Modul ke: 10 Manajemen Persediaan MRP Lot For Lot Fakultas FEB Program Studi Manajemen
Learning Objectives Describe the conditions under which MRP is most appropriate. Describe the inputs, outputs, and nature of MRP processing. Explain how requirements in a master production schedule are translated into material requirements for lower-level items. Discuss the benefits and requirements of MRP.
Learning Objectives Explain how an MRP system is useful in capacity requirements planning. Outline the potential benefits and some of the difficulties users have encountered with MRP. Describe MRP II and its benefits. Describe ERP, what it provides, and its hidden costs.
MRP Material requirements planning (MRP): Computer-based information system that translates master schedule requirements for end items into time-phased requirements for subassemblies, components, and raw materials. End products MRP Time-phase based Subassemblies Components Raw materials
Independent and Dependent Demand Independent Demand A Dependent Demand B(4) C(2) D(2) E(1) D(3) F(2) Independent demand is uncertain. Dependent demand is certain.
Dependant Demand Dependent demand: Demand for items that are subassemblies or component parts to be used in production of finished goods. Once the independent demand is known, the dependent demand can be determined.
Amount on hand Amount on hand Demand Demand Demand Stable demand Lumpy demand Time Time Safety stock Time Time
Overview of MRP MRP Inputs MRP Processing MRP Outputs Master schedule Bill of materials MRP computer programs Primary reports Secondary reports Changes Order releases Planned-order schedules Exception reports Planning reports Performancecontrol reports Inventory records Inventory transaction
MPR Inputs Master Production Schedule Time-phased plan specifying timing and quantity of production for each end item. Material Requirement Planning Process
Master Schedule Master schedule: One of three primary inputs in MRP; states which end items are to be produced, when these are needed, and in what quantities. Cumulative lead time: The sum of the lead times that sequential phases of a process require, from ordering of parts or raw materials to completion of final assembly.
Planning Horizon Assembly Subassembly Procurement Fabrication 1 2 3 4 5 6 7 8 9 10 Time Period (weeks)
Bill-of-Materials Bill of materials (BOM): One of the three primary inputs of MRP; a listing of all of the raw materials, parts, subassemblies, and assemblies needed to produce one unit of a product. Product structure tree: Visual depiction of the requirements in a bill of materials, where all components are listed by levels. Low-level coding: Restructuring the bill of materials so that multiple occurrences of a component all coincide with the lowest level the component occurs
Level 0 Product Structure Tree Chair 1 Leg Assembly Seat Back Assembly 2 Legs (2) Cross bar Side Rails (2) Cross bar Back Supports (3) Level 0 X X 1 B (2) C B (2) C 2 D (3) E E (2) F (2) D (3) E E (2) F (2) 3 E (4) E (4) E E (2)
Inventory Records One of the three primary inputs in MRP Includes information on the status of each item by time period Gross requirements Scheduled receipts Amount on hand Lead times Lot sizes And more
Inventory Requirements Net requirements: Net Requirements = Gross Requirements Available Inventory Available Inventory: Available Inventory = Projected on hand Safety stock Inventory allocated to other items
Assembly Time Chart Procurement of raw material D Procurement of raw material F Fabrication of part E Procurement of part C Procurement of part H Subassembly A Subassembly B Final assembly and inspection Procurement of raw material I Fabrication of part G 1 2 3 4 5 6 7 8 9 10 11
