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Steel drum screen printing and plate making (2)
Yang Wenliang
Third, the plate making method of steel drum screen printing
Screen printing is the basis of screen printing. If the quality of the plate is not good, it is difficult to print good quality products. Failures in printing are often associated with improper platemaking techniques and materials used in platemaking. Therefore, in order to make a good quality screen version, it is necessary to correctly master the plate making technology according to the requirements of the plate making process, and strictly select the plate making material for plate making. The main problems involved in plate making work are: how to correctly select the materials used in plate making, such as wire mesh, mesh frame, photosensitive materials, etc., and how to master the mastering, stretching, coating, printing, development, light source and so on.
(a) wire mesh
The wire mesh is a skeleton for making a screen, and is a substrate supporting the photosensitive film. Commonly used screens for steel drum screen printing are monofilament nylon mesh, thin nylon mesh and stainless steel wire mesh.
1, nylon mesh
It is a nylon monofilament woven fabric that is heat-treated and dimensionally stable after being woven. The plain weave can reach 380 mesh and the twill weave is larger than 330 mesh. Nylon mesh has the following characteristics:
(1) The nylon mesh has a smooth surface and good ink permeability, and an ink having a large viscosity and a large particle size can be used, so that a fine printed pattern can be obtained;
(2) A fine mesh mesh woven with extremely fine mesh, suitable for drawing patterns and dot printing of extremely thin lines;
(3) It has large elasticity, proper softness, good adaptability to the surface of the steel drum, and is suitable for printing on the curved surface of steel drum;
(4) good tensile strength, knot fastness, elasticity and abrasion resistance, and long service life;
(5) Good resistance to acid, chemicals and organic solvents, easy to recycle, especially strong resistance to alkali.
The following points should be noted when using nylon mesh:
(1) Compared with polyester and silkworm mesh, nylon mesh has a large elongation. In order not to cause printing failure, the tension of the stretcher should be increased, and the contraction force will also become larger. Therefore, it is required to use a strong frame and a stretcher;
(2) Because of its low heat resistance, if the net is fixed to the frame by the hot melt method, the operation is difficult;
(3) Not resistant to strong acid, phenol, cresol, formic acid, etc.;
(4) Poor light resistance, easy to degrade under the action of ultraviolet rays, and avoiding light when storing.
2, polyester mesh
Polyester mesh has the following characteristics:
(1) When the tensile force is small, the elongation is small and the elasticity is strong, and the single-wire mesh is suitable for high-precision printing;
(2) The tensile strength, structural strength, resilience and printing durability are good;
(3) It has sufficient chemical resistance, especially acid resistance and organic solvent resistance, and can be recycled and used like nylon;
(4) low hygroscopicity, almost unaffected by humidity;
(5) The heat resistance is higher than that of nylon, and it is less affected by ultraviolet rays than nylon.
The following points should be noted when using polyester mesh:
(1) A larger stretch tension is required than a nylon mesh, and a stronger frame, a stretcher, and a stronger bonding method are required;
(2) The polyester is hydrophobic, and it is difficult to bond with the plate. It is necessary to pay attention to the washing and degreasing before the plate making;
(3) The ink repellency is worse than nylon and is not resistant to strong alkali corrosion.
3, stainless steel wire mesh
Stainless steel wire mesh has the following characteristics:
(1) Excellent plane stability, stable pattern size, suitable for printing high-precision steel drum products;
(2) Excellent ink passability;
(3) Good alkali resistance and tensile strength, excellent chemical resistance;
(4) Strong heat resistance, suitable for printing of hot melt printing materials.
The disadvantages of stainless steel wire mesh are:
(1) It is vulnerable to external twists and turns;
(2) During the printing process, the wire is easily loosened due to the pressure, which affects the printing durability;
(3) It is expensive and costly.
The thickness of the screen should be compatible with the fineness of the surface pattern of the steel drum. The rough pattern can be a coarse mesh of 200 mesh or less, the general pattern is a mesh of 200 to 300 mesh, and the fine pattern is preferably a fine mesh of 300 mesh or more.
