Work-In-Progress (WIP)

WIP builds up when the rate of work entering a process exceeds the rate of work leaving it. Several common production scheduling patterns cause this imbalance. Push scheduling releases work based on forecasts rather than actual downstream capacity, often overloading bottleneck operations. Large batch sizes mean that entire batches must wait at each station, creating queues proportional to batch size. Unbalanced line speeds cause faster upstream stations to outpace slower downstream stations, creating inter-station inventory. Variable processing times mean that some cycles take longer than average, creating temporary queues that persist because subsequent cycles do not fully compensate. Machine breakdowns halt a station while upstream stations continue producing, injecting WIP into the system. Prioritization changes cause partially completed orders to be set aside for rush jobs, creating abandoned WIP. Understanding which of these causes is dominant in your factory is essential for choosing the right WIP reduction strategy.

The most effective way to control WIP is to set explicit limits on the amount of work allowed in each stage of the production process. These WIP limits — also called CONWIP (Constant WIP) when applied to the total system — create a pull-based system where new work is only released when a downstream completion creates capacity. Kanban is the most common mechanism for implementing WIP limits: each kanban card authorizes one unit (or container) of WIP, and the total number of cards sets the limit. When all cards are in circulation, no new work can start until a card returns from a completed unit. Setting the right WIP limit requires balancing two competing goals: too little WIP risks starving downstream operations (especially when processing times are variable), while too much WIP defeats the purpose of the limit. A practical starting point is to set WIP limits at 1.5 times the average throughput for each stage, then reduce them gradually as the process stabilizes and variability decreases.

Production scheduling directly controls how much WIP is in the system by determining when and how much work is released to the floor. A schedule that releases all of next week's orders on Monday morning creates a WIP spike that takes all week to process, while a schedule that releases orders daily or per-shift maintains a steady, low WIP level. LinePlanner's shift-level scheduling granularity supports this leveled release strategy by allowing planners to assign specific orders to specific shifts rather than dumping an entire week's workload at once. Sequencing rules also matter: scheduling similar products together (campaign scheduling) reduces changeover time and prevents the changeover-related WIP spikes that occur when the line produces small batches of many different products. Visual scheduling makes WIP problems immediately visible — when the production calendar shows orders stacking up at a particular line or stage, the planner can rebalance the load before WIP spirals out of control.