The construction process of metal powder coating is discussed

Abstract: The types of metal powders used in spray profiles and the production process are introduced, the factors affecting the surface quality of various types of metal powders in the spraying process are analyzed, and the main points of conventional metal powder construction are discussed.
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powder coating is one of the surface treatment methods of aluminum profiles and is popular with consumers due to its 4E properties and diverse surface effects. With the improvement of living standards and the diversified needs of the market, the use of metal powder coating profiles is more extensive, and the proportion of spray profiles is becoming

. However, due to the metal powder type, specifications, metal powder quality, powder quality is different, coupled with the profile manufacturers used spray equipment has a certain difference, metal powder construction effect is not very ideal, the main problem is the difference between the surface metal effect and standard, batch production color difference and construction instability. Based on the preparation of metal powder and the process of spraying construction, this paper analyzes the construction points of metal powder spraying according to the actual experience of the production site for reference by industry experts.
1 Metal powder spraying machine
It is well known that during electrostectrulation powder spraying, the powder coating is fluidized and delivered to the spray gun by airflow, and the spray gun is transmitted to the spray gun through a high-voltage electrostectrulator, which produces corona discharge in the electrode needle of the nozzle. Electricity, which produces a dense negative charge near the electrode, is captured as a charged powder when the powder is equisited from the gun head, flying toward the grounding work piece under the influence of airflow, electric field, and its own gravity, and adsorbed to the profile surface. The electrostitive spraying of metal powder is consistent with the principle of static spraying of conventional powder, but due to the particularity of metal powder, its spraying method needs further explanation.
Assuming that the spray gun is a horizontal pipe, the powder particles from the initial stage of the spray gun by compressed air transfer force, static electricity, we can think of it as a multi-particle system of the state of air transport and the overlay of electric field force, according to Barth derivation formula, plus electric field multi-particle transmission system power equation is:
by: (1) can be seen that the powder injection power state and air transport density ρ (flow rate), powder particle density picture (particle size), clearance ε (powder) electric field strength Ex and powder belt charge q are closely related, and with collision force, friction and vortex traction force and other resistance can be ignored. During powder e-injection, air delivery density, clearance, and electric field strength are external factors, so the difference between metal powder and ordinary powder lies in the difference between powder particle density (pictured) and charge (q).
Ordinary powder after crushing, screening, its particles close to the shape of the sphere, and metal powder is not all sperical particles, in order to obtain the coating surface flash effect, some aluminum silver powder, copper gold powder and pearly mica powder is flaky particles. In the process of movement between the gun and the profile, the effect of gravity and vortex interference is very obvious, and the flow resistance formula is: the image in the
indicates the projected area of the particle in the direction of fluid flow, and ξ represents the traction coefficient, which is related to the Reynolds coefficient of particle movement. Flaky metal powder fluctuates between the spray gun and the profile, and the picture value is always in the process of change, when the metal powder particle size is small, the resistance is relatively reduced. Secondly, the density of aluminum silver powder, copper gold powder and mica powder is not consistent, wherein aluminum silver powder density is small, copper and gold powder density is larger, due to gravity, if added to the paint in a dry mixing process is easy to separate, the metal powder in the bonding state can be consistent with the base powder. In addition, aluminum silver powder, copper gold powder and mica powder conductivity differences are very obvious, most of the unprocessed mica powder is almost insulated state, and aluminum silver powder, copper and gold powder conductivity is very good, so the difference in the spraying process is very obvious.
