Talking about the equipment and process of steel drum screen printing
Yang Wenliang
The application of screen printing in China's steel drum packaging industry has just begun, compared with the old process of engraving printing, there is a qualitative leap. It not only has a beautiful and elegant pattern, but also has large information capacity, high production efficiency, low environmental pollution and low labor intensity. Therefore, the screen printing process is booming in the application of the barrel industry. However, due to the immature technology, process, equipment, etc., some manufacturers rely on local printing companies to assist in the solution. Most of them are exploring experiments and self-made equipment. Most of them are not professional enough. The equipment efficiency is not high or the quality is difficult to guarantee. It is not suitable for steel. Most of the mass production of barrel companies is still manual. Therefore, the application of screen printing technology in the steel catching industry needs to be systematically explored to achieve high efficiency, high quality and no pollution.
Types of steel drum screen printing equipment
Steel drum screen printing equipment has a variety of shapes, but most of the printing is in the top plane of the barrel and the surface of the barrel. Therefore, the surface shape of the steel drum can be divided into manual, semi-automatic and fully automatic printing machines.
First, the barrel top flat screen printing machine
The flat steel drum printing machine is used for printing on the top surface of the barrel or the side of the barrel of the short steel drum, so the stencil is mostly a flat screen, and the printing table is also flat. The flat screen printing machine is a relatively advanced one and is being widely used. It has developed a variety of flat screen printing machines. The flat screen printing machine can be placed at the end of the production line, and the steel drum is transported on the batch conveyor as a printing machine supply barrel, which has high production efficiency.
(1) Flat net platform uncovering type flat screen printing machine. This screen printing machine is the main type of flat screen printing machine. One side of the flat screen printing plate is a hinged screen printing machine. At the time of printing, the screen printing plate is lowered, parallel to the printing table, and then the squeegee is horizontally squeezed and pressed on the printing plate to perform printing. After printing, lift the screen plate and take out the steel drum.
This screen printing machine has both manual and semi-automatic models. The manual type has simple structure, convenient maintenance, low price and wide adaptability, but the printing precision is not high, the operator's technical level is high, and the printing efficiency is low, and is suitable for small-volume steel drum printing. The semi-automatic type of screen printing machine has a fast printing speed, a convenient adjustment of the squeegee pressure, a printing selection stroke, and the like, and the printing quality is superior to that of the manual screen printing machine. Its printing work platform is generally made of stainless steel.
(2) Platform flat screen printing plate horizontal lift flat screen printing machine. In the screen printing machine, the screen printing plate is lifted up and down in a horizontal state during the printing process. When the screen printing machine is printing, the degree of solidity of the printing plate is not moved, the printing table is horizontally moved up and down, and the squeegee is horizontally squeegeeed. This machine has the advantages of smooth operation and accurate overprinting.
In addition to the above two flat printing machines, there is also a flat screen horizontal moving flat screen printing machine. The printing table is inclined horizontal screen printing machine, printing table fan-shaped screen printing machine, printing table rotary screen printing machine, etc., and can be applied to different production lines.
Second, the barrel surface screen printing machine
(1) The type and structure of the curved printing machine. Surface screen printing machines are classified according to the degree of automation. There are semi-automatic curved screen printing machines and automatic curved screen printing machines; according to the shape of screen printing plates, there are dry mesh curved screen printing machines and rotary screen printing. machine.
The printing plate of the flat net curved screen printing machine is flat, and the supporting device of the steel drum can change the shape and size of the supporting device by replacing the attachment according to the curvature and shape surface of the steel drum surface. The steel drums of such printing presses can be operated manually or by mechanical means.
The rotary screen screen printing machine is generally multi-color printing, the printing plate is cylindrical, the squeegee is installed in the cylindrical printing plate, and the printing table is multi-composite curved printing. This printing machine is widely used in foreign countries and has not been used yet in China.
(2) The operation points of the barrel surface printing. In order for the barrel surface to print smoothly, the following three requirements must be observed:
1. The centerline of the scraper must pass through the normal of the barrel surface.
2. The steel drum must be a regular cylinder or cone or the like.
3. The steel drum can rotate and the plate must be able to move.
Pay attention to the printing work: the frame should be at a constant speed and the pressure of the squeegee should remain unchanged. During the printing process, the steel drum can not be loosened before and after, and the steel drum should be kept rotating at a constant speed. In order to make the printed surface bright, it is necessary to keep a certain gap between the printing plate and the surface of the barrel, and make the steel drum easy to print and pick and place. Compared with flat printing, surface printing is generally difficult to press out of ink.
Main mechanism of steel drum screen printing equipment
Regardless of manual printing or mechanical printing, screen printing machines generally consist of a squeegee, an ink return device, a printing plate, a printing table, etc. On the automatic printing machine, there are also automatic feeding and discharging devices. Some are also equipped with equipment such as drying equipment.
First, the squeegee
A squeegee is used to squeeze the ink and pass the ink through the screen and the mesh is transferred to the steel drum. It has an important influence on the printing effect. The squeegee is made of elastic material and has various structures. It is generally divided into a squeegee for manual printing, a squeegee for a flat printing machine, and a squeegee for a curved printing machine.
The hand-printing squeegee consists of a rubber sheet sandwiched between two plywood or seven-ply panels. The rubber sheet of the mechanical printing squeegee is sandwiched by an iron plate or a fiberboard. The thickness of the rubber sheet is generally 6-9 mm, and the length is generally 10-140 mm larger than the width of the printed image.
The distance at which the rubber sheet exposes the splint can be determined according to the thickness (rigidity) of the steel drum material, the surface roughness (cold rolling or hot rolling, etc.), and the hardness of the rubber sheet. For the surface of the steel drum used for unevenness or a large thickness, the exposed portion of the rubber sheet is long, generally 15-20 mm, and a soft rubber sheet is used to make it have a certain elasticity. Generally, the squeegee for printing, the rubber sheet can be exposed as long as about 10 mm. A hard rubber sheet is longer than a soft rubber sheet.
The hardness of the rubber sheet can be determined according to the fineness of the printing plate and the material of the steel drum. The finer the printed graphic, the harder the rubber plate should be used; the larger the plate, the softer rubber plate should be used. When printing thick steel drums, a softer squeegee is used, and the printing effect is better; on the contrary, when printing a thin steel drum, a hard squeegee is used. In the case where the printing conditions permit, the hardness of the rubber sheet can be appropriately increased, because the hardness can improve the wear resistance and solvent resistance of the doctor blade, thereby improving the service life of the doctor blade.
In the printing process, since the front end of the squeegee is constantly rubbed against the screen, the squeegee is required to have good wear resistance. Fluorinated rubber sheets and polyurethane rubber sheets currently used are preferred, and their wear resistance and strength are better than those of other rubbers.
The ink contains a large amount of organic solvent, so the rubber sheet must have good solvent resistance. In general, non-polar natural rubber, nitrile rubber and ethylene propylene rubber are preferred for inks containing polar solvents (alcohols, ketones, ethyl acetate). For inks containing a non-polar solvent (kerosene), polar urethane rubber and nitrile rubber are preferred.
Second, the ink return device
The ink is returned to the starting end after one squeegeeing, and a layer of generally smooth metal squeegee is applied evenly on the screen plate, the width of which is slightly larger than the squeegee. The ink return plate should be parallel to the screen plate, and its horizontal height should be adjustable. The gap between the bottom surface of the ink return plate and the screen plate surface is the thickness of the printing ink layer. The ink return plate can float freely up and down during the movement. If the resistance exceeds its own weight due to excessive thickening, excessive thickness, dryness, etc., it should be able to automatically lift the ink layer to protect the screen from loss. .
The squeegee system and the ink returning system are usually mounted on the squeegee carriage. During the reciprocating motion, the squeegee and the ink returning plate are alternately raised and lowered to realize the ink squeezing and ink returning actions, respectively. The lifting and lowering of the squeegee is mechanically commutated on a general plane automatic machine, and pneumatic control is often used on a precision semi-automatic plane machine.
Scratch and ink return operations can sometimes be achieved with a squeegee. For example, the manual curved machine, when printing is completed, relies on a unique ink-repelling mechanism to evenly apply the ink to the screen plate during the return journey for the next printing. The ink return pressure of the ink returning knife is much smaller than the printing pressure of the doctor blade.
