A merge is where two streams become one. It is also where the fewest layout decisions have the largest consequences, and where a 20-degree change in geometry decides whether the line jams four times a shift or once a month.
Merge geometry
The merge angle sets the conflict. At 30 degrees, a package entering the merge has a velocity component along the main line, and the gap it needs is small. At 90 degrees, the package must accelerate from zero to line speed in the width of the merge, and any package already on the main line will be hit.
Use 30 degrees where space allows, 45 degrees as a compromise, and 90 degrees only with a powered transfer unit and a positive gap control.
Accumulation before the merge
A merge can only release a package when there is a gap. That gap must be created upstream, not at the merge. The rule we use: at least three package lengths of accumulation on each infeed, and a release logic that holds the second package until the first has cleared the merge photoeye by one package length.
Without upstream accumulation, the merge logic either releases too early, causing a collision, or too late, causing the main line to starve. Both look like a control problem and both are solved with buffer space.
Divert types
| Type | Best for | Limit |
|---|---|---|
| Pop-up wheel diverter | Cartons, totes, 20 to 50 kg | Needs a flat base, struggles with soft bags |
| Pusher | Rigid cartons at low rate | Damages product at high rate |
| Sliding shoe sorter | High rate, mixed cartons | High cost, needs width |
| Pivot arm | Low rate, light products | Poor at high speed |
| Transfer unit | Heavy or palletised loads | Slow, needs a gap |
The controls handshake
Every merge needs three signals: package present at the merge, main line clear, and release permitted. The failure mode to design out is the double release, where two infeeds are told to release in the same scan. Use a physical or logical token: only one infeed can hold the release permit at a time, and the permit is handed on after the merge photoeye clears.
Set the clear time from measurement. A package 600 mm long at 1.2 m/s clears in 0.5 seconds; a timer set to 1.5 seconds halves the merge capacity for no reason.
Scan tunnels and weight checks
Place the scan tunnel after the merge, not before. Scanning four streams separately needs four scanners; scanning one merged stream needs one. The exception is when a reject decision must be made before the merge, which is rare.
Leave 1.5 m of straight conveyor before the scanner so the package is settled, and 1 m after it before the next divert. A package that is still rotating when it reaches the scanner will produce a no-read.
Maintenance access
Every merge and divert needs access from both sides, and the guard must be removable without tools. A divert that takes 20 minutes to unjam will be bypassed, and a bypassed divert sends packages to the wrong destination all shift.
Design the layout with a 1 m maintenance aisle along the merge section, even if it costs conveyor length. The alternative is a permanent bottleneck that nobody can reach.
