How to choose the most appropriate inspection system to ensure the highest quality print on a particular machine type and for each type of job. A producer’s analysis.
Mirco Cremasco *
The search of the highest quality in the rotary printing process is a task no printer can afford to ignore. The performance of the newer machines (over 500 m/min) and customers’ need for guarantees of quality of the production are such that control of the production at the end of each coil as well as the use of simple display cameras are no longer sufficient.
For some time now the 100% inspection systems have constituted an increasingly performant and reliable alternative, featuring leading edge tools for the monitoring of particularly demanding production, having now become essential equipment for all rotary printing. The line quality control is not only used to identify any defective parts to be discarded but, more generally, it helps the machine operator to monitor the progress of production and be able to act quickly making adjustments or solve problematic situations. The result is a more efficient production and a reduction of waste.
This said, it should be stated that to achieve these benefits the inspection system should be chosen depending on the type of production and the type of press used. And since it is not easy to pick ones way through the technical data provided by different manufacturers, we here provide some information that should help to clarify things.
The two aspects of resolution
The first parameter to consider is the system resolution (ie the size of the material portion framed by each individual camera pixel): the higher the resolution, the higher also the ability to detect small defects.
100% inspection systems capture the image of a printed film in real time through one or more linear cameras. In particular, these acquire a sequence of images as wide as the film, a pixel high in the direction of motion of the film and synchronized with the movement of the material through an encoder. The result is an image of fixed width and infinite height; hence the resolution is then calculated in the two dimensions.
The horizontal resolution is given by the width of the film pictured, expressed in mm and divided by the actual number of camera pixels. The vertical resolution is determined by the speed of the camera – line rate expressed in kHz – namely by the amount of lines that it can capture in a second. The resolution is identified by dividing the maximum speed of the rotary press expressed in m/s by the line rate.
Here is a calculation example. If the minimum flaw or defect to be detected is 0.4 x 0.4 mm (WxH) on a 1250 mm film which moves to 300 m/min (5000 mm/sec), the camera system will have to have, overall, a number of pixels above:
"Film width (mm)/ horizontal resolution L (mm) or, in this case, 1250/0, 4 = 3125 pixels”.
The line rate, in turn, must be greater than:
"Line rate, or speed of the film (mm/sec)/vertical resolution H (mm), or in this case 5000/0, 4 = 12.5 KHz." An improper sizing of the system resolution makes it almost impossible to detect the defects one is looking for.
The sensor, a sensitive variable
To determine the correct resolution, it is important to know the technology of the camera sensor, which can be of two types.
In color Tri-linear cameras the sensor is composed of three separate lines, one for the color component RGB (red, blue and green): in this case the resolution actually corresponds to that calculated.
There are, however more inexpensive color cameras, called Bi-linear cameras because they consist of only two lines. In this case, the pixels sensitive to the single RGB component follow on from each other in a RGRGR and GBGBGBG pattern; for each image pixel two color RG or GB components are captured, and the missing one is reconstructed by interpolation, using what is known as a Bayer filter. In this case the quality of the captured image decreases and the actual resolution is reduced to about 60% of the previously calculated value.
The cost of a 100% inspection system is largely determined by the resolution offered, on the basis of which the number and the size of the cameras to be used are decided. The potential buyer will have to carefully evaluate their needs in terms of the minimum size of the defect or flaw to be detected, taking into account the performance the printing system being used, including the accuracy of the register control or the plate alignment system.
Defect detection software
Another key element of the validity of a 100% inspection system is the defect detection software. It is a delicate function because it requires the ability to distinguish the real defects or flaws from natural variations due to tolerances of the printing process (and in the case of flexible films, the software must also be able to process the deformations of the material in motion). These qualities are difficult to evaluate and also any tests on samples off-line are not capable of providing totally reliable indications.
Starting from the assumption that most of the existing systems are now able to achieve a good quality of analysis, one must carefully consider how they should be used. In fact, as already mentioned, an in-line inspection system is useful if it is able to not only show up the defects that produce waste, but it is also able to help the operator to prevent or halt incipient situations that generate waste. It is therefore essential that the system is extremely intuitive to use, with software capable of independently assessing the characteristics of the print operation and to adapt to production automatically, just leaving the setting of a very limited number of parameters to the operator.
Said otherwise, a tool difficult to use becomes impractical; it is important that the operator understands the logic of operation, hence being able to exploit the potential and appreciate the effectiveness of the same.
Important ancillary functions
Some systems have certain ancillary functions, the utility of which must be assessed case by case. For example, interfacing with the production system, the integration of automatic labeling systems to report flaws, the acquisition of the master from the PDF, if well designed can prove useful.
When working with flexible film the control of the print pitch is particularly useful. Performed by the same camera control defects or with a special sensor, this enables the operator to constantly monitor the pitch and hence to ensure compliance with the tolerances set by the customer.
Equally essential, lastly, is the ability to store images of all defects in a large archive, to consult and verify the production of each coil at a distance in time, useful in case of disputes.
*Mirco Cremasco, Technical Department Manager Visionlab Srl