Study On Processing And The Mechanical Properties Of Thermoplastic Composites With Weft-Knitted Structure

In recent years, the proportion of industrial textile in the whole textile products grows rapidly, especially thermoplastic composites which use high performance fiber as the reinforcement. Weft -knitted fabrics possess superior extensibility, compliance, conformability and high productivity. So as a reinforcement, it compounds with thermoplastic matrix and then brings composites many advantages , such as better mold ability and iMPact resistance , lower manufacture costs and so on . Compared with other textile structural composites, including warp-knitted fabric composites, the research on the thermoplastic weft-knitted fabric composites is still limited. The thesis aims at affording theoretical and experimental basis for design, manufacture and required mechanical properties of thermoplastic weft-knitted fabric composites.The commingled yarn from glass fiber yarn and polypropylene (PP)yarn was used as knitting material in the thesis. By experimental work and theoretical analysis, the author investigated the knittability of weft knitted fabrics from the commingled yarn. The knitting technology of plain, rib, milano rib and half - milano rib fabric which were produced on a flat knitting machine was explored. The author discussed some factors influencing knitting process and fabric quality, such as input yarn tension, taking-down tension, sinking depth. The key technique and optimal parameters for the shaping of the glass/PP weft knitted fabrics were found.After the weft knitted fabrics were manufactured, the hot- press method was used to produce composites .By the tests, the author obtained the suitable hot-press parameters including temperature, pressure, retentive time, the method of temperature-drop etc, then discussed the effect of the mechanical properties of the composites on the change of the hot-press parameters.Bursting and tensile tests in the wale and course directions of composites specimens were carried out to examine their stress-strain curve (or deflection stress curve), initial elastic modulus, ultimate strength, breaking elongation ratio and destructive form. The author coMPared the differences of the mechanical performance parameters and the anisotropy of the materials. The effects of fabric orientation, stitch structure, loop shape and geometrical parameters on the tensile and bursting properties were analyzed.Based on the experiment, the author made use of the multi-linear regression to investigate the influence parameters of the bursting strength of the weft-knitted structural composites. It was found that there were three main factors as following, the number of yarn in the course direction per length, the breaking elongation ratio in both wale and course directions. The linear equation of the bursting strength was explored. It is valuable for the design and produce of the thermoplastic weft-knitted composites.