الفهرس | Only 14 pages are availabe for public view |
Abstract Thesis Summary A supply chain network is a set of organizations, responsible for fulfilling the downstream requests. Coordinating decisions across the supply chain network represents an important issue in supply chain operations. In an ever changing, constrained, and competitive environment, any form of flexibility or a way for industrial organizations to reconfigure their resources is a turning point for better adaptation to changes. In this thesis, two problems were investigated: first, the lot sizing coordinated decisions in constrained supply chains and second, order quantity allocation of alternative materials. The goal of lot sizing problems in supply chains is to establish a policy that would maximize profit or minimize relevant costs when implemented. The optimal policy depends on the assumptions made about the way costs are incurred, how demand is satisfied, and how constraints and limitations are faced during supply chain operations. In real industrial problems, there might be constraints on the size and number of shipments, size and number of production lots, space and monetary limitations, etc. In the present work, the first proposed model considers constrained situations seeking feasible and practical lot sizing decisions in a multi-echelon supply chain. The demand occurs continuously at constant known rate. Integer non-linear programming was used to optimize the problem. The results show that, constraints have major role in determining the lot or shipment size at each echelon. The effect of problem parameters on the optimal decisions was studied. The parameters under consideration are holding costs, shipment/set-up costs, demand, production rate, and material percent defective. It was found that, in general, increasing demand rate increases the total number of shipments per period and based on which the material and product shipment quantities are determined. However, it does not always guarantee an increase in the number of production runs per VIII period. Moreover, an increase in the integer number of shipments per production run results in a decrease of number of production runs per period. It was also found that the manufacturer is a key decision maker in deciding the lot size at different supply chain members when maximizing the total profit of the system. The second proposed model, in this thesis, addresses one of the mitigation strategies that can help in overcoming system constraints. The developed model is based on the following assumptions: 1) alternative materials can be separately manufactured and turned into same products with same quality; 2) these materials yield different scrap percentages when manufactured, 3) they are manufactured using different manufacturing times, and 4) they have different purchase prices related to their quality. The problem is modeled in mathematical integer linear programming to determine the optimal quantity mix from alternative materials to maximize profit for certain operating conditions. Analysis of a single-period model for three-echelon supply chain and two-material setting states that cost and quality are not the only drivers for orders allocation, as capacity restriction increases. Improved supply chain performance, represented by higher profit and/or higher fill rates with required quality, may be achieved by considering alternative materials. In general, capacity limitations oblige the decision maker to go for the extra production that can result from using an expensive alternative material for higher profit, unless the materials produce similar defective percentages or require similar operational conditions. Robust optimization (seeking conservative solutions) against quality variation gave same trends at lower profits. Further analysis of a multi-period model for three-echelon supply chain considering inventory at the manufacturers was conducted. It shows that, against system constraints, inventory improves the fill rate while using alternative materials improves the profit. IX Keywords: supply chains; lot sizing; integer replenishment policy; finite production; integer non-linear programming; order quantity allocation; alternative materials; imperfect quality. |