This is the second of three articles about OCR vision applications that use CIJ printers. In the first article I discussed the typical print output achieved with CIJ printers at high speed. I showed real images of print examples and showed the high degree of variability among similar characters printed by the same printer head. I recommend reading the first article before you proceed with this one.
Identify Common Defects on the Production Line
Once you understand the nature of printer output on high-speed production lines, and have noticed the high variation between one print to the next print, you can begin to identify possible solutions. But before you design your OCR vision solution, you should become familiar with the common defects that are relevant to the specific line you intend to automate. Factory quality managers, technical staff, printer technicians, and any other factory staff who keep records of production errors can help you to identify the common defects on any given production line. The defect identification stage is the most critical. Your goal is to understand the real needs of the customer. Many times the customer needs professional help in order to know what real problems he or she is facing on the production line. It's not always immediately easy to pinpoint defects and other problems, but there are reliable ways to do so.
Some helpful questions to ask quality managers or other technical staff during this critical defect identification stage are:
- May I collect some defect samples and can you provide me with descriptions of the defects found in your production line?
- When malfunctions happen, can the line recover itself, or is the intervention of a human operator required?
- What happens when a defect is found? Is there a full production stop, or do you use warning light stacks and/or reject kickers?
- Does the inspection system activate a warning immediately after the first defect is found, or does the system only activate a warning after ~10 consecutive defects? (This question is mostly relevant for lines with " sustainable" defect detection, where all products are considered defects from the moment of printer malfunction.)
- Is there one defect type that occurs most frequently in your application, or are there several? How often does each type of defect occur, and under what conditions?
This is only a small selection of diagnostic questions that can be asked. The point here is to emphasize the importance of learning a production line's problems in order to design a best-fit vision inspection system. This discussion helps align the customer's expectations with the tangible facts of his or her situation.
Basic Concepts of CIJ Printer Operation
The following description of CIJ printer functionality will help you to understand the common problems that can occur with this type of printing.
Figure 1: Basic CIJ Printer Functionality.
In the middle of Figure 1 you will see the ink reservoir. The ink pump at the left of the diagram continuously draws ink from the reservoir and pumps it through the nozzle. Ink droplets are emitted at very high speed in the direction of the target. Most ink droplets will end up in the gutter, which is a kind of ink container with the purpose of returning ink to the reservoir to avoid waste and to keep the ink in circulation.
The charging plates at the exit point of the nozzle use electric voltage to apply a static charge to the droplets. Then the deflection plates pull the droplets upward using high voltage. When this pulling force is variable and controlled, and when it is precisely timed with the movement of the print target, the desired characters or other shapes are printed on the target surface. The droplets that are not caught in the gutter escape and strike the print target, creating the image on the print surface. A very small quantity of ink ultimately ends up on the print target, which is often paper or a similar material.
There are many sources on the web that describe the operation of CIJ printers. For more comprehensive information, especially for those who are planning to work frequently with such printers, I recommend further reading online. This link is a good place to start.
Understanding the operational concept of continuous inkjet printers will help you to understand better some of the common defects describes below. Correct product movement under the CIJ print head is critically important in achieving the desired print results. When there is a problem with the stable movement of products on a conveyor, print quality can be undermined. One of the most common reasons for unsatisfactory printing is a dirty gutter from accumulated dry ink, which results in missing dot rows. Examples of this effect are shown below.
Common CIJ printer defects
The images below demonstrate some common defects that can occur in production lines with CIJ printers. Please note that the defects in the following examples are simulated for the sake of demonstration. They are not defects from actual applications.
Figure 2: Missing bottom line of dots.
Notice that the bottom line of dots is missing. This usually happens when dry ink accumulates on the edges of the gutter, creating a barrier for the droplets being emitted from the nozzle. This type of problem can render characters unreadable by OCR algorithms.
Figure 3: Missing top line of dots.
The missing line of dots in this example is at the top. The reason for this defect is similar to the description in Figure 2.
Figure 4: Complete absesnce of print.
In this example the printer didn't manage to transfer ink to the print surface at all. This can be caused by a printer triggering problem or by accidental movement of the printer nozzle, causing it to point away from the target print area.
Figure 5: Incorrect print position.
A slight movement of the print head can cause characters to be printed in the wrong area of the target surface.
Figure 6: Ink blots that obsure characters.
This problem is less common, but can still occur as a result of dirty or contaminated print surfaces (oil or water, for example, can add to this problem).
There are a handful of common defects that can occur when using CIJ printers. After a thorough investigation of a production line's defects, you may find that 90% of the defects are of one type only "“ missing rows of print dots causing incomplete characters, for example. Knowing how to identify common defects will help you to choose the correct solution for a specific application. When you're equipped with the right knowledge, you can design the right vision solution.
Microscan offers a wide range of vision solutions. AutoVISION software features several vision tools, including OCR algorithms with trainable characters, as well as non-OCR vision tools that can solve many of the common printing problems related to CIJ printers. I will discuss all that and more in my next article, which will be coming soon. I will discuss a variety of Microscan vision solutions, summarizing the advantages of each. I will provide guidelines about the optimal vision solution for each situation. Once you have this knowledge you will be able to create powerful vision inspection systems for production lines with CIJ printers.
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