ã€China Aluminum Network】 Due to the uniqueness of the electrophoretic coating method, the resulting coating film defects are the same as the general coating film defects, but the causes and prevention methods are different. Some defects are unique to the electrophoretic coating. In this chapter, the common defects of electrophoretic coating film and its causes and prevention methods are described as follows:
1. Particles, 疙瘩
On the surface of the electrophoretic coating film after drying, there is a phenomenon of rough-feeling (or visible) hard particles.
the reason:
(1) The electrophoretic bath contains precipitates, aggregates, or other foreign matter, and the filtration of the bath is poor.
(2) The concentration of the paint in the dirty or flushing water after electrophoresis is too high.
(3) The oven is dirty and falls on granular dirt.
(4) The objects and spreaders that enter the electrophoresis tank are not clean, and the water after phosphating is not clean.
(5) Dirty painting environment.
Prevention:
(1) Strengthen the electrophoretic bath filtration. All circulation tanks should pass through the filtration device. It is recommended to use a filter bag with 25 μm accuracy to filter, to strengthen mixing to prevent sedimentation, eliminate "dead ends" and bare metal in the tank, and strictly control the introduction of PH value and alkaline substances. The resin precipitates or agglomerates.
(2) Increase the cleanliness of the rinse water and minimize the solids content in the rinse water after electrophoresis. The generation of foam is controlled and the rinse solution is filtered through a (10-15) μm precision filter.
(3) Clean the drying room, add a circulating air filter (or periodically replace the air filter material, check the tightness of the drying system.
(4) Strengthen the cleaning of white parts, remove welding spatters and grinding debris; improve spreader structure, remove loose dirt in time; strengthen filtration of pretreatment liquid, increase magnetic filtration if necessary, reduce the residue content of phosphating bath; strengthen phosphorus After rinsing, rinse the phosphating residue floating on the surface of the workpiece.
(5) The coating environment should be kept clean. Between the pretreatment and the electrophoresis equipment, a dustproof channel should be provided between the electrophoresis and the drying chamber, and no secondary pollution source should be ensured.
2. Sink hole, shrink hole
The dust, oil, etc. are attached to the surface of the object to be coated, the phosphate film or the electrophoretic wet film, or the particles not mixed with the electrophoretic coating are mixed in the coating film, and they become shrinkage centers and cause drying. The initial flattening was uneven and resulted in crater-like pits. The diameter was usually (0.5-3.0) mm and the film was in the form of defects.
the reason:
(1) Foreign matter (oil, dust) is mixed in the bath. The oil floats on the surface of the electrophoretic bath or emulsifies in the bath.
(2) The objects to be coated are contaminated (such as dust, oil that has fallen from the transport chain, iron powder, dust from surface paint, and oil stains in compressed air used to blow water).
(3) Bad pre-treatment degreased, oily on the phosphate film.
(4) Foreign matter (oil, dust) is mixed in the cleaning liquid during flushing after electrophoresis, and the purity of the deionized water is poor.
(5) The interior of the drying room is not clean or the circulating air contains oil.
(6) There is silicone contamination in the coated workpiece (white part) from the stamping and welding process to the coating shop for pretreatment, electrophoresis, or silicone accumulation in the process media.
(7) The electrophoretic bath has a poor Y/C ratio.
(8) The coating material or resin is poorly dissolved (insoluble particles).
Prevention:
(1) Set a skim filter bag in the bath circulation system to remove dirt.
(2) Keep the coating environment clean, the transport chain and hangers should be clean, and the compressed air used should be free of oil to prevent dust, paint mist and oil from falling onto the coated workpiece. Do not allow the coated workpiece with oil and dust to enter the electrophoresis tank and set a dust-proof partition.
(3) Strengthen the pre-treatment degreasing process to ensure that there is no pollution on the phosphating film.
(4) Maintain the water quality after electrophoresis, strengthen the filtration of the cleaning solution, and set a dustproof corridor between the drying room and the drying room.
(5) Keep the drying room and circulating hot air clean, and the temperature of the coated workpiece should not be too hasty.
(6) Silicone contamination is strictly prevented during the coating process and during the coating process. Stamping workshops, welding workshops, equipment and process media (such as cables, drawing oils, anti-rust oils, anti-welding slag adhesives, structural adhesive sealants, etc.) related to the manufacture of coated parts are not allowed to contain silicone; paint shop Non-metallic materials used in equipment (plastic parts, rubber parts, greases, etc.), process auxiliary materials (grinding, masking materials, filter materials, etc.) and other process-related equipment and equipment shall not contain silicone. The above range of materials must be checked before use to ensure that there is no silicone.
