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Automotive painting technology tutorial

01-06

Spraying is a skill that comes from practice and is the accumulation of long-term work experience. To master good spraying skills, operators must first operate in accordance with the correct spraying technical specifications, and continuously learn and train in practice. Spraying technical specifications, including how to select spraying air pressure, adjust the appropriate paint viscosity, spray gun moving speed, the distance between the nozzle and the surface of the object to be sprayed, and spraying points and operating points. The following Lily Color automotive paint manufacturers introduce spraying technology and process specifications one by one.

Spray pressure

Spraying air pressure refers to the amount of air pressure required to atomize the paint after the compressed air enters the spray gun and the paint is mixed during the spraying process. During construction, the spray pressure of each coating is chosen, which depends on the characteristics of various coatings and various environmental conditions.

Different types of spray guns require different air pressures when spraying. Currently widely used spray guns in the automotive repair industry are mainly suction-type (siphon-type) spray guns and gravity-type spray guns. By increasing the air pressure during spraying, the amount of paint can be increased, but too high is counterproductive. If it exceeds 0.6 to 0.7 MPa, the amount of paint is reduced. The amount of paint sprayed by the pressure-feed spray gun has little to do with air pressure, and can be adjusted within a certain range. However, such spray guns are rarely used in the automotive repair industry.

The type of coating refers to the type of coating used when spraying, such as solvent evaporation type, two-component curing type or high temperature baking type. Different coatings used during spraying have different drying speeds, and the spray pressure selection during construction should also be different.

In addition to the selection of the thinner, it should also consider its own volatilization rate in addition to the selected coating variety. Automotive refinish coating suppliers generally have multiple thinner products for each type of coating to adapt to various climate changes, such as slow-drying, fast-drying, and standard. According to the actual situation, the appropriate type of diluent should be selected during construction.

Paint viscosity also depends on the ambient temperature and humidity of the construction site when spraying, and has a certain effect on the selection of spray air pressure.

The stable air volume and air pressure are the guarantee of good quality in spraying construction. The compressed air transportation at the construction site is closely related to the length of the gas pipeline. Choosing a suitable hose length is one of the conditions to ensure the stable air pressure. A more suitable compressed air pipe is 8-10 mm in diameter and 8-10 m in length.

2. Spray viscosity

Viscosity is a measure of the interaction between liquid molecules and the resulting ability to hinder relative movement between molecules. It can also be called the resistance or internal friction of liquid flow. The thickness and thinness, thickness and thinness of coatings are often mentioned in construction, which actually refers to the changes in viscosity, temperature and the influence of solvents and thinners on the fluidity of liquids.

The viscosity of coatings used in automotive repair coatings is usually measured by the viscosity cup test results. The most commonly used coating-4 viscosity cup, according to the national standard GB / T1723-93 coating viscosity determination method, use the coating-4 viscosity cup to measure the viscosity of 10 ~ 15 s coating.

Choosing a suitable viscosity before spraying is conducive to obtaining good paint film quality after spraying. When the viscosity is too thin during spraying, the spraying amount of the coating is increased, which is prone to sag, which affects the coating hiding power. After the film is formed, the coating will lose its gloss and the topcoat is thin.

Before spraying, the viscosity of the coating should be selected according to the type used to obtain the best spray effect. Table 1 shows the spray viscosity of commonly used coatings.

The viscosity test should be carried out at normal temperature, because the temperature will decrease the viscosity, otherwise it will increase the viscosity. The viscosity of different coatings will vary with temperature. Therefore, in the daily construction, the habit of using a viscosity cup should be developed. Today, automotive refinish coating suppliers have been able to provide the ratio of each product according to different weather.

3. Spraying technology

The operator's mastery of the spray technology will directly affect the quality of the coating. Skilled spraying technology not only saves material, man-hours and labor, but also avoids many coating ills and reduces unnecessary rework, such as sagging, leaky spray and rough. The gun-holding posture, spraying distance, moving speed, and width of the atomizing surface in the basic spraying technology are the basis of spraying technology, which need to be explored continuously in long-term practice, proficient and strictly obeyed.

(1) Gun attitude

In the normal spraying process, regardless of the spray gun moving up or down or left and right, the spray gun should be kept at a 90 ° right angle to the surface of the object to be coated. When moving the spray gun, it should be kept parallel to the substrate to be coated. Do not swing it with your hands or the wrist, or you will not get a uniform coating (Figure 1).

If the spray gun is skewed upward or downward during spraying, the result will cause the spray band to flow to one side, while the other side is too thin, which will cause surface streaks. Incorrect spray angles can cause poor coating results (Figure 2).

(2) Spraying distance

The distance from the spray gun's paint hole to the surface of the object to be painted depends on the size of the spray gun. The spraying distance of the large spray gun should be 200-300 mm, the spraying distance of the small spray gun is 150-250 mm, and the spraying distance of the air atomized portable electrostatic spray gun is 250-350 mm.

