Secondary Research - Screen Printing

Intro to Screen Printing

Screen Printing (also referred to as silk screening, screen process printing, serigraph) is a unique method of transferring or printing graphic images, and is considered by many to be the one of the oldest methods of printing. A primitive form was first used by the Chinese and the Japanese to decorate clothing, reproduce artwork, and publish posters with the latest words of wisdom of the emperor. Over 1000 years ago it was discovered that silk or human hair stretched on a wood frame, with a stencil image attached to the bottom with glue, could be used to reproduce the same image over and over on different materials by forcing ink or paints through the opening in the stencil.

Ancient or Modern?

Examples of this original technique are still used today. If you ever get the chance to watch a traditional Japanese kite maker, you get a glimpse of ancient screen printing in action. The kite maker uses a set of miniature silk screens with different colours and stencil shapes to repeat fish scales, eyeballs, and other shapes that illustrate a fish on the side of a kite.
The basic components of the screen printing process remain the same as in ancient times:

• the art or design
• the frame with the mesh
• the stencil carrying the image
• the ink
• the squeegee
• the press system
• the substrate (material) to be printed

There have been many advancements made over the years in materials, techniques, and equipment that allow today’s screen printer to use the process to print almost anything anywhere.
This course is concerned with covering some of these advances in screen printing, and helping readers integrate the new ideas into their printing efforts. As you get more practice and knowledge, you will develop your ability to recognize and reduce variables and problems within the process to achieve ‘the perfect print’.

Safety

Screen printing can be dangerous. The inks are toxic, solvents and chemicals used in clean-up can burn your skin, and presses, cutters, and equipment are capable of inflicting permanent injury or death.
Following are some steps you can take which will reduce risks in your shop or studio.

WORKPLACE CLEANLINESS

• The printing workshop will be divided into areas for artwork, exposure, screen making, ink mixing, printing, and a first aid area. Maintain storage areas (a cupboard, a shelf, a pegboard wall, etc.) for the individual items used in the specific areas, and daily or between jobs, PUT YOUR TOOLS BACK IN THEIR PLACE.
• There should be first aid around so make your self familiarized with the studio.
• Clean up ink spills or anything else WHEN THEY HAPPEN.

By keeping your workplace clean and tidy, you reduce the risk of tripping or bumping into things, which account for a large percentage of workplace misshaps. Guess what? You will also notice you become more efficient, and your printing improves!!!

PERSONAL SAFETY EQUIPMENT

As well as the above mentioned which are all used AFTER THE FACT, there are a number of personal safety items which should be worn while performing different tasks in the screen shop, which will PREVENT INJURIES from happening:

• Solvent resistant gloves should be worn for screen reclaiming, and any time solvents are used. The gloves will protect skin from chemical burns or irritation, and also stop absorption through the cuticles, which are an entry point in the body for toxic liquids.
• A face shield, or goggles, as well as a protective apron or coat, should be worn during screen cleaning to protect the eyes and skin from splashes.
• Hearing protection should be worn when using a pressure washer, operating a press with a loud vacuum.
• Although it doesn’t protect you from personal injury, an apron or shop coat worn at all times in the print shop can protect you from guilt and abuse caused by getting ink on your clothes.

 

ENVIRONMENTAL CONSIDERATIONS

• By using inks which don’t require solvents or solvent evaporation, and by using citrus based or alternative wash-up products, you can reduce the impact on the air quality in your workshop.
• By switching to ‘drainsafe’ biodegradable screen reclaiming products, you can minimize your workshops’ impact on water quality. Installing a filtering/evaporation unit to process wastewater from the washout booth will clean almost all particulate from your used water. 

WHY USE SCREEN PRINTING?

• Screen printed inks are the most durable and light fast available compared to most of the other printing processes.
• Screen inks are available to adhere to a wide range of materials, and can also be formulated as metallic, florescent, glow-in-the-dark, electrically conductive, as glass frit, as glue, etc.
• Screen inks are brighter and more opaque than those used in other printing processes.
• Compared to other printing technologies, screenprinting has very low capital costs for initial set-up.
• Because of the simplicity of the process, it is easily adaptable to a wide range of uses and applications, especially when it comes to printing 3 dimensional objects, parts, and different materials.
• Screen printing is not as restricted by size of print, length of production run, or thickness and type of printable material as other printing processes.
• The screen printing process is equally adaptable to labour intensive situations or fully automated systems while remaining economical in both.

