Using the Correct Screening Method
In all types of printing from web to sheetfed, an offset printing press can only print a solid ink film, a screening method is then needed to allow the eye to see shades of color. With many styles and types of screening, which are best? And for which papers?
First to list a few names you may have heard thrown around the industry:
• Staccato (Kodak’s version of Stochastic)
• Hybrid (both conventional and Stochastic)
• Elliptical (Shape of conventional dot)
• Square (Shape of conventional dot)
• Combination (Shape of conventional dot)
• Frequency Modulation (FM)
• Amplitude Modulation (AM)
• Crystal Raster (AGFA’s version of Stochastic)
Then add to this list conventional screen rulings like 175 and 200 line or Stochastic spot sizes like 20 & 36 leading to more confusion. When you break it all down there is really only two types of screening:
1. Conventional (AM) - Where the dot size varies based upon densities and it has rows of predictable angles much like a chess board in which the squares are at 45 degree angle to each other.
2. Stochastic (FM) - Where the dot or spot size is the same but the dots are gathered together for more density and scattered for less density in a computer generated random pattern.
Stochastic vs Conventional
Stochastic is great for the elimination of moiré in patterned objects like linens, rugs and window blinds. It produces smoother curves in type when produced with tints. But it can have a problem printing on rough surfaces which can create a mottled appearance.
TIPS: Conventional is better for printing on rough surfaces giving it a smoother appearance but the different angles can cause problems with the edges of type, tint boxes and moiré of photo’s.
Some rules of thumb are:
For #1 grade gloss coated papers you can use 20u spot stochastic or 175- 200 line conventional screening. For #2 grade dull or silk coated papers you can use 175-200 line conventional screening. For uncoated papers you can use 36 spot stochastic or 150-175 line conventional screening
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