PVD Pre-treatment Cleaning
Cleaning is through chemical or (and) physical methods will be the workpiece oil, rust, dust and other clean removal, to ensure that the workpiece to obtain a good coating adhesion and the smooth production. Cleaning is essential before PVD coating procedure, as well as the most important PVD coating production technology, clean out of the question, the coating might disrupt the production have to delay, coating process, problems or coating binding force and lead to customer complaints and compensation, especially in coating equipment under the condition of technical ability is not high, cleaning out the problem, are more likely to happen the risk.
There are four factors affecting the quality of cleaning, namely: cleaning time, chemical agents, mechanical action and cleaning fluid temperature, these four factors also affect each other, the weakening of one factor can be made up by enhancing the effect of the other three factors, and vice versa. Among these four factors, the minimum cleaning time is also the goal to pursue, which can improve the efficiency of cleaning. Shorten production time and lead time.
Main cleaning methods and steps should include: chemical spray, chemical soak and ultrasonic cleaning, rinsing, drying.
1) Chemical spray
Spray is very effective for cleaning, it can remove most of the oil and contaminants on the workpiece. Especially for the workpiece with holes, it is more effective. The chemical agent sprayed on the workpiece will flow into the hole or the chemical agent will be sprayed directly into the hole to flush the inner wall of the hole. So when cleaning the loading card, be sure to let the workpiece can be sprayed. In addition, if the chemical spray on the workpiece can not immediately flow away in time, will hinder the fresh chemical continue to clean the workpiece, and in the following drying step is not easy to dry, so when loading CARDS, to ensure that the flow to the workpiece can naturally flow away.
2) chemical soaking and ultrasonic cleaning
Ultrasonic is a kind of sound wave whose frequency is more than 20 kHz beyond the range of human hearing. The propagation of ultrasonic depends on the elastic medium. When it is propagated, the particles in the elastic medium oscillate and transfer energy according to the propagation direction of ultrasonic through the medium. This kind of wave can be divided into longitudinal wave and transverse wave. In solids, both can be transmitted, whereas in gases and liquids, only longitudinal waves can be transmitted. Ultrasonic waves can cause particle vibration. The acceleration of particle vibration is proportional to the square of ultrasonic frequency. Therefore, tens of kilohertz ultrasound will produce a great force, strong ultrasonic propagation in the liquid, due to the nonlinear effect, will produce acoustic cavitation. When the cavitation bubble suddenly closes, the shock wave can generate thousands of atmospheric pressures around it, and directly and repeatedly impact the pollution layer. On the one hand, it will damage the adsorption of the dirt and the surface of the cleaning part; on the other hand, it will also cause the damage of the dirt layer and break away from the surface of the cleaning part and disperse them into the cleaning solution. The vibrations of the bubbles also scrub the surface of the solid. Bubbles can also "drill" into cracks to make them vibrate, causing dirt to fall off. For greasy dirt, due to ultrasonic cavitation, the two liquids quickly disperse at the interface and emulsify. When the solid particles are coated by oil and adhere to the surface of cleaning parts, the oil is emulsified and the solid particles fall off. Cavitation bubbles will cause the liquid itself to generate circulation in the vibration process, which is called acoustic flow. He can make the surface vibration bubble high velocity gradient and viscous stress, makes clean the dirt on the surface of the damage and loss, ultrasonic cavitation on solid and liquid surface generated by high speed micro jet can eliminate or weaken the boundary layer, the corrosion of solid surface, increase mixing effect, accelerate soluble dissolve dirt, strengthen chemical cleaning agents and cleaning. In addition, ultrasonic vibration in the cleaning fluid caused a lot of particle vibration velocity and acceleration, also make the surface of the dirt cleaning parts by frequent and intense impact
Because the ultrasonic wave will produce low pressure area and high pressure area in the transmission process, and cavitation phenomenon only occurs in the low pressure area. Therefore, the workpiece to be cleaned needs to vibrate up and down in the ultrasonic wave, so that each area on the workpiece through the low pressure area to obtain the "micro brush" cleaning effect. The velocity of ultrasonic wave in water is 1500m/s, assuming the frequency of ultrasonic transition is 30000Hz, then the wavelength of ultrasonic transition used
λ(Lambda) = velocity/frequency = 1500/30000 =0.05m=5cm
Therefore, if 30000Hz ultrasonic wave is used, the up-down vibration distance of the workpiece should not be less than 5 cm. For other frequencies. The vibration distance can be calculated in the same way. In the pure chemical agent soaking cleaning, the cleaning solution first dissolves the pollutants in the top layer of the workpiece, and gradually penetrates and dissolves into the contaminated layer. In this process, a layer of dissolved saturated layer will be formed in the surface layer of the workpiece. This saturated layer isolates the fresh chemical cleaning solution from the pollutants in the deep layer and prevents the solution from continuing to dissolve the pollutants in the deep layer. If the saturated layer cannot be destroyed and removed, the cleaning will stop. For the dirty workpiece, pure soaking cleaning method is difficult to thoroughly clean the workpiece. The use of ultrasonic "micro brush" can destroy the surface layer of dissolved saturated layer, new chemical agents to the deeper layer of pollution continue to dissolve, ultrasonic then continue to destroy the new formation of dissolved saturated layer, so the cycle, cleaning continues, until the workpiece is clean.
The purpose of rinsing is to thoroughly clean up the residual cleaning fluid or other dirt on the workpiece, so that the workpiece is cleaner before drying. The residue on the workpiece will be difficult to clean after drying. In industry. Deionized water is usually used for rinsing and deionized water is used instead of tap water for rinsing. The impurities and pollutants in the tap water can be avoided to remain on the workpiece. However, deionized water is highly corrosive and will cause corrosion to the workpiece. Therefore, a certain amount of anti-rust agent must be added to deionized water in order to rinse the workpiece. To prevent the workpiece from being corroded in the process of rinsing and drying.
After cleaning the workpiece is generally put into the oven drying, including the oven body, drainage pipe, heating system, exhaust system and temperature control system. The boiling water, evaporation temperature is 100 ℃, and baking temperature must be greater than 100 ℃, the best between 110 ℃ ~ 130 ℃, both can be rapid moisture evaporation drying artifacts. And not after drying, the workpiece temperature is too high and need a long time to cool.
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