Hao Li Sen Application Engineering Center, Hu Cheng Hao
To ensure stable coating quality and smooth painting production, the following aspects of process control management must be strengthened:
One: Strictly control the quality of electrophoretic coatings, and ensure thorough inspection of the coatings according to technical standards. Original paint inspection items: solid content, fineness, pH value, and electrical conductivity.
Two: Strengthen the management of electroplating tank liquid process parameters, strictly control the solid content, pH value, conductivity, temperature, and solvent indicators of the electroplating tank liquid.
One. Solid content
The solid content of the electroplating solution refers to the percentage by weight of the film-forming material in the electroplating solution.
The solid content of the electroplating solution is typically controlled between 10% and 15%. Generally, the higher the solid content of the solution, the greater the electrodeposition efficiency, but the greater the loss of coating removed from the workpiece. The solid content of the solution affects the coating thickness and permeation. Under the same conditions, a higher solid content can increase the coating thickness and permeation; a lower solid content results in lower electrodeposition efficiency and decreased permeation. As the electroplating solution is continuously electroplated onto the substrate, the solid content of the solution decreases over time, and it must be replenished regularly.
To maintain the solid content of the liquid in the tank within the required range, replenish the electrophoretic coating on a daily or shift-by-shift basis according to consumption. Typically, testing should be performed once per shift or per day.
Two. pH Value
The pH value indicates the changes in the balance between acidic resins and alkaline neutralizers. The acidity or alkalinity of the electrophoretic coating, along with the stability and electrophoretic properties of the electrophoretic bath, are all influenced by the pH value of the electrophoretic bath. Maintaining the correct pH value of the electrophoretic bath is crucial for ensuring its stability.
As electrodeposition progresses, the resin content in the electrolyte decreases continuously, while the neutralizing agent content increases. To control the pH value, the cathodic system must be circulated and the cathodic liquid must be discharged. If the pH value is too high, the coating will form granules and exhibit the "L" effect. If the pH value is too low, the electrolyte has poor solubility and stability, which can cause the coating to dissolve. In this case, it is necessary to adjust the pH value by adding alkaline neutralizing agents. Typically, the pH value should be checked once per shift.
III. Electrical Conductivity
The conductivity of the electrolytic bath solution is an indicator of the mobility and conductivity of ions in the solution, which is related to the concentration of neutralizing agents, solid content, and temperature of the solution. It is also related to the electrolytes present in the surface treatment solution applied to the workpiece.
Under normal operating conditions, the conductivity is controlled at (1400±400) μS/cm to ensure the electrodeposition effect. Minor variations in conductivity will not affect the coating performance, but will affect the coating thickness and permeability. If the conductivity is too high, the electrodeposition reaction will be vigorous, and the coating is prone to forming pinholes and rough surfaces. The method to control conductivity is to reduce the conductivity of the electrolyte by converting it to ultrapure water. Conductivity is typically measured once per shift.
IV. Tank liquid temperature
The temperature of the electroplating bath solution is generally maintained between 28～35°C.
As electrodeposition progresses, the tank liquid temperature will increase. When the tank liquid temperature is too high, the electrodeposition rate accelerates, the resistance of the wet coating decreases, the coating thickness increases, and the coating becomes rough and porous, leading to coating damage, reduced coating permeability, and decreased tank liquid stability. When the tank liquid temperature is too low, the electrodeposition efficiency decreases, the coating becomes thin, the viscosity of the wet coating increases, and bubbles generated on the coated surface are difficult to remove, which can cause the coating surface to become rough. The tank liquid temperature is controlled by a heat exchange system.
Five. Content of extraneous ions
Impurities in the tank liquid are typically introduced into the tank from the pretreatment process. An increase in the concentration of impurities in the tank liquid results in an increase in the liquid's conductivity, which can lead to rough surfaces, pinholes, and loss of gloss on the coating film. Methods for controlling the concentration of impurities in the tank liquid include: selecting pretreatment processes that are compatible with the electrophoretic coating; strengthening pretreatment washing to control the conductivity of the washed substrate to no more than 30 μS/cm; and reducing the concentration of impurities by adding deionized water and draining the ultrafiltration liquid.
Six. Organic solvent content
The presence of a certain amount of water-soluble organic solvents in the bath solution acts as a solvent, affecting the stability, thickness, and leveling properties of the coating. When the content of organic solvents in the bath solution is high, the solution tends to become cloudy, leading to the remelting and reduced permeation of the electroplated coating. When the content is low, it affects the stability of the solution and the appearance of the coating. As the coating production progresses, the solvent continuously evaporates, and it is also continuously reduced due to the emission of ultrafiltration liquid, therefore, it needs to be adjusted and supplemented in small amounts.
Seven. Other
To prevent acidic, alkaline, and oily substances from entering the liquid, as this can cause resin polymerization and lead to defects such as pinholes and shrinkage in the coating, affecting the stability of the liquid and the quality of the coating. Items immersed in the liquid should have their upper end at least 200mm above the liquid surface and the bottom end at least 300mm from the bottom of the electrolytic tank. The liquid level in the main and auxiliary tanks should not exceed 150mm to prevent foam formation.