Metal parts paint process Oct 11, 2021

1. Substrate:
The base materials of the hardware baking varnish are:
Cold rolled steel sheet stampings or plates
Galvanized steel stamping parts or plates
Magnesium alloy stamping parts or plates
Aluminum alloy stamping parts or plates
But the paint process is similar

2. Hardware paint process:
Pre-treatment → drying → primer coating → baking → top coat coating → baking → inspection → packaging

Three, appearance pretreatment:
〔1〕 1 Degreasing; 2 Water washing; 3 Derusting; 4 Water washing; 5 Meter adjustment; 6 Water washing; 7 Phosphating; 8 Water washing; 9 Water washing; 10 Drying;

[2] The purpose and importance of pre-processing:
The purpose of pretreatment is to obtain a good coating, because the above stamping parts will be affected during manufacturing, processing, handling, and storage.
There are grease, oxide scale, dust, rust and corrosion on the surface. If it is not removed, it will directly affect the performance of the coating film.
Appearance, etc., so the pre-treatment occupies an extremely important position in the coating process.

〔3〕The meaning of pre-treatment: pre-treatment, coating and drying are the three main processes of coating process.
The pre-treatment of coating is the basic process, which has an important influence on the quality of the entire coating, the service life of the coating, the appearance of the coating, etc. The appearance of the workpiece after degreasing, rust removal, phosphating and other processes is clean, uniform, and grease-free.

1. Degreasing
Mainly mineral oil, lubricating grease, animal, vegetable oil
The adhesion of the materials is severely reduced, which affects their dryness, and also reduces the hardness and gloss of the coating. Use alkaline solution to remove oil stains on the workpiece. The main alkali is mixed with NaOH, NaHCO3, etc.; special degreasing powder can be purchased and then formulated into an aqueous solution; the saponification of soap;

2. Wash with water
Rinse with clean water to avoid contamination of the solution in the previous process to the next process. Clear water must be alive [flowing].

3. Derusting
Yellow rust can promote the spread of corrosion products under the coating and make the coating lose its shielding and moisture impermeability. Under high temperature and high humidity conditions, it can cause early damage to the coating and metal, loose yellow rust, poor adhesion, and can fall off with the coating. There are many methods to remove rust, such as lye method, acid method, mechanical method, electrolytic method, etc. The most common one is lye method. Chemical solution is used to remove oxide scale, rust, etc. on the surface of the workpiece, generally sulfuric acid is used, but oxalic acid is also useful. It can also be supplemented by ultrasonic cleaning.

4. Table adjustment (neutralization)
Abbreviation for appearance adjustment: Use acid (or alkali) solution to remove excess alkali (or acid) in the process of removing rust, and adjust the pH as much as possible.
It may be neutral, that is, PH value=7;
Function: Speed up the phosphating speed, refine the phosphating crystal, and increase the crystallization point of phosphating;

5. Phosphating
Phosphating is the central link of pre-treatment for painting. Metal workpieces are treated with phosphoric acid-type manganese, zinc, and cadmium orthophosphate solutions.

A layer of insoluble phosphate protective film is formed on the surface of the workpiece. The formed phosphating film has a dense structure with fine pores, which increases the surface area of the workpiece, increases the contact area of the coating, and makes the phosphating film and the coating Favorable interpenetration between layers,
Greatly improve the adhesion of the organic coating to the workpiece.
The phosphating film formed by phosphating is a stable non-conductive isolation layer, which changes the appearance of the metal from a good conductor to a poor conductor.

Suppresses the formation of micro-batteries and effectively prevents corrosion of the coating. The commonly used methods of phosphating are divided into high temperature phosphating according to the treatment temperature
[70~90℃], middle temperature phosphating [50~70℃] and normal temperature phosphating [20~30℃]. The phosphating film is generally black.

6. Drying
The above pretreatment process is basically carried out in the water tank. And each process has a time regulation. Dry the moisture and prepare for painting;
Fourth, the quality inspection before hanging
Perform visual inspection of unpainted workpieces before going online; a large part of them are defective products that have been painted.
It is necessary to check whether it can be sprayed directly or to remove the old paint; the method of removing the old paint is solvent immersion cleaning or removal with a special paint remover.

