Collapsible as a noun was a neologism first used in a book entitled Collapsibles [1]. Collapsibles are objects that, in one way or another, fold out for action and fold up for storage. Collapsibles are ubiquitous. They exhibit two opposite states, one folded and passive and one unfolded and active.
They grow and shrink, expand and contract, according to functional need. To qualify as a true collapsible, an object must be repeatedly collapsible and expandable. Collapsibility can be achieved in many ways: by compression, hinging, sliding, etc. and also by folding and creasing. Creasing means folding an object along preset lines instead of wrinkling randomly [1].
One of the most famous ways of creasing and folding is the Japanese technique called origami. It has been an inspiration for many industrial and interior designers. Origami-inspired folding is often used in textile design and production as well. It can be performed using various techniques. Woven and nonwoven textiles usually exhibit a creased or folded look achieved by sewing or finishing.
Knitted products can be designed by integrating creases and folds directly into the knitted structure. Creased or folded knits can involve a wide range of structures from simple ribs and pleats to more complex 3D structures. Links-links knitting enables manufacturing of aesthetically intriguing structures which are flat-knitted but crease and fold after relaxation, forming various textures and spatial patterns.
Issey Miyake has been most famous for his experimenting with various methods of creasing and pleating fabrics that would allow both flexibility of body movement as well as easy-care and production [2]. He manipulated knitwear by infusing pleats in his 'Seashell' coat [3]. His wool blend men’s stretch knitted shirt featured an integrated collar, a front button down closure, long sleeves, seamless look and a soft foldable links-links structure [4].
There are also other fashion designers inspired by folding techniques. Using the principles of origami structure Harriet Wollard creates three-dimensional knitted garments [5]. Swedish designer Sandra Backlund explores knitted textures, volumes and silhouettes [6].
Trying to visually mimic morphological structures found in nature (i.e. fruit peel) on one hand and inspired by origami technique on the other, Darja Rant from the Department of Textiles at the University of Ljubljana designed multifiunctional foldable links-links structures (Fig 1). They could be used for foldable-extendable textile packaging for bakery products, flexible sound absorption panels or decorative folding screens. Andrej Vilar used some of the foldable structures for his knitwear collection, presented at Ljubljana 2013 Fashion week (Fig 2).
Foldable knitted structures can therefore be used for fashionable knitwear as well as for various non-clothing purposes. Some links-links foldable structures exhibit auxetic potential which adds to their applicability for special use.
Auxetic materials are different from conventional materials: they transversely expand under longitudinal strain and laterally contract when compressed [7]. This counterintuitive behaviour gives auxetic materials various beneficial properties compared with conventional ones [8, 9, 10]: improved energy absorption properties and the ability to form doubly curved surfaces or synclastic curvature which is particularly desired in materials used to construct dome-shaped structures. They could be suitable for packaging, where a synclastic curvature enables perfect fit of the textile wrap, whereas indentation resistance and fracture toughness protect the content. Enhanced acoustic properties make them suitable for sound-proofing applications.
In weft links-links knitting, auxetic effect of foldable structures is based on the structural disequilibrium of face and reverse loops which causes the fabric to crease, contract and form into a three-dimensional structure after the production process. Foldable structures shrink in both course and wale directions.
Under applied strain in the horizontal or vertical direction, three-dimensional foldable structures smooth into a flat fabric, creases unfold and the structure expands in both directions. Liu and co-workers [11] produced weft-knitted auxetic fabrics based on a specific arrangement of face and reverse loops forming a three-dimensional zigzag. They determined that the fabric which had the more folded and more closed zigzag form also had a more significant auxetic effect.
Therefore, in focusing on origami-inspired foldable weft knitted fabrics with auxetic potential at Ljubljana Department of Textiles, the experimental design has been orientated towards their ability to fold. The influence of composition of the yarn and structural parameters such as size of the repeating unit cell on the folding effect of links-links knitted fabrics was investigated. The number of the same type of loops in a course direction needed to initiate the structure folding effect was also investigated.
Two series of samples with 12 different zigzag structures were knitted. The first series of samples was produced with varying unit cell sizes both in the course and wale direction (from the smallest 2×2 to the biggest 24×24 loops in a unit cell). The second series was produced with varying widths of a zigzag line in a unit cell with a constant number of courses (from the narrower 2×24 to the widest 24×24 loops in a unit cell). Both series of knitted structures were produced on the knitting machine Shima Seiki SES 122 RT of gauge12E. Two yarns of different material compositions were used: 46.38% WO/53.62% PAN with linear density 70.54 tex and twist 243 m-1, and 86.38 %CV/13.62% PA with linear density 74.78 tex and twist 330 m-1.