Among the commonly used screen printing materials such as nylon mesh, polyester mesh and stainless steel wire mesh, nylon mesh has good resilience, good ink passability, static electricity is less than polyester mesh, and wear resistance is good, and the cost is moderate. The preferred screen for steel drum printing. However, due to the high elongation of the nylon mesh, it is not suitable for printing high-precision patterns. For screen printing such as multi-color overprinting, it is better to use polyester mesh; when the printing pattern requires particularly high precision, stainless steel mesh should be used. Due to the major shortcomings of stainless steel wire mesh, which are poor in elasticity and expensive, it is generally less used in screen printing of steel drums.
(2) Net frame and stretch net
1, the net frame
The frame is a frame for supporting the wire mesh. It is made of metal, wood or other materials and is divided into fixed and adjustable. The most commonly used is the frame made of aluminum profiles. The material of the frame is made to meet the needs of tension, firm, durable, light and inexpensive.
Square and rectangular frames are generally used for printing on the top surface of a steel drum. However, when printing on the circular arc surface of the barrel, a frame adapted to its shape is generally used.
The thickness and shape of the cross-section of the frame material are different depending on the method of fixing the screen, the use, and the strength. For example, when the screen is fixed with an adhesive, angles and corners are generally used; when physically fixed, a slotted punching frame is generally used. In the case of high strength requirements, the reinforcing ribs are reinforced on the inside of the pipe, and the pipe used must have a certain thickness. In addition, some special frames are equipped with a stretcher mechanism, and the stretcher can be used without stretching. 2 is a schematic cross-sectional view of a mesh frame of various shapes.
In order to ensure plate making, print quality and other requirements, the frame can be selected according to the following conditions.
(1) The tensile strength is strong. The material used as the frame should have sufficient strength to withstand the tension of the screen, because when the net is stretched, the screen produces a certain pulling force on the frame, and if the frame is not strong enough, the frame is flexed and deformed, so that a good print cannot be printed.
(2) It should be prestressed. After the tensioning, the bending deformation of the frame will affect the tension stability of the screen. To reduce this effect, the frame can be preprocessed. There are two methods of treatment: one is to make the frame into a convex shape according to the strength principle of the arch structure, the deflection is about 4 mm/m, and each inner angle is slightly larger than 90°; or the metal frame has been made, This shape is stretched with a special tool, and this pre-deformation treatment can resist the influence of the wire mesh tension. Another method is to perform prestressing treatment while pneumatically pulling the net, that is, the front end of the net puller is tightly pressed against the outer side of the frame, and the net frame is bent by the action of the top force, and when the net is fixed, the frame is stressed. Although the surface is moved from the outside to the top, the direction and size of the force are basically the same, so the bending is no longer increased. Due to these advantages, the pneumatic stretcher has become the most popular stretcher equipment at home and abroad.
(3) The size is suitable. In the production, different specifications of the net frame should be configured. When using, the appropriate frame can be determined according to the size of the printing size, which can reduce waste and is easy to operate.
The size of the frame should be larger than the plate image. If the plate pattern is a × b, the inner size of the frame is 2a × 2b, which can be fully printed. The selection of the frame size is based on not only the printing area, but also the need to simultaneously consider the position of the squeegee and the ink return plate during printing; the need to accumulate ink on the plate; and ensure that the screen tension of the plate portion of the plate is uniform. The need for a screen plate to rebound during the squeegee printing stroke.
2, stretch net
The stretcher stretches the screen onto the frame, either manually or mechanically. Manual stretch nets are simple and easy to use, and are commonly used methods, while mechanical stretch nets are of good quality. Whether it is a manual stretch or a mechanical stretcher, you must pay attention to the tension of the screen. The tension of the screen is too big or too small, too small, the screen is prone to slack (the stainless steel screen will also appear prematurely slack); too large, the screen is easy to break. In the actual stretcher operation, the tension of the stretcher can be determined empirically, while the screen is stretched, and the screen is pressed with a finger until the screen has a feeling of elasticity. Commonly used stretching methods are:
(1) Manual stretch net? The manual stretch net operation steps are as follows:
1 Cut a piece of wire larger than the size of the outer frame. Generally, the length and width of the wire mesh are 40~60mm larger than the size of the outer frame.
2 Soak the screen in the water.
3 The screen is placed on the frame. Generally, the warp (weft) line of the screen is parallel (vertical) to the frame, but when used for printing precision or layer printing, the (weft) line is at an angle to the frame. It is 20°~45°.
4 Place the slug a, nail the A frame, and pull the screen in the E direction (see Figure 3). Then the all-in-one a is nailed and the AB side is fixed.
