In the production process of signage, the decisive role is the process technology and printing points of the pretreatment of screen printing plates and substrates and the selection of screen printing inks.

1. Screen printing system

1. Carving method of paint film:

It is a kind of manual plate making method, which is relatively simple. It can print monochromatic patterns and text which are generally not very delicate.

(1) Spray engraved lacquer film paper: paste the tracing paper with a paste on a flat wood plate, use a pen to adjust 1 part of rubber water (volume ratio) and 1 part of gasoline, and evenly coat the paper surface ï¼Œ Spray soft varnish 3-5 times after drying. After each spray, dry in an oven at 40 Â° ~ 50 Â°. The thickness of the paint film should be sprayed to 5 ~ 6 mm. The paint film should be bright, uniform and free of bubbles. , No stains.

(2) Engraving graphics: Paste the paint film paper on the graphics, and engrave on the graphics with a carving knife, compass knife, and ruler, and gently remove the paint film.

(3) Reposting: After engraving the graphics, you can repost it on the screen frame that you have previously made. The method is to put the engraved paint film under the net frame and press it tightly, then gently rub it with cotton and a little thin material on it until the paint film and the wire mesh are firmly adhered. After drying for a few minutes, peel off the tracing paper. If the paper is not completely removed, wipe it with cotton and water. The blank part is coated with a layer of nitro lacquer to block the mesh of the wire mesh. It can be printed after drying.

2. Carbon paper printing method:

It is also a relatively simple manual plate-making method.

(1) Cutting photosensitive paper: photosensitive paper is a kind of carbon paper made by thick coating of oxo-barium base paper, which is cut according to the size of the graphic and ready for use.

(2) Sensitization: Soak in a solution prepared with 500 milligrams and 20 grams of ammonium dichromate for 3-5 minutes, remove the carbon paper, and drain the water.

(3) Exposure: Put the carbon paper flat on the clean glass with the adhesive film facing upwards, and cover the positive film polyester film negative plate on the top. Expose it with a printing machine. The exposure time is 8-12 minutes.

(4) Development: After exposure, soak in warm water at 40 ~ 50 degrees for several minutes to make the film and positive plate naturally detach from the zinc oxide base paper, and carefully develop with warm water until the force text is clear.

(5) Reposting: Put the prepared version under the frame of the pre-stretched wire mesh, cover several layers of newspaper on the wire mesh, press the glass plate, and press it with a weight of 1-2 kg / cm2. When the adhesive film is firmly attached to the silk screen, remove the heavy objects and newspapers, dry the adhesive film in an electric furnace, and then remove the base paper, and the graphic of the adhesive film will be transferred to the silk screen. Carefully check the graphics for defects. After passing, apply a layer of nitromagnetic lacquer to protect the screen around the graphics, and install the screen printing plate on the printing plate for printing. The application scope of this plate-making method is the same as the engraving method of paint film. [next]

3. Photographic plate-making method: The printing plate used for silk-screening signage requires higher precision, and generally adopts advanced direct, indirect, and direct photo-sensitive plate-making methods.

A. Direct plate-making method:

(1) The aluminum frame is used for the post frame, and it is not necessary to stretch the net and the mobile net is stretched. The air pressure is 5-6 kg / cm2. The acetal adhesive net is used. The degree of stretch is measured with tension. The tension is 5-7 kg. The size should be less than 2 mm.

(2) Cleaning the wire mesh: Use washing powder, detergent, ethanol, etc. as cleaning agents to clean both sides of the wire mesh, rinse with clean water, and dry it with hot air for use. The purpose is to make the mesh better adhere to the photosensitive adhesive.

(3) Scraper (coating hopper, applicator): The squeegee that coats the photosensitive adhesive can be made of stainless steel, or a plexiglass plate. Its length is generally slightly smaller than the inner diameter of the wire mesh frame (a pair of scrapers with different lengths are equipped as a set of spares) B. The edge of the scraper must be thin without a blade, smooth and straight, and no defects such as bumps, burrs, scars, etc. are allowed, which affects the quality of the plate making. When coating the screen photosensitive adhesive, it needs to be dried 3-4 times every three times, until the adhesive film reaches the required thickness, and then exposure.

(4) The temperature of the drying oven is ideal to use an adjustable temperature drying oven. The drying temperature of the photoresist is generally controlled at about 40 degrees. If the baking time is too long or the temperature is too high, it will affect the quality of the photosensitive layer.

(5) Safety light: The lighting source of the studio usually uses orange, yellow or red light as the safety light, but from a visual point of view, the orange-yellow light source is more comfortable. Its brightness is suitable for being able to see all objects clearly.

