Grafica News - December 2012
Master the art of screen printing right from pre-press and screen making stage*
Resolution should be 170 (pixels/inch); and maximum printable LPI is 85
- Resolution of the computer screen is measured in pixels per inch (ppi)
- Resolution of an image file is also measured in pixels per inch
- Pixel ÷ PPI = Image size (Example: 6800 ÷ 170 = 40”)
This stage mainly involves fabric stretching; fabric preparation/degreasing to make ready for emulsion coating also called as stencil making; emulsion drying; screen exposing; screen developing (washing); finally screen inspection/touch up for pinholes.
The screen making process should be executed only under yellow light condition, which prevents preexposures, in a clean, air tight set up, free of sunlight.
Fabric stretching: The requirements include fabric with appropriate mesh count as per application, glue, stretching machine, tension meter, aluminium frame as per printing size requirement.
In screen printing it is important to maintain 6” margin between screen frame ID (Inner Dimension) to print area to achieve close tolerance registration. So by maintaining 6” margin on all four sides of the print area the frame size in ID should be bigger by 12” from the print area in total e.g. screen frame size for Print Area 20” x 30” should be 32” x 42” ID. But while ordering your screen frame its advisable to order in OD (Outer Dimension) keeping in mind the screen frame holder design of the machine specified by the manufacturer.
Screen printers even print with screen, maintaining only 4” margin on all four sides to get max. utilization of the print area. But this is possible when high-tension, low-elongation polyester mesh is used to make the screens and stretched on an automatic pneumatic fabric stretcher, stretched at high tension.
The requirements of the frame are rigidity to support high tensions; chemical resistance; accepting bond surface; hermetically sealed; and durability. For high quality printing it's advisable to use good quality aluminum frames only.
- Maintain a reasonable gap between the squeegee edge and inside of frame
- Short gap will negatively affect print quality, performance, uniformity of ink deposit; and it also shortens the life of stencil, fabric & squeegee
- Squeegee should be only 2” bigger than the print area i.e. for printing 30” wide squeegee should be 32” only.
- Keep ends of the squeegee rounded to get good registration and uniform ink deposit
- Always maintain sharp squeegee while printing and use squeegee sharpener regularly.
- Storage of squeegees is also very important. Its advisable to store the squeegees vertically (squeegee rubber side up) and ensure no objects are in contact during storage.
- Good air circulation is also recommended to keep the squeegee dry after cleaning with solvents.
- Do not use aggressive solvents to clean the squeegees
- Do not wipe the ink from the squeegee after printing with any sharp metal or hard materials. This can damage the squeegee edge.
- Do not print squeegees with one edge for longer period always use both edges after few hours or impressions to maintain the edge sharpness and parallelism
Fabric/mesh selection: A screen is made of finely woven fabric called mesh, which is available in different mesh counts, ranging from 6 mesh / cm to 180 Mesh/ cm and various widths; with various numbers in between. The lower the mesh count, the less detail one can print; and the thicker it is, the ink lays down. Mesh of different count is used depending on the job to be printed and ink deposition requirement – thin layer or heavy.
Mesh is mostly available in either yellow or white colour (nylon and polyester). It is always advisable to use yellow polyester mesh for making high quality screen. Polyester Mesh is the best for high quality close tolerance screen printing buy in same cases screen printers use Nylon for printing on curve/uneven substrates like containers, molded parts because of the elastic characteristics of Nylon. Even Nylon has one major advantage to avoid ink choking while printing when using water based inks because Nylon absorbs water and keeps the mesh wet thus avoids ink choking.
Stretching A mesh is stretched tightly over an aluminium frame. While stretching and before finalising the frame, it is necessary to ensure proper tension in the frame for accurate registration. The mesh is then glued with certain special adhesives to make it tightly stick onto the frame. To determine the maximum tension level, one must consider the type of mesh; mesh specification; frame size & strength; and tensioning system. Mesh tensioning enables to stretch mesh in two directions; consistent thread direction; pre-bow frame; and accurate distribution of pressure.
