[China Aluminum Industry Network] Chen Jie, He Juncheng, Chen Xiaolei
(Sichuan Guanghan Samsung Aluminum Co., Ltd., Deyang, Sichuan, 618300)
Abstract : This paper introduces the types of metal powders used in sprayed profiles and their production processes. The factors that influence the surface quality of various types of metal powders in the spraying process are analyzed. The main points of conventional metal powder construction are discussed.
Keywords: metal powder coating; dry mix powder; bonding powder; spraying process
1 Introduction
Powder spraying is one of the surface treatment methods for aluminum profiles, and is popular among consumers because of its diverse 4E characteristics and surface effects. With the improvement of living standards and the diversification of market demands, the use of metal powder sprayed profiles has become more widespread, and the proportion of sprayed profiles has become higher and higher. However, due to the different types of metal powders, specifications, metal powder quality, and powder quality, together with certain differences in the spray equipment used by profile manufacturers, the construction effect of metal powder is not very satisfactory. The main problem is the effect and standard of surface metal. The difference between the batch production of color difference and construction instability.
This article from the preparation of metal powder and spray construction process point of view, according to the actual experience of the production site analysis of the metal powder coating construction points for reference by industry experts.
2 metal powder spraying mechanism
It is known that during the electrostatic powder coating process, the powder coating is fluidized and delivered to the spray gun through a gas flow. The spray gun passes through a high-voltage electrostatic generator to generate a corona discharge at the electrode needle of the spray head, resulting in a dense negative charge near the electrode. When ejected, the trapped charge becomes a charged powder that, under the influence of airflow, electric field, and its own gravity, flies toward the grounded workpiece and adsorbs on the surface of the profile. Electrostatic spraying of metal powder is consistent with the principle of conventional powder electrostatic spraying, but due to the special nature of metal powder, the spraying mechanism needs further explanation.
Assuming that the spray gun is in a horizontal state, the powder particles are subjected to compressive gas transfer force and electrostatic force at the initial stage of spraying from the spray gun. We can regard it as a superposition of the multi-particle system's pneumatic conveying state and electric field force, according to Barth's derivation. Formula, the dynamic equation of the multi-particle transmission system with applied electric field is:
Study on Construction Technology of Spraying Aluminum Profile Metal Powder Coating" />
Among them, Ï refers to the transport gas density, ÏP refers to the powder particle density, ε refers to the porosity, λ*Z refers to the collision pressure loss factor, fL is the fanning coefficient, c refers to the particle velocity in the pipeline, νε refers to Is the actual gas velocity, Ex refers to the actual electric field strength.
From the above formula, it can be seen that the power state of powder discharge is closely related to the air transport density Ï (flow rate), powder particle density ÏP (particle size), voidage ε (powder) electric field intensity Ex, and powder charge q, and Impact resistance, friction force, and eddy drag force are negligible. In the process of powder jetting, the air transport density, voidage, and electric field strength are external factors. Therefore, the difference between the powdered metal powder (pP) and the charged electricity (q) is the difference between the metal powder and the ordinary powder.
After the common powder is crushed and sifted, the particles are close to the sphere shape, and the metal powder is not all spherical particles. In order to obtain the flash effect on the coating surface, some of the aluminum silver powder, copper powder and pearlescent mica powder are in the form of tabular particles. During the movement of the spray gun and the profile, the influence of gravity and vortex disturbance force is very obvious. The flow resistance formula is:
AP denotes the projected area of ​​the particles in the direction of fluid flow, and ξ denotes the drag coefficient, which is related to the Reynolds number of particle motion. The flake metal powder fluctuates between the spray gun and the profile, and the AP value is always in the process of change. When the metal powder particle size is small, the resistance is relatively weakened.
Secondly, the density of aluminum silver powder, copper gold powder and mica powder is inconsistent, in which the density of aluminum silver powder is relatively small, and the density of copper powder is relatively large. Due to the effect of gravity, if it is added into the coating by dry-blending, it will be easily separated during the dusting process. The metal powder in the bonding state can maintain consistency with the base powder.
In addition, the difference in conductivity of aluminum silver powder, copper powder and mica powder is very obvious. Most untreated mica powders are almost insulated, while aluminum silver powder and copper gold powder have very good electrical conductivity. Therefore, the difference in the spraying process is also very high. obvious.
3 Metal Powder Coating Preparation
Metal powder coating production includes melt extrusion, dry blending and bonding techniques. The melt extrusion method is the same as the traditional powder preparation process except that metal pigments are added to the raw materials, then high-speed pre-dispersed, melt-extruded, tablet-broken, and milled to form a finished product. This process is simple, The metal powder and the base powder are uniformly mixed, but the production process has a high-temperature basis and high-temperature shearing, which easily leads to oxidation of the metal surface and breakage of the metal particles. After the sprayed profiles, an ideal metal effect is not obtained.
