Material Requirement Planning
Manu Manufa fact ctur urin ing g reso resour urce ce plan planni ning ng is a meth method od for for the the effe effect ctiv ivee plan planni ning ng of a manufa manufactu cturer rer's 's resour resources ces.. MRP II is compose composed d of severa severall linked linked functi functions ons,, such such as business planning, sales and operations planning, capacity requirements planning, and all related support systems. The output from these MRP II functions can be integrated into financi financial al report reports, s, such such as the busine business ss plan, plan, purchas purchasee commi commitme tment nt report report,, shippi shipping ng budget budget,, and invent inventory ory projec projecti tions ons.. It has the capabi capabili lity ty of specif specifica ically lly address addressing ing operational planning and financial planning, and has simulation capability that allows its users to conduct sensitivity analyses (answering "what if" questions). The The earl earlie iest st form form of manu manufa fact ctur urin ing g reso resour urce ce plan planni ning ng was was know known n as mate materi rial al requirements planning (MRP). This system was vastly improved upon until it no longer resembled resembled the original original version. The newer version was so fundamentally fundamentally different from MRP, that a new term seemed appropriate. Oliver Wight coined the acronym MRP II for manufacturing resource planning. In order to best understand MRP II, one must have a basic understanding of MRP, so we will begin with a look at MRP and then expand into MRP II.
MATERIAL REQUIREMENTS PLANNING Material Material requiremen requirements ts planning planning (MRP) (MRP) is a computer-bas computer-based, ed, time-phase time-phased d system system for planni planning ng and control controllin ling g the produc productio tion n and invent inventory ory functi function on of a firm firm from from the purchase of materials to the shipment of finished goods. All MRP systems are computer based since the detail involved and the inherent burden of computation make manual use prohibitive. MRP is time phased because it not only determines what and how much needs to be made or purchased, but also when. A material requirement planning first appeared in the early 1970s and was popularized by a book of the same name by Joseph Orlicky. Its use was quickly heralded as the new manufacturing panacea, but enthusiasm slowed somewhat when firms began to realize the difficulty inherent in its implementation. The MRP system is composed of three primary modules, all of which function as a form of input. input. These These are the master master product production ion schedul schedule, e, the bill-o bill-of-m f-mate ateria rials, ls, and the inventory inventory status file. Each module serves a unique purpose that is inter-rela inter-related ted with the purpose of the other modules, and produces several forms of usable output.
MASTER PRODUCTION SCHEDULE. The master production schedule (MPS) is basically the production schedule for finished goods goods.. This This sche schedu dule le is usua usuall lly y deri derive ved d from from curr curren entt orde orders rs,, plus plus any fore foreca cast st requirements. The MPS is divided into units of time called "buckets." While any time frame may be utilized, usually days or weeks is appropriate. The MPS is also said to be the aggregate plan "disaggregated." In other words, the plan for goods to be produced in aggregate is broken down into its individual units or finished goods. The MPS is initially initially developed from firm customer customer orders or from forecasts forecasts of demand before the MRP system begins to operate. The MPS is an input to the MRP system. Designed to meet market demand, the MPS identifies the quantity of each and product (end item) and when it needs to be produced during each future period in the production planning horizon.
THE MRP PROCESS. The MRP logic starts at the MPS, where it learns the schedule for finished goods (how many and when). It takes this information to the BOM where it "explodes" the gross requirements for all component parts. The MRP package then takes its knowledge of the
gross requirements for all components parts to the inventory status file, where the onhand balances are listed. It then subtracts the on-hand balances and open orders from the gross requirements for components yielding the net requirements for each component. Of course, we now know not only how many components are needed but when they are needed in order to complete the schedule for finished finished goods on time. time. By subtracting subtracting the lead time from the due date for each part, we now see when an order must be placed for each part so that it can be received in time to avoid a delay in the MPS. A manual version of MRP for a part with requirements of 100 in period 3 and 250 in period 6 and with a two-period lead time is shown in Figure 2. Notice that in order for the firm to meet demand on time (the MPS), they must place an order for 25 in Period 1 and an order for 200 in Period 4. The reader should be aware that this is an overly simplified version of MRP, which does not include such relevant factors as lot sizing and safety stock.
