Modul ke: Fakultas Program Studi Manajemen Persediaan 11 FEB Manajemen MRP EOQ
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Modul ke:
Fakultas
Program Studi
Manajemen Persediaan
11 FEB
Manajemen
MRP EOQ
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.
• 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.
MRP
End products
Time-phase based
• Subassemblies
• Components • Raw materials
MRP
Independent and Dependent Demand Independent Demand
A
B(4) C(2)
D(2) E(1) D(3) F(2)
Dependent Demand
Independent demand is uncertain. Dependent demand is certain.
• 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.
Dependant Demand
Demand
Time
Time Time
Time
De
man
d
De
man
d
Stable demand “Lumpy” demand
Am
ou
nt
on
han
d
Am
ou
nt
on
han
d
Safety stock
MRP Inputs MRP Processing MRP Outputs
Master schedule
Bill of materials
Inventory records
MRP computer programs
Changes
Order releases
Planned-order schedules
Exception reports
Planning reports
Performance- control reports
Inventory transaction
Primary reports
Secondary reports
Overview of MRP
• Master Production Schedule
• Time-phased plan specifying timing and quantity of production for each end item.
• Material Requirement Planning Process
MPR Inputs
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
1 2 3 4 5 6 7 8 9 10
Procurement
Fabrication
Subassembly
Assembly
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
Product Structure Tree Chair
Seat
Legs (2) Cross bar
Side Rails (2)
Cross bar
Back Supports (3)
Leg Assembly
Back Assembly
Level 0
1
2
X
B (2)
D (3) E
C
E (2) F (2)
E (4)
X
B (2)
D (3) E
C
E (2) F (2)
E (4) E E (2)
Level
0
1
2
3
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:
• Available Inventory:
Net Requirements = Gross Requirements
– Available Inventory
Available Inventory = Projected on hand
– Safety stock
– Inventory allocated to
other items
Assembly Time Chart
1 2 3 4 5 6 7 8 9 10 11
Procurement of raw material D
Procurement of raw material F
Procurement of part C
Procurement of part H
Procurement of raw material I
Fabrication of part G
Fabrication of part E
Subassembly A
Subassembly B
Final assembly and inspection
Time-Phased Product Structure
| | | | | | | |
1 2 3 4 5 6 7 8 Time in weeks
F (2)
2 weeks
3 weeks
1 week
A
2 weeks
1 week
D (2)
E (2)
2 weeks
D (2)
G (1)
1 week
1 week
2 weeks to produce
B (2)
C (3)
E (2)
Start production of D
Must have D and E completed here so
production can begin on B Lead time
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
Figure 14.5
Output Reports
MRP by period report
MRP by date report
Planned order report
Purchase advice
Exception reports
Order early or late or not needed
Order quantity too small or too large
Data Files
Purchasing data
BOM
Lead times
(Item master file)
Inventory data
Master production schedule
Material requirement
planning programs (computer and
software)
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 35 30 40 0 10 40 30 0 30 55
Scheduled
receipts
Projected on
hand 35 35 0 0 0 0 0 0 0 0 0
Net
requirements 0 30 40 0 10 40 30 0 30 55
Planned order
receipts 30 40 10 40 30 30 55
Planned order
releases 30 40 10 40 30 30 55
Holding cost = $1/week; Setup cost = $100/times; Lead time = 1 week
Lot-for-Lot Example
1 2 3 4 5 6 7 8 9 10
Gross
requirements 35 30 40 0 10 40 30 0 30 55
Scheduled
receipts
Projected on
hand 35 35 0 0 0 0 0 0 0 0 0
Net
requirements 0 30 40 0 10 40 30 0 30 55
Planned order
receipts 30 40 10 40 30 30 55
Planned order
releases 30 40 10 40 30 30 55
No on-hand inventory is carried through the system Total holding cost = $0 There are seven setups for this item in this plan Total setup cost = 7 x $100 = $700
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 35 30 40 0 10 40 30 0 30 55
Scheduled