Time-Phased Product Structure Start production of D Must have D and E completed here so production can begin on B Lead time 2 weeks 1 week G (1) D (2) 2 weeks 1 week 3 weeks 2 weeks D (2) E (2) 2 weeks to produce E (2) 1 week F (2) B (2) 1 week C (3) 1 2 3 4 5 6 7 8 Time in weeks A A B C D E F G 1 2 1 1 2 3 2 weeks weeks weeks weeks weeks weeks weeks
Gross Requirements Plan Week 1 2 3 4 5 6 7 8 Lead Time A. Required date 50 Order release date 50 1 week B. Required date 100 Order release date 100 2 weeks C. Required date 150 Order release date 150 1 week E. Required date 200 300 Order release date 200 300 2 week F. Required date 300 Order release date 300 3 weeks D. Required date 600 200 Order release date 600 200 1 week G. Required date 300 Order release date 300 2 week
MRP Structure Data Files BOM Lead times (Item master file) Master production schedule Output Reports MRP by period report MRP by date report Planned order report Inventory data Purchasing data Material requirement planning programs (computer and software) Purchase advice Exception reports Order early or late or not needed Figure 14.5 Order quantity too small or too large
Determining Gross Requirements Starts with a production schedule for the end item 50 units of Item A in week 8 Using the lead time for the item, determine the week in which the order should be released a 1 week lead time means the order for 50 units should be released in week 7 This step is often called lead time offset or time phasing
Determining Gross Requirements From the BOM, every Item A requires 2 Item Bs 100 Item Bs are required in week 7 to satisfy the order release for Item A The lead time for the Item B is 2 weeks release an order for 100 units of Item B in week 5 The timing and quantity for component requirements are determined by the order release of the parent(s)
Determining Gross Requirements The process continues through the entire BOM one level at a time often called explosion By processing the BOM by level, items with multiple parents are only processed once, saving time and resources and reducing confusion Low-level coding ensures that each item appears at only one level in the BOM
Net Requirements Plan
Net Requirements Plan
MRP Processing Gross requirements Schedule receipts Projected on hand Net requirements Planned-order receipts Planned-order releases
MPR Processing Gross requirements Total expected demand Scheduled receipts Open orders scheduled to arrive Projected on hand Expected inventory on hand at the beginning of each time period
MPR Processing Net requirements Actual amount needed in each time period Planned-order receipts Quantity expected to be received at the beginning of the period Offset by lead time Planned-order releases Planned amount to order in each time period
Lot-Sizing Techniques Lot-for-lot techniques order just what is required for production based on net requirements May not always be feasible If setup costs are high, costs may be high as well Economic order quantity (EOQ) EOQ expects a known constant demand and MRP systems often deal with unknown and variable demand
Lot-Sizing Techniques Part Period Balancing (PPB) looks at future orders to determine most economic lot size EPP = setup cost / holding cost Programming technique Assumes a finite time horizon Effective, but computationally burdensome
Lot-for-Lot Example Week 1 2 3 4 5 6 7 8 9 10 Gross requirements Scheduled receipts Projected on hand Net requirements Planned order receipts Planned order releases 35 30 40 0 10 40 30 0 30 55 35 35 0 0 0 0 0 0 0 0 0 0 30 40 0 10 40 30 0 30 55 30 40 10 40 30 30 55 30 40 10 40 30 30 55 Holding cost = $1/week; Setup cost = $100/times; Lead time = 1 week
Gross requirements Scheduled receipts Projected on hand Lot-for-Lot Example No on-hand inventory is carried through the system Total holding cost = $0 Net requirements Planned order receipts 1 2 3 4 5 6 7 8 9 10 There are seven setups for this item in this plan Total setup cost = 7 x $100 = $700 Planned order releases 35 30 40 0 10 40 30 0 30 55 35 35 0 0 0 0 0 0 0 0 0 0 30 40 0 10 40 30 0 30 55 30 40 10 40 30 30 55 30 40 10 40 30 30 55 Holding cost = $1/week; Setup cost = $100/times; Lead time = 1 week