During the spraying process, the distance between the spray gun and the surface of the object is too close, the painting rate will increase, but it may cause paint film defects such as sagging, orange peel, wrinkles, and metallic paint. When spraying, the distance between the spray gun and the coating object is too far, so that the diluent volatilizes too much, which will increase the paint dust. The paint dust cannot form a film on the surface of the object, which reduces the painting rate and produces defects such as matte, rough and thin paint .

In actual construction operations, the simple measurement method usually used is: open palm, bent forefinger, middle finger and ring finger, and the distance between thumb and little fingertip is between 150 and 220 mm. This is also the commonly used spray distance. Different coating intervals result in different coatings (Figure 3).

(3) Movement speed

The speed of the spray gun during spraying has a great impact on the quality of the coating after spraying. The proper moving speed of the spray gun in actual construction should generally be 30 to 60 cm / s, and it should remain stable during spraying. At the same time, a certain spraying distance must be ensured, and the spray gun must be kept at a 90 ° angle with the substrate to be coated.

According to the characteristics of the type of paint used, the moving speed of the spray gun can be reasonably adjusted. For example, when spraying nitro paint because of its fast drying speed, the moving speed of the spray gun should be appropriately slower, and when spraying the metal base paint, the moving speed of the spray gun should be appropriately fast (marks on the surface of the sprayed metal paint).

The spray gun movement speed during the spraying process should be kept balanced, not slow or fast. Before the trigger is pulled during spraying, the spray gun should have started to move, and the trigger should be released before the spraying operation is finished. This will prevent excessive paint on the head and tail during a series of spraying processes and cause sagging.

(4) Atomization surface width

The choice of the width of the atomizing surface during spraying is usually based on visual inspection. The width of the atomizing surface can be adjusted by the mist control valve. The spray control valve is fully screwed in to obtain the smallest circular jet. Turn the mist control valve all out, and you will get the largest atomization surface.

(5) Spray overlap

When spraying, the overlap between each section must be consistent. The overlap speed of the two sides of each spray should be faster than the middle section to prevent the film at the interface from being too thick and causing sagging. The coverage of the top and bottom overlaps is generally: 2/3 for pure color paint and 1/2 for metal paint.

(6) Spraying sequence

There is no fixed pattern for the spraying order, but the principle usually followed is "from high to low, from left to right, from top to bottom, from inside to outside". Here are some common spraying sequences. In the case of vehicle spraying (passenger cars), the sequence should first be spraying parts that are difficult to spray, such as the luggage compartment, the edge of the engine compartment cover, the gap and each side of the door. When finished, the door should be slightly opened to facilitate drying and prevent sticking. The outer surface can then be sprayed in the order shown in Figure 4.

4. Spraying method

It is difficult to get the best results if any type of paint (primer, middle coat, top coat and putty) is not sprayed properly. Similarly, automotive repair coating systems cannot achieve excellent coatings without proper spray application. In the construction process, in addition to knowing what kind of paint to apply for a particular vehicle, the actual operator should learn to use the correct spraying technology according to the different characteristics of each paint.

Before spraying, no matter what kind of coating should be done: the coating should be thoroughly stirred, and the construction site environment should be constant temperature and humidity (recommended temperature 20-25 ° C, relative humidity <70%), properly prepare the coating, adjust viscosity, and check And make sure all spray equipment is intact. There are several spraying methods.

(1) Transverse spraying method

Refers to the movement of the spray gun from left to right during spraying, and the lower gun presses the upper gun 2/3 (depending on the type of paint, it can also be 1/2), and the guns are sequentially arranged until the entire spraying is completed. Then move to the right and repeat the above operation method. This is the most basic spray gun method. The operation is simple, but the overlap is easy to hang.

(2) Longitudinal spraying method

The moving direction of the spray gun is from top to bottom, the rear gun presses the front gun 2/3 or 1/2 (depending on the type of paint) to spray sequentially, or the front gun presses the rear gun and moves the spray in order Until the entire surface is sprayed. This method has the advantages of labor-saving operation, more uniform coating, fewer overlaps, and difficulty in sagging. This method is easy to master and is especially suitable for coating the skins on both sides of commercial buses.

(3) Crosswise spraying method

This spraying method integrates the two methods. The first layer is sprayed horizontally. When the second layer is sprayed slightly, the vertical spraying method is used to obtain a more uniform spraying effect.

(4) Spraying interval

Interval time means that after the first coating is sprayed, there must be a certain time for the solvent to evaporate and the paint film to level. If this point is ignored during construction, too much solvent will remain in the coating after spraying, resulting in spray defects, especially when spraying metallic paint, such as spraying varnish if the base paint is too short to dry. Will cause dissolution phenomenon, causing the disadvantages of pan-flowering. Therefore, it is necessary to grasp the interval time during construction, not only according to the characteristics of the paint variety, but also according to the change in temperature.

