Causes and prevention of particles on the surface

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Causes and prevention of coating surface particles

coating as a way of product surface treatment, the main purpose is to prevent and slow down product corrosion, and at the same time, the flat and bright coating surface has a beautiful effect; However, when the coating contains particles or sundries brought into the coating construction process, it will directly affect the appearance and coating quality of products. During the spraying process of our products, the surface coating of the products is often attached with particles, which makes the coating surface feel rough when touched by hand, which not only affects the appearance, but also the protruding part of the particles on the surface of the coating is easy to be damaged and stripped by external objects, so that the coating is damaged and the protection is greatly reduced [1]. Combined with many years of working experience in coating construction, this paper analyzes the causes of particles on the surface of the coating, and puts forward preventive and solving measures

1 coating preparation

generally, coatings are composed of film-forming substances, pigments and solvents, of which film-forming substances and solvents are generally liquid organic substances, while pigments are generally 0.2 μ m-10 μ M inorganic or organic powder, which can be used as an effective coating after being evenly mixed according to the appropriate ratio. However, most of the coatings have to go through a long process from production, sales, transportation, storage to use. During this period, the pigments evenly distributed in the coatings will precipitate due to long-time standing. Some inorganic powders will also be affected by the charge to gather into larger particles, which will affect the normal use of the coatings. Therefore, the coatings must be stirred evenly and filtered before construction

according to Clause 5.3 of hb/z "painting process of" three proofing "coating of airborne equipment", the diluted primer is filtered with 80 mesh -100 mesh, and the finish coat is filtered with 180 mesh -200 mesh copper wire or silk, the purpose of which is to filter the particles in the paint. It is understood that some units adopt a low mesh (60 mesh) multi-layer filtering superposition method to achieve the purpose of rapid filtration. This method can not completely filter out the larger particles in the paint, so the correct use of filtration is a very important link. At present, the funnel-shaped filter made of 200 mesh silk is directly purchased from the market, which is cheap and easy to use. It can be used as the filter of primer and finish paint, so that the particles in primer and finish paint can be filtered, and the sprayed coating is fine, smooth and has high adhesion. For spraying another kind of special varnish coating with heat-resistant aluminum powder, it can be filtered by the superposition of two 200 mesh filters, and the effect is better

2 spraying environment

Bernoulli equation: p+1/2 ρ V2+ ρ Gh=c

where p is the pressure potential energy at a point in the fluid, and V is the flow rate at that point of the fluid, ρ Is the fluid density, G is the gravitational acceleration, h is the height of the point, and C is a constant. This equation describes that in the process of fluid flowing along the streamline, the streamlines are not intersected with each other, and the unit volume and even the venture capital institutions have clearly expressed their willingness to invest funds to expand the production range of new copper alloy materials. The pressure potential energy P and gravity potential energy of the fluid ρ GH and kinetic energy 1/2 ρ The sum of V2 is constant C, that is, the total energy remains unchanged, that is, when the fluid flows along the contour, the flow rate is large, and the pressure is small

during the actual air spraying, the coating is coated on the part surface at a certain speed under the action of the compressed gas spray pressure. In this process, the pressure on the part surface is lower than the atmospheric pressure of the surrounding environment due to the rapid movement of the coating, forming a negative pressure, making the surrounding air flow rapidly to the part surface in the low-pressure area. If there are many suspended substances such as dust or wool in the spraying plant, or there are particles with residual paint on the tooling used to protect the parts from spraying, they may be adsorbed on the surface of the parts during the spraying process to form particles. Therefore, ensuring the cleanness of the coating production environment is an important link to ensure the quality of air spraying coating. The impact of the surrounding environment can be eliminated to the greatest extent by regularly cleaning the vehicles used to place the workpiece, the tooling used for protection, and the hangers used to hang the workpiece or wrapping them with clean paper

3 pretreatment before spraying

generally, if the parts before spraying are made of aluminum alloy, they should be anodized, and the surface is generally free of particles within the effective time. However, for the parts directly sprayed after electroplating or sand blowing, there may be rough coating or residual sand on the surface. When the coating of large parts is rough or the residual sand on the surface of parts is not cleaned after sand blowing, Will directly form coating particles after spraying. In addition, for some cutting tumors produced during laser cutting, adhering to the surface of parts will affect the quality of later coating. Therefore, it is particularly important to control the surface state of the parts before coating

4 spraying method

the flow of the coating caused by external forces during construction includes tensile flow and shear flow. Viscosity is a measure of resistance to flow. For the coating, because its amplitude is 100 ⑴ 70 yuan/ton, it is a non-Newtonian fluid, that is, the fluid with non-linear relationship between its shear force and shear strain rate. It needs to be when the shear stress reaches a certain value before the fluid begins to flow. In addition, the coating has thixotropy, that is, when the shear rate is high (when brushing), the viscosity is low, which can facilitate brushing and make the coating have good fluidity. When the shear rate is low (after standing or spraying), it has high viscosity, which can prevent sagging and pigment sedimentation. One of the reasons for thixotropy is the formation of a weak structure in the static system, such as the physical crosslinking of polymers formed by hydrogen bonds and the crosslinking between polymers formed by polar adsorption of pigments as bridges. This shape structure is destroyed under the action of shear. Once the shear force is removed, the shape structure slowly recovers [2]. Therefore, the viscosity of the coating during construction is greatly affected by the shear speed, which is inversely proportional to the shear speed. If the spray gun produces less paint and the dynamic gas pressure is insufficient during spraying, the viscosity of the paint will be too high, and it is easy to gather and form particles on the surface of parts. At the same time, the distance between the spray gun and the parts is required to be 150mm-250mm during spraying. If the distance is too far, and the phases have grounding words or grounding marks, the shear force of the coating reaching the surface of the part is small, and the viscosity of the coating is large, which will also form particles. This requires that during the spraying construction, the distance between the spray gun and the surface of the parts should meet the requirements, especially for the parts with complex appearance, the distance between the spray gun and the parts should be strengthened

in the spraying process, it is also necessary to combine the extraction (exhaust) and air flow direction to prevent the rebound effect of paint mist and paint, and avoid the formation of particles on the surface of parts with high viscosity paint; And particles brought in by the construction personnel wearing unclean tooling, work shoes, etc

it is a systematic project to solve the generation of surface particles in the coating process, which involves many links such as the composition of the coating, the modulation of the coating, the construction environment, and the operation method. The specific requirements and precautions of the operation steps should be clarified from the working documents, and the control points of each link should be refined, so as to ensure that the semi-automatic test is mainly used for large industrial metal factories to certify the high-quality coating quality

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