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Prevention and Control of Common Defects for Paint Construction in Summer



1 Introduction
Excellent coating appearance needs stable technological support, and the change of process parameters will directly affect the quality of automobile body. The quality of automobile body in high temperature production in summer is a major problem that puzzles many manufacturers at present. How to produce good products in the transitional stage of process parameters change requires high temperature in summer. The following are opinions and control methods for various quality problems that occur in summer.

2. Prevention and Control of Common Defects in Paint Construction in Summer
2.1 Orange Peel

Definition: The orange peel-like ripple on the surface of the coating is more than 1mm.


Causes:
1) The viscosity of the coating is on the high side, and the leveling agent or solvent used in the coating is improper.
2) The distance between the spray gun and the coating is relatively long, the coating film is thin, and the ground is rough and uneven.
3) Paint atomization in spraying is poor due to the small amount of paint vomiting or low spraying pressure.
4) The spray chamber temperature of the car body is higher during spraying, and the leveling agent in the paint does not increase the slow drying agent according to the change of spray chamber temperature, which causes the solvent volatilization in the paint to be faster.
5) Coatings drying time is short.

Preventive measures:
1) In the process of high temperature spraying in summer, the paint viscosity should be reduced or the high boiling point solvent should be increased according to the temperature change of spraying chamber.
2) To adjust the distance between the spraying gun and the coating, it is generally required that the distance between the spraying gun and the coating be 20-750px/s, and the thickness of the paint film be increased appropriately.
3) The paint pressure is controlled between 4 and 6 bars, and the spraying parameters of the automaton and the manual spraying gun are adjusted to a good state.
4) The general drying time should be controlled in 5-10 minutes.
5) The base material is polished flat.

Remedies:
1) Check the coating condition of automobile body in time after spraying. When orange peel appears on the coating, temporary manual repairing is used on the spot, and the process parameters are reported to the technicians for adjustment.
2) grinding and polishing the orange peel or re-spraying and drying the orange peel.

2.2 pinholes
Definition: During the drying process, the surface of the paint film has been dried and formed, but because the additives continue to evaporate, the needle-shaped holes formed through the initial drying surface of the paint film or the holes like leather pores are called pinholes.

Causes:
1) The temperature of spraying room is higher, and the surface of spraying film dries faster.
2) After spraying, the car body dries too quickly, and the heating zone of the drying room rises too quickly.
3) improper leveling agents and solvents used in coatings.
4) The coated substrate has small holes which are not polished, and the top coat is sprayed directly without spraying and filling.
5) The paint contains water.

Preventive measures:
1) Observe the dry and wet condition of the coating after spraying, and increase the leveling agent appropriately.
2) Adjust the drying temperature according to the technological requirements, and require the heating curve of the drying furnace to rise steadily.
3) Choose appropriate leveling agent or solvent.
4) The body base material should be polished and smoothed according to the requirements, and then sprayed with paint after spraying.
5) Strengthen the inspection of imported paint, pay attention to the water content in the paint during the process of mixing, recycling and using.

Remedies:

1) Use wet or dry circular polishing method to eliminate pinholes on primer surface and then re-spray mid-coat. Dry topcoat body can be polished and polished.


2.3 Hanging, sinking and flowing
Definition: The phenomenon that too many coatings are solidified and firmly adhered to the vertical surface and edge of the coated surface is called "hanging". The phenomenon of local vertical flow of the coatings from finishing to drying is called "sinking" because of the semicircle and ripple of uneven thickness, and the phenomenon of large area deposition of coatings on the coated surface is called "flowing". "

Causes:
1) The distance between the spray gun and the coating is too close, the moving speed of the spray gun is too slow, and the one-time spraying is too thick.
2) The spraying pressure is too low, the spraying range is small, and the amount of paint produced by the spraying gun is large.
3) The diluent used is not compatible with the paint, the high boiling point solvent content in the paint is more, and the paint viscosity is too low.
4) The air velocity of the spray chamber is not in the range of technology; the ambient temperature is too low.
5) The drying time of wet impact wet spraying is shorter.
6) Non-uniform spraying, slow surface drying at the thickness of the film, such as its lower part is very easy to form flow hanging.

