Screen Printing


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Screen Printing

  1. 1. Md. Hasanuzzaman 2011000400007 1 | P a g e Screen Printing What is Textile Printing? Textile printing is the process of applying colour to fabric in definite patterns or designs. In properly printed fabrics the colour 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 colour, in printing one or more colours are applied to it in certain parts only, and in sharply defined patterns.In printing, wooden blocks, stencils, engraved plates, rollers, or silkscreens etc used to place colours on the fabric. Methods of printing 1) Hand block printing In this process, a design is drawn on, or transferred to, a prepared wooden block. A separate block is required for each distinct colour in the design. To print the design on the fabric, the printer applies colour to the block and presses it firmly and steadily on the cloth, ensuring a good impression by striking it smartly on the back with a wooden mallet. The second impression is made in the same way, the printer taking care to see that it fits exactly to the first, a point which he can make sure of by means of the pins with which the blocks are provided at each corner and which are arranged in such a way that when those at the right side or at the top of the block fall upon those at the left side or the bottom of the previous impression the two printings join up exactly and continue the pattern without a break. Each succeeding impression is made in precisely the same manner until the length of cloth is fully printed. When this is done it is wound over the drying rollers, thus bringing forward a fresh length to be treated similarly. If the pattern contains several colours the cloth is usually first printed throughout with one, then dried, and printed with the second, the same operations being repeated until all the colours are printed. Block printing by hand is a slow process it is, however, capable of yielding highly artistic results, some of which are unobtainable by any other method. 2) Engraved copper plate printing The printing of textiles from engraved copperplates was first practiced in the United Kingdom by Thomas Bell in 1770. The presses first used were of the ordinary letterpress type, the engraved plate being fixed in the place of the type. In later improvements the well-known cylinder press was employed, the plate was inked mechanically and cleaned off by passing under a sharp blade of steel, and the cloth, instead of being laid on the plate, was passed round the pressure cylinder. The plate was raised into frictional contact with the cylinder and in passing under it transferred its ink to the cloth. The great difficulty in plate printing was to make the various impressions join up exactly; and, as this could never be done with any certainty, the process was eventually confined to patterns complete in one repeat, and was made obsolete by roller printing.
  2. 2. Md. Hasanuzzaman 2011000400007 2 | P a g e 3) Roller printing, cylinder printing, or machine printing Roller printing, also called cylinder printing or machine printing, on fabrics is a textile printing process patented by Thomas Bell of Scotland in 1783 in an attempt to reduce the cost of the earlier copperplate printing. The advantages possessed by roller printing over other contemporary processes were three: firstly, its high productivity, 10,000 to 12,000 yards being commonly printed in one day of ten hours by a single-colour machine; secondly, by its capacity of being applied to the reproduction of every style of design, ranging from the fine delicate lines of copperplate engraving and the small repeats and limited colours of the perrotine to the broadest effects of block printing and to patterns varying in repeat from I to 80 in.; and thirdly, the wonderful exactitude with which each portion of an elaborate multicolour pattern can be fitted into its proper place without faulty joints at its points of repetition. 4) Stencil printing A stencil is a thin sheet of material, such as paper, plastic, or metal, with letters or a design cut from it, used to produce the letters or design on an underlying surface by applying pigment through the cut-out holes in the material. The key advantage of a stencil is that it can be reused to repeatedly and rapidly produce the same letters or design. The design produced with a stencil is also called a stencil. Screen printing also uses a stencil process, as does mimeography. The masters from which mimeographed pages are printed are often called "stencils." Stencils can be made with one or many colour layers using different techniques, with most stencils designed to be applied as solid colours. 5) Roller printing Roller printing, also called cylinder printing or machine printing, on fabrics is a textile printing process patented by Thomas Bell of Scotland in 1783 in an attempt to reduce the cost of the earlier copperplate printing. This method was used in Lancashire fabric mills to produce cotton dress fabrics from the 1790s, most often reproducing small monochrome patterns characterized by striped motifs and tiny dotted patterns called "machine grounds". Improvements in the technology resulted in more elaborate roller prints in bright, rich colours from the 1820s; Turkey red and chrome yellow were particularly popular. 6) Screen-printing Screen printing is by far the most used technology today. A blade squeezes the printing paste through openings in the screen onto the fabric. . About screen printing we will discuss later in this assignment. 7) Digital textile printing Digital textile printing, often referred to as direct to garment printing, DTG printing, and digital garment printing is a process of printing on textiles and garments using specialized or modified
