More and more printers realize the value of measuring and analyzing the performance of printing presses, because this is the most expensive equipment and the highest cost center in printing production. For reasons of purchase / installation acceptance and quality control and color management, tests should be conducted regardless of model size. The real benefit of this type of printing press testing is that it is a comprehensive and systematic work, and the production process needs to be evaluated as a whole. The following explanation will help the printing company successfully complete this task.
The abnormalities found by analyzing the test data of the printing press can indicate the existence of mechanical faults, and even the printing press faults that will occur. In addition to diagnosing faults, press testing can also be used as a calibration and process control tool to judge current printing performance and collect color management profile information.
Reasons to test multicolor printing presses
1. The purchase / installation acceptance printer will not buy a printing machine that does not know whether it can achieve the desired printing quality. Many companies use structured testing as part of the acceptance criteria for the purchase and installation of new printing presses, as do previously owned printing presses. Diagnostic verification can determine whether the performance of the printing press meets the factory's nominal specifications and quality tolerances accepted by the industry. Printing press testing is the first step in optimizing and extending the life cycle of major investment equipment.
2. Process review Testing the increase in primary ink density and tone value of the printing press is helpful to determine the current printing performance, and the test results can be compared with industry specifications. Printing tests can also be used to check materials such as inks, tapes, fountain solution, printing plates, and paper to make better choices.
3. Diagnostic printer testing can help determine the causes of related quality failures such as: difficulties in matching proofs, mechanical smear and ghosting, registration failures, and color shift failures.
4. Annual quality system maintenance and correction In order to comply with recognized quality control requirements, such as ISO9000 and Total Production Quality Inspection (TPM) programs, more and more printing companies test their printing machines every year or even half a year; these The test can also provide information to the factory technicians during shutdown maintenance.
5. Characterization of printing presses The increase in the tonal value (increased dots) of each printing press is different, which affects the printing effect. Knowing the dot gain characteristics of a particular printing press and using the compensation curve when outputting films and printing plates will allow the printing press operator to better control the printing quality. [next]
Printing machine testing should focus on the following issues
1. Are the main printing parts of the printing machine such as cylinders and gears damaged or aged?
2. Can the printing machine print the ink density and dot increase value accepted by the industry?
3. Can the printing machine accurately achieve stable image registration?
4. Can the printing press maintain consistent ink density and tone value throughout the printing process?
Three stages of printing press testing
The implementation of printing press test analysis requires the use of test targets (Test target) and test samples (Testform). The process includes three stages, in the following order:
1. Material stage
Because the printing press test is to determine the best degree of printing press performance, not the worst, so we must first select the material that can best optimize the printing press performance. If low-standard materials are used for testing, it cannot effectively reflect the mechanical failure of the printing press. Ensure that the materials used in printing press testing meet specific requirements for printability.
Plate: First, test the plate by printing the plate exposure control strip to determine the resolution of the image. It is the sharpness that can be measured and can be used to generate a halftone dot. Second, test the registration grids of the suction drying rack or continuous drying equipment to determine whether they meet the acceptable registration accuracy. The registration accuracy should be within ± 0.01mm.
Tape: Use cushioning and fast-rebound tape to get the clearest dots on the printing press.
Paper: Paper used for printing press testing should be of good quality. For example, powder paper commonly used by commercial printers and fly ash card paper commonly used by packaging printers are tested.
Ink: Use the ink that optimizes the performance of the printing press. At a reasonable thickness of the ink layer, the dots produced by the ink increase and the overprint rate is within an acceptable range.
Fountain solution: ensure that the fountain solution is compatible with inks and printing plates; incompatible fountain solutions will cause a larger dot increase. It is also important to determine the PH value and conductivity of the fountain solution. Please ask for help from the fountain solution supplier.
* The above requirements for materials are mainly to affirm the accuracy of the test. Of course, in actual production, it is impossible that all of them are top-grade materials.
2. Printing press setup and specification stage
Once the materials have been selected and checked, the next step is to determine whether the operating parts of the printing press are set and maintained according to the specifications of the manufacturer and the material supplier, and then verify that they can be operated correctly. This can be done by a series of checks based on the specified test sample. Inspections and tests include the following:
Ensure that the condition and settings of the ink reel conform to the specifications of the printing press manufacturer, and use a hardness tester to measure the Shore hardness of the ink reel; the Shore hardness of the new ink reel should meet the specifications of the printing press manufacturer. The setting and manufacturing technology of the rollers play an extremely important role in reducing the water streak problem. Correctly set the width of the embossed surface according to the specifications of the printing machine manufacturer; for example, the printing machine with a split format is often set to 3mm (1/8 "), and the width of the embossed surface on the printing plate is usually set to 4mm / 6 ″). Make sure that the press roll pillows are in good condition and that they are in proper contact when printing together; to check the flat sheet press, apply a thin layer of ink to the two roll pillows on the plate cylinder of the press with a 6 inch gap between them. Run 4 to 5 sheets of paper together, and then check whether the amount of ink transferred to the blanket roller roll pillow is the same. Flat sheet printing machines that do not use roller pillows need to be inspected with a probe gauge; please consult the printing machine manufacturer.
