INVENTORY CONTROL IR.H.DADANG HENDRIANA,M.SC

Definisi : Persediaan : bahan atau barang yang di simpan untuk penggunaan lebih lanjut, baik untuk di jual maupun sebagai masukan dari proses produksi.

ITEM INVENTORY MANAGEMENT Hal-hal yang perlu diputuskan : Item persediaan yang penting Bagaimana pengendalian per item Ukuran order (order quantity) Waktu pemesanan

FUNGSI INVENTORY Menyelaraskan antara penyediaan dan kebutuhan Menyelaraskan kebutuhan konsumen dengan barang jadi Menyelaraskan barang jadi dengan komponen pembentuknya Menyelaraskan kebutuhan operasi dengan output dari operasi sebelumnya Menyelaraskan komponen dan bahan baku dengan supplier material

Tujuan Manajemen Persediaan (Inventory Management) Maximum, customer service Low cost plant operation Minimum Inventory Investment

INVENTORY COST

INVENTORY COST ORDER COST. CARRYING COST. ITEM COST/PURCHASING COST STOCK-OUT COST

INVENTORY CARRYING COST CAPITAL COST. INVENTORY SERVICE COST. STORAGE SPACE COST. INVENTORY RISK COST.

Inventory Model Inventory Model dibangun sesuai dengan kharakteristik material/komoditi yang di simpan Klasifikasi Material Berdasarkan Proses Produksi : - Bahan Baku (input) - Barang Setengah Jadi - Barang Jadi Klasifikasi Material Berdasarkan Frekuensi Pemakaian : - Fast Moving - Slow Moving - Dead Stock

Inventory Model Klasifikasi Material Berdasarkan Nilai dan Jumlah nya (Klasifikasi ABC atau Pareto Classification) - Material Kelas A : Jumlah item sedikit (1 – 10% ), nilai material tinggi (40 – 70 %) - Material Kelas B : Jumlah item sedang ( 10 - 30 %, nilai material (10 – 30 %) - Material Kelas C : Jumlah item banyak ( 40 – 70 %), Nilai material rendah ( 1 – 10 %)

Inventory Model Klasifikasi Material Berdasarkan “Cara menentukan kebutuhan material” : - Independent Materials : kebutuhan yang satu tidak berkaitan dengan kebutuhan yang lain ( contoh : Atk, computer supplies, spareparts untuk maintenance, dsb) - Dependent Materials : Kebutuhan yang satu ditentukan oleh kebutuhan yang lain seauai dengan Struktur produk (Bills of materials), terdapat pada dua keadaan : material proyek, serta material untuk produksi barang-barang berstrukrur.

PENENTUAN ECONOMIC ORDER QUANTITY, RE ORDER POINT & SAFETY STOCK

Perhitungan : Economic Order Quantity ; Reorder Point dan Safety untuk “ Independent - material “ Fixed Order Quantity Inventory System (System-Q). Diketahui : Kebutuhan Material pertahun =3.000 unit (D) Order Cost = $500 Perkali pesan (O) Harga Material = $ 30 per unit (M) Storage Cost/Carrying Cost / Holding cost = $ 3 /unit.tahun (C) 1 tahun terdiri dari 300 hari kerja Lead time 10 hari

Rata-rata kecepatan pemakaian material adalah 10 unit perhari dengan simpangan baku (standard deviation)= 1,2 unit Service level : Diinginkan kemungkinan terjadinya “ Stock -Out” adalah 0,05 atau 5 %

Ditanya : a. Economic Order Quantity b. Reorder Point c. Safety Stock d. Total Inventory Cost Pertahun

Jawab : a. EOQ = b. Kebutuhan material pertahun = 3000 unit jumlah hari kerja pertahun = 300 hari jadi kebutuhan material rata-rata perhari = 3000 =10 unit/hari 300 Lead Time = 10 hari Reorder Point = kebutuhan selama lead time = 10 x 10 unit = 100 unit

c. Safety Stock = Z.SX Z = Nilai koefisien distribusi normal untuk sevice level tertentu. Bila diinginkan Probabilitas Stock Out = 0,05 (Probabilitas dapat terpenuhi kebutuhan material sebesar 0,95 ) maka Z= 1,65 Sx = Deviasi standard pemakian material perhari =1,2 unit t = Lead time ( dalam hari ) = 10 hari SS= 1,65x1,2 x ==== 6,26 unit

d. Total Inventory Cost pertahun = = 500 x 3000 + (3000 x 30) + (3 x 1000)+(6,26x3) 1000 2 TC= 1500 + 90000+1500 + 18,78 = 93018,78 OD x MD + CQ* + (SS). C Q* 2

Fixed Order Period inventory system ( system P) Untuk soal yang sama dengan system Q, bila “ Rate “ pemakaian material konstan, maka system P dan Q menunjukkan karakteristik yang sama, kecuali dalam penentuan safety stock nya. Dalam hal pemakaian material tidak sepenuhnya konstant, maka Order Quantity (Order size) bergantug kepada persediaan di gudang ( inventory on hand ) pada saat perioda pemesanan tiba. misalnya: Pada Suatu perioda pemesanan inventory on hand = 150 unit ( termasuk SS)

Safety stock = Z.S x Dimana t+r = waktu siklus = waktu rata-rata dari suatu pemesanan ke pemesanannya berikutnya dalam contoh t+r = EOQ kecepatan pemakaian perhari = 1000 = 100 hari 10 Dengan Faktor keamanan 0,95 maka diperoleh : SS= 1,65 x 1,2 x = 19,8 unit unit Order size = EOQ + SS - Inventroy on Hand + Pemakaian selama lead time = 1000 + 20 - 150 + 100 = 970 unit

JIT Inventory Management

JIT Inventory Management Principle Cut Lot Sizes and increase frequency of orders Cut buffer inventory Cut purchasing costs Improve material handling Seek zero inventory Seek reliable suppliers

Characteristics of a JIT Partnership Long-term contract Improved accuracy of order filling Improved quality Ordering flexibility. Small lots ordered frequently. Continuous improvement in the partnership.

