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Contrast sensors ensure labels are aligned on boxes of sweets

Contrast sensors ensure labels are aligned on boxes of sweets

During high-volume production of confectionery, sealed cartons of bagged candy travel by conveyor to a labelling station. Contrinex’s versatile and high-performance, RGB light-based photoelectric contrast sensor checks the label’s alignment and confirms the presence of print registration markings. If a label is blank, illegible or wrongly positioned, the carton is diverted to a holding area for investigation.

In the food-processing industry, packaged consumer products require labelling – both for identification and for batch control – before final packing. During confectionery production, sealed cartons of bagged candy travel by conveyor from carton-taping stations to a label printer-applicator, before passing to the final packing area.

The printer-applicator, which is located at one side of the conveyor, produces a continuous feed of printed labels from roll-stock. As each carton passes the applicator head, the label-stock advances, peeling a single label and applying it to the side of the carton. From time to time, misfeeds or malfunctions occur within the printer-applicator, which consequently applies labels that are either blank, illegible or wrongly positioned. Failure to detect these errors results in unidentified or untraceable products arriving at subsequent operations or, critically, at the customer’s premises.

A highly reliable non-contact sensor system is needed. The sensor must detect the alignment of labels and confirm the presence of print markings as cartons leave the printer-applicator, rejecting any defective cartons.

A photoelectric contrast sensor from the Contrinex Cubic Small range is ideal for this application. This versatile device incorporates a single-lens autocollimator, delivering a narrow optical beam that allows detection of very small features. The choice of red, green or blue LED light, auto-selectable during set-up, ensures optimal contrast resolution.

Immediately after the applicator, a single sensor is mounted beside the conveyor; adjustable mounting brackets makes for easy positioning. As the label on each carton passes the detection point, changes in contrast trigger the sensor. The transition from carton surface to label body identifies the position and alignment of the label itself, while the well-defined contrast between printed content and the label body ensures reliable detection of even the smallest print marks.

If the label is wrongly aligned or the print markings missing, the sensor triggers a reject mechanism that diverts the carton to a holding area for investigation. An operating range up to 12mm allows adequate clearance between sensor and target, while the compact 40mm x 50mm x 15mm PBTB plastic housing is both mechanically robust and small enough to accommodate any space constraints.

Set-up is accomplished either using a three-button keypad on the sensor, or via IO-Link, a standardized point-to-point serial connection protocol. Photoelectric contrast sensors are available with industry-standard push-pull 4-wire output. Connection to the customer’s control system is via a PVC-sheathed cable with the choice of an integral M12 connector or a hermetically sealed entry.

There is a video here showing the sensors in action on a box labelling application.

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