The inspection of the roller pillow of the BB type printing press against the blanket cylinder can be carried out using a special metal foil cut into strips. Put the strip-shaped metal foil between the plate cylinder and the roller of the blanket cylinder to press and roll. Then use a magnifying glass to compare the width of the strip of metal foil. If the width is different, it means that the pressure on both sides of the drum is inconsistent. The pressure must be adjusted before proceeding.
Make sure that the pressure between the plate cylinder and the blanket cylinder is suitable for the type of blanket used; traditional blankets usually have a embossed surface width of 0.05 mm to 0.1 mm; and the air cushion blanket should be 0.1 to 0.125 mm. The pressure of the BB type printing press against the blanket cylinder usually depends on the thickness of the paper.
In each unit, use cyan ink to perform solid printing and decompression test. Checking the solid can determine the streak and printing pressure. Dry decompression test can reveal the damage or repair of the drum. The density of dry solid ink should conform to the cyan density currently used in factories or the norms or guidelines accepted by the industry. Cyan ink density should be as even as possible between each sheet. Using a densitometer to measure from the mouth to the end of the plate will help determine whether the amount of ink delivered is consistent.
Dry printing test. Checking dry printing can determine the streak and printing pressure. The field decompression test can reveal the damage or repair of the cylinder. On the flat sheet printing press, perform a solid decompression test on each unit to print Evaluate the pressure, check whether the impression cylinder is parallel, and find out the abnormality of the impression cylinder. Each time the pressure of the impression cylinder decreases by 0.05mm, 25 sheets of paper are run.
Continue the decompression test until the paper just presses onto the tape or the paper finally comes off the tape roller. If different units begin to show that the pressure of the paper leaving the blanket cylinder is different, then the pressure gauge setting of the impression cylinder may be wrong. Make proper corrections and reset the pressure gauge to prevent excessive roller pressure.
Wet printing on each unit to evaluate the performance of the wetting system and water marks; the wet printing plate image should cover the entire paper (with a margin of 25mm). If water marks appear on the wet ground, it is usually the cause of the wetting unit. Check and correct the setting of the insufficiently moistened water roll, the condition of the water roll, and the concentration of fountain solution.
After completing the press setup and specification testing, determine if there are any press mechanical parts that need to be repaired or replaced. This must be done before the diagnostic printing process or before the diagnostic process reveals the characteristics. Once the printer settings and specifications are in place, the diagnostic process can be carried out. [next]
3. Printing process diagnosis and analysis
There are three steps in the diagnostic test of the printing process. The first step is to check the ghosting caused by the machine. The second step is to use the registration square film and the printing plate to test the registration accuracy of each unit. The last step is to use Test samples to determine the registration accuracy and printing performance of the press.
Check Mechanical Ghosting
This test is performed on each unit using a mechanical ghost test plate, which can be used in the dark brown color of Pantone477. If the difference in ink density displayed on the ghost printing plate from the intersection of the longest vertical bar and the horizontal bar is greater than 0.1 (measured with a densitometer), then some mechanical corrections are required. The ink transfer reel of the inking system may need to be reset, or older press designs hinder acceptable printing.
Check the registration of the printing unit. This test uses the registration grid tool, whose film negative film has precisely delineated lines and light mesh color blocks. The printing plates made with this film also have original color control strips. This test can find registration errors as small as 0.025mm.
Install the printing plate according to the keyhole positioning system. In each unit, test different colors of ink: CMYK, and spot colors of orange and green. Run the printer until the ink density is uniform between sheets, then print at least 50 sheets. The registration accuracy at the end of the first printed sheet should be within ± 1 grid width. If the set grid exceeds the ± tolerance, then check the keyhole positioning system, the flatness of the plate clamp and the zero setting of the plate cylinder.
Move the plate / cylinder to register the teeth, and the printing machine does not have the standard of image registration accuracy. The folio printing machine can achieve the point-to-point registration accuracy at the tooth mouth and there is a 2% point size error at the end of the plate. Larger full-open presses may have a half and half dot size error at the end of the plate. If the registration error in the vertical direction at the end of the plate is too large, the drum circumferential direction may be inconsistent. Replace the pads of the plate cylinder and blanket cylinder as needed to achieve acceptable tolerances for registration. Due to previous damage, the printer may need to replace the cylinder.
