The tray is an aluminium extrusion welded into a frame, roughly 1.9 m by 1.4 m, weighing 42 kg before the cooling plate is fitted. It has to be located within 0.3 mm at the welding station, and it must not be marked by the conveyor at any point.
The constraint list
The customer gave us four conditions that shaped everything else:
- Takt of 38 seconds, including two welding operations of 26 and 22 seconds.
- No contact marks on the anodised outer surface.
- Carrier must accept three tray variants without changeover.
- Line must fit an existing bay 4.2 m wide, with columns every 6 m.
Carrier design
A 60 kg carrier with a 42 kg tray is not a light load, but the difficulty was not the weight. It was supporting the tray on its machined mounting faces rather than its outer skin, because the outer skin is 2 mm thick and deflects under its own weight when supported at the ends.
The carrier uses four adjustable support pads on the mounting bosses, with polyurethane contact faces, and two pneumatic clamps that hold the tray down during transport. The pads are set once with a gauge and checked monthly; the tray sits on the same four points in every station, which is what makes the 0.3 mm repeatability possible.
Conveyor and locating
The conveyor is a heavy-duty chain plate line, 1.6 m wide, running at 6 m/min with a 38-second index. At each weld station, a lift-and-locate unit raises the carrier 40 mm off the chain and pushes it against two fixed datum pins.
Why lift? Because a carrier sitting on a moving chain is never quite still. The lift unit isolates the carrier from the chain, and the datum pins locate the tray, not the carrier. The carrier's own tolerances then drop out of the welding stack.
Measured repeatability at the weld station after commissioning was 0.18 mm, against a 0.3 mm requirement.
Welding and distortion
Two robotic MIG stations weld the frame corners and the cooling plate mounts. The heat input distorts the tray by 1 to 2 mm if the weld sequence runs continuously around the frame. The customer's process engineer specified a skip sequence, welding opposite corners in turn, which brought distortion under 0.8 mm.
The conveyor's contribution was to hold the tray consistently, so the robot program could be tuned once rather than per tray. Before the lift units, the welder was adjusting the program every few days.
Layout in a narrow bay
The bay was 4.2 m wide with columns, which ruled out a loop. The line is a straight run with a return conveyor above the operator aisle at 2.4 m, feeding empty carriers back to the loading station. The return uses the same chain type and a vertical lift at each end.
This arrangement kept the footprint within the existing building and avoided a move to a new bay, which the customer had costed at several million RMB.
Results
| Metric | Target | Achieved |
|---|---|---|
| Takt | 38 s | 36.5 s |
| Locating repeatability | 0.3 mm | 0.18 mm |
| Distortion after weld | 1.0 mm | 0.8 mm |
| Surface defects per 100 | 2 | 0.4 |
| Changeover between variants | 10 min | 0 (no changeover) |
What we would do differently
The return conveyor was added late, after the layout had been frozen, and it forced the lift units to be repositioned twice. A return path should be part of the first layout, not an afterthought once the main line is drawn.