(7) Keep the electrophoretic bath with the correct ratio of the pigment to the base and the solvent content.
(8) When adding new paint, stir it evenly to ensure that it is dissolved, neutralized, and filtered.
3. Pinhole
It refers to the phenomenon of needle-shaped pits on the coating film.
The reasons for different pinholes are as follows:
(1) The re-solubilized pinhole bathing wet coating film is not rinsed in time, and is re-dissolved by the scumming solution to generate pinholes.
(2) Gas pinholes During the electrophoresis process, due to the intense electrolytic reaction, excessive bubble generation is caused by poor debubbling; the temperature of the bath liquid is low or the stirring is insufficient. Causes the air bubbles to be wrapped by the coating film, and the air bubbles burst and pinholes appear during the drying process.
(3) Stepped pinholes with live slots occur when the live slots are severely damaged. The pinholes are exposed along the inclined line of the slot to expose the bottom plate; in addition, the pinholes of the bubbles are charged into the slot, and due to the bad wetting of the surface by the bath, some bubbles are trapped in the coating film or on the surface of the bath. The foam adheres to the surface of the coated workpiece to form bubble pinholes, which are easily generated in the lower part of the coated workpiece.
Prevention:
(1) After the coated workpiece has been electrophoresed, it should be immediately flushed with UF liquid immediately after leaving the electrophoresis tank to eliminate re-dissolvable pinholes.
(2) During electrophoretic coating, the concentration of impurity ions in the bath solution should be controlled from the process management, the content of various ions should be controlled within the specified range, and various ion concentrations within the bath should be periodically tested. Excessive discharge of UF fluid is required, and the conductivity of the polar liquid must be controlled within the range. The temperature of the bath is controlled within the range of the process, and the bath must be stirred well.
(3) In order to eliminate the stepped pinholes charged into the slot, the flow rate at the surface of the bath must be greater than 0.2m/s to avoid foam accumulation; in the energized mode with live electricity, the transport chain speed should not be too low.
(4) In order to eliminate the pinholes, it is necessary to first ensure the electroosmotic performance of the coating film, control the solvent content in the tank (can not be too high) and the content of impurity ions, and the water pressure after the flushing can not be higher than 0.12Mpa.
4. Thin film
Refers to the phenomenon that the thickness of the dry coating film on the surface of the coated workpiece is lower than the thickness of the electrophoretic coating technology or process.
the reason:
(1) The solids content of the bath is too low.
(2) The swimming voltage is low and the swimming time is too short.
(3) The bath temperature is lower than the process specification.
(4) The solvent content in the bath is low.
(5) The aging of the bath liquid makes the resistance of the wet coating film high and the conductivity of the bath liquid low.
(6) Poor plate electrode connection is poor or the effective area is low, the polar liquid conductivity is too low, and the coated workpiece has bad power.
(7) The UF solution was washed for too long after flushing, resulting in redissolution.
(8) The pH of the bath is too low (high MEQ).
Prevention:
(1) Within the scope of the process that increases the solids content and ensures the stability of the solids, the fluctuation of the mass fraction of the solids is better controlled to 0.5% or less.
(2) To increase the swimming voltage and extend the swimming time, they are controlled within the appropriate range of usage rights.
(3) Pay attention to the regular cleaning of the heat exchanger, check if it is blocked, if the temperature control system fails, and the bath temperature is controlled within the specified range or upper limit.
(4) Add solvent adjustment to make the solvent content within the specified range.
(5) Accelerate the bath renewal or add a regulator to increase the bath conductivity and reduce the wet film resistance.
(6) Check whether the plate has any loss (corrosion) or scaling on the surface. Always clean or replace the plate regularly to increase the conductivity of the polar liquid and check that the power to the coated workpiece is good.
(7) Shorten the UF solution rinse time to prevent re-dissolution.
(8) Add a coating with a low degree of neutralization so that the pH of the bath reaches the process range.
5. Film is too thick
Refers to the phenomenon that the thickness of the dry coating film on the surface of the coated workpiece exceeds the film thickness of the electrophoretic coating technology or process.
the reason:
(1) The swimming voltage is high.
