Analysis and solution of defects in powder coating

Analysis and solution of defects in powder coating

I. Introduction
There are fewer powder coating processes and fewer problems are generated. The main processes are only three processes: pretreatment, electrostatic spraying and baking. The main process affecting quality should be the pre-treatment process. However, many manufacturers have not paid enough attention to it, resulting in many troubles. Some hidden dangers are not reflected in the short term. Generally, the more advanced technology and the cost of pre-processing account for about 25% of the total cost.

Analysis of various problems arising from the production process. I think that the advanced level of equipment and the quality of raw materials play a decisive role in product quality. Both are indispensable. As an engineering technician, we should establish a consensus on this issue, and we must never use the new powder production line. Only when the money is saved at a time, using low-standard production equipment, processes, and materials will surely bring many troubles to long-term production, and even produce high-quality products. Many manufacturers have put on the powder coating production line, and the application will soon be forced to dismount or replace with new equipment, for the above reasons. We often see that the surface coating of foreign products is very beautiful. One of the main reasons is that they constantly use advanced equipment and new materials and materials.

In this sense, for the powder coating equipment and paint manufacturers, in the current market competition conditions, non-quality equipment and coatings have no future. Especially in the competition conditions of hundreds of dusting equipment and powder coating manufacturers in China. Only nearly 10 equipment and powder coating production plants in Shaanxi Province have been converted or closed down.

Second, the analysis of the production of defects

(1) Pre-processing:

1, degreasing and rust is not complete

First of all, I don't advocate the process of degreasing and rusting two in one. The two-in-one process itself is prone to this problem. Because the commonly used degreasing and descaling formulas are weakened, rather than complement each other, it is difficult to come up with a reasonable two-in-one or three-in-one process (including phosphating). In another process, the oil is used in the metal detergent, and the oil cannot be completely removed. Because the metal cleaning agent has poor oil removal effect, it can only be floated on the surface of the metal body. To remove it, a mechanical force is needed. If there is no such mechanical force, the oil will not be completely visible. Metal cleaners are not suitable for industrial mass production.

I advocate the use of alkali chemistry to remove oil. The lye is very effective in removing animal and vegetable oils because it produces a saponification reaction. However, the effect of removing mineral oil is poor. In order to remove mineral oil, an emulsifier is also needed. In addition, the quality of washing has a great influence on the oil removal effect, one is water quality, and the other is water temperature. The third is to grade and turn, all three are good, the only purpose is to clean the grease with pure water. The final stage is best washed with deionized water.

2, phosphating film roughness and other issues

Steel parts must be phosphatized before dusting, and the sand phosphating process is commonly used in foreign countries. Many of the phosphating solutions we use are not ideal, which directly affects the quality of the phosphate film. More common is the problem of phosphating film roughness. I want to analyze this from three aspects: The first is to choose a phosphating solution, which is the basis for ensuring quality. The first question involved is the type of high, medium and low temperature? From the perspective of its development history, it has begun to have a high-temperature thick film type, and has now progressed to medium and low temperature film types. Domestic low-temperature phosphating solution, because the solution is not stable enough, maintenance is difficult, the adhesion of the phosphate film is poor, and some are not even resistant to water washing, etc., and are rarely used. Some low-temperature phosphating solutions are ideal, but they are more expensive. Although low-temperature phosphating is the development direction, it has yet to achieve new breakthroughs in achieving universal application. At present, Xianghe Phosphate (Chengdu) Co., Ltd. has successfully solved the problem of room temperature phosphating (passivation) powder and has supplied it in bulk. At present, the medium temperature phosphating process is generally adopted. The second is the zinc-based, iron-based, zinc-calcium-based phosphating solution. In contrast, I think that the zinc-calcium phosphating solution is better, and the thin coating phosphating film is required for the pre-spraying treatment. The iron system is also a better choice. It has good heat resistance in addition to some advantages of the zinc system. Under moderate temperature conditions, the hydrolysis is less, the sediment is less, the solution is easy to maintain, the mechanical strength is good, the corrosion resistance is strong, and the crystal is still fine when the film is thick. The iron-based iron phosphide film has poor corrosion resistance. In short, it is better to select warm zinc-calcium phosphating solution. The second is that the phosphating solution must be adjusted at any time, mainly to adjust the acidity ratio and the amount of zinc. Both of these factors directly affect the film formation rate and the thickness of the crystal. It is necessary to adjust the solution regularly and irregularly according to the workload, and adjust it in time. Without analytical data, it is unscientific to blindly replenish the liquid. The third is to control the thickness of the film layer. Each type of phosphate film has its optimum thickness. For example, the optimum film thickness of zinc and calcium is 1.5-2 grams per square meter, while the iron system is 0.5-0.7 grams per square. Rice, in this range, has the best anti-corrosion performance. The finest grain, such as too thick, affects the flexibility and adhesion of the film, and the grain becomes thicker. A common problem in production is that the phosphate film is thick. Here are some suggestions on how to overcome the problem of coarse grain.

