A weaving machine for three-dimensional Kagome reinforcements

Abstract

Two-dimensional or three-dimensional (3D) textiles have been used as reinforcement in composite materials. Most techniques for weaving 3D textiles have been developed to obtain a compact preform so that the final product, the fiber-reinforced composite, has a high volume fraction of fibers with the least fraction of matrix for high strength. Contrarily, this article describes a novel technique for weaving a loose 3D preform called wire-woven bulk Kagome with polymer wires or threads. Firstly, the principle is explained by using a manual loom. A weaving machine is then designed with detailed mechanisms and its prototype is presented. Finally, the benefit, shortcomings, and future plans are discussed.

Keywords

three-dimensional reinforcement weaving production Kagome

In the 2000s, a new concept of cellular material named truss or lattice material was introduced.¹ Unlike previous cellular materials, such as foams, which are composed of stochastic random cells, this material is a miniaturized version of the truss structure used in civil engineering since the Greco-Roman period, that is, it is composed of regular truss cells. This truss or lattice material exhibits a stretching-dominated deformation under external loading, leading to high strength and stiffness, particularly at a low relative density.^1,2 The truss materials are classified according to the architecture of the unit cell. Among the various architectures of the truss or framework, the Kagome truss is known to provide the lowest joint connectivity between the structural elements, that is, struts.³ Specifically, each joint in the Kagome truss connects with six struts, while each joint in the pyramid and octet trusses connects with 12 struts.^1,3 As a result, the Kagome truss is less sensitive to defects at the joints than the other trusses.⁴ However, the Kagome truss is not rigorously rigid, although it consists of triangular cells and tetrahedron cells in its two-dimensional (2D) and three-dimensional (3D) architectures, respectively.⁵

Kagome is a Japanese term referring to a type of weaving pattern used in bamboo baskets. In the architecture of the Kagome, two long bamboo strips intersect each other at each joint to build the 2D Kagome. Because two strips are overlapped at single points to effectively stiffen the structure, the joints are very rigid, but not frictionless, as assumed in Maxwell’s criterion for static determinacy of ideal trusses.⁶ If a 3D Kagome truss is made of continuous wires instead of discrete struts connected with frictionless joints, the problem of non-rigidity would be eliminated because of the stiffened joints. Thus, the 3D Kagome truss made of wires was devised, called the wire-woven bulk Kagome (WBK) (Figures 1(a) and (b)). The WBK is fabricated with high mechanical properties, despite being composed of helically curved struts, so that the wires barely pass by each other at the joints. Since WBK was first introduced, several wire-woven materials, including WBK, have been studied in terms of their properties, fabrication techniques, and applications, which have been well reviewed in a recent article.⁷ Mostly, wire-woven materials are fabricated by assembling helically preformed metallic wires and then fixing the intersections between wires with metallic bonding such as brazing.

Figure 1.

(a), (b) Wire-woven bulk Kagome in the two different orientations and (c), (d) their unit cells.

2D or 3D structured woven preforms, known as textiles, have been used as reinforcements in composite materials. In particular, 3D textiles are effectively used to fabricate thick composites with enhanced resistance to delamination. Various techniques for weaving 3D textiles have been developed since the 1980s. However, most techniques are designed to obtain a compact textile so that the final product, a fiber-reinforced composite, has a high volume fraction of fibers with the least fraction of matrix for the highest strength. In contrast, techniques that can be used to weave loose 3D textiles are rare.^8–10 However, we believe that such a 3D textile with a low volume fraction of fibers (typically, the relative density is less than 0.1) and sufficient stiffness is needed in many important applications. Examples are as follows.

Ultra-light space frame structures of composite materials. Isotruss® is a good example and is well-known for its unique architecture and high bending strength.¹¹

Reinforcements for porous media, such as foams. Unlike those used with solid matrices in ordinary composites, MMCs (metal matrix composites), or CMCs (ceramic matrix composites), a porous medium does not need dense or compact reinforcements, which would increase the weight and thus sacrifice its main benefit of being light in weight.

