Three-dimensional Computer Simulation Of Warp-knitted Fabrics

CAD technology in warp knitting is a new technology that is rapidly developed accompanied by the development of computer graphics technology. In recent years, with the rapid development of warp knitting industry, this technology has been widely used in the production of warp-knitted fabric, to assist the pattern designers with pattern design, fabric simulation, process calculation, data output and so on. With the improvement of the functional requirements of warp knitting CAD software, the designers have not only limited to two-dimensional simulation of the fabric, but more likely to watch the effect of three-dimensional fabric design, and to enhance the visual perception of the fabric. Therefore, the development of three-dimensional warp-knitted fabric simulation capability becomes the current trend in the development of warp knitting CAD software. This will enable users to view the three-dimensional effect during the fabric design process, easy-on production. Based on this, the main purpose of this paper is to establish an appropriate three-dimensional structure model by studying the loop structure in tricot warp-knitted fabric, and then to develop a three-dimensional computer simulation system to achieve three-dimensional simulation of warp-knitted fabrics.First, the loop geometric structure of warp-knitted fabric was conducted a detailed study in this paper, as a basis for the establishment of a loop geometric model. There are many structures used warp-knitted fabrics. However, stitch structure and inlay structure are most used, and the two structures can meet the production demand of all the basic structures of tricot warp-knitted fabrics. According to the needle bed number and the needle number that the guide bar overlapping, the loop can be divided into common single-needle-bed loop, koper loop and double-needle-bed loop. Inlay stitch used in tricot warp-knitted fabrics is partial inlay, and according to the needs of the fabric structures it can be one-wale inlay, two-wale inlay and multi-wale inlay. The loop geometry of these types was studied in this paper. The loop physical dimensions in fabric plane and fabric profile were obtained by experimentally measurements, and they were separately defined by loop height and yarn diameter, then the geometric models of these loops were established.Subsequently, according to the geometric model of warp-knitted loops, the three-dimensional solid model of these loops were developed with NURBS curves and surfaces in this paper, and the loop offset was studied in depth. During the establishment of three-dimensional solid model of the loops with NURBS, the yarn that used in the warp-knitted fabrics was simplified into a uniform cylinder with circular cross-section. So the central axis of the yarn could determine the parameter u of NURBS surfaces, and the circular cross-section determined the parameter v. According to the structural characteristics of these types of the loops, they were described by different number of data points. Common single-needle-bed loop and double-needle-bed loop were described by 10 data points, while koper loop was described by 16 data points, and inlay by 5 data points. Then the control points were calculated by interpolation method so as to establish the solid model of loops. As the variety of warp-knitted structures, the loops which are connected to each other had a variety of morphological structure. Therefore, the loops were detailed classified in this paper according to the overlap direction and the chain notations of adjacent course loops, and then the coordinates of data points that defining each type of loop was determined by step by step derivation. At last, all forms of three-dimensional simulation of warp-knitted loops were achieved.Due to various factors in actual fabrics, the loops are showing different degrees of inclination. In this paper, the factors that caused the inclination of the loops were analyzed, and the yarn tension and the tension of underlaps were considered the main reasons for causing the loop offset. By the analysis of the force of each type of loop, the offset equations were obtained respectively. To simplify the process stress analysis, this paper used the method that first calculating the offset of single loop and then accumulating the offset to obtain the final offset of warp-knitted loops. As the data points in the loop model at different locations, the offset degrees of the points were various. Therefore, the data points were derived into a few groups according to their positions, and the offset of each point was calculated. And then the offset of each point was combined to their horizontal coordinates to obtain the final coordinates of each point. Finally the more realistic simulation of loop form was achieved. Based on the three-dimensional model of warp-knitted loop, the three-dimensional computer simulation system of warp-knitted fabrics was developed with VC++.NET and OpenGL, which could simulate the tricot warp knitting fabrics. During the development process of this system, a series of practical programming issues, such as the coordinate system conversion, the conversion of structural information, the judgment of fabric types and the reading of threading, were solved. At last, the three-dimensional computer simulation system of warp-knitted fabrics was developed with the two powerful development tools. There are five functional modules in this system, i.e. the file management module, basic parameter setting module, threading setting module and material setting module. The system can be used for the three-dimensional simulation of single needle-bed warp-knitted fabrics and double needle-bed warp-knitted spacer fabrics.