The printing test sample can now be installed with a printing plate made from a printing press test sample to carry out the actual printing press test process. The goals designed on the test samples are used to collect information on printer diagnostics, process performance studies, process control, and standardization.
Preparation for printing Preparation for printing starts with installing the inspected printing plates on the printing press. Check the registration of all color groups on the first printed sheet, and use the registration grid on the test sample to determine the registration accuracy of the positioning keyhole. Adjust the inking system to achieve the required ink distribution. CMYK and spot color orange and green field initial distribution densities can be based on the ink density of a typical printing press or industry accepted specifications or guidelines.
After printing and adjusting the water-ink balance, you can print the test sample. Use the color control bar at the end of the test sample plate to determine that the target for stable ink density is within ± 0.05 (handheld density meter or scanning density meter). During the test, the speed of the printing press (rev / hr) should be between 80% and 85% of the rated speed. For example, if a flat sheet press is rated at 15,000 iph, then the test speed should be at least 12,500 iph. Let the printing press run at the test speed of 500 to 700 rpm to ensure the balance of water and ink, so that all the test prints reach the target density. With minimal operation intervention, it continues to run, and the flat sheet printing machine has to print another 2,000 sheets. [next]
Analysis and diagnosis target
After the printing is completed, when the ink is dried early, the diagnosis target is analyzed. The target on the test sample is designed to reveal the characteristics of the mechanical system and printing. The following explanation focuses on the characteristics of the mechanical system.
Ladder targets
Place on both sides of the test sample, from the mouth to the end of the plate. The main elements of the ladder are the vertical and horizontal lines with 50% coverage. Vertical movement will cause the horizontal line to expand or smear, resulting in a darker image in the horizontal direction. The horizontal movement also affects the vertical line. Ladders can very sensitively display mechanical printing failures such as excessive dot gain, poor paper and paper transfer, loose tape, gear wear bars, gear eccentricity, poor roller contact, and ghosting and smearing. Measured with a densitometer, the difference in density between the vertical and horizontal lines at the same place should be less than 0.07 (CMY) and 0.1 (K and particularly dark colors).
Star targets
Provides visual quick diagnosis of excessive tonal value increase (dot increase). Under good printing conditions, a small dot appears in the center of the star. The large dot in the center of the abnormality indicates that the ink layer is too thick or the dots increase seriously. The oval dot in the center indicates that the dot produces smear, and its direction is perpendicular to the smear direction. The 8-character or bow in the center of the star indicates ghosting.
Vernier targets
Measurements and evaluations help determine changes in print length and paper stability between units. Measure the extension of the paper and the lengthening of the printing length at the mouth and tail. The registration between the printing units is measured within 200mm from the mouth. Accuracy error of more than 0.02mm can usually show visual contrast in the cursor.
The registration grid is in four areas, two at the mouth and two at the end of the plate, which helps to determine the registration accuracy between the units. The registration grid helps to indicate that the printing unit has an inaccurate registration.
Tonal value increase (dot increase) data can be measured with a density meter on the color control bar at the end of the plate. It is more suitable for measurement on the important screened light color block in the center of the test sample.
Use a density meter to check again whether the density is close to the target density. If the density is too heavy or too light, the hue value no longer represents what the test is to determine. High density will produce a serious dot increase, low density will produce a slight dot increase. For the increase of acceptable outlets, please refer to some typical industry specifications. A properly set printing machine is mechanically suitable for operating conditions and uses high-quality materials, so the printed image dots should be acceptable.
The change in ink density throughout the test process is another factor to be analyzed. During the test, at least 50 printed sheets were drawn continuously or randomly, and the density of each printed sheet was measured with a density meter.
Analysis of 50 consecutive sample prints will show whether the ink fountain roller and ink transfer roller are correctly manipulated. Analysis of at least 50 randomly sampled sheets will help determine whether a stable ink-water balance has been reached. If the change in ink density exceeds ± 0.05, then the inking system, type and concentration of fountain solution, and the suitability of the material should be investigated. [next]
in conclusion
The entire diagnostic analysis of the printing press is essential to determine whether its mechanical system requires maintenance. The press acceptance test results will show whether the press can print acceptable results and whether the printer signed a new or old press. Printing machine testing can also help discover the quality and suitability of printed materials, and has become a standardized procedure in quality control and maintenance systems. Printing press testing can also help optimize the life cycle of major investment products.
Finally, printing process management (Process contro1) is an essential part of the color management workflow, and comprehensive diagnostic testing of the printing press can establish a process control benchmark. Many color management efforts have failed because they did not consider that the entire workflow and previous working conditions will affect the final product. If the color management work is to be effective, the key is to stabilize the mechanical and technological elements to maintain their original characteristics.
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