(6) Check the negative version: Before the printing, you must carefully check the negative version, including the dust on the surface, the blackness of the negative version, line breaks, sand hole defects, and whether the positive and negative of the image text meet the requirements.

B. Indirect manufacturing method:

(1) Prepare the mesh frame and photosensitive film: wash the stretched mesh frame (220-260 mesh nylon wire) with 10% aqueous sodium phosphate solution to remove oil stains.

Wire mesh photosensitive film is a kind of photosensitive adhesive with polyenol glue as the main body. The film is coated with gelatin as the main photosensitive agent on a transparent plastic base of 0.12-0.06 mm. After 20 mm, wipe off the powder dirt on the film with a cloth.

(2) Exposure: Press the film and the base plate tightly in the plate-making machine, and use carbon lamp or xenon lamp for exposure. The distance between the screen frame and the light source is 50-60 cm, and the exposure time is 2-6 minutes.

(3) Development: After exposure, place the phenanthrene film on a flat plate, infiltrate the omentum with warm water for 1-2 minutes, spray with a water spray head, dissolve the non-photosensitive part (graphic part) with water, and print and develop the phenanthrene film Until the graphics are clear.

(4) Paste the film: put the exposed film surface on the silk screen, use a rubber plate or other straight board on the back of the human film, gently scrape to make the film contact with the net firmly, and immediately put it in the oven at the temperature Bake for 8-12 minutes under the condition of 55 Â± 2 degrees. After drying, fix the screen frame on the printing plate table for trial printing.

(6) Film removal method: During the exposure and development, if the plate is found to be unqualified, the film can be brushed off with warm water and remade. If the plate has been dried or the film is removed after a period of production, you can use industrial sodium hypochlorite and water, soak for 10-20 minutes. Or a 5% sodium hydroxide aqueous solution can be used to heat the screen to remove the film.

C. Straight-room plate making method:

The direct method is a mixed platemaking method of direct and indirect methods. The difference between it and the indirect method is that the indirect method first makes the film on the film and then pastes it on the screen, while the direct method is the film After posting on the silk screen, the image was exposed. [next]

2. Pre-processing of printing materials

The printing materials of the signs are mainly aluminum, copper, stainless steel, aluminum, and the most consumption of plexiglass. There are also plastics, polyester, paper, etc.

Aluminum sign is a widely used sign, which has the characteristics of beautiful, bright, high hardness, good wear resistance, wide application and large amount. Used in machinery, electronics, instruments, meters, household appliances, daily hardware, clocks, daily cosmetic packaging, etc.

There are various methods for the pretreatment of aluminum substrates. Depending on the application, there are mainly degreasing, sand blasting, wire drawing, polishing, and oxygen grease.

1. Degreasing: In order to make the surface of the aluminum substrate have certain ink affinity, the oil layer and oil stain on the surface need to be removed, and it can be wiped with cotton yarn dipped in gasoline. Alkaline solutions can also be used for chemical degreasing, such as sodium sulfate 30-50 g / l, sodium phosphate 40-60 g / l, sodium silicate 10-15 g / l, immersion at a temperature of 60-70 degrees 2-5 Degrease in minutes.

2. Sand blasting: sand blasting is to improve the adhesion between the ink and the metal material. Usually, a special sand blasting machine is used for blasting. If the aluminum plate is 0.5-1 mm thick, the sand grain diameter for sand blasting is 0.05-0.15 mm. The river sand can be used after filtering through a sieve.

3. Wire drawing: also known as twisting processing, it is to process the delicate straight lines on the surface of the aluminum plate by mechanical friction. It is a relatively new surface decoration treatment process, which can overcome the defects such as scratches on the surface of the aluminum material. Improve the use of materials. There are two kinds of wire drawing, thick wire and fine wire. Generally, stainless steel wire brush with a diameter of 0.1 mm or scouring brush is used. It can also be processed with nylon abrasive belt.

4. Polishing: It is to overcome the surface defects of aluminum-based materials and improve the surface finish. Polishing is divided into mechanical polishing and chemical polishing. Mechanical polishing uses cloth wheel polishing machine, chemical polishing is mostly carried out in acidic solution, such as sulfuric acid 15%, nitric acid 10%, temperature 90-100 degrees, time is 2-3 minutes.

5. Oxidation: The polished surface of the aluminum plate has residual polishing soap, which should be wiped off with a board yarn dipped in gasoline, then chemically degreased, and then immersed in a nitric acid solution (1: 1 ratio of nitric acid and water). Then put it in the oxidation tank for electrochemical oxidation. Oxidation solution formula: nitric acid 160-200 g / ml, temperature 10-20 degrees, time 30 minutes.