Flexible open-cell porous materials. These are not only useful to cushion an underlying object from impact, similar to a soft sponge, but are also tough because they are made of thin flexible wires. Sosanya demonstrated that the structure could be fabricated using his “3D Weaver” machine, and that the structure was flexible and tough enough to be used as the sole of a shoe.¹²

In addition, this study was actually motivated by an effort to create a framework that was used to fabricate periodic shell structures, named Shellular.^13,14 Namely, a wire-woven structure was used as a framework. Liquid resin was then infiltrated over the framework and hardened several times to obtain a template with a smooth surface. Finally, the thin shell structure was obtained by coating a hard material on the surface, followed by etching out the template.¹⁴ Because metallic wires were difficult to etch out without any damage to the hard coating, polymer wires were used to fabricate the loose structure, that is, WBK. However, polymer wires were never plastically deformed, and the previous fabrication method using helically preformed wires was not applicable. For this purpose, a new technique was needed to literally weave the flexible polymer wires, without assembling helically preformed wires.

Several techniques have been reported for weaving or fabricating a 3D textile with a low volume fraction of fibers or wires. The Isotruss® and Sosanya’s structures, mentioned above, are not actually 3D. Hence, they are not expected to be used for general purposes. The authors’ research group introduced a composite WBK¹⁵ and a woven-cored sandwich.¹⁶ However, while the former was partially made of hard rods and did not have potential for mass production, the latter did not have an actual woven structure, but had wires simply passing each other and, hence, it could not maintain strength without hard face sheets. Another technique, called Integrally Woven Sandwich or Distance fabric, which stems from traditional velvet weaving, is well-known for fabricating a 3D textile with a low fraction of fibers.^17,18 However, its product is only a sandwich structure.

This article describes a novel technique for use in weaving a loose 3D textile, the WBK with polymer wires. Firstly, the weaving principle of a manual loom is explained. The design of the weaving machine with detailed mechanisms is then explained, and the prototype is presented. Finally, the benefit, shortcomings, and future plans are discussed.

Weaving principle

The WBK consists of three out-of-plane wires and three in-plane wires, or four out-of-plane wires and two in-plane wires, depending on the orientation. The unit cells of the WBK in the two different orientations are depicted in Figures 1(c) and (d). In the second orientation, the wires in the two in-plane directions do not contact each other; this arrangement was expected to simplify the weaving process in the in-plane directions. Hence, we selected this orientation for the design of the weaver. According to the terminologies of classical weaving, the in-plane wires are regarded as wefts, while the out-of-plane wires are regarded as warps.

Firstly, a manual loom was developed to weave the WBK that was used to build the frameworks for Shellular, as mentioned above.¹⁴ The loom consisted of roughly three components: a top plate, from which warp wires hung vertically; a few layers of frames, which hold the weft wires in place to prevent spring-back of the wires after picking and battening; and a base plate with square-mesh patterned holes, which positions the ends of warp wires on the bottom. Figures 2(a) and (b) show the loom used to manually weave the WBK and a magnified view of the woven WBK fixed to the external frames, respectively. The top plate is actually a sheet of transparent polycarbonate. On the plate, the arrangements of the magnetic buttons holding the top ends of the warps through the plate were regularly changed during the weaving process. Because the movement of the changing arrangement of the magnetic buttons resembles a group of people dancing a waltz on a floor, the plate was named the “dance floor”. Figure 3 shows a series of arrangements for the magnetic buttons in a 4 × 4 matrix during a single cycle of weaving. Here, the blue lines denote horizontally inserted wefts. After switching every two neighboring buttons along the x-direction on the dance floor (whereby the couples of warps cross over each other) in Step-1 (and 5), the wefts were inserted over the cross-points in the y-direction in Step-2 (and 6) by using a rod. Specifically, after an end of each weft wire was attached to the end of the rod, the rod was inserted beneath the dance floor through the vertically hanging warps. The rod was then used again to pull down the weft upon the frame. Subsequently, the weft was adhesively bonded to a frame at both ends to prevent spring-back. This process was repeated until all the wefts in a layer were bonded on the frame. Afterward, the buttons along the y-direction in Step-3 (and 7) were switched, the wefts in the x-direction in Step-4 (and 8) were inserted, and the wefts were then pulled down and fixed. The same process was repeated until the desired number of layers was obtained. Figure 2(c) shows a schematic view of the WBK structure woven between the dance floor on the top and the base plate on the bottom, excluding the external frames. In each layer, two bars are added at both sides in the x or y-direction to fix the warp wires, the direction of which alternately changes with the layers.

Figure 2.

(a) Loom used to manually weave the wire-woven bulk Kagome (WBK). (b) Magnified view of the woven WBK fixed to the external frames. (c) A schematic WBK structure woven between the dance floor on the top and the base plate on the bottom, excluding the external frames.

