The 8 Key elements in screen printing

1.    Artwork:
The artwork you start art with is important. If you have jagged or grainy artwork, you will reproduce have jagged or grainy artwork REMEMBER; Garbage in-garbage out. Programs for doing art in-house are available or there are art services on the internet that will supply you will screen printing artwork. If you fax them a logo, they can redraw it for you and send you a file in which you can use for the making of screens.

If you can draw the artwork yourself, it is wise to use a vector art program like SmartDesign which is a Corel Draw supplementary program. This quick and easy tool ensures that you will have high quality artwork in the end in less than half the time. Photoshop can be used as well, but this is more for full color printing, etc., which is not good to start out with since there are special techniques to master before moving into this realm.

2.    Film Positives:
When you have artwork that is ready to print, you can print your artwork out on a clear film instead of paper. For every color you have, you will print it out as a black plate on a separate film. For example, if you have a logo that is Red & Black, you would separate the two colors from themselves and print each out, changing the red to black and printing and then printing the black. Each of these would be printed on their own film by a printer, inkjets have made great strides and coupled with a RIP program have made actual cameras and darkrooms almost obsolete.

This is simply known as art separations. If the colors are touching in the final version of the logo, then you need to have what is called trap or choke. Basically, it’s an overlapping of the artwork by a small margin. This small margin is just enough so you can register the colors so there is no space between them. On the films, you will also place registration marks. These marks usually are a circle with a vertical and horizontal line passing through it. These marks will be on the same spot on each film. This will help you register your colors later it’s also a good idea to include center marks so the design will be dead center on the platen.

3. Screen Making:
When screen printing began it was called silk screening. The reason for this is, the screens used to be made from silk. Since this is no longer the case (now they are made from polyester), the name changed. A screen is simply a wooden or metal frame that has a fine mesh stretched over and attached to it. Some examples would be 83, 110, 125, 200, 230 and so on with various numbers in between. The lower the mesh count, the less detail you can print and the thicker the ink lays down. Choosing the proper mesh for the job is art form, in time you’ll learn through testing and results. It’s always a good idea to keep production records for consistency from the beginning of every job.

The actual process of screen making is quite simple, just time consuming, the most
common technique is using a light sensitive liquid emulsion although Capillary film is stilled used. Emulsion can be used under a yellow light so that you still can see what you are doing, but the UV light is filtered out so as not to effect the emulsion. A scoop coater is needed. You pour emulsion into the scoop coater and place it on a vertical screen. Pressing up against the screen and pulling up, you will lay a thin layer of emulsion on the outside of the screen. Turn the screen around and do the same for the inside of the screen. Emulsion thickness EQUALS ink thickness. For thicker stencils it’s a good idea to dry between coatings.

Once the emulsion dries, you can “expose” the screen. You need a good light source for this process. The exposure unit is basically a box with a glass top and a lid that has a compression lid or a rubber blanket and a vacuum frame. The light source would be contained in this box. To expose a screen, taking the film positive you created and place it on the glass top with the right reading being up. Then take the dried screen and place it on the glass top with the film positive under it. The screen mesh will be touching the film positive. When you close the lid, the compression lid presses the foam against the screen and the positive providing contact

At this point, the length of time will be dependent on your emulsion and light source. This is something you can talk to your supplier about. It’s really a matter of testing. Most problems occur in this stage, so it is critical that you understand this process through testing, training and trial and error. The better you are in this area, the better your prints will be. For a black light unit a 5 to 3 minute exposure time is the norm but will vary as already mentioned due to emulsion thickness and mesh count.

Once the image is exposed into your emulsion, you can take the screen to a washout booth lightly spray both sides of the screen with water. A garden hose with a sprayer on the end works well for this procedure. You do not want a lot of pressure but you do want some. After waiting for a minute, you can go back and begin spraying your screen with water. 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 shine through the emulsion to get to that side. A good exposure will yield less scum (softness) on the inside.

As you spray down the screen, you will see the image on your screen. What happened is, wherever there was black on your film, the light did not shine through. Since the light could not expose the emulsion, it remained water soluble. Wherever the light shine through the emulsion, it hardened and will not wash away. Lay out newsprint and pat the inside (squeegee side) with one sheet, DO NOT wipe then let the screen(s) dry. You will want to check for pinholes (little holes caused but dust, dirt) and block out with emulsion or a commercial blocking agent, then dry. After the screen has completely dried expose again (post –hardening) for a longer lasting stencil. This can be done in your exposure unit or out in the bright sun.