The first is to make the grain finer by using a surface conditioner. That is, a surface adjustment treatment is performed before the phosphating of the workpiece, so that a layer of colloidal particles is adsorbed on the surface of the metal to form an "activation center", and then, when phosphating, the growth continues at the "activation center", so that the phosphating crystal can be made. The particles are significantly thinner, especially before low-temperature phosphating, which is an indispensable one-step treatment.

The second is to add a grain refiner to the phosphating solution, such as calcium, nickel and other salts as a modifier, preferably using a complex salt, the effect is better, and at the same time can reduce the amount of low sediment.

The third is a passivation treatment after phosphating to make the peak structure formed on the phosphating film, thereby improving the corrosion resistance and improving the adhesion of the coating.

The fourth is to adjust the acidity ratio and zinc content of the phosphating solution at any time. I have already talked about it before.

(2) Electrostatic powder spraying:

1. Uneven powder spraying is mainly caused by uneven fluidization of powder in the powder drum. The microporous plate should be replaced or the gas supply system under the microplate should be adjusted so that the powder layer returns to normal boiling state and can be eliminated. If the pressure is low, the pressure can be eliminated. Another possibility is that the pipe is blocked, including the blockage of the Venturi nozzle, and the pipe needs to be cleaned. If the venturi is often clogged, it is inappropriate for the venturi's powdered nozzle material (such as brass). If you change it into a polytetrafluoroethylene material, it can be solved. The nozzle head can also be solved by the same method. If the powder is not a fluidized bed type, it is necessary to improve the stirrer or add a vibration stirrer to prevent the powder layer from appearing voids and not being coated.

2, the powder rate is low:

(1) Low voltage or no high voltage is one of the main reasons. Need to increase the output voltage or repair the high-voltage static generator.

(2) The workpiece is poorly grounded, and the hook or low layer resistance is large; if the second pass powder is sprayed or greasy, it will make the low layer resistance large. When spraying the second pass powder with a common spray gun, the workpiece must be preheated to about 104 °C for thermal spraying, using the principle of hot melt adsorption. The hot spray operation is also conducive to the smoothing of the powder. With the chameleon spray gun, the problem of large low-layer resistance can be overcome, and the second spray can achieve cold spray. It is realized by the principle that the additional plate is used to form a constant electrostatic field in the spray chamber.

