How to screen print on plastic products?

The screen printing is the secondary processing of almost all kinds of plastic products. The reason is mainly determined by the properties of the plastic itself, such as its simple dyeability, and the week solarised colorfastness. To compensate for these shortcomings and to improve the appearance of plastic products, secondary processing is necessary. Plastic printing refers to the technical process of transferring visual original information/image onto a plastic product using a printing screen or other ways. Printing on plastic products includes gravure printing, flexographic printing, screen printing, anti-counterfeiting printing, and jet printing, transfer printing, etc special printing methods. Nowadays, it has been able to print on plastic products with different surface textures, different hardnesses, and different shapes, and is widely used for decoration of plastic packaging films and plastic containers.

There are many types of plastic products. If they are classified by the screen printing method, the sheet and plane plastic use plain screen-printed, the cambered surface use curved silk screen printing, and the abnormity is using indirect screen printing.

The plastic items contain a polar substance in its molecular structure, which has excellent chemical stability, it’s corrosion-resistant to most acid and alkali, and insoluble in common solvents at normal temperature. Moreover, additives such as antioxidants and stabilizers are added into the ink reduce the affinity between the plastic surface and the ink, also the ink dried slowly after printing. Meanwhile, the plastic surface does not have the porosity like the paper could absorb the ink. Therefore, the surface of the plastic items to be printed has to be processed before printing, that could improve the adhesion of the ink to the plastic items, make the ink layer adhesive firmly, the screen printed image more distinct, complete and vivid. Specific treatment could also improve the surface properties of plastics directly, such as antistatic property, etc.

There are mainly the following substrates of plastic products suitable for screen printing:

1. Polyvinyl chloride plastic. Polyvinyl chloride (PVC) was put into industrial production in 1931. Polyvinyl chloride plastic has the advantages of bright color, anti-break, acid & alkali resistance, and cheap price. However, the addition of some auxiliary materials during PVC production is usually toxic, so PVC products can not use for food container. PVC film has a wide variety of colors, and most of the commercially available colorful plastic sheets and artificial leathers are polyvinyl chloride products.
2. ABS plastic. ABS resin plastic is an engineering plastic. In recent years, it has been widely used in televisions, calculators, and other products as well as in many areas of the national industries. ABS plastic is a terpolymer of acrylonitrile, butadiene, and styrene, it has the typical characteristics of three components. Acrylonitrile makes the copolymer has chemical corrosion-resistant and specific surface hardness; butadiene makes the polymer have rubber-like toughness; The polymer has characterized of easy process & forming.
3. Polyethylene (PE) plastic. Polyethylene plastics are widely used and can be made into various finished products by extrusion molding, injection molding, and other molding processes. The molecular chain of polyethylene consists of a long string of methylene groups, which containing pendant groups. The longer the pendant group in the polymer chain, the lower the crystallinity of the polymer. That is to say, the higher the density of polyethylene, the more unfavorable surface printing. Polyethylene is insoluble in various solvents, however, when the temperature exceeds 70 ° C, polyethylene can be ratherish dissolved in the solvent such as toluene or amyl acetate. This makes it possible to polyethylene surface printing. Soaking the plastic into a heating solvent to destroy the surface to increase the adhesion of the ink film on the PE plastic surface. A mixture of fuming sulfuric acid concentrated nitric acid, chromic acid, and sulfuric acid could corros on polyethylene slowly at normal temperature; sulfuric acid and nitric acid can destroy polyethylene rapidly at 90~100 °C. Mix the heavy potassium plumbite, sulfuric acid & distilled water, soak the PE plastic in the mixture at 70~75 °C, which is beneficial to enhance the adhesion of the ink film on the plastic surface.
4. Polypropylene (PP) plastic. Polypropylene has been one of the essential variety of plastics in the years of development. Suitable for all molding methods. Can be processed to all kinds of pipes, boxes, containers, films & fibers, etc. Industrial polypropylene has a high spatial regularity, high crystallinity, equal index ratio 89% to 95%, contains a little of macromolecular chains of amorphous stereostructure and low crystalline stereoblock. It’s melting point 170-175 ° C, the vitrification temperature is -13 ° C -0 ° C, and the syndiotactic polymer is 0 ° C. PE plastic will be etched in concentrated nitric acid and fuming sulphuric acid, and dissolved in aromatic hydrocarbons and chlorinated hydrocarbons at higher temperatures, and will only be expanded at room temperature. Acids and esters could also dissolve them, and solvent resistance increases as the content of isotactic polymers increases.

