Screen Printing (Part Three)

In this, the third part in our series on screen printing, we will cover press set up and the actual print. We will need a few things to get started, a printer of course. A squeegee, t-shirt, ink, spray adhesive and a way to dry the ink. Let’s start with the type of printers available.

There are two major types of textile screen printers, the manual type and the high tech automatic machines that are computer controlled. Manual printers are classified by the number of colors they are capable of printing and their number of work stations.

A typical shop will have a 4 to 8 color manual printer with multiple work stations. The center of the press is two bearing type carousels. The top carousel supports the screen mounting clamps. Each color of the print requires a different screen so a four color printer will have mounting assemblies for 4 screens. The lower one supports the workstations. Each station has a platen that the t-shirts are mounted on and like the top carousel it rotates 360 degrees. As each screen or workstation is rotated it aligns with a indexing bracket so that they align perfectly with each work station. This feature is called “all arms down” and allows several people to print shirts at the same time.

Below is a typical four color, four station printer. The operator stands at one station and loads shirts onto the platens. The four work stations rotate on a carousel and as he loads a shirt he then spins the carousel so the next platen is now in front of him where he can load another shirt and prints the first color. Once all four shirts have been loaded and the first color printed the operator continues to print each successive color until the entire image is complete. Then, as the carousel advances, each shirt is removed from the platen and placed on the dryer. The process can be sped up with the addition of another person to load and unload shirts.

4 color screen printing press

The printing process starts by taking the film positive of the artwork and aligning it on the platen so that it is centered and straight. You do this by taking a your combination square and aligning it to the side of the platen and then aligning the arm to a horizontal element of the image. We are using the bottom of the text in our example below. Leave an equal distance on the left and right sides of the design as noted by “A” and “B”. Notice that the image is upside down as you are looking at it on the platen.Once you have the film positive aligned and centered, tape it into position.

The next step is to position the screen into the clamp assembly and tighten only slightly. Then lower the screen to the platen and adjust the screen so that the image burnt into the screen aligns with the image of your film positive. If your using a commercial machine there will be an adjustment screw on the bottom of the platen that you will loosen so that you can move the platen up and down the platen arm to position the screen to the positive. Any small adjustment you need to make can be done by moving the screen within the clamp. If your printer has Micro Registration, you will use these controls to make your final alignment. Once you have matched both images, tighten the clamps securely.

If you are using our homemade printer, you need to pay close attention to where the image is burnt onto your screen during the exposure process. Since you don’t have the ability to adjust the platen’s position like you do on a commercial printer, you will have to burn the image in close proximity to where you want the final print to land. You can do this by placing a t-shirt on the platen and then laying the film positive on top of the shirt in the location you want the print to be. Then measure the distance from the very back of the screen clamp to the bottom of the image on your film positive. Then when you are exposing your screen be sure to place the film positive on the exposure glass so that the image falls this same distance from the far edge of the exposure glass to the bottom of your image. One other adjustment you have to take into consideration at this point is the off contact position. This is the distance between the top of the platen to the bottom of the screen when it is in the down position. Typically you will want about 1/8 of an inch. The distance varies with how tight the screen is stretched. A tight screen allows a smaller off contact distance and a hand stretched screen will normally require a slightly larger off contact distance. A smaller off contact with a very tight screen will give the best print results. On commercial printers there are mechanical adjustments to raise and lower the screen to set your off contact distance. With a hand made printer you will have to shim the bottom of the screen in the clamp to set your off contact distance. You can also tape a small shim to the front of the platen t

Next you will need to prepare your ink for the screen. You should be using a plastasol type ink which will be very thick. I usually place the ink into a quart plastic paint mixing cup and add a very small amount of reducer and stir until the ink is back to an even consistency. Once I have the Ink reduced slightly, I add about a one inch wide ribbon across the entire width on the image along the back of the screen. First you will want to print a test print onto a piece of paper or a scrap t-shirt. Cover the platen surface with spray adhesive so the substrate or t-shirt won’t move during the print process. You will need to re-apply adhesive every so often as you’re printing. You can tell if it’s time to re-apply when you tug on the last shirt you printed and it is not well adhered to the platen. Start your print by taking the squeegee with both hands and holding it at a 60 degree angle you will make a flood stroke by pull the ribbon of ink towards you without applying any downward pressure.

This should be done prior to every print stroke to distribute ink evenly across the image. Returning the squeegee to the back of the screen you will make the print stroke in the same fashion, only this time you will need to apply downward press so that you are actually scraping the ink off the top of the screen as you go. At the end of this stroke you will make a scooping motion so you gather the ink back onto your squeegee and return it to the back of your screen. Lift the screen and check your print. Look for any small pinholes you may have missed during the block out process. You can take a small piece of scotch tape and cover the hole on the bottom of the screen as a quick fix. Be sure the tape doesn’t cover any of the image. Do another test print and if all looks well, you’re ready to start printing.

You then need to load a t-shirt onto the platen so that is is centered. Align the shoulder seams so they are even with the edge of the platen that is facing you and align the center of the neck as well. Take a felt tipped pen and mark a reference point where the edge of the neck falls on the platen and align all shirts to this mark so that the printed image falls the same on each shirt. Once you have the shirt in the place you want, take your hand and smooth the shirt out as well as securing it to the adhesive on the platen.

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January 2018
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