At present, many domestic printing manufacturers have adopted water-based varnishes. However, in the process of lithographic in-line water-based coating, many problems are often encountered. It is hoped that this article will help printers solve problems encountered in actual production and increase production efficiency.
1. Anti-wear test method
In the packaging and printing industry, the abrasion resistance test of printed materials is a basic experiment. In many cases, measuring the abrasion resistance of printed products is measuring the wear resistance of the light film layer. It is not possible to make a final judgement on the anti-wear effect of varnishes with only one experiment. A number of samples should be taken from the printed matter for experimentation, and the final composite value should be taken. Instrument for each experiment. Analytical methods and experimental conditions (including humidity, temperature, ink, paper, etc.) should be the same for comparative analysis.
Some cartons manufacturers only use a piece of white paper to lightly wipe the surface of the freshly printed product and determine whether the ink has been transferred to white paper or not. In fact, this practice is unscientific and it is recommended to use the following abrasion resistance test methods.
(1) Sutherland Rub detector
Printed specimens should be reciprocated for a certain number of times under constant weight to observe the extent of wear on the surface. Since there is no one that can be used as a standard wear value, some reference samples need to be referenced during the experiment.
(2) Anti-back adhesion test
In some cases, if the paper boxes are not stacked tightly in the packing box, friction will occur during transportation and the surface of the paper box will be worn out. Due to the hydrophilicity of the aqueous coating, the abrasion resistance of the aqueous coating is also reduced under humid conditions, and abrasion and back-tacking occur under high-heat conditions. Therefore, the anti-tackiness test provides a basis for evaluating the anti-adhesive properties of coating oils. Put two printing samples in the temperature and humidity control room, the printing surface is opposite, place under the pressure of 500g/cm2, temperature 55°C for 24 hours. This kind of experimental analysis is very reliable and can provide a basis for whether or not there is stickiness in transportation. It should be emphasized that a series of samples should be selected for experimentation, and the final comprehensive values ​​should be taken for analysis and evaluation.
(3) Vibration experiment
The vibration experiment is a simulation experiment for the actual transportation process, usually 4 to 6 hours of vibration, the requirements are very demanding, and the experimental analysis is also time-consuming and laborious, but it can help customers determine the appropriate packaging and printing materials.
It is recommended that the abrasion resistance be tested after the varnish is cured for 24 hours to achieve optimal performance and compared with the experimental results of the reference sample.
2. Drying of waterborne varnish
The first is how to ensure that the underlying ink is sufficiently dry after applying the aqueous coating to prevent blemishes. Water-based coatings do not accelerate the drying of inks. Moreover, wet inks still need to be dried through oxidized conjunctiva. Therefore, the key issue is to ensure the air permeability of the waterborne varnish, so that the wet ink layer can come into contact with the air and dry the oxidized conjunctiva.
The second is how to control the dryness of the varnish and ensure that the printing product will not be contaminated when stacked. Here are a few suggestions for your reference.
(1) In the printing production line, the operator can use the thumb to press on the dark printed area of ​​the printed product. Under normal circumstances, the fingerprint should disappear within 15 to 20 seconds after pressing. The freshly printed product should be placed on the observation table to observe whether the traces on the paper have disappeared in 10 to 15 seconds, and to determine the degree of dryness of the coating oil.
(2) When gluing on the first time, it is necessary to check the overall filthy condition of the paper 30 minutes after printing, and from the top to the bottom of the stack of printed products, check the blemishes of the ink.
(3) When spots appear on the coating, it is necessary to check whether there is excessive accumulation of varnish at the edges of the spots. Excessive varnish will cause the printings to stick together. There are several ways to avoid the accumulation of coating oil. It is recommended to dilute the coating with 1% to 2% water (by weight) to reduce the viscosity and eliminate spots.
(4) In the UV curing drying mode, the waterborne varnish can be used in combination with UV ink. When printing with UV ink, as long as the ink is properly cured, the waterborne coating can obtain better wear resistance and dryness. performance.