Why is there color difference in plastic flexible packaging printing?

[China Packaging News] How to keep the ink color of a batch of printed products or several batches of printed products consistent in printing is a basic requirement and an important problem. For manufacturers, the color of the packaging used in the product has become an important means and symbol for identifying the product (even becoming the logo color of the enterprise).

If the appearance of the product packaging looks different, consumers may think that the quality of the product is unstable and even doubt the authenticity of the product. Therefore, these well-known enterprises attach great importance to the color of product packaging and require that their ink color must be stable and consistent.

How to keep the ink color of a batch of printed products or several batches of printed products consistent in printing is a basic requirement and an important problem. For manufacturers, the color of the packaging used in the product has become an important means and symbol for identifying the product (even becoming the logo color of the enterprise). If the appearance of the product packaging looks different, consumers may think that the quality of the product is unstable and even doubt the authenticity of the product. Therefore, these well-known enterprises attach great importance to the color of product packaging and require that their ink color must be stable and consistent.

The consistency of colors can only be said to be relative. It is more difficult to strictly control the color difference of the same batch, but it is more difficult to strictly control the color difference between large batches or different batches. Plastic flexographic packaging gravure printing is mostly multi-batch printing and high-volume printing. Controlling chromatic aberration is a very important issue for plastic flexible packaging printing companies.

1. Ink

Ink is one of the main factors that determine the color consistency of printed products. The ink itself also has the same batch of color difference and the color difference between different batches, which is an important indicator of ink quality.

To solve the color difference problem of plastic gravure printing, we must first select the ink with stable quality and no hue deviation. A batch of printed products is best printed with the same batch of ink (it is best to prepare a sufficient amount of time when preparing spot color ink), because it is difficult to ensure that there is no difference in hue between different batches of ink. If the ink hue deviation is too large, it is difficult to control the consistency of the printing ink color by other methods, especially when printing the gradation version.

Secondly, the amount of ink added to the ink tank each time during the printing process is required to be less, in order to keep adding new ink frequently. Especially for the patterns with less ink, if too much ink is added at a time, the ink will be oxidized and become stale after being exposed to the air for a long time. For example, the black ink used for a long time is gray and not as black as new ink; The red ink used for a long time will become dark red, not as bright as new ink. In the printing process, the volatilization rate of the mixed solvent used in the ink must be strictly controlled. When the volatilization rate is too fast, the transfer rate of the ink may be reduced, thereby affecting the color of the printed product.

Furthermore, when using two-component inks, they must be fully stirred, and less frequently used. Otherwise, it is difficult to maintain the stability of the ink viscosity, and may also cause other printing failures.

2. Scraper

The contact position, angle and pressure of the scraper and the plate cylinder have a great influence on the ink color of the printed product, especially the ink color of the shallow layer, so the ideal state is that the scraper is in the same position from beginning to end. However, due to various reasons, the position, angle, and pressure of the squeegee had to be adjusted during the printing process, so we must understand the influence of the change on the color of the printed product and take corresponding measures.

The contact position between the blade and the plate cylinder determines the distance between the blade, plate contact point and the imprint point. The length of this distance should be adjusted according to the printing speed, ink viscosity, mesh depth, and plate quality. In general, the longer the distance, the greater the degree of ink drying in the cell, and the lower the ink transfer rate. In terms of its impact on the printing effect, it is equivalent to the part of the ink color faded. At the same time, when the distance is longer, it is also a test for the adaptability of the ink. On the contrary, when the distance is short, the transfer rate of ink is relatively high, and at the same time, some fine lines on the layout are transferred to the printing material.

The blade angle is the angle between the tangent of the contact point between the blade and the plate cylinder and the blade. Increasing the blade angle makes it easy to scrape off the ink on the plate surface to make the picture clear, but it may also cause phenomena such as too thin ink layer and poor gloss of the ink film, and it is also easy to damage the plate cylinder. Conversely, if the angle of the doctor blade is reduced, the printing ink layer becomes thicker and the gloss of the printed product is good, but the corners of the picture and text are easily blurred.

