The object of metal printing is a non-absorbent material, a low-carbon tinplate, commonly known as tinplate. The printing principle of tinplate is similar to that of lithographic offset printing, but there is a special aspect in the process, such as:
1. After the tinplate is printed, the ink stays on the surface and must be dried and solidified by some form of heat energy;
2. The tin layer on the surface of the tinplate is bright and shiny, and its spectral characteristics are considered as the black phase. Therefore, before the color printing, the surface of tinplate must be coated with white paint. Of course, depending on the special artistic effect of the product pattern, it may be partially coated with white paint or directly using the tinplate surface itself;
3. Tinplate after printing the finished product. Need to go to the next process for machining, made into a variety of shapes of containers or other packaging. In the machining process, the ink coating on the tinplate surface must be resistant to mechanical impact and extension. Special products such as cans and containers, the ink coating also has the physical properties of autoclave resistance. This requires excellent adhesion between the ink coating and the tinplate surface.
The quality of the ink coating on the surface of tinplate has always been a long-term discussion and careful study by professionals in practice. It is related to the surface properties of tinplate itself, the types of ink used in the printing process, the type of coating, the baking temperature, and the thickness of the ink coating. Now on how to enhance the adhesion between the ink coating and tinplate, talk about personal superficial views.
1. Relationship between surface properties of tinplate and adhesion of ink coating Tinplate is a low carbon steel sheet with steel plate as base material and both sides coated with pure tin to make it glossy and bright. Tin-plated steel sheet is divided into hot-dip tinplate and tin-plated plate by tin plating process.
The structure of tinplate is composed of a steel substrate, a tin-iron alloy layer, a tin layer, an oxide film, and an oil film. as the picture shows:
From the structural point of view of the tinplate, its surface is coated with a thin layer of oil film in order to prevent the tinplate from being rusted during storage and transportation, so that it is easy to lift the tinplates of the tinplate one by one during printing. Most oil film components are dioctyl sebacate or acetyl tributyl citrate. At present, the tinplate produced in various countries in the world has no certain standard for the thickness of the oil film. Generally, the amount of oil applied is controlled at 2 to 4 mg/m2, but some are at 6 mg/m2 or more. If too much oil will affect the quality of tinplate erosion, so that the coating, ink layer can not completely wet the surface of tinplate, in the curing of the ink layer after the formation of circular or oval pore defects, commonly known as "eyehole" . The "eyehole" will cause the adhesion between the ink layer and the tinplate to decrease. Therefore, when the tinplate surface oil is over 6mg/m2, the tinplate must be pre-baked once before printing to make excessive oil film volatilize. The pre-baking temperature is generally between 200°C and 15min, and the speed is 4200r/h.
In addition, due to the poor production environment, such as the tinplate covered with fine water droplets before printing due to moisture, the seasonal climate is too cold, etc., will prompt the tinplate surface coating and ink coating insoluble, affecting Adhesion. Similar to this phenomenon, pre-baking before printing is basically ruled out.
The easiest way to detect the quality of the tinplate surface oil film is the liquid tension and casting test of the paint. If you take a small amount of paint, pour it slowly onto the flat surface of the tinplate and observe the paint. At this point, the coating liquid spreads rapidly to the surroundings, and the radius of the center point of the liquid is approximately equal to the circumference. Then, the tinplate is lifted up to make a 45-degree angle with the plane. At this time, the liquid flows evenly to the other end. Then the tinplate The amount of film meets the printing requirements. If the paint spreads on the surface of the tinplate or spreads irregularly to the surroundings, the amount of oil film does not meet the requirements before printing, and the tinplate should be pre-baked and printed.
The second layer of the tinplate surface structure is an oxide film. After tinplate tin plating process is completed, there is a very thin oxide film on the surface of tin. Its composition is mostly divalent tin or tetravalent tin oxide and its hydrate, and the structure is very unstable. Some people think that the thinner the oxide film, the better the adhesion, and vice versa. This argument is not without reason. After the tinplate is tin-plated, passivation treatment is performed. The purpose of the passivation treatment is to stabilize the surface of the tinplate and prevent the growth of the surface oxide film. Excessive oxide film will discolor tinplate during long-term storage, especially after printing and baking. Due to the structural instability, the ink layer on the oxide film is peeled off, the ink adhesion performance is lost, and the product quality problem occurs. Electroplated tinplate surfaces are usually chemically or electrochemically passivated. Hot dip tinplate is not passivated during production. Therefore, when producing various kinds of exquisite packaging metal containers, the manufacturer must correctly select the material of tinplate according to the characteristics of product modeling and machining.
Second, printing paint. Ink and the relationship between the force of tinplate printing, also known as Indian iron, is a lithographic offset class, the basic principle is to use the oil and water insoluble physical phenomenon to print. Therefore, in addition to the printability of offset printing inks, iron printing inks must also possess several properties such as heat resistance, solvent resistance, processability, and resistance to retort. The team printed tinplate products should use paints and inks selectively.
1. Primer Coatings Primer coatings are the first coating necessary for the tinplate surface printing process. Its main function is to firmly adhere to the surface of the tinplate, and at the same time, it can be easily attached to and adhered to the various inks on the tinplate so that the tinplate printing product can be bent, impacted, stretched, crimped, etc., and the ink is formed. The coating will not be damaged by machining. Therefore, the choice of primer coating requires the paint to have good wettability on the surface of the gold crumb, and it has a very good mechanical strength after film formation. It is generally believed that alkyd, epoxy and other resins have these properties. This is due to the fact that the resin structure contains aliphatic hydroxyl groups, ether groups, and extremely reactive epoxy groups. The polarity of the hydroxyl group and the ether group causes the electromagnetic attraction between the resin molecule and the adjacent surface, and the epoxy group reacts with the surface of the metal containing the active hydrogen to form a chemical bond, so the adhesion is particularly strong. (To be continued)