1 Preface

As one of the surface treatment methods of aluminum profiles, powder spraying is deeply loved by consumers due to its 4E characteristics and diversified surface effects. With the improvement of living standards and diversified market demands, metal powder spraying profiles are used more widely and occupy a higher proportion in spraying profiles. However, due to the different types, specifications, quality and quality of metal powder and the different spraying equipment used by profile manufacturers, the construction effect of metal powder is not very ideal. The main problems are the difference between the surface metal effect and the standard, the color difference of batch production and the unstable construction.

From the point of view of the preparation of metal powder and spraying construction technology, this paper analyzes the construction points of metal powder spraying according to the actual experience of the production site, for the reference of industry experts.

2. Metal powder spraying mechanism

As we all know, in the process of electrostatic powder spraying, the powder coating is conveyed to the spray gun through air flow after fluidization. The spray gun generates corona discharge at the electrode needle of the spray nozzle through a high-voltage electrostatic generator, which generates dense negative charges near the electrode. When the powder is sprayed from the gun head, the captured charges become charged powder. Under the action of air flow, electric field and its own gravity, Fly to the grounding workpiece and attach it to the profile surface. The principle of metal powder electrostatic spraying is the same as that of conventional powder electrostatic spraying, but due to the particularity of metal powder, its spraying mechanism needs further explanation.

Assuming that the spray gun is a horizontal pipeline, and the powder particles are subject to compressed gas transmission force and electrostatic force at the initial stage of ejection from the spray gun, we can regard it as the pneumatic transmission state of the multi particle system and the superposition of electric field force. According to Barth's formula, the dynamic equation of the external electric field multi particle transmission system is:

Wherein, ρ refers to the density of the transported gas, ρ P refers to the density of the powder particles, ε refers to the porosity, λ * Z refers to the collision pressure loss coefficient, fL refers to the fanning friction coefficient, c refers to the particle speed in the pipeline, νε refers to the actual speed of the gas, and Ex refers to the actual electric field strength.

From the above formula, it can be seen that the dynamic state of powder ejection is closely related to the air transport density ρ (flow rate), powder particle density ρ P (particle size), porosity ε (powder content), electric field strength Ex and powder charge q, while the resistance to collision force, friction force and eddy current drag force can be ignored. In the process of powder ejection, air transport density, porosity and electric field strength are external factors. Therefore, the difference between metal powder and ordinary powder lies in the difference of powder particle density (ρ P) and electric charge (q).

After the ordinary powder is crushed and sieved, its particles are close to spherical shape, while the metal powder is not all spherical particles. In order to obtain the flash effect of the coating surface, some aluminum silver powder, copper gold powder and pearlescent mica powder are flake particles. During the movement of such particles between the spray gun and the profile, the influence of gravity and eddy current interference force is very obvious, and the flow resistance formula is:

AP represents the projected area of particles in the fluid flow direction, ξ represents the drag coefficient, which is related to the Reynolds coefficient of particle motion. The flaky metal powder fluctuates between the spray gun and the profile, and the AP value is always changing. When the particle size of the metal powder is small, the resistance is relatively weak.

Secondly, the density of aluminum silver powder, copper gold powder and mica powder is inconsistent. The density of aluminum silver powder is small, while the density of copper gold powder is large. Due to the effect of gravity, if the powder is added into the coating in a dry mixing way, it is easy to separate during the powder spraying process, and the metal powder in the bonding state can be consistent with the base powder.

In addition, the conductivity of aluminum silver powder, copper gold powder and mica powder is very different. Most untreated mica powder is almost in the insulation state, while aluminum silver powder and copper gold powder have very good conductivity, so the difference in the spraying process is also very obvious.

3 Preparation of metal powder coating

The production of metal powder coatings includes melt extrusion, dry mixing and bonding technology. The melt extrusion method is the same as the traditional powder preparation process, except that metal pigments are added to the raw materials, and then the finished products are made by high-speed pre dispersion, melt extrusion, tablet crushing, grinding and screening. Although this method is simple and the metal powder and the base powder are evenly mixed, the production process has a high temperature basis and high temperature shearing, which is easy to cause metal surface oxidation The metal particles are broken and deformed, and the sprayed profiles often fail to obtain the ideal metal effect.

The dry mixing method is to add the metal powder into the pre prepared base powder and make the finished product by high-speed mixing. The advantage of this method is that the metal powder is not easy to be damaged, and the metal effect after spraying is fully exerted. Its obvious disadvantage is that the separation between the metal powder and the base powder is very obvious, especially in the spraying process, due to the difference between the gravity, shape, charge and the base powder particles, The color difference and metal effect of the profile surface are inconsistent, and the powder pump, venturi tube, spray gun electrode needle and flat nozzle are easily worn.

