1 electrophoretic coating process principle
Electrophoretic painting is divided into anode electrophoresis and cathodic electrophoresis, and the type of electrophoretic coating is selected depending on the type of substrate material or the purpose of coating. The cathodic electrophoresis process generally uses polyurethane coatings to coat the jewelry surface. In the anode electrophoresis process, the predecessor coating is an epoxy resin coating and is now mainly used for the surface treatment of automotive chassis; the second generation is an acrylic coating for the surface treatment of aluminum profiles. Acrylic acid formula is CH2CHCOOH, and the polymerized acrylic resin is a mess, 70% of which is replaced by amino group. Because of the presence of -COONHR in the resin, the resin is water-soluble, and the amino resin is cross-linked at high temperature. Curing reaction. The uniformity of the coating molecules has a great influence on the process operation. In general, the better the emulsification, the more homogeneous the molecules.
The principle of the electrophoretic painting process of aluminum alloy is that after the surface of the substrate is anodized, a porous cellular protective layer composed of Al12O3 and A12(SO4)3 is formed. Under the action of the direct voltage, the aluminum alloy acts as the anode, and the current is electrolyzed through the oxide film to produce H+ and O2. At the same time, the electrophoretic coating fluid moves to the anode coating under the action of the electric field, reacts with H′ and deposits on the coating. Paint on. Under the action of the electric field, the water molecules in the membrane penetrated out, and the moisture content in the membrane was as low as 2% to 5%. After baking, cross-linking reaction hardens. Electrophoretic painting functions as a closed porous oxide film.
Electrophoretic painting is generally considered to have 4 processes:
(1) Electrolysis: Acrylic amine R-COONH2→R-COO-+NH2
Water H2O→H-+OH-
(2) Electrophoresis: Charged particles move in the conductive medium under the influence of a potential. The migration rate of particles is proportional to the ZETA potential of the particles, the size of the electric field, and the dielectric constant of the medium, and is inversely proportional to the viscosity of the paint.
(1) Process parameter requirements:
Degreasing: sulfuric acid 180g/L~200g/L, time 2min~5min
Alkaline hydrazine; sodium hydroxide 40 g/L~50 g/L, time 2min~5min
Neutralization: sulfuric acid 180 g/L~200 g/L, time 2min~5min
Oxidation: sulfuric acid 150 g/L~180 g/L, Al3+<150 g/L, bath temperature (20±5)°C, current density 150 A/m2~180 A/m2.
Coloring: The coloring voltage is less than the oxidation voltage, other parameters are according to the requirements of the matching tank
Electrophoretic coating: solid mass fraction 5% to 7%, PH value 7.6~8.4, temperature (23±3) °C, conductivity 590us/cm~900 us/cm at 25°C, painting time 40s~180s, painting Voltage 40v~160v
Drain: 15 min~20min
Curing: temperature 170°C~180°C, time 30min
(2) Electrophoretic coating performance index list 1
Electrodeposition: R-COO-+H+→RCOOH
3R-COO-+Al+→(RCOO)3→(RCOO)3Al↓
(1) Electro-osmosis: A film of paint deposited on the anode, constructed like countless capillaries. There is an electrical double layer at the interface between the capillary and the water. Under the effect of the electric field, the water is dehydrated through the coating film, and the opposite charge is exchanged to cause the penetration phenomenon, which is called electroosmosis.
2 aluminum electrophoretic painting process
(1) The production process is shown in Figure 1.
1 Gloss: The use of human reflection angle 600 gloss meter, the target is set to 100;
2 hardness required scratch hardness of 3H;
3 immersed in mt with "(NdOH) I% aqueous solution XIh protection level greater than 9.5;
4 in 20 ~ C ~ "(Hcl) 5% aqueous solution soak 20h protection level greater than 9.5;
5 After soaking in boiling water for 8 hours, the loss of light was less than 5%.
3 electrophoretic painting process equipment
(1) Electrophoresis power supply: The requirement for the electrophoresis power supply is mainly the ripple factor. According to national standards, the ripple factor must be less than 6%. A series of defects such as pinholes, orange peel, and loss of light occurs due to voltage instability.
(2) Recycling system: The paint liquid in the washing tank is recovered to realize pollution-free production. The key components are high-pressure pump, RO membrane, etc. It is a closed recovery system composed of electrophoresis tank, electrophoresis sub tank, two washing tanks after electrophoresis, and recycled membrane tubes. The advantage of this system is that the composition of the bath liquid entering the membrane tube is stable and has a protective effect on the membrane tube. When carrying out the paint recovery, the dynamic flow balance of each tank is achieved, as shown in FIG. 2 .
