The crusts formed during the normal ink storage process and the crusts that occur during the printing process often bring a lot of annoyance to printing companies and operators of printing presses, such as the reduction of printing costs and the quality of printed products. It is of great significance to printing companies to be able to effectively control the ink skinning phenomenon before and during printing.
The so-called crust of the ink refers to: the surface layer of the packaging printing ink is in contact with the air during storage or printing due to drying at room temperature due to oxidation, penetration, volatilization, evaporation, etc., the oxidation of the vegetable oil or the volatilization of the organic solvent leads to the ink system Gel formed by polymerization and other actions. When the concentration of the printing ink increases to a certain value, its surface will be covered by a layer of molecules. At this time, even if a solvent or grease is added to reduce the concentration of the ink, the surface that has been skinned (gel) is no longer possible. Accommodate more molecules.
When the ink skinning phenomenon is serious, it will have a great impact on the stability of printing costs. According to calculations, the lighter waste 1%, and the heaviest 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 skins during the printing process, the ink skin will be distributed to each link of the ink transmission under the effect of rolling, which has a great impact on the normal ink delivery. At the same time, when the ink skin adheres to the ink roller against the plate, it will Abrupt changes in the density of the graphics on the printing plate; when the ink skin is transferred to the printing plate and the blanket, it will cause ring marks on the printed product; when the ink skin adheres to the water roller, it will cause water to be transferred Abnormal, dirty phenomenon; ink skin may also be directly attached to the paper surface.
From the above we can see that it is necessary to prevent and deal with the phenomenon of ink crust. So how does the crust of ink appear?
As we mentioned above, the ink skinning phenomenon is a phenomenon of oxidative filming of the connecting material in the ink. The mechanism of conjunctiva is the same as that of ink drying. It is also affected by factors such as temperature, humidity, oxygen in the air, and the content of desiccant in the ink. Simply put, the crust phenomenon of ink is actually an oxidative conjunctive phenomenon of ink that we do not want to see (because conjunctiva occurs when we do not want conjunctiva).
There are many factors that can cause the ink to form a skin, but it comes down to the following categories: the dry oil content in the ink is too high; due to temperature; due to the long-term contact between the ink and the air. Below we will do some analysis.
1. The content of dry oil in the ink is too high
In general, such mistakes may occur during the process of manufacturing ink or during the printing process. Adding desiccant must be combined with printing conditions and ambient temperature to add an appropriate amount, otherwise the ink will dry (skin) when you do not want to dry. As for the use of desiccant, it will not be repeated here.
2. Due to temperature
The crust of ink caused by temperature is mainly caused by high temperature. When the temperature is too high, the activity of unsaturated molecules in the ink increases, especially the part of the surface that is in contact with air is easier to oxidize the conjunctiva under the action of oxygen. The high temperature mentioned here has the following aspects:
a) The ambient temperature is too high. According to our experience, the ink skinning phenomenon in summer is much more obvious than that in winter, and it will be more obvious if there is no air conditioning in the workshop. It is necessary to control the temperature of the working environment reasonably. Generally speaking, it is an ideal temperature to control the temperature of the workshop at about 20 ℃, which can ensure the good fluidity and transfer performance of the ink, and can control the skin of the ink A relatively low degree.
b) When the temperature on the machine (such as between the ink rollers) is too high, the ink will also cause skinning during the printing process. When the contact pressure between the ink rollers is too large, the temperature rise is more obvious. At this time, the ink is likely to form a skin on the ink fountain and the ink roller.
Accurate adjustment and control of the pressure between the ink rollers is very important to prevent the ink from forming during the printing process.
c) The air leaking from the dryer dries the surface of the ink, or the hot air blows the ink into the ink fountain.
d) There are other abnormal heating caused by the machine.
3. Due to the prolonged contact between ink and air
The binder material in the ink can undergo redox reaction with oxygen in the air. Of course, if it is exposed to the air for a long time, it will of course be oxidized and skinning will occur. Generally this kind of crust mainly has the following types:
a) The ink that is not used temporarily is exposed in the air, and a thick oxide film will appear on the surface for a long time. We all have this experience in printing houses. When holding ink from the ink cartridge or the ink tank, we should try to keep the surface of the ink as flat as possible. The reason for this is to minimize the contact between ink and air. Area, so that even the waste of ink after film formation will be as small as possible. Then, the remaining ink is covered with a protective film to block the contact with the air, which can also effectively prevent the ink from forming a skin.
b) When the printing machine is shut down for a long time, the ink path is not cleaned, causing the ink to skin in the ink roller and the ink fountain. In the work, we must pay attention to clean the ink road when the machine is shut down for a long time. When the shutdown time is not very long, you can choose to spray ink anti-crust agent on the ink road.
