Real time film inspection with speeds up to 1,200 m/min
Witten-based OCS GmbH is one of the world’s leading manufacturers of production-integrated film inspection systems. Up to ten cameras operate side by side in scalable systems enabling production speeds of up to 1,200 metres per minute. Machine vision components from STEMMER IMAGING make up the optical solution.
Plastic films can be found in a wide variety of different thicknesses and colours – transparent or opaque, structured or coated – in almost all industries and areas of life. Applications range from flat screens, cell phone displays, pharmaceutical tablet blisters, food packaging to baby diapers.
Films are produced in huge plants 24/7, around the globe. Their quality depends on many parameters. Quality monitoring and quality assurance during production have a significant impact on the end product’s quality. There are a variety of standards: the specific requirements are agreed individually between manufacturers and users. Type and number of permissible errors are reflected in the production costs and, consequently, in the prices for the end users.
There are a variety of standards: the specific requirements are agreed individually between manufacturers and users. Type and number of permissible errors are reflected in the production costs and, consequently, in the prices for the end users.
„In simple terms: nobody likes to spot sealed insects in packaged cheese; there shouldn’t be any turbidities on TFT screens, and you definitely do not want to find any holes in the insulation of high-voltage cables,” exemplifies Mendo Gusevski. He is a product manager at OCS, a company which specialises in the optical quality assurance of polymers and the films produced from them.
The measurement range of defects in extruded films mostly depends on their intended use:
- Packaging film (PET) requires reliable detection of defect sizes between 150 μm and 500 μm.
- In technical film the defect sizes range from 50 to 200 μm.
- In surface protection film they range from 60 μm to 160 μm.
- In film with specific optical characteristics it may be necessary to reliably detect defects ranging from 25 μm to 100 μm.
Defects occur during the extrusion process due to incorrectly melted material (so-called specks or burn marks), fisheye, thin places, holes, streaks, flow lines or simply dirt. Whether quality monitoring should classify inhomogeneities as defects depends on the subsequent use of the film. It would be impossible to detect all these defects at the required speed without the use of machine vision.
Over the years of working together with STEMMER IMAGING, OCS have developed a scalable, modular inspection system, which includes up to ten cameras operating side by side. In practice, most applications use systems with two cameras for the inspection of film widths ranging from 1500 mm to 2000 mm.
In addition to the line scan camera’s sensor size of up to 8192 pixels (8k) the camera distance defines the attainable resolution. For example, films for the packaging industry generally do not require resolutions over 150 μm to 200 μm, which corresponds with the use of one camera per metre of film width. Technical films are inspected from double to triple the resolution and therefore require a greater number of cameras.
The mere number of pixels is not the measure of all things. The sensor elements in 8k cameras are smaller and consequently less light-sensitive. In the case of less light-permeable material you would be more likely to opt for a 4k camera with a larger sensor diameter. This simple consideration shows that plant engineers must bring along a great deal of experience in order to optimally meet the technical requirements and minimise the costs.
Each camera with its own frame grabber
In order to manage the amounts of image data resulting from real time production speeds of up to 1,200 metres per minute, each camera is equipped with its own frame grabber. Camera and frame grabber are both included in a common housing attached to the machine. These work stations offer the necessary modularity and short signalling pathways between camera and frame grabber. Integrated cooling fans generate a slight overpressure to prevent the deposition of production-related dust on the lens. In harsh and extremely dirty environments, hermetically sealed housings are used, which are then cooled by a separate dust and moisture purge air circuit.
OCS and STEMMER IMAGING have agreed on a number of internal standards for cameras, cabling and illumination. The collaboration led to special firmware versions regarding JAI and Teledyne Dalsa cameras as well as Teledyne Dalsa frame grabbers successfully delivering around 90 % of the applications.
STEMMER IMAGING has long left their role as pure component distributor: „In order to better support our key customers we have built up profound expertise in the pre-assembly of modules“, explains Georg Schelle, Senior Key Account Manager at STEMMER IMAGING, who has been supporting OCS for many years. „These modules save our customers a lot of integration time compared to individual components. Especially for OCS we are working with highly complex optical modules with Qioptiq lenses and filters from Schneider Kreuznach or Midwest Optical Systems. Our clients benefit from shorter delivery times, higher operational safety and reduced cost.“
The LED lines lights for OCS with lengths ranging from 100 to 3,600 mm are now manufactured by STEMMER IMAGING supplier CCS in Japan. Today's design is based on a proven standard product, that has continuously been improved. "This is reflected in a number of characteristics: for example, the homogeneity of the LED light output, the diffuser properties in all three axes, the concept of connectors and maintenance as well as the IP54 protection class," explains Georg Schelle. "Basically, we deliver a modular platform for OCS solutions across all machine vision elements, which can be adapted to specific applications."
Pre-testing in OCS in-house test and training centre
A rewinder located in OCS’s in-house test and training centre is available for pre-testing the equipment of a planned system. Sample film material can be verified under real conditions at speeds of up to 400 m/min and system designs can be optimised according to the customer’s needs.
OCS are constantly testing new technologies with this system, such as a specific Truecolour illumination with controllable LED elements which allow the setting of different colours and, if necessary, even the use of near infrared light. The Mitsubishi Electric Contact Imaging Sensors, innovative polarisation cameras as well as the uncooled Teledyne Dalsa Calibir LWIR thermography cameras have all been tested with this system.
„We normally use traditional monochrome line scan cameras in our systems”, explains OCS expert Mr Gusevski. „For special applications we use JAI cameras with three sensor lines and an RGB prism.“
However, the possibilities of a plant engineer like OCS are constantly growing with new, innovative technologies. This is why OCS always thinks outside the box in order to find new technologies for future systems at an early stage. *„Here again, working together with STEMMER IMAGING pays off for us“, emphasises Mr Gusevski.
Inspection for process control
In current systems all errors are recorded as individual images, which can be evaluated retrospectively. The analysis software developed by OCS allows operating personnel the use of different kinds of error trends, which help derive alarms and specific modifications of process parameters in the production process. Trend data show tendencies as a result of modifications.
This way, problems can easily be detected whether it is with certain additives fed, with extraordinary dirt ingress or when changing the pellet tanks, and can then be allocated to certain sections. The defective film sections can then be cut out in subsequent commissioning processes.
A few years ago the QS management was satisfied with an inspection rate of 10% the web width. Today, as computing power constantly increases, there is a growing demand for 100% inspection which can be reliably provided by OCS.
„Currently the widest web section we are able to inspect with our FSP 600 system at the speed of 1,200 m/min, is 870 cm wide. This enormous amount of data, which sums up to 400 MByte per camera, can only be handled on a hardware level – and, if necessary, we talk directly with Teledyne Dalsa developers about details that do not affect other users with less time-critical processes”, explains Oliver Hissmann, Sales Manager at OCS. „The highest resolution required by a customer is currently 25 µm at a web width of 1,500 mm. This is solved with ten synchronised cameras.“