A joint economic production quantity (EPQ) and delivery quantity model for a production system is investigated in this paper. More specifically, an EPQ policy is implemented in the production system, while a smaller shipping quantity is periodically dispatched to the customer. The production system is also responsible for the shipment cost, i.e. a delivery price-based procurement from the customer. The considered cost includes setup cost to launch the batch production, inventory carrying cost, and transportation cost, where the transportation cost is a function of the delivery quantity. A per unit time cost model is developed and analysed to determine the optimal production and delivery quantities. Under some mild conditions, it can be shown that the joint cost function is convex with respect to the production quantity; and the number of delivery is an integer in each replenishment cycle. Computational study has demonstrated the significant impact of the joint decision model on the operating cost. In particular, the reduction in total cost can be more than 15% when inventory carrying costs, and/or transportation costs, are high.Keyword: economic production quantity; periodic delivery; transportation cost; joint cost model
IntroductionThe inventory replenishment problem of simultaneously determining optimal production and delivery strategies in a production facility is addressed in this paper. This study is motivated by the following observations. A sequential decision process is most often observed in the production-replenishment-delivery operation management. First, an economic production quantity is determined in the production system (as a supplier) at the upper echelon, with the objective to minimise the production-inventory cost to meet the demand. This is often addressed in the well-known production inventory models. Second, the optimal shipping quantity to the customer at the lower echelon is determined to minimise the delivery or transportation cost, which can be frequently found in the study of logistic or transportation management. Third, the integration of production (supply) and delivery (demand) in these systems is often stressed and discussed in many researches, but lack of a formal methodology to address the coordinated production-replenishment-delivery problem. The popularity of this decision process suggests that a joint analytical decision model that integrates both production and delivery strategies with transportation cost will be useful.The economic production quantity (EPQ) model, e.g. see Hadley and Whitin (1963) or Silver, Pyke, and Peterson (1998), is a classical inventory replenishment policy where both production and demand rates are assumed deterministic and constant. Each replenishment cycle starts when there is no on-hand inventory and a fixed quantity (lot size or batch) is launched in the production facility. An analytical model is developed to determine the optimal EPQ so that the sum of setup cost and inventory carrying cost is minimised. For an extended study of implementing thi...