Study on the process of coating PVF coating on steel drums for new foods (3)

Study on the process of coating PVF coating on steel drums for new foods (3)

Third, baking plasticization process and safety technology

The sprayed barrels can be placed indoors for several hours to one day, but they are often dusted for a long time, so it is best to send them in a timely manner.

Baking and plasticizing is to heat the sprayed workpiece in the drying tunnel or drying room to evaporate the volatile components such as dimethyl ester, which is a key process for plasticizing the film of polyvinyl fluoride resin.

This is a physical process. The factors affecting are mainly temperature and time. Therefore, it is easier to master the baking technology and the quality is more stable. Although it has its own special aspects, it requires the operator to use it for the first time.

1. Baking and plasticizing equipment can be used in continuous drying tunnel or fixed drying room according to the size and quantity of the coated barrels. Continuous drying tunnels should be used for all units in condition. In order to ensure a good coating film and achieve safe production, the drying tunnel or drying room should meet the following conditions: a, can reach a sufficiently high temperature (average is greater than 270 ° C); b, should be set a good exhaust vent; c, there should be no open flame in the furnace; d, temperature control is sensitive, and can be adjusted.

According to the heating energy, it can be classified into electric heating tunnel and fuel drying tunnel. Fuel heating is more economical than electric heating. 1Kg of fuel is 10 degrees of heat, and the fuel is heated and maintained. In addition, the fuel heating device can also burn a part of the solvent volatilized in the paint to ensure the national environmental protection requirements, and the heat generated by the solvent combustion also saves fuel. We recommend the use of fuel drying tunnels to be economical and reliable. Nowadays, the production of such drying tunnels in China is based on the Beijing Aerospace Xinyuan Coating Research Institute. After many uses, the technology is advanced and the quality is stable.

If you use the original electric oven and drying tunnel, you must meet the above conditions. If you do not need to carry out the transformation, you can start from the following aspects.

1) Heat or modify the heating element to increase the temperature

2) Set a good exhaust vent

3) Replace or thicken insulation materials

4) Check the interface of the heating element to prevent short circuit and open flame

5) Calibration thermometer

2. Baking and plasticizing operation procedures

1) Preparation before production

a, check the voltage or the burner is normal

b, check whether the temperature is normal

c. For the electric heating tunnel and drying room, check the closing condition of the exhaust valve, close it when heating up, and adjust the opening degree of the exhaust valve according to the number of barrels in production.

2) Determination of baking temperature and residence time

The baking temperature and time of the drying tunnel are determined according to the degree of heat absorbed by the barrel and the bottom cover and the movement state in the drying tunnel to determine different temperatures and baking times. The following is a description of the different operating procedures for the fuel drying tunnel and the electric heating tunnel.

a. Fuel drying tunnel, because the forced hot air circulation in the drying tunnel, the temperature in the entire drying tunnel is relatively uniform.

The temperature is generally set to 250 to 260 °C. (hot air temperature in the drying tunnel)

The effective residence time is controlled in 10 to 15 minutes.

Precautions:

1) The drying tunnel should be free of black smoke

2) Control the concentration of dimethyl ester vapor in the drying tunnel to maintain sufficient displacement.

b, electric drying tunnel

Generally determined by the following conditions:

Baking object

Front temperature

Middle temperature

Back temperature

Barrel body

260~270°C

240 to 260 ° C

230 ° C

Bucket bottom cover

240 to 270 ° C

240 to 260 ° C

230 ° C

Baking time, that is, speed as a major adjustment factor, is affected by many conditions, such as the thickness and thickness of the paint film, the number of barrels is more or less, and a preferential condition can be selected according to the completion of plasticization of the coating film. The solvent must be completely evaporated in the channel. The general effective time is controlled in about 10 to 15 minutes.

If the temperature of the drying tunnel cannot reach the above temperature, the time can be extended appropriately, not more than 20 minutes. However, if the temperature of the front and middle sections is lower than 230 °C, the quality cannot be guaranteed, so it cannot be lower than this temperature.

