Press some Silly Putty against the Sunday funnies and then wrap the Silly Putty around a soup can. Notice how the characters are now elongated and distorted. That, simply put, is the distortion problem in flexography, or flexo, printing. The flat but flexible printing plates that give the process its name must be wrapped around cylinders on the printing press. This stretches the plate and thus distorts the image that the plate carries. Printers calculate the amount of distortion so that the image on the plate can be altered during platemaking. Then when the plate is put on the press, it "distorts" to give the image the right dimensions.
Items you will need
- Tape measure
Measure the repeat length of the print job. Assume for our example it's a length of 30 inches.
Measure the thickness of the printing medium on the printing plate, which usually consists of a photopolymer coating attached to a polyester support backing. Assume here that the plate is 0.115 inches thick, including a backing that measures 0.005 inches; the photopolymer (which carries the image and does most of the stretching) is 0.110 inches.
Multiply 2 by the value of pi, truncated to four decimal places, or 2 X 3.1415. Multiply the result, 6.2830, by the photopolymer thickness, 0.110, giving the result 0.69113. This is known as the constant for a particular plate thickness.
Divide the constant, 0.69113, by the repeat length, 30. Round off the result, 0.0230376667, to three decimal places, then multiply 0.023 by 100 or move the decimal point two places to the right. The result, 2.3 percent, is the distortion factor, and the length of the film negative used to make the printing plate must be foreshortened by that amount.
The repeat length measures the size of each individual printed page or wrapper coming off the rotary press. Usually, the repeat length is provided by the customer, who knows the size of the product he wants, and the printer adjusts the cylinder rolls to accommodate the printing. Repeat length is important to the calculation because usually it's the length, not width, of the printing plate that's being distorted.
Anamorphic lenses, the optical cousins of fun house mirrors, are used to create the negative of the printed image for platemaking -- reducing the length of the image without reducing the width. Photopolymer plates are made by contact printing, negative against plate, so the size of the negative determines the dimensions of the image.
Online distortion calculators and distortion charts are also available, such as those on the website of Flexo Exchange, a consulting service for flexo printers. Instead of directly measuring the distorted printing plate, these formulas precisely measure the press cylinder that's doing the distorting.
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