Figure 3.

Series of arrangements for the magnetic buttons in a 4 × 4 matrix during a single cycle of weaving. (Color online only.)

Design of the weaving machine

Dance floor

To realize the switching motion on the dance floor, we modified the mechanism suggested by Tsuzuki et al.¹⁹ for 3D braiding of compact composite structures. In fact, braiding has been identified for some time as one of the most commonly used techniques to produce ropes. The most well-known is the Maypole braiding machine, which is widely used to produce axisymmetric composite structures (2D), such as shafts or tubes.²⁰ On the other hand, Tsuzuki et al.’s mechanism consists of star-shaped rotors arranged in a matrix of multiple rows and columns, and it can produce any shape of 3D braider.

Figures 4(a)–(d) show a series of motions of star-shaped rotors and oval-shaped carriers in the bottom view of the dance floor. Table 1 shows the detailed sequence of movements of the star-shaped rotors that are classified in the four different colors. Here, the oval-shaped carriers act as the magnetic buttons, shown in the manual loom, and the star-shaped rotors act as the operator. The star-shaped rotors are divided into five groups, identified by the different colors. While the one group marked in white is fixed, the other four groups are driven by individual motors. Specifically, the rotors marked in light goldenrod and light green rotate through 180° to interlace couples of the nearby warp wires, corresponding to the switching motions. Figures 4(a)–(d) indicate the motions corresponding to the switching motions of Sequences-1, 3, 5, and 7 listed in Table 1, respectively, and Steps-1, 3, 5, and 7 shown in Figure 3, respectively. However, the switching motions in two alternate directions by this mechanism are not as simple as in the manual loom. That is, after one switching motion in the x or y-direction, the carriers need to be rearranged before another switching in the perpendicular direction, that is, y or x-direction. Therefore, Sequences-2, 4, 6, and 8 listed in Table 1 are for the rearrangements. The rotors marked in light blue and dark brown rotate through +90° and −90°, respectively, to rearrange the carriers and move the warp wires to the next cells. The rotors marked in light blue and dark brown can be driven by a single motor, because they operate simultaneously. As a result of Sequences-1–8, the carriers move along specific routes by the action of the rotors. Figure 4(e) shows four different patterns of the routes of the carriers during a single cycle of weaving.

Figure 4.

(a)–(d) Series of motions of star-shaped rotors and oval-shaped carriers in the bottom view of the dance floor during a single cycle of weaving. (e) Four different patterns of the routes of the carriers. (Color online only.)

Table 1.

Control sequence of star-shaped rotors on the dance floor (color online only)

Figure 5 shows a more realistic design with a gear train above the back panel used to drive the dance floor by electric motors. This design is for 10 by 10 warps. The star-shaped rotors in an identical color rotate simultaneously, as their motions are synchronized by the gear train. Hence, three motors are used to derive the four groups of rotors for the entire dance floor, as shown in Figure 4. The gear train consists of three layers of gears, as shown in the side cross-section in Figure 5(a). The top and the third layers are depicted in Figures 5(b) and (c), respectively. The gears are arrayed in a matrix, which reduces back lash by increasing the interference between the gears. Figure 5(c) depicts an array of the rotors and carriers observed from the bottom. Bobbins that supply the warp wires are mounted under the carriers so that the bobbins move with the carriers during the weaving process.

Figure 5.

Realistic design of the dance floor with a gear train above the back panel: (a) side view showing the three cross-sections for figures (b)–(d).

Weft-inserter

The weft-inserter is developed to continuously insert the wefts in two perpendicular directions, and is mounted on a turntable. In a traditional weaving machine, a weft-inserter with a rapier or shuttle is designed to weave a compact woven structure; consequently, it cannot be used for our purpose. A totally new mechanism is thus needed. The weft-inserter has four main components: a turntable, a filler-inserter, a shedding mechanism, and a shuttle operator. The weft-inserter components are indicated in gray color in the overall view of Figure 6. The working mechanism is as follows.

Figure 6.

Overall view of the weft-inserter (gray) and take-up unit (ivory). (Color online only.)

Turntable

The turntable is designed to rotate 90°, which enables insertion of wefts in two perpendicular directions, that is, the two directions of in-plane wires shown in Figure 1(b). The filler-inserter, shedding mechanism, and shuttle operator are all mounted on the turntable.

