Pad printing technology and high-grade printing ink

The pad printing (mother) plate is to etch the pattern to be printed on the surface of a steel plate or a washed photopolymer film. When printing, the scraper (usually 0.25-0.5mm thickness is made of steel or copper sheet with good elasticity and absolutely straight edge) will scrape ink into the etched picture, and the remaining ink will be scraped back to the ink Bad. During pad printing, the flexible, soft pad printing head transfers the ink from the stencil to the substrate.

The rubber head made of silicone rubber transfers the ink on the steel template to the substrate. The plastic head must be soft and free of stain. So can be printed on different shapes, different hardness and different quality of the substrate, such as lighters, arc-shaped pens, toys, home appliances, switches, keyboards, buttons and other electronic and automatic equipment.

Pad printing advantages and hiding power characteristics

advantage:

- Variety of substrates to choose from - Fast cylinder printing

- Can print hard, hollow objects - can be synchronized with assembly and production lines

- Printable fine lines - closed system operation, slow solvent evaporation, no need to spend too much time on ink, such as ink viscosity

- Easy to operate

- Multi-color printing - Fast drying, low energy costs

- Short change time, especially suitable for small objects - The operating space is not too large, saving space

- Low printing costs

Hiding power features:

- Can print differently shaped objects

- Can be printed on different types of surfaces

2, printing and requirements

2.1 printing plastic head shape and comparison

The printing head is an important part of the pad printing machine and the pad printing process, which directly affects the ink transfer rate and printing quality. All standard plastic head shapes have one thing in common: The printed surface is convex, with a certain angle from the edge to the middle of the printed area. This convex surface affects the transfer of the ink but maintains the cleanliness of the ink print. Semi-circular, conical plastic head is ideal, can rotate evenly to prevent bubbles, but also easy to transfer ink from the steel template to the substrate, but the shape of the plastic head can sometimes be square or rectangular. If it is a square, you must ensure that the middle is a cone, so that the plastic head can be rotated. The result of printing with a rectangular rubber head is not ideal because the rubber head can only move in two vertical directions.

2.2 The size of the plastic head

To ensure the sharpness of the lines and the blemishes of the printing, the rubber head must be larger (that is, the effective projection area of ​​the plastic head is larger than the area of ​​the graphic on the steel template). Because the smaller the deformation of the plastic head, the sharper the printed lines, and the plastic head is slightly larger than the actual printed image, especially for some special printing, but the pressure required by the large plastic head is also greater and the price is more expensive.

2.3 The hardness of the plastic head

The hardness of the plastic head is usually from 2 to 18 degrees, and the special hardness is from 0 to 40 degrees. The higher the number, the greater the hardness. Hardness affects printing quality and printing life, hard plastic head printing effect is good, long life. However, in most cases, the rubber head is not too hard because it will damage the printed material; similarly, the soft head can be used for uneven surfaces, such as workpieces with large curvature of the printing surface. Therefore, the choice of hard plastic head, depending on the pressure when printing, the greater the pressure, the harder the plastic head. However, the pressure of the plastic head should not be too large. Otherwise, the substrate will have small cracks and prone to blemishes. In short, when the printed lines are thin, the plastic head is relatively soft, and the plastic head may be harder when printed on the plane, and the softer rubber heads should be used for the surface arc or uneven surface of the printed surface.

2.4 The quality of silicone rubber used in rubber heads and the transfer of ink:

The pad printing head is made of silicone rubber (Nanjing University ND-347 room-temperature vulcanized silicone rubber) and a flexible material such as polyvinyl chloride resin. There are two main types of silicone rubber: polymeric and polymeric.

From the physical properties such as anti-pulling, anti-solvent, plus polymerization head is better than the polymerization of plastic head, of course, the price of raw materials is also expensive. The smoothness of the surface is a decisive factor in determining the quality of the printing. For example, small stains or air bubbles can affect printing. At the beginning of pad printing, the new plastic head may not be able to absorb the ink. You can try it on the paper or use alcohol to clean it. Can also be used diluent cleaning, plastic head can immediately suck ink from the steel template; if the ink transfer is not very good, the surface of the adhesive tape can be used to glue, stick the dust off the plastic head. At the same time, the degree of rise and fall of the print head also affects the amount of ink transferred.

2.5 plastic head life

Usually a plastic head can print about 5,000 to 10,000 times, but it also depends on the type of ink used and the print quality requirements. With one-component inks, the life of the plastic head is longer and the two-component ink is shorter. The size, shape, hardness, and material shape of the plastic head all affect the service life. If the printing angle is sharper or the object is particularly curved, the rubber headprint will be damaged from 1000 to 5000 times. Silicone cleaning silicone oil.

3, the requirements of the steel template

Steel stencils can be used to transfer printed texts. The type of steel stencil used depends on the quality and quantity of the print. To ensure the fluency of the printing, the distance between the graphic on the steel template and the edge is at least 25 mm and it is fixed in the middle. If it is printed long lines or fine lines, the angle between the squeegee and the steel stencil is 5-15° to ensure that the squeegee will not scrape off too much ink. Several tests have shown that the etching depth of 25um is most suitable for pad printing because the amount of ink transferred during each transfer is small. If the etching depth is 25um, the thickness of the ink after printing is 12um, and the remaining ink remains on the steel plate. in. Because the ink contains 40-60% thinner, the ink film thickness after drying is 5-8um, sometimes only 2um. The etching depth of small fonts is 15 um, while it can reach 30 um in large areas. The materials commonly used to make steel formwork include T8, T10 tool steels and CrwMn, 9Mn2V alloy steels. According to the provisions of GB 230-83, the hardness between HRC=>45-60 is determined. No matter which type of steel is used, the etching diagram The roughness Rz value of the grinded and polished surface should be less than 0.8-0.2 um. Therefore, before the etching, it is important for the plate grinding.

