Analysis and solution of common quality problems in powder spraying.

1, coating impurities
common impurities mainly from particles in the powdering environment, as well as other factors caused by impurities, is summarized below.
1.1 impurities in the curing furnace. The solution is to thoroughly clean the inner walls of the curing furnace with a damp cloth and vacuum cleaner, with a focus on the high-speed construction of the

. If it is a large black particle impurities need to check the air pipe filter for damage, there is a timely replacement.
1.2 powder indoor impurities. Mainly dust, clothing fibers, equipment grinding particles and powder system scale. The solution is to use compressed air to blow the powder system before starting work every day, and thoroughly clean the powder spray equipment and powder room with a damp cloth and vacuum cleaner.
1.3 hanging chain impurities. Mainly hanging chain oil plate and a lifting water plate (material for hot galvanized plate) by the pretreated acid, alkali vapor corrosion products. The solution is to clean these facilities
1.4 powder impurities on a regular basis. Mainly powder additives too much, pigment dispersion is not equal, powder is squeezed caused by powder points. The solution is to improve the quality of the powder and improve the way the powder is stored and transported.
1.5 pre-treatment impurities. Mainly caused by phosphate slag large particle impurities and phosphate membrane yellow rust caused by small pieces of impurities. The solution is to clean up the slag in the phosphate tank and spray line in time and control the concentration and proportion of phosphate tank liquid.
1.6 water quality impurities. It is mainly impurities caused by the sand content and salt content of water used in pre-treatment. The solution is to increase the water filter and use pure water as the last two levels of cleaning water.
2, coating shrink hole
2.1 pre-treatment oil removal is not clean or after oil washing is not clean resulting in surface active agent residue caused by shrink holes. The solution is to control the concentration and proportion of pre-skimmed tank and degreasing tank fluid, reduce the amount of oil in the work parts and strengthen the washing effect.
holes caused by the high oil content of 2.2 water quality. The solution is to increase the inermissible filter to prevent the water supply pump from leaking oil.
hole caused by the water content of the compressed air at 2.3. The solution is to discharge compressed air condensate in a timely manner.
caused by moisture in the 2.4 powder. The solution is to improve the conditions of powder storage and transportation, increase the dehumidifier to ensure the timely use of recycled powder
2.5 suspension chain oil pollution by air conditioning blowing down on the work piece caused by the shrink hole. The solution is to change the position and direction of the air conditioning air port.
hole caused by 2.6 mixed powder. The solution is to thoroughly clean the powder system
3, coating color difference
3.1 powder pigment distribution caused by uneven color difference. The solution is to improve the quality of the powder, to ensure that the L, a, b difference between the powder is not large and positive and negative unity.
difference caused by different curing temperatures of 3.2. The solution is to control the set temperature and the speed of the conveyor chain in order to maintain the consistency and stability of the curing temperature and time of the work piece.
difference caused by uneven thickness of the 3.3 coating. The solution is to adjust the powdering process parameters and ensure that the powdering equipment is running well to ensure uniform coating thickness
4, coating adhesion difference
The solution is to strengthen the washing, adjust the parameters of the skimming process, and prevent the degreasing fluid from entering the phosphate tank.
adhesion caused by the yellowing, flowering or local phosphation-free membrane of 4.2 phosphate film. The solution is to adjust the concentration and proportion of phosphate tank fluid and increase the phosphate temperature.
adhesion caused by the non-clean drying of the corner moisture of the 4.3 work piece. The solution is to increase the drying
large area adhesion of the coating caused by insufficient 4.4 curing temperature. The solution is to increase the
4.5 deep well water oil content, salt content is too large caused by the adhesion difference. The solution is to increase the water indation filter and use pure water as the last 2 cleaning water. In a word, there are many application methods of powder electrostectrectrectrectrectrectrectrative spraying, which need to be used flexibly in practice.
5, powder-coated orange peel
5.1 powder coating orange peel appearance of the judgment method:
(1) visual method
in this test, the model is placed under a double-tube fluorescent lamp. The reflected light source of the model can be obtained by placing the model appropriately. Qualitative analysis of the clarity of reflected light can visually assess the nature of flow and leveling. In the case of poor mobility (orange peel), the two fluorescent tubes appear blurry and unclear, while high-mobility products provide clear reflections.
