یک مدل هزینه / سود برای سرمایه گذاری در موجودی و نگهداری و تعمیرات پیشگیرانه در یک سیستم تولیدی ناقص

In this research, a cost/benefit model is developed for supporting investment strategies about inventory and preventive maintenance in an imperfect production system. The effect of such investments on the return is expressed as a function of measurable variables. Using this model, the decision maker can decide whether investments in inventory and preventive maintenance are necessary and how much to invest. This investment model is developed for an imperfect production system with imperfect product quality and supplied quantity. Investments in inventory and preventive maintenance increase service level for the customer and reduce the proportion of defective products, and hence affect stockout and backlog of supplied products and the delivery time to the customer. This model includes in its scope investment in inventory and preventive maintenance, manufacturing cost, inventory cost, backlog cost, stockout cost, and delay cost. This model can be used to evaluate the effects of investments on the financial cost/benefit and other relevant critical performance measures. This model can be solved by an iterative process using the Sequential Quadratic Programming Method. The optimal investment in inventory with respect to the service level and the optimal investment in preventive maintenance with respect to the proportion of defective items can be obtained first, and then other relevant costs can also be obtained.

Inventory can be used to protect the manufacturer against the randomness in production, respond to variable customer demand, and keep higher availability of goods to maintain high quality customer service. The amount of inventory needed should depend on the safety stock as so to protect against the demand uncertainty, and to achieve a high service level for satisfying customers' demand. Thus, the proper inventory level should be set based on the relationship between the investment in inventory and the service level. On the other hand, the investment in preventive maintenance will reduce the process variance and the deviation of the process mean from the target value of the measured quality characteristic, and hence reduce the proportion of defective items. However, few efforts reported in the literature aggregately link the investment in inventory to service level and link the investment in preventive maintenance to proportion of defective items. Most of the researchers focus on a perfect manufacturing system and a perfect service level, and do not present the effect of the service level and the proportion of defective items on relevant performance measures and costs. In this research, the investment model is developed for an imperfect production system with imperfect product quality and imperfect supplied quantity. The investment in inventory increases service level for the customer, and hence affects stockout and backlog situation of supplied products. The investment in preventive maintenance reduces the proportion of defective items, and also affects the delivery time to the customer. In this paper, the investment in preventive maintenance and the investment in inventory are jointly linked to relevant performance measures related to quality, delivery time, service, inventory, and costs in an imperfect production system. The development of cost/benefit models for supporting investment strategies in inventory and preventive maintenance is crucial because it can help manufacturers in evaluating the effectiveness of their investments and in selecting optimal investment opportunities. The impact of the investment on cost/benefit should be considered and related to management performance of a company, e.g. financial performance, so that the investor can select optimally from alternative projects, including quality improvement projects, productivity improvement projects, and customer satisfaction projects. Investments in inventory and preventive maintenance should be based on their impact on quantified measures of performance, e.g. service level and quality. Therefore, what is needed is a way of expressing the quantified performance measures as a function of investments in inventory and preventive maintenance.

A cost/benefit model is developed for supporting investment strategies in inventory and preventive maintenance in imperfect production system in order to increase product and service quality. Investments in inventory and preventive maintenance can increase service level and reduce defects respectively, and hence affect stockout cost, backlog costs, and relevant costs in an imperfect production system with imperfect product quality and supplied quantity. In this research, analytical models are developed to quantify the effects of investment in preventive maintenance and inventory projects on tangible performance measures. Our approach is unique due to the hierarchical structure used in studying the effect of investments. The specific investment opportunities are the investment in preventive maintenance and the investment in inventory, which are at the first level. The second level includes items that are immediately affected by the investments of the fist level. These include the service level and the proportion of defective items. Safety stock is included at level three. The manufacturing quantity at level four affects the delivery time at level five. The next level includes the internal and external costs related to manufacturing cost, inventory cost, stockout cost, backlog costs, and delay cost. The final result of this research includes an aggregate cost model that can capture the return on the investment in preventive maintenance and inventory projects in one common metric. The service level and the proportion of defective items are the decision variables that affect the amount of investments and relevant costs. The critical performance related to quality, costs, delivery time, inventory, and service are also affected. This structure enables the user to include a wider range of cost elements than traditional models. The final quality investment models are used to predict the benefits of investment before it is made and justify investment in quality improvement projects. The resource allocation model can help the industries to make optimal selection of quality improvement projects for investment. The Sequential Quadratic Programming (SQP) method can be used to solve this resource allocation problem. Some researchers have shown that the SQP method outperforms every other tested method in terms of efficiency, accuracy, and percentage of successful solutions over a large number of test problems.