How to ensure the quality of the coating on the surface of the steel drum (2)
Coating film defects during the coating process or after coating are generally related to the pre-coating state of the steel energy surface, the coating process, the paint quality, the coating method and operation, the coating equipment and the environment. Common defects and the causes and improvements are shown in the table below:
Common defects
Causes
Improvements
1 sag, sag and flow marks
Such defects cause uneven streaks and flow marks in the paint film due to the abnormal flow of the paint on the vertical surface. Sagging is generally proportional to the specific gravity of the coating, inversely proportional to the viscosity, and proportional to the cube of the thickness of the coating.
1 The solvent evaporates slowly. The temperature is too low, almost no ventilation, and the solvent vapor content in the surrounding air is high.
2 paint blending viscosity is too low.
3 thickness is too thick. The spraying distance is too close, the spraying angle is not appropriate, and the spray width is too large to overlap too much.
1 Strictly control the temperature, properly ventilate, in line with the process standards.
2 Strictly control the application viscosity of the paint and mix it according to the standard.
3 Control the spraying distance and the effective spray width, and move the spray gun parallel to the surface of the steel drum to make the spray thickness uniform and control it at 20~25 microns.
2 coarse grain, 疙瘩
This defect is caused by the formation of protrusions on the dry paint film, which are distributed in the form of particles on a part of the surface of the steel drum.
1 There is dust in the painting room, and the air in the drying room is not clean.
2 The coating is not well filtered, the easily precipitated coating is not well stirred, the patent leather is mixed in the paint or the modified coating is used.
1 The dust removal in the painting room should be sufficient, and the air in the drying room should be strictly filtered and purified.
2 The paint preparation must meet the process requirements, carefully filter, fully stir, and strictly prevent the paint and other mixing.
3 exposed, poor coverage
This kind of defect mainly refers to the phenomenon of leaking or coating too thin and bottoming out.
1 The effective spray width of the spray gun is too small to leak the spray at the center of the top and bottom of the steel drum.
2 The viscosity of the coating is too thin, the coating is too thin; the mixing and mixing of the coating is insufficient, and the precipitate is not stirred and is put into use.
1 Reasonably adjust the effective spray width of the spray gun.
2 According to the process requirements, the paint should be mixed. The paint must be stirred evenly before being put into use.
4 pinhole
Pinhole refers to the production of needle-like holes or pores like leather on the paint film, with a pore size of about 100.
1 Coating: The surface tension of the coating is too high, the fluidity is poor, the flattening property is poor, and the bubble release is also poor.
2 Painting: The steel drum paint is not well treated beforehand, and the surface remains water, oil and other debris; the paint is stirred vigorously for a long time, mixed with air to generate numerous bubbles, and the viscosity is too high; the air pressure during spraying is too high, and the solvent balance is destroyed; The drying solvent evaporates too quickly and the heating rate is too fast.
An effective way to prevent pinholes from appearing on the surface of the dried paint film is to eliminate the aforementioned causes. Strictly control the construction viscosity, reduce the viscosity and control the parameters of the coating process in actual production, which can effectively eliminate the large number of pinholes.
5 orange peel
When spraying, a smooth dry paint film cannot be formed, and the orange-like shape has an unevenness of about 3
1 The coating viscosity is too large and the leveling property is poor.
2 The spraying pressure is insufficient, the atomization is bad; the spraying distance is too far, the spray gun runs too fast, and the spraying thickness is not enough.
3 The drying temperature is too high, or enter the high temperature zone too early, and the flux evaporates too quickly.
1 Adjust the viscosity of the coating application.
2 control the compressed air pressure to achieve good atomization; adjust the spraying distance and the speed of the spray gun so that the spray thickness is not less than 20
3 Control the drying specifications to meet the requirements of the process standards.
6 wrinkles
The phenomenon that the paint film produces wrinkles like streaks during drying is called wrinkling.
1 The paint film is too thick and is beyond the normal.
2 The viscosity of the coating is too large.
3 Suddenly high temperature accelerates baking and drying, and the paint film will have a greater tendency to wrinkle.
4 The solvent evaporates too quickly and the coating is easy to wrinkle.
1 Strictly control the thickness of the spray.
2 According to the process requirements, the coating is formulated to meet the requirements.
3 According to the drying technical specifications, the process conditions for gradual heating and drying are formulated.
