Case analysis of shrinkage cavity in the hottest p

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Case study of shrinkage cavity caused by powder coating case study of shrinkage cavity caused by powder coating June 13, 2019 shrinkage cavity caused by spray of catenary oil in a refrigerator factory, as the backbone of Shandong new material industry, the oil pipe connecting the catenary refueling tank fell off, compressed air discharged the catenary oil, and atomized the last small amount of catenary oil into oil drops that can be floated, which entered the spraying system close to the height. During the period of catenary oil leakage, the sprayed workpieces are all shrinkage cavities

the obvious characteristic of the shrinkage cavity is that oil enters the uncured powder coating layer to form a free low surface tension center coating. Such shrinkage defects can be reproduced by shooting diesel oil and gasoline into the powder coating layer before curing with fingers, but the diesel oil and gasoline shot on the film of the national large-scale experimental machine manufacturer of phosphating Jinan new era Gold Testing Instrument Co., Ltd. will not appear shrinkage after being covered by powder coating, and the oil will not appear shrinkage after being poured into the powder coating and then sprayed

indicates that the presence or absence of low surface tension centers is directly related to the presence or absence of shrinkage cavities

2. Shrinkage cavity occurs when paint thinner is added to the spray pretreatment phosphating solution

when painting at the site of a refrigerator factory, a phosphating solution barrel was used to contain paint thinner. The workers mistakenly added the thinner into the phosphating solution, resulting in the generation of batch shrinkage. The shrinkage cavity disappeared after the phosphating solution was replaced

the uniformly dispersed low surface active material on the substrate will not appear shrinkage, also because there is no isolated low surface tension center. However, the non-uniformity of solvent distribution in phosphating solution and the non-uniformity caused by spray system lead to shrinkage defects

the reason in this case is simple and clear

3. Shrinkage caused by adding emulsifier into the pretreatment phosphating solution

the trough degreasing effect of a radiator manufacturer is not good. The suppliers of pretreatment raw materials separately provide emulsifiers to the manufacturers, especially some technical enterprises, to the degreasing tank to enhance the degreasing effect. Because the operator understood the reason, he mistakenly added the emulsifier to the phosphating solution, resulting in serious shrinkage at the solution aggregation

in this case, the water-soluble surfactant is not evenly dispersed and cannot be cleaned. After drying, the welding of pipe fittings can be seen The appearance is similar to that of the oil. It is also a surfactant. Due to its uniform dispersion and easy cleaning in degreased grains, shrinkage cavities will not occur in normal production

4. Shrinkage caused by abnormal drying equipment

2013, the compressor of the equipment used for compressed air freeze-drying was damaged, and the first level of refrigeration purification could not be carried out during maintenance. There are continuous shrinkage defects during this period. After cleaning, it can be sprayed normally for a certain time, but shrinkage defects will occur again soon

in addition, the cooling fan of the freeze dryer was damaged, and the non heated (or slightly heated) regenerative adsorption dryer worked abnormally, resulting in excessive shrinkage defects. Pure water will not cause shrinkage in any form, but water and oil in compressed air exist as mixed emulsion, and oil is the material causing shrinkage

5. Shrinkage due to powder interference

2013, after trying out the powder raw materials from a new manufacturer, the workpiece was coated with the original coating, resulting in a large number of fine shrinkage holes with a diameter of about 1 mm; However, when another powder coating was applied to another coating for interference test, a large number of shrinkage cavities with a diameter of nearly 5 mm appeared, "said Ralf usinger, innovation and industrialization manager of ruag spac e (CH), while the template was normal when spraying in equal proportion

interference between powders is a common occurrence. Using different resins, leveling agents or other additives, each particle of powder may become a center of low surface tension and cause shrinkage, while the residual powder with high surface tension will not affect the powder coating with low surface tension. If the tension data of each kind of powder can be obtained, the interference between powders can be avoided. Directly adding a very small amount of powder to be tested (less than 1%) to the background powder can visually check whether there is interference. The degree of interference between powders determines the cleanliness of the site to be cleaned. For filter elements that are difficult to clean, only replace them with new ones to eliminate shrinkage

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