Screen printing color change

This is a basic article written for inexperienced beginners, but the content contains an important lesson that in order to obtain stable and high-quality prints in the current environment of a printing plant, standards must be established. .

The hardest part is often to answer questions that seem simple. Some beginners often ask why colors change when printing. Why is it so difficult for a printer to keep a job exactly the same as the color of another job? The only answer is to use careful process control and automatic adjustment techniques to minimize color changes. This is a suitable answer when you just started working on screen printing and know nothing about the process. However, from the point of view of an experienced production and printing worker, they may not necessarily all have a clear understanding of the reasons for the color change.

If you print a simple line job, which is monochrome printing, this time you can not consider the four-color mesh tone, this answer makes sense within a certain range. Specifically, the main factors to be considered should be the following points.

Ink mix is ​​not good

If, assuming that the pigment in the ink is properly formulated, the usual cause of the color change is the added solvent. In a well-controlled shop, the ink should be ready to be supplied to the press at any time. That is, the printer should not dispense ink. In many companies, the ink is not adjusted for use by the printer, but is left to the printer to adjust, and they add and mix the ink according to their own feelings. As a result, the pigment balance in the ink is broken. For water-based ordinary inks or UV inks, the water in the ink acts in the same manner as the solvent in the solvent ink. Adding water will thin the dried ink film and affect the color of the ink, thereby reducing the density of the color. . The reasons for such problems can be further traced. In ink warehouses, ink distributors do not use weighing devices and rely on their own judgment to add the right amount of solvent, or to mix them inappropriately at first, or to change the mixing amount of ink during printing, so that the mixed ink will Produce different colors. When this job is printed again later, this situation becomes even worse, unless there is a sufficient ink ratio record, it is almost impossible to reproduce a color.

Inappropriate screen selection

Any screen printer who thinks that the number of screen meshes is only a variable of the screen structure will only affect the deposition of the printing ink, and he will have troubles. The diameter of the wire mesh and the weaving method, i.e. plain or twill, have a great influence on the thickness of the printed ink film. Wire mesh suppliers will provide detailed technical information on the wire mesh, the most important of which is the theoretical ink volume, which represents the amount of ink that passes through the screen mesh under certain printing conditions, and is generally expressed in cm3/m2. For example, a 150 mesh/cm screen with a mesh diameter of 31 μm will be capable of passing 11 cm3/m2 of ink. A screen with a mesh diameter of 34 μm and a mesh of 150 mesh will pass 6 cm3 of ink per square meter, corresponding to 11 and 6 μm thick wet ink layers. It can be seen from this that the simple expression of 150 mesh will give you a distinctly different thickness of the ink layer, which results in a great difference in color.

With the improvement of the screen weaving technology, to get a certain number of diagonal twill screens instead of plain weave screens, although sometimes this possibility, but the possibility is small. Sometimes wire mesh suppliers store some old twill meshes. In general, these meshes have a theoretical 10% change in ink volume. If you print fine-grained images with twill weave screens, fine lines break more than plain screens.

Screen tension

The lower tension of the screen will cause the screen and the printed surface to depart slowly, which will affect the ink staying on the screen, and the effect of color unevenness and the like, so that the color appears to have changed. To solve this problem, it is necessary to increase the pitch, that is, increase the distance between the horizontally placed screen plate and the printing material. Increasing the net pitch means that the pressure of the squeegee is also increased. This will affect the amount of ink passing through the screen and cause further color changes.

Squeegee setting

We know more about how the squeegee works. Recent studies have shown that during the printing operation, the squeegee has some very interesting phenomena, which are described in detail later. Based on the current state of the art, the softer the squeegee used, the more ink will pass through the screen. The greater the pressure acting on the squeegee, the faster the squeegee blade wears during printing. This will change the contact point between the squeegee and the print, which will also change the amount of ink passing through the screen. Causes color changes. Changing the squeegee angle also affects the amount of ink deposited. If the doctor blade runs too fast, this will reduce the thickness of the attached ink layer.

The setting of ink-knife

You will be surprised to find that many screen printers do not understand that the setting of the ink-removing knife will have a major impact on the amount of ink deposited. The role of the ink-return knife is to fill the screen holes with a stable amount of ink. Adjusting the ink blade pressure, angle, and sharpness of the knife edge will cause the mesh to fill too much or too little ink. Too much pressure on the ink-return blade will force the ink through the mesh and cause the ink to adhere excessively. The pressure of the ink-returning blade is insufficient, which will cause the mesh hole to be partially filled with ink, resulting in insufficient ink adhesion. The operating speed of the ink-return knife is also very important. If it runs too slowly, the ink will overflow; if it runs too fast, it will cause serious ink shortage, which is similar to the effect of changing the operation speed of the scraper.

The setting of the machine

Serious process control is the biggest key factor. The stable and consistent adjustment of the machine means that the color is stable and consistent. If the adjustment of the machine is uncertain, then the color will lose control. This problem usually occurs when the printers change classes, or later the printers adapt their own habits and freely change the settings on the printing press, which will cause the color to change. The latest type of multicolor screen printing machine is automatically controlled by a computer, eliminating this possibility. These stable, consistent set-ups for the press are made and these settings are kept constant throughout the print job.

Impact of printing materials

In the screen printing industry, the often overlooked aspect is the consistency of the printing materials to be printed. Paper, cardboard, and plastics used in printing are generally produced in batches. A high-quality supplier can guarantee that the entire batch of materials they provide has a good surface smoothness, but this is not always the case. Any slight change in the process during the processing of these materials will change the color of the material and Surface finish. Once this happens, the color of the print will look different, although nothing has changed in the actual printing process. When we want to print the same graphic as a promotional advertisement from a variety of materials, such as corrugated plastic panels and fine art paperboard, the printer will encounter these practical difficulties. Another problem that is also often encountered is that our screen printing has to catch up with offset images. If we do not pay attention to process control, we have no chance. Careful process controls include accurate color measurements, the use of a spectrophotometer to measure line color, and densitometers to determine the three primary colors so that we can print consistent color images on a variety of materials. If we also use the traditional intuitive method, the color change may be so large that the process is completely out of control.

Observe the light source

Under different light sources, the colors look different, and human eyes are very sensitive to these changes. This can be reduced by ensuring that the colors of the pigments used in the entire printing job are exactly the same. If you change suppliers, this may be a disaster. Color measurement and perception is a very complex area. To achieve the best control, there must be a closed loop consisting of ink manufacturers, ink distribution, proofing, and accurate measurements during the printing process.

The effect of drying

Color changes sometimes occur due to improper adjustment of the dryer. When printing paper or cardboard, if the drying temperature is adjusted too high, the white is generally yellow. The glass and ceramic industries are the most troubled by the color change during drying or baking. The pigment used here has to be completely changed from the printed color to the sintered color. These sintered colors are affected not only by the baking temperature but also by the reduction of air quality in the oxidation or baking zone.

The above are just a few of the many causes of color change, and one of the areas that makes screen printing so interesting and challenging. All of this comes down to a point of good process control.



· Source: Screen Printing