(5) Primer spraying method

The primer basically has no filling ability, so there should be no sagging during spraying, and it can not be sprayed in one thick layer. The best spray results come from a uniform, thin, wet single layer.

(6) Midway paint spraying method

Varieties with fast drying speeds are best sprayed with a spray gun with an inner diameter of 2.2 mm, which can maintain the correct film thickness without overlapping junction points. For spraying synthetic resin midway paints with a high solid content and slow solvent evaporation, spraying with a spray gun with a nozzle inner diameter of 1.8 mm should be used to obtain the correct film thickness and speed.

(7) Spray method of putty

There are two types of putty that can be sprayed: one-component and two-component. The single-component putty must be properly diluted before spraying, and the drying time between the layers must be kept at an appropriate interval when spraying, so as to improve the drying quality and obtain a good filling effect. Two-component acrylic polyurethane putty is divided into high-thickness film spray putty, general primer putty, and wear-free midway putty according to the dilution ratio and the number of sprayed layers. If high fillability is required during construction, it is necessary to spray 4 single-film coatings with high viscosity, the film thickness can reach 220 μm, and it has good drying properties.

5. Coating dry

Drying the coating during spraying is an indispensable procedure in the spraying quality system, and its mastery will directly affect the quality of the entire coating. There are many ways to dry the coating. Here are some common drying methods.

(1) Naturally dry

Also called air drying or air drying, that is, the coating film is dried at room temperature. Generally, the ideal drying conditions are 15-30 ° C, and the relative humidity is not more than 80%. However, it should be noted that when working in the open air in winter, the phenomenon of slow drying of the coating film often occurs.

(2) Forced drying

In automobile painting, in order to speed up the production speed and shorten the painting construction cycle, the painted vehicle is often placed in a baking room and accelerated drying at a certain temperature (generally 50 to 80 ° C). The accelerated drying film has better performance in hardness and adhesion than natural drying.

(3) Radiation drying

The radiation drying method is also called an infrared drying method. Infrared lamps are commonly used for auto-repair coatings for radiation drying, which has a significant effect on small-area repairs and drying of atomic ash and varnish at low temperatures.

(4) Other drying methods

① UV drying

Ultraviolet drying, also known as light curing, is a specific drying form of photosensitive coatings, photosensitive paints (Figure 5). Photosensitive paint is a new type of coating that can be cured quickly into a film within a few seconds or minutes under ultraviolet radiation. This drying technology can effectively improve the repair efficiency and is suitable for small or fast repair processes.

②Pulse radiation curing

Refers to irradiating paint with pulsed radiation with high energy to make it dry.

③ electron beam curing

It is to use thousands of kW electron accelerator to obtain electron energy, and use high-speed electron beam radiation or accelerated electron flow radiation coating film to make it dry.

④Induction drying

That is, the object to be dried is placed in an electromagnetic field, and the electromagnetic energy is converted into thermal energy inside the object to be dried, so that the object to be dried is first heated, and then transmitted to the coating film to be dried. In automotive painting construction, spraying is a highly technical and highly demanding job. In order to obtain a smooth, smooth, uniform thickness and a mirror-like surface after spraying, in addition to the variety and quality of the coating and the underlying foundation of the coating, spraying experience and correct operating techniques are also very important. In addition, sprayers' self-protection cannot be ignored (Figure 6).

The coating construction technology has its own characteristics and emphasis according to the type of car. Passenger cars are more demanding than commercial vehicles in terms of surface decoration or bottom protection. Its surface coating belongs to first-class decorative precision, has a beautiful appearance, and is as bright as a mirror or smooth surface, free of fine impurities, abrasions, cracks, wrinkles, blistering and visible defects, and should be sufficient Mechanical strength. The bottom coating is an excellent protective layer, which should have excellent rust resistance, corrosion resistance and strong adhesion. Partially or fully scraped putty with good adhesion and high mechanical strength will not rust or fall off after several years of use.

In the past, automobile spraying was mainly based on pure color paints, and the relative requirements were not harsh. With the continuous development of the coating industry, the variety of coatings has continued to increase, and the grade of automotive coatings has become higher and higher. In particular, the rapid growth of metallic paint and pearl paint has continuously improved the quality of automotive repair coatings, which has put forward higher requirements for operators' spraying technology.

For example, there are different degrees of difficulty in controlling the construction stability and color difference of sprayed light-colored metallic paint. In particular, it is much more sensitive than the dark paint in the "angle of view flash effect". Therefore, when spraying, the operator should master the spray gun in accordance with the process and technical specifications, and master the most basic spray gun adjustment and use methods.

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