The oxidized aluminum plate is fully washed with water and dried, and an oxide layer with uniform pores, that is, an oxide film, is formed on the surface to avoid contamination by dirt. The oxidized aluminum plate is best printed immediately. If it cannot be printed immediately, it should be wrapped with soft paper septa or placed directly on the air-dry rack. The storage time should not exceed one week.

The oxide layer on the surface of the aluminum plate can absorb dyes, moisture and other salts, so do not touch the surface with your hands or oily things during screen printing. When the ink is printed on the oxide layer, the ink penetrates into the pores. The next step of heating is to accelerate the ink to penetrate into the pores until all the surface pores covered by the ink are filled.

The ink printed on the surface seldom diffuses or does not diffuse, which ensures the accuracy and clarity of characters and patterns.

6. Sealing: Its function is to seal the ink firmly in the pores, and never overflow or spread. For the parts that are not covered with ink, the function of sealing is to fill the gaps of the oxide film and make it impermeable to water, and enhance the corrosion resistance of the surface. Nickel sulfate solution is generally used for sealing. Formula: 10-12 ml of nickel citrate, temperature 80-90 degrees, 10-15 minutes. After sealing the hole, touch the surface with your hands or other oily things, and then wash it off with a general cleaner. [next]

3. Screen printing ink (coating)

In today's metal signage or non-metallic plate screen printing, more printing inks can be used, but it is not easy to choose a suitable ink (coating). The commonly used inks for signage screen printing are as follows:

1. Baking enamel:

It is a single-liquid formula, which crosslinks when heated and has good storage performance. It can be stored for 6 months to one year. It has good finish, wear resistance, chemical resistance and weather resistance, so it is most commonly used. Alkyd, acrylic and polyester have these characteristics. Epoxy resins have a tendency to break and powder, and are generally not used outdoors, but they dry quickly and have good chemical resistance. Alkyd enamels have the most balanced physical properties, such as chemical resistance and weather resistance, formability, hardness and gloss, and are most widely used.

2. Air-drying enamel:

It is usually made of drunk acid, and it needs to use the oxygen in the air to achieve solidification through reaction. The performance of drunk acid enamel depends on the type and quantity of oil used. The longer the oil drying time, the longer the enamel drying time. When the drying speed is the condition, paints with a long drying time generally have good surface durability. These paints are mostly used for outdoor metal signs.

3. Melted volatile ink:

Only rely on the solvent to achieve drying. The final properties of the coating are based on the inherent properties of the polymer before it is formulated into a liquid. Such polymers contain thermoplastic acrylic. Screen printing inks with a predominant nature are often damaged by dissolution. There are some acrylics that are insoluble in gasoline and alcohol, where the film surface can be in contact, so inks based on thermoplastic acrylic, vinyl and other soluble polymers are mostly used on plastic substrates.

4. Two-liquid ink:

Common is epoxy tree finger or polyurethane, although there are many others. The two-liquid system that is often used in wood is the most sensible choice for energy saving. Its coating quality is high, but mixing is inconvenient. In addition, it should be selected according to the use of the product. If the product may be used outdoors and it has some flexibility, then use urethane ink with aliphatic hydrocarbon as the main body. The two-component epoxy tree inks on the market now have almost the same performance as baked enamels, but the formability is not good enough.

5. Ink additives:

Like other inks, screen printing inks for metals also contain additives. This is a small amount of chemicals added during production to improve the performance of the ink film. Some can accelerate curing or initiate curing, while others can cause bubbles to burst and ink Uniform quality reduces the possibility of conjunctiva. The manufacturer mixes the additives with the ink according to the specific formula. What the printer needs is to dilute the ink to a consistency suitable for printing. [next]

4. Printing

The quality of screen printing, the thickness of the ink layer are closely related to the thickness of the screen, the viscosity of the ink, the hard filling of the blade, the angle, and the method of making the plate.

1. Silk screen and positioning: aluminum plate material is positioned with three points. When register printing, first print light color, then print dark color.

2. The distance between the printing plate and the substrate is 1-2 mm, and the distance is related to the selection of the screen. The nylon screen has good elasticity and large scalability, generally 2-3 mm.

3. The polyurethane rubber sheet used for the scraper has a hardness of 70-75 degrees and a thickness of 8-9 millimeters. The scraper angle during printing is preferably 70 degrees, and the speed of the scraper should be linear at a uniform speed.

4. Adhesive protective film: After printing, in order to avoid trauma during cutting and transportation, a transparent pressure-sensitive adhesive protective film should be attached to the surface, and double-sided adhesive paper should be attached to the back.

5. Cutting: With a small shearing machine, forming can be performed with hole positioning or edge positioning punching.

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