Effects of injection molding parameters on shrinkage and weight of plastic part produced by DMLS mold

Purpose - The purpose of this paper is to present the results of an investigation into the effect of injection molding process parameters on the performance of direct metal laser sintered (DMLS) mold in producing quality Zytel nylon 66 plastic parts with consistency in part shrinkage and shot/part weight. Design/methodology/approach - The injection mold for an industrial component (hub gear) was fabricated in EOS M-250 machine using bronze-based material. The effect of four injection molding parameters (injection pressure, melt temperature, speed, and injection time) on part shrinkage and weight were studied experimentally using L-g orthogonal array. The weight of the part just after ejecting from the cavity, and the average shrinkage measured after cooling, were used in grey relational analysis technique to assess the effect of each molding parameter. Further, surface properties such as surface finish, wear, scratch and corrosion resistance tests were conducted on DMLS mold material samples, in order to evaluate its use in rapid manufacturing applications. Findings - The study found that injection speed and melt temperature have significant influence on part weight and shrinkage. The optimized molding process variables were slightly more in the case of DMLS molds as compared with the parameters suggested in the plastic datasheet. Scanning electro microscope (SEM) analysis of the mold surface after producing 5,000 glass filled Nylon 66 (Zytel) moldings did not indicate any surface degradation, confirming the use of DMLS mold in rapid manufacturing of few thousands of moldings. Research limitations/implications - The grey relational analysis does not compute the effect of any two or more variables together unlike ANNOVA. Second, this study alone is not enough to estimate life of DMLS mold, although 5,000 glass filled nylon 66 moldings are successfully produced without any damage on mold surface. Practical implications - This investigation demonstrates a generic approach of using grey relational analysis to quantify the effect of different molding process variables on selected quality parameters. This method can be easily extended for new processes and materials. The preliminary tests on surface finish, scratch, wear and corrosion resistance performed on DMLS mold samples have highlighted the need for improving surface properties to enhance their life. The authors are currently working on hard coating of DMLS molds as one of the solutions. Originality/value - Use of grey relational analysis is new to the problem of injection molding process optimization. Moreover, effect of injection molding parameters on part weight and shrinkage in DMLS mold has not been studied previously. This study helps while considering DMLS molds for manufacturing few thousands of parts.