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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?

 
At Graphic FX we mainly screen print custom t-shirts so these FAQ questions are answered with that process in mind and may not apply to other substrates

Q: What are halftones?
A:

Halftones are a pattern of tiny dots that can simulate different shades of color using varing percentages of a single ink. Visually, halftones create the illusion of a continuous tone image by using spots of varying size and density to represent darker or lighter color values.

Halftones work by fooling the eye into seeing the combination of the ink color and the color of the shirt they are printed on. When seen from a distance, the colors blend together and the dots merge with the background color of the shirt. If you look closely at or magnify the print, the separate dots are quite clear. You can see good examples of halftones if you magnify a picture in a magazine or a print from a color printer or even if you look closely at your TV screen. All these are made up of tiny dots.

In screen printing we use halftones for three main purposes:

1. To create a tint or lighter shade of a color. This will allow more "colors" in the design without adding more screens.

Example:

  100% Blue 50% Blue 20% Blue

Desired
Colors

Halftones  
Enlarged
Halftones
 

One color
image
Halftones Enlarged

 

2. To create a gradient or the appearance of a continuous tone of color:

Example:

1 Color
Gradient

Halftones Enlarged
Halftones

 

3. To create overlapping screens of different ink colors that combine to simulate more colors.

Example:

Simulated
Solid Color
Halftones Enlarged
Halftones
     

2 Color
Gradient

Halftones Enlarged
Halftones
     
4-color
process
Halftones Enlarged
Halftones
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Q: What is line count, screen frequency, and lpi?
A:

These terms all refer to the number of vertical lines of halftone spots per inch or lines per inch (lpi). Halftones are created using a grid of cells. Each cell contains one halftone spot. These spots vary in size depending on the shade of color being represented but only one fits in a cell. The size of these cells determines the lpi.

  Individual
Cell

Cell Grid
~3 lpi

Cell Grid
~12 lpi

10%
Halftone

50%
Halftone
90%
Halftone

Various lpi numbers are used for different types of printing. Magazine pictures may have 100-130 lpi, newsprint is typically 85 lpi, a 300 dpi laser printer is around 55 lpi, and billboards might be 3-6 lpi. When we are using halftones for screen printing we use 45 lpi for basic designs and 65 lpi for detailed and process type designs.

The correct lpi to use is a function of the detail you want to produce and the distance from which a print will be viewed. When reading a magazine, the 100 lpi halftones will be invisible at 12 ". Likewise, if you view a 45 lpi t-shirt print from 3-4 feet you won't notice the halftone dots.

You must also take into account what screen count (see next question) you plan to use. To calculate the proper LPI for a particular screen count you take the count and you divide by a constant, usually between 3 and 5. So, let's say a 200 mesh screen and the constant we use is 4. That gives us a LPI of 50 (200/4=50). We arrive at this constant through experimentation and it may be different for other printers.

 

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Q: What is screen count or mesh count?
A:

The mesh count is the number of threads per inch (tpi) used to weave the mesh. Typical mesh counts for screen printing t-shirts range from 85 tpi to 355 tpi. The mesh count defines basically two things: The thickness of the ink deposit and the size ( or lpi ) of halftones that can be printed.

The lower mesh counts, 85-110 tpi, have a relatively large thread diameter and more space between the threads ( called "open area" ). This allows for a thicker deposit and more ink to pass through the screen. Typically, lower mesh counts are used for specialty inks ( like glitter ) and when a thick deposit is needed on dark garments.

The high mesh counts, 305-355 tpi, have a very small thread diameter and less open area. These are used when fine detail and high halftone lpi are needed.

In the middle, 200-255 tpi, are general purpose screens for spot color, good detail and acceptable halftones.

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Q: What is an underbase?
A: An underbase is a layer of ink, usually white, that is printed under the other ink colors when printing dark garments. When printing most colors on dark garments the color of the shirt will show through the ink slightly. For instance, yellow ink printed directly on a royal blue shirt will look very green. To prevent this, a thin layer of white ink is printed, then "flash" dried, and the yellow is printed on top. This gives the top colors a good neutral base and reduces or eliminates the shirt color showing through.
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Q: What is flashing or flash drying/curing?
A: Flash curing is the process of "gelling" a layer of ink with a spot heating unit while still on the press. To "gel" the ink layer the temperature is raised to the point where the ink begins to dry but is not completely cured. The ink will be dry to the touch and will form a solid surface to print additional colors on. When the garment is run through the drier the flashed layer will cure completely and bond to the ink layers on top to form a solid film.
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Q: How many colors can you print?
A: Strictly speaking, we can print six different ink colors at a time. However, we use several techniques to increase the number of perceived colors in a print. We can produce full color prints on both light and dark colored garments. ( See "What is 4-color process?", "What is simulated process?" and "What is index color?" for examples. )
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