Printing
Content
Printing: Textile printing is the process of applying color to fabric in definite patterns or designs. In properly printed fabrics the color is bonded with the fiber, so as to resist washing and friction. Textile printing is related to dyeing but, whereas in dyeing proper the whole fabric is uniformly covered with one color, in printing one or more colors are applied to it in certain parts only, and in sharply defined patterns. In printing, wooden blocks, stencils, engraved plates, rollers, or silk screens are used to place colors on the fabric. Colorants used in printing contain dyes thickened to prevent the color from spreading by capillary attraction beyond the limits of the pattern or design. Traditional textile printing techniques may be broadly categorized into three styles: • Direct printing, in which colorants containing dyes, thickeners, and the mordant or substances necessary for fixing the color on the cloth are printed in the desired pattern. • Resist dyeing, in which a wax or other substance is printed onto fabric which is subsequently dyed. The waxed areas do not accept the dye, leaving uncolored patterns against a colored ground. • Discharge printing, in which a bleaching agent is printed onto previously dyed Fabrics to remove some or all of the color METHODS OF TEXTILE PRINTING: Block Printing Stencil Printing Batik Printing Screen Printing Transfer Printing Roller Printing
Screen printing: Screen-printing is the most flexible printing process. It can be used to print on a wide variety of substrates, including paper, paperboard, plastics, glass, metals, fabrics, and many other materials including paper, plastics, glass, metals, nylon and cotton. Some common products from the screen-printing industry include posters, labels, decals, signage, and all types of textiles and electronic circuit boards. The advantage of screen- printing over other print processes is that the press can print on substrates of any shape, thickness and size.
An important characteristic of screen-printing is that a greater thickness of ink can be applied to the substrate as compared to the other printing techniques. This allows for various interesting effects that cannot be achieved through the other printing methods. Because of the simplicity of the application process, a wider range of inks and dyes are available for use in screen-printing than for use in any other printing process. Utilization of screen printing presses has begun to increase because production rates have improved. This has been a result of the development of the automated and rotary screen printing press, improved dryers, and U.V. curable ink. The major chemicals used in screen-printing include screen emulsions, inks, and solvents, surfactants, caustics and oxidizers used in screen reclamation.
Screen-printing consists of three elements • The screen which is the image carrier • The squeegee • Ink The screen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Proper tension is essential for accurate color registration. The mesh is made of a porous fabric or stainless steel. A stencil is produced on the screen either manually or photo chemically. The stencil defines the image to be printed. In other printing technologies this is referred to as the image plate. Screen printing ink is applied to the substrate by placing the screen over the material. Ink with a paint-like consistency is placed onto the top of the screen. Ink is then forced through the fine mesh openings using a squeegee that is drawn across the screen, applying pressure thereby forcing the ink through the open areas of the screen. Ink will pass through only in areas where no stencil is
applied, thus forming an image on the printing substrate. The diameter of the threads and the thread count of the mesh will determine how much ink is deposited onto the substrates. Many factors such as composition, size and form, angle, pressure, and speed of the blade (squeegee) determine the quality of the impression made by the squeegee. At one time most blades were made from rubber, which, however, had a tendency to warp and distort. While blades continue to be made from rubber such as neoprene, most of them are now made from polyurethane, which can produce as many as 25,000 impressions without significant degradation of the image. If the item is to be printed on a manual or automatic screen press the printed product will be placed on a conveyor belt, which carries the item into the drying oven or through the UV curing system. Rotary screen presses feed the material through the drying or curing system automatically. Air-drying of certain inks, though rare in the industry, is still sometimes utilized. The rate of screen-printing production was once dictated by the drying rate of the screen print inks. As a result of improvements and innovations in the printing technology, the production rate has greatly increased. Some specific innovations, which affected the production rate and have also increased screen press popularity include: • Development of automatic presses versus hand operated presses, which have Comparatively slow production time. • Improved drying systems, which significantly improve production rate. • Development and improvement of U.V. curable ink technologies • Development of the rotary screen press, which allows continuous operation of the press. This is one of the recent technology developments.