2 Metal Powder Coatings
metal powder coatings include molten extrusion, dry mixing, and bonding technology. Melt extrusion method is consistent with the traditional powder preparation process, only in the raw materials to add metal pigments, and then high-speed pre-dispersion, melt extrusion, chip crushing, grinding powder screening and make the finished product, although the process is simple, metal powder and base powder are evenly mixed, but the production process has a high temperature foundation and high temperature shearing, easy to cause metal surface oxidation, metal sheet fragmentation, sprayed profile often can not get the ideal metal effect. Dry mixing method is to add metal powder into the pre-prepared base powder, through high-speed mixing to make a finished product, the advantage of this method is that the metal powder is not easy to be destroyed, after spraying the metal effect to give full play to, its obvious deficiency is the separation of metal powder and base powder phenomenon is obvious, especially in the Spraying process due to gravity, shape, charge and base powder particles are not consistent, resulting in the profile surface color difference, metal effect is not consistent, at the same time easy to powder pump, Wenqiu tube, spray gun electrode needle, flat nozzle caused wear. Bonding technology is improved on the basis of dry mixing method, metal powder and base powder mixed evenly, under the protection of inert gas, the temperature will be uniform and stable rise to the resin softening point, so that the base powder particles and metal powder particles adhere to each other, and even partially wrapped metal particles, so that metal powder and base powder materialization performance tend to be consistent, which makes the spraying process, metal powder can be well dispersed on the profile surface, to achieve better metal effect. Compared with bonding technology, dry mixing metal powder can only be controlled at less than 7%, while bonding technology can be added to 20%, as its process matures, bonding technology will be more widely used.
in order to achieve the user's requirements of the surface effect, metal powder materials, specifications, quality must be carefully selected. At present, there are more pigment factories around the world, and different types of metal powders are available. The more common are aluminum silver powder, copper gold powder and pearly mica powder. Among them, aluminum silver powder is divided into floating and non-floating type, floating aluminum silver powder is easy to float to the coating surface, forming a metal surface, but the resistance to chemicals and weather is very poor; The tendency of non-floating float is relatively weak, but can be good combination with the base powder, the coating has good physicolic performance, and can achieve better metal effect, non-floating aluminum powder is usually surface treatment, the common TiO2 cladding treatment, sol coating treatment and organic polymer treatment. Copper gold powder composition is mainly copper or copper and zinc alloy, in order to achieve a better copper-gold effect, copper powder particle surface is often covered with a protective film, and the thickness of the protective film affects the reflection of its light diffraction, resulting in its surface appears gold, red, bronze or turquoise color. Mica powder cross-section is similar to the structure of pearls, artificial mica powder according to this principle, in the mica chip wrapped in a layer of metal oxides with high refractive index, such as TiO2, Fe2O3, high refractive index of metal oxides and low refractive index of mica chip parallel arrangement, resulting in a visual rainbow color, if the thickness or ratio of metal oxides control, mica powder color is richer.
3 Metal powder construction process
3.1 spray process parameters
powder spray process parameters can be found in Table 1.
According to the actual operation experience of the production site, metal powder spraying and conventional powder spraying process is about the same, but in order to achieve a more ideal metal effect, but also according to the type of metal powder, metal powder particle size to adjust the gun distance, voltage, powder volume, atomized air pressure and other process parameters. Due to the recycling of recycled powder, the metal effect fluctuates, and the curing process has a very obvious effect on some metal powders.
In order to study the influence of these factors, through orthosis test, according to the metal powder type, particle size distribution, gun range, voltage, powder volume, atomized air pressure, recovery powder addition, curing process 8 factors to compare the spray model surface metal effect and standard style consistency.
3.2 Test Method
According to the type of powder is divided into dry mixed powder and bonding powder, subdivided into aluminum silver powder, copper gold powder, mica powder three categories, respectively, sampling 5 kinds, test particle size distribution, in turn adjust the gun distance, voltage, atomized air pressure, powder volume, recovery powder addition, curing process 6 factors, each test 3 test plates, to compare the test results with the standard board.
The number of metal flashes per unit area of the spray surface is observed using a low multiplle mirror, the metal effect consistency is the ratio of the number of metal flashes of the test board to the standard plate, and finally visual comparison verifies the accuracy of the data, resulting in a series of process factors affecting the metal effect.
3.3 gun distance on the effect of metal
take dry mixed aluminum powder, copper gold powder, mica powder according to the gun distance of 250 to 400mm to adjust, the voltage is set to 80kV, powder volume, atomized air pressure, do not add recycling powder, curing process fixed unchanged, to obtain the gun distance and metal powder effect relationship.
can be seen from Figure 1, the metal powder construction process, the metal powder effect with the gun distance, the worse the effect. Among them, aluminum powder in the gun distance at 250mm when the metal effect is very close to the standard plate, the best effect, and copper and gold powder due to its better charged effect and its own gravity, in the shorter gun distance, the metal effect is better than the standard plate, but the test board appeared anti-ionization phenomenon, so the gun distance is too close to the surface comprehensive effect is not ideal; The mica powder, however, has a relatively poor flash effect because it is covered with a coating when sprayed.
