Come si risolvono i problemi di formulazione e stampa dell'inchiostro per rotocalco?
In the field of gravure printing, ink formulation is like an art form, which involves a lot of skills and considerations, and is related to the final printing effect and product quality.
A detailed analysis of the formulation of dark and light inks
Ink formulation can be divided into two camps: dark and light. First, let’s look at dark inks, which are formulated using only primary inks without any reducers. When mixing, the amount of ink used for printing must be taken into account, and the main and secondary colors must be determined using color analysis and then blended evenly. Dark ink mixing can be subdivided into three cases: monochrome, intermediate colors, and multiple colors. Monochrome means mixing with only one primary color ink, as we can see in some minimalist-style printed matter, where only one primary color is used to highlight the purity of the color. Intermediate colors are created by mixing two primary colors, such as red and yellow to produce orange, which is used to create a unique visual effect in the reproduction of traditional artworks. Full-color printing is the result of mixing all three primary colors, which creates a more complex and richer color palette. This method is often used in high-end printing where color depth and richness are required.
Light-colored inks are made by adding a thinner. There are three key points to note when mixing light-colored inks. First, white ink should be used in preference to thinner, as it has the advantage of maintaining the original properties of the ink. For example, in some high-quality poster printing, the use of thinner may lead to poor adhesion of the ink to the paper. Second, white ink should be used as the main ingredient in the mixture, and a small amount of colored ink should be added to it. This allows precise control of the shade and tone of the color. Third, the color selection must be accurate, which requires the color mixer to have a keen sense of color, just like a professional painter’s grasp of color. Any slight deviation may affect the final result.
A multi-dimensional strategy to enhance the gloss of the ink film
To enhance the gloss of the ink film, it is necessary to start from multiple aspects. Adding an appropriate amount of toner oil within the scope allowed by the color concentration is like injecting a layer of “gloss agent” into the ink film. Coating the printed film surface with a varnish or ink conditioner forms a smooth protective film on the surface, enhancing light reflection. At the same time, increasing the transparency of the ink is also key. You can choose inks with high transparency or add a moderate amount of ink conditioner. And you should make good use of the smoothness of the smooth, highly reflective substrate printing surface to allow light to better reflect between the ink film and the substrate. In addition, preventing the substrate surface from absorbing is also crucial, which requires reasonable improvement of the smoothness of the substrate printing surface.
The temperature and humidity of the printing environment have a significant effect on the gloss of the ink film. For example, when gravure printing on plastic is carried out in high humidity and low temperature conditions, the solvent evaporates faster, absorbing the heat around the ink film, causing rapid condensation of water vapor in the air and forming fog on the surface of the printed ink film, which greatly reduces the gloss of the ink layer. This is especially noticeable during the rainy season. According to a large number of practical cases, it is generally best to keep the room temperature at 21-23°C and the humidity at 40%. Such environmental conditions can ensure the most accurate reproduction of colors. Just like precision printing in a laboratory environment, every parameter affects the final result.
The unique art of mixing special color inks
Mixing gold and silver inks involves adding a suitable amount of gold or silver powder to the ink oil and stirring well. The gold and silver powders must be of the right fineness, and the varnish must be chosen according to the substrate. The gold and silver inks must be mixed just before printing, otherwise they will settle and separate after a while. Interestingly, gold ink can also be made by mixing silver powder, transparent yellow and varnish. For example, when mixing gold ink, a good result can be achieved by using a ratio of gold powder (greenish): varnish: transparent yellow = 1:3:2. To prepare gold ink with a reddish hue, simply replace the gold powder with red powder, depending on the customer’s requirements.
Pearlescent ink is prepared by mixing pearlescent pigment with a suitable proportion of high-transparency ink or ink-mixing oil until a uniform mixture is obtained. However, it should be noted that if the ink has too strong a covering power, the pearlescent effect will be lost. This is like covering the pearl with a thick cloth, and the original luster of the pearl cannot be displayed.
