Product category:
Vision and Colour Sensors
News Release from: Cognex UK | Subject: In-Sight 2000
Edited by the Engineeringtalk Editorial
Team on 25 March 2003
Vision sensors are key to in-mould
labelling
The process of in-mould labelling poses a number of challenges with respect to correct label placement, and machine vision sensors are crucial to its successful implementation.
Pioneered in Europe in the early 1970s, in-mould labelling (IML) is the process whereby a label becomes forged into the wall of a container during injection moulding Although IML can often be expensive to implement, it offers a number of advantages over conventional glue-on labelling
This article was originally published on Engineeringtalk on 20 Mar 2000 at 8.00am (UK)
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For example, because the labelling occurs during the moulding process, no extra labelling steps or equipment is required.
Additionally, without the need for a separate labelling station, line throughput can often be improved.
Among US companies using IML is Precise Technology, a contract injection moulder serving the packaging, consumer/industrial and healthcare markets.
The company's Bridgeport, New Jersey plant is currently the largest moulding facility in the states using IML, where it uses the process on Pampers baby wipes containers.
"A key advantage of using IML for us is product safety", explains Jim Bruynell, Maintenance Manager at Precise Bridgeport.
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"Because the label becomes part of the container during moulding, it can't be peeled off, which can be a safety hazard in terms of kids choking.
The other benefit is that the entire container can be recycled, since the label and the tub are both made of polypropylene".
Bruynell also points out, however, that the IML process poses some challenges with respect to correct label placement.
"We use robots to place each label into the mould cavity, and the robot creates a electrostatic field around the label so that it automatically sticks to the tub.
If the field around the label is not strong enough, it may cause an arc in the label when we inject the plastic, which can cause the label to shift to the left or right".
While improper label registration is the main problem with which to contend, other problems can occur.
For example, labels which have been misloaded into the moulder may be applied backwards.
And because the plant produces containers for more than seven different styles of baby wipes, occasionally the wrong label might be applied.
In addition to contending with these problems, Precise needs to ensure that a blank white box printed on the label, where a lot code is later applied at the end-customer's facility, is in the same position every time.
To ensure that each and every container leaving the Bridgeport facility has a perfect label, the plant relies on In-Sight 2000 machine vision sensors from Cognex.
The sensors are used to verify label position within a +/-2mm tolerance, ensure each label is the correct one for the container, and inspect the lid hinge to make sure the lid has been properly snapped into place.
In-Sight vision sensors are compact, general-purpose vision sensors featuring a digital machine vision camera, high-speed vision processing unit, a full library of vision software tools for measuring and inspecting the containers, and a vision spreadsheet interface for application set up.
The set up process involved selecting vision tools and parameters from simple drop-down menus.
The spreadsheet then automatically generated tool results into worksheet cells, which were then linked together to set up the measurements.
According to Steve LeBlanc of Serview, a Bristol, Pennslyvania based systems integrator that helped Precise implement the vision solution, vision sensors were an appropriate choice over PC-based vision systems for this application.
"The sensors had the right vision tools for the job, and were also fairly straightforward to use for those who may not have had previous experience with machine vision", he said.
"Also, the sensors are less expensive than PC-based systems, making the overall installation more cost-effective for the company".
Two vision sensors are used to provide full inspection coverage of the left and right hand portions of each container.
The cameras are mounted in a light-shielding cabinet approximately 450mm above the inspection point.
Once a container has been moulded and the lid has been snapped into the top, the entire assembly is flipped upside down (so the tub bottom faces up) and moves down a conveyor towards the inspection point.
Looking down at a 45-degree angle, the cameras, triggered by photo-optic sensors, capture an image of each side of a container, and the images are immediately transferred to the vision processor.
There, they are analysed using the In-Sight PatFind pattern-matching tool, which compares each image to a pretrained reference image of what the label should look like.
Once this match has occurred, various edge detection and inspection tools are used to measure the exact x, y and theta position of the label, and to ensure that the lid has been properly assembled to the container.
More than 30 containers are inspected per minute.
To optimise the accuracy and reliability of the inspections, LaBlanc used multiple lighting techniques to highlight specific part features.
"The application involves a myriad of inspections, from measuring label position to assembly verification of the container.
There were also a variety of part appearance variations to deal with, such as glare on the part, different coloured plastics, and label graphic variations.
So, we implemented a highly engineered illumination system involving back lighting, front lighting, and structured lighting techniques.
What comes out of all this are really nice, high contrast images that work very well for the vision sensor's measurement and inspection tools".
If a container passes inspection, it proceeds down the conveyor to a diverter and is then placed into a stacker.
Containers that fail inspection are ejected off the conveyor and discarded.
A VGA monitor above the inspection shows a live image of the inspections as they occur, green and red LEDs indicating pass or fail, a readout of the total number of parts inspected per shift, as well as the number of bad parts per shift.
According to Bruynell, since the vision sensors were installed, they have been able to inspect the labels 24/7 with a high degree of accuracy and repeatability.
"The accuracy is superb, and we know the labels we end up shipping are the same every time.
This allows us to ensure 100% product quality for our customer, and ties in with our overall continuous improvement strategy".
To date, the company has implemented the two-vision sensor solution on four lines, and currently has plants to tie the inspection results data into their ERP system for improved production reporting. Request a free brochure from Cognex UK ...
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