Inner can coating technology and application

Almost all metal containers are internally coated so that the package has a good shelf life, maintains the taste of the package, and prevents the metal from being corroded. ?

The inner coating technology is an important part of the production of cans. It uses a non-pneumatic pump and nozzle system to spray paint onto the rotating surface of the can body and dry it to form a thin film coating process. Grasp the spraying technology can reduce the can manufacturing cost and improve the product packaging performance. To control the quality of internal coatings, first of all, find out the factors that affect the internal coating process. Some common important influencing factors are:

1, coating temperature;

2, spray pressure;

3, nozzle model and spray gun position. ?

However, there are some other factors that have a great influence on the inner coating process. These factors include the performance of the drying oven, the washing performance of the cans before the inner coating and the tanks are not contaminated, the characteristics of the coating, the rotation speed of the can during spraying and no deformation during rotation. ?

First, the effect of paint temperature?

As the temperature of the coating increases, the viscosity decreases, the flow rate increases, and the amount of spray increases. Therefore, the temperature has a great influence on the internal coating process. The use of heaters can overcome seasonal changes. Factors such as production shifts also have a certain influence on the coating temperature. Use the heater to set a constant temperature value to heat the coating. The coating temperature can be changed. Control within 5°C. When using water-based paint, the temperature should be set above the factory ambient temperature, usually 38 to 41 °C.

Second, spray pressure?

1. What are the signs of pressure changes?

In the spraying pressure control, we must pay attention to two issues, first, the actual pressure of use, the second is the pressure of normal stability, compare the two. Most of the tankers use a spray pressure of 600-900 PSI. In the absence of spraying, the pressure of the spray gun is checked. Usually, when the pressure fluctuates within 100 PSI, there will be a delay of 50 mS. Time, this time can help us check whether the pressure changes. Of course there are many reasons for pressure fluctuations, but most pressure fluctuations can be understood by checking the noise in the system's guns, valve closing-closing motions, and pump movements under normal operating conditions. Through the comparison of the normal value and the actual value, the pressure change is discovered and processed in time. ?

2, paint accumulation problem?

Pipeline plugging is a common problem in interior coating systems. In particular, water-based paints are more noticeable due to the accumulation of paint on the inner wall of the pipe and the pump room, especially at the bends of pipe joints. Accumulation problems can cause the system pressure to fluctuate above 100 PSI, while lowering the pressure of 100 PSI will correspondingly reduce the spraying volume of 10 mg. As a result, the spraying quality is not good. Therefore, bends should be minimized when designing pipes, and the function of the paint pump used should be slightly larger.

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In order to overcome the problem of fluctuations in the pressure, it is often difficult to control the amount of coating. In the process, the amount of change in the amount of coating is always set in a wide range. In the actual operation, the total amount of coating is guaranteed to be in the middle line or upper limit of the average value of the upper and lower limits. When the pressure is reduced, the coating amount is not lower than the lower limit. In order to get enough paint, but doing so can meet the coating quality requirements but wasted a lot of paint. Therefore, controlling the pressure is the fundamental solution to the problem. ?

Another problem with paint accumulation is that the paint inside the tank will be affected by residual paint at the edges of the filling port and at the flanges of the tank mouth, so that a certain amount of overcoating will accumulate on the spray gun nozzle, also called the gun stack. These overcoatings are scraps, but in order to ensure the integrity of the can's coating, overcoating is necessary. The best method is to control the spray pressure, both to achieve a complete coating and to reduce the amount of overcoating to reduce the loss. The higher the pressure is, the higher the atomization of the paint is, and the wider the fan-shaped surface is, and the narrower is the opposite. Generally, the change range of the fan-shaped surface is allowed to be 2% to 6%. For ease of operation, the internal coating system should be equipped with a pressure regulator to adjust the pressure of the spray gun to solve the problem of nozzle build-up and use the test device to compare the pressure of the spray gun with the system pressure.

3, pressure control?

The use of spraying pressure control device in production can effectively control the amount of coating to obtain a lower amount of coating, which is conducive to the use of different coating amounts for different packages. It can also compensate for the effect of nozzle wear on the amount of coating. As the nozzle wear aperture becomes larger, too much paint is sprayed into the tank, relying on pressure adjustment can make up for this deficiency, and also extend the service life of the nozzle.