Types of screen print:
Hand Screen Printing:
The silk screen is a wooden or aluminum frame with a fine nylon or silk mesh stretched over it. The mesh is coated with a light sensitive emulsion or film, which - when dry - will block the holes in the mesh. The image that needs to be printed is output to film either by camera or image-setter. This film positive and the mesh on the screen are sandwiched together and exposed to ultraviolet light in a device called a print-down frame. The screen is then washed with a jet of water which washes away all the light sensitive emulsion that has not been hardened by the ultraviolet light. This leaves you with an open stencil which corresponds exactly to the image that was supplied on the film. Next the fabric
to be printed is pinned on a wooden table so that it is evenly stretched and there are no ripples. Alternatively a wax table is used. The surface of the table is covered with wax. Below there are a network of pipes through which steam is passed. This causes the wax to soften and the fabric is just firmly pressed on to the table. The wooden frame of the screen is fitted with metal handles which will fit onto to corresponding wooden protrusions on the table. This is to aid placement, when two or more colors are being used. The dye is poured on the screen A rubber blade with a wooden handle is firmly pulled across the top of the screen; it pushes the ink through the mesh onto the surface of the fabric which is being printed. Another person stands at the other side of the table. He takes hold of the rubber blade and repeats the process. Since the nonprinting areas on the silk screen are blocked out, the ink is pushed through only the porous areas corresponding to the design and is thus transferred to the fabric. If more colors are used, the process is repeated with another screen and color. The screen is always washed with a lot of water immediately after use. If this is not done, the dye dries on the screen and clogs up the design.
FLAT BED SCREEN PRINTING A screen printing press comprising a main frame, a printing bed supported on the main frame, a rectangular printing frame having a rear end pivoted to said main frame for movement between a first printing position generally parallel to said printing table and a second flood position angled with respect to said printing table, said printing frame including elongated side members, a front member and a rear member, each of said side members defining an elongated track, a squeegee and flood bar carriage engaging said elongated tracks and movable along said side members, said carriage including a pair of spaced side plates, a pair of slides, each slide extending from one of said side plates and into a respective one of said tracks, a pair of pivot plates, each pivot plate pivoted to one of said side plates, a squeegee support member extending between said pivot plates and a flood bar support member extending between said pivot plates in spaced, parallel relationship, said squeegee and flood bar support members being engaged by said pivot plates so that pivoting of said pivot plates alternately raises and lowers said squeegee and flood bar support members.
pivot plate actuating means operatively connected to said main frame and engaging at least one of said pivot plates for automatically lowering said flood bar support member when said printing frame is raised to its flood position and for automatically lowering said squeegee support member when said printing frame is in its printing position, said pivot plate actuating means including an elongated, rigid actuating member pivoted to said printing frame and operatively connected to said main frame, said elongated actuating member extending along and parallel to one of said side members of said printing frame a pivot bracket supporting said actuating member, said pivot bracket being pivoted to said printing frame; shifting means fixed to said pivot bracket and operatively engaging said main frame for pivoting said pivot bracket as said printing frame moves between said first and said second positions; and means operatively connecting said actuating member to one of said pivot plates. • Mechanization of hand screen process • Fabric glued to blanket • Screens rise and fall • Printing done while screen in down position • Rod or blade squeegee system • Up to four strokes possible • Slow process ROTARY SCREEN PRINTING In rotary-screen printing, continuous rotation of a cylindrical screen while in contact with the fabric ensures genuinely continuous printing. Print paste is fed into the inside of the screen, and during printing is forced out through the design areas with the aid of a stationary squeegee. The design of most machines follows the pattern established for fully automatic flat- screen machines: an endless driven blanket, screen positions along the top, and blanket washing and drying effected underneath during the return passage. Provision for the use of a thermoplastic adhesive is common on rotary machines, with a curved-surface heating plate to heat the fabric before it is pressed on to the blanket. The cylindrical screens can be much closer together than is possible with flat screens and so the blanket is shorter (for a given number of colors). The fabric dryer, however, must be longer to enable the printed fabric to be adequately dried at higher running speeds. Typically, speeds of 30–70 m min–1 are used depending on the design and the fabric quality. It is quite possible to run the machine faster than this, the limitations often being the
length and efficiency of the cloth and blanket dryers and the difficulty of observing Printing faults at high running speeds. Print paste is often poured into flat screens by hand, even in fully automatic machines, but the continuous movement of a cylindrical screen and the restricted access necessitates automation of this operation. The print paste is pumped into the screen through a flexible pipe from a container at the side of the machine; inside the screen, the paste pipe has a rigid structure as it also acts as a support for the squeegee. Holes in the pipe allow the paste to run down into the bottom of the screen; since the paste is pumped in from one end, the holes need to be larger at the end furthest from the pump to achieve an even spread across the full width of the Screen. A sensor (level control) actuates the pump when the paste level falls below a preset height
Printing Paste Preparation for Screen Printing:
Screen printing: Screen-printing is the most flexible printing process. It can be used to print on a wide variety of substrates, including paper, paperboard, plastics, glass, metals, fabrics, and many other materials including paper, plastics, glass, metals, nylon and cotton. Some common products from the screen-printing industry include posters, labels, decals, signage, and all types of textiles and electronic circuit boards. The advantage of screen- printing over other print processes is that the press can print on substrates of any shape, thickness and size.
