Paint IV in steel drum for the hottest food

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Coating in steel drums for food (IV)

precautions: there should be no black smoke in the drying channel; Control the dimethyl vapor concentration in the oven: maintain sufficient exhaust volume

electric heating oven: it is generally determined according to the following conditions:

temperature of the baking object in the first period of time, the middle period of time and the later period of time

barrel body ℃ ℃ 230 ℃

barrel bottom cover ℃ ℃ 230 ℃

baking time, i.e. speed, as a major adjustment factor, is affected by many conditions, such as the thickness and thickness of the paint film, and the number of barrels in. You can choose a condition according to the completion of plasticization of the outgoing film, Volatilize the solvent completely in the oven. Generally, the effective time is controlled at about 10 ~ 15min. If the oven temperature cannot reach the above temperature, the time can be extended appropriately, not more than 20min, but if the temperature of the front and middle section is lower than 230 ℃, the quality cannot be guaranteed

precautions and safety work: ① prevent open fire. Dimethyl vapor evaporated from the coating during baking reaches a certain concentration, and contact with open fire will cause combustion. Therefore, attention must be paid in the construction: the electric heating wire should be controlled to be used slightly below the rated voltage; Prevent heating elements from reddening; Prevent electric spark caused by poor contact of power connector. ② It is forbidden to close all exhaust outlets during normal production, which will cause danger. ③ Control the vapor concentration of volatile gas dimethyl ester in the drying channel (or drying room) and maintain sufficient exhaust air volume. ④ Control the number of coated parts entering the drying channel and drying room within a unit time, and do not put too much and too dense. In case the local concentration of solvent gas is too high and unsafe

3. Discussion on the influence of baking temperature and holding time on coating quality

① temperature: the workpiece should be sent after rising to the specified furnace temperature. Generally, the available temperature range is 240 ~ 270 ℃. In fact, most of them take 240 ~ 260 ℃, which depends on the specific situation. Baking operation, one is the temperature factor, the other is the time factor, which affects each other. The operator should first decide a suitable temperature, and then find the corresponding suitable time. The appropriate temperature depends on the barrel, that is, the thickness of the material, the quantity of each furnace, the condition of the material, etc. If the material is thick and the quantity is large, the heat capacity is large and the temperature rise is slow, so a higher baking temperature is required. Therefore, in actual production, the temperature often exceeds 240 ℃, and the temperature for processing thick barrels even needs more than 300 ℃

practice has proved that it is not feasible to reduce the temperature duration. There is a minimum temperature for baking plasticization temperature. If it is lower than the temperature, defective products will be baked. This minimum temperature increases with the increase of the heat capacity of the workpiece. At a low temperature, the baking quality problems are mainly manifested in the cracking of the coating, ranging from cracks at the corners to cracks in the whole coating, and the adhesion is significantly reduced

② time: generally 5 ~ 20min. After the workpiece is put in for about 1 ~ 5min, it starts to smoke a lot (dimethyl vapor). In principle, it can be discharged immediately after the latent solvent volatilizes and the baking has been completed. Baking cycle is affected by furnace temperature, thickness and quantity of barrel materials, film thickness and other factors. When the furnace temperature is the same, it is affected by factors such as wall thickness, large quantity, film thickness, baking temperature, etc. For the same kind of workpiece, the number is the same. If a higher furnace temperature is specified, the baking cycle will be shortened. On the contrary, if a lower furnace temperature is specified (not too low), the baking cycle will be extended. However, unlimited delay is not allowed. Generally, it should be completed within 20min. What if you can't finish it in 20min? The answer is to raise the furnace temperature and speed up the heating rate. But it's not that the higher the temperature, the safer it is. In fact, when the furnace temperature is too high, the time factor becomes very sensitive, and it is easy to cause coating coking. We need an appropriate furnace temperature. Within this temperature range, baking can be completed within 20min, generally within 10 ~ 20min. The construction quality is stable and good. Such an optimal drying condition is not difficult to master. Operators should find out the best condition according to the equipment conditions that it is difficult for different small manufacturers to compete with large manufacturers. In the appropriate temperature range, the quality problem caused by insufficient time is mainly the poor adhesion of the coating. The extension of time will cause the coating yellowing, or coking, but the slight yellowing will not affect the use quality

4. Cooling

after baking, the barrel parts are discharged for cooling. The cooling methods include natural cooling and accelerated cooling. The accelerated cooling can be blown by the exhaust fan. The advantage is to shorten the production cycle and reduce the chances of dust and collision damage of the hot and soft coating. In addition, accelerated cooling can reduce the crystallinity of DSM polyamide materials with rich color selection polymers, so as to make the coating more flexible. Due to the thermoplasticity of PVDF, the coating just baked is still soft. Do not touch it immediately. After it is basically cooled, it can be stacked with each other or sent to the next process for processing (such as crimping, stamping, etc.). It is better not to touch or touch the barrel parts that need to be coated for the second time, and keep them dry and clean

IV. quality inspection methods

the qualified coating surface is flat, free of cracks, dirt and flaws, uniform color, no embrittlement, no trace of nail scoring, sharp scraping with nails at the corners to prevent bulging and peeling, and further cutting with a knife can not be peeled from the breach

PVDF coating has excellent anti-corrosion performance. At present, it has been tested to contain more than 100 kinds of chemical products. The different processes of surface treatment, spraying and baking of the plates used by barrel manufacturers have an impact on the PVF coating. The existence of such products can provide the most effective assistance for many industries. Therefore, necessary filling tests should be carried out for each new product before filling, so as to eliminate the hidden dangers of product quality in the factory

the anti-corrosion coating should also be inspected through pinholes to avoid reducing the anti-corrosion effect due to pinholes. Pinhole inspection method is as follows:

① 5% copper sulfate solution is soaked at room temperature for more than 24h, and copper precipitation on the surface indicates that there are pinholes

② 1000V hand pendulum megger, one pole is in contact with the metal of the workpiece, and the other pole is connected with the brush dipped in salt water, which moves on the coating surface. When the pointer jumps, it indicates that there is a pinhole at the position. It can also be replaced by a 1.5V dry battery, equipped with a sensitive galvanometer or microammeter

③ pinhole detection instruments can be purchased directly from the market, such as spark pinhole detection instruments

adhesion inspection (cross cut method): Cut 6 parallel scratches with a length of about 10 ~ 20cm and a spacing of 1mm on the paint film, and then turn it 90 °, and cut it into a square grid with the same method. The adhesion of PVF coating is required to be above grade I. Refer to international gb325-91 for details

v. treatment methods for common defects

1 Common quality problems and treatment methods (see the magazine for detailed tables)

2 Coating repair and removal

the coating quality is unqualified, so it is much easier to rework before baking. Just wash it with alcohol or gasoline, and it is much more difficult to remove the coating after baking and plasticization. In view of this, the thermoplastic characteristics of the polyvinyl fluoride coating should be used as much as possible, and the repair method should be adopted instead of fading. But there are often four situations that need to be considered to fade; Large area of scorched, the appearance needs to be good-looking, which belongs to the matching surface that requires flatness and has been coated for many times or repeatedly repaired. (end)

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