What's involved in specifying a vision system for a printing press?
There are three key figures that decide the cost of your inspection system.
The three key factors are:
- Line speed – does your line run at 100mpm or 10mpm?
- Product widths – what is the maximum width of product to run on that line?
- Minimum defect size – what is the smallest defect that must be detected?
Printing process is key as it defines the issues
You need to know your process. Let look at the key process. CI Flexo, process flexo or gravure. The defect list is critical. A gravure inspection solution should have a resolution of 170 dpi where flexo can drop to 120 dpi. The reason is simple. The rotagravure process can have blade streaks caused by the doctor knife. To reliably detect these defects the system requires a higher resolution. You then take the line speed and the web width and you can determine the resolution required. Lets take a 1200mm web running at 300 mpm. The system will require close to 10,000 pixels. Lets compromise and go for two 4096 cameras. This will give a resolution of 150um. If the line frequency is 50 khz then the top line speed is 50,000 * 0.150* 60 = 450 mpm.
The next step is to decide what lights to use. If you are running film, then you will need a back light. If you are running metallic inks or foil you will need a bright field light. A dark field light is the minimum standard. So for a standard flexible packaging system you will require two cameras and three LED lights.
Next up is some good 100% inspection software
The next requirement is a vision inspection package that can interface with the camera and control the lights. The most common algorithm is the golden template approach. This is where the operator will tell the system to start inspecting once the printing is in register. The software then takes a golden template and compares all subsequent images to the template.
All printing process involve the application of layers of ink. Depending on the method, sheet or web, gravure or offset, there is always the task of print registration. The 'D' below shows an image with a degree of Miss-register. However no printing process is perfect and a print inspection system must allow for a certain amount of register. It does so by detecting the edges as show above, and then building a threshold image from those edges.
The three images below illustrate the effect. The edges on the print are detected. The last image shows that these edges are increased. This is the image that is then used as a threshold.
The question is can you build your own print inspection solution. If you are to build a system that works, you need to understand the variables.
The following are fundamentals.
- Your type of printing process and the register tolerance of your process.
- The resolution of the scanning system in your vision system.
- The algorithm being used.
OneBoxVision are industrial automation experts for web and sheet manufacturers. The founders have a combined experience of over 50 years in print inspection and have developed a suite of solutions for flexo, offset, gravure and screen printing. Download our latest white paper on Print Inspection Requirements and do not hesitate to contact us if you would like more information.