  3. 3. Md. Hasanuzzaman 2011000400007 3 | P a g e inkjet technology. Inkjet printing on fabric is also possible with an inkjet printer by using fabric sheets with a removable paper backing. Today major inkjet technology manufacturers can offer specialized products designed for direct printing on textiles, not only for sampling but also for bulk production. What is Screen Printing? Screen printing is the most multipurpose process among all the printing processes. It is utilized on different mediums of substances like paper, paper board, metals, fabrics, and numerous other substances like plastics, glass, nylon and even cotton. The products produced as finished goods from the printing press include a wide range of products like posters, labels, signage and all types of textiles and electronic boards. Screen Printing Process  Hand screen.  Semi-automatic flat screen.  Rotary Screen. Screens  Made of cotton, silk, nylon cloth, polyester or metal gauze  Screen mesh refers to the number of threads per inch of fabric. The more numerous the threads per inch the finer the screen.  The usual mesh of screen employed for cotton and silk printing is 80 threads per inch.  The finer the screen the sharper are the outlines but more effort is needed to force the printing paste through the screen. Screen Frames  There are two types of screen frames, metal and wood.  Screen frames for commercial use are usually made of steel, or a lighter metal, with a hollow cross section to provide rigidity with minimum weight.  Screen frames are usually 26" x 55" (measured externally) and 23" x 52" (measured internally) for printing 45" wide cloth
  4. 4. Md. Hasanuzzaman 2011000400007 4 | P a g e Screen Fabric There are two types of threads for screen fabric:  Monofilament - single strands weaved into fabric  Primarily used in commercial printing and other applications  Advantage: Monofilament is easier to clean than multifilament  Multifilament - multiple strands wound together like a rope, then weaved into fabric.  Primarily used in textile printing.  Disadvantage: ink tends to build up on screen, more difficult to clean. Monofilament mesh has become the industry standard. Screen Fabric Types 1) Silk - multifilament weave  loses toughness with frequent use  reclaiming chemicals containing bleach or chlorinated solvents destroy the silk  Today silk is primarily used for printing art, not commercial use as before 2) Nylon - multifilament or monofilament  good for stretching  compared to polyester, lacks stability  less rigid than polyester  unsuitable for closely registered colors 3) Polyester - multifilament or monofilament (calendared monofilament polyester, metallized monofilament polyester)  primary material used in commercial screen printing  Polyester is strong and stable when stretched 4) Other screen materials - carbonized polyester  glass
  5. 5. Md. Hasanuzzaman 2011000400007 5 | P a g e  wire mesh  stainless steel Screen Preparation Photochemical method is most widely used for preparing the screen. This is based on the principle that when a coating of a solution of ammonium dichromate-gelatine or ammonium dichromate-polyvinyl alcohol is dried and exposed to light, Insolubilisation takes place Other method for screen preparation is lacquer and laser screen. Photochemical method  Coat the flat screen with light-sensitive polymer, and dry it in the dark.  Position a positive transparency of the pattern on the polymer-coated screen.  Expose the screen to ultraviolet light. Ultraviolet light rays pass through the transparent (non-pattern) areas of the transparency on to the screen and harden the polymer.  Wash the screen in warm water to remove the polymer from the unexposed (pattern) areas of the screen through which the printing paste will pass.  Dry the screen Preparation of Sensitising solution Sensitising solution may be prepared as follows: (1) Chrome-Gelatine Solution Solution A: 200 g Pure gelatin 500 g Boiling water Total 700 g
  6. 6. Md. Hasanuzzaman 2011000400007 6 | P a g e Solution B: 70 g Ammonium dichromate 150 g Boiling water 80 g Liquor Ammonia Total 300 g Solution A and Solution B are mixed in a dark room. (2) Chrome-Polyvinyl Alcohol Solution 600 g polyvinyl alcohol (15% solution) 120 ml Ammonium dichromate (33% solution) 240 ml Cold water 1 litre with cold water Squeegee system  Rubber Squeegee  Double Squeegee  Magnetic rod Squeegee 1) Rubber Squeegee  These vary in Shore hardness from 55 (soft) to 70 (hard). Softer blades give a heavier print. The edge shape of the rubber blades is chosen to suit requirements.  Round ones [Figure (a)] suit, for example, wool and fleece fabrics, where a heavy print is needed to penetrate the fibrous surface.  Long, tapered edges [Figure (c)] are used when penetration is not important as on flat and woven fabrics.  The stubby edge [Figure (b)] is good for one-stroke printing on interlock. The chisel shape  [Figure (d)] is used to flood the screen with printing paste while the screen is raised in preparation for the print stroke when only one print stroke is to be used.
