考量安全庫存下運用滾動排程於多期多產品生產規劃之研究

Abstract

隨著企業間的競爭越來越激烈，公司為了能有更多的獲利空間，除了提升收入外成本控管更是重要的目標。存貨管理對於製造業者來說是一個相當重要的課題，正確的存貨政策除了對成本有影響外更會影響顧客服務水準，存貨可以提升顧客服務水準但也會造成龐大的管理成本，而太少的存貨則容易造成缺貨的風險。生產規劃模型可以幫助工廠在有限的產能下規劃每一期最佳的生產數量，同時計算每一期最適存貨量而達到成本最小。 過去的文獻多在需求已知的情形下做生產規劃，大部分考慮到缺貨的可能性。本論文以某食品公司為例，以實際的銷售數據做需求預測並運用滾動排程於模型上；此外建立生產規劃模型時考量到安全存貨的重要性，為了降低缺貨機率而設計兩階段懲罰機制，分別是低於安全存貨的基本懲罰成本及缺貨所造成的額外懲罰成本。 利用本論文所建立的模型，考量生產規劃時發生的不同成本項目，並對結果進行分析，此外也對不同參數做敏感度分析研究其影響，而得出結論如下： 1. 透過兩階段懲罰機制，工廠可以在產能的限制下且在一定服務水準之上，妥善運用其生產規劃，並在存貨持有數量與缺貨的權衡中做最好的決策。 2. 隨著安全庫存水位的提高，總成本與基本懲罰成本也會提高且增加的幅度皆有上升的趨勢。而安全庫存設定的改變對於缺貨沒有影響。 3. 服務水準的調整對於總成本來說影響很小。但從75%服務水準提升至85％時缺貨的情形獲得大幅度的改善，若進一步提高至95%，缺貨情形則維持不變。 4. 隨著基本懲罰成本的提高，總成本與存貨持有成本也會隨之提高，但是總成本上升的幅度變小。而缺貨的情形則沒有影響。

Cost management is important to generate higher profit for companies in intense competitive environments. Inventory management is especially vital for manufacturers, and a good inventory policy will not only influence cost but also customer service. While customer service levels will rise, it will also cause huge management costs. However, less inventory will easily generate a higher probability of backlog. Production planning provides factories a method to effectively plan the production units under limited capacity and meanwhile calculate the best inventory level each period to minimize total costs. In the past study of production planning, demand levels are provided and most of them consider the possibility of shortage. For example, a food company is examined in this study, where future demand forecasting is based on real sales data with a rolling schedule applied to the model. Besides this, to lower the probability of shortages, while establishing a model of production planning we consider the importance of safety stock, design two-stage penalty mechanism, basic penalty cost and extra penalty cost. Considering different cost items in this study, we implement the model and analyze the impact by sensitive analysis as well, and conclude as following paragraph: 1. Under a two-stage penalty mechanism, factories can effectively plan production schedule and make better decisions based on tradeoffs between holding inventory and backlog. 2. With the increase of safety stock, total cost and basic penalty costs will rise at an increasing rate. However, safety stock has no influence on backlog. 3. The adjustment of service level only has a minor impact on total costs. While the service rate moved from 75% to 85%, it shows a great improvement on backlog, but no further changes happen if the rate advances to 95%. 4. With the rise of basic penalty cost, both total cost and holding cost will increase, but total cost increase at a decreasing rate. On the other hand, change of basic penalty cost has no relation to backlog.