(3) The excessive or small amount of powder charge also affects the powder rate. The results of the epoxy powder test; when the powder charged electricity is 1.6 microcoulomb / gram, the powder rate can reach more than 95%. The corona discharge spray gun can only charge the powder to 0.8-0.9 microcoulomb, so the powder rate is only about 85 percent. This number shows the effect of the charge on the powder rate. Therefore, the friction spray gun is not as high as the electrostatic spray gun. Generally, the control current does not need to be too large, and about 60 microcoulombs is enough. Otherwise, the charge is too large, and it is easy to cause overcurrent and power failure. If it is too large, it will easily cause the Faraday screen to block the effect. Generally, the charge rate affects the powder rate, and the charge is small. In addition to the difference in the self-ionization effect of different powder coatings, the main reason is that due to the failure of the high-voltage electrostatic generator, maintenance is required to restore the high pressure.

(4) Excessive air pressure: it will offset the electrostatic attraction. The powder paid on the workpiece is blown off to reduce the powdering rate. The experimental results show that when the amount of air delivered is 3 liters/second, the rate of powdering is about 95%, and when the amount of air is increased, the amount of powdered air is increased by one liter/second. Reduce by two to three percent.

3. Fire during construction

This problem generally does not exist, because high-voltage electrostatic generators are now mostly equipped with constant voltage, constant current protection lines. Old-fashioned spray guns have a problem. The reasons for the fire are as follows:

(1) The distance between the gun and the workpiece is too close and the concentration of the powder in the electrostatic field is large, exceeding the limit concentration, which is an important reason.

(2) The internal resistance is small, resulting in excessive current.

(3) The voltage is too high: The experiment proves that when the field strength is 1 kv/cm, the adsorption efficiency is close to saturation. After that, the field strength increased by a factor of three, and the coating efficiency increased by only three to five percent. Therefore, the voltage is too high to be desirable, but it is easy to cause a fire. General control at 60kv is sufficient.

4. The coating has shrinkage pits:

(1) In most cases, the surface of the workpiece is not removed from the bottom, and some are caused by oil in the compressed air. Because the oil point significantly affects the surface tension of the powder layer when it is cured.

(2) The internal factors of powder coatings are mixed with powders from different manufacturers or batches. The introduction of silicon dust also affects the uniformity of surface tension during curing and causes shrinkage.

5, the coating has pinhole foaming:

The gas under the coating reaches the surface of the coating during the drying process. The hole that breaks through the interface is a pinhole, and it is too late for the bubble to be excluded. The gas in the coating may be air, water vapor or hydrogen (caused in a galvanized layer) or the like. The fundamental solution is to completely remove the gas before spraying. A small amount of gas that cannot be removed can also be used to control pinning or air bubbles by controlling drying and spraying conditions. It has been calculated that it takes 26 seconds to remove the air from the coating and heat up slowly during the safe melt leveling section (100-135 degrees Celsius) before the film begins to solidify. Give enough exhaust time. Or take the method of powder preheating after the workpiece is preheated, all have effects.

6, the coating has orange peel:

The main reason for the production of orange peel is the poor leveling of the powder coating itself. Construction also has an impact and is a secondary factor.

(1) The leveling property of the powder coating mainly depends on the added leveling agent. The leveling agent generally has two effects, namely the wetting benefit and the leveling benefit: the wetting benefit should be when the powder is at about 100 degrees Celsius. Increase the leveling of the machine, the surface tension is as small as possible. The main effect of the leveling effect is greater than 150 degrees Celsius. The surface tension of the powder should be as large as possible to increase the smoothness. This is the reason why leveling agents generally contain more than two materials.

(2) Poor construction can also cause orange peel phenomenon: once the coating is too thick or the baking temperature rises too fast, the orange peel will be obvious. The coating thickness is preferably about 60u. In addition, the control of the safe melt leveling period is longer, which is beneficial to the leveling of the powder coating.

