Controlling color difference in packaging bags is crucial for product quality

Color difference in packaging bags has always plagued many customers. Whether the color difference can be effectively controlled is an important indicator of the product quality, reputation of the packaging bag manufacturer, and the skill level of the operators. Today, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will share its production experience and explain how to control color difference in packaging bags.

Color difference in packaging bags is roughly divided into two types: one is that the color of the same batch of products is consistent, but there is a color difference with the sample; the other is that there is a color difference within the same batch of colors, some consistent with the sample, but others inconsistent with the sample.

Shunxingyuan Packaging will now discuss and analyze this with you from the following aspects.

1. The color of the same batch of products is consistent, but there is a color difference with the sample

1. Human factors

It is not related to the skill level of the press operator, but it is related to the sense of responsibility of the operator. Since the color of the same batch of products can be kept consistent, it shows that the operator's skill level is still not low, but why would they dare to print it if it is inconsistent with the sample? Was the first sample signed off? This is entirely a matter of the operator's sense of responsibility.

It is also possible that there was a mistake in signing off the sample. If so, this is related to the sense of responsibility of the person who signed off the sample.

2. Raw material color

Different film materials have different effects on the color presentation of the printed ink layer.

Because different whiteness is equivalent to adding different amounts of black, red, blue, or yellow to the ink, even if the ink amount and hue do not change during printing, the actual ink has a certain degree of transparency, and the color effect varies with the whiteness of the film material, resulting in different color differences.

When cutting materials, the same batch number of film materials must be used. Because although the specifications and dimensions are the same, the production batch dates are different, the whiteness of the film materials will have some differences, thus leading to color differences in the packaging bags. Therefore, film materials with the same whiteness must be used as the printing film for the same product.

3. Gloss and smoothness of raw materials

The gloss of the printed matter depends on the gloss and smoothness of the film material.

In gravure printing, when light is incident on the surface of the film material, the light is reflected to the retina of the human eye, and the color is seen through the photosensitive cells' reception of light.

If the gloss and smoothness of the film material are high, the color we observe is basically the color reflected through the ink layer, and the saturation of the main color light is high.

If the surface of the film material is rough and the gloss is low, diffuse reflection will occur, which will reduce the saturation of the main color light, making the color of the printed matter appear lighter to the human eye.

The density value is measured with a densitometer using the same amount of ink. Film materials with high smoothness and gloss have high density values, while those with low smoothness and gloss have low density values.

4. Surface treatment of printed sheets

After surface treatment such as laminating, glazing, calendering, oiling, and printing, the film material will have different degrees of hue changes. Some of these changes are physical changes, and some are chemical changes.

Physical changes mainly manifest in the increased specular reflection on the product surface, which has a certain impact on the color density. For example, gloss lamination, UV coating, and calendering will increase the color density. Matte lamination and matte oiling will reduce the color density of the printed matter.

Chemical changes mainly come from laminating adhesives, glazing agents, UV oils, etc. These materials contain various solvents, which will cause chemical reactions in the printed ink layer, resulting in color changes.

Therefore, for gravure-printed packaging bags, if there are post-printing processes, the physical and chemical changes of the post-printing process must be considered when printing to determine the density and Lab values of the ink layer.

5. Dry-back density

After gravure printing, the ink is still wet, and the density value measured at this time is different from the density value measured after the printed matter is dry.

The density value is high when wet and low when dry, which is the dry-back density phenomenon.

This is because the freshly printed ink layer has a certain leveling property. It is characterized by specular reflection on the surface, making the colors look bright and glossy. When the ink layer dries, there is diffuse reflection on the surface, and the gloss naturally appears duller than when it was freshly printed.

In normal printing, in order to minimize color difference in the same batch of products, we use the same wet density test method to control. That is, the first printed product is in a wet state, and the wet density and L*a*b* values are measured immediately after the customer signs and approves. When printing this product later (wet state), the color is controlled according to the density and L*a*b* values measured for the first time.

Of course, if a densitometer with a polarizing device is used, it can eliminate the light caused by specular reflection on the surface of the ink layer. The wet density measured is relatively close to the dry density, which can also help reduce color difference control.

6. Printing pressure

Printing pressure is one of the conditions for ink transfer. Because the surface of the printing plate, rubber blanket, etc., cannot be absolutely flat, and the surface of the film material inevitably has slight unevenness or thickness variations.

