Nowadays the conductive printing materials used in screen printing have increased a lot than in the past. Some printers who often use conductive printing materials are well aware of the importance of controlling static electricity. Understand the knowledge of electricity, master the common sense of static electricity, and know how to avoid or reduce static electricity, which will improve the printing quality.
Generally, humidity, grounding, static neutralization, squeegee speed / printing pause time, screen tension and screen spacing are all effective factors to reduce or avoid static electricity.
1. The concept of static electricity
From an atomic point of view, plastics, paper and other materials are usually neutral. The positively charged nucleus in each atom is neutralized with the negatively charged electron cloud around it. When the two materials are separated by pressure or frictional contact, electrons will free the surface of the Material, creating an electrostatic area. The surface of the material with reduced electrons is positively charged, while the surface of the material with excess electrons is negatively charged. These two charged materials will attract the oppositely charged materials to neutralize the electrical properties. If the charged materials are conductive, static electricity The charge will soon be transferred to the ground through the material. If the material is charged during printing, the static charge will be transferred to the surface of the machine. If the material is an insulator, the static charge will gradually leak out after a few hours, weeks or even months.
It is easy to generate static electricity when the humidity is low, because most material surfaces will have conductivity if there is a certain humidity. The humidity in the surrounding environment will increase and the conductivity will increase, thus reducing static electricity. Usually in winter or in an air-conditioned room, static electricity is easily generated due to reduced humidity.
If a factory wants to reduce static electricity in the working environment, it is best to install humidity and temperature control equipment, which can not only effectively control static electricity, but also reduce or eliminate various factors that affect printing quality.
Since the environmental conditions of the entire plant remain unchanged, the preparation time before printing will be reduced. Under constant humidity and temperature, the printing ink will also volatilize at a certain known rate, which reduces the printing and drying. Many uncertain factors.
According to the survey, if the working environment is comfortable and stable, the employee's work performance will also be good, with fewer operational errors. Some problems caused by static electricity, even though they are the same material, have different performances due to the different speeds of the printing press. Sometimes the problem occurs with almost no reason.
Each piece of paper 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 manifestations:
â— After the squeegee leaves, the ink clumps on the screen and sticks together;
â— The printing materials are pasted together and cannot be separated;
â— Printing materials cannot be stacked together or hardly stacked;
â— The printing material is pasted on the bottom of the silk screen;
â— The paper cannot be fed normally on the automatic printing machine;
â— The operator was shocked.
2. Static Eliminator
Some static eliminators use metal ground wires to neutralize static charges. The two most commonly used are metal foils and conductive brushes.
The static eliminator generates two kinds of positively and negatively charged ions in the air. When any charged body passes by, the correspondingly charged ions are neutralized with it to eliminate static electricity.
The static eliminator with power supply is composed of a power supply and an antistatic rod. The length of the antistatic rod can be arbitrarily customized from the manufacturer. In general, the length is the same as the width of the material to be processed.
It is easy to remove dust on the surface of the material that has been eliminated of static electricity, because dust will not be re-adsorbed to the surface of the material due to static electricity.
There are also vibration-free static eliminators on the market, especially when a transmitter is accidentally grounded due to dust or moisture, and the anti-static rod is still working, this eliminator is very effective. There is a conductive brush that uses the existing electrostatic voltage to ionize the air molecules. This high electrostatic voltage is generally generated on the surface of the material that moves rapidly under high pressure. For example, under the pressure of a high-speed scraper, the surface of the polyester mesh will There is electrostatic voltage.
Another container-type static eliminator can eliminate static charge within a specified range.
Third, the static control methods used by printing workers
At present, the general and basic methods for controlling static electricity are mainly the following three:
â— Earth static materials;
â— Use antistatic paint / cleaner;
â— Ionize the air around static materials.
1. It is not practical to ground static-charged materials, because it is impossible to make the materials always contact with grounded objects, such as metal printing tables. When the printing material is separated from the grounded objects, static electricity will be generated. Therefore, this method is meaningful only when the surface humidity of the printing material is 60% to 70% and it is conductive, and the printing machine is grounded.
In a clean environment, you can also use other grounding objects, such as conductive floor mats / wheels, conductive table mats, grounding lugs on shoe soles, etc.
2. Antistatic coatings / cleaners are mainly applied to the graphic areas with electric charges, where the generation of electrostatic charges is the main problem. The antistatic coating is a solution of hydrocarbons with high volatility, containing low-valent metal ions, low particle content, and no damage to the surface of polyethylene, stainless steel, PVC material or plastic film.
Although the effect of antistatic coating is obvious, it is not practical, because the coating is expensive, and the antistatic coating needs to be repeatedly applied on the graphic or equipment. Antistatic coatings reduce the frictional force of the surface of the material, so that the charge flows to the ground, and reduce the ability of the material to generate static charge. Most antistatic coatings are non-corrosive and non-flammable.
3. Ionizing air molecules frees the electrons of oxygen molecules, thereby generating 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: powered, unpowered, and nuclear neutralization. Nuclear neutralization uses radioisotopes to emit particles to ionize air molecules. Such particles are mainly alpha particles with shorter wavelengths, which are easily absorbed by oxygen molecules. The alpha particles collide with oxygen molecules to separate the nuclei of the oxygen molecules from the electrons. Radioisotopes emit tens of thousands of alpha particles per second, and each alpha particle can generate thousands of ions in a short stroke, namely positively charged oxygen ions and a dense layer of electron clouds. The advantage of nuclear neutralization is that it does not require an additional power source and can be moved from one machine to another. It is very suitable for use in the presence of flammable solvents.
The neutralizing electrostatic charge of the nucleus uses polonium. This method has an obvious inconvenience, that is, the distance between the surface of the material and the neutralizer is between 1in and 1.5in (inches).
A brush used in screen printing also utilizes the principle of nuclear neutralization. Putting a polonium sheet on the hard surface of the brush can not only reduce the static charge, but also remove dust, hair and other particles.
4. 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 free and generates static electricity. Therefore, only when the printing speed of the scraper is slow and the pressure of the scraper is suitable can static electricity be avoided.
Screen tension is considered to be a key factor for successful printing, reducing the screen pitch and squeegee pressure, thus avoiding friction and reducing static electricity.
In the screen printing industry, static electricity is the main factor that affects the normal progress of printing, because static electricity makes the printing operator inconvenient and creates a fire hazard.
If a printing plant has insufficient or no control over room temperature and humidity, static electricity will be generated, which makes the automatic printing machine unable to continue paper feeding and high-speed operation. It is difficult to say which equipment is suitable for each different plant, which can effectively reduce static electricity. Please contact the company that sells static elimination equipment, ask them to take a look at your plant, check the cause of static electricity, and then purchase equipment to eliminate static electricity .
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