1 type of printing ink
Special inks for pad printing differ from general solvent-based inks, water-based inks, and UV-curable screen printing inks. Although pad printing inks and screen printing inks have some similar characteristics, they are still quite different. Unlike screen printing inks, pad printing inks dry faster, which is the most prominent feature of pad printing inks. At the same time, pad printing ink is often printed on the film. This application causes the pad printing ink to be easily affected by factors such as temperature, humidity, and static electricity. The inks specially formulated for pad printing include single-component inks, two-component inks, baked-dry, oxidized, and sublimated inks.
1) One-component ink
The drying of single-component inks (also called solvent-evolving inks) is volatilized by solvents, which do not necessarily have to be added to the catalyst. One-component inks are glossy and non-glossy, they are mainly used for the printing of plastic substrates.
2) Two-component ink
Two-component inks are also called chemical reaction inks. Before the printing, the catalyst must be added. The catalyst reacts with the resin in the ink to achieve the purpose of ink drying through the polymerization reaction.
When two-component inks are used in pad printing, it is necessary to add a certain amount of catalyst, usually measured by weight. The addition rate of the catalyst must be strictly controlled. If too much is added, the “acclimation period†of the ink will be greatly reduced, which may result in the ink not achieving the best drying performance when it is dried. In general, if the proportion of the ink and the catalyst is not properly formulated, the ink may be unevenly adhered and the durability of the printed image may not be sufficient.
3) Dry type ink
Dry-type inks are divided into two types: one is the addition of additional catalysts based on standard two-component inks; the other is the ink specially formulated for substrates such as glass, ceramics and metals. As its name suggests, baked dry inks must be warmed and dried at a certain temperature. When using this kind of ink to calculate the drying time, the higher the temperature, the shorter the drying time, but the drying temperature is too high will make the ink film brittle, so the ink can be baked at a temperature that can maintain the flexibility of the ink.
4) Oxidation type ink
The oxidized ink absorbs oxygen in the surrounding environment to perform a polymerization reaction to form an ink film without adding a catalyst. Oxidation inks are typically used for flexible packaging substrates and synthetic substrates such as rubber or keyboards. Oxidation inks have a limited amount of use due to their slow drying speed and long time.
5) Sublimation ink
Sublimation inks require special processing during use, that is, they are heated after printing to make the substrate porous, so that when the dye comes in contact with the heated substrate surface, the dye in the ink becomes gaseous. Then enter the surface of the substrate, which actually changes the surface color of the substrate. Once the substrate cools, the ink sticks to the surface of the substrate.
The most common applications for sublimation inks are computer keyboard printing, and those prints that require higher oil and abrasion resistance, which are not achieved with two-component inks. Of particular note is that since the sublimation ink actually changes the surface color of the substrate, the matching between the substrate and the ink color is more difficult. Therefore, the color of the substrate must be lighter than the color needed for final printing, because sublimation inks tend to have less visible color change on the darker surface of the substrate.
6) Special inks and additives
Pad printing specialty inks include edible inks, silicone resin inks, lubricating inks, corrosion-resistant inks, conductive inks, and UV-curable inks. The ability of these inks to quickly become sticky will determine whether ink can be effectively transferred to the surface of the substrate. . At the same time, in order to adjust the printability and performance of the ink, in addition to the addition of solvents and catalysts, it is also possible to use many additives, such as viscosity inks, antistatic agents, rheological agents, and the like. However, the use of additives will have a great impact on the printing performance of inks, and must be used consistently.
2 printing process
The first step is to add ink to the etch pits of the plate, which marks the beginning of the printing cycle (Figure 1). Some pad printers use open inked holes, while others use closed ink holes (such ink holes prevent solvents from escaping in the ink holes). The viscosity of the ink at this time must be sufficiently low so that the ink can spread flatly on the surface of the transfer printing plate and can enter the recessed etching image area.
In the second step, the ink on the raised portion (blank portion) of the printing plate is scraped off with a squeegee to allow the ink to enter only the etched recessed image area. Due to the high volatilization rate of the solvent, this will cause the ink on the surface of the image depression to be more viscous than the underlying ink.
In the third step, the print head applies pressure evenly to the rolling gravure while squeezing air from the gravure. At this time, since the viscosity of the surface layer ink in the depressed region of the image is large, the ink can be moved away from the depressed region and adhered to the print head.
In the fourth step, the solvent in the ink on the surface of the printing head evaporates after the printing head has left the printing intaglio, which makes the ink on the surface of the printing head more viscous.
