Discussion on Several Problems in Electrophoretic Coating of Wheels
At present, there are many types and manufacturers of automobiles, agricultural vehicles, home appliances, and light industrial products. In the manufacturing process of body and parts, various manufacturers use cathode electrophoresis paint as coating for improving coating quality and reducing cost. With the anti-corrosion treatment of the primer, it can be said that the electrophoretic coating has been widely used in various industries due to its high utilization rate of paint, easy operation, and high degree of automation. Electrophoretic coating can coat the inner cavity of the workpiece, the seam and other parts, which can greatly improve the corrosion resistance of these areas. In the wheel coating process, it is also constructed according to the requirements of medium and high-grade coating. The main processes are degreasing, phosphating, electrophoresis, topcoat (dusting) / varnishing. However, the wheels are mainly formed by steel plate welding and welding, which is a typical workpiece with crevices, and the plates are mainly hot-rolled plates, with more scale, or more rust, etc. , derusting treatment. In a continuous production line, a pickling process is often employed.
The wheels have cracks and need to be pickled. Now, the problems that often occur on such coating lines are analyzed, and improvement measures are proposed to improve the quality of painting and reduce the drawbacks of painting.
1 wheel coating process
In a large domestic wheel production line, the coating process of the wheels is: upper part, pre-degreasing (spraying), degreasing, washing, washing, pickling (hydrochloric acid), washing, washing, washing, neutralization, washing, conditioning, Phosphating, water washing, water washing, water washing, pure water washing, pure water washing, cathodic electrophoresis, 0 times UF water washing, UF1 water washing, UF2 water washing, pure water washing, pure water washing, baking, and lower parts (transfer surface coating line).
The whole line is produced by continuous assembly line. The dip tank is a boat-shaped tank. The pre-treatment, electrophoresis and drying tunnel are into a circulating conveying system, that is, a conveyor chain is shared.
2 common problems in painting
2.1 The problem of foaming and rusting of wheel crevices
In the production line, there are often rusting of the phosphating film at the wheel crevices, welds, etc., and the phosphating film is incomplete. After electrophoresis, there is a blistering of the paint film in the crevice of the wheel or a substance like lacquer. In addition, after the electrophoresis wheel is placed outdoors for a long time, yellow rust water sometimes flows from the wheel nip. In production, the smaller the wheel gap, the more prone to such problems.
2.1.1 Cause Analysis
(1) Degreasing is not thorough
From the process point of view, the two processes of pre-degreasing and degreasing, spraying and dipping, can completely remove the oil on the workpiece for the general workpiece, but from the actual effect, there is a phenomenon that the oil is not cleaned at the seam.
This is related to the poor performance of the degreaser. If the degreaser contains not only the conventional basic components, but also the 0P-10 emulsifier (poor cleaning) and sodium dodecylbenzene sulfonate as the surfactant, then this The type of degreasing agent is simple, the combination effect is not strong, and the high-efficiency and strong penetrating agent is lacking, so it is difficult to remove the oil in the crack, which directly affects the phosphating effect.
(2) Effect of residual acid ions
When the wheel is pickled, the radius of the inorganic acid ions such as C1- and SO2-4 is very small, and it is easy to seep into the interior of the wheel crevice. It is also difficult to clean in the subsequent washing, and these acid ions become a hidden danger of corrosion. When the alkali is neutralized, the iron salt residue in the crack will be neutralized by acid and alkali, and rapidly form insoluble Fe(OH)2 and Fe(OH)3. These flocculents will seal the iron salt in the crack and accelerate. The crack is corroded and interferes with the phosphating reaction, making the phosphate film incomplete. Eventually, the phosphating film is porous and rust-prone.
The residual acid ions in the crack also lead to the local electrical conductivity at the interface between the workpiece and the cathodic electrophoretic paint, which causes the local electrolysis reaction to intensify and generate a large amount of gas, which affects the adhesion of the paint film and causes the wet paint film to foam and fall off. Even at the crack, the electrode reacts too much, causing the paint film to break down, and the paint liquid agglomerates to form a substance similar to the paint residue.
(3) Phosphating aspects
If the pH is high in the nip of the wheel, the acid-base neutralization reaction occurs at the interface between the workpiece and the solution. On the one hand, it hinders or even stops the initial reaction of phosphating, and it is difficult or impossible to form a phosphate film; on the other hand, due to the consumption of H+ on the metal surface, phosphate is caused. The dissociation equilibrium of each stage moves to the right, and the supersaturation of the phosphate precipitate is too large, which is not conducive to the phosphating reaction, resulting in coarse phosphating film and no binding force, which may also cause the stability of the phosphating solution to deteriorate. If the pH value is low in the crack, the metal corrosion is accelerated and aggravated, and a large amount of iron ions are generated, resulting in more phosphating slag and poor phosphating quality.
Phosphating film and cathodic electrophoresis are poorly matched. The original selection of phosphating in the production line is a zinc-phosphorus film, which has poor alkali corrosion resistance and a large amount of dissolution (the interface pH between the workpiece and the paint solution can rise to 12-13 when the cathodic electrophoretic coating is short), and the phosphating in the crack. The membrane binding force is poor, the amount of dissolution is larger, and the dissolved metal ions are present between the paint film and the cathode interface, which further aggravates the electrolysis reaction and is more likely to cause foaming or even breakdown of the paint film.
