How to deal with the skinning phenomenon of offset ink
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The crust formed during normal ink storage and the crust that occurs during the printing process often cause many problems for the printing company and the operator of the printing machine, such as a reduction in printing costs and a decrease in the quality of the printed product. It is of great significance for printing companies to be able to effectively control the skinning of ink before and during printing.
The so-called skinning of the ink means that the surface layer is in contact with the air during storage or printing due to oxidation, penetration, volatilization, evaporation, etc. at normal temperature, and the oxidation of the vegetable oil or the volatilization of the organic solvent leads to the ink system. A gel formed by the action of polymerization or the like. When the concentration of the printing ink is increased to a certain value, the surface is covered by a layer of molecules. Even if a solvent or grease is used to reduce the concentration of the ink, the surface of the skin (gel) is no longer possible. Hold more molecules.
When the phenomenon of ink crust is serious, it will first have a great impact on the stability of printing costs. According to projections, the lighter wastes 1%, and the heavy ones are nearly 4%. This tangible consumption adds a heavy economic burden to packaging and printing companies. Secondly, the impact on the quality of printed products cannot be ignored. When the ink is crusted during the printing process, the ink will be distributed to all parts of the transfer ink under the action of rolling, which has a great influence on the normal ink transfer. At the same time, when the ink is adhered to the ink roller, Abrupt changes in the density of the printed matter on the printing plate; when the ink is transferred to the printing plate and the blanket, an annular mark appears on the printed product; when the ink is adhered to the water roller, the water is transferred. Abnormal, there is a dirty phenomenon; the ink may also adhere directly to the surface of the paper.
From the above we can see that it is necessary to prevent and treat the skinning phenomenon of the ink.
So how does the ink crust appear?
As we mentioned above, the phenomenon of ink crust is the phenomenon of oxidative conjunctiva of the binder in the ink. The mechanism of the conjunctiva is the same as the mechanism of ink drying, and is also affected by factors such as temperature, humidity, oxygen in the air, and desiccant content in the ink. Simply put, the crusting of the ink is actually a phenomenon of oxidative conjunctiva of the ink we don't want to see (because the conjunctiva occurs when we don't want the conjunctiva).
There are many factors that can cause ink crusts, but there are mainly several categories: 1. The dry oil content in the ink is too high; 2. Due to temperature reasons; 3. Due to long-term contact between ink and air. Below we will do some analysis separately.
1. The dry oil content in the ink is too high. Such errors may occur during the manufacture of the ink or during the printing process. Adding a desiccant must be done in combination with the printing conditions and ambient temperature, otherwise the ink will dry out when you don't want to dry. As for the use of desiccant, we have already discussed it in the previous article, and we will not repeat it here.
2. Due to temperature reasons. The skin of the ink caused by temperature is mainly caused by high temperature. When the temperature is too high, the activity of the unsaturated molecules in the ink is enhanced, and especially the surface exposed to the air is more susceptible to oxidation of the conjunctiva under the action of oxygen. The temperature mentioned here has the following advantages:
a) The ambient temperature is too high. According to our experience, the skinning phenomenon of the ink in summer is much more obvious than that in winter. If there is no air conditioner in the workshop, it will be more obvious. It is necessary to properly control the temperature of the working environment. Generally speaking, it is an ideal temperature to control the temperature of the workshop at around 20 °C, which can ensure the good fluidity and transfer performance of the ink, and can control the crust of the ink. A relatively low degree.
b) When the temperature on the machine (such as between the rollers) is too high, it will also cause the ink to crust during the printing process. When the contact pressure between the ink rollers is too large, the temperature rise is relatively obvious, and at this time, the ink is liable to crust on the ink fountain and the ink roller. Accurate adjustment and control of the pressure between the ink rollers is important to prevent the ink from crusting during the printing process.
c) The air leaking from the dryer dries the surface of the ink, or the hot air blows into the ink in the fountain.
d) Also caused by other abnormal heating of the machine.
3. Due to the long-term contact of the ink with air. The binder component in the ink can undergo a redox reaction with oxygen in the air, and if it is exposed to the air for a long time, it will be oxidized and crusting will occur. Generally, there are the following types of crust:
a) The ink that is not used temporarily is exposed to the air, and a thick oxide film appears on the surface for a long time. We have the experience in the printing factory. When ink is contained from the ink cartridge or the ink tank, the surface of the ink should be kept as flat as possible. The reason for this is to minimize the contact between the ink and the air. The area, so that even if the ink is wasted, it will be as small as possible. Then, the remaining ink is re-covered with a protective film to block the contact with the air, which can effectively prevent the skin from sticking.
b) When the printing machine is stopped for a long time, there is no cleaning of the ink path, causing the ink to crust in the ink roller and the ink fountain. In the work, be sure to clean the ink path when the machine is stopped for a long time. When the downtime is not very long, you can choose to spray the ink anti-skinning agent on the ink path.
