Vacuum gripper loses grip: is the new carton the cause?
A vacuum gripper suddenly starts producing misgrips after a carton change. The cause is almost always the same: the new surface seals less well or lets more air through. The existing vacuum system can no longer build up enough holding force. Porosity, coating and stiffness of the corrugated cardboard determine whether the vacuum gripper still matches the packaging.
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Why the new carton stops the gripper
The short answer to the title question: a vacuum gripper is an end effector that holds cartons by means of vacuum. The vacuum level inside the suction cup multiplied by the sealed area produces a force that exceeds the weight of the box. If the vacuum level drops because of leakage, the holding force drops with it. That is exactly what happens when a supplier change or new packaging alters the surface.
3 properties of the new carton are the usual triggers:
- More porous corrugated cardboard. Air flows in through the material. Leakage rises, the vacuum level collapses. Recycled cardboard without a coating is typically more porous than coated carton and is considered difficult to grip.
- A different coating. A varnish or seal can suddenly make a difficult carton easy to grip. If a familiar coating is missing, the opposite applies: a tight carton turns into a porous one.
- Changed stiffness. A softer top layer is drawn into the suction cup during pick-up, deforms and opens leakage points at the edge of the sealing lip.
Leakage is the airflow that streams into the cup through porous material or leaky spots. What matters for shift operation: leakage varies from carton to carton, even within the same quality grade. That is why the system works in the test and fails later in live production.
The holding force of a vacuum gripper follows the formula F = Δp × A: differential pressure times sealed area. A new carton attacks both factors, because leakage lowers the differential pressure and an uneven surface shrinks the effective sealing area.
Porous cartons need flow rate, not deep vacuum
The flow rate is the volume of air per minute that the vacuum generation moves. The most common misconception with misgrips is the assumption that a deeper vacuum would produce more holding force. On porous corrugated cardboard that is wrong: a higher target vacuum creates no additional holding force, because leakage limits the achievable level. As soon as air flows in through the carton, the vacuum generation has to replace that flow rate just to hold the level.
Piab states as a principle that porous materials need a high flow rate instead of a deep vacuum. For corrugated cardboard, OnRobot explicitly recommends a low vacuum level, for example 20 %, because a high level generates no additional lifting force.
A field measurement on corrugated-handling systems described by Piab found up to 43 % oversizing of the vacuum generation, because systems are conventionally sized for the worst case. The generation is therefore oversized relative to the average carton and still remains the bottleneck on the leakiest one. The orders of magnitude show why. A compact ejector like the SCPLi from Schmalz delivers up to 1,140 litres per minute of suction capacity. A gripper with integrated electric generation like the VGP20 from OnRobot reaches 48 litres per minute. The more porous the carton, the more suction capacity the generation has to supply.
The effect of leakage can be reduced mechanically without changing the carton:
- Shorter lines. Vacuum generation directly at the cup eliminates flow losses in long hoses.
- Larger cross-sections. For permeable workpieces, the pneumatics manufacturer SMC recommends more generous line cross-sections and ejectors with high suction capacity.
- Switchable suction circuits. A flow control valve is a valve that automatically blocks a leaking cup. Models like the SVK from Schmalz or piSAVE sense from Piab isolate every cup that does not seal. A single leaking cup at the edge of the carton can then no longer drag down the vacuum level of the whole area.
3 adjustments: cup, seal, vacuum generation
When the vacuum gripper produces misgrips after a carton change, 3 adjustments are available. The order is deliberate: the cheap component first, the generation last.
1. The suction cup. The sealing lip is the soft rim of the cup that seals the contact area against the carton. Bellows cups compensate height differences and uneven surfaces, because the bellows settles in before the sealing lip closes. A field case from the packaging industry shows the effect. At Smurfit Kappa, the existing system sporadically lost perforated cartons, and the bellows cups wore heavily. The solution from the cup specialist Fipa: bellows cups with a soft sealing lip and a stable body, combined with ejector boxes of high suction capacity. Bellows cups specialised in carton handling like the BCP from Piab work with a stroke of about 25 mm, which compensates height variations. The lip, about 40 mm in diameter, seals even on slightly dusty surfaces.
2. The seal. An area gripper (foam gripper) is a vacuum gripper with a large contact surface and sealing foam instead of individual cups. It covers varying carton sizes with a single tool. The sealing foam adapts to the surface and seals small irregularities. The FMP area gripper series from Schmalz is explicitly designed for highly porous workpieces. What that means in practice is shown by the palletizing cell at Seeberger. The FXCB area gripper from Schmalz used there covers all carton sizes on site with a single tool. Details are in the Seeberger case study.
3. The vacuum generation. If cup and seal alone are not enough, the system needs more suction capacity or decentralised generation at the gripper. Multi-stage ejectors based on Piab's COAX principle deliver up to 3 times more vacuum flow than conventional systems. A side effect: energy consumption falls with the level. Lowering the vacuum from −65 to −55 kPa saves 25 to 30 % energy according to the manufacturer.
When misgrips follow a carton change, the order of checks is: first cup and sealing lip, then lines and valves, finally the vacuum generation. In most cases, adjusting the first of the three is enough.
