Spray processing
Spraying is usually divided into powder spraying and paint spraying. The processing cost, adsorption method, surface effect, and application range of the two spraying methods are different.
Powder spraying process
The powder spraying process, also called powder coating, is a new type of coating process that has developed rapidly in recent decades. The raw material used is plastic powder. Some countries started research experiments as early as the 1940s, but progress was slow. In 1954, James of Germany successfully coated polyethylene with the fluidized bed method. In 1962, the French company Symes company invented powder electrostatic spraying, and powder coating began to be formally adopted in production. In recent years, due to the environmental protection of various countries The importance of protection, powder coatings that do not pollute water and atmosphere, have been developed rapidly.
Outstanding advantages
1. A thicker coating can be obtained by one coating, such as coating a coating of 100-300μm, using ordinary solvent coatings, about 4 to 6 coatings, while powder coatings can achieve this thickness in one pass . The corrosion resistance of the coating is very good.
2. The powder coating does not contain solvents and has no pollution from three wastes, which improves labor and hygiene conditions.
3. It adopts new technology such as powder electrostatic spraying, which has high efficiency and is suitable for automatic assembly line coating; the powder has high utilization rate and can be recycled.
4. In addition to thermosetting epoxy, polyester, acrylic, there are still a large number of thermoplastic grease resistant powder coatings, such as polyethylene, polypropylene, polystyrene, fluorinated polyether, nylon, polycarbonate and various Fluorine resin, etc.
Powder coatings began to be used in protection and electrical aspects. With the development of technology, it has been widely used in the automotive industry, electrical insulation, corrosion-resistant chemical pumps, valves, cylinders, pipes, outdoor steel components, steel furniture, castings, etc. Surface coating.
China began experimental research on powder coating in the 1960s, and it has been applied in production, and it has been widely used up to now.
Electrostatic powder
This is an important construction process that is currently developing fastest in powder coating.
Fundamental :
A high-voltage corona discharge electric field is formed between the spray gun and the workpiece. When the powder particles are ejected from the nozzle of the spray gun and pass through the discharge area, a large amount of electrons will be collected and become negatively charged particles. Adsorb to the positively charged workpiece. When the powder adheres to a certain thickness, the effect of “same repulsion” will occur, and the powder can no longer be adsorbed, so that the thickness of the powder layer of each part is uniform, and then the powder layer will flow into a uniform film after heating and curing. Layer; powder electrostatic spraying process, the typical powder electrostatic spraying process is as follows: upper part → degreasing → cleaning → rust removal → cleaning → phosphating → cleaning → passivation → powder electrostatic spraying → curing → cooling → lower part; affecting powder The main factor of electrostatic spraying quality.
In the electrostatic spraying, in addition to the quality of the surface pretreatment of the workpiece, there are spraying time, spray gun form, spraying voltage, powder spraying amount, powder conductivity, powder particle size, powder particle size, powder and air mixture. The speed gradient and so on.
Resistivity of powder:
The resistivity of the powder is ideally 1010～1016 ohm/cm. If the resistivity is too low, it is easy to cause the powder to disperse. If the resistivity is too high, the coating thickness will be affected.
Powder spraying amount:
At the beginning of spraying, the amount of powder sprayed has a certain effect on the thickness of the film. Generally, the amount of powder sprayed is small and the deposition rate is high. The amount of powder spraying is generally controlled within the range of 50 g/min to 1000 g/min.
Velocity and gradient of powder and air mixture:
The velocity gradient is the ratio of the velocity of the powder-air mixture at the outlet of the spray gun to the spraying distance. In a certain spraying time, the film thickness will decrease as the spraying gradient increases.
Spraying distance:
The spraying distance is a main parameter of the thickness of the resisting film. It is generally controlled at 10~25 cm from the workpiece, and is mostly determined by the spray gun type.
Spraying time:
Several parameters such as spraying time and spraying voltage, spraying distance, and spraying volume have mutual influence. When spraying time increases and spraying distance is large, the influence of spraying voltage on the limit value of film thickness decreases. With the increase of the powder spraying time, the influence of the powder spraying amount on the growth rate of the film thickness is significantly reduced.
Water dispersible powder coating:
Water-dispersed powder coating is a stable dispersion of powder coatings in an aqueous medium. It has the advantages of water-based coatings and powder coatings. All conventional methods including dipping, brushing, spraying and electrostatic coating can be used in the process.
Powder electrophoretic coating:
It is the product of comprehensive powder coating and electric current coating, and has the characteristics of both. The basic principle is to disperse powder particles (generally required to be less than 40μm) in an aqueous solution containing electrophoresis resin, use aqueous electrophoresis resin as a carrier, and powder particles as a film-forming substance to charge the powder particles in a DC electric field Electrophoretic deposition to form a film. It is suitable for the construction of workpieces with complex shapes.
Painting process
The spray paint is dispersed into uniform and fine droplets by means of a spray gun or a disc atomizer with the help of pressure or centrifugal force, and is applied to the surface of the object to be coated. It can be divided into air spraying, airless spraying, electrostatic spraying and various derivative methods of the above basic spraying forms, such as large flow and low pressure atomized spraying, thermal spraying, automatic spraying, multi-group spraying, etc.