EOQ Lot Size Example Week 1 2 3 4 5 6 7 8 9 10 Gross requirements Scheduled receipts Projected on hand Net requirements Planned order receipts Planned order releases 35 30 40 0 10 40 30 0 30 55 35 35 0 43 3 3 66 26 69 69 39 0 30 0 0 7 0 4 0 0 16 73 73 73 73 73 73 73 73 Holding cost = $1/week; Setup cost = $100/times; Lead time = 1 week Average weekly gross requirements = 27; EOQ = 73 units
Scheduled receipts Projected on hand Planned order receipts Planned order releases EOQ Lot Size Example Annual demand = 1,404 Total cost = setup cost + holding cost Total cost = (1,404/73) x $100 + (73/2) x ($1 x 52 weeks) Total Gross cost = $3,798 /year requirements Cost for 10 weeks = $3,798 x (10/52) = $730 Or 1 2 3 4 5 6 7 8 9 10 35 30 40 0 10 40 30 0 30 55 35 35 0 0 0 0 0 0 0 0 0 Total Net cost = setup cost + holding cost requirements Total cost = 4 x $100 + 318 x ($1 /weeks) Total cost = $718 0 30 0 0 7 0 4 0 0 16 73 73 73 73 73 73 73 73 Holding cost = $1/week; Setup cost = $100/times; Lead time = 1 week Average weekly gross requirements = 27; EOQ = 73 units
PPB Example 1 2 3 4 5 6 7 8 9 10 Gross requirements Scheduled receipts Projected on hand Net requirements Planned order receipts Planned order releases 35 30 40 0 10 40 30 0 30 55 35 35 0 50 10 10 0 60 30 30 0 0 30 0 0 0 40 0 0 0 55 80 100 55 80 100 55 Holding cost = $1/week; Setup cost = $100; Lead time = 1 week ; EPP = 100 units
Gross requirements Scheduled receipts Projected on hand Net requirements Planned order receipts Planned order releases PPB Example Trial Lot Size Periods (cumulative net Costs Combined requirements) Part Periods Setup Holding Total 2 30 0 2, 3 70 1 2 40 = 340 x 14 5 6 7 8 9 10 2, 3, 4 70 40 = 40 x 1 35 30 40 0 10 40 30 0 30 55 2, 3, 4, 5 80 70 = 50 x 1 + 10 x 2 100 + 70 = 170 2, 3, 4, 5, 6 120 230 = 90 x 1 + 50 x 2 + 40 x 1 Combine periods 2-5 as this results in the Part Period closest to 35 the EPP 6 40 0 6, 7 70 30 = 30 x 1 6, 7, 8 70 30 = 30 x 1 6, 7, 8, 9 100 120 = 60 x 1 + 30 x 2 100 + 120 = 220 Combine periods 6-9 as this results in the Part Period closest to the EPP 10 55 0 100 + 0 = 100 Total cost 300 + 190 = 490 Holding cost = $1/week; Setup cost = $100; EPP = 100 units
Lot-Sizing Summary For these three examples Lot-for-lot $700 EOQ $730 PPB $490 Wagner-Whitin would have yielded a plan with a total cost of $455 for this example
Example 1 2 3 4 5 6 7 8 9 10 Gross requirements Scheduled receipts Projected on hand Net requirements Planned order receipts Planned order releases 35 30 40 0 10 40 30 0 30 55 35 35 0 50 10 10 0 30 0 0 55 0 30 0 0 0 40 0 0 30 0 80 70 85 80 70 85 Holding cost = $1/week; Setup cost = $100; Lead time = 1 week ; EPP = 100 units
Lot-Sizing Summary In theory, lot sizes should be recomputed whenever there is a lot size or order quantity change In practice, this results in system nervousness and instability Lot-for-lot should be used whenever economical Lot sizes can be modified to allow for scrap, process constraints, and purchase lots
Lot-Sizing Summary Use lot-sizing with care as it can cause considerable distortion of requirements at lower levels of the BOM When setup costs are significant and demand is reasonably smooth, PPB, Wagner-Whitin, or EOQ should give reasonable results
Updating the System Regenerative system Updates MRP records periodically Net-change system Updates MPR records continuously
MRP Primary Reports Planned orders - schedule indicating the amount and timing of future orders. Order releases - Authorization for the execution of planned orders. Changes - revisions of due dates or order quantities, or cancellations of orders.