The ink for plastic printing is different from the ink for paper, and the two cannot be mixed. To get the ink adapt for plastic substrates, it mainly uses plastic resins such as polyamide, synthetic solvents, organic solvents and colorants to mix the plastic printing inks. It has excellent colloidal fluid properties after being fully ground and dispersed. The ink for plastic printing is a volatile ink, which has the characteristics of excellent printing performance, stable adhesion fastness, and drying rapidly.

All screen printing requires the screen as a substrate and framed screen with the graphic process. The five major elements of screen printing: screen printing screen, squeegee, ink, printing plat, and substrate. The silkscreen mesh on the screen frame could be transmitted through the ink, and the non-image area of the mesh can not. When printing the plastic product, pour the ink at one end of the screen printing screen, pressing and slipping the squeegee with the ink to the other end of the screen, and the ink is scraped through the screen mesh onto the plastic substrate to get the required image and characters.

Taking the character printing on the ABS plastic keyboard as an example, let’s look at the printing of specific plastic products. ABS plastic is usually used to make the keyboard because of its glossy appearance, low water absorption, excellent impact strength and surface hardness under specific temperature range, and excellent dimensional stability. Most of the ABS resins plastic remain favorable impact strength and toughness at -40 ° C. The ABS resin products are widely used at -40 ° C to 60 ° C temperature. ABS plastic has a high molecular weight, good physical properties, good molding property, and excellent printability.

The word keys are rapped frequently, it requires excellent adhesion of the characters to the plastic keys, good wear resistance, solvent resistance, and sweat resistance. The word key has a very small area (the minimum size is 6.8mmx3.5mm), which need high printing precision and large quantity. It is inappropriate to use the reverse paste method. Therefore, screen printing is optimal. Jig the word keys to solve the problem convex surface during the screen printing.

For the ink choosing, due to the less brightness and color of plastic inks, easy to block the screen, and low adhesion, it’s not adapt to the keyboard. The key point to choose the ink for ABS product is pleasant brightness, moderate viscosity to anti-blocking screen, and strong adhesion to the ABS plastic. The ink can not be high viscosity; otherwise, the printed words can be rubbed off with fingers by normal force easily. Also, after the ink drying, a thin layer of varnish should be sprayed onto the surface, to improve the brightness and protect the ink layer significantly.

The ABS keyboard silk screen printing process is as below:

1. Silk screen printing mesh preparation. Use 230 mesh count, mesh stretching, and cleaning.
2. Screen making. Use photosensitive adhesive screen making or film screen making method.
3. Pre-processing of plastic parts. Soaking with degreaser and washing.
4. Screen printing. Set the ABS plastic keys with special fixtures, and screen printing use the dedicated ink.
5. Initial inspection. Cleaning the unqualified with alcohol and re-screen printing.
6. Drying. Oven-baked at 60 ° C for 0.5 hours or at room temperature for 2 hours.

The plastic products decorated by silk screen printing has thick ink film with substantial cover ability, adapt to various surface shape and area dimension. In brief, silk screen printing for plastic items has the advantages of:

1. Silk screen printing can be executed not only on the flat but also on a curved or spherical surface;
2. Not only adapt to printing on small objects but also for large;
3. The printed plastic products will not fade, tear, stain, wrinkle or peel off;
4. Steam resistant and waterproof;
5. Clean look, without label edges;
6. Extremely cost-efficient;
7. Easily recyclable;
8. No creative restrictions;
9. Fully Customized;
10. Easy operation, fast delivery;

This printing method has excellent flexibility and broad applicability and becomes more and more applied in every corner of our lives.

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