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In contrast to a push style of production control is pull. In a basic pull system, production does not commence at a specific processing step until the downstream process sense the signal, usually in the form a certain level of inventory between the processes. Product might be waiting in front of that processing step, but production cannot commence until the downstream process demands it (Spearman and Zazanis 1990).

2.3.2 MRP:

Material requirements planning (MRP) is a computerized system that is typically laid on top of a traditional “push” manufacturing system with a functional layout. A functional layout is one where like machines are grouped together in “work centers” or “departments”. Products usually need to pass through many of these work centers, and managing what product goes to what work center becomes a tough task as the number of work centers and products increase.

Many companies have implemented an MRP system to handle this complexity. An MRP system can be programmed with the routings of all products and a reasonable time for each product to pass from one work center to the next. By combining these routings and processing times, a promised lead time can be generated for every product that starts on the factory floor.

MRP systems can run into problems because of the variability that is present in every manufacturing system. Any delay in production, such as machine going down or a process taking quite a bit longer than expected starts a ripple in the system that usually magnifies. The product that gets behind will now reach the next work center at a different time than originally planned. This work center might now be working on the next product that MRP has planned, so either this must be broken into or the other product must be delayed further. As the variability effects add up, products get behind schedule and need to be expedited through the system (Plenert 1998).

2.3.3 Kanban:

Kanban, is a production control method used for lean manufacturing and the most widely used form of “pull”. In order for Kanban to be implemented, production must be organized in such a way that the product or group follows the same sequence through the factory on dedicated equipment.

Kanban works by limiting the inventory between each step in a production process. Cards are created for each process step. Work cannot begin at a particular step until that step receives a card from the upstream process. Work therefore does not build up between work centers because the number of cards is limited. Kanban is the ultimate form of “pull” production because every step waits from the downstream step to signal or pull material rather than just produce what is placed in front of the process (Gravel and Price 1988).

Kanban creates a very tight production system that is intolerant of variability. The failure of one step in the chain will quickly ripple through the system and shut down the other process in the chain. A similar kanban system is shown in later chapters of this work.

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