How to test the new carton before the changeover
A pick test is a suction test on the real system with the real carton. A reliable pick test follows 5 rules. The pick test belongs before every production changeover to a new carton. For permeable workpieces, the pneumatics manufacturer SMC mandates a suction test on the real system with the real workpiece. The reason: depending on the generation, the target vacuum is not reached in operation. OnRobot puts it more briefly: the simplest way to check whether a carton is tight enough is the test with the gripper.
- Test with the new carton, not the old one. Request sample boxes from the new supplier before the batch reaches production.
- Test aged cartons. Fresh samples are smoother and stiffer than cartons after weeks in the warehouse. Pick tests with fresh samples miss exactly the properties that later cause misgrips.
- Test at the edge of the spectrum. The test has to include the heaviest carton, the softest batch and the most unfavourable grip position. The average part is not sufficient as a test carton.
- Measure leakage, not just the outcome. Grippers with an IO-Link interface (serial sensor interface) output the leakage rate and make cup wear and leaks visible before boxes are dropped.
- Document the limits. Which carton variants the gripper holds reliably belongs in the system documentation. The next packaging changeover is then checked against a list instead of gut feeling.
Some suppliers support such tests with loan components. Piab, by its own account, provides test components for customer projects. When selecting new grippers, a look at the range helps: 35 vacuum grippers are listed in the marketplace, from bellows cups to area grippers with sealing foam. What the designs cost is answered in the post on the prices of good robotic grippers.
Sizing holding force and safety factor correctly
The static holding force follows the formula F = Δp × A with 2 quantities: differential pressure in pascals times effective cup area in square metres gives the force in newtons. For moving cycles with acceleration and lateral force, Schmalz extends the calculation. Mass, gravity and acceleration are added, as well as a friction coefficient between cup and carton. The friction coefficient has to come from a test; tables only give guide values. SMC quotes 0.1 for oiled surfaces, 0.2 to 0.3 for wet ones, 0.5 for wood, metal and glass, and 0.6 for rough surfaces.
The safety factor is a multiplier on the calculated holding force that absorbs dust, ageing and batch variation. On the safety factor, the manufacturers disagree:
| Source | Recommendation |
|---|---|
| Schmalz | At least 1.5 for smooth, tight workpieces; at least 2.0 for porous, rough or oiled ones; 2.5 and above when swivelling |
| Festo | 1.5 for vertical and horizontal movement, 2.0 for rotation |
| SMC | 1.5 for smooth, non-permeable workpieces, at least 2.0 for permeable ones, at least 2.5 for vertical lifting; in device selection additionally 4.0 horizontal and 8.0 vertical |
| Accident prevention regulation | Binding 1.5 as the minimum |
Depending on the source and load case, the recommendations thus range from 1.5 to 4.0, and in SMC's device selection up to 8.0 for vertical lifting. For porous workpieces, Schmalz and SMC both state at least 2.0. The documentation of the respective gripper manufacturer is authoritative. Anyone sizing with the smallest permissible factor has no reserve in shift operation for dust, ageing of the sealing lip and soft batches. The relationship between gripper, cycle time and system performance is covered in the post on the throughput of palletizing robots.
Frequently asked questions
Why does a vacuum gripper hold recycled cardboard less well?
Recycled cardboard without a coating is typically more porous than coated carton. Air flows in through the material, leakage rises and the vacuum level inside the cup collapses. The holding force falls below the weight of the carton, and the gripper drops the box or stops the cycle.
Does a deeper vacuum mean more holding force on corrugated cardboard?
No. On porous cartons, leakage limits the achievable vacuum level, so a higher target vacuum brings no additional holding force. What matters is a high suction capacity of the vacuum generation that replaces the leakage. OnRobot explicitly recommends low vacuum levels for corrugated cardboard, for example 20 %.
What is the difference between a bellows cup and an area gripper?
A bellows cup is a single flexible cup whose bellows compensates height differences and uneven surfaces. An area gripper (foam gripper) combines a larger contact surface with sealing foam and handles varying carton sizes with one tool. Bellows cups suit individual cartons at varying positions; area grippers suit mixed formats and full layers.
Does the gripper have to be adapted at every carton change?
Not every time. If the new carton stays within the limits that cup, seal and vacuum generation are designed for, a pick test is enough to confirm. Components are only replaced when porosity, coating or weight lie outside these limits. A documented pick test per carton variant answers the adaptation question before the changeover.
Can I calculate the holding force of my gripper myself?
The static holding force is calculated as differential pressure times effective cup area, F = Δp × A. For moving cycles, mass, acceleration, friction coefficient and a safety factor are added, which the manufacturers specify differently. The calculation does not replace the suction test on the real carton; it only sizes the starting point.
When is an adjustment enough, when does the gripper have to be replaced?
An adjustment is enough when the new carton can be gripped reliably again using the 3 adjustments: a different cup, a different seal, more suction capacity. The gripper is replaced when even that is not enough, for example because the design does not match the carton size or weight. The pick test with sample boxes shows which stage is needed.
