Introduction to the three major systems of
spraying and painting
Solvent-based coatings contain solvents,
which emit a large amount of VOC (Volatile Organic Compounds) during the
production and construction process, endangering human health and causing
environmental pollution. According to statistics, the global annual VOC
emissions to the atmosphere are up to several million tons.
VOC is a major pollutant in the paint shop,
including benzene, styrene, formaldehyde and acetaldehyde, which are harmful to
human body and even have the risk of cancer. Therefore, it is necessary to wear
labor protection supplies, especially breathing apparatus, when entering the
painting workshop.
Many companies have tall chimneys erected
in the painting workshop, which is the symbol of the painting workshop. These
chimneys are used to emit harmful gases, and with the increasingly strict
environmental regulations on VOC emission standards, reducing VOC emissions has
become the most important thing for companies. So where do VOC emissions mainly
come from?
According to the process of VOC emission,
it is not difficult to find that source control and end-of-pipe treatment are
the more appropriate ways to control VOC, and many domestic enterprises reflect
a high level of VOC control. In fact, the workshops of these enterprises also
adopt the source + end method to control VOC, and water-based painting is the
best way to control VOC emission from the source. For water-based paint,
companies have developed water-based coating process, which reduces VOC
generation during the process. And in the production line of water-based
coating, the following systems are included.
Conveyor and paint mixing system.
Since water is a corrosive medium, it will
cause metal corrosion, cause equipment damage and produce metal ions, which
will not only yellow the color of the paint film, but also reduce the
anti-corrosion ability of the paint film. For this reason, the paint storage
container and transport pipeline of water-based paint should be made of
stainless steel and should be treated by chemical passivation. The paint mixing
system, mixer and back pressure regulating valve in the painting line should be
of low shear type. The connection of paint pipeline should avoid dead ends, and
the conveying system should be better with two lines, and the pipeline should
be enlarged to increase the pressure and flow of circulating material pump. The
circulating speed of the mixer should be 0.15m/s. The temperature between paint
mixing should be controlled at (3±1)℃ lower than the paint. The storage
stability condition of water-based paint: 5~30℃/6 months.
Spraying system.
Spraying system is roughly divided into: 1,
high-pressure airless spraying; 2, air spraying; 3, electrostatic spraying.
1、High-pressure
airless spraying
The principle of high-pressure airless
spraying is the use of high-pressure plunger pump to continuously transport
paint to the closed paint pipeline, so as to pressurize the paint to 210 kg /
cm2 in a closed space to form a high pressure, and then release the gun trigger
connected to the end of the paint tube, so that the high-pressure paint flow
forced through the extremely small nozzle and the formation of atomization, so
as to shoot the surface of the object to be coated. Its advantages are: 1,
suitable for spraying high viscosity paint; 2, spraying efficiency is about 2
times higher than ordinary spraying; 3, excellent surface quality: it is the
paint pressure spraying into tiny particles, so that it is evenly distributed
on the surface of the wall, so that the paint in the workpiece surface to form
a smooth, smooth, dense coating, which is incomparable to the original method
of brushing, rolling, etc.; 4, improve the adhesion of the paint and extend the
life of the coating. High-pressure spray atomization makes the paint particles
gain strong kinetic energy, and the paint particles use this kinetic energy to
reach the pores, thus making the coating more dense, enhancing the bite force
with the workpiece, improving the adhesion, and effectively extending the
coating life.
2、Air spraying
Air spraying is a kind of coating process
which is widely used in coating construction. Air spraying is the use of
compressed air flow, flowing through the gun nozzle hole to form a negative
pressure, negative pressure so that the paint from the suction tube inhalation,
sprayed by the nozzle, the formation of paint mist, paint mist sprayed to the
surface of the painted parts to form a uniform paint film. Air spraying can
produce uniform paint, coating fine and smooth; its advantages are: easy to
operate, for the parts of the more hidden parts (such as crevices, concave and
convex), can also be uniformly sprayed. But in the construction efficiency and
out of the paint is slightly worse than high-pressure airless spraying.
