Abstract. In this paper injection process for industry production is just a grey system which have problems of complex representation and discrete data. Orthogonal experiments have been designed to obtain the value of shrinkage and warp. The lower value of the two objects is better. After data obtained in experiments are dealt with by grey theory, relational degree had been calculated. With variance analysis the optimal level of the factors influencing the value of the two objects have been find out, then a optimal group of the process parameters has been obtained. Through simulation in computer, the warp is less than 0.5 mm, the shrinkage is less than 0.5%. The requirements of the product have been satisfied. In this method the factors had been considered before mould is tested comprehensively. Time and cost of product development have been decreased greatly. It is believed that the method is valuable in actual product.

An automobile bumper is a large scale thin-wall plastic part. It is difficult for the weld lines of product to be removed by conventional injection method. Five valve gates in sequential injection have been used to control successfully weld lines to the border, but the warp degree of the product is too large. Through grey theory the packing pressure and time of valves opening and closing have been optimized. The grey relational degree have been calculated, the optimal process parameters group have been obtained by variance analysis. The result has been verified successfully in Moldflow software, the warp degree of product has been decreased greatly and satisfied the requirement of industry production. The injection method can be applied in actual production and increase corporate profits.