Preventive measures:
1) Strictly according to the process requirements for spraying, for the higher requirements of the film can be divided into multi-channel spraying, each spraying needs to be air-dried before the next spraying.
2) Make adjustment of spraying gun to ensure that spraying equipment is in normal condition.
3) Check the viscosity of the coating and the spraying pressure in the normal process range.
4) Raise the temperature of the spray room within the required range, and measure the wind speed to ensure that the wind speed meets the requirements.
5) Choose the right diluent.

6) Make sure the surface is clean when spraying.


Remedy: After drying the film, wet grinding and polishing the convective hanging place with fine sand paper, scraping the larger convective hanging place with a scraper, then grinding the convective hanging place to no flow mark and re-spraying and drying.

2.4 chromatic aberration
Definition: There are differences in hue, brightness, purity and standard color plate after spraying or measuring with color difference meter.

Causes:
1) There are differences among batches of coatings.
2) In spraying, the spraying gun cleaning is not clean and mixes or crosses colors appear.
3) Inappropriate matching of primer and topcoat, and over-baking during drying.

Preventive measures:
1) Strengthen the inspection of paint entering the factory, and the inspection of paint color difference shall be strictly carried out in accordance with the company's standards.
2) Implement process inspection and operate in strict accordance with the operation instructions.

3) Optimize the matching relationship between the colors of the top coat and the middle coat according to the requirements, and solidify after the color difference is no problem.


Remedy: Painting should be done in time after the color difference occurs in the field spraying. When the color difference is serious, fine sandpaper should be used to polish, spray and dry again, and standard plate comparison or color difference meter should be used to measure the color difference of the car body to ensure that it meets the requirements.

2.5 Shrinkage, Shrinkage, Fisheye
Definition: Due to the presence of foreign substances on the surface of the coated or mixed into the coatings (such as oil, water, silicon chamber, etc.), the paint can not adhere evenly, resulting in shrinkage and exposed to the coated surface, or the phenomenon of uneven film is called shrinkage hole. The larger and irregular dew area is called shrinkage, commonly known as "laughter"; the round (more than 0.1-2 mm in diameter) is called shrinkage hole; and the particles in the round hole are called "fisheye".

Causes:
1) Employees do not wash their hands before operation, wear work clothes, gloves and other unclean, there are pollution sources.
2) The surface of the coating is contaminated by residual stains such as frog chamber, water, oil, soap and detergent.
3) Employees are not clean when painting or spraying tools and equipment, as well as gas pipelines, resulting in pollutants mixed into the paint.
4) High surface tension, poor leveling and poor shrinkage resistance of the paint itself.

Preventive measures:
1) Employees should clean their hands before operation and wear clean and tidy work clothes.
2) Ensure that the surface of the coating is free from pollution, and that the compressed air is clean, oil-free and water-free after pollution occurs.
3) Ensure that the painting environment is clean and there should be no dust, oil mist and paint mist floating in the air.
4) Painting equipment, tools, paint pipes and auxiliary materials for production should not contain harmful substances causing shrinkage, especially organosilicon compounds.

Remedies:
1) Grinding to the substrate to eliminate active pollutants, requiring primer to be repaired before drying.

2.6 Undercover
Definition: insufficient film thickness or poor covering power of paint, can see the phenomenon of background color.

Causes:
1) The inadequate mixing of the paint before use results in slurry precipitation or poor covering power of the paint used.
2) improper selection of additives, low construction viscosity of paint.
3) Failure to carry out multi-channel spraying or insufficient single-layer paint film in accordance with process requirements.
4) The color difference between primer and topcoat is great.

Preventive measures:
1) Paint should be fully stirred before use, appropriate diluents should be selected, inspection of paint materials should be strengthened, and high-quality coatings with strong covering power should be selected.
2) Spraying according to the technological requirements and meeting the technological requirements of the single layer film thickness.
3) Improve the proficiency of spraying operation and operate cautiously.
4) Reasonable matching of primer and topcoat color, light-colored primer matching light-colored topcoat, dark-colored primer matching dark-colored topcoat.

Remedy: When the wet film is found to be exposed, it should be repainted and covered in time. When the dry film is found, it needs to be polished and re-sprayed to be dried.

2.7 Hair Flowers
Definition: The color of the film surface is not uniform, and the appearance of patches, stripes and color confusion.

