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Screen Printing FAQ

Frequently asked questions about screen printing


What is screen printing?
What is spot-color?
What is 4-color process?
What is simulated process?
What is index color?
What are half tones?
What is line count, screen frequency, and lpi?
What is screen count or mesh count?
What is an underbase?
What is flashing or flash drying/curing?
How many colors can you print?

 
Q: What is screen printing?
A:

Screen printing is a process though which ink is mechanically applied to a substrate via the use of a screen and squeegee. In its basic form, screen-printing is a very simple process. Over time rather sophisticated software, machines and techniques where developed and we now use those to produce our product. Screen-printing can be used to "decorate" many different substrates from ancient eastern wall hangings to Andy Warhol's fine art to signs, billboards, computer circuit boards and more. At Graphic FX, we print, almost exclusively, custom designed t-shirts. The questions and answers in this FAQ are, therefore, slanted in the t-shirt direction.

In our process, we start with the artwork. Each color of the design requires an individual screen so we must separate the design into its component colors. This is done on the computer and each color separation is printed to a transparent sheet.

  Original Black Yellow
Design Colors
Separations  

 

Next we must prepare the screens. The screen is a rigid frame of wood or aluminum that has a fine monofilament nylon mesh stretched over it. This mesh is much stonger, more durable and more consistent then the silk mesh used in the early days of screen printing. (The silk is what gave the process the name "silk screening" that is still sometimes used today.) This mesh is then coated with a light sensitive emulsion that will become the stencil through which the ink will pass when printed.

Blank Screen Coated Screen

The screen is then mounted, with the separation, in an exposure unit. This machine exposes the screen to high intensity UV light.

Exposing the Screen

When the UV light hits the emulsion a chemical reaction hardens the emulsion making it water and solvent resistant. The separation acts as a shield to block the light in certain areas of the screen. These soft areas are then rinsed away with water to create the open area of the stencil.

Black Screen Yellow Screen

The screens are then mounted in the press and registered,or aligned, so that each color prints in the proper location relative to the other colors. Ink is loaded into the screens and squeegees are installed. The actual printing is accomplished by pushing ink through the screen and onto the shirt with the squeegees. As the squeegee scrapes across the screen it fills the stencil with ink while simultaneously bending the mesh down to transfer the ink to the shirt.

To create the composite image on the shirt, individual colors are printed then the shirt is moved to the next color. After test prints are run to check alignment, shirts are loaded one by one and printed.

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Printing Mechanics Colors Print In Sequence

Once all the colors have been applied to the shirt it is removed from the press. The ink on the shirts is still "wet" at this point and needs to be "dried".

The ink we use for t-shirts is the variety called "Plastisol" and is not actually"dried" but cured with heat. Plastisol is made up of polyvinyl chloride resins (PVC), plasticizer and pigments. When plastisol ink is heated the PVC resin particles swell and absorb the liquid plasticizer and these swelled particles merge with each other and form a solid film. Curing of plastisol ink is accomplished by rapidly bringing the ink up to curing temperature ( ~330° F ) with electric or gas infrared heaters.

To cure the shirts we run them through a "drier" that utilizes a conveyor belt to pass the shirts under infrared heating panels. The shirts spend between 30 seconds to 1 minute in the dryer, and when they come off the belt they are done and ready to be folded and packed.

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