Screen Printing

Screen Printing

Overview

Screen printing is a decoration process that uses a fine mesh with a hard emulsion blocking some of the mesh. Your design makes up the parts of the mesh that are not blocked by the hard emulsion. Ink is pressed through the mesh onto the t-shirt. The inked t-shirt is then heated in a conveyor dryer to over 330 degrees to cure the ink. The ink used in screen printing is called plastisol. Plastisol is a rubbery plastic based ink that is very durable and will last as long as the shirt. Plastisol ink washes well and is perfect for any shirt that will see repeated use.

The ClassB Screen Printing Process

Screen printing as a decoration process dates back to ancient China where it was developed as an alternative to applique embellishment. The screen printing process is much more complex today. The main components of the screen printing process are the artwork, screen frame and mesh, ink, and printing equipment. The best way to tell the story is to follow an order through the screen printing cycle.

The artwork

Artwork used in screen printing garments is much different than artwork used in other printing processes. Compared to metal or paper, garments come in many different colors, have a rough surface, and are soft and pliable. Our artists take into account all these factors when they prepare your artwork for screen printing. We remove fine details that are too small to print, calculate how to reproduce tones properly on the color of shirts ordered, and separate multicolor designs into a series of layers (one for each ink color). Tints of a color must be created that work well with the variety of garment textures and ink viscosity. Once the artwork is complete, it is ready to image on to the screen frame.

Emulsion applied to a screen	Screen with emulsion	Image printing on screen
Exposing the screen	Screen with image	Screen printing squeegees

The screen frame and mesh The screen frame is a rectangular wood or metal frame that has a mesh stretched and secured to it. We use tubular metal frames that are tightened after each use. We use different types of mesh with holes that range larger than a window screen to so small you can’t see the holes. The mesh is stretched as tight as a drum with a pneumatic machine. The tighter the mesh, the better the final image. After stretching, emulsion is put in the screen. Emulsion is a photosensitive (light-activated) chemical suspended in a base similar to white glue. A machine moves a tray filled with emulsion evenly across the surface of the mesh, coating the screen surface. The more consistent the layer of emulsion, the more consistent the quality of the imprint. Humidity-controlled dark rooms are used to dry the screen.

Putting the image on the emulsion Since emulsion is activated by light, the image must be transferred to the frame so that light can reach some parts of the emulsion and not others. In the early 1980’s, this was done by taping a clear carrier sheet on which the design was printed in opaque ink. Modern technology applies the design directly to the emulsion by a special ink-jet printer that uses an opaque wax. Next the screen is placed in a machine that has a high power light bulb. This causes the parts of the emulsion that are exposed to the light to harden. The parts of the design under the wax image remain soft and will dissolve with water. We place the exposed screen in a backlit washing cabinet and use water to remove the unhardened emulsion. This leaves the image areas open and the non image areas blocked. The Ink Most our our ink colors are mixed in-house from bases that are free of lead and phthalates. Different inks have different opacity and require different mesh sizes to flow freely onto the garment.

The Printing Process

This is where everything comes together! Screens, ink, and shirts are moved to one of our state-of-the-art automatic presses. An experienced press operator sets up the press for each run by placing each screen in the press one at a time and loading ink into each screen. Alignment is verified with a series of test prints to ensure that the image will be correctly positioned on the garment and relative to other colors.Screen printing multicolor designs is like putting together a coloring book page where each color is a separate piece of paper. You have to get all the colors in the right place, without gaps or overlaps. This process is known as registration and, depending on complexity, can take a few minutes or a few hours.

Once all the shirt sizes and colors are verified, the press operator begins placing shirts onto the press. Each shirt rotates under each screen which is then lowered to meet the garment. A hard rubber blade draws across the screen forcing ink through the open areas of the emulsion and onto the shirt below. The screen rises and the next shirt rotates under the screen. Each ink color is printed individually one shirt at a time and takes about 5 seconds.Finally, each shirt is placed in a long conveyor-belt dryer that looks a lot like a pizza oven. The shirt must reach 330 degrees to cure the ink. Temperature sensors ensure a proper cure every time. Each shirt is individually removed at the end of the conveyor, inspected, folded, counted and shipped.

