Manufacturing Planning: From Sales Orders to Production Schedules

Every manufactured product started as a plan. The plan determined what to make, how many to make, when to make them, and what resources were needed. Manufacturing planning is the bridge between customer demand and production execution. When planning is done well, factories run smoothly, customers receive orders on time, and inventory is under control. When planning is poor, the factory lurches from crisis to crisis.

Manufacturing planning operates at multiple time horizons. Long-range planning covers years and involves capacity decisions. Medium-range planning covers months and involves production rates and workforce levels. Short-range planning covers weeks and days and involves scheduling and dispatching.

Aggregate Planning

Aggregate planning determines the production rate, workforce level, and inventory level over a planning horizon of 6 to 18 months.

Planning Strategies

Level strategy maintains a constant production rate regardless of demand fluctuations. Inventory builds during low-demand periods and is consumed during high-demand periods. Level strategy is stable and easy to manage but requires significant inventory capacity.

Chase strategy adjusts production rate to match demand. Workforce is hired and laid off, overtime is used, and subcontracting supplements capacity. Chase strategy minimizes inventory but creates instability for the workforce.

Mixed strategy combines elements of both. A common approach is to maintain a stable core workforce and use overtime and subcontracting to handle demand peaks. The optimal strategy balances inventory costs against capacity adjustment costs.

Aggregation

Products are aggregated into product families — groups of products that share production resources and have similar cost characteristics. Planning at the aggregate level reduces complexity. Detailed planning for individual products happens later.

Aggregate units measure total production volume. If a factory makes 10 products with different labor content, aggregate output is measured in labor hours or machine hours rather than units. The production systems design article discusses aggregate planning in the context of capacity decisions.

Master Production Schedule

The MPS disaggregates the aggregate plan into specific products, quantities, and dates. It is the driving input for material requirements planning.

Time Fences

The MPS is divided into time zones with different levels of flexibility. The demand time fence — typically 1 to 2 weeks — is frozen. Changes within the DTF require executive approval because materials have been ordered and production has been scheduled.

The planning time fence — typically 4 to 8 weeks — allows changes within limits. Resources are committed but not yet consumed. Beyond the PTF, the schedule is tentative and can be adjusted as demand forecasts change.

Available to Promise

ATP is the quantity of finished goods available for new customer orders. It is calculated as the MPS quantity minus customer orders already committed for each time period. ATP tells the sales department whether a new order can be accepted for delivery in a given period.

Material Requirements Planning

MRP calculates the quantities and timing of component materials needed to execute the MPS.

Bill of Materials

The BOM is the complete list of components, subassemblies, and raw materials needed to make one unit of the finished product. It is structured as a tree — the end product at level 0, major subassemblies at level 1, components at level 2, and purchased parts at the lowest level.

Each BOM entry specifies the quantity required per parent item. A bicycle might require two wheels, one frame, one handlebar assembly, and one seat assembly. Each wheel requires one tire, one rim, one hub, and 36 spokes.

MRP Explosion

MRP explosion calculates gross requirements for each component based on the MPS quantities and BOM structure. Net requirements are gross requirements minus on-hand inventory and planned receipts. Planned orders are created for the net requirements, offset by lead times.

If the MPS calls for 100 bicycles in week 10 and each wheel requires 36 spokes, the gross requirement for spokes in week 10 is 7,200. If 1,000 spokes are in inventory, the net requirement is 6,200. With a lead time of 3 weeks, the planned order release for spokes is week 7.

Lot Sizing

Lot sizing determines the quantity of each planned order. Lot-for-lot orders exactly the net requirement. It minimizes inventory but maximizes ordering frequency. Fixed order quantity uses the economic order quantity regardless of net requirements. Period order quantity covers demand for a fixed number of periods.

Capacity Planning

Capacity planning ensures that the factory can execute the production plan.

Capacity Requirements Planning

CRP calculates the capacity required to execute the MPS and MRP plans. It compares required capacity to available capacity. If capacity is insufficient, the planner must either increase capacity, reduce the production plan, or find alternative resources.