DRAWBACKS TO USING SCREEN PRINTING

• When compared to high speed litho presses or other automated printing processes, screen printing can be seen as very slow and laboured.
• Traditionally, images must be converted from a negative, then exposed on the screen, then set up and printed. New digital imaging, especially in short runs, often allows for cheaper unit costs with higher resolution when compared to screen printing.
• Some inks and chemicals used are highly toxic.
• Quality standards vary widely. Many printers are poorly trained and badly equipped.
• Fine details and half-tone or process colour printing are harder to print because of technical limitations in the process.

WHAT IS IT USED FOR?
IN-PLANT PRINTING / CONSUMER GOODS

• Examples of consumer products using screen printing as an integrated part of manufacturing:
• Control panels on washers, dryers, stereos, back lit instrument panels on cars, machines, etc.
• Heater systems on back windows of cars, pinball and video games, frosted or decorated glass shower doors
• Ceramic tiles, mugs, plates, beer bottles, lab beakers and coffee pots, shampoo and lotion containers
• Hockey sticks, skis & snowboards, basketball backboards, decorated tablecloths & patio umbrellas, wooden gift boxes
• Radiant heating panels, circuit boards, membrane switches, keyboards and telephones, compact discs, wallpaper
• Plastic, metal, wood, fabric, leather, and other material.

GARMENTS & TEXTILES

• In the garment and textile field, screen printing is used to
  print t-shirts, hats, jackets, and other articles of consumer wear.
  Most patterned cloth found in fabric stores or used in shirts, dresses,
  or bed linens, has been screen printed.

SIGNS & COMMERCIAL GRAPHICS

• The widespread use of plastics or vinyls in outdoor promotional
  advertising, and the demands of marketers and advertisers for large
  durable images create a huge demand worldwide for screen printed
  graphics.
• Billboards, chain store signage, gas station pump toppers, large
  sale posters at grocery and department stores, transit ads on buses and
  bus shelters
• Logos on boards at hockey games, banners and graphics at car races, football and soccer games, ski races.
• Road signs, traffic control devices, license plates and stickers, car and truck graphics
• Displays and exhibits for museums, science centres, movie houses
• Advertising specialties

FINE ART

• Screen printing is the process used to produce limited edition
  serigraphs. Some artists work on their own, designing and printing the
  work themselves in their studio. Many artists choose to work with
  printers who specialize in limited edition screen printing.

Art for Screen Printing

The original ‘art’ or design’ to be printed can come in many forms, but must eventually be converted to a stencil image on the screen. In screen printing, only one colour can be printed at one time, and so multi-coloured print jobs require a separate piece of film to create each colour’s stencil.
When we talk about art for screen printing, we really mean 3 different things:

#1 The Original Art or Design

Can be in the form of a sketch, a layout on a computer, a painting or drawing, a typewritten rough, or a finished camera-ready design.
From this original you can determine what you are going to print, the size, location on the substrate, and number of colours required to reproduce the image.
Decide at this point if the art is reproduceable or usable for screen printing.

#2 Mechanical Art

This is an interim step where the elements of the design are composed or laid out with proper type, artwork is changed to camera ready images, or scanned as digital information in a computer. At the end of this
stage, all design elements are in place relative to each other, colours have been separated to individual overlays or plates, in register, and trapping considerations where colours meet have been built in. The term
‘camera ready’ is used to describe the art at the end of this step.
The term mechanical art refers to the physical re-scaling and building of the separate colour plates, with type and half toned photos. In the old days of paste-up and camera shots, this was done as separate steps, and then laid out on a board or overlays.

#3 Film Positive

A film positive carries an opaque image on a transparent film which is identical to the image to be printed. The film is used to make a stencil.
 

ORIGINAL ART OR DESIGN

• Line drawings (or line art, which is art or type with no tonal shading) should be in black on white, regardless of the final colour.
• Photos or works of art need to be scanned (digitalised) to prepare them for conversion to halftone art, or separation for printing process colour.