Five, polishing
Non-essential process, remove the external granular impurities and increase a certain degree of roughness to increase paint adhesion;
The first time: 400# water sandpaper
Second time: 800# water sandpaper or finer

Six, hang up
Hook the product on the hanger, the hanger is conductive and moves with the assembly line;
The hanger should be conductive and grounded (for electrostatic coating);
The hanger is generally customized according to the different characteristics of the product; the quantity is large, and the entire assembly line is used for turnover

Seven, blowing dust
Remove external dust, dander, etc.; compressed air; electrostatic dust removal gun; flame burning;

8. Manual spraying
Before automatic spraying, pre-spray the places with low potential (such as shadows, recesses, etc.), but it is also a necessary choice when there is no automatic spraying equipment. The efficiency is not high, the production capacity is limited, there are many unstable factors, and the defect rate is high. The equipment for manual spraying of uneven paint film thickness mainly includes air compressors, air pipes, air oil-water separators, spray guns, paint containers, spray booths, etc. )

The types of spray guns are:
1. Gravity type;
2. Suction type;
3. Pressure feed type

Nine, Disk spraying
Disk automatic spraying belongs to the category of electrostatic spraying. It uses the high voltage difference between the spraying device and the grounded workpiece to generate electrostatic force. Disk uses the centrifugal force generated at a fixed speed to throw the paint out, and the electrostatically atomized paint is applied to the workpiece. Disk up and down Move to cover the entire workpiece range;
Disk spraying equipment is actually a rotary electrostatic spraying equipment, which can also be called centrifugal electrostatic atomization equipment. The rotary speed of the rotary disk is 5000~60000r/min, the working voltage is 70~120KV, it is driven by an air motor, and it is electrostatically painted. The electrostatic paint used, the resistance value of the paint should be controlled within a certain range;

Ten, touch up paint
Check Disk spraying defects (less paint, thin paint, etc.) for repairs. Through the inspection, the defects of Disk spraying can be found in time and adjusted, and the parts that Disk cannot be sprayed can be sprayed;

11. Bake in the preheating furnace
Bake at a lower temperature (such as 80~150℃) to partially evaporate the solvent and level the wet paint film

12. Main oven baking
Forcibly bake at a higher temperature (such as 180~185℃) to dry the paint. The faster the speed of the assembly line, the more the temperature needs to be adjusted.

13. Quality Inspection
Check the finished product;

14. Down-hanging packaging
Take off the workpiece and pack; enter the next processing procedure or finished product storage;

Liquid coating and powder coating

1. Liquid coating:
Coatings and solvents
[Or add hardener] after mixing and construction, atomizing with compressed air or electrostatic force, flying to the workpiece and attaching, the solvent evaporates [or reacts to each other] and then solidifies into a film;
The high content of VOC (Volatile Organic Gas) is harmful to the human body;
The paint utilization rate is very low, only about 50%; the film thickness formed by one spraying is 15-25um;

2. Powder coating:
The paint is in the state of fine powder and adheres to the workpiece through electrostatic action. When passing through the high-temperature oven, the paint melts, leveled, and cross-linked and solidified; the VOC content is zero, but pay attention to the hazard of dust; the paint can be recycled and the utilization rate can be More than 99%; equipment investment
Large capital; suitable for thick film coating, the film thickness of one spray is 50~150um;

3. Film performance
Mechanical properties: impact strength 50Kg/cm, no cracks on the coating; more than cupping / 1mm extrusion per second; adhesion (100 grid) test, no peeling of the coating [grading]; bending resistance 1mm→, 180° bending , No cracks in the coating; pencil hardness 2H, allowing slight traces; chemical properties: resistance to boiling water boiling water, 1hr soaking, no abnormal; alkali resistance 5% NaOH, no change for 240hr; acid resistance 3% HCL, no change for 240hr; Solvent resistance to xylene, no change in 24hr; pollution resistance, lipstick, no abnormality in 24hr at room temperature;
Heat resistance: 180℃xlhr
Color difference E<0.8,
Excellent gloss retention;
Corrosion resistance: no change in salt spray resistance for 1000hr;
No change in heat and humidity resistance for 500h;

Principle of Ultrasonic Cleaning
Ultrasonic cleaning is a physical cleaning. The cleaning liquid is put into the tank, and ultrasonic waves are applied in the tank. Because ultrasonic waves are dense and dense vibration waves like sound waves, the pressure of the medium changes alternately during the propagation process. In the negative pressure area, a tearing force is generated in the liquid, and a vacuum bubble is formed. When the sound pressure reaches a certain value, the bubble grows rapidly, and the bubble collapses and closes due to the pressure in the positive pressure area. At this time, the liquids collide with each other to generate a powerful shock wave. Although the displacement and velocity are very small, the acceleration is very large, and the local pressure can reach several thousand atmospheres. This is the so-called cavitation effect.

Fundamental Principles of Phosphating
Phosphating is a process of forming a phosphate chemical conversion film by chemical and electrochemical reactions. The phosphate conversion film formed is called a phosphate film. The main purpose of phosphating is: to provide protection to the base metal and prevent the metal from being corroded to a certain extent; for primer before painting to improve the adhesion and anti-corrosion ability of the paint film layer; to reduce friction in the metal cold working process Use for lubrication.



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