EXPANDING INTO MRP II With MRP generating the material and schedule requirements necessary for meeting the appropriate sales and inventory demands, more than the obvious manufacturing resources for supporting the MRP plan was found to be needed. Financial resources would have to be generated in varying amounts and timing. Also, the process would require varying degrees of marketing resource support. Production, marketing, and finance would be operating without complete knowledge or even regard for what the other functional areas of the firm were doing. In the early 1980s MRP was expanded into a much broader approach. This new approach, manuf manufac actu turi ring ng reso resour urce ce plan planni ning ng (MRP (MRP II), II), was was an effo effort rt to expa expand nd the the scop scopee of production resource planning and to involve other functional areas of the firm in the planning process, most notably marketing and finance, but also engineering, personnel, and purchasing. Incorporation of other functional areas allows all areas of the firm to focus on a common set of goals. It also provides a means for generating a variety of reports to help managers in varying functions monitor the process and make necessary adjustments as the work progresses. When finance knows which items will be purchased and when products will be delivered, it can accurately project the firm's cash flows. In addition, personnel can project hiring or layoff requirements, while marketing can keep track of up-to-the-minute changes in deliv deliver ery y time times, s, lead lead time times, s, and and so on. Cost Cost acco account untin ing g info inform rmati ation on is gath gather ered, ed, engineering input is recorded, and a distribution requirement planning is performed. An MRP II system also has a simulation capability that enables its users to conduct sensitivity analyses or evaluate a variety of possible scenarios. The MRP II system can simulate a certain decision's impact throughout the organization, and predict its results in terms of customer orders, due dates, or other "what if" outcomes. Being able to answer
these "what if" questions provides a firmer grasp of available options and their potential consequences. As with MRP, MRP II requires a computer system for implementation because of its complexity and relatively large scale. Pursuit of MRP or MRP II in a clerical fashion would prove far too cumbersome to ever be useful. In additi addition on to its its effici efficient ent perfor performan mance ce of the data proces processin sing g and file file handlin handling, g, a computer also allows the system to run remarkably quick, providing near-immediate results and reports when asked to simulate a decision.
CLASSES OF FIRMS USING MRP AND MRP II MRP and MRP II users are classified by the degree to which they utilize the various aspects of these systems. Class D companies have MRP working in their data processing area, but utilize little more than the inventory status file and the master production schedule, both of which may be poorly used and mismanaged. Typically, these firms are not getting much return for the expense incurred by the system. Class C firms use their MRP system as an inventory ordering technique but make little use of its scheduling capabilities. Class B companies utilize the basic MRP system (MPS, BOM, and Inventory file) with the addition of capacity requirements planning and a shop floor control system. Class B users have not incorporated purchasing into the system and do not have a management team that uses the system to run the business, but rather see it as a production and inventory control system. Class A firms are said use the system in a closed loop mode. Their system consists of the basic MRP system, plus capacity planning and control, shop floor control, and vendor scheduling systems. In addition, their management uses the system to run the business. The system provides the game plan for sales, finance, manufacturing, purchasing, and engineering. Management then can use the system's report capability to monitor accuracy in the BOM, the inventory status file, and routing, as well as monitor the attainment of the MPS and capacity plans. Class A firms have also tied in the financial system and have developed the system's simulation capabilities to answer "what if" questions. Because everyone is using the same numbers (e.g., finance and production), management has to work with only one set of numbers to run the business.
Material requirements planning (MRP) has become a centerpiece for all manufacturing syst system ems. s. The The key key to succ succes essf sful ul prod produc ucti tion on and and oper operat atio ions ns mana manage geme ment nt in a manufacturing company is the balancing of requirements and capacities. It’s that simple and yet very challenging. To understand it is essential and to practice p ractice it can be a lot of fun. Remember what you are trying to do: Meet the needs of your customers. How? By having the product available when it is wanted. In production management, we do this by knowing in advance what our requirements are now and in future and planning ahead to have the capacity available. To begin with we shall define MRP Material requirements planning (MRP) a system of planning and scheduling the time phased materials requirement for production operations. PLANNING FOR MATERIALS NEEDS
In recent years material requirements planning systems have replaced reactive inventory systems in many organizations. Managers using reactive systems ask,” What should I do now?” Whereas managers using planning systems look ahead and ask.” What will I need in the future? How much and when?” Improved customer service and other advantages come at a cost, however. They require a system for accurate inventory and product buildup information. They also require a realistic master production schedule (MPS) to specify when various quantities of end items will be completed. DEMAND DEPENDENCY Demand dependency is an important consideration in choosing between reactive and planning systems. Demand dependency is the degree to which the demand for some item is associated with the demand for another item. With independent demand, demand for one item is unrelated to the demand for others. In the dependent demand situation, if we know the demand for one item, we can deduce the demand for one or more related items. APPLING MRP AS A SCHEDULING AND ODERING SYSTEM MRP is a system of planning and scheduling the time-phased materials requirements for produc productio tion n operati operations ons.. As such, such, it is geared geared toward toward meeti meeting ng the end-ite end-item m output outputss prescribed in the master production schedule. MRP OBJCTIVES AND METHODS MRP provides the following: 1. Inventory reduction: MRP determines how many of component is needed and when, in order to meet the master schedule. 2. Reduct Reduction ion in produc productio tion n and delive delivery ry lead lead times: times: MRP identi identifie fiess materi materials als and components quantities, timings, availabilities, and procurement and production actions required to meet delivery dealings.
