The difference is clear

MACHINE BUILDERS and end-users are well practised at detecting transparent objects, such as clear glass, plastic bottles and jars, plastic blister packaging, and plastic films. Traditional transparent object detection sensors are typically tuned for each production run when the shape or colour of the target packaging changes, because the sensor can usually only see the thicker front and rear of the packaging being filled.

Imagine a labelling machine looking at clear glass jars – it would first detect the rear edge of a jar, then the front face of the next jar, before placing the label on that jar. If a water droplet strays onto the packaging, a traditional sensor might falsely trigger on the droplet, and upset the count, so that from now on, the label machine is applying the labels to the gap between the jars.

Features on the side of the packaging, like raised lettering or the strengthening ribs in plastic trays used, for example, for meat, can be problematic. The packaging machine needs to ignore the signals from traditional sensors, which will detect the ribbing as well as the packaging’s front and rear faces.

Contrinex offers conventional transparent object detection sensors that use red light, as well as a family of patent-protected sensors that uniquely use ultraviolet light and special reflectors. These utilise the fact that UV light is blocked by plastic and glass. Unlike red light, UV light is consistently blocked by thin films or thick objects, removing the need to tune traditional transparent object-detection sensors for every packaging design. These unique sensors provide unparalleled reliability and operational benefits for packaging and bottling lines.

The thermoforming challenge

Blister packaging is used widely because of its low cost and suitability for automated filling. The process involves in-line vacuum-forming of clear thermoplastic material into semi-rigid trays, which are then filled and sealed. Maintaining the correct tension of the continuous reel-stock material as it feeds into the loading station is crucial for consistent product quality and line efficiency.

Traditional photoelectric sensors that use visible light can struggle with the material's inherent transparency. They often cannot reliably distinguish between "material present" and "material not present," especially when the plastic sheet varies in thickness, shape, or speed. This can lead to erratic feeding, material waste, and frequent production interruptions.

The breakthrough lies in the use of polarised ultraviolet (UV) light at a specific wavelength (275 nm). At this wavelength, commonly used transparent plastics exhibit significantly higher absorption of UV light compared to visible light. This fundamental difference is what allows the new sensors to detect even the thinnest transparent materials with exceptional accuracy.

Contrinex C23 transparent-object reflex sensors are mounted between the reel feeder and the loading station to constantly monitor the presence of the transparent material loop constantly.

Why this UV approach is so effective:

- High Absorption Factor: The strong absorption of UV light by transparent plastics produces a clear, distinct signal, ensuring a high operating reserve even with very thin films.

- Insensitive to Light Distortion: Unlike visible light, UV light at this wavelength is largely unaffected by the target's transparency, thickness, shape, or speed, eliminating multiple reflections on a single target.

- Elimination of Blind Zones: Auto-collimated optics ensure there are no blind zones, enabling reliable detection near the sensor or through small notches in the material.

- Contamination Tolerant: These sensors boast very low sensitivity to dust, liquid droplets, and splashes – common challenges in factory environments – further enhancing their reliability. 

For manufacturers and end-users, UV transparent-object sensors offer reliable and accurate detection, even of the thinnest plastic materials, helping to minimise false detections and unplanned stoppages. This reduces the need for constant operator intervention, maximises uptime and maintains production throughput, delivering potential cost savings.

Setup and maintenance are also simplified, with one- or two-step teach procedures and IO-Link enabling remote retrieval and adjustment of sensitivity parameters. A stability alarm can also highlight reduced sensitivity, allowing preventive maintenance to be carried out before performance is affected or costly breakdowns occur.

Advanced features

Beyond the core UV technology, these sensors offer features suited to modern factory automation, including IO-Link integration on PNP versions, enabling remote retrieval and adjustment of sensitivity parameters to simplify integration and maintenance. Ecolab-certified plastic housings with an IP67 rating provide robust, durable performance in demanding environments, including medical packaging, while a sensing range of up to 1200mm, adjustable via the teach button or IO-Link, accommodates a wide range of machine geometries.

For packaging operations using transparent packaging, the adoption of Contrinex’s UV transparent-object sensors represents a significant leap forward in reliability and efficiency. PLUS Automation represents Contrinex in the UK and Ireland, and helps engineers #MakeSenseofSensors every day to optimise high-speed production, minimise downtime, and save money.

Mark Weymouth is managing director of PLUS Automation

plusautomation.co.uk