Filler-inserter

For this weaving machine, flexible wires, threads, or yarns that cannot maintain their shapes without an external support are used as the raw materials. Consequently, they need supporting fixtures, that is, the filler to maintain the woven structure, that is, the pitch and the height of the WBK cells. In fact, the filler has a number of parallel arranged bars, each of which supports a series of intersections of warps, as shown in Figure 7(a). The fillers are placed by the filler-inserter (Figure 7(b)) below the weft alternately in the two perpendicular directions.

Figure 7.

(a) Two layers of filler arrays assembled with woven warps and wefts. (b) Filler-inserter.

Shedding mechanism

The shedding mechanism separates the warps into two divisions to form a tunnel known as a “shed”, which provides room for the passage of the shuttle. The shed is created by a shedding head at the end of each rotating bar, as illustrated in Figure 8(a). The shedding head rotates one time as the rotating bar advances by one pitch. When the shedding is completed and the rotating bar is recovered, the “shed” for the shuttle would be prepared, and the weft could go through the shed. As the result, the weft is inserted in a helical pattern, as shown in Figure 8(a). The number of wefts differs in the two perpendicular directions: five and six in this specific design for 10 by 10 warps. Hence, the shedding mechanism is set on a slide, which can change the number of working shedding heads by shifting the shedding mechanism to either end of the slide, as illustrated in Figure 8(b).

Figure 8.

(a) Schematics of operation of the shedding heads at the end of each rotating bar. (b) Overall view of the shedding mechanism, which is set on a slide.

Shuttle operator

The shuttle is used for inserting the wefts and is composed of a shuttle body and a flat spool, onto which a weft wire is wound. Unlike conventional weaving, a constant gap (i.e. one pitch) is ensured between the wefts, which are arranged in parallel. As the shuttle hangs from a shuttle guide (actually a linear bearing) under a frame, as shown in Figure 9(a), it runs on a zigzag track during the inserting process. Namely, after the shuttle travels in the longitudinal direction from one end to the other end, it shifts by one pitch in the lateral direction, and then travels back in the longitudinal direction again. The track is marked as the red dashed line in Figure 9(b).

Figure 9.

(a) Shuttle operator with a shuttle hanging from the shuttle guide. (b) Top view of the shuttle operator and the woven structure. The red line indicates the zigzag track of the shuttle during the weft-inserting process. (Color online only.)

Take-up unit

The take-up unit has three main components: a fence, a take-up unit, and a container. The components of the take-up unit are marked in ivory in the overall view of Figure 6. The working mechanism is as follows.

Fence

The fence surrounds the square area of the woven structure, as shown in Figure 10. The fence consists of circular columns with sharp caps and thin rods at the top. The columns line up along the four sides with a constant gap. During weft insertion, the weft wire passes through the gap between the thin rods while being pulled by the shuttle in the areas indicated by the red dashed lines in Figure 10(b). After each pass, the weft wire winds halfway around a rod before it takes another pass in the opposite direction, making a zigzag pattern of the weft track. The interval between the rods is the same as half of the pitch of the WBK. Also, the fence plays a role in maintaining the tension in the wefts after weaving. The sharp caps at the top of the circular columns are detachable. Hence, if necessary, the caps and rods can be separated from the fence.

Figure 10.

(a) Isometric view of the fence surrounding the square area of the woven structure. The horizontal line segments on the outer surface of the fence indicate the wefts. (b) Areas indicated by the red dashed lines where the weft wire passes through the gap between the thin rods while being pulled by the shuttle during weft insertion. (Color online only.)

Take-up unit

After the weft insertion, the take-up unit pulls down each layer of the woven structure from the weaving area and stores the layer in the container. The wefts are then forced to wind the columns with the larger diameter, and are tightly stretched within the fence, while the warps in each layer of the woven structure are supported by a layer of filler. The take-up unit mechanism has four spiral push rods, which are mounted on the four corners of the upper frame of the take-up unit, as shown in Figure 11(a). Once the weft insertion is carried out in one layer of the woven structure, the spiral push rods operate to push down the layer (assembled with an array of fillers) below the weaving area, and the layer is ratcheted by the spring hooks mounted on the container to keep the filler just below the weaving area.

Figure 11.

(a) Take-up unit with four couples of spring hooks and spiral push rods mounted on the four corners of the weaving area. (b) Assembly of the container, woven structure, and fillers; the assembly is separated from the weft-inserter, after weaving.

