Only inks, substrates, and plates are all ready, and we can start large-scale screen printing.
The preparation work includes the following: Is the ink type used appropriately? Whether the viscosity and thinner of the ink are suitable? Have the size, type, and quantity of the substrate determined? Does the surface of the substrate need to be processed? Is the screen printing machine running smoothly? Is the surface of the screen printing platform flat? Whether the size of the screen accurate? Whether you choose the adapt silkscreen mesh count? Moreover, so on.
Before printing, please mix the screen printing ink to ensure excellent printability.
The ink color matching is an indispensable part of the screen printing production. The principle of mixing colors is based on color theory, such as the addition of chromatic light and reduction of coloring matter. Color matching is the combination of two or more colors or a small amount of color to blend the primary color.
Before mixing the inks, please calculate the consumption of each color to get an appropriate volume of the mixed ink. The ink mixing should proceed under a steady natural light to bring out the correct color, do not observe multiple colors at the same time to avoid visual illusion. When fresh mix ink, the addition of dark ink should be gradual, please don’t add too much and then use light ink to remedy — also, the more parvus the hue, the better to reduce the grayscale.
Moreover, pay attention to the color change before and after the ink drying. Such as on the light-colored substrate with a large volume of ink absorption, the dried ink color tends to lighter. There is a significant color difference of the wet, dry, and foaming of the foamed ink. Moreover, the hue changes hugely after the ceramic ink burned. Such as the white ink turns yellow under high temperatures. This kind of discoloration rules requires screen printers to accumulate experience and samples in practice and make their color mark.
Precautions for mix screen printing ink:
- When mixing ink, do not use too many colors. The less color of the ink, the better mixture.
- Take the principle of “from light to dark,” no matter for light or bright-colored inks mixing, be careful when the hue close to the sample. Do not to mix the inks produced by different manufacturers; otherwise, the color going uneven easily and maybe scrapped.
- Some screen printing inks need to be dried by oven. Whether the color tone of the ink differs before and after drying is a little troublesome question. Light-colored inks may become lighter after drying, and dark becomes darker than before. Generally, plastic, metal, paper, glass and other substrates dried under room temperature, the color will not change; however, the ink for ceramics that are colored after burning and oxidizing could only be toned by experience. An ink mixing machine could be used for a large amount of ink mixing.
Requirements for screen printing workers to mix inks:
- Master the various specifications and performance of the ink, make the ink suitable for the production.
- Master the rudiments and rule of color mixing.
- For the multi-colors screen printing, analyze the multiple colors of the image, figure out the correct color printing sequence to avoid the syncretize.
- Pay attention to the ink brightness. Use the primary ink for the higher brightness color.
Use the solvent, thinner and surfactant to adjust the viscosity, surface tension, liquidity, and drying rate of the ink, and so on, it should be according to the image, screen mesh count, printing speed, working temperature, substrate surface tension & ink absorbing ability, etc.
For example, when printing fine lines and dots, make the ink less fluid and drying slowly; when screen-printing by lower mesh count, use the high viscosity and fast-drying ink; For high-speed screen printing, the ink should be in a little higher fluidity and drying speed; In the higher temperature workshop, make the ink in high viscosity and slower drying; For the small surface tension of the substrate, the ink surface tension should be smaller as well; For the substrate of high ink absorptivity, use the ink with high fluidity and surface tension, and slow-drying.
To get the excellent printability of the substrate, pre-processing of the substrate before screen printing is necessary.
- Hanging the paper and other substrates which temperature and humidity sensitive, make them in similar temperature and humidity with the workshop, to ensure proper color accuracy.
- Pre-heating the substrates which need to be oven drying, to make the color difference before and after drying as small as possible.
- For those plastic substrates with small polarity, the ink film is challenging to adhere. Make surface pre-process by mechanically, physically or chemically roughened, oxidized or polarized before printing.
