以再訂購點與訂購量為決策變數下TFT-LCD設備零件存貨模式之研究

Abstract

設備零件的存貨並非是出售給客戶的終端產品，因此其庫存管理方法也不同於在製品庫存或其他的庫存。而TFT-LCD設備的零件具有種類繁多、使用頻率差異大…等特性，而實務上也會依照零件的需求頻率與零件價格採用不同的庫存管理政策來管理。在使用頻率高的零件大多假設其需求呈現常態分配，但是容易忽略了零件需求的產生與設備發生故障的機率有密切關係；而卜瓦松分配較適合用於估計設備發生故障的機率。本論文採用永續盤存制中的(s,Q)存貨政策，針對快速流動的零件提出一簡單存貨模式，在供應商的前置時間確定下假設需求呈現複合卜瓦松(Compound Poisson)分配，來推導出最適的安全庫存因子進而決定最適的再訂購點與最適訂購量使得整體的採購成本為最小。研究的結果顯示： (1). 平均需求數量直接的影響了再訂購點、再訂購量與總成本。 (2). 前置時間的縮短對總成本的下降，並無顯著的幫助，但可以有效的降低庫存水準，並且使每次的訂購量下降。 (3). 當缺貨成本增加，會使得再訂購點上升而訂購量下降。因此當訂購成本低而缺貨成本高時，採購最適當的備料模式會趨向於 (S-1,S)，即缺一補一模式。

Spare parts inventories are not the final products to be sold customers and the policy which govern spare parts inventories are different from WIP and some other inventories. Spare parts of TFT-LCD equipment have characteristics of multi-items, high difference of usage and high difference of prices; hence managers usually use different inventory policy according to the usage and price of spare parts to govern spare parts inventories in practice. For those fast-moving items, most literatures assume that the demand is Normal distribution. It will overlook the relation between the demand of spare parts and malfunction of equipment. In this study, we develop a simple inventory model for the fast-moving spare parts under the continuous (s,Q) inventory policy, lead-time constant and demand follows compound Poisson distribution . Then, we find the optimal reorder point and the reorder quantities that make total cost minimal. The study result shows: (1). Average demand quantities affect reorder point, reorder quantity and total cost directly. (2). Lead time reduction does not mean the total cost reduction, but can help to reduce inventory level and reorder quantities. (3). The shortage cost increasing will lead the reorder point up and reorder quantities down respectively. Therefore, when the ordering cost is low and the shortage cost is high, the optimal purchasing decision will close to (S-1,S), that means lot-for-lot policy.