The Pallet That Collapsed Halfway Through the Stack
A robot palletizer picked cases from a conveyor and stacked them on a pallet. The pattern was a simple grid (2×3 per layer). On the floor, the stack leaned and toppled after 4 layers. The problem wasn’t the robot path — it was the case weight distribution. The cases weren’t uniform weight. Some were full (heavy), some were empty (light). The robot placed them randomly, and the heavy cases on one side pulled the stack off balance. We added a pattern that alternated heavy and light cases, and we used a slave pallet (a stack of empty pallets that feeds one at a time). The stack stood straight. The issue wasn’t the robot — it was the pattern and the pallet handling.
Robotic palletizing and depalletizing is about more than programming the robot to pick and place. The pallet pattern, the end-of-arm tool, the slip sheets, and the pallet handling all determine whether the stack stays upright. This article is how I design palletizing cells that stack stably and don’t topple.
The Cell Layout: Three Zones
A palletizing cell has three zones.
Infeed (Product Input)
The product (cases, bags, bottles) arrives on a conveyor. The conveyor presents each case to the pick position. A sensor detects the case and tells the robot to pick it.
The infeed should have a queue (accumulation) so the robot always has a case waiting. If the infeed is empty, the robot waits — lost cycle time. A short accumulation conveyor (5–10 cases) buffers the supply.
Pick / Place (Robot)
The robot picks the product from the infeed and places it on the pallet. The robot’s reach must cover both the infeed and the full pallet height (including the tallest stack).
Pallet Handling
Empty pallets are fed into the cell. Full pallets are removed. Two approaches:
- Fixed pallet position: One pallet position. The robot fills it, then stops while the operator removes it and replaces it with an empty pallet. Simple, but the robot is idle during pallet change.
- Dual pallet position: Two positions. The robot fills one while the full pallet is removed from the other. Continuous operation. A powered roller conveyor or a pallet chain moves pallets in and out.
For production, dual position is standard. The robot doesn’t wait for pallet changes.
End-of-Arm Tool: Picking Cases, Bags, or Bottles
The EOAT depends on the product.
Case Gripper (Vacuum or Forks)
For corrugated cases, a vacuum gripper (multiple suction cups on a plate) picks the case from the top. The cups span the case. The vacuum holds the case while the robot places it.
For heavy or plastic-wrapped cases, a mechanical gripper (fingers that clamp the sides) may be needed. Vacuum works for light cases (under 15 kg). Mechanical for heavy cases.
Bag Gripper (Bags of Product)
For bags (seed, pet food, chemicals), a bag gripper has a clamp that squeezes the bag sides. It lifts the bag and places it with controlled drop (the bag flops into place). A vacuum gripper doesn’t work well on flexible bags.
Layer Gripper (Whole Layers)
For high-speed palletizing, a layer gripper picks an entire layer of cases (from a layer former) and places it on the pallet in one move. Faster than picking individual cases. But it requires a layer former (a station that arranges the cases into a layer before the robot picks it).
| EOAT Type | Product | Speed | Best For |
|---|---|---|---|
| Vacuum (top pick) | Cases (light–medium) | Medium | General case palletizing |
| Mechanical clamp | Heavy cases, plastic bins | Medium | Heavy or non-porous products |
| Bag clamp | Flexible bags | Medium | Bagged products (seed, food) |
| Layer gripper | Whole layers | High | High-speed, layer-former cells |
Pallet Pattern: Stability Comes From the Arrangement
The pattern (how cases are arranged per layer) determines stack stability. A random stack topples. An interlocked pattern stands.
Common Patterns
- Block (grid): All cases aligned the same direction. Simple, but the vertical seams line up between layers. The stack can split along the seam. Least stable.
- Brick (interlocked): Alternating layers rotate 90° or shift by half a case. The seams are staggered. More stable. Standard for most palletizing.
- Pinwheel: Cases arranged in a pinwheel pattern (each case perpendicular to the next). Maximum stability for odd-sized loads. Used for mixed-size cases.
Stack Height and Overhang
The cases must overhang the pallet evenly. A case that hangs off one side pulls the stack that way. The pattern must keep the load centered on the pallet.
The max stack height is determined by the product (a 2 m stack of cases) and the robot’s reach. The robot must place the top layer within its vertical reach. If the stack is too tall, the robot needs an elevator or a longer arm.
Slip Sheets
For unstable products (bags, light cases), a slip sheet (a thin cardboard or plastic sheet) between layers spreads the load. It prevents the cases from sinking into the layer below. The robot places a slip sheet between layers (a slip sheet magazine feeds them).
The stability rule: After the stack is complete, push it with your hand. It shouldn’t wobble. If it does, the pattern is wrong (not interlocked, overhang uneven, or layers not aligned). Fix the pattern before blaming the robot path.
Depalletizing: The Reverse Problem
Depalletizing (taking cases off a pallet) is the reverse, but with added complexity.
Layer Detection
The robot must find the top layer. A vision system (or a height sensor) tells the robot where the top of the stack is. As cases are removed, the top lowers. The robot adjusts its pick height automatically.
Mixed Pallets
If the pallet has mixed cases (different sizes, weights), the robot needs to know which case is where. A vision system (2D or 3D) identifies each case’s position. This is “random depalletizing” — harder than uniform depalletizing.
Robot Selection: Reach and Payload
The robot must reach the corners of the pallet (not just the center) and lift the heaviest case.
Reach
A standard pallet is 1200 × 1000 mm. The robot must reach the far corners from its mounting position. A typical palletizing robot has a 2–3 meter reach. Mount it 1–1.5 meters from the pallet center.
Payload
The robot payload must handle the case + the EOAT. A 20 kg case + 10 kg gripper = 30 kg payload. Size the robot with margin (40–50 kg) for acceleration forces.
Speed
Palletizing is a point-to-point motion (pick from infeed, place on pallet). The robot’s speed at these distances determines the cycle rate. A typical palletizing robot does 8–12 picks per minute. Layer grippers do 1 layer per 10–15 seconds.
Safety: The Cell Is Fenced
A palletizing robot moves heavy cases at speed. The cell must be fenced.
- Fence with interlocked gate: The cell is enclosed. The gate opens for pallet changes (empty pallets in, full pallets out).
- Light curtain at infeed: The infeed conveyor enters the cell. A light curtain stops the robot if someone reaches in.
- Pallet exchange zone: The full pallet exits on a conveyor. The operator doesn’t enter the cell. The exchange is automatic (rolls in/out).
A Palletizing Cell Checklist
- What is the product? (Cases, bags, bottles?)
- What is the case size and weight? (EOAT selection.)
- What is the required palletizing rate? (Cases per minute.)
- What is the pallet pattern? (Block, brick, pinwheel?)
- Is a slip sheet needed?
- Single or dual pallet position? (Continuous operation.)
- Does the robot reach the pallet corners? (Reach check.)
- Is the robot payload sized for case + gripper?
- For depalletizing: is layer detection (vision/height) set up?
- Is the cell fenced with interlocked gate and light curtain?
- How are empty pallets fed and full pallets removed?
- Has the pattern been tested for stability?
The Bottom Line
Robotic palletizing cell design isn’t programming a robot to pick and place. The pattern determines stability (interlocked, not block). The EOAT matches the product (vacuum for cases, clamp for bags). Dual pallet positions keep the robot running during pallet changes. Vision or height sensing finds the top of the stack for depalletizing. The cell is fenced. The stack that toppled wasn’t a robot error — it was a block pattern on uneven cases. Interlock the layers, balance the weight, and the pallet stands upright through shipping.