- Clean the substrate surface that non-absorbent such as metal, glass, and plastic, since they are easy stained by dust and grease, which will affect the firmness of the ink film.
- Cloth, cotton paper, and other soft substrates which easily deformed need to be shaped and set before printing; it helps to facilitate the calibration and positioning of the multiple printing screens.
- Some warp-shaped plastic sheets, such as PVC and plexiglass sheets, heat-setting them before printing to make them flat.
Screen printing station Installation
- A vacuum printing station is commonly used for paper, hard plastic and other substrates. Its drawback is the thin paper, and the plastic film is easy to dent by vacuum adsorption and thicken the ink film.
- The flat plate is the simplest printing station supported by four legs. Glass or thick acrylic plates could be installed on the station. Commonly to apply adhesive or double-sided tape on the printing plates fix the substrate. However, it’s not suited for glass, propylene, metal sheets, circuit boards.
- For the thin substrate, the flatness of the printing plate is essential. For example, screen printing a thin paper on a slightly uneven plate, more ink will in the recess, less ink in the convex. Also, if the printing plate is uneven seriously, the opposite is, no ink will in the recess, resulting in the uncomplete image.
Screen printing tables come in many forms and shapes to meet various substrates requirements.
The positioning of the substrate
There are two ways to position the substrate: fixed screen, moved substrate, and fixed substrate, a mobile screen, which mostly used for textile printing. In the printing of fixed screen, the substrate located by blocking position, and the positioning pin for the Mobil screen location.
1. Fixed block position
When substrate positioning, move the original image on the printing table to make it superposition with the image to be printed, to get the purpose of substrate positioning. For the scattered original image, splicing the original pieces and sticking on the correct position of the substrate.
After fixing the substrate on the printing platform, please attach a small piece (metal, plastic, paper, etc.) which has the same thickness as the substrate on both sides ( three sides for oblong) of the substrate as a fixed block, and the thickness could be slightly thinner than the substrate but cannot be thickner，otherwise the screen and the fixed blocks will damage squeegee during printing production.
Generally Semi-automatic and automatic screen printing machines provide a positioning system, such as automatic feeding positioning, fixed blocks, molding position and so on. The fixed block positioning for the automatic screen printing machine is the same as manual printing. In the chromaticity printing, When printing registration, the worker usually moves the screen printing table by adjusting the screws to make the master and the screen plate coincide with positioning the substrate.
Fixed blocks deploy for most of the regular edges substrates. Various types of fixed blocks suitable different substrate, respectively.
a. The sheet fixed-block adapt for slice substrate.
b. The paper-folding block has a certain elasticity to accommodate thicker substrates.
c. Cube fixed-block suit for thick substrates.
d. Pin fixed-block adapt for pre-perforated circuit board, ruler, dashboard, etc. of exact printing.
Also, When the printing table and the substrate have a specific light transmissivity, it is not necessary to make a positioning device, only adjust the substrate and plate by the light- transmits under the table.
2. The positioning of the moving printing screen
For the substrates such as fabrics that are soft, easily deformable, and porous, it’s difficult to position them by the fixed-blocking method. The substrate needs to be affixed on a strip printing plate. When screen printing, moving the screen to print one by one.
Test printing is necessary before each production to check the uniformity to the designed or sample image. If discrepant, adjust the screen distance, squeegee angle, printing pressure, and the ink viscosity slightly. In multi-color overprinting, mark the position of the screen after the first screen printing qualified, and record the distance between the printing plate and the screen. When mass printing start, find the best one as a sample for proofreading. Mark the position of the fixed-block on the proofread sample exactly which as a reference after the fixed-block movement. The proofread sample will be used for every time of color version changed, screen fixed, and proof.
After the first lot of printing, wash the screen immediately to prevent the residual ink drying up to block the mesh. For the volatile-dry ink, when the ink dry and block the mesh during printing, clean the silk mesh fabric by cotton plugging with the solvent and scrubbing from squeegee side.
After all the above preparations are well done, start your mass screen printing!