Squeegee pressure also has a certain influence on the ink color. When the pressure becomes larger, the ink color becomes lighter; when the pressure becomes lighter, the ink color of the printed matter tends to become darker.

3. Plate cylinder

The influence of the printing plate cylinder on the consistency of ink color is mainly manifested in the following aspects:

1. Blocking: Blocking is caused by clogging of the mesh holes (caused by solidified materials, high-viscosity materials, or insoluble materials in foreign materials, etc.) for some reason and causes a decrease in transfer rate. At this time, the amount of ink transfer is reduced, which affects the hue. When blocking occurs, use a solvent or a special cleaning agent (using ordinary decontamination powder is easy to cause other failures) to clean the plate cylinder, and take corresponding measures according to the cause of blocking to make it return to normal.

There are many factors that affect plate blocking. In addition to the two-component inks and other inks that easily cause chemical reactions and chemical changes that cause insolubility, other reasons can be summarized as the ink is dry on the plate surface, mixed with impurities, and the quality of the plate making is low. factor. In addition, during the printing process, if water or other types of ink are mixed into the ink, it will affect the fluidity of the ink, cause the re-agglomeration of the pigment, and will also cause blockage failure.

In addition, a detailed inspection of the plate cylinder is required before printing. If the printing plate cylinder adheres to oil stains, etc., printing failures similar to blocking will occur during printing.

2. Wear of the printing plate cylinder

When the volume of the printed product is large, the degree of wear of the plate cylinder is large, and the color of the printing ink will change. Generally speaking, it will tend to become lighter (because the mesh hole becomes lighter, the amount of ink is reduced). Therefore, after a certain number of prints, you need to re-plate. Although the cylinder of the printing plate cylinder becomes lighter, although it can be adjusted by other processes to maintain the stability of the color, it will lose the value of re-plating after excessive use, and only re-making the plate.

Various faults and damage may also occur during the use of the printing plate cylinder. Due to many influencing factors during plate-making, it is almost impossible to completely repeat the original plate-making process conditions when re-plate-making, so that even after the plate-making is re-made, it is often difficult to obtain the same printing effect as the original even under the same printing conditions. In order to solve this problem, if it is a printing job with a large printing volume and is frequently printed, you can consider making two sets of plates or even multiple sets of plates to solve it.

3. Printing plate design and color management

In the design of printing plates, we must fully consider the impact of printing and post-press processing technology. For example, the color of some dot areas is very easy to be graded and graded, and it must be dealt with when making a plate. Many plate-making companies have begun to use color management software, which has played a positive role in improving the quality of color reproduction.

4. Printing conditions and environment

In the printing process, the color of the ink film mainly depends on the ink concentration and transfer rate. For a specific substrate and a specific ink, the concentration of the ink is relatively fixed, the transfer rate depends on the ink viscosity, dryness, and the state of the mesh on the plate, and is also affected by the printing speed, printing pressure, blade pressure, blade Factors such as location, blade angle and environmental conditions (temperature and humidity). It should be pointed out here that when these conditions change, the degree of influence on the highlight, dark tone, and middle tone of the printed matter is different.

1. The viscosity and drying speed of the ink change

Ink viscosity changes can directly affect the color of the printed product. When the viscosity is high, the color density is relatively high. Conversely, when the viscosity is low, the color density is relatively low. This effect is sometimes very obvious and must be highly valued. According to the requirements of the printing process, the viscosity of the ink should be moderate. If the viscosity is too large, it is not conducive to the transfer of the ink, and it is easy to produce a paste phenomenon; if the viscosity is too small, it is easy to produce static electricity, and it is not conducive to the control of the ink color.

The change in ink viscosity has a subtle effect on the migration area caused by ink migration or dot flow, especially in multi-color printing. Therefore, when the printing speed is constant, the viscosity of the ink is required to be consistent from beginning to end. It is best to use an automatic viscosity controller.