receipts
Projected on
hand 35 35 0 43 3 3 66 26 69 69 39
Net
requirements 0 30 0 0 7 0 4 0 0 16
Planned order
receipts 73 73 73 73
Planned order
releases 73 73 73 73
Average weekly gross requirements = 27; EOQ = 73 units
Holding cost = $1/week; Setup cost = $100/times; Lead time = 1 week
EOQ Lot Size Example
1 2 3 4 5 6 7 8 9 10
Gross
requirements 35 30 40 0 10 40 30 0 30 55
Scheduled
receipts
Projected on
hand 35 35 0 0 0 0 0 0 0 0 0
Net
requirements 0 30 0 0 7 0 4 0 0 16
Planned order
receipts 73 73 73 73
Planned order
releases 73 73 73 73
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 cost = $3,798 /year Cost for 10 weeks = $3,798 x (10/52) = $730 Or Total cost = setup cost + holding cost Total cost = 4 x $100 + 318 x ($1 /weeks) Total cost = $718
Average weekly gross requirements = 27; EOQ = 73 units
Holding cost = $1/week; Setup cost = $100/times; Lead time = 1 week
PPB Example
1 2 3 4 5 6 7 8 9 10
Gross
requirements 35 30 40 0 10 40 30 0 30 55
Scheduled
receipts
Projected on
hand 35 35 0 50 10 10 0 60 30 30 0
Net
requirements 0 30 0 0 0 40 0 0 0 55
Planned order
receipts 80 100 55
Planned order
releases 80 100 55
Holding cost = $1/week; Setup cost = $100; Lead time = 1 week ; EPP = 100 units
PPB Example
1 2 3 4 5 6 7 8 9 10
Gross
requirements 35 30 40 0 10 40 30 0 30 55
Scheduled
receipts
Projected on
hand 35
Net
requirements
Planned order
receipts
Planned order
releases
Holding cost = $1/week; Setup cost = $100; EPP = 100 units
2 30 0 2, 3 70 40 = 40 x 1 2, 3, 4 70 40 = 40 x 1 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 the EPP
Combine periods 6 - 9 as this results in the Part Period closest to 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 + =
10 55 0 100 0 100 Total cost 300 190 490
+ =
+ =
Trial Lot Size Periods (cumulative net Costs Combined requirements) Part Periods Setup Holding Total
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 35 30 40 0 10 40 30 0 30 55
Scheduled
receipts
Projected on
hand 35 35 0 50 10 10 0 30 0 0 55
Net
requirements 0 30 0 0 0 40 0 0 30 0
Planned order
receipts 80 70 85
Planned order
releases 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 non-existing 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
Material Checking & Balancing
9 8 7 6 5 4 3 2 1 0 Week
A
B C
D
E
F
G
H I J K
Assembly diagram
D N O S A J J M A M F J
Target Plan
Acc
um
ula
tive
pro
du
ctio
n
K J I H G F E D C B A
Balancing chart
Resource Requirements Profile
Lot 1
Lot 2
Lot 4
Lot 7
Lot 8 Lot
3 Lot 5
Lot 10 Lot
13
Lot 9 Lot
12
Lot 14 Lot
16
Lot 6
Lot 15
Lot 11
Available capacity
Capacity exceeded in periods 4 & 6
Lot 1
Lot 2
Lot 4
Lot 7
Lot 8 Lot
3 Lot 5
Lot 10 Lot
13
Lot 9 Lot
12
Lot 14 Lot
16
Lot 6
Lot 15
Lot 11
Available capacity
Lot 6 “split” Lot 11 moved
200 –
150 –
100 –
50 –
–
1 2 3 4 5 6 7 8
Period (a)
Sta
nd
ard
la
bo
r h
ou
rs
200 –
150 –
100 –
50 –
–
1 2 3 4 5 6 7 8
Period (b)
Sta
nd
ard
la
bo
r h
ou
rs
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
• Food catering service
– End item => catered food
– Dependent demand => ingredients for each recipe, i.e. bill of materials
MRP in Services
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
• Expanded MRP with emphasis placed on integration
– Financial planning
– Marketing
– Engineering
– Purchasing
– Manufacturing
MRP II
Market Demand
Production plan
Problems?
Rough-cut capacity planning
Yes No Yes No
Finance
Marketing
Manufacturing
Adjust production plan
Master production schedule
MRP
Capacity planning
Problems? Requirements
schedules
Ad
just
mas
ter
sch
ed
ule
MRP II
• 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
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
Terima Kasih
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