(2) Form measurement method
in this method, the surface shape is recorded by the offset of the highly sensitive probe. This allows for a quick distinction between rough, orange peels caused by shrink holes, pinholes or dirt, and poor flow caused by
5.2 avoiding orange peels
the coating appearance becomes increasingly important in new equipment manufacturing coatings. Therefore, one of the main objectives of the coating industry is to achieve optimum coating performance according to the final requirements of the user, which also includes a satisfactory surface appearance. Surface conditions affect visual effects through factors such as color, gloss, fog and surface structure. Gloss and image clarity are often used to control the appearance of the coating. However, by using a coating film with a high gloss, the surface fluctuations also affect the appearance of the entire coating film, and it is believed that gloss measurement can not control the visual effect of fluctuations, this effect is also known as "orange peel".
or micro-fluctuations are corrugated structures with dimensions between 0.1mm and 10mm. On high-gloss coated surfaces, areas of wave-like, light and dark can be seen. Two different levels of fluctuations can be distinguished: long fluctuations, also known as orange peels, which can be observed at distances of 2 to 3 degrees, and short fluctuations or microscopic fluctuations, which are observed at a distance of about 50 cm. It should be pointed out that sometimes in order to cover the surface defects of the substrate or to obtain a special coating surface appearance, the purposeful design of a certain degree of volatility or ripple structure.
therefore, "orange peel" can be defined as "the wave structure of a high-gloss surface", which gives the surface of the lacquer layer a patterned, unflowing visual appearance.
control of the visual appearance (gloss, fog, flat orange peel) of the coating film is very important, especially when assembling different parts of the coating.
factors that affect the flow and appearance of coatings in powder coatings: in industrial coatings, phase changes in the preparation and film-forming process of powder coatings are unique. The lack of solvents to moisturize and improve film fluidity makes powder coatings more difficult to remove surface defects than liquid coatings. Although the main components of the two are similar, but compared to liquid coatings, thermo-solid powder coatings based on a very different process.
powder coating is a solvent-free system. During preparation, pigments and other components are dispersed and partially encased in low-molecular solid resins through melting mixing. Powder coatings are used to transport powder through air to the end of the material (powder suspended in the air) and then by charge to attach it to the substrate. Heating at a predetermined temperature, so that powder particles melt, gather together (cluster), flow (film), and then level, this period through a mucous liquid phase wetting the surface, and finally chemical crosslinking to form a high molecular weight coating film, which is the film-forming process of powder coating. the orange peel problem of
powder coating
factors affecting the flow and appearance of the coating film: the
film-forming process can be divided into melting fusion, the formation of the coating film, leveling three stages, at a given temperature, the most important factors to control the melting polymerization speed are the melting point of the resin, the viscosity of the molten powder particles and the size of the powder particles. In order for the flow to work best, the melting fusion should be completed as quickly as possible so that there is a longer time to complete the leveling phase. The use of curing agents reduces the time required for flow and leveling, so the coating film formed by those extremely active powders often presents orange peels.
factors affecting film flow and leveling are the molten viscosity of the resin, the surface strength of the system and the thickness of the film. In turn, the viscosity of the melt depends in particular on the curing temperature, curing speed and heating rate.
the factors mentioned above, together with particle size distribution and film thickness, are usually determined by the required coating performance, coated objects and powder construction conditions.
flow and leveling when applying powder is driven by the surface pressure of the system, as mentioned earlier. This force is the opposite of the intermoleceral gravity applied to the coating film, resulting in greater resistance against flow and leveling if the viscosity of the melt is higher. Therefore, the difference between surface stress and gravitational forces between molecules determines the degree of coating film leveling. For well-liquid coatings, it is clear that the surface pressure of the system should be as high as possible and the viscosity of the melting should be as low as possible. These can be achieved
additives that increase the surface pressure of the system and resins with low molecular weight and low melting points.
coatings prepared according to the above conditions can have excellent fluidity, but due to their high surface strength will lead to shrink holes, and due to the lower melting viscosity will produce a flow hanging, and the corner coating is poor. In practice, the surface strength and melt viscosity of the system are controlled to a specific range, so that a qualified coating surface appearance can be obtained.