5 Pull the screen in the F direction, put the molding c, and then nail the C end. Then, while pulling the screen toward the G, nail the molding c to make the CD side fixed.
6 In the same way, nail the b and d, fix the BC and AD sides, and the operation of the stretched net is completed.
(2) Instrument stretcher? Instrument stretcher is also known as self-tensioning net. This kind of stretch net method uses a relatively simple device to assist the manual stretch net, and the net frame is called a self-tensioning net frame. This kind of frame is mostly made of metal, and its outstanding feature is that it can be pulled at any time. After the stencil is slack, it does not have to be replaced, and it can be used again after being tensioned again. In recent years, the roll-type self-tensioning net has been successfully used for precision printing, and the market has expanded rapidly. Among them, the roll frame has the best performance. It consists of four aluminum tubes. There is a simple buckle device in the tube to clamp the wire mesh. The ends of each frame tube can be twisted by the wrench to complete the stretch.
(3) Mobile stretch net? The mobile stretch net uses a mechanical stretcher. The mechanical stretcher has a push rod type, a screw type and a gear strip type. Large stretchers are generally maneuverable, while medium and small stretchers are mainly manual.
The most important feature of the manual mechanical stretcher is that it does not require electricity or gas source. The quality of the stretched net is significantly higher than that of the manual stretcher, which is close to the quality of the pneumatic stretch net.
(4) Pneumatic stretch net? Pneumatic stretch net machine uses compressed air as the air source to drive multiple cylinder pistons, synchronously push the net clamp for relative contraction movement in the longitudinal and transverse directions, and produce uniform and uniform pulling force on the screen. Generally, the tension of the nylon mesh can be controlled at 0.8~1.0MPa; the tension of the polyester mesh can be controlled at 0.8~1.0MPa; the tension of the stainless steel mesh can be controlled at 1.0~1.3MPa.
When using the pneumatic stretcher method, first place the frame on the stretcher table, then lay the screen horizontally close to the frame and fix the mesh around the chuck. Tighten according to the required tension, and then apply adhesive on the interface between the frame and the screen. This method gives a uniform tension.
3, the fixing of the wire
There are two methods for fixing the screen to the frame, namely the clamping method and the bonding method.
(1) Clamping method? The clamping method is to sandwich the screen between two objects and fix it by friction. The wire mesh of this fixing method is not easy to loosen, but the tension of the stretcher is not easy to control, and a manual stretch net is commonly used.
(2) Bonding method The bonding method is to fix the screen with an adhesive. The screen tension of this fixing method is easy to control, but after a period of use, the screen will be loose. Commonly used adhesives are as follows.
1 photosensitive adhesive
The formulation of the photosensitive adhesive is as follows: epoxy phenolic methacrylic resin 300 mL; benzoin butyl ether 10~12 mL; absolute ethanol 150 mL.
How to use: Apply the adhesive to the bonding surface of the frame so that the tight mesh is tightly bonded to the surface, and a layer of adhesive is applied to the bonding between the mesh and the frame. After coating, it is cured by ultraviolet light, that is, it is irradiated with 1000W ultraviolet high-pressure mercury lamp, the distance is 500mm, the time is 5~8min, and it is cooled by electric fan.
2 polyvinyl acetal glue? Dilute 4 parts of JSF-4 glue with 1 part of absolute ethanol, brush on the bonding surface of the frame, apply it to the tight mesh and then brush it once, dry naturally or use Infrared drying. This adhesive is often used for the bonding of wooden frames and metal mesh frames.
3 shellac? Dissolve in 3 parts of absolute ethanol with 1 part of insect film to make shellac. In order to facilitate the bonding, the shellac glue may be pre-coated on the bonding surface of the frame, and dried for use. When sticking the net, apply a sufficient amount of absolute ethanol directly on the screen of the adhesive layer to dissolve the shellac on the frame. It can be used after drying.
In addition, there are phenol-chloroprene, nitrile rubber, nitrile rubber, nitrocellulose rubber, perchloroethylene rubber, and the like.
Usually, the adhesive mesh glue is purchased from the market. The commonly used imported adhesive mesh glue has Japanese W-18 and Italian UG-3 adhesive mesh glue, which makes the operation very simple, the tension of the stretch net is easy to control, and the mesh and the mesh frame are sticky. Strong.
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