Screen Printing :: Pre-Press Operations (Part Two)

In this second part of our series, we are going to cover the steps and materials required to create the actual screen used for printing. A screen consists of a rectangular frame made of wood or aluminum which has nylon cloth stretched across it very tightly. The cloth can be stapled, glued or held in place with cotton cord that is driven into a grooves of a wooden frame. On commercial type roller frames, the cloth is attached via mechanical means and you are able to retention the screen fabric after each use.

Types of Screen Fabric to Use:

The type fabric you use is one of the biggest variables in determining the thickness of the ink deposit on your shirt. The smaller the mesh number the more ink it allows to pass thru it. For most spot color applications printed onto a light color t-shirt, a 110 mesh will work just fine. If you want a softer feel or “hand” as it’s called. You would use a 156 mesh. For speciality inks and printing on dark t-shirts you will want a fabric with more open area such as 41 to 86 mesh. Most fabric you will be using will be white. Finer mesh fabrics that are used to print fine detail and four color process halftones are dyed orange or yellow to help with light scatter but are not necessary for most spot color type t-shirt printing. Consult the fabric manufacturer’s website or catalog for specialized information specific to your print application.

Types of Screens:

Homemade Wooden Screen: If you have some woodworking skills, a frame can be fashioned from 2×2 lumber. I have used soft woods such as pine but no doubt a hardwood like oak will produce a more stable frame and will hold higher tension longer. You will need to coat the frames with sanding sealer to waterproof them prior to applying fabric. Screen fabric can be attached using a staple gun and canvas stretching pliers. Use cardboard upholstery strips on top of the fabric as you are stapling and it will not be torn by the staples. (See this article for directions on stretching your own screens).

Factory Built Wooden Screen: You can purchase factory built frames that you can stretch and attach your own cloth too, or they can be bought with the fabric pre-stretched and glued to the frame. This is by far the best choice for the beginning printer as it will give you a screen with tension that will be much greater than you can achieve by hand stretching. Higher screen tension gives you a cleaner, more detailed print.

Factory Built Retensionable Roller Frames: These are by far the best option over all as they will provide you with a completely stable frame that you can set and adjust the screen tension to the exact specification of the job at hand. A must for anyone wanting to print t-shirt using the four color process method. Drawback is there cost. I’ve used Neumans and Diamond Chase and like both manufacturers.

Now that we have our screen the next step in the process is to prepare it for coating with a photographic emulsion. This is done by washing the screen fabric with a degreasing solution. A good old fashion dish washing solution and a good scrubbing with a nylon brush has always done the trick for me, but there are commercial solutions available for this step. This step is to remove any sizing residue left from the manufacturing process and to ruff up the strands of fabric so the emulsion will adhere better. After the screen has been throughly washed and rinsed of any detergent, you can set it aside to dry. Commercial shops have specially fabricated drying cabinets that are heated with forced air. You can simply place a fan so that it blow across the top of the screen to speed the drying process.

Next we will be coating the screen with a photographic emulsion. Sold in quarts or gallon containers, the emulsion is a two part solution that is light sensitive. The main solution must be mixed with a catalyst and a small amount of water. (follow the manufacturers instructions) This process and any others involving the emulsion must be done in the dark with only dark room lights for vision. Coated screens must be stored in a light proof box until you are ready to expose them.

Once you have mixed the emulsion it has about a 3-4 month shelf life and lasts longer if refrigerated. You will need a Scoop Coater to spread the emulsion evenly. It is an “V” shaped aluminum container that has rounded edges and 2 plastic end caps that control the angle at which the emulsion is applied. Fill the scoop about 3/4 full. Holding the screen upright, apply emulsion from the bottom up. Tilt the scoop coater towards the screen until the plastic end caps are flush with the screen and gradually pull upwards towards the top of the screen, returning the coater to it’s normal position at the top of the sweep. Apply two coats to both the inside and outside of the screen. Wet screens should be stored horizontally until dry. Then they when dry they can be stored vertically in your light proof cabinet for up to 4 months. You can build a wooden rack to separate screens so you can blow air across them while they are drying or then can be separated with blocks of wood.

The next step is to expose the screen to a light source using our film positive to block areas of light to create our image. Most commercial shops use exposure units that are constructed of a banks of special flourescent bulbs under a glass top. The film positive is taped to the glass and the screen is placed on top of that. Then the top of the unit, which is a vacuum blanket is closed. All the air around the screen is removed and the screen and film positive are held closely together while being exposed to the light source. You can construct a inexpensive exposure box from plywood with a glass top. A number 2 photoflood bulb mounted in a aluminum diffuser will work as your light source. (See sketch for homemade exposure unit.)

Exposing Screens:

Using your homemade exposure unit you will start by tape your film positive, right side up in the center of the glass top. Next, under darkroom light you will place the coated screen on top of your film positive so that the fabric is touching the film positive. Then you will want to place some weight on top of the screen to hold it tight to the film. You then turn your light source on and set your timer. You will have to experiment with exposure times as they will depend on the type of emulsion and the distance from the light source. For starters try a 3 minute exposure time.

The next step is to take your exposed screen to your washout sink. Using a spray nozzle, wet both sides of the screen. Wait 10-20 seconds and start washing the image out with your spray nozzle. You will only need moderate water pressure. Areas of the emulsion that were exposed to light will be fixed and not washout. Areas that were blocked by the black of the film positive will not be fixed and should wash out. A under exposed screen will wash out areas that were meant to be fixed. An over exposed screen will be difficult to wash out at all. Adjust your exposure times to compensate for difficulties. Inspect your screen to see if all areas of the image have washed out. If so, you can turn on your regular light and set the screen in your rack to dry.

Once the screen is dry, we need to seal off any unwanted open areas of the screen. These are the areas next to the frame where no emulsion was applied and any pin holes that might has been caused by dust or dirt on the screen. Using a thick quick drying solution called blockout, take 3″x 5″ piece of cardboard and spread this solution along the edge of the frame filling any open areas that you dont won’t ink to pass through. Then you will want to hold your screen up to a light source and look for any pin holes. With a paint brush, fill any of these with blockout also. Again set your screen aside to dry.

The final step in preparing your screen for printing is to tape off the frame area with masking tape to make clean up after printing easier. Tape the frame top and inside edge as well as about 2-3″ of the fabric all away around the inside of the frame. This will keep ink from getting into the crack between the frame and fabric which is almost impossible to clean if it is not taped. I usually tape the open area on the bottom side of the screen as well. Your now ready to mount your screen in your printer and start whipping out tees.

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