If the printing pressure is insufficient or uneven, the ink layer is prone to uneven density. Therefore, the process requires "three flatness," that is, the surface of the printing plate, rubber blanket, and substrate and backing must be relatively flat to obtain a relatively uniform ink color on the printed matter with a thinner ink layer through the action of uniform printing pressure.

When the printing pressure is insufficient or uneven, and there is poor contact between the backing and the printing plate surface, the ink supply must be increased to meet the visual effect of the plate inking.

However, this will not only increase ink consumption but also easily cause color difference and dirty printing. Therefore, by ensuring "three flatness" (ink roller flat, water roller flat, cylinder flat) and using uniform and constant printing pressure, color difference caused by inappropriate pressure can be prevented.

7. Light source of the proofing table

When observing the color of a gravure printed product, a light source is essential; without light, no color can be seen. However, if the characteristics of the light source differ, the color difference will be significant.

Generally, color observation should be conducted under natural light (i.e., standard light source). If an ordinary light bulb is used as the light source, the color will appear yellowish, making accurate color identification difficult, and ultimately resulting in significant color deviation in the printed product. Additionally, the intensity and angle of the light will also affect color identification.

Under the same light source, the intensity of light reflected from the illuminated sample is mainly determined by the distance between the sample and the light source. Closer means stronger, farther means weaker.

Therefore, in our daily work, whether it's day or night, we must pay attention to two points: firstly, use a standard light source (wavelength around 400-700), and secondly, ensure the light source intensity and irradiation angle are consistent.

For products with high color requirements, the ink color sample of the printed sheet should be placed on the standard sample for comparison. In this way, due to the small difference in observation angle, the color identification is relatively accurate. Therefore, practical experience tells us that the characteristics of the light source and the difference in observation angle will directly affect the accurate identification of color.

II. Color difference exists within the same batch of colors; some are consistent with the sample, but others are not.

1. Human factors

If the color of the first few sheets is considered accurate, and printing begins before the ink and water are balanced, continuously adjusting the water and ink amounts during the printing process will inevitably cause color differences within the same batch of printed products. This is largely related to the operator's skill level, experience, and personality, and falls under the category of insufficient ability leading to color differences in the same batch of products.

2 Printing Speed

The speed of printing will affect the lightness and darkness of the printed product's color. This is because the speed affects the balance of ink and water. The amount of water on the printing plate varies with the speed; fast printing has less water evaporation, while slow printing has more water evaporation.

Variations in water content lead to an imbalance of ink and water, inevitably causing changes in the ink color. Therefore, during printing, a constant speed must be maintained; alternating between fast and slow speeds will inevitably cause color differences in the printed product.

At the same time, the most stable speed must be adjusted according to the machine's condition. For example, some imported offset presses have a maximum speed of 16000 sheets/hour, while domestic machines have a maximum speed of 8000 sheets/hour. These speeds are not stable printing speeds.

Generally speaking, for both imported and domestic machines, the ideal stable speed is 70% of the maximum speed: approximately 11000 sheets/hour for imported machines and 5600 sheets/hour for domestic machines. This speed makes it easier to control the ink-water balance and reduce color differences.

3 Poor Ink Rollers

In the printing process, the quality of the rollers plays a crucial role in obtaining uniform ink color. Generally, there are three types of rollers: inking rollers, distributing rollers, and impression rollers, all of which significantly affect the ink density.

The elasticity, viscosity, concentricity, hardness, and surface smoothness of the rollers largely determine the quality of the printed ink color. If the impression roller has poor elasticity, insufficient viscosity, eccentric roller body, excessively hard roller, or aged roller, the inking process is prone to uneven or unstable inking, resulting in inconsistent ink color on the printed sheets.

Therefore, the quality of the rollers must be ensured during printing to ensure that the ink applied to the printing plate in each printing cycle is basically consistent. Daily cleaning of the rollers is necessary to remove paper dust and impurities that remain on the rollers, ensuring uniform ink transfer. The contact pressure between the rollers and between the rollers and the printing plate should also be adjusted; otherwise, uneven inking will occur, resulting in significant color differences.

4 Equipment Maintenance

Maintaining good machine performance is a reliable guarantee for improving the quality of printing ink color.

If the machine is unstable, such as inconsistent pressure or unstable paper feeding, and frequent blank or multiple sheets occur during printing, this will inevitably cause color differences in the printing. Therefore, regular maintenance and lubrication of key machine parts are necessary to prevent increased wear and tear and maintain stable equipment operation. Offset printing workers should strive to be "three good" (manage well, use well, repair well) and "four able" (able to use, able to maintain, able to inspect, able to troubleshoot).

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