Color matching in pad printing technology

In pad printing technology, the correct restoration of colors plays an important role in printing quality. In color matching, the color matching problem related to color measurement equipment is an objective scientific problem, which can be expressed by data; while the color matching problem that is not related to the device is a subjective problem, which is limited by the level of artistic accomplishment and observation. How to establish the relationship between ink colors in printing can correctly express colors. It has a relationship with subjectivity. Even if the correct method is strictly followed, the most color matching method is used to produce the customer’s own visual defects (such as color blindness or eye fatigue). Rejected. Therefore, in order to meet the needs of customers, not only the impact of objective data must be considered when printing, but also the desired artistic effect must be considered.

We know that the observation target, the observation light source or light and the observer (human vision or a certain color measuring device) are the three elements necessary to form a certain color, and none of them are indispensable. The observer’s ability to perceive colors is affected by many factors. Limitations. Almost all variables are included in the three elements of observing color. These variables affect the color matching ability. Any change in any one of the variables will have an effect on the color of the image after the transfer. The influence of these variables will be introduced below.

1 Influence of printing materials

A certain color required by customers usually comes from a color matching (color matching) system such as Pantone or a standard color sample. However, due to the limitation of inheritance, the color transferred in pad printing is not a complete reflection of the color in the color swatch. For the impact of substrates, the following four aspects are mainly considered:

(1) The influence of the substrate color and surface characteristics of the printing material or medium

(2) Printing times required to reach a certain hiding power

(3) The influence of ink additives, diluents and catalysts

(4) Produce a printing method (offset printing, silk printing or pad printing) that is compatible with a certain color code

1.1 The color and surface characteristics of the base

The surface color of the printing material is an important factor that affects the color reproduction of the secret image of the pad printing technology. When observing the printing colors, the colors seen are formed by the reflection of light from the surface of the substrate and the printing ink superimposed layer to the human eye. Due to the thick ink layer accumulated on the substrate in pad printing, light usually passes through the ink layer to reach the base, and then is reflected on the surface of the base, passes through the ink layer again, and is reflected in the eyes of people. The amount of light reflected on the substrate of the printing material is related to the color of the material itself, especially the color of the printing material itself is darker, and the influence is more obvious when observing the lighter tones printed on it.

The surface characteristics of the printing material, such as smoothness or flatness, and porosity also play an important role in the color reproduction effect. Observing the same color printed on two media with the same color but different flatness, the visual effect obtained is The difference: the color printed on a smooth base looks brighter and has higher sharpness; the same color printed on a porous rough surface such as a fabric looks darker. This is caused by the difference in the light reflected by the two hostages. When the light hits the smooth and flat surface of the medium, most of the light is reflected at the same angle, so most of the reflected light enters the human eye; on the contrary, rough, rough, The light from the porous surface is reflected from different angles, so much less light enters the human eye.

Therefore, the closer the color match between the printed material and the substrate, the better the pad printing effect: a certain color can be better reproduced on white paper media with or without film, while on darker plastic media The reproducibility is very poor.

In addition, under certain printing conditions, one or more layers of white ink should be pre-printed as the base color, otherwise a better matching effect will not be obtained. For example, for transparent bright pigments, in order to avoid losing characteristics, a layer of white ink should be pre-printed before printing any color.

1.2 Number of prints

As mentioned earlier, while observing the color on the printed product, you can also see the color of the substrate itself. In order to reduce the influence of the substrate, the selected ink should be as opaque as possible. In pad printing, the thickness of the printing ink layer after drying is about 20% of the corrosion depth of the printing plate. If the etching depth is 0.001 inches, the thickness of the film after drying is between 0.0002 inches and 0.00025 inches, which is thinner than silk screen printing. Therefore, a film that is too thin is not enough to conceal the effect of the color of the printing material. One of the solutions is to increase the etching depth, but when the depth exceeds 0.0015 inches, the effective transferability of the ink will be greatly reduced, the image will become blurred, and start to blur, so we can only adopt another method, which is to increase the number of printing. In pad printing , This process is called double printing or multiple printing.

The use of multiple printing will undoubtedly extend the production time. Therefore, before taking multiple printings, it is necessary to determine whether this can achieve the required productivity. Even if a single printing does not achieve the desired color matching, it is not necessarily at the expense of production time. The cost of multiple printing can be achieved by reducing the amount of diluent in the ink or adding pigments. In addition, before using multiple printing, it should be determined whether the image can still maintain good detail clarity and will not be deformed or bent during the printing process when the number of printing is increased.

