Effect and Measurement of Conductivity in Quality Control of Offset Printing

The development of offset printing technology should be considered very mature today, but many failures often occur in the actual printing production process. For example: the same batch of prints, paper, ink, blankets, fountain solution, printing presses, The temperature, humidity, and operator have not changed, but the printing quality has undergone great changes. One of the important reasons is that the conductivity of the dampening fluid, which is usually not noticed by people, is changed. The composition of the electrolyte added to the water when the solution was diluted changed.

Offset printing is based on chemical methods, so the three factors that affect the ink balance are water, fountain solution, and ink, and water is the most easily overlooked. Electrolytes in water generally contain calcium, magnesium, and iron ions, as well as chlorides and fluorides. Iron ions are the main factor in the non-graphic part of the corrosion plate. Calcium is a “hard” metal, which can damage the chemical composition of the fountain solution, form insoluble calcium, erodes the plate, rubber rollers and blankets. Rubber aging, printing plate text part does not ink, but the graphic part does not adhere ink. Therefore, controlling the fountain solution cannot only measure its pH value, but also control the conductivity of the electrolyte in the fountain solution. As shown in Figure 1, when the concentration of the fountain solution increases, the conductivity value increases linearly and the pH curve decreases. In a certain range, although the concentration increased, there was little change in the pH value. It can be seen that the conductivity is more accurate than the pH value to express the concentration change of the fountain solution.

What is the conductivity

Conductivity is the degree to which the solution electrolyte can conduct electrons. The unit is micro ohms/cm (μΩ/cm). It can show the interaction of water, fountain solution, printing plate, ink and paper, and at the same time, the water and dampening. Liquid measurement and control are also more accurate. The fountain solution used in our normal printing is a mixture of water and a concentrated fountain solution in a certain proportion, in which the electrolyte composition changes as the electrolyte of the fresh water used and the electrolyte of the fountain solution increase or decrease. In this way, the electrolyte contained in the fountain solution mainly depends on the formulation and water quality of the fountain solution. The formula is different, and the conductivity is not the same; different water sources, the conductivity will vary, some are 300 ~ 1000μΩ/cm, some are 2000 ~ 4000 μΩ/cm, usually 1000 to 2000 μΩ/cm. Generally speaking, water of 0-200μΩ/cm is called soft water; water of conductivity 200-400μΩ/cm is called neutral water; water of 400-550μΩ/cm is called hard water; water of more than 550μΩ/cm is very hard. Water.

Effect of Conductivity on Print Quality and Measurement

1. Effect of Conductivity on Print Quality

Conductivity too high or too low will affect the normal performance of the PS plate.

Conductivity is too high: a. Ink concentration is not easy to control, the full background color printing difficulties; b. Ink emulsification, the impact of outlets to restore, resulting in fuzzy printing points; c. Obstruction part of the printing plate graphics, causing ink bad; D. Easy Make the red ink and blue ink seep out; e. Damage graphic portion of the printing plate.

The conductivity is too low: a. The non-graphic part of the printing plate gradually deteriorates, or adheres ink; b. The lines of the graphic part of the printing plate are easy to paste; c. The ink of the blanket is accumulated; d. The printing plate layout is prone to dirt.

2. The setting of the conductivity value

Since we know the importance of electrical conductivity, can we just follow the calibration of the electrical conductivity? In actual fact, there is no absolute standard conductivity reference value provided for different water quality and different printing processes to follow, we in addition to the fountain solution suppliers request the relevant parameters of the concentrated fountain solution to understand the conductivity of the electrolyte, In addition to latitude, the standards adopted by the printing shop for the conductivity of fountain solution can only come from long-term testing and experience. If there is any fouling or scumming, it means that the conductivity is low, and the electrolyte needs to be increased. If there are light ink or ink emulsification on the edge of the printed dot, the value of the conductivity should be reduced.

The fountain solution conductivity is generally 800 ~ 2000μΩ / cm, if the measured value of the conductivity is large, to add a certain buffer in the fountain solution. Because the buffer contains citric acid, the iron and calcium ions in the fountain solution chemically react with the citric acid to form an organic salt, thereby eliminating the effects of excess iron, calcium ions, and the like.

If the measured conductivity value is small, the dampening solution stock solution can be properly added to increase the conductivity of the fountain solution.

In addition, we must regularly clean the wetting system and check whether the circulation system of the water tank is contaminated. When adding water substitutes, detergents, lubricants, and fiber softeners to the water tank, we must always check whether the conductivity of the water bath liquid is within the normal range. . In the case of alcohol wetting systems, a 2% change in isopropanol will cause a 8% change in conductivity; a 1% change in temperature will cause a 3% change in conductivity.

How to measure the conductivity

Measuring the conductivity is best used as a modern micro-processed conductivity meter or as a conductivity meter. The measurement steps are:

1) Measure the conductivity of water. Note that the temperature of the water is close to the temperature of the fountain solution because the conductivity is affected by temperature changes.

2) In the range of 0.5% to 5%, the concentration and conductivity of the fountain solution were measured at intervals of 0.5%.

3) In actual application, according to the percentage and electrical conductivity recommended by the fountain solution supplier, the electrical conductivity must first be subtracted from the conductivity of the water. If the alcohol-wetting system, add appropriate alcohol, and then measure the conductivity again, after all the printing conditions are stable, the conductivity is the standard. Since alcohol or alcohol substitutes are non-conductive substances, the conductivity will decrease when they are added.

4) According to different printing machine production of conductivity control chart, when the fluctuation of the conductivity is found to be 100 ~ 200μΩ/cm, it shows that the working performance of the fountain solution begins to deteriorate, we must check whether the work of the wet cycle system Normally, when the difference is larger, the fountain solution should be replaced.
The general configuration reference standards for dampening fluid for sheetfed lithographic printing machine alcohol wetting systems are:

Temperature setting: 12~14°C
Alcohol ratio: 10% to 12%
Additive dosage: 2% to 3%
Conductivity: 800 to 1200 μΩ/cm
pH: 4 to 5.5

In short, we must strictly control the management of fountain solution, in addition to controlling the concentration and pH of the fountain solution, the temperature and humidity of the environment, and the amount of alcohol added, we must pay special attention to the conductivity of the fountain solution, so as to produce beautiful printed matter.

Shanghai Publishing and Printing College Pan Jie

Source: "Printing Technology - Equipment and Special Printing"