MRP Secondary Reports Performance-control reports system evaluation, deviation, late delivery, stockouts Planning reports useful for forecasting future inventory, assess future material requirement Exception reports late or overdue orders, excessive scrap rate, requirement of nonexisting parts
Material Checking & Balancing Use for monitoring of amount of part and product during processes Needs information to balance materials Accumulative production planning or target plan BOM or Assembly diagram Normally periodic checked
Accumulative production Material Checking & Balancing B C G Assembly diagram D A E H I J K F 9 8 7 6 5 4 3 2 1 0 Target Plan Week Balancing chart J F M A M J J A S O N D A B C D E F G H I J K
Standard labor hours Standard labor hours Resource Requirements Profile 200 Capacity exceeded in periods 4 & 6 200 Lot 6 split Lot 11 moved 150 100 50 Lot 1 Lot 2 Lot 3 Lot 4 Lot 5 Lot 6 Lot 7 Lot 8 Lot 11 Lot 9 Lot 12 Lot 10 Lot 13 Available capacity Lot 15 Lot 14 Lot 16 1 2 3 4 5 6 7 8 Period (a) 150 100 50 Lot 1 Lot 2 Lot 3 Lot 4 Lot 5 Lot 6 Lot 7 Lot 8 Lot 9 Lot 12 Lot 10 Lot 13 Available capacity Lot 11 Lot 15 Lot 14 Lot 16 1 2 3 4 5 6 7 8 Period (b)
Smoothing Tactics 1. Overlapping Sends part of the work to following operations before the entire lot is complete Reduces lead time 2. Operations splitting Sends the lot to two different machines for the same operation Shorter throughput time but increased setup costs 3. Lot splitting Breaking up the order into smaller lots and running part ahead of schedule
Other Considerations Safety Stock Lot sizing Lot-for-lot ordering Economic order quantity Fixed-period ordering
MRP in Services Food catering service End item => catered food Dependent demand => ingredients for each recipe, i.e. bill of materials
Benefits of MRP Low levels of in-process inventories Ability to track material requirements Ability to evaluate capacity requirements Means of allocating production time Ability to easily determine inventory usage by backflushing Backflushing: Exploding an end item s bill of materials to determine the quantities of the components that were used to make the item.
Requirements of MRP Computer and necessary software Accurate and up-to-date Master schedules Bills of materials Inventory records Integrity of data
MRP II Expanded MRP with emphasis placed on integration Financial planning Marketing Engineering Purchasing Manufacturing
Adjust master schedule MRP II Finance Market Demand Manufacturing Master production schedule Marketing Production plan MRP Adjust production plan Rough-cut capacity planning Capacity planning Yes No Requirements No Yes Problems? Problems? schedules
ERP Enterprise resource planning (ERP): Next step in an evolution that began with MPR and evolved into MRPII Integration of financial, manufacturing, and human resources on a single computer system.
ERP Software ERP software provides a system to capture and make data available in real time to decision makers and other users in the organization Provides tools for planning and monitoring various business processes Includes Production planning and scheduling Inventory management Product costing Distribution
MRP in Services Service applications such as: Professional services Postal services Retail Banking Healthcare Higher education Engineering Logistical services Real estate
ERP Strategy Considerations High initial cost High cost to maintain Future upgrades Training
Daftar Pustaka Richardus Eko Indrajit, (2005), Manajemen Persediaan, Grasindo, Jakarta Heizer Jay, B.Rander, (206), Manajemen Operasi, Salemba Empat, Jakarta Hani handoko, (2002), Manajemen Produksi dan Operasi, BPFE, Yogyakarta Siswanto, (2005), Riset Operasi, Erlangga, Jakarta M. Syamsul Ma arif, (2003), Manajemen Operasi, Grasindo, Jakarta Sofyan Assauri, (2001), Manajemen Operasi, BPFE, Jakarta Martinich, (2003), Operation Manaement, Prenice hall, New Yory
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