The dry blending method is to add the metal powder to the pre-prepared base powder and make the finished product through high-speed mixing. The advantage of this method is that the metal powder is not easily destroyed and the metal effect after spraying is fully exerted. The obvious deficiency is metal powder. The phenomenon of separation from the base powder is very obvious, especially due to the inconsistence of gravity, shape, charge quantity and base powder particles in the spraying process, which causes the surface color difference of the profile and the metal effect to be inconsistent. At the same time, it is easy for the powder pump, the venturi tube, the electrode needle of the spray gun, and the flat Muzzle wear.
Bonding technology is improved on the basis of the dry-blending method. After the metal powder is uniformly mixed with the base powder, under the protection of the inert gas, the temperature is evenly and steadily increased to the softening point of the resin, so that the base powder particles and the metal powder particles interact with each other. Adhesion, and even partial encapsulation of metal particles, so that the metal powder and the basic powder properties tend to be consistent, which makes the spraying process, the metal powder can be well dispersed in the profile surface, to achieve a better metal effect. Compared with the dry-blending method and the bonding technique, the amount of dry-mixed metal powder can only be controlled below 7%, and the bonding technology can be added to 20%. With the maturity of its technology, the application of bonding technology will be more extensive.
In order to achieve the surface effect required by the user, the material, specification, and quality of the metal powder must be rigorously selected. At present, there are many pigment factories around the world and the types of metal powders provided vary. More common is divided into aluminum silver powder, copper powder and pearlescent mica powder. The aluminum silver powder is divided into floating and non-floating type, and the floating aluminum silver powder is easy to float to the surface of the coating to form a layer of metal surface, but the chemical resistance and weather resistance are poor; the tendency of non-floating float is relatively weak. However, it can be well combined with the base powder, the physical and chemical properties of the coating are better, and a good metal effect can be achieved. The non-leafing aluminum powder is usually subjected to surface treatment, and there is a common TiO2 coating treatment, sol coating treatment and organic Polymer treatment.
The composition of copper powder is mainly copper or copper-zinc alloy. In order to achieve a good copper-gold effect, the surface of the copper powder particle is often covered with a protective film, and the thickness of the protective film affects the reflection and diffraction of light, resulting in the surface of the copper powder. Gold, red, bronze or blue gold. Mica powder cross-section similar to the pearl structure, artificial mica powder according to this principle, in the mica chip coated with a high refractive index metal oxides, such as TiO2, Fe2O3, etc., high refractive index metal oxides and low Refractive index mica chips are arranged in parallel, resulting in a visible rainbow color. 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 Powder Coating Process Parameters
According to the actual operation experience at the production site, the metal powder coating and the conventional powder coating process are almost the same, but in order to achieve a more satisfactory metal effect, the distance, voltage, powder amount, and fog must be adjusted according to the type of metal powder and the particle size of the metal powder. Pressure and other process parameters.
Due to the recycling of recycled powder, the metal effect has a phenomenon of fluctuation; and the effect of the curing process on some of the metal powder is also very obvious. In order to study the influence of these factors, through the orthogonal test, according to the metal powder type, particle size distribution, gun distance, voltage, powder volume, atomization pressure, recycled powder addition, curing process 8 factors to compare the surface effect of metal spraying model and Standard swatch consistency.
4.2 Test methods
In accordance with the powder type is divided into dry powder and bonding powder, subdivided into three categories of aluminum silver powder, copper powder, mica powder, respectively, 5 kinds of samples, test particle size distribution, in order to adjust the gun distance, voltage, atomization pressure, powder Quantity, recycled powder addition, curing process 6 factors, 3 test plates for each test, used to compare test results with standard plates.
Use a low power microscope to observe the number of metal flashes per unit area of ​​the surface. The consistency of the metal effect is the ratio of the number of metal flashes on the test board to the standard board. The accuracy of the data after comparison with the visual verification is later, and a series of processes affecting the metal effect are obtained. factor.
4.3 Effect of Gun Distance on Metal Effect
Take dry-blended aluminum powder, copper powder, and mica powder in accordance with the gun distance of 250-400mm, the voltage is set to 80kv, the powder volume, atomization pressure, no added recovery powder, and the curing process are fixed, and the distance between the gun and Metal powder effect relationship.
As can be seen from Figure 1, during the metal powder construction process, the farther away from the gun distance is, the worse the effect is. The aluminum effect is similar to the standard plate when the distance between the guns is 250mm, and the effect is better; and because of the better electrification effect and the self-gravity effect of the copper powder, when the distance between the guns is shorter, the metal effect is better than the standard plate. Better, but there is a phenomenon of deionization on the test plate, so the comprehensive effect of the gun too close to the surface is not ideal; while the mica powder is coated by the coating, the flash effect is relatively poor.
