Printed iron coating common defects and analysis

With the rapid development of packaging technology and coating industry, the application range of tinplate packaging has become wider and wider, and it has been widely used in the packaging of canned foods, beverages, cosmetics and petrochemical products. In the food and beverage industry, many products have strong acidity, such as tomato sauce, pickled cucumbers, etc. Some foods contain a large amount of protein, which produces S2- ions and H+ ions during high-temperature sterilization, and has strong corrosion to the inner wall of the tank. Sexuality not only affects the shelf life of canned foods, but also affects the flavor of the products, which can easily lead to spoilage and deterioration of the products. The same applies to cosmetics and petrochemical products. Most of them are highly corrosive to the inner wall of the tank, and therefore have higher requirements for the inner coating of the tank. In order to prevent atmospheric and environmental media from corroding the outer wall of the tank, to avoid or reduce the appearance of mechanical damage, to protect the printing surface and improve the glossiness of the tank, the outer wall of the tank generally has an outer coating. In order to enhance the anti-corrosion capability of the inner and outer coatings of the cans and to truly realize the function of the coatings, various defects of the coatings must be prevented. Poor coatings can cause them to lose their protective effect and even accelerate the corrosion of the tank, and some can also affect the appearance of the tank.

1 Defects arising from improper selection of raw materials

There are many quality indicators of tinplate, among which tin plating quantity, tempering degree, cupping value and other indicators have a greater impact on the quality of machining. When these indexes do not match the mechanical processing adaptability of the product, the coating will wrinkle due to uneven deformation of the iron sheet itself. The surface condition and surface quality of tinplate are the main factors affecting the adhesion of the coating. If defects such as scars, pits, wrinkles, dust, and rust are found on the surface of the tinplate, the coating quality will be greatly affected and the thickness of the coating will not be produced. The lack of adhesion, insufficient adhesion, etc., can cause blushing, blistering, shedding, etc. when boiled or boiled.

In order to prevent the tin layer from being oxidized by the air and tinplates from rusting, the tinplate surface often has a film of oil. The presence of this oil film will greatly reduce the adhesion of the coating. If the oil film is not removed cleanly during coating, the adhesion of the lacquer layer to the iron surface is insufficient, resulting in cracking of the coating.

The surface roughness of tinplate is also one of the influencing factors. The state of supply of tinplate is mainly smooth, stone surface and hemp surface. Although some organic coatings have stronger adhesion when the surface is smoother, the specific surface area of ​​the stone surface and the matte surface increases, which is beneficial to the mechanical bonding and adhesion of the coating to the surface of the iron sheet, most of the coating and the stone surface. Or the combination of pockmarking is strong. If the product requires high temperature cooking, high temperature sterilization, deep drawing, repeated bending and other aspects of processing, and the coating adhesion requirements are high, the need to carefully consider the adaptability of the tinplate surface and coating, otherwise there will be coating cracking Shedding phenomenon.

There are various types of coatings and they all have different compositions. According to the different occasions, it can be divided into inner coating and outer coating. According to the performance of the coating, it can be divided into acid-resisting coating, anti-sulfur coating, anti-stick coating, high-temperature cooking coating, high-temperature sterilization coating, deep-resistance coating and other special coating and so on. Production must be based on the use of the product and mechanical processing requirements to select the appropriate coating, otherwise even if the amount of coating, curing state meets the requirements, the coating will not achieve the corresponding protection, there will be a variety of defects.

The viscosity of a coating is a property that hinders the relative flow within the coating. When the viscosity is large, the coating can produce excessive coating thickness under the same coating system. At the same time, the leveling property of the coating is poor, and the coating is easily caused. Uneven thickness, wrinkling or local accumulation, sometimes "orange peel" phenomenon. In the local accumulation area, partial curing may also occur incompletely, resulting in poor local adhesion, boiled water, and falling off. When diluting the paint, you should also choose the appropriate dilution solvent for the paint: if the solvent is used improperly, the coating will produce “small bubbles”, “pinholes” and “crease”. When the viscosity of the coating is too low, on the one hand, the thickness of the coating film is low, the denseness of the coating is poor, and the conductivity value of the coating is increased. On the other hand, at the time of coating, due to the poor adhesion of the coating material to the coating drum, under the action of the centrifugal force, the coating material will directly drip on the iron sheet, resulting in a phenomenon of dripping.