Precautions and safe work:

1 to prevent open flame, the dimethyl ester vapor evaporated from the film during the baking process reaches a certain concentration and contact with open flame will cause burning. Therefore, the following points must be paid attention to during construction: a. The heating wire should be controlled to be used slightly below the rated voltage; b. Prevent the heating element from being red; c. Prevent the electric power connector from being in contact with the electric spark.

2 It is forbidden to close all exhaust vents during normal production, which is dangerous.

3 Control the concentration of volatile gas dimethyl ester vapor in the drying tunnel (or drying room) to maintain sufficient air discharge.

4 Control the number of painted parts in the drying tunnel and drying room within the unit time, can not be placed too much, too dense. It is not safe to prevent the local concentration of solvent gas from being too high.

3, baking temperature and holding time to discuss the quality of the coating

1) Temperature: It should be sent to the workpiece after it is raised to the specified furnace temperature. The generally usable temperature range is 240-270 °C, and in fact, most take 240-260 °C. It depends on the specific situation. The baking operation, one temperature factor, and the other time factor, affect each other. The operator should first determine a suitable temperature and then find the appropriate time. The appropriate temperature is determined by the barrel, ie the thickness of the material, the number of furnaces, the material, and so on. The material is thick, and the quantity is large, the heat capacity is large, the temperature rise is slow, and the baking temperature is high. Therefore, the temperature in actual production often exceeds 240 ° C, and even thicker barrels need to be more than 300 degrees.

Practice has proved that the method of lowering the temperature duration does not work. The baking plasticizing temperature has a minimum temperature. Below the temperature, the baking must be defective. This minimum temperature rises as the heat capacity of the workpiece increases. The problem of baking quality at low temperature is mainly caused by cracking of the coating, light cracking at the corners, and cracking of the entire coating, and the adhesion is significantly reduced.

2) Time: Generally 5 to 20 minutes. After the workpiece is placed for about 1 to 5 minutes, a large amount of smoke (dimethyl ester vapor) is started. In principle, the solvent is evaporated and the baking is completed, and it can be immediately discharged. The baking cycle is affected by factors such as the temperature of the furnace, the thickness and quantity of the material of the barrel, the film, and the thickness. When the furnace temperature is the same, the furnace temperature cycle is prolonged due to factors such as wall thickness, quantity, coating thickness, baking temperature, etc.; the same kind of workpiece, the same quantity, such as the specified higher furnace temperature, the baking cycle Shorten, conversely, a lower furnace temperature (not too low) is specified and the baking cycle is extended. But pay attention to the time does not allow unrestricted delays, generally can be completed in 20 minutes. What if I can't finish in 20 minutes? The answer is to increase the furnace temperature and speed up the heating rate. However, it does not mean that the higher the temperature, the more insurance. In fact, when the furnace temperature is too high, the time factor becomes too sensitive and the coating coking phenomenon is likely to occur. We need a proper furnace temperature. In this temperature range, baking can be guaranteed within 20 minutes, usually within 10 to 20 minutes. The construction quality is stable and good, so that an optimal drying condition is not difficult to master. Personnel should find a corresponding best condition according to different equipment conditions. In the appropriate temperature range, the quality problem caused by insufficient time is mainly due to poor adhesion of the coating. The prolonged time will cause yellowing of the coating and heavy coking, but slight yellowing will not affect the quality of use.

4, cooling

After the baking is completed, the barrel is cooled. The cooling method has natural cooling and accelerated cooling, and the accelerated cooling can be blown by the exhaust fan. The hot barrels that are baked allow for sudden quenching. The benefits of quenching are shortening the production cycle, reducing the chance of hot and soft coatings being stained with dust and collisions, and quenching reduces the crystallinity of the polymer, making the coating more flexible. . Due to the thermoplasticity of polyvinyl fluoride, the coating that has just been baked is still soft. Do not rub it immediately. After basically cooling, it can be stacked and sent to the next process (such as crimping, stamping, etc.). Need to continue to apply the second bucket, it is best not to touch it by hand, keep it dry and clean.