3、Electrostatic
spraying
Electrostatic spraying refers to the use of
corona discharge principle to make atomized paint in the high-voltage DC
electric field under the action of the negative charge, and adsorbed in the
positively charged substrate surface discharge coating method. Electrostatic
spraying equipment is composed of spray gun, spray cup and electrostatic
spraying high voltage power supply. Water-based paint resistance is less than
0.1M, there is a high-voltage electrical leakage, should use special
electrostatic spraying equipment, usually using an external electric paint
delivery system, the advantages of the paint delivery system and color change
and the traditional way the same. There are spraying companies develop and use the
cup type electrostatic spray gun with electricity inside, the advantage is to
improve the utilization of paint. Robot magazine-type spinning cup spraying
system, the transit paint container into an independent paint container (called
magazines), can be quickly combined with the spinning cup, pressure feeding
paint. The advantages are: it saves cleaning solvent when changing colors and
can be applied to both solvent-based and water-based coatings for electrostatic
spraying. The advantages of electrostatic spraying compared with other spraying
methods are: 1. High utilization rate of paint, when spraying in high voltage
electrostatic field, the paint particles with negative charge are coated to the
surface of the workpiece along the direction of the power line, therefore,
basically there is no paint jet rebound and paint mist scattering phenomenon,
paint mist loss is very small, the utilization rate of paint can reach 85% to
90%. 2. The effect of electrostatic field, paint particles dispersion is high,
and the distribution in the jet is more uniform, so the coating formed on the
surface of the workpiece to be decorated is also more flat and uniform, and the
gloss and adhesion of the paint film are higher.
Drying system.
Drying system is roughly divided into: 1、 Hot air drying; 2、 Infrared drying; 3、 Microwave drying.
1、Hot air drying
Hot air drying is a heating and drying
method that adopts convection principle and takes hot air with temperature of
50~90℃ as the heat carrier to transfer heat energy to
the coating on the surface of workpiece, and the coating absorbs energy and
cures into film. Electricity or steam is commonly used as the heat source to
heat the air first, and the heat is transferred from the hot air to the surface
of the coating through convection form to make the coating get dried quickly.
Using hot air drying process can obviously accelerate the drying speed of the
coating and has the characteristics of strong adaptability, so it is a more
widely used form of drying.
2、Infrared drying
Infrared curing can be used to dry the
coating of water-based wood coatings. The so-called infrared curing actually
refers to the painted furniture panels and their coatings under the irradiation
of infrared light, absorbing radiation energy and being converted into heat
energy, so as to achieve the curing of the coating. Infrared ray is a kind of
invisible ray, between visible light and microwave, the wavelength is 0.72-1000
um. According to the wavelength range, it can be divided into "near",
"medium", "far "infrared, commonly used far infrared drying
coating. Infrared curing has the advantages of fast curing speed, rapid
temperature rise, good curing quality, etc. However, when drying the coating
with infrared heating, the coating has an obvious temperature gradient, and its
drying is extended from the surface to the inside, which makes it unsuitable
for drying thicker coating film; infrared drying can only heat the area that
can be irradiated by infrared light, and cannot be used to dry three-dimensional
objects.
3、Microwave drying
Microwave refers to the wavelength of
1mm-1m, frequency of 300MHz-300GHz, with penetrating electromagnetic waves,
commonly used microwave frequency of 915-2450MHz. microwave heating uses the
principle of dielectric loss, and the dielectric constant of water is much
larger than dry matter, the vast majority of the energy released by the
electromagnetic field is absorbed by the water in the coating. The microwave
field periodically changes the direction of the applied electric field at a
high speed of several hundred million times per second, causing the water
molecules to swing rapidly and producing a significant thermal effect, thus
causing the temperature of the paint interior and surface to rise rapidly at
the same time. The advantages of microwave heating are that the drying speed is
particularly fast, and different substances have selective absorption for
microwaves; there is no shape requirement for the object to be dried; the
heating for the coating film is very uniform, and there is no temperature
gradient, which can dry thick films.
With the continuous and rapid economic
growth, the construction of a large number of infrastructure, the gradual
increase of people's awareness of environmental protection, coatings will certainly
go to the road of health, environmental protection, water-based, and
water-based coatings industry will usher in a broader space for development.