When the proportion of solvent in the ink changes, the drying speed of the ink changes, which in turn affects the transfer rate of the ink and the color of the printed product. For some inks with a small amount of ink, after being placed and used in the ink tank for a period of time, due to the automatic volatilization of the solvent, it will not only affect the viscosity of the ink, but also affect the proportion of the solvent and the rate of volatilization, so the solvent should be replenished in time.

2. Environmental temperature and humidity changes

If the room temperature and the liquid temperature of the ink change significantly, the ink migration state will change, and the color tone will also change accordingly.

This is because the temperature change affects the ink viscosity. At the same time, high temperature and low temperature weather will have a significant impact on the ink transfer rate of highlights. Therefore, when printing high-end products, the temperature and humidity of the printing workshop must be controlled anyway. In addition, when using ink in winter, preheat in advance to reduce the temperature change of the ink itself.

3. Separation of synthetic colors

This phenomenon is prone to occur in light colors and mixtures of different colors such as organic pigments and inorganic pigments, which differ greatly in specific gravity. Especially when the ink in the ink tank is not flowing, the precipitation will promote the separation of the synthetic color, so pay attention to make the ink circulation smooth. In addition, if the stirring is insufficient during color adjustment, as the printing progresses, the density of a certain color tends to increase. Therefore, before the official printing, a mechanical or other method such as a dissolver should be used to sufficiently stir the ink.

4. The deterioration of ink makes the color development worse

Due to the long-term printing, the ink solvent composition changes, or the moisture in the air is mixed into the ink to cause the ink to deteriorate, which deteriorates the color development.

In order to prevent the temperature drop of the ink caused by the heat of vaporization, a slow-drying solvent can be used together. In high temperature and high humidity weather, a certain amount of water may be mixed into the ink. When an abnormality occurs, new ink should be replenished or replaced completely. The remaining ink that has been used many times should be filtered or discarded regularly due to the mixing of a lot of dust. .

5. Other printing conditions

Discoloration may occur due to the overlap of the dots and the irregular wind blowing on the layout, or changes in printing speed and drying speed.

Five, the impact of post-press processing

1. Changes in post-press processing technology

The use of different post-press processing (such as extrusion compound and dry compound) also has a certain effect on the hue of the printed product. At the same time, when extruding the compound at high temperature, it sometimes causes the change of the pigment. In addition, when heat sealing is performed at a temperature of 200 ° C. or higher, the hue of the heated portion may become darker. This is due to the melting of the heated colorant and the formation of microcrystals during cooling and recrystallization, which causes an increase in coloring power.

2. Changes in the substrate after printing

Changes in the substrate after printing, such as changes in the aluminum layer of the aluminum coating, will directly affect the overall hue. Of course, the color deviation of the printed substrate will also have a slight effect on the overall hue.

3. Photochemical reaction

When the light resistance and weather resistance of the ink colorant are poor, the color of the printed product may change due to the photochemical reaction.

4. Impact of packaging contents

When the content contains active substances with volatility and migration, the color of the packaging print may also change due to corrosion. Therefore, the appropriate ink should be selected according to the final use of the packaging print (or negotiate with the ink manufacturer).

6. Visual inspection and inspection using color control instruments

Customers want a high degree of consistency in the ink color of packaging prints, but everyone's observation of color is not exactly the same. What customers need is persuasive data, that is, quantitative data of color. Although the naked eye is good at viewing colors, it is not good at remembering colors (if the products are not placed side by side, the naked eye cannot accurately determine the color), so the naked eye is no longer considered to be a reliable guarantee for judging the color of the ink, and the instrument shows its unique importance. From the naked eye to the densitometer, colorimeter to spectrophotometer, to the current desktop spectrophotometer, the color measurement results are more accurate, enough to compensate for human eye fatigue effects, visual retention, visual memory, etc. Adverse effects. Of course, it will take time for color control instruments to be widely used in China.

As part of the color management process, color inspection is an indispensable step and the first step in color management. At the same time, because the stability of color will be affected at any time during the operation of the printing press, it will be an important development trend to use online color control devices to measure and monitor color stability.