The RO membrane tube adopts high-pressure reverse osmosis technology and continuously permeates water from the input liquid. The good membrane tube has a large amount of water and the water quality is clear. Some manufacturers also use seawater desalination instead. When the output is large or the production of titanium gold yellow profiles, the membrane tube is easy to be blocked, and the quality of the effluent is poor, so the recovery rate of the paint cannot be raised. The high-pressure pump should use the products of a professional manufacturer, and the stable inlet pressure is a necessary condition for the normal operation of the membrane tube.
(3) Electrophoresis refining system: It consists of Yin Tower and Yang Tower. The cation resin is a macroporous weak acid resin, and the anion resin is a strong basic resin or a macroporous weakly basic resin, and the resin is used to exchange impurities and cations to generate H+. And OH-, play a role in the purification of paint. Good resin has a close relationship with producing good products, so pay attention to the selection of resin grades.
(4) Cycling Filtration System: Since the electrophoresis tank generates a large amount of heat during production, heat exchange must be performed in time to maintain a constant temperature of the bath liquid, and the circulation speed should be such that the generated heat can be fully exchanged. In general, the cycle speed is 4 times/h~6 times/h. At the same time, the circulating filtration system has the function of eliminating suspended impurities and bubbles.
(5) Poles: Digestion of a large number of bubbles generated by the electrolysis reaction and reduction of spot defects in profiles. In addition, the pole plate can be separated from the profile after infiltrating the bath into the bath to avoid electric shock.
4 Problems and Countermeasures in Production Practice
Electrophoretic painting leaves any minor surface defects on the profile exposed under lacquer and cannot be reworked using the process line on the production line. Electrophoresis profiles have a long production process, and they have a high consistency in flow operations. Often, the same type of defects are present in the products produced in a certain period of time. The defects produced in each stage of the process have a sudden exposure after the curing process. The yield of electrophoretic profiles is lower than that of ordinary oxidation profiles. The causes of common defects in production and countermeasures are as follows:
(1) Outstanding film defects such as orange peel and blister bubble
The reason for this is that the electrodeposition is not uniform. Therefore, it is necessary to analyze the factors that accelerate the electrodeposition rate, such as the bath temperature, conductivity, and painting voltage. These factors accelerate the migration of charged particles. Once they fluctuate to critical values, they increase the probability of defects. In particular, when the bath temperature is greater than 25°C, the probability of such defects is 63.7%. Therefore, the process parameters should be kept as stable as possible.
(2) Inclusions on the surface of the paint film
Different reasons lead to differences in the location of the inclusions. If the upper inclusion of a hanging profile is obvious, it is generally caused by more impurities in the washing tank. This is because the oxide film has a positive charge, while the particles in the water have a negative charge, which is not easy to remove once adsorbed. If the inclusions are on the outside of the hanging profile, it is usually the result of the electrophoretic profile being exposed to dirty air. When it is carried out to the draining stage after electrophoresis, because the paint film has a certain viscosity, when the dust in the air is more, it is easy to adhere to the paint film. Of course, this type of defect can also occur when an uncleaned oven is used or if the oven filter is broken. If the paint film from the front of the oven exiting the oven is not clean and then normal again, it is certainly because the oven cleaning process does not guarantee quality.
(3) Freckle defects often appear
Causes of variegation are as follows: The profile is not placed on the shelf, or bubbles on the surface of the profile cannot be discharged during oxidation, electrolytic coloring, or electrophoretic painting. The countermeasure is: when hanging, the profile of the profile faces upwards, the smooth side facing downward without gully, and at the same time the angle hung on the profile should be appropriate; the profile is inclined at a large angle obliquely out of the process tank, and needs to be slow after entering the process tank with additional current. Shake the profile to dislodge air bubbles and soak it properly before applying electricity so that bubbles have escape time:
(4) The surface of the finished product is cloudy and opaque.
At this time, it is necessary to carefully observe whether the turbidity surface is an oxide film or a paint film. Under normal production conditions, the aluminum oxide film is transparent. If the surface of the profile before oxidation is not clean or the oxidation process parameters are biased towards the critical value, for example, if the concentration of Al3 is more than 20g/L, the oxidation current density exceeds 180
A/m2, surface temperature and other factors during the oxidation of the profile will all lead to deterioration of the oxide film, resulting in turbidity of the finished product surface. The main reason for the turbidity of the paint film is the hot water washing process, which includes insufficient temperature, insufficient time, and problems with the quality of the hot water. When the thickness of the oxide film is greater than 20um, the phenomenon of cloudiness of the paint film may also occur when the conventional hot water washing process is performed. The reason for this is that the oxide film is not thoroughly cleaned, and the impurities contained in the oxide film are involved in the formation of the film.
