Technical Characteristics of Silicone Rubber Key Screen Printing Process

Abstract: This paper introduces the silicone rubber screen printing process, analyzes its differences from the common screen printing process, expounds its technical characteristics, and summarizes its own views on technical control.

With the rapid development of communication and electronic industries, rubber button production has also developed into a mature technology. These key products are molded by silicone rubber through high temperature molding to form the required key surface, and then formed by spraying, screen printing, punching and stripping and other processes. Here I make a preliminary discussion on this screen printing process.

1、 Features of silicone rubber keys

As the key material, the hardness of silicone rubber is generally about 45 °, and the single key surface of the printer is convex, concave and flat, all of which are printed in line. In addition to accurate position and clear picture and text, printing ink is required to be suitable for printing silicone rubber. The ink layer reaches a certain thickness, has strong adhesion and wear resistance, and does not change color after baking at high temperature. Therefore, it puts forward special technical requirements for screen printing process.

2、 Screen printing process

There are many methods of screen printing plate making, mainly using the photo sensitive plate making method, that is, directly coating the photoresist on the net, and forming a certain thickness of photo sensitive film after drying to become a graphic carrier. The process flow is as follows:

Screen selection → frame selection → screen stretching → screen cleaning → drying → coating photosensitive adhesive → exposure → development → printing

The specific process is as follows:

1. Mesh frame: generally made of aluminum alloy, easy to operate, strong and not easy to deform. Wipe the mesh frame with cyclohexanone before stretching the mesh, remove the residual grease and dirt, wipe with industrial alcohol, clean with water, and then dry the mesh frame. In the screen printing plate making operation, it needs to be dried for many times, so the drying condition and cleaning should be strictly controlled.

2. The screen shall be stretched with silk screen first, and nylon screen shall be used for rubber screen printing. The screen passing rate of the ink is good and the tension is constant. The screen that can cover the edge of the screen frame and extend 6~10Cm beyond the periphery of the screen frame shall be selected. The screen shall be stretched with a pneumatic screen stretching machine, and tested with a six point method with a tensiometer. After reaching the required tension, the universal adhesive shall be coated to make the screen stick to the screen frame. The drying shall be accelerated with hot air, and the screen stretching machine can be turned off after about 5mInR is stuck, Stick the excess screen on the outside of the screen frame to maintain the bonding fastness of the screen. Cut off the excess screen, clean the screen surface with cyclohexanone and industrial alcohol, and then dry it. If the tension of the stretching screen is unstable, the font may be offset and blurred, and the substrate with chordal surface may cause the difference of ink layer thickness. The flat type printing needs a screen plate with high tension, and the printing surface with curved surface needs a screen plate with low tension. The screen plate tension range for process application is 3N/Cm ～ 7N/Cm. Generally, the screen plate with large tension has a short service life. In the process, it should be selected according to needs: 100 mesh straight screen for printing conductive ink, and 200~300 mesh oblique screen for printing orthographic characters and color blocks to ensure the ink layer thickness and integrity of images and texts.

3. Coating of photosensitive adhesive: it is usually applied manually, lean the blank screen against the wall, apply it 2~3 times with a stainless steel coating groove at an angle of about 50 °, observe the bottom color of the adhesive layer, and measure the thickness with a film thickness gauge. The emulsion thickness on black lettering and base color scraper is 20~25 μ m. The thickness of hollow printing screen emulsion with radian is 28 ± 3 μ m. The emulsion thickness of different processes is different. The material shall be measured and the emulsion thickness must be uniform. The drying temperature shall be controlled at about 40 ° in the oven to avoid the influence of too high temperature on the light application performance. During the drying process, attention shall be paid to the safety of light. Yellow light should be used for lighting, and white light should not be emitted into the room. The screen plate shall be taken out for exposure after drying for 10min.

4. Exposure: exposure is to make the photosensitive adhesive film of non graphic parts react chemically after seeing the light, and the molecular structure between the adhesive films is cross-linked and hardened. From soluble to insoluble, the light blocking layer (hardened part) and the image part of the transparent mesh (no photo sensitive adhesive dissolved in water) are formed after development, so that the ink surface passes through the mesh to form a printing pattern. The exposure uses a vacuum printer. Use transparent tape to paste the film on the screen plate, with the film face up. Stick the template to the film, cover the screen frame with black cloth, put down the leather cover, and start the air extraction device (the air pressure is about 110-120PA). When the air pressure is stable, activate the exposure device after the leather cover is stuck to the glass, and set the exposure time according to the nature of the photoresist, the film thickness, and the thickness of the image and text. It is usually controlled between 20S and 60S.

5. Development: soak the exposed screen plate with water>, and then wash both sides of the adhesive film under a water pipe under a certain pressure until the film of the image part is washed clean and the mesh forming the image is completely transparent. After the image and text parts and non image and text parts appear, touch the surface gently with your hand and there should be a sense of smoothness without traces of dissolution. Filter the water drops and send them to the drying oven for drying. Second exposure is generally required for drying screen to enhance its service life.

6. Dispensing ink: The silicone rubber printing ink must be suitable for the physical and chemical properties of silicone rubber, and can be firmly attached to the printing surface. Like other inks, it is made by mixing pigments, binders, and solvents in a certain proportion and grinding with three liquid wheels. Kerosene is usually used as the diluent. Vulcanizing agent (trace) is added to the single liquid ink to enhance the adhesion and wear resistance of the print. Double liquid ink is used for direct printing. The storage temperature of the ink shall be controlled at 0>~10 ℃, and the ink shall be dried at high temperature after printing.

7. Printing: Before printing, prepare the fixture, screen plate and ink, clean the printing surface without oil stains, and if the surface is too smooth, it must be treated with flame and corona to ensure good ink adhesion. The sunned screen plate fixture is fixed on the machine table, and the key for printing is fixed in the fixture. The hardness of the scraper should be equivalent to that of the rubber (about 55 °). The gauge line should be positioned. The gauge line on the screen plate should be aligned with the gauge line on the cover plate, Overprint the sample with accurate position. Adjust the distance between the screen and the printing surface so that the scraping screen can contact the printing substrate well. The coverage of the scraping stroke must be 2~5 larger than the pattern surface. The angle of the scraper shall be controlled at about 75 °. During printing, the first article must be checked, the samples must be compared, and the sampling inspection shall be carried out on time to achieve the overall control of product quality.

3、 Discussion on process technology

The copying position of the screen printing film must correspond to the fixture. When the screen plate is made, the film and the screen plate are in close contact to prevent the position from moving. During the design process, the fine line characters should not be too close, otherwise it is easy to cause adhesion. In case of printing with background color, the background color is generally 0.1~0.2m smaller than the surface color, so that the printing surface can be fully covered and the product quality can be ensured. In the above process of duplicating screen printing, because the substrate is soft and the tension of the screen plate is small, it is easy to have fuzzy fonts, inaccurate positions, and uneven ink layer thickness. In the process of solving these problems, careful analysis must be made. The screen plate with smaller tension is used for the substrate surface with radian, and the scraper should not be too hard, and the fluidity of the ink is good (but the font should be controlled to become thicker), The scraping speed of the doctor blade is controlled at 12~24Cm/S. During the baking process of printed products, the oven temperature and baking time must be controlled to improve the adhesion of products.