An important characteristic of screen-printing is that a greater thickness of ink can be applied to the substrate as compared to the other printing techniques. This allows for various interesting effects that cannot be achieved through the other printing methods. Because of the simplicity of the application process, a wider range of inks and dyes are available for use in screen-printing than for use in any other printing process. Utilization of screen printing presses has begun to increase because production rates have improved. This has been a result of the development of the automated and rotary screen printing press, improved dryers, and U.V. curable ink. The major chemicals used in screen-printing include screen emulsions, inks, and solvents, surfactants, caustics and oxidizers used in screen reclamation.
Screen-printing consists of three elements • The screen which is the image carrier • The squeegee • Ink The screen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Proper tension is essential for accurate color registration. The mesh is made of a porous fabric or stainless steel. A stencil is produced on the screen either manually or photo chemically. The stencil defines the image to be printed. In other printing technologies this is referred to as the image plate. Screen printing ink is applied to the substrate by placing the screen over the material. Ink with a paint-like consistency is placed onto the top of the screen. Ink is then forced through the fine mesh openings using a squeegee that is drawn across the screen, applying pressure thereby forcing the ink through the open areas of the screen. Ink will pass through only in areas where no stencil is
applied, thus forming an image on the printing substrate. The diameter of the threads and the thread count of the mesh will determine how much ink is deposited onto the substrates. Many factors such as composition, size and form, angle, pressure, and speed of the blade (squeegee) determine the quality of the impression made by the squeegee. At one time most blades were made from rubber, which, however, had a tendency to warp and distort. While blades continue to be made from rubber such as neoprene, most of them are now made from polyurethane, which can produce as many as 25,000 impressions without significant degradation of the image. If the item is to be printed on a manual or automatic screen press the printed product will be placed on a conveyor belt, which carries the item into the drying oven or through the UV curing system. Rotary screen presses feed the material through the drying or curing system automatically. Air-drying of certain inks, though rare in the industry, is still sometimes utilized. The rate of screen-printing production was once dictated by the drying rate of the screen print inks. As a result of improvements and innovations in the printing technology, the production rate has greatly increased. Some specific innovations, which affected the production rate and have also increased screen press popularity include: • Development of automatic presses versus hand operated presses, which have Comparatively slow production time. • Improved drying systems, which significantly improve production rate. • Development and improvement of U.V. curable ink technologies • Development of the rotary screen press, which allows continuous operation of the press. This is one of the recent technology developments.