Third, the printing plate device
The flat printing plate is relatively simple, consisting of a frame and a wire mesh. The screen printing plate must be fixed on the printing plate device in the screen printing machine, and in the printing process, the uncovering type is lowered or horizontally raised and lowered.
In order to improve the printing precision, it is required that the partial screen which has just passed through the ink after the squeegee is smeared away from the steel drum, and this action is often realized by the screen plate detaching device, in addition to the elastic force of the screen itself.
In addition, the plate unit has the following components:
l, plate holder. The clamping is required to be firm and the frame is not damaged at the clamping point. There are many clamping methods, but it is widely used that the groove body is provided with the presser foot clamping.
The screen printing machine of the book type landing printing plate generally adopts a cam mechanism, or a pendulum bar mechanism: a screen printing machine of a horizontal lifting type landing plate, generally adopting a cylinder guide column structure or a cam guide column mechanism. The structure can be diversified, but one requirement must be met, that is, the relative position of the plate to the platform should remain unchanged each time the printing action is completed and the plate is again dropped to the working position. Therefore, the axial and radial clearance of the hinge rotary shaft must be strictly controlled, generally not more than 0.05 mm; the radial clearance of the horizontal lifting guide column is generally not more than 0.03 mm, which are guaranteed by the manufacture of the equipment.
Generally there are lighting version, agency version, electronic version. The plate mechanism is placed on the printing table or placed in the plate device. The position of the platform when moving against the plate is generally achieved by mechanical threading and should have reliable locking devices and shifting guides. Shift has two purposes. One is that the coordinate position of the steel drum printing part is correct. One is that the printing position of the steel drum always maintains this correct position during the printing process, which is one of the steps to improve the printing precision.
4. Printing platform. It is used to fix steel drums. The printing platform should have a high degree of flatness and can guarantee the printing precision; the printing table should have a steel barrel positioning device; in order to adapt to different steel drums and maintain a certain screen spacing, the height of the printing table should be adjustable; The platform should be adjustable in the horizontal direction.
The printing table used for the surface of the barrel should be placed between two parallel rollers to allow the drum to roll. In order to reduce the frictional resistance during the printing of the steel drum, the printing table uses four rolling bearings to facilitate the rotation of the steel drum under the wire mesh.
Steel drum screen printing process
First, the steel drum plane printing machine operation
1, steel drum manual printing
(1) Using a good quality double-sided adhesive tape, fix the plate on the printing table and make the plate and the surface of the steel drum 3-5 mm apart.
(2) Place the original on the printing table to align the image on the plate with the original.
(3) After finding the position, set the rules. The rules can be made of hard plastic and should be slightly thicker than the steel drum.
(4) Place the steel drum on the printing table and put the ink in the frame.
(5) The ink placed on the frame needs to be slightly blended with a squeegee, and then the ink is applied to the plate with a squeegee. At the same time, the ink is returned. The operation must ensure that the ink always covers the surface of the plate to prevent the plate from drying out. Lift the plate frame when returning to the ink.
(6) After the printing plate is dropped, use a squeegee to press the plate and use a squeegee to maintain a certain inclination angle with the squeegee. The angle is generally between 45° and 90°. The speed of the squeegee should be appropriate.
(7) First test printing, after the printing quality meets the requirements, can be transferred to the official printing.
(8) The printed product can be naturally dried or placed in a drying tunnel for forced drying.
(9) After the printing is finished, the squeegee, plate, etc. must be cleaned, and turpentine can be used for cleaning. It is easy to damage the printing plate when scrubbing on the back side of the printing plate. Therefore, when scrubbing the back side of the printing plate, the ink should be washed off with a brush and the printing plate is transparent. Finally, use a dry sponge to clean the surface of the plate.
2, mechanical printing
The manually printed squeegee pressure and the squeegee movement speed can be arbitrarily changed, and the mechanical printing pressure and speed are not changed after adjustment.
In mechanical printing, the pressure of the squeegee and the viscosity of the ink should not be suddenly increased. It should be slowly adjusted to make it in an appropriate state. Otherwise, unexpected accidents such as damage to the layout may occur.
Second, the barrel surface screen printing operation
Contrary to the flat printing, the screen-printed squeegee is fixed, the plate moves, and the printing is completed by the plate movement. Surface printing is divided into cylindrical curved printing and conical curved printing.