In the fifth step, when the surface of the print head touches the surface of the substrate, the viscosity of the ink on the surface of the print head is relatively large, so that the ink leaves the print head and adheres to the surface of the substrate. At this point, the print head is designed to be curved and flexible, despite the fact that the print head may exert extreme pressure, so that the image can be brought into contact with the substrate in a curved rather than a flat manner. In fact, a well-designed print head does not form a zero contact with the substrate. Corners, which prevent air from remaining between the print head and the substrate. If air remains in the image, it can cause incomplete image transfer.
In the sixth step, the head moves away from the surface of the substrate and returns to its original state. If the change factors involved in the six steps are properly controlled, the print head will leave the substrate cleanly, at which time a complete pad printing cycle will be completed and the pad head will be ready for the next pad printing cycle.
From the above six steps, it can be seen that one of the distinctive features inherent in pad printing inks is the ability of the inks to become sticky, and the pigment particle size and the proportion of pigments are very different from other types of inks. The depth of the gravure image etched in many pad printings is 25 μtn, or even less. In this case, the pigment particles contained in the pad ink must be smaller than most screen printing inks. In addition, since the thickness of the ink film for pad printing is about 20% of screen printing, the pigment concentration in the pad ink must be sufficiently high in order to obtain sufficient hiding power.
Since pad printing inks have a very high viscosity change requirement, the solvent used in pad printing inks can be vaporized at a much faster rate than the solvents used in conventional screen printing inks. Otherwise, it is difficult to obtain the desired printing effect.
3 Pad Printing Ink Dispensing
In general, the ink manufacturer will give guidance on which substrate the ink is suitable for, but this opinion is for reference only. To choose a suitable ink, you need to understand a few things: what substrate to use; whether the substrate needs pre-treatment before printing; what kind of color needs to be printed; the abrasion resistance, chemical resistance, and weather resistance of the ink. Printing requirements such as printing; and inks are recommended for drying or curing.
Once you have selected an ink that is suitable for the substrate, remember that it must be formulated according to the ink manufacturer's recommended method. A lot of ink manufacturers have provided a specification of technical data, above which there are catalysts, additives and solvents that the ink can be compatible with, and the correct ratio of the ink formulation. Many ink formulations are formulated on a weight basis rather than on a volume basis, so it is best to use a weighing accuracy of at least O. 1 g digital instrument to accurately weigh. If you want to match colors, you should use an O. O1g instrument to weigh, this can avoid the waste of ink in the color test.
Properly mixing the ink with the auxiliaries is the best time to dilute the ink consistency, which can be diluted with a suitable solvent or a mixture of several solvents. Standard pad printing inks usually have a variety of suitable diluting agents, including three diluting agents with fast, medium, and slow volatilization rates. Ink manufacturers generally have a range of values ​​for diluting agents. Generally, diluting agents are added at a rate of 10-20% of the ink.
Deciding which diluting agent to use and how much diluting agent to add is a guess for most people. This is actually incorrect. Before the start of printing a batch of jobs, the diluting agent test should be carried out. First, the print speed of the printing machine is adjusted to the required speed, and the pressure of the print head is adjusted to look optimal. It can then be tested by first adding a diluting agent with a moderate volatilization rate and adding a ratio to take the median of a recommended range of values. For example, if the recommended range is 10-20%, add 15% diluter. Try to print several times so that the etched gravure is fully wetted and the print head is properly expanded. At least try to print on the substrate 12 times, and if necessary, adjust the pressure of the print head. If the printed image has quality problems, the following two steps can be used to test.
The first step: Take an image printed at the normal print speed and turn off the press. Check the image on the print head and use a small mirror to observe the image under the print head. Check that the image on the print head is complete and that the image is in the correct position. If there is no problem with the image on the print head, continue to the second step. (Note: If the part of the image on the print head is missing, or the edges of the image appear to be jagged, the ink is generally too thick. Add a certain amount of diluting agent, repeat this work until it can be on the print head Get an acceptable image: If you find that the image on the print head is not in the position it should be,
The position of the print head is adjusted. ) Second step: If there is no problem with the image on the print head, then the image is printed onto the substrate at the normal operating speed. If the quality of the print is OK, you can begin the formal printing production; if the printed image looks a bit dirty, transparent, or fuzzy, and the ink is still attached to the print head, the ink is too thin. You can add a small amount of ink, you can also reduce the printing speed.
In the process of controlling ink performance, it should be noted that the influence of temperature and relative humidity on the performance of the ink is very large. The higher the temperature, the faster the solvent volatilizes and the faster the ink becomes sticky. However, this is not the case with the two extremes of relative humidity (very high humidity and very low humidity). The two extremes of relative humidity are a headache for printers when controlling ink performance on the press.
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