4 control the solvent evaporation rate.
7 poor drying
After painting, it is dried according to the drying specifications specified by the technology, but the film is not completely cured, the film is wet, the hardness is low or the surface is not dry, which is poor drying.
1 Remaining water or oil on the surface of the steel drum.
2 The compressed air used for spraying contains oil.
3 The spray coating is too thick.
4 Drying oven has poor ventilation, low temperature, high humidity and short drying time.
1 Strengthen the surface treatment of the paint to remove oil and moisture from the compressed air.
2 Control the thickness of the spray.
3 Ensure that the technical status of the drying room is good.
8 rusty
Shortly after the steel drum was painted, red silk appeared under the paint film or rust spots appeared through the paint film.
1 The steel drum has problems in pre-painting treatment, the surface quality is poor, and it has rust and has not been removed.
2 The coating is incomplete. If there are defects such as pinholes and leaky coating, the corrosion resistance of the paint film is poor.
3 When the stack is damaged, the paint film is damaged, and the storage air humidity and temperature are too high.
1 The surface of the paint must be cleaned, and it is possible to perform phosphating.
2 to ensure the integrity of the coating.
3 Do not damage the coating when stacking, do not roll on the site; the warehouse should pay attention to ventilation, and it is strictly prohibited to stack in the open air.
9 becomes brittle
The phenomenon that the paint film loses its elasticity or the elasticity is deteriorated is called brittleness, which is a prelude to the cracking or peeling of the paint film.
1Overheating drying, that is, the drying temperature is too high, and the drying time is too long to make the paint film brittle.
2 The paint is out of date, and the poorly painted paint film is also easy to become brittle.
3 The paint film is used under too low temperature conditions.
1 Drying according to the drying conditions recommended by the process regulations or technical conditions. If the drying room needs to be repaired for a long time due to failure, the drying temperature of the drying room should be lowered or the steel drum dried in the drying room should be taken out in time.
2 Use high-quality paint, pay attention to the expiration date.
3 Steel drums used under low temperature conditions should be coated with special coatings.
Third, the steel surface coating requirements for the environment
Another important factor to ensure the quality of the surface of the drum is to have a suitable coating environment, which includes the following:
1. Spraying and drying indoors have good lighting and uniform brightness
Appropriate brightness should be provided in the coating operation area (spraying room) on the surface of the steel drum, but direct sunlight and too strong light should be avoided to facilitate operation.
2. The temperature and humidity in the spray booth are moderate.
The temperature and humidity of the air have a great influence on the surface coating performance of the steel drum. If it is not properly controlled, it will cause various defects in the surface coating of the steel drum. Different coatings require different temperatures and humidity when applied due to their different volatility and workability. Below are the ambient temperature and humidity requirements for some coatings.
Paint name
Temperature (°C)
Relative humidity%
Remarks
Alkyd resin coating
Nitrocellulose coating
Modified amine curing epoxy coating
Polyamide curing ring coating
Polyurethane coating
10~30
15~20
10~30
15~30
5~30
≤85
≤70
≤85
≤85
≤70
High temperature
Over temperature and humidity are not good
High temperature
High temperature
Bad humidity
3. When spraying the surface of the steel drum, the indoor air should be cleaned.
The indoor air sprayed on the surface of the steel drum should be kept dry, clean, dust-free or dust-free. Because the dust in the spray booth adheres to the paint film on the surface of the steel drum during spraying, it not only affects the appearance quality of the steel drum, but also reduces the coating performance and durability of the steel drum surface. Therefore, the indoor air spray should be strictly controlled during the production process. Circulation cleanliness.
4, spray drying room should be well ventilated, appropriate ventilation
The air in the spray drying chamber should be continuously flowing. On the one hand, the coating can be accelerated to dry, so that the concentration of harmful substances in the spray drying chamber can be diluted, and the sanitary conditions in the workshop can be improved.
5, spray drying room should be fireproof and explosion-proof
A large amount of solvent vapor is volatilized during the surface coating process of the steel drum. These solvent vapors are mixed with air to form an explosive gas at any time, and are prone to explosion and combustion if exposed to fire. Therefore, open flames are not allowed in areas with flammable liquid volatiles, and all electrical equipment is explosion-proof. A sufficient number of fire extinguishers must be installed to ensure safe production.
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