The object of metal printing is non-absorbent material-tinplate tinplate. Its printing method is similar to lithographic offset printing, but has its own unique aspects in the process, such as:

1. The tinplate ink stays on the surface after printing, it must be heated to dry and solidify;

2. Because the tin layer on the surface of tinplate is bright and bright, it is customary to apply a layer of white paint on the iron sheet before color printing. Of course, this depends on the requirements of the printed matter, and it can also be directly coated on the front side of the iron sheet, which is commonly referred to as the tinned surface;

3. After all the processes are printed, it needs to be processed into packaging of various shapes and sizes. After the coating is processed, it must meet the requirements of impact resistance and elongation resistance; if it belongs to food Grade varieties also need to have high pressure cooking conditions; this requires excellent adhesion between the coating and the substrate. The adhesion of the coating to the substrate is related to the surface properties of the tinplate itself, the type of ink, the baking temperature, and the thickness of the ink coating.

Now on how to enhance the adhesion between the ink coating and the substrate (ie tinplate tinplate), learn from and exchange with industry experts:

1. The relationship between tinplate surface properties and ink coating adhesion

Tinplate is based on a thin steel plate, coated with pure tin on both sides to make it shiny and bright; according to the different processes, it can be divided into two types: hot dip plate and electroplated plate; mainly composed of steel substrate, tin-iron alloy layer, oxide film, Oil film and tin layer.

First of all, the surface of tinplate is usually coated with a very thin film of oil, most of which are composed of dioctyldioic acid ester or acetyl tributyl citrate. The purpose is mainly to prevent the tinplate from rusting, and to facilitate the separation of piles of tinplate one by one during printing. Generally, there is no fixed standard for the thickness of the oil film, and it should be controlled at about 3-5mg / m2. It should not be too thick so as not to affect the wettability of the coating on the surface of the substrate, to prevent printing failures such as shrinkage after printing and drying. Due to seasonal changes in the environment, tinplate is also susceptible to moisture, which directly affects the coating The incompatibility of the layer leads to poor adhesion between layers. For this reason, many can-making plants pre-treat tinplate with oil film volume exceeding 5-7mg / m2 to make the oil film completely volatilized before use. The pretreatment temperature is generally within 200 ℃ / 15min, and the vehicle speed is controlled at 4000r Within / h is appropriate.

Normally, the simplest and most direct way to check whether the amount of tinplate oil film is qualified is to take a small amount of paint and pour it on a flat tinplate to observe its diffusion. Generally, the radius of wetting from the center point to the surrounding is approximately equal. At this time, lift the tinplate into a corner At an angle of 45 ° C, the paint can flow evenly in the other direction, which means that the amount of the oil film meets the printing requirements. If the condensation is not scattered or spreads irregularly to the surroundings, it means that it does not meet the printing requirements and should be pre-baked before use.

The second layer of tinplate is an oxide film. Generally, after the tinplate tin plating process is completed, a thin oxide film is left on the surface of the tin. Its structure is very unstable, which easily causes the coating to peel off and affect its adhesion. Therefore, the tinplate should be passivated every time after tin plating is completed, the purpose is to increase its stability; too thick oxide film is easy to change the color of tinplate, especially after high temperature baking; usually, electroplating The board is passivated, while the hot-dipped board is just the opposite. Therefore, to choose the applicable tinplate, it must be decided according to the different requirements of customers on printed matter.

Second, the relationship between printing paint Yao ink and adhesion

The printing of tinplate is commonly known as printing iron. The production of this kind of product is called printing iron ink. It belongs to the lithographic offset printing method and uses the working principle of oil and water repulsion. This kind of product has relatively high requirements and must have the following properties: , Solvent resistance, mechanical processing resistance and cooking resistance, etc. Therefore, the printing plant should be required to use printed iron products selectively according to the different requirements of the printed matter.

1. Priming paint

Priming coating is the first process in tinplate printing, it helps the adhesion between the top coating and ink, so that the coating should not be damaged during various mechanical forming processes, so choose primer The coating must first consider its wetting properties on the metal surface, and has good mechanical processing properties after film formation.

It is usually formulated with alkyd, acrylic and polyester resins, supplemented by epoxy resins. Because the structure of these resins contains aliphatic hydroxyl groups, ether groups and extremely active epoxy groups, the resin molecules and Electromagnetic suction is generated between adjacent surfaces, and the epoxy group reacts with the metal surface containing active hydrogen to form a chemical bond, so the adhesion and water resistance are particularly strong. There is no generation of by-products, no bubbles, and the shrinkage rate is also small. , Will not deteriorate due to heat.