2.1.2 Improvement measures
(1) Improve the degreasing effect
To improve the degreasing effect, it can be considered from the following aspects: (a) In terms of process, a hot water spray cleaning process can be added at the forefront, which can effectively remove more than 90% of the mechanical impurities on the surface of the workpiece and reduce the load on the degreasing process. In addition, this will also improve the residual acid problem in the wheel joints to some extent; (b) choose the optimized degreasing agent variety. According to the different effects of the components of the degreaser, the ratio is appropriately adjusted, and the content of sodium metasilicate is appropriately reduced (its cleaning property is poor, and it is extremely harmful when the acid is formed into a silica gel which is difficult to wash off). Give full play to the combined effect of surfactants, add JFC penetrant (good permeability), TX-15 emulsifier (strong hydrophilicity), can maximize the removal of oil in the crack; (c) strengthen the degreasing bath management. The effective concentration of the degreasing liquid is controlled by the free alkalinity, but the free base cannot be used to judge the effect of the degreasing liquid, because although the alkalinity is not lowered, the effective concentration of the surfactant is lost or invalid, so alkalinity sometimes occurs. Normal and degreasing effect is not good. Therefore, in addition to the need to add degreaser every day, it is necessary to change the tank regularly. When the tank is changed, the secondary degreasing liquid is used as the pre-degreased liquid, and the second degreasing liquid is re-dispensed, so that the utilization ratio of the degreasing liquid is improved, and the degreasing liquid is always maintained in an optimum state. At the same time of production, the parameters such as degreasing temperature and time are strictly controlled according to the process requirements.
(2) Strengthen the washing of the workpiece after degreasing
Strengthen the water washing after degreasing, frequently update the water for washing, prevent the dirt in the washing tank from causing secondary pollution to the workpiece, and also prevent the workpiece from rusting during the process.
(3) Selection of pickling solution
Commonly used pickling solutions are hydrochloric acid, phosphoric acid, sulfuric acid, and the like. It has been verified by field experiments that when the rust is removed by sulfuric acid, the foaming rate of the paint film is high; the rust removal of hydrochloric acid is slight; the rust removal by phosphoric acid is most advantageous. Of course, it is also necessary to consider the choice of pickling solution from the cost and the pace of adapting to the automated production line, but considering the problem of cracks, it is recommended not to choose sulfuric acid.
[NT:PAGE](4) Neutralizer concentration for controlling the neutralization process
Under the premise of ensuring complete neutralization of the residual acid on the wheel surface, crevices, etc., the concentration of the neutralizing agent is reduced as much as possible to prevent the lye from remaining in the nip of the wheel.
(5) Improve the effect of the adjustment
The surface adjustment effect of the workpiece should also be taken seriously. An excellent surface conditioning agent should be selected, which can play a good auxiliary role in improving the quality of phosphating.
(6) Selection and control of phosphating agent
Improve the quality of phosphating, improve its anti-rust performance, control various parameters, prevent phosphating film from blooming, rough, ash and other issues. Especially when the workpiece is rusted before phosphating, more attention should be paid to the control of the free acidity, acid ratio and temperature of the phosphating solution. Improve the compatibility of phosphating and cathodic electrophoresis.
The selection of low-zinc zinc, nickel and manganese ternary systems, and the phosphating solution with good cathodic electrophoresis can improve the quality of phosphating and the overall performance of the coating after electrophoresis.
(7) Strengthen manual cleaning and cleaning of seams, welds, etc.
In order to reduce the residual chemical liquid at the workpiece joints, welds, etc., before the workpiece enters the electrophoresis tank, it can be arranged to manually clean the gaps, welds, etc. with high-pressure clean pure water. At the same time, it is recommended to use artificial compressed air to blow water on the workpiece gap, weld seam and pocket. On the one hand, it can reduce the contamination of the electrophoresis bath with the liquid on the workpiece, and on the other hand, it can reduce the coating of the workpiece, weld seam, etc. The emergence of ills.
2.2 Incomplete problem of partial paint film in the wheel
The paint film on the inner side of the wheel hub and the bottom of the curved part are incomplete, and some paint film is exposed.
2.2.1 Cause analysis
(1) The paint film at the top of the wheel is incomplete. The main reason is that there is air bubble at the top of the arc when the workpiece enters the groove, so that the paint cannot be electrophoresed on the workpiece.
(2) The paint film on the inner side of the wheel is incomplete. The main reason is that the bottom pocket liquid is not drained before entering the electrophoresis drying tunnel. More water accumulates in a short time, and the water liquid boils to damage the paint film.
2.2.2 Improvement measures
(1) Adjusting the angle of the hanging on the workpiece, on the one hand, designing and improving the hanging device, on the other hand, making full use of the porous structure of the wheel itself, so that the hole of the wheel itself becomes the venting hole of the workpiece after the groove, so that the workpiece can be made No foaming during the groove, so that every part of the workpiece can be processed.
In addition, at the inlet of the electrophoresis tank, a moisturizing nozzle for the workpiece after the trough can be added under the liquid surface to drive off the bubbles at the top of the workpiece arc.
(2) Before the wheel enters the drying tunnel, increase the artificial compressed air to blow dry the water in the workpiece. On the one hand, it can avoid the defects of the paint film caused by the accumulated water, and on the other hand, it can save the energy consumption of the drying of the drying tunnel.
2.3 The appearance of the paint film has foreign matter such as particles and rust.
There are many foreign materials such as particles and rust on the appearance of the paint film. Before painting (powder), the workpiece has to be polished and cleaned with a large amount of manpower, which increases the production cost.
2.3.1 Cause Analysis
Because the line is pre-treatment, electrophoresis shares a conveyor chain with the drying tunnel, and the pickling uses hydrochloric acid which has strong volatility, which has strong corrosion to the transport catenary and hangers. The rust layer of the catenary and the hanger is detached, especially the high temperature in the drying tunnel is more likely to cause the rust layer to burst, and the rust layer and the granule directly affect the appearance quality of the paint film.
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Tags: Wheel Phosphating Film Drying Channel Degreaser Workpiece
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