c) If the ink is not stirred for a long time in the ink fountain, skinning will occur. In the printing process, we often talk about "three diligence", in which the ink in the ink fountain is very important. First, it can maintain the good fluidity of the ink in the ink fountain, and secondly, it can make the surface ink and the internal ink. The ink is exchanged frequently to avoid the ink in contact with the air always being the same part. The standard operation of ink supply adjustment, the basic principle of ink supply adjustment is: 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 stagnant part that does not flow. As mentioned in b, this will also cause a certain portion of the ink to be in constant contact with the air, so that skinning is inevitable. [next]
So how to effectively deal with the failure of ink crust in printing? The following are the more commonly used methods in printing operations.
(1) Do not use ink that dries too fast; (2) Add additives such as slow-drying solvents; (3) Replace the deteriorated ink; (4) Keep stirring in the ink fountain; (5) Set on the ink fountain A closed top cover; (6) Adjust the hot blowing angle. Anyone with a little common sense of printing knows 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 static electricity, the speed of the printing machine is adjusted fast, which prevents the ink from drying on the printing plate in advance, and overcomes In addition, the crust formed by the printing ink of the packaging printing ink is eliminated, and the marking failure of the picture and text is also eliminated.
In the work of preventing ink crust, adding ink anti-crust agent to the ink is a very important method. The following is a brief introduction to the application scope and precautions of ink anti-crust agent in printing:
The oxidation of printing ink, the oil of modified resin, the amount of pigments, fillers, driers used, the environmental temperature and humidity during the storage of the finished ink have a certain effect on the crust of the ink, so they are used in When anti-crusting agents are used, the above factors must be taken into consideration. At the same time, the basic knowledge of the following seven aspects such as the amount of addition should also be noted:
1. Amount added
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, while also considering the influence of other components.
Usually the amount of addition is between 0.1-0.3%. Because of the difference in temperature and humidity throughout the year, there is a certain difference in the speed of skin formation. Therefore, it is 0.1% in winter, 0.3% in summer, and 0.2% in spring and autumn, depending on the ingredients of the formula.
2. Ink film drying rate
We have proved in practice: in offset inks made by alkyd resin, the amount of methyl ethyl oxime is generally 1%. Although the drying speed is slightly longer, it is still within the standard range of printing regulations. This is because the complex formed by the adjuvant and the drier causes the drier to temporarily lose its activity, so it is different from the traditional principle of using a phenolic compound to capture the activity and freely terminate the polymerization reaction. Therefore, the influence of such compounds on the drying rate is not great. If the amount is too large, the volatilization time is prolonged. After the complex is disintegrated too slowly, the film printing speed of packaging printing ink is often affected.
3. Yellowing
Once the anti-skin agent exceeds 0.3%, it will often cause yellowing of the printed graphic ink film. Especially when used in white ink, yellowing will occur after long-term storage, so strictly controlling the amount of addition or changing to butyraldehyde oxime can avoid the occurrence of yellowing.
4. Color
After adding anti-crust agent to the ink, it often directly affects the color of the ink. Usually, the adjustment ink affects more than the color ink, and the white or yellow ink affects the dark ink. Generally, this phenomenon is difficult to observe by visual inspection and has nothing to do with the amount of addition. Although we already know that the printed image and text film drying has no effect on the hue of the ink film application, but in the presence of certain pigments (such as amphoteric pigments) and driers, the color tone of the color graphic ink film for packaging printing There will be a significant difference between them, which has disturbed the hue. When we encounter this kind of situation, we should make continuous proofs during the ink making process and make the color difference test first. Its purpose is to avoid affecting the appearance quality of the ink film due to color change after printing.
5. Printing ink film status
Although the anti-skin agent will not have any effect on the state of the printed ink film, when the ink is stored for a long time, crystals will be precipitated, resulting in turbidity of the ink after film formation and affecting the gloss of the ink. If the material and the binder, pigment, filler and drier in the ink system are not properly designed and matched, larger particles will appear. This should be tested before printing.
6. Gloss and weather resistance
After adding the anti-skin agent, the gloss of the printing ink will not only be affected, but also the gloss retention. Second, after adding the auxiliary agent, the weather resistance of the printed graphic ink film has been significantly improved and improved.
7. How to use
Anti-crusting agents are generally added during ink adjustment and are added with stirring at room temperature. For volatile inks, in order to control the ink, the coloring material will form a skin during the grinding, dispersion and storage, especially during the printing process. It is advisable to add a part of the ink during printing, and add a part before printing.
Above we have made some discussions on the causes and treatment of ink skinning. In actual work, we may encounter various problems of one kind or another. After the principle of drying), it should not be difficult to analyze and judge.
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