Causes:
1) Poor dispersion of pigments in paints or inadequate mixing of more than two kinds of paints, insufficient solubility of solvents or inappropriate construction viscosity.
2) The thickness of the film is uneven, and the pigments in the film are convective on the inner surface.
3) poor spraying technology. Not in accordance with the three principles of spraying (spraying distance, spraying speed, pattern overlap width) operation.

Preventive measures:
1) Choose pigments with good dispersibility and mutual solubility.
2) Choose appropriate solvents, and adopt viscosities and film thickness that meet the technological requirements.
3) The same type of paint should be used when mixing composite paint, and the same type of paint produced by the same manufacturer should be better used.
4) Apply good spraying technology. The width of spray pattern should be kept constant, and the degree of overlap is 1/4-1/3 of effective spray.

Remedy: Flowering appears in the wet film of the car body after spraying paint. After the paint surface is slightly dried, the correct spraying technology is used to spray a topcoat. After drying, the car body should be polished and repaired to dry.

2.8 Poor gloss and low gloss
Definition: Gloss represents a 0-100 m ripple, i.e. the coating does not achieve the desired gloss after drying or the coating appears gloss decline and haze shortly after coating.

Causes:
1) The miscibility of resin is poor without using diluent in summer and winter.
2) Coated surface is rough, the amount of paint absorbed by the coating is large and uneven.
3) Surface contamination before spraying.
4) Excessive atomization due to high spraying pressure or low paint viscosity.

5) the poor exhaust of the paint room or improper air flow causes the spray to fall back to the surface that has been sprayed.


Remedy: Let the film dry and solid thoroughly, and polish and polish, so that the luster can be reproduced.

2.9 Poor Freshness
Definition: Represents the ripple between 100m~2mm, which is manifested in freshness, image depth, image clarity, smoothness and luster of the film surface. It is one of the decorative properties of the film. Poor freshness is the poor decorative property of the coating.

Causes:
1) The surface of the coated material has poor smoothness, rough surface and inadequate Roughness Grinding precision.
2) The selected paint has poor leveling or its own gloss and fineness are not up to the standard, and its freshness is not good.

3) The coating environment is poor, and the surface of the coating produces particles or lacks luster. 

4) When spraying, the paint atomizes badly and the orange peel is serious.

5) The thickness of the coating is insufficient and the fullness is poor.

Preventive measures:
1) Improve the accuracy of surface preparation and leveling and grinding process to make the coated surface smooth and smooth.
2) Choose paints with good leveling, fineness and luster.
3) Improving the painting environment, high decorative coating should be carried out in clean and dust-free spraying room and dry place. The air cleanliness of high decorative spraying room is generally measured by laser ion analyzer. The cleanliness standard is generally controlled by 2.83L/min dust in 1-5 micron < 50 000; 6-10 micron < 10; and above 10 micron is zero. 。
4) Choose spraying gun with good atomization performance, master correct spraying technology and construction viscosity, and make paint achieve good smoothness.
5) High decorative coatings generally use multi-coating coating system to increase the thickness of the coating, in order to improve the fullness and smoothness of the coating.

Remedy: If the coating is dry and has poor fresh reflectivity after polishing and polishing, a repairing paint with better fresh reflectivity should be selected and re-sprayed.

2.10 Sandpaper grain and grinding mark
Definition: After the body is sprayed and dried with topcoat, the effect of sandpaper grinding marks on the appearance can still be seen on the coating surface. The deeper grain marks left on the coating surface are grinding marks.

Causes:
1) The selected sand paper model does not match the paint surface.
2) The condition of polishing tools is not good.
3) When polishing the car body, the local force is too strong or polished with one finger.
4) When the thickness of the topcoat film is insufficient, it often shows "abrasion marks".

5) The surface of the coating is not completely dry.


Preventive measures:
1) Choose the appropriate type of sandpaper, 400_water sandpaper is used for general electrophoretic coating, 800_1000
2) Maintain and use tools in time and select good equipment.
3) The body is polished with the palm of the hand.
4) Increase the coating thickness to the range of technology.
5) After the coating is dried thoroughly, it is grinded again.

Remedy: Sand paper grain found in spraying should be covered by repairing paint in time, and not too thick repairing paint. Dry film should be grinded and re-sprayed to drying.

3 Concluding remarks
Common paint defects of painting body need to be prevented in advance, and the production department should establish an implementable long-term management system to improve production efficiency.