Features
The spraying operation has high production efficiency and is suitable for manual operation and industrial automation production. It has a wide range of applications, mainly in the fields of hardware, plastics, furniture, military industry, and ships. It is the most common coating method used today; spraying operations require environmental requirements There are dust-free workshops ranging from one million to one hundred. The spraying equipment includes spray guns, spray booths, paint supply rooms, curing furnaces/drying furnaces, spraying workpiece conveying equipment, anti-fogging and waste water, waste gas treatment equipment, etc.
The main problem in spraying is the highly dispersed paint mist and volatilized solvents, which not only pollute the environment, is not good for human health, but also waste paint and cause economic losses.
Large-flow and low-pressure atomization spraying is a low atomization air pressure and low air jet velocity. The low atomization coating running speed improves the rebound of the coating from the surface of the coating. The paint rate has been increased from 30% to 40% of ordinary air spraying to 65% to 85%. In the light leather coating, spray the coating on the leather surface with a spray gun or spray machine.
Common problems and solutions for painting operations:
1. Phenomenon: graining
Reason: the job site is dirty, dust is mixed into the paint; after the paint is mixed for too long, the paint and curing agent have produced copolymerized particles; the oil output of the spray gun is too small, and the air pressure is too large, which makes the paint poorly atomized or the spray gun is off the surface Too close
Solution: Clean the spray booth and cover the paint bucket; the paint should not be left for too long after the paint is mixed; adjust the spray gun to make it in the best working condition, and determine the distance between the muzzle and the surface of the object is 20-50CM.
2. Phenomenon: vertical flow
Reason: Excessive thinner makes the paint too low viscosity and loses its viscosity; too much oil output, too close to the surface of the object or too slow spray operation; too much and too thick each injection or too short interval between resprays; Uneven, especially the streamlined shape is easy to sag
Solution: proportion according to requirements; control the oil output to ensure that the spray paint is separated to increase the operating speed of the spray gun; each injection should not be too thick, and it is better to master the interval spray time in two; control the oil output to reduce the thickness of the paint film; Instructions for use
3. Phenomenon: orange peel
Reason: Too much curing agent makes the paint film dry too quickly and the reaction is violent; the spraying air pressure is too large, and the paint film can not be leveled by wrinkling; the temperature at the job site is too high, so the paint film should be violent
Solution: According to the instructions for use; adjust the air pressure, not too high; pay attention to the on-site temperature, and add a slow-drying diluent to suppress the drying speed
4. Phenomenon: whitening
Reason: The temperature and humidity at the job site are high, and the paint film reacts violently, which may combine with the moisture in the air to produce whitening; excessive curing agent, too much and too thick spray at one time
Solution: Pay attention to the humidity of the site, add anti-white water to prevent the phenomenon of whitening; adjust according to the proportion, and the spray should not be too thick at one time
5. Phenomenon: blistering
Original solid: The compressed air has water mixed on the paint film, the temperature at the job site is high, and the paint dries too fast; the surface moisture content is high, and the air humidity is high; one spray is too thick
Solution: Separate oil and water, pay attention to drainage; add slow-drying thinner; clean the surface, add anti-white water to the paint; not too thick at one time
6. Phenomenon: shrinkage
Reason: The primer or intermediate coating is not fully dried before applying the top coat
Solution; spray according to the recommended thickness of each spray gun spraying layer
7. Phenomenon: wrinkles
Reasons: the drying time is too short or the paint film is too thick; the curing agent in the primer or putty is improperly selected; the primer putty is not complete; when spraying the top coat, a gun is too thick, the internal deep agent is not volatilized in time, and the outside is dry and the inside is dried Quit.
Solution: Give enough drying time between each coat; only spray the second coat or wet spray after it dries.
Precautions and safety measures
1. Spraying workers must be trained in safety technology, and they are not allowed to work without training.
2. All painting equipment and tools must be thoroughly inspected before painting operation, and work can only be performed when there is no problem.
3. The ventilation and purification of the painting work site should meet the requirements of GB 6415.
4. During the operation, the safety technical department of the enterprise should assign a dedicated person to regularly measure the oxygen content and combustible gas concentration in the air in the confined space. The oxygen content should be more than 18%, and the combustible gas concentration should be less than 10% of the lower explosive limit.
5. At least two people are required to operate the paint spraying in the cabin. If the working place is too small and can only accommodate a single person, the other person should be responsible for supervision.
6. The spray painting operations at heights should comply with the “Safety Regulations for Ship Construction at Heights.”
7. When working with multiple spray guns at the same time, the spacing (about 5m) must be opened and sprayed in a certain and the same direction.
8. For special coating operations, the spraying equipment and hoses should be managed by special personnel. If the equipment fails or has abnormal conditions, first notify the cabin personnel to evacuate out of the cabin, and the maintenance personnel will oversee them.
9. The high-pressure jet and leakage of airless spray paint can cause serious injury accidents. During operation, the following should be done:
a. Do not point the nozzle of the spray gun to others and yourself at any time;
b. Do not touch the nozzle with your fingers or peer into the muzzle;
c. When cleaning the spray gun, cut off the pump drive source, release the pressure, and sell the safety device to the spray gun before cleaning;
d. When the spray gun is out of use, the safety latch must be locked.
10. After the operation is completed, the spray gun must be withdrawn from the cabin in time and continue to ventilate until the paint film is completely cured.
11. After the painting operation is over, the workplace should be cleaned up in time, and the remaining paint and solvents should be sent back to the warehouse in time, and it is not allowed to leave them randomly.
12. The operator must wear protective masks, rubber gloves, eye masks and other related protective equipment.