Impact of JIT Cost Reduction on lot Size a. Basic large Lot EOQ Cost Graph

b. Impact of JIT Reduced Ordering Cost

C. Subsequent Impact of JIT Reduced Carrying Cost

Methodology Assumptions 1. Only one product is to be considered in the model 2. Annual total demand requirements are known 3. Annual demand usage is spread evenly to achieve a fairly constant usage or demand rate from customers 4. Order delivery lead time is constant 5. Each order is received in a single delivery 6. There are no quantity discounts

A Methodology for Implementing JIT in a EOQ Environment 1. EOQ Order Quantity (Q *) 2. EOQ Total Annual Cost (T*) =(Carrying Cost + Ordering Cost) + Q*=Cost minimizing order quantity in units under EOQ System O = Ordering costs in dollars per order D = annual demand in units C = Carrying costs in dollars per unit T*= Minimized total annual cost in dollar under EOQ system

JIT/EOQ Models 3. JIT/EOQ Order Quantity 4. JIT/ EOQ Total Annual Cost 5. JIT/EOQ Delivery Quantity

JIT/EOQ Models 7. JIT/EOQ Optimal Number of Deliveries 6. Saving by Switching to JIT/EOQ Qn = Cost minimizing JIT order quantity in units per each “n” delivery n = Optimal number of delivery during the year Q*= Cost minimizing order quantity in units under EOQ system T*= Minimized total annual cost in dollars under EOQ system q = Optimal number of units per delivery Tjit= minimized total annual cost in dollar under JIT systems nm= Optimal number of delivery with “m” maximum inventory capacity limitation Q*=Cost minimizing order quantity in units under EOQ system m=maximum inventory capacity level

JIT/EOQ Models 8. JIT/EOQ Optimal Number of Delivery na= Optimal number of deliveries with “a” targeted level of average on-hand inventory Q*=Cost minimizing order quantity in units under EOQ system a= a specific targeted average inventory level in units 9. JIT/EOQ Optimal Number of Delivery np= Optimal number of delivery with a prespecified percentage of “p” saving in total costs p= a desired prespecified percentage of total costs savings

To illustrate the JIT/EOQ modeling or order quantities, let’s look at an example. An organization is currently using a large-lot EOQ system of ordering units of a single item inventory component. Their product is sold on a contractual basis and yearly demand is known to be 40,000 units. The carrying costs per unit is $1 and the per order costs of purchasing the units is $50. The firm decides that they want to shift their operation to a JIT system. To start the JIT implementation, a long-term JIT purchasing contract has just been signed committing the organization to an overall quantity to be delivered during a period of a year. The overall quantity is based on known demand of the organization’s customers and the known production capacities of the manufacturing operation. The supplier is flexible as to when and how many units will be shipped, but requires that once the number is fixed, the order quality will remain fixed for a designated period of time. The supplier is also willing to make as many deliveries as requested by the purchaser, but again, once the delivery quantity is set, it should remain set for a mutually agreeable period of time. Given this background information, let’s look at the following examples :

Example 1 What are the optimal EOQ quantity and total annual cost ? As we can see the resulting EOQ is 2,000 units, which in turn results in a total annual cost of $2,000. In this system the lot size of the delivery quantity is the EOQ of 2,000 units EOQ Order Quantity (Q*) EOQ Total Annual Cost (T*)

Example 2 What would the optimal JIT/EOQ order quantity be if the firm has chosen to reduce the lot size by splitting the order quantity into two deliveries (I.e., n=2) ? JIT/EOQ Order Quantity (Qn) JIT/EQO Total Annual Cost (Tjit)

JIT/EOQ Delivery Quantity (q) Saving by Switching to JIT/EIQ(S)

Example 3 What would the optimal JIT/EOQ order quantity be if the firm has s maximum inventory storage limit of only 500 units ? JIT/EOQ Optimal Number of Deliveries (nm) JIT/EOQ Order Quantity (Qn) JIT/EOQ Total Annual Costs (Tjit)

Example 4 What would the optimal JIT/EOQ order quantity be if the firm has set an average inventory target of having no more than 200 units on hand? The number of deliveries has to increased to 25 achieve the 200 units goal. The resulting JIT/EOQ optimal order quantity than becomes 10,000 units, resulting in a total annual cost of only $400. This again reinforces the JIT Principle of how reduction in on-hand inventory can reduce costs. JIT/EOQ Optimal Number of Delivery (na)

JIT/EOQ Order Quantity (Qn) JIT/EOQ Total Annual Cost (Tjit)

Example 5 What would the optimal JIT/EOQ Order quantity be if the firm has prespectified a desired to achieve a 30 percent total cost saving from the original EOQ T* of 2,000? JIT/EOQ Optimal Number of Deliveries (np) JIT/EOQ Order Quantity (Qn) JIT/EOQ Total Annual Cost (Tjit)

Implementing Strategy for JIT Purchasing Commitment Changing the system Selection of Suppliers Building Relationships