The solid content of the paint is also an important performance indicator of the paint. When the solid content is large, there are many film-forming substances. Under the same coating conditions, the coating thickness is large, and the protective performance of the coating is improved, but the mechanical processing performance is reduced, and at the same time, when the amount of single-use coating is large, Because the curing conditions are difficult to guarantee, poor curing, insufficient adhesion, cracking and shedding of the coating can easily occur. With high solids content, the leveling properties of the coating will be reduced to a certain extent, and defects such as “wrinkling” will occur accordingly.

2 Defects caused by poor coating

When coating, in order to cure well and enhance the adhesion of the coating, the thickness of the coating should be as uniform as possible to reduce the thickness of the same plate. Poor coating is divided into several cases: one is uneven coating, the same thickness of the coating film is larger, so that the curing conditions are difficult to meet the requirements of various thicknesses, prone to local due to excessive thickness and poor curing, Adhesion drops. In this way, the boiled boiled acid will appear on the scene. The reason for this phenomenon is that the coating system is defective, the colloidal hardness of the coating drum is not good, and the transfer performance is not good. The cylindricity of the cartridge is too large and the rotation of the drum is not smooth. When installing and adjusting the drum, the axes of the two drums are not parallel, and along the axis direction, the gap between the two drums is inconsistent.

The second situation is that the amount of coating film once coated is too large, the coating thickness is large or even the “orange peel” phenomenon occurs. Even if the curing temperature is increased and the curing time is increased, the fully cured state cannot be achieved. Acid whitening, low adhesion, and even shedding. The reason for this phenomenon is that in addition to the improper setting of the process, the gap between the upper and lower coating cylinders is too large (such as carefully adjusting the gap after refitting the cartridge after trimming the cartridge): too high a viscosity of the coating will cause this phenomenon. Before production, paint performance test should be done first to determine the most used viscosity of the coating, and carefully adjusting the roller gap can effectively prevent this phenomenon from happening.

The third situation is that there is a leak coating point, the coating film has a high conductive value, the corrosion resistance of the coating decreases, and even the protection effect is lost. The drum is not clean with foreign matter such as oil, solid dust particles, colloidal hardening off; the tin itself has foreign matter such as solid dust particles, the transmission of the oil leaking drops on the iron or rubber roller; coating itself has foreign matter, etc. are caused this The cause of a phenomenon. During the production process, it should be carefully observed whether there are any foreign materials in the rubber roller, metal sheet, and paint. If necessary, the paint should be carefully wiped or filtered.

The fourth case is that the coating is too thin, the conductivity value is high, and the corrosion resistance is low. The reason for the formation is the opposite of the second case, which is mainly caused by the reason that the clearance between the upper and lower rollers is too small or the viscosity of the paint is too low.

The fifth case is "coating with paint," which means that because the roller scraper is not sharp or the scraper installation position is not correct, the remaining paint on the roller is not clean. When the next sheet is coated, the reverse side of the iron foil is contaminated, and the reverse side is also brought. There is paint change. When this phenomenon is found, the condition of the squeegee should be carefully checked.

The sixth situation is “Lack of pollution”. The need for welding requires that some areas need to be left blank during coating. The side positioning system of the iron sheet and the rear pawl adjust the jaws; Leave empty pollution.

The seventh case is a cup mark, due to high humidity and other reasons, the metal suction cup is damp, there is a water mark on the contact point of the suction cup when the iron is sucked, and the coating is poor in the adhesion of the paint at the water mark. mark. The conductivity is increased by the presence of helium: suction cup marks.

The eighth case is a roller mark, which is mainly caused by blemishes: the coating has a large viscosity, poor leveling properties, or is caused by surface hardening of the roller glue. There are many defects caused by poor coating, as long as you can always find it through careful observation. (to be continued)