Types of screen print:
Hand Screen Printing:
The silk screen is a wooden or aluminum frame with a fine nylon or silk mesh stretched over it. The mesh is coated with a light sensitive emulsion or film, which - when dry - will block the holes in the mesh. The image that needs to be printed is output to film either by camera or image-setter. This film positive and the mesh on the screen are sandwiched together and exposed to ultraviolet light in a device called a print-down frame. The screen is then washed with a jet of water which washes away all the light sensitive emulsion that has not been hardened by the ultraviolet light. This leaves you with an open stencil which corresponds exactly to the image that was supplied on the film. Next the fabric
to be printed is pinned on a wooden table so that it is evenly stretched and there are no ripples. Alternatively a wax table is used. The surface of the table is covered with wax. Below there are a network of pipes through which steam is passed. This causes the wax to soften and the fabric is just firmly pressed on to the table. The wooden frame of the screen is fitted with metal handles which will fit onto to corresponding wooden protrusions on the table. This is to aid placement, when two or more colors are being used. The dye is poured on the screen A rubber blade with a wooden handle is firmly pulled across the top of the screen; it pushes the ink through the mesh onto the surface of the fabric which is being printed. Another person stands at the other side of the table. He takes hold of the rubber blade and repeats the process. Since the nonprinting areas on the silk screen are blocked out, the ink is pushed through only the porous areas corresponding to the design and is thus transferred to the fabric. If more colors are used, the process is repeated with another screen and color. The screen is always washed with a lot of water immediately after use. If this is not done, the dye dries on the screen and clogs up the design.
FLAT BED SCREEN PRINTING A screen printing press comprising a main frame, a printing bed supported on the main frame, a rectangular printing frame having a rear end pivoted to said main frame for movement between a first printing position generally parallel to said printing table and a second flood position angled with respect to said printing table, said printing frame including elongated side members, a front member and a rear member, each of said side members defining an elongated track, a squeegee and flood bar carriage engaging said elongated tracks and movable along said side members, said carriage including a pair of spaced side plates, a pair of slides, each slide extending from one of said side plates and into a respective one of said tracks, a pair of pivot plates, each pivot plate pivoted to one of said side plates, a squeegee support member extending between said pivot plates and a flood bar support member extending between said pivot plates in spaced, parallel relationship, said squeegee and flood bar support members being engaged by said pivot plates so that pivoting of said pivot plates alternately raises and lowers said squeegee and flood bar support members.
pivot plate actuating means operatively connected to said main frame and engaging at least one of said pivot plates for automatically lowering said flood bar support member when said printing frame is raised to its flood position and for automatically lowering said squeegee support member when said printing frame is in its printing position, said pivot plate actuating means including an elongated, rigid actuating member pivoted to said printing frame and operatively connected to said main frame, said elongated actuating member extending along and parallel to one of said side members of said printing frame a pivot bracket supporting said actuating member, said pivot bracket being pivoted to said printing frame; shifting means fixed to said pivot bracket and operatively engaging said main frame for pivoting said pivot bracket as said printing frame moves between said first and said second positions; and means operatively connecting said actuating member to one of said pivot plates. • Mechanization of hand screen process • Fabric glued to blanket • Screens rise and fall • Printing done while screen in down position • Rod or blade squeegee system • Up to four strokes possible • Slow process ROTARY SCREEN PRINTING In rotary-screen printing, continuous rotation of a cylindrical screen while in contact with the fabric ensures genuinely continuous printing. Print paste is fed into the inside of the screen, and during printing is forced out through the design areas with the aid of a stationary squeegee. The design of most machines follows the pattern established for fully automatic flat- screen machines: an endless driven blanket, screen positions along the top, and blanket washing and drying effected underneath during the return passage. Provision for the use of a thermoplastic adhesive is common on rotary machines, with a curved-surface heating plate to heat the fabric before it is pressed on to the blanket. The cylindrical screens can be much closer together than is possible with flat screens and so the blanket is shorter (for a given number of colors). The fabric dryer, however, must be longer to enable the printed fabric to be adequately dried at higher running speeds. Typically, speeds of 30–70 m min–1 are used depending on the design and the fabric quality. It is quite possible to run the machine faster than this, the limitations often being the
length and efficiency of the cloth and blanket dryers and the difficulty of observing Printing faults at high running speeds. Print paste is often poured into flat screens by hand, even in fully automatic machines, but the continuous movement of a cylindrical screen and the restricted access necessitates automation of this operation. The print paste is pumped into the screen through a flexible pipe from a container at the side of the machine; inside the screen, the paste pipe has a rigid structure as it also acts as a support for the squeegee. Holes in the pipe allow the paste to run down into the bottom of the screen; since the paste is pumped in from one end, the holes need to be larger at the end furthest from the pump to achieve an even spread across the full width of the Screen. A sensor (level control) actuates the pump when the paste level falls below a preset height
Printing Paste Preparation for Screen Printing:
