Weerasinghe et al. (2017) claimed that the gravitational pull to which the vertically inserted wefts are subjected, make the dual weft insertion mechanism impractical, which led to the use of the current design. However, the experimental device developed by Khokar (2001), employs the dual weft insertion mechanism, through positively driven shuttles, powered by a pneumatic system. Despite the direction of weft insertion, positive control of the weft carriers is necessary to guide the weft carriers across the sheds in a linear path, within the limited space available. Therefore, even if shuttle picking is employed in 3D fully interlaced preform weaving, the shuttles are not ‘picked’ as it is done on conventional two-dimensional weaving machines. However, the use of rapiers instead of shuttles by Weerasinghe et al. (2017) allowed achieving a weft insertion with more precision and stability. Furthermore, the use of rapiers eliminates the need for a reed with a guiding element, reduces power consumption and eliminates the need for additional machinery (i.e. pirn winders). However, direct selvedges cannot be produced unlike in the device developed by Khokar (2001) and a separate mechanism is needed to form the selvedges.
The key advantage of the rotating disk shedding mechanism, over the linear–linear and linear–angular shedding mechanisms, lies in the fact that the two sets of orthogonal wefts can be inserted in one shed formation. This would significantly increase the efficiency of the weaving process, compared to the linear–linear and linear–angular shedding mechanisms, which require the insertion of the two sets of orthogonal wefts during separate stages. Furthermore, the system provides the benefits of its ability to form smaller shed heights, due to the use of rapiers for weft insertion. As shuttles are not used for weft insertion, an additional unit, which comprises of a locking thread needle, a locking thread and a crowbar, is required for producing the selvedges of the fabric. However, it allows the formation of closed selvedges, similar to the ones produced by shuttle weft insertion, at both the picking end and the receiving end of the fabric.
There is no mention of the beat-up mechanism employed in the experimental device disclosed by Khokar (2001) and the author claimed that at the time of development of the device, no suitable reed had been developed to be employed in the device. The experimental device disclosed by Weerasinghe et al. (2017) employs a comb-type reed. Due to the rotating motion of the shedding, warp let-off and fabric take-up mechanisms of the device as well as the insertion of wefts in two sets of sheds formed in two directions, a conventional reed cannot be used. The comb-type reed is employed as a separate beat-up unit and can be lowered into the multi-layer warp only when required, so it does not disturb the rotating elements. At weft insertion, the reed is lowered and once the rapiers are fully retracted, the reed performs a horizontal linear motion to press the newly inserted wefts to the fabric fell. Once the beating-up is completed, the reed is raised and retracted from the multi-layer warp to allow the next cycle of weft insertion.
As discussed above, the shuttle weft insertion has several limitations. To overcome these limitations, a novel weft carrier which can be used for weft insertion in 3D fully interlaced preform weaving devices was disclosed by Khokar (2005). Additionally, the weft carrier is equipped with a reed dent. Hence, the novel weft carrier is capable of performing the beat-up operation during the weft insertion, which results in improved weaving efficiency. Khokar claimed that the weft carrier can be employed in uniaxial noobing devices as well. To overcome the issues associated with the large shuttle height, the novel weft carrier was made thinner and wider, so that it was compact and could carry a relatively large amount of yarn. This had been made possible by arranging the yarn about two axes of rotation, inside a cartridge-like yarn supplying device. The yarn is arranged on a positively driven flanged belt which runs on two wheels. The novel weft carrier has several benefits over shuttles. It is less bulky than shuttles and can accommodate more yarn, making the weaving process more efficient. During withdrawal, it does not impart a twist into the yarn, unlike in the shuttles and due to the positive drive of the belt, the yarn is released with fewer tension variations. The weft carrier allows the weft to be laid close to the fell of the fabric. The yarn is protected from contamination as the cartridge-like yarn supplying device is enclosed. The shuttle has tips at the two corners, which guide the shuttle during its movement across the shed. These tips are arranged in a linear alignment and consequently, the back and forth movement of the shuttle has to be done in a rectangular path to lay the weft either in the upper/lower or right/left shed of a given warp yarn layer. This requires more space among the layers of the multi-layer warp. However, the novel weft carrier is equipped with tips that are offset oppositely about the central horizontal axis. This arrangement guides the carriers to lay the yarn in two different paths, relative to a layer of warp yarns in the multi-layer warp while traversing back and forth in the same linear path.