Exposure

Exposure is the process of transferring the positive image on the film to the stencil, and includes the washout of the image on the stencil. A UV light from a light source reacts with photopolymer elements contained in the photo stencil emulsion, and causes a chemical cross-linking and hardening of the exposed areas of the stencil. When washed with water, these areas stay adhered to the mesh, and the areas blocked by the opaque image on the film, which has shielded the underlying stencil from the UV light, wash out. The end result should leave an image identical to the film positive image as open areas in the mesh, surrounded by a layer of hardened stencil adhered to the mesh.
Components of the exposure process are a light source, a vacuum frame, and a washing booth. An exposure calculator, timing device, and hot and cold water are also required.

Light Source

The light source supplies UV spectrum light rays in the 330-440 nanometer range which cause the photosensitisers in the emulsion to react. Light sources range from photo floods and black light tubes, which give off low amounts of UV light through carbon arc to enclosed exotic bulbs such as metal halide, which use a transformer to jump the electricity wattage and ignite the gasses contained in the bulbs. Other light sources include the sun, and direct projection devices. The light sources can be run on a timer, or more commonly a light integrator, which reads the amount of UV light reaching the screen.

Vacuum Frame

This consists of a piece of untempered glass, a sealed flexible rubber blanket, and a vacuum motor. When the vacuum motor is turned on it holds the film positive in direct contact with the stencil, and pressed tight to the glass. The UV light source is then aimed at the glass, passing through it and the transparent areas of the film, exposing the stencil.
Vacuum frames come as stand alone units mounted on a wall, or on wheels. Glass in the exposure frame must be kept clean and free of dust or dirt which will start showing up unwanted on stencils.

Washout Booth

The washout booth is used for both the exposure and reclaiming of screens. It is constructed to contain water over spray, rest the screen against the back so it is on a diagonal, and it should have a back lit translucent panel to allow close inspection of the screen mesh during washing and reclaiming. The booth needs hot and cold water, and should have a pressure washer for washing.

Prepress

Prepress is the name given to the point in the screen printing process where the art and screen are given a final check, the stock or substrate is prepared and arranged for printing, the ink has been mixed and thinned, and the drying system and items needed during the print run are assembled close at hand and ready to use. It is important to carry out these prepress activities before the ink hits the screen, otherwise delays in printing will result, often stopping the print process or requiring cleaning of the screen.

ART & SCREEN CHECK

Using the film as a guide, a final inspection of the stencil should be made. Pay special attention to pinholes, or lines caused by film edges on the stencil, or any other imperfections in the stencil or image. These should be touched up before the screen is put in the press.

Block-out and Touch-up

Touch-up or block-out is applied to the underside of the screen using a brush for spots, or a small flat spreader for larger areas. If touched up on the top-side of a screen, you risk wearing nicks or gouges in the squeegee rubber.
Ensure that the screen is in good shape, with no serious rips or holes that will let loose during the print run. If there is any doubt that a stencil is suspect, or the image is flawed, re-shoot the screen. It saves time, money, and frustration if the problem is fixed at this point of the process.
Taping off screen gutters along the sides of the frame, or around images, can be carried out at this time, or if it’s easier, tape the screen once it is in the press. Use thin packing tape, never paper or masking tape. A good tape job makes production printing easier because it controls where the ink goes, and makes clean-up a snap. On long production runs, especially with water base, or when you know a stock edge is going to wear on the screen, a line of tape on the underside can protect the stencil from wearing thin from squeegee pressure along an edge. Make sure the tape goes on flat with no creases or folds.

Stock Prep

Whatever the stock or substrate is, it needs to be prepared for printing. This includes cutting down in size from master sheet to press sheet size, or may mean removing wrapping or packaging, or cleaning dust or other residue from surfaces. The stock should be prearranged or stacked on a convenient table close to the press.

Inks

Inks used in screen printing are more varied and complex than most other printing processes because of the wide range of different materials a modern screen printer can print on. Basically, inks are made up of a carrier, which is a thick clear liquid, plus finely ground pigments for colour, and different thinners used to make the ink flow.
Job number 1 for any ink is to stick to the substrate, and it follow that due to the many different materials screen printers have to print, and their different molecular make-ups, there are a large assortment of different ink types, manufactured by many different manufacturers, specifically designed to bond to various plastics, papers, glass, metal, or whatever you are trying to print. Many manufacturers produce a multi-purpose ink that will work well on a range of different materials.
A colour system unique to each manufacturer is common, with the colours having different names and code numbers, available in each of the manufacturers’ lines and types of ink.
Special formulation inks, which include conductive metals, acid resists, adhesives, or ground glass, are used when screen printing is being used as part of an industrial process. These inks are usually supplied by specialized manufacturers, or formulated by the user.