4.    Printing Press:
Choosing a printing press is as critical, although you are looking for a quality press. To be honest, you may want to stay away from all-in-one units and similar machines. They are a waste of money. Even though you can print just as good of a print with these machines, they are costly and they slow your process down. When your first starting, you could use a one platen machine but a 4 color-2station press is just about what you will need. Very rarely will you ever need to print anything more than a 4 color design. Later, as you grow purchase more machines that will allow for more colors. In the first years of your business, you may only have 1-6 color job. When you need to expand will suit production requirements perfectly. What you are looking for in a press is a solid frame, joystick or micro registration and rotating platens. Outside of this, startups don’t need much more than that.

5.    Conveyor dryer and flash unit:
To actually cure the ink, you need a heat source to reach 320 degrees for your ink. If you can reach 320 degrees in 1 second, it’s cured. If it’s 10 seconds, it’s cured. As long as it reaches 320 degrees, you are good. A flash unit is a unit that you place over your platen (the arm that you place the shirt on). This flash unit is meant to flash the ink just long enough where it is not cured and it is not wet. This will allow you to print colors on top of colors if needed, and you will need it! If you have a flash unit over a platen, it has to reach 320 degrees to cure the ink.

Raytek makes a heat gun that when the shirt comes out, you point the laser beam at the ink and it will give you a temperature reading, 320 is the magic number! This heat may eventually warp your platens, it will also heat your platen up enough that when you put another shirt on it and print, it might semi cure the ink in your screens, causing a clogging and poor printing so allow cool down time on long runs. The reason for this is, you would print white on a shirt, flash it, and then when the shirt comes back around to you, and it needs a second print. This gives you a good vibrant white.

If you are printing a color on a dark shirt, you would also print a white under base, flash it, then print an exact image with a different screen over top of it with the color you need. To increase your output a conveyer dryer is needed to increase your production. When you finish printing a shirt, you pull it off of your platen and place it on the conveyer dryer. Basically, it’s a dryer that has a belt on it that goes through a tunnel of heat. When it comes out the other side, it is cured. Again, use a heat gun when the shirt is about to come out, you point the laser beam at the ink and it will give you a temperature reading. Remember, 320 is the magic number!

6. Inks and miscellaneous:
The ink you will use is a Plastisol ink. There are so many manufacturers and types of inks, it’s good to find one and stick with them. Consistency is the key to success
You will also need squeegees. A squeegee is basically a handle with a rubber blade on the end. This blade is what you use to push or pull the ink through the screen and onto the shirt. There are so many miscellaneous items that it would be good to talk to Ryonet about what you need to get started

7.    Screen prep and registration:
Your scoop coater cannot reach all areas of the screen, so you want to tape out the areas that did not have emulsion, there are special tapes made to do this.
If you have a one color design, placing the screen on your press is quite simple. If you have more than one color, this is where the registration marks are needed. After placing your first screen on the press, you would do a test print. Place some ink on the screen and rest a squeegee on the frame close to the head. Pull the ink across the screen and onto the shirt then flash it. Next, take your second screen and place on the next head.
Align the registration marks on the screen to the marks on the print you made. Once in place, you can lock them in and adjust the joystick or micro registrations if necessary. Once locked in, do a test print. If everything is registered, you can tape up the registration marks on your screen and you are ready to print.

8.    Your first print:
You will be working upside down when you print t-shirts. The collar will be closest to you. After placing the shirt on the platen, pull your screen down, look between the shirt and the platen, you should have a gap. This is called your off-contact. You need about 1/8” between the screen and the platen. This will give you just enough room to make a print and allow for the screen to snap away from the shirt which gives you a clean print. Some people will push their squeegee and others will pull, whatever is comfortable to you is best for you.

Most printers pull the squeegee, which means that when you pull down the screen, grasp the squeegee and pull the ink towards you. You want to have the squeegee at an angle, if you go too much of an angle, you will get a heavy print The good thing is, if the first print does not work out for you, you can print it again, right over top of it. The registration of the machine will be the same so even if you rotated the press and came back to it, it would still print good. Rule of thumb; Angle and slow speed for light inks, less angle and a faster print for dark inks, especially printing on an under base.

~ by ryonet on May 7, 2008.