自有與外租兩倉庫存貨決策之研究

Abstract

本文所探討的是製造業完成品存貨的批量與租倉決策問題。基本的經濟製造批量模式(Economic Production Quantity, EPQ)是在企業倉容充足的假設下，求取存貨系統中整備成本與存貨持有成本等相關成本最低的製造批量，但當企業面臨倉容不足時，批量決策上除自有倉庫持有成本需考慮外，外租倉庫的租倉成本亦成為影響決策的重要因素之一，二成本間的差異並將影響到批量大小與租倉程度，此時的存貨決策與基本經濟製造批量模式明顯有所不同，文獻上習以兩倉庫存貨決策稱之。 此論文之主題為批量與租倉決策，以製造業為探討對象，並依決策內涵之不同將論文中的模式區分為固定產品需求與時間相依產品需求兩大類，在這兩類下分別構建模式推廣前人的兩倉庫存貨研究: (一)、思考Sarma(1987)以及Pakkala & Achary(1992)的固定產品需求兩倉庫存貨決策問題，將存貨週期做為決策變數，構建一個能同時決定最佳存貨週期與缺貨水準的製造業存貨決策模式。若依Naddor(1966)對存貨系統的分類，本部份所構建的存貨模式將是一個「批量與存貨水準」(lot size-order level)模式。 (二)、延申Goswami & Chaudhuri(1992)的兩倉庫模式，構建製造業時間相依需求兩倉庫存貨決策模式，並假設自有與外租倉庫皆可直接發貨，此模式並未限定所適用的時間相依產品需求函數類別，將較Goswami & Chaudhuri的存貨模式為一般化。 我們在整備成本、存貨持有成本、儲存損耗、產銷環境與/或缺貨成本等因素的考慮下，構建兩倉庫存貨決策模式，並透過範例，說明模式的性質與操作過程，經由模式解析與範例說明得到諸多性質可做為存貨管理者面臨批量與租倉決策時之參考。

This thesis explores the two-warehouse inventory decision issue of finished goods for a manufacturer that adopts central warehousing policy. To this kind of manufacturers, providing storage space is more important function for a warehouse than its location. Furthermore, storage spaces are usually limited in the plant site due to land scarcity, creating a frequent problem of capacity shortage in the own warehouse and making it necessary to rent an external public warehouse. Consequently, a manufacturer has to deal with the issue of simultaneous management of the own warehouse and the rented warehouse. This thesis explores such a two-warehouse strategy for a business. As the rented warehouse in a two-warehouse inventory system incurs out-of-pocket cost, it is mainly used to store inventory overflows not accommodated by the own warehouse; and due to limited space of the own warehouse, the space to rent must be simultaneously determined with the lot size for each production cycle. To be more specific, this thesis studies the two-warehouse inventory decision problem facing a manufacturer with limited space in its own warehouse. Extended models are proposed to enhance previous researches in dealing with both constant and varying demand functions: (1) Extend the two-warehouse inventory system with constant demand studied by Sarma (1987) and Pakkala & Achary (1992), to explore the impact of setup cost on the optimal inventory policy. A manufacturer inventory decision model is constructed that can simultaneously determine the optimal inventory level and shortage level. The system falls under the lot size-order level model according to Naddor's classification (1966). (2) Reconstruct Goswami & Chaudhuri's (1992) two-warehouse model with time-dependent demand to build a two-warehouse inventory system in which both the own warehouse and the rented warehouse are able to dispatch goods for a manufacturer with form-free demand function. We construct two-warehouse inventory decision models by taking into account such factors as setup cost, holding cost, deterioration cost, demand and production rate and/or shortage cost. Through examples, we explored the operation of this model and found many properties that are useful for making optimal decisions.