Printing quality control - the role of ink system and wetting system

There are many factors that affect the quality of printing, and the effects of the inking system and the wetting system on print quality are particularly important. Different prints require different amounts of ink and water. Therefore, the ink system and the wetting system need to be adjusted and controlled in real time. Only in this way can high-quality prints be printed.

Factors affecting the inking system

1. The nature of the ink roller

The surface accuracy and hardness of the ink roller have an impact on the performance of the ink system. If the hardness of the ink roller is too large, its ink-transmitting property is poor, and the printing plate is easy to wear; if the hardness of the ink roller is too small, it will also affect the sharpness of the dot, and it is easy to produce a paste plate. For this purpose, the surface hardness of the ink roller should be continuously tested to ensure the quality of the printed product.

The ink roller should be cleaned frequently, and the cleaning agent also affects the surface properties of the ink roller. If used improperly, it will accelerate the aging of the ink roller, so choose the cleaning agent carefully to prolong the service life of the ink roller.

2. Ink roller pressure

The pressure between the ink roller has a great influence on the ink transfer performance. Too much pressure will accelerate the wear of the ink roller and reduce the service life of the ink roller. The pressure is too small to meet the requirements of ink transfer, so the pressure must be adjusted properly. The size of the pressure and the timing of adjustment should be determined based on the use and cleaning of the roller during the printing process. The pressure between the rollers should be accurate, reliable, and can be accurately measured. There are two commonly used measurement methods.

One method is to experience the operator's sense of hand, by constantly pulling a plastic foil sandwiched between two rollers, to determine the resistance of the plastic foil roller drag generated when dragging. According to the operator's experience and feel to adjust the pressure, and repeatedly experience this process until the pressure is appropriate. This method is only an estimate, and there is a certain gap between accurate measurements.

Another method is to estimate the pressure value by measuring the width of the "ink mark" (indentation). When the two ink roller pairs performing the ink transfer are rolled, the two ink rollers contacting each other produce an indentation under pressure, and the pressure value is determined by measuring the size of the indentation. This method is simple to operate and measures at stop without removing the ink roller. Usually, a small amount of yellow ink can be applied to both ends and in the middle of the ink roller. After stopping and stopping, the indentation can be easily recognized and measured. If the width of the three indentations is not the same, indicating that the pressure of the ink roller is uneven, it is necessary to re-align the ink roller position.

The operator determines the size of the pressure by the width of the indentation and does not have to increase the amount of ink if the pressure is too low. However, it should be noted that when manually clutching the ink roller, the width of the indentation is increased due to the inertia of the ink roller and the force applied by the hand, so this does not truly reflect the pressure between the ink rollers.

However, the same method of operation cannot be used to measure the pressure between the ink roller and the plate cylinder. Since a uniform ink is applied to the plate, the required ink and coating time are difficult to determine, so the indentation that occurs on the plate when the ink roller is pressed off should be measured. The clutch of the ink roller is usually achieved by a landing gear whose swinging arm rotates around a fixed axis. If the closing time is too fast, a recoil force will be generated in a short time, and the indentation will be expanded due to the inertia of the roller to bend the swing arm. The indentation thus obtained does not reflect the real situation. In order to accurately obtain the indentation, we can adjust the pressure to match the system response time, reduce the inertia force of the ink roller and the vibration of the system, using pneumatic landing gear. This method is easy to adjust and accurately measures the indentation of the ink roller on the plate. It is important to note that the cylinder is always working under pressure.

The manufacturer of the ink roller should provide the relationship between the width of rubber deformation of the ink roller surface and the force, or provide the relationship between the width and pressure of the ink roller indentation, and the pressure setting value, which is very useful to the user. of.

In addition, the separation of the ink path and the ink layer of the inking system, the temperature of the ink roller, and the like have an influence on the indentation. Therefore, it is necessary to comprehensively consider.

Factors affecting the wetting system

Factors affecting the wetting system include various water rollers and the pressure and dampening fluid between them. The detection and adjustment of the water roller and its pressure are similar to those of the ink roller. The following is only the effect of the dampening solution on the wetting system. Explained.

The quality of fountain solution is important for the wetting system, and it has a great influence on the printing quality. Excessive water quality of fountain solution can affect the normal operation of the wetting system. For untreated water used for the first time, it should be tested several times a day, several times a week, until the hardness of the water is stable, and then measured once a week. Always pay attention to the quality of the local water supply and need to install equipment that stabilizes water quality, such as penetrating equipment that softens or hardens water. There are several ways to treat the water used to moisten the fountain solution.

The first method is to mix the untreated water with the softened water, and add soft water in proportion to reduce the hardness of the water to obtain water with the required hardness. This method is only applicable to water that has not been treated with high hardness, and the hardness of the water does not change much. For example, the hardness of untreated water is generally 14 degrees (American Water Quality Association standards), which is relatively high, and the hardness of water is required to be 6 degrees. In order to achieve such hardness, it is necessary to equalize the amount of untreated water and Soft water is mixed with permeability. In this case, the hardness of the untreated water will be reduced, which means that if the original water hardness varies between 10 and 14 degrees, the hardness of the mixed water is between 5 and 6 degrees. Between changes. Once the hardness changes too much or the hardness of the untreated water is too small, it is necessary to use a different solvent to change its hardness.

The second method is the use of additives that quantitatively and continuously add additives to the softened or hardened water. This solvent can completely eliminate the hardness change of water, but its cost is higher. When used, dilute the additive to a certain concentration and place the mixture in an interim reservoir, stirring constantly to prevent sedimentation.

The third method is to use an aqueous fountain solution matching the water. The fountain solution manufacturer has a proven and reliable water treatment method: First, take a sample of water, usually in the laboratory, draw the dissolution curve of the solution of the fountain solution in this water to obtain the smallest solution on the surface. The amount of fluid required for the fountain solution during tension is a condition that satisfies the print compatibility in the printing process. Second, the conductivity of the fountain solution is provided to the user so that the concentration of the fountain solution can be determined by testing in a simple manner.

If there is no water treatment system, the conductivity of the fountain solution must be measured before printing. This is a simple method of measuring the content of the fountain solution in water. Similarly, this method can also be used to determine the nature of water, especially untreated water. If the nature of the untreated water changes, its conductivity will also change.

Pay special attention to the dissolution of fountain solution. The use of different additives requires different doses, which rely on the law of dissolution of the fountain solution to provide data. Therefore, the quantity of the fountain solution supplied to the pump is measured. The cooling system must also be inspected. This is because the stability of the temperature of the fountain solution plays a decisive role in the stability of the printed product during the printing process. In particular, when printing a long version, when the temperature increases, it will seriously affect the dampening. Dissolved liquid solution.

In order to ensure the normal operation of the wetting system, in addition to regular inspections, at least one more thorough cleaning must be performed every month to prevent bacteria, such as filters, from growing in the conveying water pipes. When cleaning, the filter should be cleaned separately and replaced if necessary.

Source "Printing Technology"