(2) The bath temperature is high.
(3) The solids content of the bath is too high.
(4) Bathing time is too long (such as the stop of the transportation chain).
(5) The solvent content in the bath is too high.
(6) The circulation around the coated workpiece is not good.
(7) The bath conductivity is high.
(8) The cathode/anode ratio is not good, and the anode (for cathodic electrodeposition coating) is not properly positioned.
Prevention:
(1) Lower the swimming pool voltage.
(2) The bath temperature cannot be higher than the process specification, especially the cathodic electrodeposition coating. When the coating temperature is too high, the stability of the bath liquid will be affected and the bath temperature will be maintained within the temperature range specified by the process.
(3) Reduce the solids content to the technical regulations. If the solid content is too high, not only the coating film is too thick, but also more bath liquid is brought out on the surface, which will increase the difficulty of subsequent flushing.
(4) Control the swimming bath time, avoid chain stoppage as much as possible during continuous production.
(5) Control the solvent content in the bath solution, discharge the UF solution, add deionized water, and extend the ripening time of the new preparation solution.
(6) The circulation of the tank fluid around the coated workpiece is usually caused by the blockage of the pump, the filter and the nozzle, and should be repaired and adjusted.
(7) Discharge the UF solution, add deionized water, and reduce the content of the heteroions in the bath.
(8) Adjust the pole ratio and the position of the anode arrangement.
6. Traces of water
After the electrophoretic coating film is dried, spots with uneven spots on the partially coated surface are called drop marks.
the reason:
There is a drop of water on the wet electrophoretic coating film, and the water droplet boils when drying, and the water droplets produce uneven surface.
(1) Before drying, there is a drop of water on the surface of the wet electrophoretic coating film, and water droplets attached after the washing are not evaporated or have not been blown off.
(2) Drops of water dripping from the rack before drying.
(3) After washing with electrophoresis, there is accumulation of washing solution on the coating.
(4) The amount of water that is washed by deionized water is not enough.
(5) The formed wet electrophoretic coating film (excessive thickness, loose texture, poor electroosmotic resistance, etc.) has poor water drop resistance, and the water drop resistance can be obtained by the water drop trace test method.
(6) Heated too quickly after entering the drying room.
Prevention:
(1) Blow off the water before drying.
(2) Take measures to prevent water droplets on the hanger from falling on the object.
(3) blow off the accumulated cleaning water or process hole, or change the mounting method to solve the problem of water accumulation on the object.
(4) Provide a sufficient amount of deionized water.
(5) Change the process parameters or coating composition to improve the wet-resistance of the wet coating film.
(6) Avoid overheating when entering the drying room, or increase the time for preheating (60°C - 100°C, 5min - 10min).
7. Abnormal attachment
The conductivity of the surface of the substrate or the phosphating film is not uniform. When the electrophoretic coating is performed, the electrical convection density is concentrated on a portion where the electrical resistance is small, causing the coating to intensively grow on these portions. As a result, the phenomenon of accumulation in these portions is called an abnormality. Attached.
the reason:
(1) The surface of the surface of the object to be coated is not uniform, resulting in excessive local current density. The following aspects can cause this phenomenon; phosphate film contamination (fingerprints, spot printing, pickling residue); surface contamination of the object to be coated (yellow rust, cleaning agent, welding slag, etc.); abnormal pretreatment. Such as poor degreasing, inadequate washing, degreasing fluid and phosphating solution residues; phosphate film has blue spots, yellow rust spots. The resulting abnormal attachment is referred to as pre-treatment abnormal attachment.
(2) impurities in the tank pollution, excessive conductivity, high solvent content in the bath; ash is too low.
(3) The bathing voltage is too high and the bath temperature is high, causing damage to the coating.
Prevention:
(1) Strictly control the quality of the surface of the object to be coated (white article), and there should be no rust, welding slag, etc. And we must strictly control the pre-processing of various processes, improve the pre-treatment process to ensure good degreasing, uniform phosphating, full washing, should be no yellow rust, blue spots.
(2) Strictly control the content of the impurity ions in the bath. Prevent mixing of impurities. Effluent UF solution, plus deionized water to control the impurity content and solvent content. If the ash is too low then paste is added.
(3) The voltage of the swimming bath cannot exceed the process specification, especially the initial voltage when the coated object enters the tank. Reduce the temperature of the bath and use milder electrophoretic coating conditions to avoid too close spacing.
