11.3 Inventory Management

Inventory management is crucial for businesses to balance costs and meet customer demand. It involves tracking raw materials, work-in-progress, finished goods, and maintenance supplies. Effective management optimizes stock levels, minimizes costs, and ensures smooth operations.

Key concepts include economic order quantity, reorder points, and inventory control systems. Just-in-time techniques and ABC classification help businesses streamline processes and focus on high-value items. Understanding these principles is essential for efficient inventory management.

Inventory Management

Types of inventory in production

• Raw materials inventory
  • Unprocessed materials used to create finished products (lumber, steel, plastics)
  • Serve as inputs for the production process
  • Must be purchased and stored until needed for production
• Work-in-progress (WIP) inventory
  • Partially completed products in various stages of the production process
  • Represents the value of materials, labor, and overhead incurred but not yet turned into finished goods
  • Ties up capital and requires storage space until completion
• Finished goods inventory
  • Completed products ready for sale to customers
  • Represents the final output of the production process
  • Must be stored and managed until sold and delivered to customers
• Maintenance, repair, and operating (MRO) inventory
  • Supplies and materials used to support production but not directly incorporated into finished products (spare parts, lubricants, cleaning supplies)
  • Ensures smooth operation of production equipment and facilities
  • Requires regular monitoring and replenishment to avoid production disruptions

Economic order quantity calculation

• Economic order quantity (EOQ)
  • Optimal order quantity that minimizes total inventory costs
  • Balances the trade-off between ordering costs and holding costs
  • Formula: $EOQ = \sqrt{\frac{2DS}{H}}$
    • $D$ = Annual demand in units
    • $S$ = Fixed cost per order (setup costs, transportation costs)
    • $H$ = Annual holding cost per unit (storage, insurance, opportunity cost)
• Reorder point (ROP)
  • Inventory level at which a new order should be placed to prevent stockouts
  • Considers lead time and safety stock to ensure continuous supply
  • Formula: $ROP = (Daily\ demand \times Lead\ time) + Safety\ stock$
    • Daily demand = Average number of units sold per day
    • Lead time = Time between placing an order and receiving the inventory
    • Safety stock = Extra inventory held to prevent stockouts due to unexpected demand or supply delays

Inventory control system analysis

• Periodic inventory system
  • Inventory levels are reviewed and orders placed at fixed intervals (weekly, monthly)
  • Benefits: simplicity, less frequent ordering, suitable for low-value items
  • Costs: higher average inventory levels, increased risk of stockouts
• Perpetual inventory system
  • Inventory levels are continuously monitored and updated with each transaction
  • Benefits: real-time inventory tracking, lower risk of stockouts, better decision-making
  • Costs: higher setup and maintenance costs for technology and systems
• ABC inventory classification
  • Categorizes inventory items based on their value and importance
    1. A items: high value, critical to production (20% of items, 80% of value)
    2. B items: moderate value, less critical (30% of items, 15% of value)
    3. C items: low value, least critical (50% of items, 5% of value)
  • Benefits: focuses management attention on high-value items, optimizes inventory control efforts
  • Costs: time and resources required for classification and differentiated management

Just-in-time inventory techniques

• Just-in-time (JIT) inventory management
  • Production and inventory strategy that aims to minimize inventory holding costs by receiving materials just as they are needed
  • Key principles:
    1. Pull system: production is triggered by customer demand, not forecasts
    2. Small, frequent deliveries from suppliers
    3. Continuous improvement to reduce waste and inefficiencies (lean manufacturing)
  • Benefits:
    • Lower inventory holding costs
    • Reduced space requirements
    • Improved cash flow
  • Challenges:
    • Requires strong supplier relationships and reliable deliveries
    • Increased risk of stockouts if supply chain disruptions occur (natural disasters, labor strikes)
    • Higher transportation costs due to frequent deliveries