The effect of surface tension and molten viscosity on film flow can be seen in Figure 2. As can be seen in Figure 2. It can be seen that too low surface tension or too high molten viscosity will prevent the film flow, resulting in poor film flow, and when the surface pressure is too high, shrink holes will appear in the film process. Melting viscosity is too low will make the physical storage stability of the powder worse, the corner coating is poor during construction, and the construction on the facade produces a flow hook.
In summary, it is clear that the final surface condition, defects and deficiencies (e.g. orange peel, poor mobility, shrink holes, pinholes, etc.) obtained from the powder coating coating are closely related to each other and are also controlled by the fluid forces involved in phase change in the film-forming process.
the distribution of powder particle size also affects the surface appearance of the coating film. The smaller the particle, because of its low thermal capacity of larger particles, so its melting time is shorter than the large particles, the grouping is also faster, the formation of a coating film surface appearance is better. The melting time of large powder particles is longer than that of small particles, and the resulting coating film may have an orange peel effect. The powder electrostation construction method (corona discharge or friction discharge) is also a factor leading to the formation of orange peel.
can reduce or avoid the orange peel effect
promote flow and leveling can reduce or avoid the orange peel. The system uses lower melting viscosity, extended leveling time during curing, and higher surface stress to improve flow and leveling. Controlling the surface stress gradient is an important parameter to reduce the orange peel, while also controlling the surface tone of the coating surface is uniform in order to obtain the smallest surface area.
, flow promoters or leveling agents are often used in practical work to improve the appearance of the coating film to eliminate surface defects such as orange peels, shrink holes, pinholes, etc. Good-performance flow promoters reduce melt viscosity, which helps melt mixing and pigment dispersion, improves the wetting of the substrate, smooths the flow of coatings, helps eliminate surface defects and facilitates air release. The relationship between the amount and effect of flow modifiers should be examined. Insufficient dosage can lead to shrink holes and orange peels, while excessive dosage can lead to loss of light, fog, and the problem of re-applying the upper layer. Typically, flow modifiers are added during premixing. They are either made of resin masterens (the ratio of resin to the additive is 9/1 to 8/2), or are absorbed in powder on an inorderable carrier. The dosing of the additive in powder coatings is 0.5 to 1.5% (in effective polymers based on substations), but may also work well at lower concentrations.
polyacrylate resins are most widely used in
flow modifiers, such as polyacrylate butyl esters ("Acronal4F"), acrylates-acrylic acrylic acetate co-polyesters and acrylates-acrylates co-polymers. They can be used in a wide range of concentrations. Polyacrylates generally have little effect on surface stress and can help the coating form a relatively uniform surface. They do not reduce surface stress compared to additives that reduce surface stress, such as silica, and can therefore be used to accelerate leveling. Additives that reduce surface stress include surfactants, fluorinated alkyl esters, and sioxane. They are very sensitive to the amount added. Resting incense is a degassing agent, but also has the effect of reducing surface pressure, is widely used to improve the surface appearance of powder coating coating.
6, there are bubbles in
the main reason: the powder contains volatile substances and water. The surface of the work piece has water, compressed air has oil or water
method: strengthen the storage of powder, moisture protection, drying the surface moisture of the work piece. De-oiling compressed air, dehydration
7, coating pinholes, concave film
reasons: the coating is too thick, resulting in static rejection, spray gun from the work piece too close, resulting in fire break-through, the surface of the work piece has grease and moisture, powder moisture, compressed air contains oil or water, the work piece itself has pinholes,
8, coating thickness is not equal
powder spray speed is not equal, compressed air is unstable, powder supply device fluidization effect is not good, the speed of the conveyor chain is not stable. The fluidization effect caused by the moisture-trapped powder is good.
9, coating flow
cause: the coating is too thick, heating too fast, curing temperature is too high, the coating before baking curing is not uniform,
10, poor coating gloss, color change
powder heat resistance performance is poor, curing temperature is too high or too long, powder and curing process time interval is too long, pre-treatment desoluing is not clean, powder supply and powder system, recycling system, such as cleaning is not clean, mixed with other varieties or colors of powder.
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