8. Low penetration
Complex coated workpieces and box-shaped (sandwich) structures or parts facing away from the electrodes are not painted or coated too thin.
the reason:
(1) The throwing power of the selected electrophoretic paint itself is poor or the throwing power is poor.
(2) The swimming voltage is too low.
(3) The solid content of the bath is low.
(4) Insufficient agitation of the bath.
Prevention:
(1) In order to make the box structure (such as the car body or the cab) coated with paint, the more basic measure is to use materials with high throwing power, at least 75% of the throwing power of the FAW steel pipe method. Strictly detect the penetration power of the electrophoretic coating and the bath solution.
(2) Applicable to increase swimming voltage.
(3) Add paint in a timely manner to ensure that the solid content is within the scope of the industrial process.
(4) Strengthen the stirring of the bath.
9. Dry paint
Due to the time between the coating material being discharged from the electrophoresis tank and the washing after electrophoresis, or the washing after electrophoresis is insufficient. As a result, the bath adhered to the wet electrophoretic coating film is dried, and the surface of the coating film is stained after drying.
the reason:
(1) The time between electrophoresis and the post-rinsing zone is too long.
(2) Incomplete initial washing, insufficient washing after electrophoresis.
(3) The bath temperature is high.
Prevention:
(1) During the process design, it should be noted that the time from the discharge of the coated material from the electrophoresis tank to the initial flushing should not exceed 1 min.
(2) Intensify the initial washing to completely clean the object.
(3) Lower the bath temperature appropriately.
10. Secondary flow mark
After electrophoresis, the liquid contained in the slot structure of the object to be coated (such as a weld seam) flows out during the drying process, causing a clear flow mark at the slit.
the reason:
(1) Poor washing after electrophoresis.
(2) The solids content of the bath is too high; the coating content in the rinse water is also high.
(3) The structure of the object to be coated.
(4) The temperature rises too quickly when entering the drying room.
Prevention:
(1) Intensify water washing after electrophoresis and increase the immersion cleaning step.
(2) Properly reduce the solids content of the bath; reduce the coating content in the rinse water.
(3) Improve the structure of the object to be coated.
(4) Add pre-heating (60°C - 100°C, 5min - 10min) procedure before drying so as to avoid boiling in the slit to extrude mucilage during rapid temperature rise.
(5) After the electrophoresis flushing, add a blow water step to blow out the fluid at the weld seam.
11. Redissolve
The wet coating film deposited on the object to be coated is re-dissolved by the bath solution or the UF cleaning solution, resulting in thinning of the coating film, loss of light, pinholes, and exposed bottom.
(1) Improve the phosphating process, use dense film type phosphating film, and strengthen the water quality management of phosphatized water washing and washing water.
(2) Raise the solids content of the bath liquid and control it to enhance the full filtration of the bath liquid within the range specified by the process, especially to remove heavy particles that easily settle.
(3) It is usually to reduce the amount of colorant and adjust the base ratio of the bath.
(4) Discharge the UF solution, add deionized water, reduce the content of the hetero-ion in the bath and the conductivity.
(5) Add solvent to increase the solvent content of the bath.
(6) Strengthen circulation and stirring of the bath during electrophoresis.
(7) The temperature of the bath is strictly controlled within the scope of the process.
12. Charged into the slot ladder ills
In the case of continuous production of the material being charged into the groove, the surface of the surface of the object suffers from the disadvantage of a porous step-like stripe film.
the reason:
(1) There is foam floating (or accumulation) on the liquid surface of the tank, and the foam is adsorbed on the surface of the surface of the material to be coated with the deposited paint.
(2) The surface of the object to be coated is not uniform or has water droplets.
(3) The voltage in the slot section is too high, causing a strong electrolytic reaction and generating a large amount of electrolytic gas on the surface of the object to be coated.
(4) The transport chain (into the tank) is too slow or pulsating.
Prevention:
(1) Increase the flow rate in the liquid surface of the tank and eliminate the foam on the liquid surface.
(2) Blow off the water droplets on the surface of the object to ensure that the object is completely dry (or evenly wet) into the electrophoresis tank.
(3) reduce the voltage into the slot; in the slot or no electrode.
(4) Accelerate the speed of the transportation chain. Generally, the chain speed is less than 2m/min and it is easy to generate electricity and enter the slot. The chain speed should be uniform.
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