Carton manufacturers world wide live with high volumes of rejected product. Returned by the truckload and stacked on palettes, it's simply not practical to inspect and sort failed cartons, which are routinely discarded to the pulp mill.

The costs are high. Carton manufacturers must allow for extra production capacity to replace product rejected by the client. They may also have to pay contract penalties to clients and overtime rates to operators. And then of course there's the prospect of losing major supply contracts or incurring damage to client relationships and brand names.

A STICKY PROBLEM
Carton production uses rectangular sheets of cardboard that are fed through a die cutter. The resulting carton shape passes along a conveyor, where glue strips, in this case three, are applied onto tabs along the edges. Continuing along the conveyor, panels are folded up before one edge of the carton is squeezed onto another, with the glue strips in between. The flat-finished cartons are then stacked onto a palette, before being shipped to the client.

Problems arise however, when the glue is incorrectly applied:

· When there's too much or too little glue.
· Where glue strips start and / or stop at the wrong point along the tab.
· When the carton is skewed so that glue strips are either inside or outside the edge of the tab.

The consequences can be serious for clients, who may discover failed units when it's too late. When glue is improperly placed, the inside layers of a carton may stick together so that the carton will not erect. Too much glue may mean client machinery picks up more than one carton. Too little glue may mean it's possible to erect the carton and put a product inside, but once a batch reaches the loading bay, the palletiser picks up a carton and the carton collapses, together with its client contents.

Carton production lines typically run at web speeds of eight metres per second and more, while the working environment involves vibration, wet glue, dust and flying fragments of cardboard. During slow speed idling, when checking the first carton, it's possible for human eyes to see the quality of glue strips. But once the line is cranked up to full speed, perhaps with a safety cage, it's impossible to monitor.

RE-WRITING THE STANDARD
Most previous attempts to fix this type of problem have depended on discreet sensors, lasers or similar devices to detect the presence or absence of each of the glue strips. However the strips are small, and although discreet sensors see an absence if they move out of line, not all misplacements are detected, and the volume of glue cannot be adequately monitored.

Due to some recent cost / performance improvements of DVT's SmartImage smart sensors a customer decided to review how they could use these self contained inspection systems as they provide a visual data source with low connectivity costs, high speeds, seamless data exchange throughout a smart factory enterprise. Once a series of DVT bench tests had given positive results the first test system was installed over a weekend and with no interruption to production, was an instant success.

The first inspection image, clearly showed just hairlines of glue it meant it was possible to re-write the standards for the packaging industry. With a 100% integrity check, why were three stripes needed? Why not two, or even one? The savings could be considerable!

But this was only the beginning…

ACCESS PRIVILEGES
Because it's not easy to stop a large high-speed line, occasional defects could not easily be removed. Experiments were carried out with a variety of techniques: stopping the line when five successive defects were identified; even focusing attention on installing a bomb-door reject mechanism. However it was preferred to provide best value by removing the cause of defects, rather than just reject them from the line.
The DVT camera was supplied with a fast PLC, connected to photoelectric sensors and a line shaft encoder, also connected to an HMI panel. This system provided a graphic display for the operators to validate the dimensions of the carton and the location of the glue strips.

In an industry as mature as carton production, such entrenched waste problems can encourage non high-tech minded machine operators to react to new solutions with a degree of scepticism. Ultimately, operators can easily choose to operate a by-pass switch to disable the camera inspection.

Using standard Ethernet communications, SmartImage Sensors support TCP/IP, Modbus/TCP and EtherNet/IP. Profibus and DeviceNet are supported with the optional SmartLink module. As part of the trial installation, it was ensured that 100BaseT Ethernet TCP/IP was connected to the factory WAN so providing access to regularly check on progress using the internet.

Before an operator had even reported any problems it was therefore possible to detect off-site, that 'off-cuts' from the tabs had attached themselves to the glue gun and so during trials contact the plant to suggest that operators remove the obstructions. Similarly, over a period of time stalactites of dried glue grew downwards from the glue gun and needed to be removed. Crucially, whether connecting in response to receiving an operator report by telephone or unprompted DVT could always validate the camera's performance and operation.

SYSTEMATIC FINE TUNING
Having got the basics in place, the system began to detect a number of systemic faults, each of which the team was able to set about correcting. First, at full line speed, the glue strips were often hairline and never full specification in thickness. The first fix was to apply more pressure to the glue accumulator, followed by installation of an improved pressure regulator.

In addition, because of data received from the DVT system, the glue solenoids were rebuilt to prevent spluttering and spray at the start and stop of glue lines. The system also detected that too many cartons were skewed on the line, leading to paying attention to the conveyor's drive-belt system. Finally it was also possible to determine that a recurrent problem of too little glue was in fact due to a batch of glue that was off- spec and overly viscous.

Until the system was installed, different faults appeared to be random. But the DVT camera made it possible to isolate causes and enable specific fixes. Because of this, defects are now down to zero which means that need for high cost reject mechanism is off the agenda. This system the SM-VIDMON glue detection and rejection system, was developed (now patented) by the company SMOFAD and is revolutionising the packaging industry.