Orange peel is a common and difficult leveling problem in the powder coating application process. There are many factors that greatly affect the smoothness of the coating film. The orange peel-like surface has a great influence on the decorative appearance of the product.
Orange peel is an urgent problem for powder coating institute manufacturers, using powder coatings. The occurrence of orange peel in powder coatings is inseparable from the characteristics of powder coatings, that is, leveling performance, the influence of equipment in coating production, and the control of process parameters in the coating process.
1. Influence of powder coating characteristics
1.1 The influence of different molecular weights resins in powder coatings
The choice of the molecular weight of the resin in the synthesis of powder coatings is important. Although the high molecular weight resin has good weather resistance, when the powder is leveled, the powder in the molten state has a higher viscosity and is more difficult to level, resulting in the appearance of orange peel. From the point of view of viscosity, the choice of low molecular weight resin is beneficial to improve the leveling performance of powder coatings. However, the low molecular weight resin has poor weather resistance, which requires powder coating manufacturers to comprehensively consider the selection of synthetic raw materials.
1.2 The influence of powder particle size distribution
Generally speaking, when the proportion of powder with particle size greater than 75 μm is too much, it will cause the leveling performance of the powder-coated film to deteriorate and easily produce the orange peel phenomenon. At the same time reduce the powder unit spraying area, and in the construction process, the electrostatic effect of the powder coating becomes worse, easy to cause the powder off.
When the particle size of less than 10 μm powder accounted for a large proportion, due to the powder coating dry powder flow is slightly worse, will cause the spraying of powder, sticky gun and other spraying construction problems.
1.3 The influence of leveling agents
Besides resin, there are many other materials that need to be melt mixed together, for example, flow modifiers, leveling agents, and several other additives. These materials will all influence the powder coatings.
Leveling agent is an important additive in the production of powder coatings, manufacturers in the production of powder coatings, are adding leveling agents in order to improve the leveling performance of powder coatings and even coating performance, improving the flatness of the coating film, reducing the orange peel phenomenon.
The stability of powder coating explains the phenomenon of cross-linking reaction of powder coating molecules under storage conditions; the more intense the cross-linking reaction of powder occurs, the larger the molecular weight of powder becomes, reflecting the increased viscosity of powder under curing temperature, the leveling characteristics become worse, and the orange peel is easy to appear.
2. Cause analysis and solution of orange peel problem caused by equipment in the powder coating process production
As the gun nozzle wear, resulting in poor spray gun atomization, making the powder adsorbed on the surface of the workpiece thickness gradient is larger, thicker coatings leveling more difficult, and easy to produce orange peel phenomenon. For this situation, you can use to reduce the amount of powder and increase the amount of compressed air output to improve the atomization performance of the gun.
2.2 The influence of the hanger
When the workpiece enters the powder spraying chamber, due to the deformation of individual hangers, the workpiece deflects with the shape of the hanger, so that the surface of the workpiece is not perpendicular to the spray gun, and the part far from the spray gun has too thin powder coat film, poor leveling, and serious orange-peel effect.
Although the part closer to the spray gun has a thick coating film, which is conducive to leveling, the impact force of the compressed air makes the thick coating film produce larger ripples, which worsens the leveling and causes an orange-peel effect.
In production, if the hanging workpiece is found to be twisted and deformed, it is necessary to replace the hanger in time, ensure that the workpiece is in a good hanging state on the hanger, and ensure that the surface of the workpiece is perpendicular to the spray gun during powder spraying.
2.3 Influence of Nozzle
In production, due to the collision between the powder particles and the friction between the powder particles and the nozzle, the generated heat energy causes a small number of powder particles to scale in the nozzle, causing the nozzle to block and the ejection pressure to be insufficient. Output decreased.
Corresponding to the decrease in the surface powder thickness of the workpiece sprayed from the spray gun, the orange-peel effect also occurs. Therefore, the nozzle of the spray gun needs to be cleaned at the end of each production to remove the scaling in the nozzle in time.
3. Influence of process conditions on the problem of orange peel in the powder coating process and the solution
3.1 The effect of time
If the leveling time is too short and the powder has not leveled, the resin molecules in the powder and the curing agent will undergo polycondensation and addition reaction to form a macromolecular network structure. This inhibits the leveling of the powder, makes the leveling worse, and is prone to the orange peel phenomenon. By reducing the line speed and prolonging the leveling time, a smooth powder coat surface can be obtained and the occurrence of orange peel problems can be reduced.
3.2 Influence of temperature
4. Cause analysis and solution of orange peel problem caused by environmental factors
During the spraying construction, due to excessive ventilation, the air velocity in the powder spraying room is too fast, so the powder particles ejected from the spray gun are not only pushed by the electric field force, electrostatic charge, gravity and compressed air, but also affected by the external wind force, and the movement mode of the powder is changed.
This causes the uneven thickness of the powder adsorbed on the surface of the workpiece, affects leveling and produces orange peel. Therefore, during the coating production, the ventilation in the powder spraying room should be properly maintained to reduce and avoid the occurrence of orange peel problems caused by the influence of external factors on the powder spraying operation.
5. Cause analysis and solutions of orange peel problems caused by other factors
During metal fabrication, metal can conduct the circuit through the charged powder. But the surface of the workpiece is not smooth, which affects the leveling of the powder coating and is easy to produce orange peel. Therefore, before construction, the uneven parts of the workpiece shall be polished to a uniform finish to meet the requirements, so as to reduce the occurrence of orange peel.
5.2 Influence of phosphating film
If the phosphating film is rough and uneven or the phosphating film is discontinuous, orange peel can also appear after powder spraying. In this case, the workpiece needs to be phosphate again after pickling. Powder spraying shall be carried out after the phosphating film meets the spraying requirements.
The orange peel phenomenon in the coating process of powder coating is caused by many factors. Especially for UV light(ultraviolet light) and the sun in outdoor applications, even if the durable finish and hard coating will also suffer certain damage. In order to reduce and eliminate the orange peel problem, powder coating manufacturers and users need to work together to solve it.
Manufacturers can adjust the powder coating formula to improve the leveling performance of the powder coating itself, so as to avoid orange peel problems; Powder coating manufacturers can reduce the orange peel phenomenon by strengthening equipment management and daily maintenance; At the same time, the spraying process is improved to effectively control the orange peel problem and achieve the appearance decoration of powder coating.
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