Bonding technology is improved on the basis of dry mixing method. After the metal powder and base powder are mixed evenly, the temperature is raised to the softening point of resin evenly and stably under the protection of inert gas, so that the base powder particles and metal powder particles adhere to each other, or even partially wrap the metal particles, so that the physical and chemical properties of the metal powder and base powder tend to be consistent, which makes the spraying process, The metal powder can be well dispersed on the profile surface to achieve better metal effect. Compared with bonding technology, the addition amount of dry mixed metal powder can only be controlled below 7%, while bonding technology can be added to 20%. With the maturity of the process, bonding technology will be more widely used.

In order to achieve the surface effect required by users, the material, specification and quality of metal powder must be carefully selected. At present, there are many pigment factories in the world, providing different kinds of metal powders. It is commonly divided into aluminum silver powder, copper gold powder and pearlescent mica powder. The aluminum silver powder is divided into floating type and non floating type. The floating aluminum silver powder is easy to float to the coating surface, forming a layer of metal surface, but has poor chemical resistance and weather resistance; The tendency of non floating aluminum powder is relatively weak, but it can be well combined with the base powder. The coating has good physical and chemical properties, and can achieve a better metal effect. Non floating aluminum powder usually undergoes surface treatment, including TiO2 coating treatment, sol coating treatment, and organic polymer treatment.

Copper gold powder is mainly composed of copper or copper zinc alloy. In order to achieve better copper gold effect, the surface of copper powder particles is often covered with a protective film, and the thickness of the protective film affects the reflection and diffraction of its light, resulting in its surface showing gold, red, bronze or turquoise colors. The cross section of mica powder is similar to the structure of pearls. According to this principle, the artificial mica powder wraps a layer of metal oxide with high refractive index on the mica chip, such as, TiO2, Fe2O3, etc., metal oxides with high refractive index and mica chips with low refractive index are arranged in parallel, resulting in visible rainbow colors. If the thickness or ratio of metal oxides is controlled, the color of mica powder is more abundant.

4 Metal powder construction process

4.1 Spraying process parameters

Table 1 Process Parameters of Powder Spraying

According to the practical operation experience on the production site, the metal powder spraying process is roughly the same as the conventional powder spraying process, but in order to achieve a more ideal metal effect, the process parameters such as gun distance, voltage, powder amount, atomization pressure, etc. need to be adjusted according to the type of metal powder and the particle size of metal powder.

Due to the recycling of recycled powder, the metal effect appears a phenomenon of cascade fluctuation; Moreover, the curing process also has a significant impact on some metal powders. In order to study the influence of these factors, the consistency of the metal effect on the surface of the spraying sample plate with the standard color plate is compared by orthogonal test according to eight factors, namely, the type of metal powder, particle size distribution, gun spacing, voltage, powder amount, atomization pressure, addition of recycled powder, and curing process.

4.2 Test method

According to the type of powder, it can be divided into dry mixed powder and bonding powder, which can be subdivided into three categories: aluminum silver powder, copper gold powder and mica powder. Five kinds of powder are sampled respectively. After the particle size distribution is tested, the gun distance, voltage, atomization pressure, powder amount, addition of recycled powder and curing process are adjusted in turn. Three test plates are tested for each type to compare the test results with the standard plate.

Observe the number of metal flashes per unit area of the spraying surface with a low-power mirror. The consistency of the metal effect is the ratio of the number of metal flashes on the test plate to the standard plate. Finally, verify the accuracy of the data by visual comparison, and obtain a series of process factors affecting the metal effect.

4.3 Impact of gun distance on metal effect

The dry mixed aluminum powder, copper gold powder and mica powder are adjusted according to the gun distance of 250~400mm, the voltage is set to 80kv, the powder amount, atomization pressure, no addition of recycled powder, and the curing process are fixed, and the relationship between the gun distance and the effect of metal powder is obtained.

It can be seen from Figure 1 that during the construction of metal powder, the effect of metal powder will be worse as the gun distance is farther. The metal effect of aluminum powder is very close to the standard plate when the gun distance is 250mm, and the effect is the best; The copper gold powder, due to its better electrification effect and its own gravity, has better metal effect than the standard plate when the gun distance is short, but the anti ionization phenomenon appears on the test plate, so the comprehensive effect of gun distance too close to the surface is not ideal; The mica powder is covered by the coating during spraying, so the flash effect is relatively poor.

4.4 Influence of voltage on metal effect

The dry mixed aluminum powder, copper gold powder and mica powder are adjusted according to the voltage of 40~100kv. The gun distance, powder amount, atomization pressure, no addition of recycled powder, and the curing process are fixed. The relationship between the spraying voltage and the effect of metal powder is obtained.

