Method for judging the appearance of orange peel of powder coatings

Update:08 Aug,2022
In the daily powder coating work, we will often encounter some problems that are more troublesome to solve, and will also affect the quality of work. Let's take a look at the problems we will encounter and the methods to solve them.

1. Coating impurities
Common impurities mainly come from particles in the dusting environment and impurities caused by various other factors, which are summarized as follows.​​

Impurities in the curing oven. The solution is to thoroughly clean the inside of the curing oven with a damp cloth and vacuum cleaner, focusing on the hanger chain and air duct gaps. If it is black large particles of impurities*, you need to check whether the air supply duct filter is damaged, and replace it in time.​​

Dusting indoor impurities. Mainly dust, clothing fibers, equipment abrasives and dusting system fouling. The solution is to use compressed air to purge the dusting system every day before starting work, and thoroughly clean the dusting equipment and dusting room with a damp cloth and a vacuum cleaner.​​

Dangling chain impurities. It is mainly the product of the suspension chain oil baffle and the primary spreader water receiving plate (material is hot-dip galvanized sheet) corroded by pre-treatment acid and alkali vapor. The solution is to regularly clean these facilities

Powder impurities. Mainly due to excessive powder additives, uneven pigment dispersion, and powder spots caused by powder extrusion. The solution is to improve powder quality and improve powder storage and transportation methods.​​

Pre-treatment impurities. Mainly are large particles of impurities caused by phosphating slag and small impurities caused by phosphating film yellow rust. The solution is to clean up the slag in the phosphating tank and the spray pipeline in time, and control the concentration and proportion of the phosphating tank.​​

water impurities. It is mainly impurities caused by excessive sand content and salt content in the water used for pretreatment. The solution is to add a water filter and use pure water as the final two-stage cleaning water.​​

2. Coating shrinkage
Shrinkage cavities caused by residual surfactants caused by unclean pre-treatment degreasing or unclean water washing after degreasing. The solution is to control the concentration and proportion of the pre-degreasing tank and the degreasing tank liquid, reduce the amount of oil on the workpiece and strengthen the washing effect.​​

Shrinkage cavities caused by excessive oil content in water. The solution is to increase the water inlet filter to prevent the oil supply pump from leaking.​​

Shrinkage cavities caused by excessive water content in compressed air. The solution is to discharge the compressed air condensate in time.​​

Shrinkage cavities caused by moisture in the powder. The solution is to improve the powder storage and transportation conditions and increase the dehumidifier to ensure the timely use of the recovered powder.

Shrinkage cavities caused by the oil stains on the suspension chain being blown onto the workpiece by the air-conditioning wind. The solution is to change the location and direction of the air conditioning vents.​​

Shrinkage caused by powder mixing. The solution is to thoroughly clean the powder spraying system when changing the powder

3. Coating color difference
Color difference caused by uneven distribution of powder pigments. The solution is to improve the quality of the powder to ensure that the L, a, and b of the powder are not much different and the positive and negative are unified.​​

Color difference caused by different curing temperature. The solution is to control the set temperature and conveyor chain speed to maintain the consistency and stability of the workpiece curing temperature and time.​​

Color difference caused by uneven coating thickness. The solution is to adjust the powder spraying process parameters and ensure that the powder spraying equipment operates well to ensure that the coating thickness is uniform.

4. Poor coating adhesion
Incomplete pre-treatment water washing results in poor adhesion caused by residual degreasing agent, chromed slag on the workpiece, or the washing tank is polluted by lye. The solution is to strengthen water washing, adjust the degreasing process parameters and prevent the degreasing liquid from entering the washing tank after phosphating.​​

Poor adhesion caused by yellowing, blooming or partial absence of phosphating film. The solution is to adjust the concentration and proportion of the phosphating bath and increase the phosphating temperature.​​

Poor adhesion caused by unclean moisture in the corners of the workpiece. The solution is to increase the drying temperature

Poor adhesion of the coating on a large area caused by insufficient curing temperature. The solution is to increase the curing temperature

Poor adhesion caused by excessive oil and salt content in deep well water. The solution is to add a water inlet filter and use pure water as the last 2 cleaning water. In short, there are still many powder electrostatic spraying technology and its application methods, which need to be used flexibly in practice.

5. Powder coated orange peel
Judgment method of orange peel appearance of powder coating:
(1) Visual inspection method
In this test, the panels were placed under double-tube fluorescent lamps. The reflected light source of the template can be obtained by placing the template appropriately. Qualitative analysis of the clarity of reflected light* allows a visual assessment of flow and leveling properties. In the case of poor flow (orange peel), the two fluorescent tubes looked blurry and unsharp, while high-flow products achieved clear reflections.​​

(2) "Appearance measurement method"
In this method, the surface shape is recorded by the deflection of a highly sensitive probe. This makes it possible to quickly distinguish roughness, orange peel and poor flow caused by craters, pinholes or dirt.

Avoid orange peel
Coating appearance is becoming more and more important in new equipment manufacturing coatings. Therefore, one of the main goals of the coatings industry is to maximize the performance of the coatings according to the final requirements of the user, which also includes satisfactory surface appearance. Surface condition affects visual appearance through factors such as color, gloss, haze, and surface structure. Gloss and image clarity are often used to control the appearance of coatings. However, even with highly glossy coatings, the fluctuation of the surface affects the appearance of the entire coating, and it is believed that gloss measurement cannot control the visual effect of fluctuations, which is also known as "orange peel".

Orange peel or micro wave is a corrugated structure with a size between 0.1mm and 10mm. On high gloss coated surfaces, one can see wavy, light and dark areas. Two different levels of fluctuations can be distinguished: long fluctuations, also known as orange peel, which are fluctuations that can be observed at intervals of 2 to 3 distances; short fluctuations or micro fluctuations, which are observed at about 50cm distance to the fluctuations. It should be pointed out that a certain degree of undulation or corrugation is purposely designed in order to cover the surface defects of the substrate or to obtain a special coating surface appearance.

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