c) If the ink is not stirred in the ink fountain for a long time, skinning may occur. In the printing process, we often talk about "three Qin". The ink in the ink fountain is very important. First, it can maintain the good fluidity of the ink in the ink fountain. Secondly, it can make the ink on the surface and the interior. The inks are often exchanged to avoid the same parts of the ink that come into contact with the air. For the normalized operation of the adjustment of the ink supply amount, the basic criterion for the adjustment of the ink supply amount is that the gap between the ink fountain blade and the ink fountain roller is as small as possible, and the rotation angle of the ink fountain roller is as large as possible. Part of the reason for this is based on the above considerations.
d) The structure of the ink roller is not good, resulting in a non-flowing retained portion. As mentioned in b, this also causes a certain part of the ink to always be in contact with the air, so that the skin is inevitable.
So how to effectively deal with the failure of ink crust in printing? The following are the more common methods used in printing operations.
(1) Do not use too fast drying ink
(2) Add slow-drying solvent and other additives
(3) Replace the deteriorated ink
(4) Stir in the ink fountain
(5) Set a closed top on the ink fountain The cover
(6) adjusts the angle of the hot air blow. Some people with a little common sense of printing know that when the drying is too fast, the ink concentration is too light, the flow is poor, the ink thixotropy is large, or the ink is static, the speed of the printing machine is adjusted faster, that is, the ink is prevented from drying on the printing plate in advance, and the ink is prevented from being overcome. The skin of the packaging printing ink is formed into ink, and the patterning fault of the graphic is also eliminated.
In the work of preventing ink crusts, it is an important means to add an ink anti-skinning agent to the ink. Here are some introductions to the scope of use and precautions for ink anti-skinning agents in printing:
The oxidation of the printing ink, the oil of the modified resin, the amount of the pigment, the filler, the drier used, the environmental temperature and humidity during storage of the ink product have certain influence on the skin of the ink, and therefore are used. Anti-skinning agents must take into account the above various factors, but also pay attention to the basic knowledge of the following seven aspects:
1. Add amount. Generally calculated according to the ink material (because the main component of the skin is the main raw material of the ink) or calculated according to the total amount of printing ink, but also the influence of other ingredients. Usually, the addition amount is between 0.1 and 0.3%. Because of the difference in temperature and humidity throughout the year, the speed of skinning is different. Therefore, it is 0.1% in winter, 0.3% in summer, and 0.2% in spring and autumn. It is scientific depending on the composition of the formula.
2. Ink film drying rate. In practice, we have proved that in the offset printing ink made of alkyd resin, the amount of methyl ethyl hydrazine is generally 1%, although the drying speed is slightly longer, it is still within the standard range of printing. This is because the complex formed by the auxiliary agent and the drier is caused by the temporary loss of activity of the drier, so it is different from the conventional principle of freely terminating the polymerization by the phenolic compound-trapping activity. Therefore, the effect of such compounds on the drying rate is not great. If the amount is too much, the volatilization time is prolonged, and the disintegration of the complex is too slow, which often affects the film forming speed of the packaging printing ink.
3. Yellowing. Once the anti-skinning agent exceeds 0.3%, it tends to cause yellowing of the printed graphic film. Especially when used in white inks, yellowing occurs after long-term storage, so strict control of the amount of addition or change to butyraldehyde oxime can avoid yellowing.
4. Color degree. After the anti-skinning agent is added to the ink, it often directly affects the color of the ink. Usually, the effect of the varnish is greater than that of the color ink, and the white or yellow ink affects the ink larger than the dark color. Generally, this phenomenon is difficult to observe by visual inspection and is independent of the amount added. Although we already know that the printed image has no effect on the hue of the ink film after drying, but in the presence of certain pigments (such as amphoteric pigments) and drier, the color of the printed graphic film is color. There will be significant differences between them, which will bother the color tone. When we encounter this situation, we should continue to proof during the ink making process, first test the color difference, the purpose is to avoid the appearance quality of the ink film due to discoloration after printing.
5. Print the ink film status. Although the anti-skinning agent does not have any influence on the state of the printing ink film, when the ink is stored for a long period of time, crystals are precipitated, which causes turbidity after the film formation of the ink and affects the gloss of the ink. If the above materials are improperly matched with the binder, pigment, filler and drier in the ink system, large particles will appear. In this regard, it should be tested before printing.
6. Gloss and weather resistance. After the anti-skinning agent is added, the gloss of the printing ink is not affected, but also has gloss retention. Second, after the addition of the additive, the weather resistance of the printed graphic ink film is significantly improved and improved.
7. How to use it. The anti-skinning agent is generally added during the ink adjustment, and is added while stirring at normal temperature. For volatile inks, in order to control the ink, the colorant is crust during the grinding dispersion and storage, especially during the printing process, and it is preferable to add a part at the time of ink making, and it is preferable to add a part before printing.