INK TYPES

The golden rule of inks is TEST if you are unsure if the ink will stick on a specific material. The best idea when dealing with a material you have not printed before is to test it. Saves buying ink that doesn’t work.
If you are wanting to print onto plastic this gives the most problems. There is a world of difference between polyeurethane and polyethelene, PVC and acrylic. This is the most common ink:

MULTI-PURPOSE or GENERAL-PURPOSE

• These inks will stick on lots of things, from card and paper stock to vinyls, styrenes, acrylics, and sometimes chloroplast.
• Big Advantage is you don’t have to carry a full range of material-specific ink types, plus different thinners and colours.

Press Types

In its simplest form, a screen press holds the frame in a fixed position over the substrate while the print stroke is pulled. The table or base usually has a grid of small holes hooked to a vacuum to hold the stock in place. Presses designed to print 3D parts or objects may have a jig set-up to hold the stock while printing. The screen is swung away by means of a hinge system so that the printed material can be removed and another piece inserted. The screen is then returned to the original printing position and another print is pulled, and so on.
The ideal press locks the screen frame into position, and returns it to the exact same printing position every time. All the other functions found on different presses allow the operator to gain more and more repeatable control over the different actions and variables that come into play during the act of printing. The physical limitations of an individual pulling a squeegee have resulted in some interesting advances in press design.

Some typical press classifications:

By lifting style

• Clamshell – most hand presses and many automatics use this. the screen hinges at the back, with the front lifting to allow the stock to be placed. Most print front to back, or back to front on older autos.
• 2 pole and 4 pole – these presses are automatic, and the masterframe and squeegee/floodbar mechanism A lifts vertically. On two pole presses (smaller) the head lift high enough for a part or material to be placed on the platen. ON 4 pole presses, during printing, the head only moves an inch or so up and the table comes out to the front for loading/unloading. ON 3/4 automatics grippers hold the material when the table moves away, and drops the material on a conveyor.
• Parallel lift – the master frame and squeegee swing up and back allowing clearance and access to the platen. this type of press is used for one arms, and large format industrial printers, as it allows easy entry and exist from the sides. Plus the screen stays parallel, so less ink movement.

By automation

• Manual – the screen is lifted and dropped by hand. the squeegee is pulled by hand. the material is placed and removed by hand.
• Semi automatic – the screen lifting and squeegee stroke is activated by a foot switch or a timer. the stock is still placed and removed by hand.
• 3/4 Automatic – As above with the semi -automatic, but either including a feeder or a take off system.
• Fully automatic – feed, screen lifting, squeegee and flood stroke, and stock takeoff are all automated. Many fully automatic presses are linked in a line to print multi-coloured.

Some variations

• T-shirt carousel – with 4 or more screen holders, this type of press allows the screens to be rotated into position, and can print multi-colours on after another until the shirt is finished. It is then removed and dried.
• Automated oval – A larger version of a t-shirt carousel, these can be round or an elongated oval. they have from 8 to 16 heads holding screens, and an equal number of t-shirt platens. Once the shirt is loaded, the platen proceeds in order under the different coloured print heads, with spot drying in between.
• Web – these press are designed to accept roll stock, which feed through the print head
• Cylinder press – In smaller configurations, these are designed to print bottles or round or oval objects. the object rotates in sync with the screen, while the squeegee stays in one place. In larger models, they print flexible sheet stock, which is drawn over a large rotating vacuum cylinder. they are among the fastest screen presses, capable of 200 or more impressions per hour.

Different press types have been adapted for special printing on a wide range of objects. Many people only think a screen press is a small homemade hand setup. The reality is screen presses run the gamut from simple to extremely complex. They may include optical alignment sensors, specially designed tables with blowers to ‘float’ sheets of glass or heavy materials into place, or systems where a screen printing head is added to a a flexo print line to lay down a thick coat of white or other specialty inks.



Screen Prints using creative inks.

 Heat Reactive Ink.

 Glow In The Dark Ink.

 Metallic Inks.

Florescent Inks.

Glue to Flock.