Capacity can be increased through overtime, additional shifts, subcontracting, or capital investment. The manufacturing planning process must respect capacity constraints — an unconstrained plan that exceeds capacity will not be executed.

Rough Cut Capacity Planning

Before running detailed MRP, RCCP checks the MPS against major capacity constraints — critical workstations, bottleneck operations, and key resources. RCCP uses simplified capacity calculations to identify obvious problems early, before detailed material planning is done.

Production Activity Control

PAC executes the production plan on the factory floor.

Dispatching

Dispatching releases orders to the factory floor and assigns them to workstations. Priority rules determine the sequence in which orders are processed. First come first served is simplest but ignores due dates. Earliest due date prioritizes orders that are due soonest. Shortest processing time minimizes average completion time.

Input-Output Control

Input-output control monitors the flow of work to and from each workstation. Actual input is compared to planned input. Actual output is compared to planned output. Backlogs indicate that input exceeds output. Load is the total work awaiting processing at a workstation.

Finite Scheduling

Finite scheduling creates a detailed schedule that respects capacity constraints. Each operation is scheduled on a specific machine with specific start and end times. Finite scheduling provides realistic delivery dates but requires accurate data on setup times, processing times, and machine availability.

Sales and Operations Planning

S&OP aligns demand, supply, and financial plans across the organization. It is a cross-functional process involving sales, marketing, operations, finance, and supply chain.

Monthly S&OP Cycle

The S&OP cycle runs monthly. The first week updates the statistical demand forecast with market intelligence from sales and marketing. The second week runs supply planning — capacity constraints, inventory targets, and supplier capabilities are evaluated against the demand plan.

The third week holds the pre-S&OP meeting where demand and supply are balanced. Alternatives — overtime, inventory buildup, outsourcing — are evaluated. The fourth week holds the executive S&OP meeting where the plan is approved, resources are committed, and performance is reviewed.

Benefits of S&OP

Effective S&OP improves forecast accuracy by 10 to 30 percent through collaboration between functions. It reduces inventory by 15 to 30 percent by aligning supply plans with actual demand. It increases on-time delivery by improving visibility and coordination.

S&OP is particularly valuable in industries with long lead times, seasonal demand, or volatile markets. Consumer packaged goods, pharmaceuticals, and electronics manufacturers rely on S&OP to balance supply and demand across complex product portfolios.

Integrated Business Planning

IBP extends S&OP to include strategic planning, financial planning, and risk management. The planning horizon extends to 3 to 5 years. Strategic initiatives — new products, plant expansions, acquisitions — are evaluated for their impact on supply and demand. Financial targets are integrated with operational plans to ensure that the plan is both operationally feasible and financially acceptable.

Frequently Asked Questions

What is the difference between MRP and ERP? MRP — material requirements planning — calculates material needs. ERP — enterprise resource planning — integrates MRP with finance, human resources, sales, and other business functions. ERP systems include MRP as a module along with financial accounting, order management, and payroll.

How do you handle rush orders in manufacturing planning? Rush orders disrupt the plan. They consume capacity and materials planned for other orders, causing delays. Rush orders should be evaluated for their impact and approved only when the value of the rush order exceeds the cost of disruption. In practice, many rush orders could have been anticipated with better planning.

What is the role of safety stock in manufacturing planning? Safety stock buffers against demand and supply uncertainty. In MRP, safety stock for components provides protection against yield losses, supplier delays, and unexpected demand. Safety stock for finished goods buffers against demand variability. The inventory management article discusses safety stock calculation.

Can manufacturing planning be automated? Planning software automates the calculations, but planning judgment cannot be fully automated. Planners make decisions about priorities, tradeoffs, and exceptions that require understanding of the business context. The role of the planner is shifting from calculation to exception management and decision making.

Production Systems Design — Inventory Management — Project Management for Industrial Engineers