It can be seen from Figure 2 that since the conductivity of aluminum powder is better than copper gold powder and mica powder, when the voltage is very low, the metal effect of aluminum powder is more obvious, and the voltage required to achieve the metal effect of standard plate is also the lowest; With the increase of voltage, the difference between the metal effect and the standard plate becomes smaller and smaller. When the voltage exceeds 80kv, the anti ionization phenomenon appears on the test plate of aluminum powder and copper gold powder, the powder coating is broken down, and the surface metal effect is not ideal. Because the aluminum powder is more charged, the anti ionization phenomenon is more obvious, and the metal effect also drops faster; Due to the weak conductivity of mica powder and its own gravity, the metal effect becomes more and more obvious as the voltage continues to increase, and the difference between mica powder and standard plate gradually becomes smaller.

4.5 Influence of atomization pressure on metal effect

The dry mixed aluminum powder, copper gold powder and mica powder are adjusted according to the atomization pressure of 0.05~0.20MPa respectively. The gun distance, voltage, powder amount, no addition of recycled powder, and the curing process are fixed. The relationship between the atomization pressure and the effect of metal powder is obtained.

It can be seen from Figure 3 that the metal effect is the best when the atomization pressure is 0.05MPa under the condition that other construction parameters remain unchanged. With the increase of the atomization pressure, the metal effect decreases. Due to the influence of gravity (mica powder>copper gold powder>aluminum powder), the greater the gravity is, the faster the powder drops, and the worse the metal effect is.

4.6 Influence of powder spraying amount on metal effect

The amount of powder spraying determines the thickness of the coating. Under the condition that other construction parameters remain unchanged, the surface effect of the coating is affected by the section of the profile, the way and density of the hanging material, the speed of the spraying chain, and the conductivity of the equipment. The difference between metal powder construction and ordinary powder is that with the increase of powder spraying amount, the pigment in the powder is absorbed by the profile faster than the resin, and the surface metal effect is covered by the resin, which is significantly different from the standard plate; In addition, some metal pigments with small particle size are subject to weak electric field force, and are prone to edge during spraying, affecting the coating quality.

4.7 Effect of recycled powder on metal effect

During the construction of metal powder, the reasonable use of recycled powder is very important, which directly affects the metal effect of the coating and the production cost (powder utilization rate). Inconsistent addition proportion of recovered powder will cause quality defects such as color difference of batches and low content of metal powder, especially dry mixed metal powder. Pigments are easy to be crushed in the recovery system, and the specific gravity of pigments per unit weight is reduced, particle size is smaller, and electric capacity is weakened. In addition, the specific gravity of powder pigments in batches may be inconsistent, and the coating metal effect is not ideal.

In general, the mixing ratio of recovered powder and new powder is 1: (4~6), and the powder should be recycled while producing as much as possible. The metal powder that cannot be directly used should be recycled after the plate is confirmed to be qualified in time. Some metal powder with poor quality should be returned to the manufacturer by the construction party for rework before use. For metal powder with bright color, the proportion of recycled powder should be appropriately reduced.

4.8 Effect of curing process on metal effect

The curing process of metal powder is basically the same as that of ordinary powder, except for high gloss and texture. During film curing, the profile must be fully cured. The maximum curing temperature of the curing furnace must be set in advance, and the heating speed of the curing furnace, the heating speed of the profile surface and the holding time must be mastered. These parameters directly affect the metal color effect of the film. For example, high gloss, hammer and sand texture metal powder requires fast curing temperature rise rate and higher curing temperature, while texture metal powder requires fast curing temperature rise rate, metal texture will become smaller and finer, and slow temperature rise rate will become larger and coarser, so strict and stable curing process is necessary for metal powder construction.

5 Conclusion

Because metal powder contains metal pigments, electrostatic spraying construction is quite different from ordinary powder, and it is difficult to ensure the coating quality and process stability. The preparation process of metal powder is different from the electrostatic spraying equipment of profile manufacturers, and its construction parameters and metal effects are very different; Through further combing the electrostatic spraying mechanism of metal powder, it is concluded that the difference between metal powder and ordinary powder lies in the difference of powder particle density (ρ P) and electric charge (q); Combined with construction site experience and orthogonal test data, 8 factors affecting the construction effect of metal powder are summarized, which provides a basis for various metal powder construction. Click the original text to read and view the past history

Guangzhou Tuoshen Surface Treatment Co., Ltd., the spraying manufacturer, is preferred for purchasing spraying, Spray processing Factory, trustworthy