In a palletizing cell, fault reports rarely start at the robot. They start at the interface between the robot and the product: a dropped case, a bag that slides, a layer set down out of line. The gripper is the most application-dependent part of the system and the one that touches the product every cycle, which is why it belongs in the design conversation from the start rather than being specified after the robot is chosen.
Five holding principles
Cortiva's gripper technologies page defines five holding principles, and the selection is made from within those five:
- Vacuum gripper: cases, cartons and trays with flat, non-porous surfaces.
- Clamp gripper: pails, drums and block loads that need to be gripped from the side.
- Combined vacuum and clamp: cells that handle more than one product type.
- Layer gripper: high-throughput palletizing where a full layer moves in one cycle.
- Bag-specific gripper: holding arrangements designed for soft loads that change shape.
The list looks short, but the choice is narrowed jointly by the product surface, the strength of the pack, how the weight is distributed, the release behaviour and the target rate. The sections below are that narrowing, in order.
Surface comes first
Vacuum needs a surface that is flat enough and effectively airtight. Laminated cases and trays meet that condition; porous kraft board, vented cases and creased film packs erode the holding force. When the surface is unsuitable, the answer is not more vacuum but a different holding principle: clamping from the sides, a bag-specific arrangement designed around the product, or a combined gripper that uses both.
Surface condition also changes during a shift. A case coming out of chilled storage carries condensation; a tray that has passed a dusty filling area carries a thin layer of it. If holding force was measured on a clean sample rather than under those conditions, the cell works on paper and drops product on site.
Pack strength is the limit on holding force
There is a ceiling on holding force, and it sits in the packaging rather than the robot. Vacuum pulls at the surface: on weak corrugated board the top liner can delaminate, a thin tray lid can collapse inwards, a film pack stretches. With a clamp the force comes from the sides and the product can be crushed inwards. In both cases the result shows up not in one cycle but a few thousand cycles later, as a pack quality issue.
A gripper conversation therefore cannot be separated from a packaging conversation. If board grammage, film type or lid construction is going to change, the gripper decision has to be revisited. Lightweighting projects usually run without telling the palletizing side, and their effect is seen first at the cell.
Weight, gripping area and centre of gravity
Payload alone is not a sufficient criterion. Where the weight sits, and how much of the product the gripper can actually reach, matter just as much. A bag whose contents move behaves nothing like a rigid case of the same weight: the centre of gravity shifts in motion and the load spreads on release. Cortiva systems handle bag palletizing from 5 to 50 kg at up to 20 bags per minute, and purpose-built palletizing cells up to 57 kg at up to 30 products per minute, but within the same weight class the holding arrangement still changes with the product.
The usable gripping area is often smaller than the product itself: a label zone, an overhanging lid edge or a film that has ballooned all reduce the vacuum surface. With a clamp, what matters is how much of the side wall stays free. If two products standing side by side on the pallet leave no gap for the clamp to descend into, the pattern has to change, which means the gripper decision also shapes the pallet pattern.
The gripper's own mass comes off the payload
A robot's payload is not the product but everything at the wrist: gripper frame, vacuum generator, valve island, cable and hose package, and the product itself. A combined gripper carries two holding principles at once and is therefore heavier than a single-principle arrangement, and that mass comes straight off what is left for the product. Close to the limit, a change of gripper concept can change the robot class, which is why the two decisions are not taken separately.
Release matters as much as grip
How cleanly a pallet pattern comes out depends less on how the product is picked than on how it is released. If the product drops, slides or nudges its neighbour as the gripper opens, layer alignment degrades and the error compounds as the stack grows: a few millimetres in the first layer becomes an overhanging corner in the eighth, and the load then compresses unevenly during wrapping. Guide plates, controlled opening and supporting the product through the last few millimetres are therefore part of gripper design, not a correction added afterwards.
With bags, release is also a forming operation: the bag lands on the layer below rather than on a flat surface, and its contents are meant to spread. Where one cell handles both cases and bags, release behaviour is defined separately for each.
More than one product in the same cell
If cases, pails and bags all run through one cell, there are two routes: a combined gripper covering several product types, or separate grippers exchanged by product family. The combined route removes changeover stops but adds mass. The exchangeable route stays light, but every change is a stop and the exchange station takes floor space of its own. Which is right depends on how often the product mix actually changes during a shift: a mix that changes twice a day and one that changes twice an hour do not give the same answer.
Loads gripped from the side, such as pails and drums, favour a clamp arrangement; pail and drum palletizing cells use vacuum and clamp together.
Throughput drives the concept
At higher rates, moving product one unit at a time may not be enough, and row or full-layer grippers come into play. A layer gripper moves far more per cycle, but it requires the layer to be formed correctly before the robot picks it. The high-level palletizer is built on exactly that arrangement: the layer is formed below and transferred to the pallet in one cycle.
Layer forming has a price, and it has to be counted. Adding a mechanical aligner to the line costs both speed and format flexibility; on a line with many formats that loss can exceed what the layer gripper gains. The alternative is vision-guided picking, which reads the position of the product every cycle and resolves alignment in software rather than mechanically. In other words, the gripper decision also settles the design of the infeed conveyor and the layer-forming station.
How a wrong choice comes back
- Insufficient hold: the product drops mid-cycle. The visible result is damaged product; the invisible one is an operator entering the cell and the cycle stopping.
- Excessive force: the pack deforms, the top liner delaminates, the lid collapses. It gets logged as a packaging complaint rather than a palletizing fault, and the cause goes unfound for years.
- Uncontrolled release: layer alignment degrades, the error compounds with stack height and the load compresses unevenly during wrapping.
- A heavy gripper: close to the payload limit, the compensation is a slower cycle and the rate target is lost.
- A concept that ignores format changes: every product change needs a gripper change, and the cycle time gained is handed back in downtime.
Questions worth asking on site
- How many different products run on the line, and which are the heaviest, lightest, largest and smallest?
- What is the pack surface: laminated, porous, vented, film-wrapped?
- How much of the top face is genuinely free of labels, overhanging lids or loose film?
- Do the contents move, and does the centre of gravity shift while the product is carried?
- Is the packaging going to change soon, in board grammage or film type?
- How many times does the product mix change during a shift?
- Can the target rate be met by picking single units, or is a row or a full layer required?
- In what position and with what repeatability does the product reach the cell?
Common misconceptions
"If it does not hold, we turn up the vacuum." On a porous or vented surface, more vacuum only feeds the leak; the limit is the pack, not the generator.
"Products of the same weight use the same gripper." Weight is one criterion, not the only one: surface, usable gripping area and the behaviour of the centre of gravity call for different arrangements within the same weight class.
"A combined gripper is always better." The combined arrangement removes changeover stops but adds mass, and close to the payload limit that mass comes off what is left for the product.
"The gripper is specified after the robot." The holding decision affects the robot class, the pallet pattern and the infeed design; left until later, all three get redrawn.
Working through it in order
A workable order: define the product and its packaging first, then choose the holding principle, then work the release behaviour and the pallet pattern together, and only then decide between single, row and layer handling against the target rate. The gripper sits in the middle of that sequence rather than at the end of it, and the rest of the system is designed around it. That is why the cells in the palletizing family share a method rather than a robot.