4, printing ink

To obtain the desired printing effect, you must use a printable ink. Because the amount of ink transferred per shift is small, the color content of the pad ink is required to be high. Ink features include:

- Easy to operate - large color concentration

- long shelf life - good coverage and delicate ink

- Does not harm health - Short silk head, no wire drawing

- Protecting the environment - Fast drying (dry for about 3 seconds in an instant)

— Easy to clean - lower than substrate surface tension and ideal viscosity

- Adhesion is good, material does not require pretreatment and post-treatment

- Wide range of applications for various substrates

Note: The requirements for the ink vary depending on the application.

4.1 Composition and Function

• Inks include binders, pigments, solvents, and auxiliaries.

• The adhesive contains one or more resins. In most cases, the resin is generally powdery and must be dissolved in a suitable solvent or a mixed solvent. The choice and synthesis of the resin determine the print characteristics of the ink, such as transfer and adhesion to the substrate, gloss and chemical resistance.

·Solvents are the decisive factors for the drying speed of pad printing inks. At the same time, the printing speed and adhesion are all influencing factors.

The effect of slow drying agent on the pad printing is not too great, because the pad printing speed is generally fast, only when using fast drying ink, such as when using Baolong KSU series ink. Slow drying agents evaporate slowly, so they do not have much effect on printing.

The diluent is contained in a solvent and the diluent is mixed with the ink to obtain a desired viscosity, which facilitates ink transfer and transfer.

• Pigments reflect the hue of the ink and determine its background color. Can be divided into organic and inorganic pigments, requiring pigments free of heavy metals, color concentration.

• Auxiliaries are generally used in small quantities, and their role is to adjust the properties of the ink such as flowability, sticky handling, or surface properties. Additives are leveling agents, thickeners and so on. No matter what kind of ink, it should be how much with each use, and fully mixed, in order to ensure that the printable viscosity, printing ink should be properly adjusted to ensure the continued printing.

4.2 Features

The following are the different requirements for printing:

— The matte, semi-glossy, glossy, or specular surface is required on the printed surface.

— The ink is chemically resistant and UV resistant.

- Lightfastness, scratch resistance or salt water resistance.

— The printed surface needs to be transparent or have good coverage.

4.3 Ink Type

— Physical drying ink (single component)

— Chemically curable inks (two-component)

- Thermal curing ink

- UV curing ink

— Water based ink

* The one-component ink can be added thinner to get the ideal printing viscosity. The drying method is physical evaporation type. Some thermoplastic substrates such as PS, PC, and PVC will have slight dissolution. This dissolution will make the ink directly bond with the substrate. Therefore, it is scratch-resistant and has good adhesion.

* The chemical resistance of two-component inks is higher. Generally, hardeners are added to inks. However, care must be taken to add them before printing production. If they are added for too long, the effect is not good. Whether or not it is completely dry depends on the ambient temperature; it takes several days to dry in a room temperature of 20°. Therefore do not perform scratch and adhesion tests prematurely.

* Thermosetting inks are generally two-component inks. Resin bonding of such inks does not occur at room temperature, but only at elevated temperatures.

* Oxidation type inks react by air oxidation. Generally, the reaction speed is slow. It is 1 to 2 days at room temperature.

* The transfer of printing ink mainly depends on the evaporation of the solvent, evaporation makes the ink film adhesive, while adjusting the ink adhesion. Good hiding power is achieved through the transfer of the ink film layer. Due to the lack of solvents in UV inks, the ink film layer cannot be changed. This shift is "water transfer" and the transfer is difficult.

UV curing inks are generally used for high quality printing.

The advantages of UV inks are:

— The ink on the steel template is not dry

- quality assurance

— Less wear on steel formwork

- Fast drying

- Solvent-free

The disadvantages of UV inks:

- High energy requirements require UV curing equipment.

— High requirements for cleaning rubber heads, less stringent than solvent inks

- Finer printing than solvent ink

* Water-based inks have not been used for pad printing since the yield is low, and the drying speed, viscosity, adhesion, and evaporation rate are not as good as solvent inks.

4.4 Preparations before printing

The amount of diluent and hardening agent must be carefully weighed; the substrate should be treated; some substrates have oil or silicon on the surface. The surface must be processed before printing. The simplest method is to use alcohol.

PP or PE can be treated with corona or flame. The corona treatment is 20,000 volts and the flame can be adjusted during flame treatment. Another method is to treat the surface with PP water.

Some plastics have a strong electrostatic surface and can be treated with an ionizer.

In multi-color printing, the drying can be accelerated by the ventilation system. However, it must be noted that the ink should not be blown directly when printing, otherwise the diluent will evaporate too quickly. Surface treatment improves the adhesion of the ink.