Static control in screen printing

The conductive substrates used in screen printing today are much more extensive than in the past. Some printers who often use conductive substrates are very aware of the importance of controlling static electricity. Understanding electrical knowledge, mastering electrostatic knowledge, and knowing how to avoid or reduce static electricity will increase the quality of printing.

In general, humidity, grounding, electrostatic neutralization, squeegee speed/printing pause time, screen tension and grid spacing are all effective factors to reduce or avoid static electricity.

First, the concept of static electricity

From an atomic point of view, plastics, paper, and other materials are usually neutral. The positively charged nuclei in each atom neutralize negatively charged electron clouds in their surroundings. When the two materials, whether separated by pressure or by frictional contact, electrons free the surface of the material, creating an electrostatic zone. The electron-reduced material surface is positively charged, while the material with excess electrons is negatively charged. These two types of charged materials will attract the oppositely charged material to be electrically neutralized. If the charged material is conductive, the static electricity The charge is quickly transferred to the surface by the material. If the material is charged during printing, the static charge will transfer to the surface of the machine. If the material is an insulator, the static charge will gradually leak out after hours, weeks, or even months.

Static electricity is easily generated when the humidity is low, because most materials have conductivity if they have a certain humidity, and the surrounding environment has increased humidity and conductivity, which reduces static electricity. In the winter season or in an air-conditioned room, it is easy to generate static electricity due to reduced humidity.

If a factory wants to reduce static electricity in the work environment, it is better to install humidity and temperature control equipment. This not only can effectively control static electricity, but also can reduce or eliminate various factors that affect print quality.

As the environmental conditions of the entire plant are controlled, the time for preparation before printing will be reduced. Under the constant humidity and temperature, the printing ink will volatilize at a certain known speed, thus reducing the printing and drying. Many uncertain factors.

According to surveys, if the working environment is comfortable and stable, the employees' performance will be good and there will be less operational errors. Some of the problems caused by static electricity, despite being the same material, are not the same due to the different speeds of the printing press, and sometimes there is almost no cause for problems.

Each sheet is separated within one minute and then stacked together. Since plastic is a fairly good insulating material, it is quite sensitive to static electricity.

The problems caused by static electricity mainly have the following performance:

● After the squeegee leaves, the ink clumps on the screen and sticks together;

● printing materials stick together and cannot be separated;

● printing materials can not be stacked together or can not be stacked;

● The printing material sticks to the bottom of the screen plate;

●Automatic printing machine cannot feed paper normally;

● The operator was shocked.

Second, static eliminator

Some static eliminators neutralize electrostatic charges with metal ground wires. The two most commonly used are metal foils and conductive brushes.

The static eliminator generates two kinds of positive and negative ions in the air. When any charged body passes there, the corresponding ions with opposite charges neutralize it, eliminating static electricity.

The static eliminator with a power supply consists of a power supply and an antistatic bar. The length of the antistatic bar can be freely ordered to the manufacturer. In general, the length is the same as the width of the material to be processed.

Elimination of static electricity on the surface of the material can easily remove the dust, because the dust will not re-adsorb on the surface of the material due to static electricity.

There is also a vibration-free static eliminator on the market, especially when an emitter is accidentally grounded by dust or moisture, and the antistatic bar is still in operation. This type of eliminator is very effective. A conductive brush uses existing electrostatic voltage to ionize air molecules. This high electrostatic voltage is generally generated on the surface of a material that moves rapidly under high pressure. For example, under the high-speed blade pressure, the surface of the polyester wire mesh will be There is an electrostatic voltage.

Another container-type static eliminator eliminates static charges within a specified range.

Third, the use of electrostatic control methods used by printers

Nowadays, the basic and basic methods for controlling static electricity are mainly the following three:

● Ground the statically charged material;

● Use anti-static coating/cleaning agent;

● Ionize the air around the electrostatically charged material.

1. It is not practical to ground grounded materials because it is impossible to make the materials always come in contact with a grounded object, such as a metal printing station. When the printed material is separated from a grounded object, static electricity is generated. Therefore, this method makes sense when the printing machine is grounded only when the substrate has a surface humidity of 60% to 70% and conductivity.

Other grounded objects can also be used in a clean environment, such as conductive floor mats/pulleys, conductive mats, grounding pads on shoe soles, and the like.

2. The antistatic coating/cleaning agent is mainly applied to the charged image area, where the static charge generation is the main problem. Anti-static coating is a hydrocarbon solution, high volatility, containing low-valence metal ions, low particle content, no damage to the surface of polyethylene, stainless steel, PVC material or plastic film.

Although the effect of the antistatic coating is obvious, it is not practical because the coating is expensive, and the antistatic coating needs to be repeatedly coated at the graphic or equipment. Antistatic coatings reduce the surface friction of materials and allow charge to flow to the surface, reducing the ability of the material to generate static charges. Most antistatic coatings are non-corrosive and non-flammable.

3. Ionizing air molecules liberate the electrons of the oxygen molecules, resulting in negatively charged electrons and positively charged oxygen ions. If the material is positively charged, it can be neutralized by negatively charged electrons; conversely, if the material is negatively charged, it can be neutralized by positively charged oxygen ions.

Ionized air molecules are divided into three methods: power supply, no power supply, and nuclear neutralization. Nuclear Neutronization uses radioactive isotopes to emit particles that ionize air molecules. The particles are mainly short-wavelength alpha particles that are easily absorbed by oxygen molecules. The alpha particles collide with oxygen molecules, which separates the nuclei of the oxygen molecules from the electrons. A radioactive isotope emits tens of thousands of alpha particles per second. Each alpha particle produces thousands of ions in its short stroke, namely positively charged oxygen ions and a dense layer of electron clouds. The advantage of nuclear neutrality is that it does not require additional power and can be moved from one machine to another. It is ideal for use with flammable solvents.

Nuclear Neutralization and electrostatic charge make use of germanium. This method has the obvious inconvenience that the distance between the surface of the material and the neutralizer is between 1 and 1.5 in (inches).

A brush used in screen printing also uses the principle of nuclear neutralization. Mounting a bract on the hard surface of the brush not only reduces static charge, but also removes dust, hair and other particles.

Fourth, screen tension and printing speed

Screen tension and printing speed are two variables that are often overlooked.

The speed of the squeegee printing is reduced, and the electron cloud around the nucleus is in a relatively stable state, but even if the printing speed is slow and the squeegee pressure is too high, the electrons around the nucleus will be in an unstable state and the electrons will be under the pressure of the squeegee. It is liberated and generates static electricity. Therefore, static electricity generation can only be avoided if the speed of the squeegee is slow and the squeegee pressure is suitable.

Screen tension is regarded as a key factor for successful printing, reducing web spacing and squeegee pressure, which also avoids friction and thus reduces static electricity.

In the screen printing industry, static electricity is the main factor affecting the normal printing process, because the static electricity makes the printer inconvenient to operate, and it also creates fire hazards.

A printing plant can generate static electricity if it does not control the room temperature or humidity or does not control it at all. As a result, the full-automatic printing press cannot run normally, and it cannot run at high speed. For each different plant, it is difficult to say which equipment is suitable, which can effectively reduce static electricity. Please contact the company that sells the static elimination equipment. Ask them to look at your plant, check the causes of static electricity, and then purchase the equipment to eliminate static electricity. .