Appliance assembly has an awkward combination of properties: the product is large, the takt is short, and the model mix is wide. The conveyor type is usually chosen on price and then blamed for the output.
The three line types
Paced. The conveyor indexes every takt, and every station must finish within the index time. Work-in-process is exactly one unit per station. Output is predictable and floor space is minimal.
Buffered. Stations are decoupled by accumulation zones holding two to six units. A stopped station does not stop the line until the buffer empties. Output is higher on unstable processes and floor space is 20 to 40 percent larger.
Hybrid. Critical stations (test, refrigerant charge, leak test) are buffered; manual assembly is paced. This is the most common arrangement on appliance lines above 500 units per shift.
What the model mix does to the choice
A single-model line at a stable takt is well served by pacing. Everyone does the same work in the same time, and a buffer would only hide imbalance.
A line running four models with different work content needs buffers, because the work content varies by 20 to 40 percent between models. Pacing that line forces every station to be staffed for the worst model, which means idle time on the other three.
The practical test: if the standard time for the longest model exceeds the shortest by more than 15 percent, pace only the automated stations.
Buffers and honesty
A buffer absorbs variability, which is its purpose and its danger. A 20-unit buffer in front of a test station can hide a failing test station for an entire shift, and the problem is discovered when the buffer empties at the worst possible moment.
Set a rule: when the buffer exceeds 70 percent full for more than 15 minutes, an andon is raised. The buffer is a shock absorber, not a warehouse.
| Factor | Paced | Buffered | Hybrid |
|---|---|---|---|
| Floor space | Lowest | 20 to 40% more | 15 to 25% more |
| Output with unstable process | Low | Highest | High |
| Model mix flexibility | Poor | Good | Good |
| Problem visibility | Immediate | Delayed | Mixed |
| WIP | One per station | Buffer size | Buffer at key stations |
Large and heavy units
Refrigerators and washing machines weigh 60 to 120 kg. Carriers must support the unit at the base, not the sides, and the conveyor must handle the moment load when the unit is tipped for base access.
On a paced line, a heavy unit that is not located correctly will be dragged by the index. Use a lift-and-locate station at every automated operation, and check the carrier's locating pins for wear monthly.
Changeover
Model changeover on an appliance line is dominated by fixture and program changes, not by the conveyor. Design the carrier so the model-specific parts are a removable top plate that can be swapped in under two minutes, and keep the plates on a cart at the line head.
A line that needs 30 minutes to change models will be run in large batches regardless of what the planning system says.
Which to specify
Start with the bottleneck station. If its cycle time is stable and below takt, pace the line. If it varies by more than 10 percent, buffer it and pace the rest. Then check the floor: a buffered line needs the space, and a buffer drawn on a layout with no room is a fiction that survives until installation.