3.4 voltage on the effect of metal
take dry mixed aluminum powder, copper gold powder, mica powder according to the voltage 40 to 100kV to adjust, gun distance, powder volume, atomized air pressure, do not add recovery powder, curing process fixed, to get the spray voltage and metal powder effect of the relationship.
can be seen from Figure 2, because aluminum powder conductivity is better than copper gold powder, mica powder, when the voltage is very low, aluminum powder metal effect is more obvious, to achieve the standard plate metal effect required voltage is also the lowest; With the increase of voltage, the difference between metal effect and standard plate is getting smaller and smaller, when the voltage exceeds 80kV, aluminum powder and copper gold powder test board appear anti-ionization phenomenon, powder coating is pierced, the surface metal effect is not ideal, because aluminum powder is more charged, the anti-ionization phenomenon is more obvious, the metal effect also decreases faster; Mica powder has weak conductivity and its own gravity effect, the metal effect with the voltage continues to increase and become more and more obvious, and the difference between the standard board gradually becomes smaller.
3.5 Effect of atomized air pressure on metal effect
Take dry mixed aluminum powder, copper gold powder, mica powder according to atomized air pressure 0.05 to 0.20 MPa, gun distance, voltage, powder, do not add recovery powder, curing process fixed, to obtain the relationship between atomized gas pressure and metal powder effect.
can be seen from Figure 3, under the condition that other construction parameters are guaranteed to remain unchanged, when the atomized air pressure is at 0.05 MPa, the metal effect is the best, and with the increase of atomized air pressure, the metal effect decreases. Due to gravity (mica powder> copper gold powder >alum powder), the greater the gravity, the faster the powder drops, the worse the metal effect.
3. 6 Effect of powder spray on metal effect
The size of the powder amount determines the thickness of the coating, and 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 spray chain, and the conductivity of the equipment. The difference between metal powder construction and ordinary powder is that, with the increase of powder spray, the pigment in the powder is absorbed faster than the resin, the surface metal effect is covered by resin, and the difference with the standard plate is obvious;
3.7 Effect of recycled powder on metal effect
During the construction of metal powder, the rational use of recycled powder is very important, which directly affects the metal effect of the coating and relates to production cost (powder utilization). The proportion of recovery powder added is not consistent, will cause batch color difference, metal powder content is less and other quality defects, especially dry mixed metal powder, pigment in the recovery system is easy to be crushed, the proportion of pigments within the unit weight is reduced, the particle size is smaller, the proportion of charge is reduced, coupled with batch powder pigment proportion may not be consistent, the coating metal effect is not ideal. In general, the proportion of recycled powder and new powder is 1:4 to 6, and as far as possible side-by-side recycling use, can not be used directly to timely plate to confirm qualified recovery, part of the poor quality of metal powder returned by the construction party to the manufacturer, rework qualified to use, bright color metal powder, recycling powder ratio should be appropriately reduced.
effect of the 3.8 curing process on the metal effect
The curing process of metal powder is basically the same as that of ordinary powder, with the exception of high gloss and texture. When applying film curing, we must ensure that the profile is fully cured, set the maximum curing temperature of the curing furnace in advance, grasp the heating speed of the curing furnace, the heating speed of the profile surface and hold time, these parameters directly affect the metal color effect of the coating film. Such as high gloss, hammer grain, sand-type metal powder requires fast curing heating, curing temperature is higher, and textured metal powder curing temperature heating speed metal texture will become smaller and thinner, slow heating speed will become terrified, so strict and stable curing process is necessary for metal powder construction.
4 conclusion
Metal powder due to the metal pigments lead to static spraying construction and ordinary powder is very different, coating quality and process stability is difficult to ensure. Metal powder preparation process and profile manufacturers electrostectrical spraying equipment is not the same, its construction parameters and metal effects are very different;
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