In the normal printing process, in addition to choosing an excellent primer, scientific and reasonable production processes, the thickness of the paint film, uniform coating, and baking temperature are all key, especially the thickness of the paint film If the coating is thin, it will not work. If the coating is thick, it will easily cause the shrinkage of the coating film and affect its adhesion. The baking temperature should theoretically be lower than the actual temperature, but the range should not be too large, generally controlled at 10 ~ 15 ℃ Left and right, artificially leaving some unreacted polar groups, so that when overprinting colors, the organic solvent in the ink can swell the primer, so that the polymer chains between the two interfaces between the coatings are more encrypted, enhancing the Adhesion. [next]

2. Printing ink

Since the tin plating layer of tinplate is relatively smooth, although a layer of primer has been applied, if the adhesion of synthetic resin to its surface is ignored during the production of ink, the produced ink cannot meet its printing adaptability. .

Generally, metal-modified inks are more suitable for oil-modified alkyd resins. These resins are modified and polymerized with vegetable oils or fatty acids. According to their properties, they can be divided into two categories: dry and non-dry; according to their content The amount of vegetable oil or fatty acid can be divided into short, medium and long oil; the dry resin mainly includes soybean oil, dehydrated castor oil and other components, which can be dried by itself or can be used together with amino resin. After baking and curing, the characteristics of firm adhesion, high gloss, strong hydrophobicity, good light retention and color retention can basically meet the requirements of tinplate printing suitability and product mechanical processing.

The printing of tinplate is due to the uniqueness of its process. The printing of products often goes through multiple overprinting of colors, drying, and it is a non-absorbent and smooth surface. Therefore, the thickness of each color ink layer should be controlled during the printing process. If the layer is too thick, the cohesion between the layers will be large, which will affect the adhesion of the entire coating to the surface of the substrate, which will directly reduce the adhesion of the entire coating. The proportion of titanium dioxide is relatively large, and the thickness of the ink layer should be controlled at 14 ± 2g / m2. Only in this way can the chromaticity of the white ink be guaranteed and the adhesion of the ink layer can be maintained.

3. Overcoat

After overprinting in multiple colors, a protective film needs to be applied on the surface of the ink, which is a varnish. After the varnish is applied, it not only increases the aesthetics of the entire printed matter, but also achieves the ideal adhesion. The key is that its gloss, flexibility, wear resistance, impact resistance, etc. have been greatly improved.

Commonly used varnishes are mostly composed of the following three types of resins:

A. Epoxy resin: because of its relatively high polarity and fatty hydroxyl and ether bonds that are not easily hydrolyzed, it not only has good adhesion and high chemical resistance; but also because its structure is a rigid benzene core and The flexible hydrocarbon chains are arranged alternately, which promotes the enhancement of their physical and mechanical processing performance. However, due to the presence of polar groups, the coating film has a certain hydrophilicity, and its water resistance can generally be solved by the isolation effect of the cross-linked structure.

B. Alkyd resin: due to its medium and short chain products containing castor oil, coconut oil and synthetic fatty acids, it has the characteristics of high saturation and good oxidation resistance; it is made by copolymerization with amino resin, epoxy resin, etc. The coating has the characteristics of firm adhesion, toughness, heat resistance, weather resistance and solvent resistance, and is widely used in the printing iron industry.

C. Acrylic resin: This kind of resin generally adopts hydroxyacrylate, such as hydroxyethyl acrylate and acrylic acid copolymerization, introduces hydroxyl and carboxyl groups in the resin, and uses amino resin as a cross-linking agent. , It has the characteristics of acid resistance, alkali resistance, water resistance, high temperature resistance and no yellowing, and meets hygienic standards.

In fact, no matter which type of varnish has its own advantages and disadvantages, the appropriate varnish should be selected according to the different requirements of the printed matter, but no matter how you choose it, you must first understand the various technical parameters of its products. So that printing can proceed normally.

3. The effect of baking temperature on adhesion

In general, the coating of printed iron products is dried in the form of heat curing, which is also one of the main differences between offset inks and different. Since the raw materials used in each product are different, the baking temperature is also different. Variety. Throughout the printing process, the baking temperature is the key to affecting the product quality:

a. If the temperature is too low, the chemical molecular bonds in the resin are not fully cross-linked and cured to form a film, which not only does not have adhesion, but also causes the back side to be smudged and reverse printing occurs; and in the next printing process, due to the ink Difficult to separate sheets due to adhesion; especially the key is that the back of the printed sheet is the inner wall of the container. If it is a food can, it will directly contaminate the? Hot Smoke? / P>

b. If the temperature is too high, the molecular bond will break, the coating will be brittle, and the coating will not withstand mechanical processing and high temperature cooking to produce explosive paint. Therefore, the error value of the baking temperature should be controlled within ± 3 ℃, and the circulating air volume should also be controlled at 21600m3 / h. In addition, the speed of the machine should be regularly checked to ensure that each coating There is a suitable baking time.

All in all, the quality of its attachment involves many factors, both objective and subjective, and the answer is also ingenious and unique. The above text is only representative of my personal point of view. Due to the limitation of knowledge, there are certainly unsatisfactory areas. I sincerely ask colleagues in the industry to criticize and correct and study together.

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