Tensioning systems These include mechanical tensioning and pneumatic tensioning systems. In pneumatic and a degreaser. This prepares the surface so that the stencil adheres well; cleans the mesh of impurities or other contaminants; acts like a dish soap and removes oil & grease.
Modern stencils are created photographically. Emulsion can be used under a yellow light, so that the UV light is filtered out so as not to affect the emulsion thus avoids premature exposure. One can pour emulsion into the scoop coater and place it on a vertical screen. Pressing up against the screen and pulling up, one can lay a thin 'photo-finish' layer of emulsion on the outside of the screen and turn the screen around and do the same for the inside of the screen.
While coating blank screens or applying first coat then emulsion tends to develop thickness on the opposite side. Since the thickness is required on the print side its advisable to once coat on the print side and there after on the squeegee side and keep in screen dryer for drying the wet emulsion o for 30 to 40 mins @40 C. Never o dry screens over 40 C. Drying over o 40 C makes screen washing difficult or sometimes impossible.
Always dry the screen horizontally after applying each coat. Face coats are required after drying the first coat mainly on the print side to improve the smoothness (To achieve lower Rz value) of the emulsion and to achieve film type quality. Number of face coats depends on the type of job and application. Lower the Rz value i.e. higher smoothness by multiple face coats results in fine edge definition thus avoids saw tooth edges.
Fully automatic emulsion coating machines are now available to make things convenient and also avoid skilled and experienced manpower. It requires a screen prepared with a light-sensitive coating, a film positive and a light source that will enable to transfer the images onto the positive of the light-sensitive screen that has been prepared.
- Always avoid dichromatebased emulsions
- For halftone screen printing it's always advisable to use Dual-Cure Photopolymer Emulsions.
- For fine and quick exposure single pot Photo Polymer Emulsion is highly recommended.
- Use higher emulsions, which has higher solid contents to minimize number of coats and face coats where high emulsion thickness is desired.
The screen is washed off thoroughly. The areas of emulsion that were not exposed to light dissolve and wash away, leaving a negative stencil of the image on the mesh.
- Dry in total darkness preferably using a horizontal screen dryer.
- Do not exceed 40 C
- Dry horizontally squeegee side up on initial coats
- Dry screen horizontally, print side up to dry after each face coats.
- Store coated screens in a cool, dark, dry area
In an area away from light and heat, set the screen to dry horizontally. This will provide the most even, flat 'film' on the underside of the screen. Create a smooth, even thin coating by applying face coats and repeat the drying process away from excess heat and light. Once the sensitised screen is dry, it must remain in a dark area, until it is ready to be exposed.
Once the emulsion dries, the screen is exposed with the help of an exposure unit. To expose photopolymer emulsions Metal Halide Lamps are the best source of light to achieve proper and faster exposure.
To expose a screen, the film positive in screen-printing should have emulsion side with the right reading being up in offset printing its emulsion down. Then the dried screen should be placed on the glass top and the film positive below it with emulsion side of positive touching the stencil. The exposing machine should have a good vacuum system to ensure proper contact between emulsion and positive to achieve fine and sharp edge.
The length of time will depend on the emulsion and light source. To know the exact exposing time it's recommended to use Exposure Calculators. These calculators are generally available with all emulsion manufacturers. Every mesh count with different coating thickness requires specific exposure time.
- Use proper vacuum blanket exposing system instead of keeping weights to get contact between positive and emulsion.
- Expose screen using Metal Halide Lamps with Light Integrator to avoid over/under exposure, which happens due to voltage fluctuations or due to lamp ageing.
Once the image is exposed, both sides of the screen should be lightly sprayed with water in a special washout booth. Spray on the outside of the frame, or the side that was touching the film. The inside will naturally be softer because the light had to pass through the emulsion to get to that side. Now the image on the screen becomes visible. It is important to let the screen dry before transferring the ready-to-use screen to printing department.
* This article, authored by Bhargav Mistry, was published by Modern Packaging& Design magazine.