1.3 Ink additives

The additives in the ink components will also affect the color of the ink. Thinners, catalysts, and flow agents are generally colorless, which will cause the hiding power of the ink to decrease. For example, adding a certain catalyst (such as a hardener) to the ink at a ratio of 2:1 will reduce the hiding power by 50%, which is almost as obvious as the difference between single printing and multiple printing. Therefore, it is very important for people who implement color matching to understand the required additives and the specific ratio when adding them, so that the correct addition can be completed before the color is developed.

1.4 Production of color samples

The standard color swatch is the guarantee for the correct implementation of printing. Due to the difference in the thickness of the resulting ink film in each printing method, whether the color sample is obtained by screen printing or offset printing, it must be corrected during use. The color samples used for reference in pad printing also need to be corrected.

2 Observe the environment

Light is produced by electromagnetic radiation, so light comes from the expression of part of the electromagnetic spectrum. The spectral range that the human eye can observe is called visible light, which ranges from red light in the longer waveband to blue-violet light in the shorter waveband. Since the light color seen by the human eye is only light of a certain wavelength in the visible light wave range, the light conditions in the observation environment have a great influence on the observation result. If the color to be matched is observed during the day and the color is obtained under fluorescent light conditions, then this matching is not a true accurate match.

In addition, the environment in which the observation target turns around also affects the color of the observation image. For example, the color perception obtained by observing the same image under a darker background and a lighter background is different. This is more important when displaying samples to customers for comments. It is necessary to ensure that the customer’s observation conditions are consistent with the environmental conditions of the ink process during printing, or change the lighting conditions during production to be consistent with the customer’s observation conditions.

3 The observer can be a human observer or some kind of color measurement equipment (such as a spectrophotometer)

It is impossible for any type of observer to observe the same image in the same way. Human vision can distinguish 10 million colors, and an individual’s psychological condition affects his or her perception of colors. When a person is in a healthy state, the observed color is different from the color perception he gets when he is tired, sick, or depressed; the feeling of an adult is different from that of a child. Moreover, certain types of people are less sensitive to color changes in a certain wavelength range, and some are even color-blind patients. Therefore, judging the correctness of a color should be based on the visual characteristics of multiple people.

Color measurement equipment, such as a spectrophotometer, measures the reflected light from an object in a simulated three-dimensional color model, also known as color space, in the form of a set of color stimulus matching. In the process of measurement, each device is calibrated under certain given conditions to complete the color measurement of the color sample. These given conditions include lighting conditions or light source type, viewing angle, whether to consider the effect of gloss and so on.

In theory, the measurement results of different devices under the same environmental conditions will have the same color matching performance in the color space. But in fact, there are differences between different devices, and the results produced are also different. Even the same device itself, without changing any conditions, may have different results for two consecutive measurements. In most cases, these subtle differences will not have much impact on the results, but for different devices under different conditions or the specified color tolerance is too small, there will be problems. Although the printing department and the customer may use the same equipment, we cannot guarantee that these equipment will observe colors and complete the measurement in the same way after calibration, so discrepancies often occur. For example, the person who regulates or sets the color takes into account the gloss factor when measuring the color sample, while the printer observes the color matching situation and does not consider the gloss factor when measuring the color sample. Then, although the naked eye observes The matching result of is acceptable, but it will be unacceptable from the measurement result of the instrument. Therefore, when matching a color within a specific tolerance range generated by a computer, it is important to ensure that the measuring device and the device used for setting the color are observed in the same environment.

Color measurement plays a very important role in color matching. The measurement results can generate a lot of useful information, which can directly indicate the position of the matched color in the color space, and provide help for reliable matching. However, in other cases, although the matching effect is very good from the measurement results, it is not ideal from the visual effect or artistic effect. This is because in the evaluation of color matching, human vision still occupies a major position, so not only the role of measurement tools in color matching must be considered, but also the subjective perception of human vision.

When the factors involved in the observation target and the influence of the observation environment and the role of the observation subject (measurement equipment and humans) are fully considered, color management becomes simple. With the understanding of variables and the implementation of effective control, it makes people The headache of color matching has become easy and efficient. I believe that in the near future, the ability to control color will be more powerful.

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