7, electrostatic shielding effect:

When the workpiece of a complicated structure is dusted, the coating film is thin at the dead angle due to the electrostatic shielding (also called Faraday) effect, and even the bottom is not covered. This effect is insurmountable for ordinary electrostatic spray guns. The friction spray can be better improved. Some electrostatic spray guns abroad have basically overcome this effect and obtained a more uniform coating film. For example, a chameleon spray gun is said to be able to spray a 3 mm deep groove for a more uniform coating. For the general electrostatic spray gun, it is advocated to overcome the electrostatic screen covering effect by increasing the air pressure. My practice proves that, on the contrary, the gas should be smaller, because the powder at the corner of the corner is not adsorbed, and the pressure is increased, but the adsorbed powder layer is blown off, which does not increase the thickness of the coating. I analyzed the reason why the chameleon can overcome the electrostatic shielding effect. In addition to the auxiliary anode, there may be a flow of air through the spiral body in the gun body, which increases the forward momentum of the powder without increasing the air pressure, effectively solving the above air pressure. The contradiction of size has received good results.

(3) Control of baking curing conditions:

I want to talk about three points for your reference:

1, oven or drying room I do not advocate the use of far infrared as a heat source, should be used ordinary electric heating direct baking or other heat source indirect circulation heating baking. This is determined by the characteristics of the far infrared radiation. There are two main characteristics: First, it is directly absorbed by the workpiece to heat the workpiece. A layer of about one millimeter thick steel plate or aluminum plate has a far-infrared absorption rate of about 75 percent. Therefore, the workpiece heats up very quickly, generally 10-20 degrees higher than the temperature of the furnace, and the far-infrared radiance of the workpiece behind it is very low, resulting in a large temperature difference, and the third layer of the workpiece is even worse. Therefore, the oven or the drying room cannot use the far-infrared heat source, and the other feature is that the far-infrared radiation path is radiated and advanced, which determines that the intensity of the radiation at each point in the furnace is inversely proportional to the square of the distance from the component, which determines its The best baking distance is between 200-400mm. It can be seen that only the drying tunnel is suitable for far infrared baking.

2, far-infrared components I advocate the use of milk-white quartz tube as a thermal energy conversion carrier, preferably without the surface coating of far-infrared conversion coating plate, tube and other components. The reason is that the former far infrared conversion lifetime is theoretically infinite, while the latter's conversion efficiency is more than 20% a year. Not only does it not save energy after decay, but it also wastes energy because its heat transfer efficiency is lower than ordinary electric heat.

3, the production of COSCO infrared baking control temperature and time, should be determined according to the specific circumstances. It depends on the efficiency of the far-infrared furnace and the thickness of the workpiece. A variety of factors such as the type of powder. A simple method for determining the control conditions is to use a test piece of similar thickness, and to determine the optimum control conditions by measuring the adhesion and impact strength of the coated film after curing, resulting in the disadvantage of baking over-fire or under-fire. We have experience in production. Powder coatings require harsh curing conditions. The optional range of fluctuations is very narrow. If it is slightly exceeded, it will cause brittle fracture or shedding. Temperature and time must be strictly controlled.

(4) Rework of non-conforming products

1. Generally unqualified products: If there are dirty spots, virtual spray, flow plastic, scratches, etc., sandpaper dry grinding method can be used. After smoothing and drying, the whole method of heavy spray is used for rework. It can not be repaired by local refilling.

2, the surface of the product has pits or bumps and other defects, the overall coating film needs to be removed, after scrubbing, the overall heavy spray for rework.

3, a large area of ​​rickets or coated film has been baked over the fire of the workpiece, you need to remove the entire film, wipe clean and re-spray. Film removal method: a film remover can be used, and concentrated sulfuric acid can also be used. The former is a mixed organic solvent, which is volatile and expensive, but has no corrosion on its material and is quick to remove. The base mechanism is to destroy the bonding force between the coating film and the body. Blocking off. I advocate the use of a film remover, which is cheaper and slower to remove the film. The mechanism is to directly destroy the epoxy coating to form a resin sulfonate. However, careless handling can cause excessive corrosion and result in scrapped parts.

4. The defects of the coating film at the dead corner can be solved by partially repairing the nitromagnetic lacquer or amino baking varnish of the same color.