A constraint buffer is the inventory reserve that is positioned directly in front of a bottleneck operation (the constraint). This buffer keeps the bottleneck operation from being forced to shut down by ensuring that there is always a constant feed of materials being forwarded from upstream workstations.
The constraint buffer must be of a sufficiently large size to weather any shortfalls in parts flow from these other workstations. The size of the buffer can be based on an historical analysis of the largest parts shortfall that has impacted the bottleneck in the past, or it could be based on a more theoretical (and larger) estimate of what would happen if there were a catastrophic shutdown of the upstream workstations for a certain period of time.
Though it may be tempting to try to eliminate all of the issues that can cause problems with upstream workstations, the fact is that some variability will always be present, so some constraint buffer will always be necessary. If ongoing industrial engineering efforts have succeeded in reducing inconsistencies in the flow of goods to the bottleneck, then the constraint buffer can be relatively small. Conversely, if there is still considerable variability in the flow of parts to the bottleneck, then it will be necessary to protect the bottleneck with quite a large inventory buffer.
The existence of a large constraint buffer is particularly important if a company does not have a sufficient amount of excess capacity upstream of the bottleneck operation, since these upstream operations will not be able to easily build up a surge of new parts to rebuild the buffer. In this case, a production snafu may clear out most of the buffer, and then put the business at risk of another bottleneck shutdown during the extended period when the buffer is being rebuilt back to its former size.
A large constraint buffer cannot substitute for the addition of production capacity to upstream workstations, especially since it takes lots of excess capacity to build the buffer in the first place. Ideally, there should be sufficient available capacity to rebuild the buffer in short order, if a production snafu requires that inventory be withdrawn from the buffer.
Alternatives to the Constraint Buffer
It may be possible to invest in only a small constraint buffer and instead buy a large amount of excess capacity in the upstream workstations. Doing so ensures that a large volume of parts can be sent downstream to the bottleneck operation in short order. However, the cost of extra capacity is usually so much higher than an additional investment in the constraint buffer that the capacity investment is not seriously contemplated.