1, cylindrical barrel surface printing
Cylindrical steel drum surface printing is to place the steel drum on two parallel rotatable rollers. The printing plate is located on the upper part of the barrel, and the squeegee is pressed up and down. When pressed, the printing plate is in contact with the surface of the steel drum. The squeegee is fixed, the plate moves horizontally, and the steel drum rotates as the plate moves, while ensuring that the line speed of the surface of the steel drum is equal to the speed at which the plate moves.
The operation of cylindrical curved printing is as follows:
The steel drum to be printed is placed on a backup roll, the support roll is raised, and the printing plate is moved for printing. After printing, the support roller is lowered, the steel drum is taken out, and the other holding steel drum is placed thereon, and the above process is repeated for printing. The printing plate should be moved at the speed of printing, and the pressure of the squeegee should remain unchanged. During the printing process, the steel drum can move back and forth, left and right. To keep the steel drum rotating at a constant speed, there must be a certain gap between the printing plate and the surface of the steel drum.
In the case of multi-color printing, the position of the printing plate must be carefully adjusted before printing to ensure accurate overprinting.
2, conical steel drum surface printing
First, the fan-shaped development of the conical steel drum is obtained, and the center of the development map is obtained. The plate size is the same as the size of the conical barrel. The palladium plate is fixed on the plate holder of the mandrel, and the center of the unfolded picture coincides with the mandrel, and then printed like a cylindrical barrel.
Manual conical surface printing, the table can be raised and lowered. When the steel drum is placed or removed, the workbench is in the lowered position, and the workbench rises when printing and is returned. The plate is mounted on the plate holder and the doctor blade is mounted on the doctor blade holder.
Operation steps: First adjust the height of the four bearings on the workbench so that the steel drum is in contact with the plate; then install the plate and fix it on the plate holder. In order to accurately align the graphic on the printing plate with the printing position, the position of the table can be finely adjusted from front to back.
After adjusting the height and position of the printing table, adjust the position of the doctor blade so that the plate is in line contact with the steel drum.
After the above parts are adjusted, put the ink on the printing plate, lower the workbench, place the holding steel drum on the bearing, then raise the workbench, put down the squeegee, apply pressure to the printing plate, and the printing plate is The mandrel is centered and printed horizontally for printing. After printing, the workbench is lowered, the printed product is taken out, and then another printed steel is placed. Repeat the above steps to print the next steel drum.
Third, steel drum screen printing process control points
The main factors affecting the quality of screen printing are: printing pressure, the gap between the printing plate and the steel drum, the angle to the ink knife and the moving speed.
1, printing pressure
In screen printing, the printing pressure applied to the printing plate by the doctor blade is too large or too small. If the size is too large, the tip of the doctor blade is too curved, so that the doctor blade cannot sufficiently scrape the ink, the amount of ink to be pressed is reduced, and the contact area between the printing plate and the steel drum is increased, the sharpness of the printed image is lowered, and the printing pressure is further reduced. If the ink is too large, the ink cannot be transferred to the steel drum. When the printing plate is in line contact with the steel drum, the printing pressure at this time is the most suitable.
2, printing clearance
The plate must be kept at a certain gap with the steel drum so that it can be contacted only when the plate and the surface of the steel drum are only used by the doctor blade during printing, and the doctor blade leaves after passing. The size of the printing gap has a great influence on the printing quality. If the gap is not properly adjusted, it is impossible to print a high quality product.
If the printing gap is too small, it is easy to cause ink squeezing in the direction of the doctor blade. If the gap is too large, it is necessary to increase the printing pressure, which will make the printing plate excessively stretch and slack, and the printed image will be out of shape, and the graphic size on the steel drum is larger than the printing plate. The printing gap is generally 1-6MM, the printing precision is high, the frame area is large, the screen is stretched tightly, or the surface printing value is small; the general printing, the frame area is large, the screen is stretched loose, or the plane printing value is large. .
3, printing speed and squeegee angle
The angle of the squeegee formed by the squeegee and the plate and the speed at which the squeegee moves should be kept constant. The squeegee angle is generally 45°-90°, and the printing speed is generally 100-300 MM/s. The smaller the angle between the printing plate and the squeegee, the slower the moving speed of the squeegee, the more ink is pressed, and the smaller the squeegee angle, the better the ink is pressed out.
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