Pull is one of the key principles of lean thinking. There are essentially two different types of pull systems: sequential pull and replenishment (or supermarket) pull. In sequential pull, the downstream customer pulls parts from the upstream supplying process in the sequence in which the supplying process produces; that is, the supplying process dictates the sequence of work.
Sequential pull limits the amount of inventory between the two processes. In replenishment pull, the downstream customer pulls from a supermarket according to what it needs (based on a schedule dictated by its customer). The supermarket is replenished by the supplying process. In this article, I will describe a case study in which a California manufacturer implemented replenishment pull between an injection-molding operation and two assembly cells.
The company in question had 12 plastic injection-molding machines. Each produced a number of components that were assembled into a finished product by the assembly cells. Before the implementation, the company had approximately 10 days of injection-molded inventory (work in process, or WIP). There were also a significant number of material shortages affecting the productivity of the assembly cells.
A kaizen team was formed, consisting of a materials person, assembly and injection-molding operators, an industrial engineer, an area supervisor, and a few employees from outside the area. After spending day one in training and mapping out the current state process, the team discovered significant waste in the process:
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Daily material shortages resulting in significant overtime costs/reprioritization
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Inefficient planning for changeovers in the injection-molding department due to shifting schedule/priorities
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Out-of-cycle work: Operators were doing their own material handling and preparation
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Excessive WIP (10 days)
After analyzing material usage and variability in usage, the team created and properly sized a supermarket of plastic materials. The system would work as follows:
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Material handlers would pull from the supermarket into the assembly cells, replenishing point-of-use inventory using a two-bin system. That is, as bins of plastic parts are emptied, the empty bin was used as a signal to replenish material to the cells.
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As materials in the supermarket were consumed (moved into assembly), a trigger point (visually indicated by the number of bins remaining) was reached. When the trigger point was reached, a kanban card would be pulled and delivered to the injection-molding lead.
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The injection-molding lead would place the kanban on a scheduling board in sequence.
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The injection-molding machines would run product according to the kanban on the scheduling board and replenish the supermarket.
The new system resulted in a 90 percent reduction in shortages and 70 percent reduction in WIP (from 10 days to three days).
About the author:
Darren Dolcemascolo is an internationally recognized lecturer, author and consultant. To learn more, visit www.emsstrategies.com or call 866-559-5598.