3. Realistic commitments: Realistic delivery promises can enhance customer satisfaction. By using MRP, production production can give marketing marketing timely information information about likely likely delivery delivery times to prospective customers. 4. Increased efficiency: MRP provides close coordination among various work centers as products progress through them.
BILL OF MATERIALS (BOM)
The The BOM BOM ident dentiifies fies how how each each end end prod produc uctt is manuf anufac acttured ured,, spec speciifyi fying all all subcomponents items, their sequence of buildup, their quantity in each finished unit, and the work centers performing the buildup sequence. The bill-o bill-of-m f-mate ateria rials ls is a file file made made up of billsbills-ofof-mat materi erial al (BOM). (BOM). Each BOM is a hierarchical listing of the type and number of parts needed to produce one unit of finished goods. goods. Other Other inform informati ation, on, such such as the routi routings ngs (the (the route route through through the syste system m that that individual parts take on the way to becoming a finished good), alternate routings, or substitute materials may be also be contained with the BOM. A tool known as a product structure tree is used to clarify the relationship among the parts making up each unit of finished goods. A product structure tree for a rolling cart might appear on a bill-of-material. This cart consists of a top that is pressed from a sheet of steel; a frame formed from four steel bars; and a leg assembly consisting of four legs, each with a caster attached. Each caster is made up of a wheel, a ball bearing, an axle, and a caster frame. The bill-of-material can be used to determine the gross number of component parts needed to manufacturer a given number of finished goods. Since a gross number is determined, safety stock can be reduced because component parts may be shared by any number of finished goods (this is known as commonality). The process of determining gross requirements of components is termed the "explosion" process, or "exploding" the bill-of-material. Assuming we need 100 rolling carts, we can use our example product structure tree to compute the gross requirements for each rolling cart component. We can easily see that in order to produce 100 rolling carts, we would need 100 tops, which would require 100 sheets of steel; 100 leg assemblies, which would require 400 legs and 400 casters (requiring 400 wheels, 400 ball bearings, 400 axles, and 400 caster frames); and 100 frames, which would require 400 bars. A BOM can be displayed in the following formats: •
A single-level BOM that displays the assembly or sub-assembly with only one level of children. Thus it displays the components directly needed to make the assembly or sub-assembly.
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An indented BOM that displays the highest-level item closest to the left margin and the components used in that item indented more to the right. modular bill of materials (modular BOM) is type of bill of materials (BOM) and a critical element in defining the product structure of an end-item Modu Modula larr BOMs BOMs defi define ne the the comp compon onen entt mate materi rial als, s, docu docume ment nts, s, part partss and and engineering drawings needed to complete a sub-assembly. While the terms BOM and and modu modula larr BOM BOM are are most most commo commonl nly y used used in asso associ ciat atio ion n with with phys physic ical al products, the concept can be used in a variety of industries (e.g. software, medical records). Modular BOMs are used by modern information systems to serve a variety of purposes, such as defining the components needed to produce a subassembly, and providing cost information for each component and "rolled-up" cost information for the overall sub-assembly. For option option-ba -based sed,, config configura urable ble produc products ts (e.g. (e.g. automo automobil biles, es, PCs) PCs) compani companies es needed to plan every combination and permutation of options to ensure they could fulfill customer demands. With a modular BOM structure, companies can plan the demand for each module independent of the end-item demand by estimating the popularity of the module sub-assembly.
INVENTORY STATUS FILE The MRP system must retain an up-to-date file of the inventory status of each item in the product structure. This file provides accurate information about the availability of every item controlled by the MRP system which can then maintain an accurate accounting of o f all inventory transaction, both actual and planned.
THE MRP PROCESSING LOGIC The MRP processing logic accepts the master schedule and determines the components schedules for successively lower-level items of the product structures. It calculates for each item in each product structure structure and for each time period (typically (typically one week) in the planning horizon how many of that item are needed (gross requirements), how many units from inventory are already available, the net quantity that must be planned on receiv receiving ing in new shipme shipments nts (plann (planned ed order order receip receipts) ts),, and when orders orders for the new shipments must be placed (planned order releases) so that all materials arrive just when needed. THE MRP COMPUTATIONAL PROCEDURE The MRP computational procedure uses the input information to calculate the current records for each component and item. INFORMATION PROCESSING SEQUENCE The MRP processing logic is applied first to the high-level items (end products) in the product structure, then to the items on the next lower level.
