When packages are handled by suction, selecting the right cup and vacuum generator may be critical to proper performance of the machine.

 

Among tasks suction cups are used for are: feeding leaflets, knock-down (KD) cartons and other items from magazines; erecting folding cartons and shipping cases; placing bags for filling; putting slip sheets on pallets; placing packages in cases; placing cases on pallets; and transferring cut blisters. For each application, design parameters for vacuum handling should be based on the properties of the item being handled and functional requirements of the application.

 

The item to be handled may be porous, if so, the porosity of the material will allow air to enter the vacuum zone, reducing the holding force. This must be compensated for by drawing air out through the vacuum generating device. Vacuum generators for such applications should be selected for high flow rate rather than high negative pressure. A vacuum pump or a single-stage pneumatic vacuum generator is a better choice when handling porous materials than a two-stage pneumatic vacuum generator. For example, a generator designed to produce 15 inches of mercury (Hg), approximately -7.5 pounds per square inch gauge (psig), with high air flow may perform better than one designed for 25 inches Hg (about -13 psig) that doesn't move air fast enough to maintain its vacuum level.

 

Corrugated paperboard is highly porous and its porosity varies with the natural variation of paper fibers. Jointly published by the Fiber Box Association and the Packaging Machinery Manufacturers Institute, "Handling, Feeding and Set-up of Corrugated Fiberboard Boxes on Automatic Packaging Machines Using Vacuum Equipment" provides guidance on the amount of vacuum induced airflow through the paperboard into the suction cup that is needed to securely hold a case, as a function of the paper porosity.

 

It is important to know the force required for each application and design the vacuum handling system to comfortably sustain that force. Anyone who's seen cases drop before they get to their position on a pallet or cartons pop off their cups in the process of being erected will understand why. The holding force applied by each suction cup can be readily calculated. It equals the negative pressure maintained inside the cup multiplied by the area of the substrate actually exposed to that negative pressure. Cups with internal ribs are useful, because the ribs keep the cup from collapsing, so they can sustain a large negative-pressure zone.

 

Bellows shaped cups are useful for objects that vary in shape or are at a variable distance from the picking device. They will have a range of distance available to contact the object, and collapse when vacuum is applied. They do not, however, control the position of the object well, allowing it to sway in handling. Whenever possible, the handling mechanism should be designed to contact the product correctly rather than counting on bellows cups to correct an imperfect condition.

 

Releasing the hold of a suction cup may be important to machine performance, especially in high-speed operations. A shot of positive pressure air through the vacuum line will deplete the vacuum and break any adhesion between the cup and substrate. On cartoning machines, for example, this is often done through a rotary valve in which the injection of reverse air can be timed in the machine cycle.

 

Many vacuum generators have a built-in blowback feature, triggered by an electrical signal. Blowing back can also help prevent clogged vacuum lines by removing dust or paper fiber sucked in off the substrate.

 

Although a suction cup looks like a simple, low-tech item, it requires proper design consideration.

 

As a consultant, I have investigated applications where a low productivity condition on an otherwise well-built packaging machine was solved by modifying the vacuum system that controls package handling. The modifications are always done to better accommodate the properties of the packaging material being handled.

 

How to select the suction cup material

 

Suction cups are a wear item and should be replaced on a scheduled basis to assure consistent performance. They are made from polyurethane, silicone rubber, synthetic rubbers, vinyl and other materials, which are available in different hardnesses. A softer cup will conform to surfaces better for better performance on difficult substances. Hard cups will wear better, needing less frequent replacement. Materials compliant with the Food and Drug Administration may be required for direct or indirect product contact applications. High or low temperatures, or chemical exposure, may also drive material selection.