4.4 Effect of Voltage on Metal Effect
Take dry-blended aluminum powder, copper powder, and mica powder in accordance with the voltage of 40 ~ 100kv respectively, the gun distance, powder amount, atomization pressure, no added recovery powder, curing process is fixed, and the spraying voltage and metal powder effect are obtained. relationship.
As can be seen from Figure 2, because the conductivity of aluminum powder is better than copper powder and mica powder, when the voltage is very low, the effect of aluminum powder metal is more obvious, and the voltage required to reach the standard plate metal effect is also lower; with the voltage With the increase, the difference between the metal effect and the standard plate is getting smaller and smaller. When the voltage exceeds 80kV, deionization occurs on the aluminum powder and the copper powder test plate, and the powder coating is broken down. The surface metal effect is not ideal due to aluminum. The powder is more electrically charged, the deionization phenomenon is more obvious, and the metal effect is also falling faster; the conductivity of mica powder is weak and its own gravity, the metal effect becomes more and more obvious as the voltage continues to increase, and the standard plate The difference between them gradually becomes smaller.
4.5 Effect of atomizing air pressure on metal effect
Take dry-mixed aluminum powder, copper powder, and mica powder in accordance with the atomization pressure 0.05 to 0.20 MPa, respectively, gun distance, voltage, powder, no added recycled powder, curing process is fixed, the atomization pressure and metal powder The effect of the relationship.
As can be seen from Figure 3, under the conditions that other construction parameters are unchanged, when the atomization pressure is 0.05 MPa, the metal effect is better, and with the increase of the atomization pressure, the metal effect is decreased. Due to the influence of gravity (mica powder> copper powder> aluminum powder), the greater the gravity, the faster the powder drops and the worse the metal effect.
4.6 Effect of Powder Spraying on Metal Effect
The amount of powder sprayed determines the thickness of the coating. Under the condition that other construction parameters are not changed, the surface effect of the coating is affected by the profile 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 the amount of powder sprayed, the speed of absorption of pigment in the powder by the profile is faster than that of the resin, and the surface metal effect is covered by the resin, which is obviously different from the standard plate; in addition, some of the metal with small particle size The pigment is weak by the electric field, and it is easy to be marginal when spraying, which affects the coating quality.
4.7 Effect of Recovered Powder on Metal Effect
During the construction process of metal powder, the rational use of recycled powder is of great importance, directly affecting the metal effect of the coating, and relates to the production cost (powder utilization rate). The inconsistent proportion of recycled powder will cause quality defects such as batch color difference and metal powder content. Especially for dry-mixed metal powders, the pigments are easily crushed in the recycling system, and the specific gravity of the pigment per unit weight decreases, the particle size becomes smaller, and the Reduced power, combined with the inconsistent proportion of batch powdered pigments, results in unsatisfactory coating metal performance.
Under normal circumstances, the mixing ratio of recycled powder and new powder is 1: (4-6), and as far as possible, the edge is used for recycling. If it cannot be directly used, it must be promptly played after the board is confirmed to be qualified, and some of the metal powder with poor quality is recovered. The construction party can only use it after returning it to the manufacturer for rework. For brightly colored metal powders, 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 the exceptions of high gloss and texture. When the coating film is cured, it must be ensured that the profile is fully cured, the higher curing temperature of the curing oven is set in advance, the heating speed of the curing oven is grasped, the heating rate and the holding time of the profile surface are directly affected, and these parameters directly affect the metallic color effect of the coating film. . Such as high-gloss, hammer, sand grain type metal powder required curing speed, high curing temperature, and texture metal powder curing temperature, the temperature will increase the speed of metal texture will become smaller and thinner, the slower heating rate will become larger and thicker Therefore, strict and stable curing process is very necessary for metal powder construction.
5 Concluding remarks
Metal powders containing metal pigments cause large differences from common powders when applied to electrostatic spraying. Coating quality and process stability are difficult to guarantee. The preparation process of metal powder and the electrostatic spraying equipment of the profile manufacturers are not the same. The construction parameters and metal effect are quite different. Through the further combing of the mechanism of metal powder electrostatic spraying, it is concluded that the difference between metal powder and ordinary powder is the powder particle density (pP ), the difference between the charge (q); combined with the construction site experience and orthogonal test data, summed up the eight factors that affect the construction effect of metal powder, providing the basis for all types of metal powder construction. Click on the original reading to view the history of the past
references
[1] Hao Xiaolin, powder flow mechanism and experimental research in pneumatic conveying system, master's periodical, Qingdao University of Science and Technology, 2006.4.30.
[2] Ruan Bing, Metal Powder Coatings, 2007.2
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