(5) Electrophoretic materials produced in dry environments are prone to dry marks
Contrary to this kind of defect is the statement that the water film on the surface of the profile before the electrophoresis tank is not uniform. It is reported that Japan generally adopts the electrophoresis process after the profile surface is air-dried, which can prevent the generation of water marks. One of our methods is to fully cool the profile after hot water washing, and then enter the pure water tank before electrophoresis; then replace a certain amount of pure water in time to ensure that the pure water tank temperature is consistent with room temperature; third is to quickly transfer the profile from a pure water tank to Electrophoresis tank. The coordination of operator actions can avoid such defects.
(6) The finished film is not glossy or sticky
Checking the hardness at this time is generally not enough, need to check the condition of the oven. If there is no abnormal change in the oven current and voltage, and the time for reaching the constant temperature point becomes longer, it is usually caused by the uneven thermal cycle of the oven, and the ventilation windows can be adjusted again.
(7) Electric shock on the profile
If the wound incision has electrophoretic paint, it is a problem of the oxidation process, or because the pole cover of the electrophoresis plate is loose, or the profile at the bottom of the electrophoresis tank is falling, and the improper operation leads to the contact between the profile and the plate. Excessive electrophoretic voltage and other factors may cause such defects.
(8) Operation of conductive beam cleaning may affect product quality
The electroconductive material of the anode conductive beam must be washed, wiped into the electrophoresis process, otherwise it will have the following problems: 1 water droplets cover a good film, resulting in partial film paint; 2 not thoroughly clean, conductive After the beam is exposed to chemical substances, the eroded surface of the beam is baked to produce a powdery substance that will contaminate the paint film.
(9) The maintenance of the electrophoresis process tank is quite important
The profiles in the sink should be salvaged promptly. Otherwise, the lacquer film on the surface of the profiles will be “dead paint†due to the immersion in the sink. Dead paint is often suspended in tank liquids and is not easily salvaged due to its small size. "Dead paint" attached to the surface of the profile will form a local clot, often resulting in the scrapping of the hanging profile.
(10) Scratches and scratches on the paint film in the hanging and packaging process
Accumulation during the hanging process of the profiles causes the paint films to rub against one another, or the ends are misaligned and scratch each other. Any irregularity in the process is detrimental to the decorative effect of the paint film. The paint film has the characteristics of capillary pores and has the function of penetrating. When using a thermoplastic package, the seal is good, the moisture is not easy to discharge, and the phenomenon of water droplets between the paint film and the substrate occurs. Although the use of the profile is not affected, The customer is reluctant to explain it, so it is better to avoid using thermoplastic packaging.
5 Management in the production process
According to statistics, the qualified rate of silver white electrophoretic paint profiles is 96%. Among the waste products, about 43.3% of the surface of the substrate had defects such as scratches, 12.5% ​​due to over-thickness of the oxide film and turbidity of the oxidized surface, and 15.7% of the paint film containing inclusions. About 15.6% of the face was scratched, and 12.9% were other defects.
Taking the production of silver electrophoretic painted sections with low production difficulty as an example, we believe that establishing a reasonable process control point is an effective way to increase the electrophoretic qualification rate. The following is the establishment mode of the control point:
(1) Establish a control point for the substrate on the shelf to check whether the surface of the substrate has obvious defects such as scratches. Control points: Determine the orientation and number of shelves on the substrate, reduce the occurrence of electrophoretic waste due to the quality of the substrate, and prevent the generation of piebald spots.
(2) Anode oxidation process control point, the purpose is to reduce the defects of the oxidation surface of the profile. Control elements: 1 Check the effect of the pre-treatment. At this time, the surface of the profile must be smooth and ash-free; 2 the oxide film after oxidation of the profile must be transparent.
(3) The surface of the treatment material before entering the electrophoresis tank is clean without impurities, and the electrophoretic bath solution is clean without floccules. When producing the coloring material, it is necessary to strictly check whether the amount of color is matched and to reduce the quality defects of the paint surface.
(4) Process control point of the cell: The process emphasizes the stable condition of the bath (including the concentration of each component, the temperature of the bath), the water quality of the washing bath is qualified, and the production line is free from dust and acid-alkali fog.
6 Conclusion
Electrophoretic painting profiles are high-grade decorative materials, which bring considerable economic benefits to the manufacturers, and their technical content and production difficulties also greatly exceed the ordinary profiles, only to realize this, and objectively analyze the problems and solve problems. After the implementation of the actual work, in order to effectively improve the passing rate of electrophoresis profiles.
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