Screen Printing

Screen Printing

Screen printing is arguably the most versatile of all printing processes. 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.

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

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

• 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

• Nylon - multifilament or monofilament

• good for stretching

• compared to polyester, lacks stability

• less rigid than polyester

• unsuitable for closely registered colors

• Polyester - multifilament or monofilament (calendared monofilament polyester, metallized monofilament polyester)

• primary material used in commercial screen printing

• Polyester is strong and stable when stretched

• Other screen materials - carbonized polyester

• glass

• 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 gelatine

500 g Boiling water

Total 700 g

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

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.

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.

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)

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.

Flat bed 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.

• F
  or 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

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 flat bed 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

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.