Offset printing inks are mainly dry, oxide conjunctiva dry and volatile drying several ways. Poor ink drying can affect the quality of printed products.
Factors that affect ink drying
The osmotic drying mainly absorbs the ink through the capillary action of the paper surface, so it is related to the tightness of the paper, the surface characteristics of the papermaking fiber, the content of the filler and the rubber material. The paper absorbs ink too strongly, the ink is easy to pulverize; if the ink absorption is too small, the back of the paper is likely to be dirty.
In addition, the pH of the paper can also affect the drying of the ink. Alkaline paper has a smaller effect on the ink drying than the acid paper.
2. Temperature and humidity
Temperature and humidity also affect the drying of the ink, especially the drying of the oxidized conjunctival ink. Under standard conditions, the relative humidity does not change, and the temperature of oxidative polymerization will double when the temperature is increased by 10Â°C. If the humidity is too high, the oxygen-capturing ability of the oil will be reduced and the drying speed of the ink will be reduced. Humidity also has a great effect on the drying of penetrating inks. Because of the high humidity, the paper absorbs water and hinders ink penetration.
Many printers now use infrared and hot air to speed up ink drying. When infrared drying is used, the energy absorbed by the ink itself is not enough to complete the polymerization, so it is often necessary to assist with other drying methods, such as volatilizing the solvent in the ink or using the heat to continue the oxidative polymerization when the paper is stacked. Infrared drying is ideal for the drying of plain paper. However, when the aluminum foil is dried, stickiness may still occur. Hot air drying only has significant effect on inks containing a large amount of solvents.
3. Fountain solution
If the amount of fountain solution is too large, the paper will absorb too much water and the ink will have too much moisture, which will affect the absorption of ink on the paper and hinder the oxidation and conjunctiva of the ink. The pH of the fountain solution is too low (less than 3.8) and the ink drying is significantly delayed.
The commonly used desiccants are red dry oil and white dry oil, and their drying methods are drying from the outer layer to the inner layer and drying from the outer layer to the outer layer (preferably, two types of dry oil should not be mixed at the same time). Note that the amount of desiccant added is not as good as possible, and its drying ability is limited, not proportional to their content in the oil, and the general amount is controlled below 3%. The choice of desiccant should also be determined based on the ink properties and the ambient humidity. When the humidity is high, more desiccant containing cobalt should be added to the ink.
In addition, adding too much ink to the ink, detackifier, will also hinder the process of oxidation conjunctiva, thus affecting the speed of ink drying.
Frequent problems with printed products drying
1. After the color is not printed or false
In the case of the printed light-colored full-color version, the post-color printing may not be printed or may be false. This may be due to excessive dry oil addition or over-provisioning or semi-finished product stacking time, and the loss of adsorption of the glass-like smooth film on the surface of the ink layer. When printing ordinary inks on gold and silver inks, it not only requires the ordinary inks to be less viscous, but also requires the fixing to be quicker, otherwise there will be hair imaginary failures.
2. Color mixing, hair loss, skinning
In multi-color printing, the interval between the front color and the rear color is short, and it is usually performed in a â€œwet-press wetâ€ manner. When the viscosity of the latter color ink is greater than the viscosity of the previous color ink, the previous color ink layer will be peeled off, and if the used paper is poor, the hair peeling phenomenon may also occur. When glazing on line, the highly viscous water-based varnish will cause large areas of the imprint on the aluminum foil to peel off.
3. Scuffed and dirty on the back
When printing on aluminum foil, the scratches that appear on the dragged part of the delivery are usually caused by small and uneven paper. However, most of the other bruises are caused by the movement of the prints without thorough drying. The back of the printed product is mainly because the surface of the paper is too smooth, the moisture content of the ink and the paper is too large, or the overprinting of the printed material is not easy to obtain oxygen from the air.
4. Other problems caused by drying
(1) excessive dusting
If water-based glazing is to be done after printing, excessive dusting will make the surface of the printed product rough and not shiny, and it will also cause the printed product to become bloated when it is covered with a film, and the bronzing cannot be hot.
(2) Imprinting is not allowed
With hot air, infrared-dried printed sheets, accurate overprinting is generally not possible at the second overprint.
(3) Hue changes
Whether it is red or white, it will affect the original hue of the ink more or less, and this should be taken into account when proofing or printing high quality products.
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