Robust process design for enhancing manufacturing performance

Abstract

Brakes India Ltd. Foundry Division is the largest exporter of automotive Iron castings from India. About 7000 tonnes per annum of automotive castings are being exported to U.K- Germany and France. The majority of the automotive castings that are being exported are caliper brake castings. The average weight of these castings is about 3 Kg and these castings require to meet string ent quality demands. As a result, only lower yield of about 40% is achieved with these castings. Foundry division has been supplying two bridge caliper castings to LUCAS Bouzon ville, France for the past 6 months. The total business potential for these castings is about 350 tonnes per month, out of which the present Brakes India foundry's share is 160 tonnes per month. There is an opportunity for increasing the share by offering the most competitive price. However, the current yield of these castings has been low, about 30%, incurring high manufacturing cost. The international casting prices are extremely competitive and there is persistent pressure from the customers to offer annual price reduction. Therefore, to remain price competitive, there is an immediate need for reducing the manufacturing cost. A significant reduction in manufacturing cost can only be achieved by improving the yield. A modified version of the bridge pattern has been expected to give an overall yield of 40%. The trials taken on this pattern in the past gave inconsistent results in terms of quality characteristics viz. casting dimensions and internal soundness. It is utmost important to achieve consistency in these characteristics. The strategy is to minimize manufacturing cost while delivering a specified quality level product. This requires optimization of process parameters that has influence on these quality characteristics. However, the relation between the numerous parameters and the response is not known and must be observed experimentally. It has been suggested to use robust process design technique to identify the significant parameters at appropriate levels from a large number of process. parameters. The objective is to achieve the highest yield and at the same time minimizing the product-to-product variation in the customer specified characteristics like dimensions, mechanical and metallurgical properties and internal soundness. Another important aspect of the need for the study is that by identifying the parameters and their levels controlling the variability in the process, it would be possible to optimize the yield. Currently, larger than the optimum levels of metal feeding parameters are being used as compared to the benchmarked values to accommodate the fluctuations in quality levels from batch to batch and time to time. The successful implementation of the robust process design techniques for all other products would improve the overall yield of the foundry. New product introduction cycle time can also be reduced by way of achieving the product characteristics in the least number of trials and corrections. Taguchi's parameter design concept is used to determine the best settings for the parameters that have minimum effect on the manufacturing cost. The objective is to minimize the sensitivity of the quality characteristic to the noise factors and bring the mean value on to the target. If at the end, the quality characteristics meet the specification requirement with minimum variability, a lowest cost product can be produced. By using the techniques of orthogonal arrays and Signal-to-noise ratios, which are an integral part of robust design method, the experimental cost is reduced substantially.