Research On The Permeability Of Composites Affected By Knitting Structure

VARTM—Vacuum Assisted Resin Transfer Molding is a newly low-cost molding technology for large composites. And it is widely used in both military and civilian applications.In VARTM process for producing large composites, the infusion net and separate film are used in order to raise the infusion velocity of the resin, so as to improve the productivity. However, this will bring two problems as follows: 1) the infusion net and separate film cannot be reused, this may cause waste and high cost. 2) the resin will adjoin to the infusion net. this is not only wasteful, but also cause environmental pollution.This paper focus on the feasibility by using knitted structure made of fiberglass to replace the infusion net which consists three parts: first part is the knitting of the knitted structure, second part is the testing of the permeability, third part is the analysis of the mechanical property.The first part is the study of fiberglass's knitting property and the process of it on the Computerized Flat Knitting Machine. Due to the high modulus, low breaking elongation ratio, the fiberglass is hard to knitting. In this part, we discussed the difficulty of the knitting technology of fiberglass, the influence factors and the improvement of knitting feasibility. The knitting technology of knitting structure is a difficult point in this issue, by changing the technology parameter, such as yarn input tension, cam setting and take down force, we can gain the best knitting process of fiberglass in CFKM, and also knitted 5 different knitted fabrics (jersey, spacer, rib, interlock, milano), they all have a good appearance.The second part is the testing of permeability. In order to compare the permeable property between knitted structure and the infusion net, the syrup and vinyl were used as the testing fluid. The permeability of biaxial warp knitted fabric as well as that added with knitted structure or infusion mesh have been tested.The permeability of the biaxial warp knitted fabric including 1,4,16 layers as well as these laminated fabrics by adding knitted structure or infusion mesh have been tested using syrup. By adding knitted structure or infusion mesh, the permeability of biaxial warp knitted fabric are remarkably improved, and the rise rate of permeability decrease with the increase of the layers. Resin are used as the matrix of composites in practical applications, so the permeability of 4 layers of biaxial warp knitted fabric and that by adding knitted structure or infusion net have also been tested using resin as the testing fluid. It is shown that the permeability of different fluid has a little difference, but the overall trend is the same, the permeability has a marked increasing when added with knitted structure, and the permeability is a little lower than that of infusion mesh.The third part is the mechanical test. The mechanical property of knitted structure is a little lower than that of biaxial warp knitted fabric, so it is very important to test the changing of mechanical property when added with knitted structure. The tensile, flexural and impact resistance properties were tested in this paper.The tensile, flexural and impact resistance properties of the biaxial warp knitted fabrics in transverse direction are better than that of vertical direction, because of the surface density of transverse direction is larger than that of vertical direction. The mechanical properties of knitted fabrics in vertical direction are better than that in transverse direction due to the direction of the knitted loop, so the differences of mechanical properties between the two direction become smaller when the biaxial warp knitted fabric and knitted structure are laminated together for composites.The tensile and flexural properties of the composites decrease gradually after the addition of knitted structure, and the impact resistance property increase gradually .In general, the permeability of reinforcement has increased significantly when added with knitted structure, but it is lower than infusion mat. By laminated with knitted structure, the mechanical property has some change, such as tensile strength decreased about 20%, flexural strength decreased about 10%, and impact resistance strength increased about 25%. Overall, the knitted structure has a good prospect of application in the areas of that not require a high mechanical performance.