In pad printing technology, the correct reproduction of colors plays an important role in print quality. In color matching, the problem of color matching related to color measuring equipment is an objective scientific problem, which can be expressed in data; and device-independent color matching is a subjective problem, subject to limitations in artistic accomplishment and observation level. In order to meet the needs of customers, both the impact of objective data and the desired artistic effect must be considered when printing.

Observing the target, observing the light source or light, and the viewer (human vision or a certain color measuring device) are three essential elements for forming a particular color. They are indispensable. The observer's ability to perceive color is limited by many factors. All the variables are included in the three elements of the observation color. These variables affect the matching ability of the color. The change of any one of the variables will affect the color of the image after transfer. The influence of these variables will be introduced below.

Impact of printing materials

A customer's desired color is usually from a color matching (color matching) system such as Pantone or a standard color sample. 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) The printing method required to achieve a certain hiding power

(3) Effect of ink additives, diluents and catalysts

(4) Produce a printing method (offset, silkscreen, or pad printing) that suits a certain color standard

Substrate color and surface features

The surface color of the printing material is an important factor affecting the color reproduction of the images obtained by the printing technology. When the printed color is observed, the colors seen are formed by reflection of light from the surface of the substrate and the overprint layer of the printed ink into the human eye. Due to the thicker layer of ink deposited on the substrate during printing, the light is usually transmitted through the ink layer to the substrate, and then reflected on the surface of the substrate, once again through the ink layer, reflected in the human eye. The amount of light reflected on the substrate of the printing material is related to the color of the material itself, and in particular, the color of the substrate itself is dark, and when the light color printed on it is observed, its influence is more obvious.

The surface properties of printing materials such as smoothness or smoothness, 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 Different: Colors on glossy substrates look brighter and taller; the same colors printed on porous, rough surfaces such as fabrics appear gray. This is due to the difference in light reflected by the two hostages. When the light strikes the surface of a smooth, flat surface, most of the light is reflected at the same angle, so most of the reflected light enters the human eye; instead, it is rough, Porous surface light is reflected from different angles, and much less light is injected into human eyes.

Therefore, the tighter the color of the printing and the substrate is, the better the resulting printing effect is: a certain color can be reproduced on a coated or uncoated colored paper medium, and the reproducibility on a dark plastic medium is very high. difference. In addition, under certain specific printing conditions, one or more layers of white ink are preprinted as the base color, otherwise a better matching effect will not be obtained. For example, for a clear bright pigment, a white ink is preprinted before printing any color in order to avoid losing the feature.

Printing times

As mentioned earlier, while observing the color of printed products, the color of the substrates themselves is also seen. In order to reduce the influence of printing materials, the selected ink should be as opaque as possible. At the time of pad printing, the thickness of the printing ink layer after drying is about 20% of the etching depth of the printing plate. If the etching depth is 0.001 inches, then the thickness of the film after drying is between 0.0002 inch and 0.00025 inch, which is much thinner than silk screen printing. Therefore, underly thin film layer is enough to mask the influence of the color of the substrate. One of the solutions is the etch depth, but when the depth exceeds 0.0015 inch, the effective transferability of the ink is greatly reduced, the image becomes blurred, and the paste starts to appear. Therefore, only one other method is needed, that is, the number of times of printing is increased. This process is called double printing or multiple printing.

The use of multiple prints will undoubtedly increase production time. Therefore, before taking multiple prints, it is necessary to determine whether this can achieve the required productivity, even if a single print does not achieve the desired color matching, nor does it have to sacrifice production time. The expense of multiple prints can be achieved by reducing the amount of diluent in the ink or by dipping the pigment. In addition, prior to using multiple prints, it should also be determined whether the details of the image are still kept clear in the case of increasing the number of prints without distortion or bending in the printing process.

Ink additives

Ingredients in the ink component also affect the color of the ink. Diluents, catalysts, flow agents, etc. are generally colorless, and therefore, the hiding power of the ink is reduced. For example, adding a certain catalyst (such as hardener) to the ink in 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 important for a member who is performing the process to understand the required additives and the specific ratio at the time of addition so that the correct addition can be completed before the coloration.

Color sample production

Standard color samples are the guarantee for proper printing. Since the thickness of the obtained ink film layer is different in each printing mode, the color sample is obtained by either silk screen printing or offset printing and must be corrected when it is used. Color samples used for reference in pad printing also need to be corrected.

Observing the environment

Light is generated by electromagnetic radiation, so light is expressed from a portion of the electromagnetic spectrum. The spectral range that can be observed by the human eye is the main visible light, and many long-wavelength red light to shorter-wavelength blue light. Since the light color seen by the human eye is only a certain wavelength of light in the visible light wave range, the light conditions in the observation environment have a great influence on the observation result. If the colors to be matched are observed during the day and the colors are obtained under fluorescent conditions, then this match is not a true match.

In addition, the environment in which the target turnover is observed also has an effect on the color of the observed image. For example, in the dark background, the color perception obtained by observing the same image with a bright background is different. This is even more important when presenting samples to customers for comments. It is important to ensure that the customer's observation conditions are consistent with the environmental conditions during the printing process, or that the light conditions during production are the same as the customer's observation conditions.

Observer

It can be a human observer, or it can be a color measurement device (such as a spectrophotometer). It is impossible for any observer to observe the same image in the same way. Human vision can distinguish 10 million colors, and the individual's psychological condition influences his or her feelings about color. When a person's card is healthy, the observed color is not the same as the color he feels when he is tired, sick, or depressed. Adults' feelings are not the same as children's feelings. 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 whether a color is correct or not is based on the visual characteristics of many people.

Color measuring equipment such as spectrophotometers are measured by measuring the reflected light from an object in a simulated three-dimensional color model, also known as the color space, in the form of a set of color stimulus matches. During the measurement process, each device is calibrated to the color of the color sample after being calibrated under certain given conditions. These given conditions include lighting conditions or light source types, viewing angles, consideration of the influence of gloss, and the like.

For the results of different device measurements under the same environmental conditions, the same color matching performance will be achieved in the color space. But in fact, there are differences between different devices, and the results are different. Even if the same device itself, the results of two consecutive measurements may be different without changing any conditions. In most cases, these subtle differences will not affect the result too much, but there are problems for different devices under different conditions or the color tolerance is too small. Although the printing department and the customer may use the same equipment, since we cannot guarantee that these equipments are calibrated to observe the colors in the same way and complete the measurement, differences often occur. For example, for a person who specifies or sets the color, the gloss factor is taken into consideration when making the color sample, and the printer observes the color matching condition and does not consider the gloss factor when measuring the color sample. Then, although the naked eye observes The matching result is acceptable. Therefore, it is important to ensure that the measuring equipment and the equipment used to set the colors are observed in the same environment when matching the colors within a specific tolerance range generated by the computer.

Color measurement plays an important role in color matching. The measurement results can produce a large amount of useful information, which can directly indicate the location of the matched color in the color space, and provide help for reliable matching. However, in other cases, although the matching effect is good from the measurement results, it is not ideal from the visual or artistic effects. This is because human vision still occupies a major position in the evaluation of color matching, so it is necessary not only to consider the role of measurement tools in color matching, but also to consider the influence of human subjectivity on subjective perception.