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Section IV
Lean Tools Needing a Different Approach
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10
Finding, Managing, and Improving Bottlenecks
One of the primary objectives of lean is to achieve smooth continuous flow of material through the process. In order to make progress toward this goal, bottleneck resources must be identified, managed, and improved. A bottleneck is any resource whose capacity is less than or equal to the planned throughput, any machine or process step where the takt rate equals or exceeds capacity.
BOTTLENECKS IN PROCESS PLANTS
Bottlenecks tend to have different causes and to have more severe implications in the process industries. In parts manufacture and assembly, people tend to be the rate-limiting factor in many steps, so managing bottlenecks is often a matter of managing people, by appropriate staffing and task leveling. In process plants, throughput in most manufacturing steps is limited by equipment capability, not by labor. In cereal manufacture, throughput can be limited by baking times or by flake extrusion rates; in papermaking by the lineal speed capability of the forming, bonding, or calendaring machinery; and in paint making by the time required to complete the chemical reaction in resin production. With equipment rather than operating labor causing the bottleneck, throughput limitations can’t be resolved by bringing in additional labor or by scheduling overtime. And because many process plants run around the clock on a 24/7 schedule, scheduling extra shifts is often not the answer. Further, because equipment tends to be expensive and relatively inflexible, replacing or upgrading equipment is not often a viable option. So, managing the bottleneck is a matter of optimizing the performance of the bottleneck resource itself, protecting the bottleneck from upstream and downstream problems, and optimizing bottleneck scheduling.
Because OEE factors (see Chapter 6) all have variability, what is not a bottleneck at some times may become one at others. For example, a 90 percent yield value shown on a value stream map is an average, and doesn’t mean that you lose 10 percent every day. That process step may run at 99 percent yield on a good day and 80 percent on a bad day. The step may have plenty of capacity on the 99 percent day but be a bottleneck on the 80 percent day. Similarly, variability in equipment reliability on a day-by-day basis can cause non-bottlenecks to become bottlenecks for periods of time.
It is important to recognize that throughput can be limited by factors directly related to a piece of equipment and its performance, either its inherent processing rate, its downtime, capacity lost to yield losses, time lost to changeovers, or all the above. But throughput can also be limited by the manner in which a piece of equipment is scheduled and how well its flow is synchronized with upstream and downstream process steps. In Synchronous Manufacturing, Umble and Srikanth explain this distinction, referring to the former as bottleneck resources and the latter as capacity constraint resources (CCRs): A “capacity constraint resource [is] any resource which, if not properly scheduled and managed, is likely to cause the actual flow of product through the plant to deviate from the planned product flow.”
As an example of a CCR, consider the case of a cereal plant that manufactures two families of cereal, one formed into thick shapes like stars and circles, and one formed into relatively flat flakes of various shapes. The plant can be divided into three major areas (see Figure 10.1), shape manufacturing, flake manufacturing, and packaging, which includes bagging, boxing, cartoning, and palletizing.
FIGURE 10.1
High-level view of a cereal plant.
From the data boxes contained on a more detailed map, packaging has a utilization of only 75 percent, even though it takes the full output of both cereal production areas. However, in real life the storage silos often became full and forced a production line to go down. Analysis revealed that although the packaging area appeared to have excess capacity, it was being scheduled with no coordination or synchronization with either production area, so it became a constraint.
Product wheel scheduling, discussed in detail in Chapter 12, is frequently employed to improve scheduling in a way that opens up the capacity constraint.
Most of the discussion in this chapter on moving bottlenecks, hidden bottlenecks, root causes of bottlenecks, and managing and improving bottlenecks applies to CCRs as well as to bottlenecks.
The following summarizes the characteristics specific to bottlenecks found in process plants:
- The root cause is generally in equipment capacity and performance, not in labor staffing.
- Plants often run around the clock, so additional shifts are not a feasible solution.
- Root causes include yield losses, reliability issues, run rate limitations, changeovers, and minor stops after a changeover as well as limitations in inherent capacity.
- Non-bottlenecks can become bottlenecks due to variability of OEE factors.
- Bottlenecks may move with product mix.
- Bottlenecks can be hidden; the resulting waste is usually hidden.
MOVING BOTTLENECKS
Identifying and managing bottlenecks can be difficult in process plants because the bottleneck may be at a different process step for one material being produced than for another material; the bottleneck may move as the process cycles through the various products being made. As an example, consider one sheet forming and one bonding machine from the process mapped in Chapter 4, shown in Figure 10.2. As can be seen from the data boxes, each has effective capacity greater than the takt requirement, so neither is a bottleneck. However, the values shown in the data boxes represent the averages taken across the full product lineup at the typical mix. When forming sheet with high basis weight, the forming machine must run at much slower lineal speeds, so for that product the capacity will be less than takt and the machine becomes a bottleneck, as depicted in Figure 10.3. With other products, forming may have excess capacity while bonding may become the bottleneck. For products that must be bonded at higher temperature, the line speed must be slower to allow the sheet to be in contact with the heated bonding roll long enough for complete heat transfer from the roll to the sheet. So, when making products requiring high bonding temperatures, bonding becomes the bottleneck, as illustrated in Figure 10.4.
FIGURE 10.2
Forming and bonding utilizations based on average effective capacity.
FIGURE 10.3
Product that causes forming to be a bottleneck.
FIGURE 10.4
Product that causes bonding to be a bottleneck.
In a process spinning synthetic fibers, the threadline winding machine may be the bottleneck when producing fine fibers, while the metering pump feeding the extrusion die may be the bottleneck with thicker fibers.
In a salad dressing bottling line, the bottle filler will usually be the bottleneck operation when filling the larger bottles, but the label applicator can become the bottleneck when packaging into smaller bottles. The carton erection and filling operation may have significant excess capacity when packaging in large cartons for the “big box” discount retailers but become the bottleneck when filling small cartons for convenience stores.
The fact that the bottleneck may move during the production cycle must be recognized so that appropriate bottleneck management strategies can be used with all process steps that can be bottlenecks.
RECOGNIZING COVERT BOTTLENECKS
To manage bottlenecks appropriately, it is necessary first to identify where they exist in the process. One way to find bottlenecks is to look for locations where inventory tends to build up. But keep in mind that inventory buildup can be for reasons other than bottlenecks, for example, cycle stock of materials produced infrequently. Nonetheless, a large inventory at a point in the process is a clue that the next step might be a bottleneck resource or a CCR.
In many process plants, however, the in-process inventory is not vis...