Color 1: Red	Red Ink Applied	Color 2: Blue
Blue Ink Applied	Color 3: Green	Green Ink Applied

After the shirts are printed

Once the garments are printed, all the remaining ink is removed from the screen using an automated machine. The organic, water based solvent is recycled and all ink particles are trapped for proper disposal. The emulsion is removed from the mesh and the screen is re-tensioned so we can begin the process again. Frames are typically reused the following day. All waste is monitored and neutralized before disposal.

Other methods of screen printing

Overview ClassB uses modern, environmentally responsible screen printing methods. There are many methods in use in the marketplace and you should be informed of the affect those can have on quality, cost, and the environment. We’ve been printing since 1982 and have seen and tried many advancements in our industry. Some were revolutionary and others were headaches. Here are some other methods you should be aware of. The artwork In the beginning of apparel screen printing, artwork was cut in a red film material called rubylith with fine lines inked with opaque black ink. This was done with a very sharp knife, a variety of ink pens with different width tips, and a set of drafting tools. Details were only as good as the steady hand and gifted eye of the artist. When you see some reproductions of 1960 designs, you will often see an attempt to recreate this hand cut and hand drawn method. When computers became readily available, the industry switched over to laser printers printing to a semi-opaque vellum paper. This provided a huge advantage to hand cutting but image sharpness was lost due to the paper not being clear. The industry then moved to large format inkjet printers using a clear polyester film. Images were sharper but generated a large waste stream of spent film, ink cartridges, and chemicals. Modern technology (described above) uses no film, no paper, no cartridges and no chemicals; only a water-soluble wax. The screen frame and mesh Historically, permanent screens are made from 2″ x 2″ metal or wood. With permanent frames, it is difficult to stretch the mesh and time consuming to remove the old image, so it’s cost-effective at first to just store and reuse screens over and over. After each use, the mesh gets looser and looser and the image quality suffers. Even the smallest shops will have hundreds of these fixed frames in racks. By 1992, we had already accumulated over 800 wood frames. If we had continued with this model, we would have over 10,000 frames. Modern retensionable metal frames (described above) save space, improve print quality, and save trees. The Ink There are many different kinds ink available in the market place. Some inks used on textiles contain bleach, phthalates, or other hazardous chemicals. Often ink is selected purely for low cost with little regard for safety or the environment. Have you ever seen a young child with part of their t-shirt in their mouth? It’s worth asking what kind of ink is being used.

Putting the image on the emulsion Preparing a stretched frame without technology is an arcane task. Emulsion must hand coated on to the mesh. This can result in ripples or uneven emulsion and is entirely dependent on the operator. The operator must maintain the applicator scoop at a uniform angle and move it a a uniform speed to get a uniform layer of emulsion on the mesh. If you have an old t-shirt that has a band where the ink washed off quicker than other parts, this is usually due to a poor quality hand scoop of the emulsion. For many years the only person we had that could properly apply emulsion was the founder of ClassB, Terry. Briefly, the industry used emulsion that was applied like a sticker. It worked well, but was expensive and had a large waste stream of leftover film. The Printing Process Printing presses can be classified into manual or automatic presses. In a manual press, the ink is forced through the stencil my hand with a rubber blade. Each ink color in the design is printed in sequence with one or more applications of the ink. Manual presses are typically built to accommodate from 1 to 6 different screens and from 1 to 6 shirts loaded at one time. A manual press is a light weight machine and is run completely by hand. Difficulties with manual printing are errors in multicolor registration, inconsistent ink layers from variations in squeegee angle and speed, and high levels of operator fatique. Long time manual press operators are likely to develop Carpal Tunnel Syndrome. Sadly, many small shops still use manual presses for the majority of their printing, risking their operators long term health. After the shirts are printed The largest area of environmental concerns in screen printing occurs after the t-shirts have been printed. There are toxic chemicals used in ink and emulsion removal and the risk of a solid waste stream tainted with uncured ink and solvents. The solvents used to remove ink ranges from organic solvents based on orange peels or soy beans to mineral spirits and acetone. The U.S. Environmental Protection Agency has done case studies into these issues. Post-processing of screens is a complicated and intense process.

• EcoTee Program
• SHARP Certified