  7. 7. Md. Hasanuzzaman 2011000400007 7 | P a g e 2) Double Squeegee This system is easier to make than a single squeegee, which must be lifted over the pool of print paste at the end of each stroke. 3) Magnetic rod Squeegee A rolling rod (a) or a pair of rods (b) is moved by a driven electromagnet moving under the printing blanket. The diameter of the single rod is small enough to allow print paste to flow over and round it at the end of a pass. The twin rods form a well of paste, the volume of which depends on rod spacing and diameter. Fundamental characteristics of screen printing  In screen printing process – hydrodynamic pressure is built up in the print paste between the squeegee and the screen surface through which the paste is passed.  The hydrodynamic pressure appears to be inversely proportional to the radius of the pore i.e. Hydrodynamic pressure  1/rn (n<2)
  8. 8. Md. Hasanuzzaman 2011000400007 8 | P a g e Here the pore radius greatly affects the amount of paste flowing through screen; Hydrodynamic pressure is also proportional to the viscosity of paste.  The percentage of open area of the screen also plays a role. More open screens allow more paste to pass.  The fabric is to be printed forms a three dimensional structure with the screen where the absorbency of the fibers and penetration capacity between yarn also affect the uptake of the paste.  The usual hexagonal openings are larger at the outside of the screen than at the inside, the capillarity and surface tension forces etc. result in a printing with actually more color deposited in the areas between holes than opposite holes. Flatbed screen printing  An automated version of the older hand operated silk screen printing  The flat-bed screen process is a semi-continuous, start-stop operation.  For each color in the print design, a separate screen must be constructed or engraved  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  Productivity is in the range of 15-25 yards per minute.  The design repeat size is limited to the width and length dimensions of the flat screen.  Currently accounted for apprx. 15-18% of printed fabric production worldwide  Slow process
  9. 9. Md. Hasanuzzaman 2011000400007 9 | P a g e Factors affecting Print paste passing through the screen  The ‘mesh’ (threads per inch) of the screen fabric  The fraction of open area in the screen fabric, this not only depends on the mesh but also on the yarn diameter and the effect of subsequent treatments, such as calendaring  The hardness and cross section of the squeegee blade; a hard rubber squeegee with a sharp cross section is suitable for outlines, whereas soft, rounded blade applies more paste and is suitable for blotches  The hardness of the printing table, if the top of the table is firm a soft squeegee is probably necessary, whereas with a resilient table surface a harder squeegee is preferable.  The viscosity of printing paste  The number of squeegee strokes; from two to four strokes are usually applied  The speed of the squeegee stroke Flat bed to Rotary screen  Modification of flatbed screen printing: from semi continuous to continuous, low productivity to high productivity.  Quality of end result.  Amount of color that can be applied. Note that the screen area consisting of holes is smaller in rotary screens than in flat screens.  Evenness of color.  Ability to produce fine lines and half-tones. Half-tones are tone graduations within one colored area.  More compact than flat screen machines for the same number of colors in the pattern  Rotary screen machines are highly productive, allow for the quick changeover of patterns, have few design limitations, and can be used for both continuous and discontinuous patterns  Typical speeds are from 50-120 ypm (45-100 mpm)  High investment cost and the machines are generally not profitable for short yardages of widely varying patterns.  Controls approximately 65% of the printed fabric market worldwide
  10. 10. Md. Hasanuzzaman 2011000400007 10 | P a g e Rotary screen printing In basic operation, rotary screen and flat screen-printing machines are very similar. Both use the same type of in-feed device, glue trough, rotating blanket (print table), dryer, and fixation equipment. The process involves initially feeding fabric onto the rubber blanket. As the fabric travels under the rotary screens, the screens turn with the fabric. Print paste is continuously fed to the interior of the screen through a color bar or pipe. As the screen rotates, the squeegee device pushes print paste through the design areas of the screen onto the fabric. As in flat-bed screen printing, only one color can be printed by each screen. After print application, the process is the same as flat screen printing. Estimates indicate that this technique controls approximately 65% of the printed fabric market worldwide. Defects on screen printing  Out of registration – pattern out of fit.  Glue streaks – from the rubber blanket.  Color smear.  Color out – from a lack of print paste.  Creased fabric.  Pinholes in any screen.  Damage to the screen leading to misprints.  Lint on the fabric causes pick-off.  The prints may come out lighter in the middle and deeper towards the selvedges. This occurs when too much cloth is steamed in one batch or when the cloth is very thick. Conclusion At the end of this assignment we can say that we got or learn a details information about screen printing through this assignment. References