The conventional reed with vertically oriented dents is not effective in beating-up the vertical wefts as they can slip through the space between the dents. The reed dent attached to the weft carrier is capable of overcoming this issue (Khokar 2005). As the weft carrier moves across the shed, the reed dent attached to it pushes the wefts inserted during the previous shed formation, to the fabric fell, while laying the new weft. Therefore, the weft carrier can combine the picking and beating-up operations, which improves the weaving efficiency and reduces the number of working elements in the machine. Despite the benefits of this novel weft carrier, there could be certain limitations associated with it. As the yarn is arranged on a belt, there could be disturbances to the smooth withdrawal of the yarn as several layers of yarn may tend to come off the surface of the belt. Furthermore, since the picking and beat-up mechanisms are combined, it may not be possible to adjust the beat-up force independently.
In the device disclosed by Fukuta et al. (1982), no positive warp-let off mechanism had been included. A negative warp let-off mechanism had been used, the motion of which was generated by the fabric take-up mechanism. The warp sheet is fed neither through a beam nor from packages. Instead, warp ends of a finite length are freely hung from a support plate 2 with weights 6 attached to the lower ends of each warp yarn to hold them under tension, as shown in Fig. 1. This imposes serious limitations to the weaving device. The length of the fabric preform produced is limited, the tension on each warp end may not be equal and tension variations can occur due to the negative warp let-off that takes place. The produced fabric cannot be wound onto a roller, as in the conventional two-dimensional weaving processes. As can be seen from Fig. 1, the device employs a linear take-up system, with an opening 17, which determines the outer dimensions of the produced fabric preform.
No details can be found on the let-off mechanism of the weaving device developed by Khokar (2001), but the warp ends can be fed using a set of beams or directly from packages on a creel. A similar device was disclosed by Khokar (2014). This type of mechanism allows the warp let-off to be controlled positively and the tension of the warp ends can be kept uniform across the warp sheet. This allows the production of a preform with a more uniform distribution of density and yarn crimp. A linear fabric take-up unit is employed in the device, and to hold the leading ends of the warp yarns according to the cross-section of the fabric preform to be produced, a profile holder is integrated into the take-up unit. The mechanism used to give motion to the take-up unit is not disclosed.
In the experimental device disclosed by Weerasinghe et al. (2017), the warp ends are directly taken from packages on a creel and fed to the weaving zone. Tensioners are used to positively control the tension of each warp end. A linear take-up unit similar to the one employed in the device by Khokar (2001), is used and the warp ends are suspended by a profile holder in the take-up unit. The profile holder is fixed to a screw, through which the profile holder can be moved to take-up the produced fabric. Weerasinghe et al. (2017) proposed that, using differential take-up and let-off systems, with the use of a rack and pinion would be a better option and that it would allow the concurrent motion of the two systems. Since the shedding, warp let-off and fabric take-up units need to be rotated after every shed formation, all the cylindrical units enclosed together are connected through a casing connector, to provide the same concurrent rotational movement. This concurrent movement minimizes the possibilities of any undesirable stresses from being developed in the warp ends. However, undesirable torsional stresses may still be developed in the length of yarn between the let-off frame and the supply packages, due to the rotational movement. Furthermore, there is a possibility of tension variations in the warp ends. Wear of the bearings on which the cylindrical casings are mounted may also disturb the concurrent movement of the three casings, leading to the development of undesirable stresses in the warp ends. The novelty approach of this prototype in terms of weft insertion has great potential for the future developments of the 3D fully interlaced preform weaving process. However, the possibility of developing this experimental prototype into a fully functional 3D weaving machine is yet to be investigated.
The use of a linear take-up system limits the length of fabric that can be produced, however, since the 3D weaving process is not used for producing large continuous lengths of fabrics unlike the conventional two-dimensional weaving process, this problem may not be highly significant. However, the linear take-up system increases the space taken up by the weaving machine, compared to a conventional take-up roll. Despite these limitations, using a linear take-up unit is necessary to maintain the consistency of the structure of the produced preform.
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