Luminous ink, also known as phosphorescent ink, has the magical effect of absorbing light energy and then glowing for a certain period of time. It is often used in printed materials with special visual requirements, such as posters or safety signs that glow at night. It is usually just a matter of dispersing the luminous pigment in a highly transparent ink-modifying oil and stirring well.
It is worth noting that none of these special inks can be ground. Grinding gold ink or silver ink can damage the grinding equipment, while grinding pearlescent or luminous ink can destroy its surface or crystal structure, resulting in the loss of the hue effect. This is like destroying the internal structure of a precision instrument, making it unable to function normally.
Key considerations for toning plastic printing
There are some important principles to follow when toning plastic printing. Ink toning should, as far as possible, use set inks of the same hue produced by the ink manufacturer, as the hue of set inks is more saturated than that of inks toned with two colours. This is like using original parts to match the equipment better than using a mixture of parts. If you want to mix one or two inks, you should try to use a set ink with a similar color as the base. And when mixing colors, try to reduce the number of ink types as much as possible, because the more types of ink you mix, the higher the proportion of cyan, which will reduce the brightness and saturation of the ink color. If you can mix two colors well, don’t use three.
Plastic film is a non-absorbent material, so you cannot dilute the color ink with thinner. You should add white ink to dilute it. Inks from different manufacturers and different varieties should not be mixed, otherwise it will affect the gloss, purity and drying speed of the white ink. Mixing different types of ink will directly change the original resin hue, resulting in poor ink performance. This is like mixing different brands and formulations of chemical reagents, which may produce unexpected chemical reactions, which will seriously damage the printing quality.
When mixing ink colors, every step must be strictly controlled in order to produce a color that meets the customer’s requirements, and to ensure that the ink color of different batches remains consistent. This is an important requirement for the stability of printing quality.
Exploring the root cause of printing ink adhesion problems and solutions
Ink adhesion is a common and difficult problem in the printing process. First of all, over-tightening the winding is an important cause. During printing, the film tension should not be too high, and the film should not be wound too long and too tightly. Imagine if the film is over-compressed like a spring, and as the printing press continues to run, the winding continues to increase, especially if the local cells are too deep and too large, then it is prone to bulging and adhesion. This is like when finishing wires, if they are over-twisted, the wires will stick to each other.
Improper ink mixing can also cause stickiness. For example, hardeners, which are low molecular substances and inherently sticky, are added to inks that require special properties (heat and oil resistance). In addition, problems can arise when preparing gravure inks if the resin has poor heat resistance. Generally speaking, resins with a universal softening point of 105°C to 110°C should be used. A softening point that is too low means that the ink is not heat-resistant. This is similar to the choice of building materials: unsuitable materials cannot withstand the test of a particular environment.
Another factor that can lead to adhesion is poor adhesion between the ink and the substrate. This problem can be solved in two ways. On the one hand, the ink should be selected correctly and its quality strictly controlled so that it can adapt well to the substrate. On the other hand, the surface of the substrate should be treated to improve its affinity for the ink. It is best to treat one side only, avoiding a situation where the tension on both sides is good, because electro-treatment of both sides may cause the ink surface to be too adhesive and cause it to stick back, just like objects coated with glue on both sides, which easily stick to each other.
The poor heat resistance and stickiness of the ink itself are intrinsic factors that cause adhesion. When a large amount of solvent remains in the printed ink, it is like the paint is not dry. Although it looks dry to the naked eye, it is actually still sticky. After the printed matter is rolled up, the residual solvent is difficult to volatilize, and the resin in the ink cannot dry and solidify, so the finished product will be seriously stuck together. If the stuck product is measured by gas chromatography, the residual solvent content is often as high as tens of thousands of PPM. Moreover, the residual solvent will make the product smell, which not only affects the lamination strength, but also has a negative impact on food flavor and hygiene standards. Therefore, the problem should be solved from the aspects of dryer performance, drying conditions, and load. Secondly, fast-drying solvents should be used as much as possible. Finally, the printed materials should be stored in a moisture-proof place to prevent the resin in the ink from swelling and becoming viscous due to the moisture in the raw materials. This series of measures is like establishing a solid defense for the printing process to ensure printing quality.
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