Application of screen printing in cosmetic packaging
Concept of screen printing
Screen printing is a technology that uses screen as plate material to make printing plate. The text (image) part formed on the printing plate can be copied to the substrate through ink leakage. (substrate refers to the products that need to be printed. Generally, we refer to bottle and cover.)
Characteristics of screen printing
Not limited by the size and shape of the substrate;
The layout is soft and the printing pressure is small;
Strong ink layer coverage;
Adapt to various types of inks;
Good light resistance;
Flexible and diverse printing methods;
Convenient plate making, low cost and easy to master plate making technology;
Classification of screen printing
Plane printing: a printing method in which the printing plate is fixed and the substrate is fixed, and the ink is leaked to the substrate by moving the scraping glue left and right and squeezing the printing plate.
Curved surface printing: a printing method in which the printing plate and the substrate move synchronously under the action of mechanical force, scraping and squeezing the printing plate, so that the ink leaks onto the substrate.
Printing process (taking PP and PE materials as an example)
Because most of the plastic substrates we print are made of PE and PP, the surface of these two plastics is waxy, very smooth and low surface tension during the molding process, which makes the silk screen printing ink difficult to adhere. In order to achieve ideal adhesion, flame treatment is required before re printing.
Flame treatment is to make the temperature of flame treatment reach more than 1100 ℃ under the action of compressed air through the combustion of gas. The substrate contacts with high temperature in an instant, damaging the waxed layer on its surface, forming an oxide film and improving the surface tension of the substrate. Flame treatment factors account for about 80% of the quality of printed words. Therefore, flame treatment is very important. How to check that the substrate after flame treatment meets the treatment requirements? It is to soak the treated substrate in water for 10 seconds and then take it out, and the water film can be evenly distributed on the surface of the substrate.
The adjustment of flame treatment shall follow the principle of flame from weak to strong and assembly line speed from fast to slow.
Screen ink consists of solvent, connecting agent and colorant. The solvent determines the drying speed of the ink to a certain extent. The linker is a natural resin or synthetic resin dissolved in the solvent. Its function is to connect the colored particles of the colorant and promote the adhesion between the ink and the substrate.
Some principles of color blending:
The fewer kinds of colors, the better the mixing effect;
Adopt the principle of from shallow to deep, and the inks of different manufacturers cannot be mixed;
Pay attention to the light in color matching. The color of ink is different before and after drying. Generally, the light color is lighter after drying, and the dark color is deeper after drying.
Viscosity allocation: usually, the ink delivered from the factory has high viscosity and is easy to block the mesh, so viscosity allocation is required. The temperature in the workshop also affects the viscosity. High temperature and low viscosity; Low temperature and high viscosity.
Formulation of additives: add an appropriate amount to improve the performance of ink.
Main defective items and causes in the printing process
Insufficient flame treatment;
The ink configuration time is too long;
Insufficient UV irradiation;
The surface of the substrate is polluted;
The printing surface itself is rough and dry;
Ink viscosity is too low;
The rubber scraping is aging and too soft;
Ink viscosity is too high;
The substrate itself is uneven;
Uneven scraping force;
The printing plate becomes soft;
Screen printing is now divided into manual printing and automatic screen printing