April 1, 2008

By Darren Dolcemascolo

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 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 about 10 days of injection molded inventory (WIP). There were also a significant number of material shortages affecting the productivity of the assembly cells.

A kaizen team consisting of a materials person, assembly and injection molding operators, an industrial engineer, an area supervisor, and a few employees from outside the area was formed. After spending day one in training and mapping out the current state process, the team discovered significant waste in the process:

Daily material shortages resulting in significant overtime costs/reprioritization

Inefficient planning for changeovers in the injection molding department due to shifting schedule/priorities

Out of cycle work: Operators were doing their own material handling and preparation

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:

Material handlers would pull from the supermarket into the assembly cells, replenishing point-of-use inventory using a 2-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.

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.

The injection molding lead would place the kanban on a scheduling board in sequence.

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% reduction in shortages and 70% reduction in WIP (from 10 days to 3 days).

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