Definition
The method for keeping Flow uninterrupted when a product must leave the cell for processing on external equipment (heat treatment, plating, etc.); number of Curtains = Curtain process one-piece cycle time ÷ Takt Time, rounded up. Two official notes ( p84): delivery time must include round-trip transport time; when the Monument is a batch process, add ×2 to the quantity. In Chinese it is uniformly called "Curtain"; another glossary entry for "Curtain" (using external process delivery time in the numerator) refers to the same thing—REV3 p84 uses "cycle time" while the textbook 18 p47 uses "delivery time"; both wordings coexist, so please check against entry [[curtain]] to verify which version of the numerator is being used.
In detail
The Curtain Effect is a method to maintain flow in cellular production. When products must leave the cell for external equipment processing, buffer inventory (curtain) is set to maintain continuous flow within the cell. The curtain quantity equals the curtain process single-piece cycle time divided by takt time, ensuring external processing does not interrupt cell production. Unlike Kanban pull, the curtain focuses more on physical buffering; compared to batch production, it maintains single-piece flow. It typically outputs curtain quantity calculations and buffer inventory setup plans.
Example in practice
An assembly cell needs to send components to the heat-treat workshop. The heat-treat process has a one-piece cycle time of 20 minutes, and the assembly takt is 5 minutes: 20/5=4, rounded up = 4; the heat-treat furnace is a batch-process Monument, and the official definition (REV3 p84) is plus ×2 = 8 pieces. If there is also transport time between the cell and the heat-treat workshop, it must first be included in the cycle time before calculating. By setting up 8 Work in Process buffers, the assembly cell can continue producing without waiting for heat treatment to finish, avoiding stagnation within the cell.