In the process of screen printing silicone, to ensure that the ink is evenly attached to the substrate, it is necessary to comprehensively control the substrate pretreatment, ink mixing, screen printing equipment and operation, environmental control and curing treatment. The following is a detailed introduction:
Substrate pretreatment
Surface cleaning
Principle: If there is oil, dust, impurities, etc. on the surface of the substrate, it will hinder the direct contact between the ink and the substrate, resulting in a decrease in the ink adhesion or even failure to adhere.
Method: Select a suitable detergent according to the material of the substrate. For example, for plastic substrates, alcohol or special plastic detergent can be used; for metal substrates, degrease can be used to remove oil first, then rinse with clean water and dry. After cleaning, wipe with a clean dust-free cloth to ensure that there is no residual detergent and impurities on the surface.
Surface modification
Principle: Some substrates have low surface energy and insufficient affinity with ink. Surface modification can increase their surface energy and enhance the adhesion of ink.
Methods:
Corona treatment: Applicable to substrates such as plastic films. Air is ionized by a high-voltage electric field to generate plasma to bombard the surface of the substrate, causing changes in the molecular structure of the surface, increasing surface roughness and polar groups. For example, after corona treatment, the surface energy of polyethylene (PE) film can be increased from 32 dynes/cm to 38-40 dynes/cm, and the ink adhesion is significantly enhanced.
Flame treatment: High-temperature flame is used to cause oxidation reaction on the surface of the substrate to generate polar groups and increase the surface energy. It is often used to treat plastic materials that are difficult to adhere to, such as polypropylene (PP). However, it is important to control the flame temperature and treatment time to avoid deformation or burning of the substrate.
Chemical etching: Use chemical reagents to slightly corrode the surface of the substrate to increase the surface roughness. For example, for metal substrates, dilute hydrochloric acid or dilute sulfuric acid can be used for a short soaking treatment, then rinsed with clean water, neutralized, and dried.
Ink preparation
Choose the right ink
Based on the material of the substrate: Substrates of different materials have different adaptability to ink. For example, silicone inks are usually divided into self-adhesive and non-self-adhesive types. Self-adhesive silicone inks can be directly used on silicone substrates with good adhesion; for some special materials, such as fluororubber, special fluororubber inks need to be selected.
Consider the use environment: If the product is used outdoors, you need to choose an ink with good weather resistance that can resist ultraviolet rays, wind and rain erosion; if it is used in food contact areas, the ink must meet food safety standards and be non-toxic and harmless.
Accurate mixing ratio
Strictly follow the formula: Silicone inks are usually composed of a main agent, a curing agent, a diluent, etc., and must be mixed strictly according to the formula ratio provided by the supplier. For example, a certain brand of silicone ink requires a mass ratio of the main agent to the curing agent of 10:1. If the ratio is unbalanced, the ink will not be fully cured or the performance will decrease, affecting the adhesion.
Fully stir evenly: Use a stirrer to stir the ink thoroughly to mix the ingredients evenly. The stirring time is generally not less than 3-5 minutes, and excessive bubbles should be avoided during the stirring process. If there are many bubbles, a vacuum degassing machine can be used for degassing.
Silk-screen equipment and operation
Silk-screen selection and production
Screen mesh matching: Select the appropriate screen mesh according to the fineness of the printed pattern. When the pattern lines are thicker and the details are less, you can choose a screen with a lower mesh (such as 80-120 mesh); if you want to print fine patterns and text, you need to use a high mesh screen (such as 200-300 mesh).
Screen tension control: The screen tension must be uniform and meet the requirements. Generally, the screen tension is between 18-22N/cm. If the tension is too small, the screen is easy to deform during the printing process, resulting in uneven ink penetration; if the tension is too large, the screen is easy to break.
Uniform coating of photosensitive adhesive: When coating photosensitive adhesive on the screen, ensure that the thickness of the adhesive layer is uniform and there are no defects such as bubbles and pinholes. The quality of the adhesive layer can be ensured by multiple coating and drying.
Scraper adjustment
Scraper pressure: The scraper pressure should be moderate. If the pressure is too low, the ink cannot evenly penetrate the screen, and the printed pattern is light and not full; if the pressure is too high, the screen will be scratched, and it may also cause ink leakage, making the pattern edge blurred. Generally, it is adjusted according to the mesh count, ink viscosity and substrate characteristics. You can first conduct a small batch test print and gradually adjust to the optimal pressure.
Scraper angle: The angle between the scraper and the screen is usually between 70° and 80°. If the angle is too small, the ink penetration will be reduced; if the angle is too large, the friction between the scraper and the screen will increase, accelerating the wear of the screen.
Scraper speed: The scraper speed should be selected according to the size and complexity of the pattern. For large-area patterns, the scraper speed can be appropriately increased; for fine patterns, the speed needs to be slowed down to ensure uniform ink penetration. The general scraper speed is between 20 and 40cm/s.
Printing pressure and speed control
Printing pressure: The printing pressure of the screen printer should be uniform and stable, which can be achieved by adjusting the screen spacing and pressure adjustment device of the screen printer. Appropriate printing pressure can make the screen plate contact well with the substrate and ensure uniform transfer of ink.
Printing speed: The printing speed should be uniform, avoiding fast and slow. If the speed is too fast, the ink will not have time to evenly penetrate the screen; if the speed is too slow, the ink may accumulate on the screen, affecting the printing quality.
Environmental control
Temperature and humidity
Temperature: The suitable temperature of screen printing silica gel is generally between 20℃ - 25℃. If the temperature is too high, the solvent in the ink evaporates too quickly, which may cause the ink to dry up on the screen, affecting the printing effect; if the temperature is too low, the viscosity of the ink increases, the fluidity becomes worse, and it is difficult to evenly adhere to the substrate.
Humidity: It is more appropriate to control the relative humidity at 50% - 65%. If the humidity is too high, the ink is easy to absorb moisture in the air, resulting in changes in ink performance and decreased adhesion; if the humidity is too low, static electricity is easy to generate, dust is adsorbed, and printing quality is affected.
Cleanliness: The screen printing workshop should be kept clean to reduce the presence of pollutants such as dust and impurities. Air purification equipment such as air filters and laminar flow hoods can be used to filter dust particles in the air. Operators should wear protective equipment such as dust-free clothing and dust-free gloves to prevent human hair, dandruff, etc. from falling on the ink or substrate.
Curing treatment
Choice of curing method
Thermal curing: Applicable to most silicone inks, the curing agent in the ink undergoes a chemical reaction by heating to achieve curing. The curing temperature and time should be set according to the type of ink and the characteristics of the substrate. For example, the curing temperature of a silicone ink is 150°C and the curing time is 3-5 minutes. During the curing process, ensure that the temperature is uniform to avoid local overheating or overcooling.
UV curing: Some silicone inks can be cured by UV, which has the advantages of fast curing speed and high efficiency. UV curing requires the use of UV lamps, and the wavelength and energy of the UV lamps should be matched according to the requirements of the ink. During curing, pay attention to controlling the distance and irradiation time between the UV lamp and the substrate to ensure that the ink is completely cured.
Post-curing inspection: After curing, the printed product needs to be tested for adhesion. The grid knife test method can be used. Use a grid knife to cut 10×10 1mm×1mm small squares on the printed pattern, then stick 3M tape tightly on the small squares, quickly tear off the tape in a vertical direction, and check the pattern shedding. According to the standard requirements, the pattern shedding area should be less than 5% to be qualified. If the adhesion does not meet the standard, it is necessary to analyze the cause, adjust the curing parameters or reprint.