Container

The multiple layers of a woven structure are stored in the container under the take-up unit. The container has an outer frame and four main columns at the square corners. The circular columns composed of the fence and the four main columns stand on the base of the outer frame to hold the woven structure, as shown in Figure 11(b).

After weaving is completed, the entire assembly, consisting of the container, fillers, and fence, shown in Figure 11(b), is simply separated from the weaving area by pulling downward. Because the woven structure is composed of flexible wires, it cannot maintain its shape without the fillers and fence. Hence, a resin can be used to infiltrate the wires and intersections between wires, and the resin is then cured to stiffen the structure. For this purpose, all parts of the container, fillers, and fence can be coated in advance with a non-stick spray. After curing, the outer frame and fillers are separated from the assembly, and the wire-structure is finally pulled out and upward from the fence. Figure 12 shows the separation sequence of a woven structure from the assembly.

Figure 12.

Separation sequence of a woven structure from the assembly.

Prototype machine and samples

Figure 13(a) shows the overall and partial views of the prototype of the weaving machine built according to the design described above. This machine has 10 by 10 carriers on the dance floor to weave a WBK with 5 by 5 cells.

Figure 13.

(a) Overall and partial views of the prototype of the weaving machine built according to the design described above. (b) A sample woven by this machine.

While the weft-inserter and take-up unit are manually operated in this work, while the dance floor was operated by three step motors mounted on the top plate, and controlled by a personal computer as follows. A user interface (UI) was used for controlling the dance floor in an intuitive way. The controller was developed based on open-source hardware, the Arduino mega 2560 microcontroller board. The UI program was written in MATLAB and ran on a computer that was connected with the Arduino board. A control signal was sent from the computer to the control board that collected information from the sensors on the machine. Also, the control board managed the collected information, and then sent a feedback signal to the computer and a driving signal to the motors.

Figure 13(b) shows a sample that was woven by this machine using cotton threads of 1 mm diameter, and then infiltrated and hardened by low viscosity epoxy (E 205, Konishi Co., Japan). The pitch is 30 mm and the overall dimensions are approximately 120 mm high, 150 mm wide, and 150 mm deep. The sample of the WBK has 5 by 5 by 5 cells.

Discussion and concluding remarks

By changing the software, this machine can be used to weave other structures such as wire-woven bulk diamond (WBD),²¹ as shown as Figure 14. In this case, the number of warps and wefts can be more than doubled, and a much denser structure can be obtained from the same machine. For example, a dance floor used to weave a WBK with 7 by 7 rotors, 4 by 4 carriers, and 1 or 2 wefts can be modified into one that weaves a WBD by changing the numbers of the carriers and the wefts to 6 by 6 and 6, respectively, while the number of rotors remains constant.

Figure 14.

Schematics of (a) a wire-woven bulk diamond (WBD) structure woven between the dance floor on the top and the base plate on the bottom, excluding the external frames. (b) Magnified view of the WBD.

A conventional weaving machine does not have moving bobbins; instead, it has a moving weft-inserter, such as a shuttle or rapier, and all the bobbins are fixed on a stand. On the other hand, a braiding machine does not have a weft-inserter, but has moving bobbins mounted on the carriers. In this paper, we employed the merits of both the weaving machine and the braiding machine. Namely, our weaving machine has not only moving bobbins on the dance floor, but also has the weft-inserter.

As mentioned above, the 3D structures woven by this machine can be used for many important applications such as ultra-light space frame structures of composite materials, reinforcements for porous media such as foam, and flexible open-cell porous materials. For simplicity, the weft-inserter and take-up unit were temporarily designed to be manually operated in this work. For full automation, the design should be changed while adding more actuators and sensors. In the near future, we will fabricate truss or space frame structures of composite material using this machine and carbon fiber yarns, and we will then examine the mechanical properties to evaluate its feasibility for engineering applications. We expect that the space frame can be used as a basic material with ultra-lightness and high strength. Also, we will continue to work on a more practical machine that can fully automatically weave loose 3D structures with a considerably larger number of cells. In this weaving machine, the warp wires hang from the carriers under the dance floor. If a spool is attached under each carrier, this machine might be used for continuously producing the woven structure. However, because this machine is designed to stand vertically for smooth operation of the dance floor, it is not good for continuous production. To overcome this shortcoming, a totally new mechanism, which allows a horizontal layout, is needed for moving bobbins.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) (grant number NRF-2015R1A2A1A01003702).

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