Experience has shown that with the pressure of 800 PSI, the atomization degree of the paint is higher, and the coating film with better uniformity can be obtained. However, the amount of over-spray coating that is sprayed on the tank is also large (it means that the sprayed can splashes into the tank). With a pressure of 300 PSI, with the same type of nozzle, good atomization can be achieved, and a uniform coating film can be obtained, apparently with less overspray. In short, to find a suitable pressure, get a uniform distribution of the coating, usually 250PSI pressure on it, with 300PSI pressure is not necessarily a good thing. ?

According to experiments, three different coating weights are obtained by adjusting the spraying time or adjusting the pressure with the nozzle of the same specification.

Under PSI pressure, the conductivity will be slightly higher, but it is in line with beer packaging requirements. ?

This test concluded that the use of low pressure reduces overspray and reduces cleaning and maintenance time. Since the sprayer needs to use a recovery tank to recover waste paint, the recovery tank must be replaced and cleaned frequently. In addition, the cleaning time of the spray gun should be reduced, which will help improve production efficiency, and more importantly, low pressure and easy operation control. Therefore, the use of low pressure and corresponding control of the working hours can achieve good results. ?

Although the pressure monitoring device is a commonly used pressure monitoring tool, its use in the internal spraying system can play an important role, such as nozzle wear, clogging, improper pump performance, filter clogging and other factors caused by pressure changes can be The pressure gauge reads out. Some manufacturers have developed a more advanced pressure monitoring system such as CANWORk. This control system automatically corrects the pressure of the gun by monitoring the pressure of the gun when a problem occurs in the gun system, so as to ensure that each gun is in good working condition, and at the same time input raw data to the computer. You can know whether the traffic is too high or too low, and the data is collected and corrected at any time, which helps to find the problem. There are two sets of information applied to the system, one is synchronization information; the other is spray gun pressure information. ?

In addition, another important factor affecting the inner coating process is the simultaneous setting. The cans are in a rotating state during spraying. The spraying working time must be determined according to the rotation speed of the cans, especially the low pressure spraying amount. In short, in order to obtain a good coating film, an incomplete coating film may cause corrosion of the bare metal, or waste due to too much paint. The ideal coating is less and more uniform. Therefore, the core problem is that the working time must be synchronized with the rotation speed of the tank. ?

Third, the choice and location of the nozzle?

There are three factors to consider when choosing a nozzle:

1 sprayed fan-shaped surface, that is, the surface covered by the sprayed paint;

2 flow, that is how much paint can be sprayed per minute;

3 The distribution of spray, that is, one side has more paint than the other side;

Scattering-type spraying can achieve uniform coating on both sides with the distribution surface as the center line, mainly used for spraying vehicles or equipment, and can obtain even thickness coating. ?

However, in terms of cans, due to the close proximity of the cans to the nozzles, the fixed position of the nozzles is related to the cans, and the result of the flat-shooter nozzles is that they are less toward the bottom of the can, and this is a problem when used for low paint spraying. The use of an operating fan nozzle is characterized by the ability to spray more paint on one side and less paint on the other side. Since the bottom of the tank is the furthest from the nozzle, more paint is needed to cover a large area at the bottom of the tank. Therefore, the side with a larger amount of nozzle coating is facing the bottom of the tank, and the side with a smaller amount of distribution is facing the upper part. Thick and uniform coating can be obtained, suitable for various types of cans and coatings, and more importantly, it is suitable for spraying with low coating amount. ?

The distance between the nozzle and the tank and the angle formed are very important. The location of the spray gun relies heavily on the type of nozzle used and the system pressure, and is also highly dependent on the properties of the coating, such as viscosity, temperature, and vacuum performance of the spray equipment. These factors will vary greatly from device to device and from factory to factory. Single gun spray, angle of about 15 to 200, using two guns spraying, the first gun to paint to the bottom of the tank wall and the bottom of the tank, the angle of about 100, the second gun to paint all parts of the tank, the angle of about 200 .

In summary, controlling such variable factors as coating temperature, system pressure, spraying time, nozzle type, and gun position can improve production efficiency, improve the inner coating process, reduce coating costs, and increase market competitiveness.