After relaxation, the dimensions of the samples in horizontal and vertical directions were measured. Considering the repeat sizes (number of loops in each repeat), the width/loop and the height/loop values were calculated to estimate the folding potential of the samples. The smaller value of width/loop or height/loop, respectively, means better folding of the structure.
Zigzag knitted structures with varying repeating unit cell sizes (1st series of samples) fold in both the course and the wale direction. The folding effect appears in all sizes of a unit cell for structures produced from both yarns, except for the smallest zigzag knitted structure with a 2×2 repeat. The result shows that these structures are more closely folded in the course direction rather than in the wale direction. As the width/loop and height/loop values do not vary substantially for different repeat sizes, it can be assumed that the folding effect of these structures in both directions is good.
The width/loop values of the zigzag structures with varying unit cell sizes increase with decreasing unit cell sizes. It signifies that knitted structures with smaller unit cell sizes are less folded in the course direction. Nevertheless, even the knitted structure with the 4×4 unit cell size is well folded. The height/loop values of zigzag knitted structures with varying unit cell sizes are more or less constant and don’t change much with variation in unit cell size. Knitted structures made of WO/PAN fold better in the wale direction, while in the course direction the folding is better for the knitted structures made of CV/PA yarn.
Figure 6: Comparison of width/loop and height/loop values for knitted structures made from different materials with varying unit cell sizes
Regarding knitted structures with varying widths of a zigzag line and a constant number of courses (2nd series of samples), the differences in the width/loop values are more substantial; reducing the width of the zigzag line quickly increases the width/loop values. The 4×24 knitted structure is very poorly folded. The height/loop value of structures with varying widths of zigzag lines does not vary considerably.
Structures made of WO/PAN evenly fold for the unit cell sizes from 24×24 to 16×24, while structures made up of CV/PA evenly fold from the unit cell size 24×24 to 14×24. Knitted structures with narrower zigzag lines are poorly folded. Hence, the width of zigzag lines for the structures with a constant number of courses substantially influences the ability to fold. A certain number of the same type of loops in a course direction is needed to provide sufficient folding effect in the relaxation process after knitting. Merely six (for the CV/PA structures), seven (for the WO/PAN structures) or fewer loops in this kind of arrangement of face and reverse loops don’t provide sufficient folding force; thin zigzag lines are too narrow for the fabric to fold.
Figure 7: Comparison of width/loop and height/loop values for knitted structures made from different materials with varying width of a zigzag line and a constant number of courses
It can be concluded that the yarn material compostition, size of the repeating unit cell and zigzag line width at the constant number of courses in the repeat, significantly influence the folding ability of a links-links knitted structure. Therefore they also have an impact on auxetic potential [11].
Besides multi-functionality, foldable knitted structures offer new, aesthetically intriguing relief surfaces. Therefore, they could be particularly appropriate for use in the clothing sector, interior design, shock- and compression-absorbing and packaging materials that are visually appealing.
Dr Alenka Pavko-Cuden, Darja Rant and Andrej Vilar are researchers in the Department of Textiles at University of Ljubljana, Slovenia
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References
[1] Mollerup Per. Collapsibles : A design Album of Space-Saving Objects. Thames and Hudson, 2001, London.
[2] available at http://www.isseymiyake.com/en/
[3] available at http://www.metmuseum.org/toah/works-of-art/2003.79.16
[4] available at http://www.farfetch.com/shopping/women/issey-miyake-men-stretch-shirt-item-10295400.aspx
[5] available at http://www.artsthread.com/p/HarrietWoollard/
[6] available at http://www.sandrabacklund.com/
[7] EVANS, Kenneth E., NKANSAH, M. A., HUTCHINSON, I. J. and ROGERS, S. C. Molecular network design. Nature, vol. 353, no. 6340, pp. 124, (1991).
[8] EVANS, Kenneth E. and ALDERSON, Andrew. Auxetic materials: functional materials and structures from lateral thinking! Advanced Materials, vol. 12, no. 9, pp. 617–628, (2000).
[9] LIU, Yanping and HU, Hong. A review on auxetic structures and polymeric materials. Scientific Research and Essays, vol. 5, no. 10, pp. 1052–1063, (2010).
[10] YANG, Wei., LI, Zhong-Ming, SHI, Wei, XIE, Bang-Hu and YANG, Ming-Bo. Review on auxetic materials. Journal of Materials Science, vol. 39, no. 10, pp. 3269–3279, (2004).
[11] LIU, Yanping, HU, Hong, LAM, Jimmy K. C. and LIU, Su. Negative Poisson’s ratio weft-knitted fabrics. Textile Research Journal, vol. 80, no. 9, pp. 856–863, (2010).