INDENTED BILL OF MATERIALS To do its levellevel-byby-lev level el calcul calculati ations ons,, the MRP proces processin sing g logic logic obviou obviously sly needs needs information about an end item’s relationship to all its subcomponents. PRODUCT EXPLOSION To create a parent item we often need multiple units of a lower-level item. LOW-LEVEL CODING Often a single item is in the product structure of several end items, or it exists in several levels of one product structure. USING MRP OUTPUTS FOR MATERIALS DECISIONS MRP merely indicates what actions are needed to meet the MPS goal; now management must act to “make things happen”- to cause (control) the productive system to execute so that management gets the results it wants. KEEPING MRP CURRENT IN A CHANGING ENVIRONMENT MRP is not state; it is responsive to new job orders from customers and current shop conditions, as well as changes anticipated an ticipated for the future. 1. Pegging: The process of tracing through the MRP records and all levels in the product structure to identify how changes in the records of one component will affect the records of other components. 2. Cycle counting: Counting on-hand inventories at regular intervals to verify inventory quantities shown in the MRP 3. Regenerative method: A procedure, used at regular intervals, to update the MRP by completely reprocessing the entire set of information and recreating the entire MRP 4. Time fence: A designated length of time that must pass without changing the MPS, to stabilize the MRP system; afterward, the MPS is allowed to change. LOT SIZING The MRP system generates planned order releases, which trigger purchase orders for outside suppliers or work orders for internal component production. 1. Lot-for-lot ordering: A lot sizing policy in which order quantity equals Net requirements for the period. 2. Part-period method: A lot sizing policy in which order quantity varies according to a comparison of holding versus ordering costs. DETAILED CAPACITY PLANNING Each time the MRP system is updated managers must ask whether shop capacity is sufficient to implement the current plan. Detailed capacity planning (also called capacity requirements planning) is a technique that addresses this question and it does so in more
detail detail than the rough-cut rough-cut method. New information information from MRP permits refinements refinements that were not possible at the rough-cut level. A document that shows the routing of a component, including the work centers and an operation time, through its production processes is called route sheet. To visualize visualize the time-phased time-phased capacity requirements, requirements, we first first construct construct the operation setback chart for the end item. LIMITATIONS AND ADVANTAGES OF MRP The limitations of MRP stem from the conditions that must be met before it can be used. A comput computer er is necess necessary ary;; the product product structur structuree must must be assemb assembly ly orient oriented; ed; bill bill of materials and inventory status information must be assembled and computerized; and a valid master schedule must be prepared. Another limitation has to do with data integrity. Unreliable inventory and transactions data from the shop floor can ruin a well-planned MRP system. Training personnel to keep accurate records is not an easy task, but it is critical to successful MRP implementation. In general, the system must be believable, accurat accurate, e, and direct directly ly useful useful or else else it will will become become an expensi expensive ve orname ornament nt that that is bypassed in favor of informal, ad hoc methods. The dynamic nature of the MRP system is a vital advantage. It reacts well to changing conditions; in fact, it thrives on change. Changing conditions from the master schedule for several periods into the future can affect not only the end item but also hundreds, even thou thousa sand nds, s, of comp compon onen ents ts.. Beca Becaus usee the the prod produc ucti tion on-i -inv nven ento tory ry data data syst system em is computerized, management can make a new MRP computer run to revise production and procurement plans that react quickly to changes in customer demands as reflected in the master schedule. MRP USER EXPERIENCES Some users reported implementation problems- lack of communication about MRP within the company, lack of company expertise, and inadequate support from marketing and manufacturing personnel – all of which were viewed as more severe than comput computer er hardw hardwar are/ e/so soft ftwa ware re probl problem ems. s. When When aske asked d about about the the majo majorr prob proble lem m in implementing MRP in their firm, the respondents most frequently answered “education of personnel” and “top management support.” Product Production ion/in /invent ventory ory control control manage managers rs rated rated the accura accuracy cy of inform informati ation on in their their produc productio tion n proces processes ses.. These These manager managerss felt felt the least least accura accurate te inform informati ation on they they had, overall overall,, was on capacit capacity y (and (and capacit capacity y planni planning) ng),, market market foreca forecasts sts,, and shop shop floor floor control control.. Their Their most most accurat accuratee inform informati ation on was bill bill of materi materials als record records, s, follow followed ed by master production schedule and inventory records. Finall Finally, y, users users assess assessed ed the benefi benefits ts of MRP. MRP. They They cited cited greate greaterr invent inventory ory turnove turnover, r, shorter shorter delive delivery ry lead lead time, time, better better-ke -kept pt delive delivery ry promis promises, es, and fewer fewer adjust adjustmen ments ts in intern internal al produc productio tion n to compen compensat satee for unavail unavailabl ablee materi materials als,, and fewer fewer materi materials als expediters. It is evident from the result of this study that MRP is an improvement over previous production planning and control systems for many users. Its applications are growing as operations managers continue to develop better methods for materials management.
