What is Web Growth?
Web presses typically experience a quality-control problem commonly known as Web Growth. Web Growth (also known as web stretch) is a physical distortion occurring to paper printed on a web press thus the name web growth. The deformation (stretch) of the paper when printed on the various color units of a typical web press occurs as the paper absorbs ink and water causing the paper to stretch. This stretching and or shrinking combination becomes accelerated due to the existence of pressure and tension of physically moving paper at a high speed through the various cylinders of a modern web press.
Causes of Web Growth:
- Type of press Heat-set Cold-set
- Press Configuration Speed
- Paper Stock Newsprint to glossy
- Humidity Pressroom humidity both of paper and environment at press
- Ink Coverage Intensity of ink front to back and top to bottom
- Conditions of Blankets General press conditions
- Tension & Press General press conditions
Mechanical adjustments to press cylinders including air jets on moving media to provide the slackening or tilting of paper to compensate for Web Growth. Mechanical dampeners and adjusting mechanisms achieve some success in dealing with Web Growth. Some platesetters provide a mechanism to speed up or slow down the imaging speed of the drum or imaging head to provide some basic level of overall Web Growth compensation, which generally affects the number of plates, produced in an hour.
All mechanical adjustment accuracies vary widely with changes in paper, humidity, and pressroom conditions. They may impose extensive labor costs when frequent changes occur in the pressroom.
The Apogee Prepress Web Growth option allows users to define web growth compensation profiles, wherein characteristics of the web growth are specified. The user may define a web growth compensation profile for different presses and/or printing media. The user should select a web growth compensation profile for the specific press. Apogee Prepress adjusts the image on the fly when outputting to a plate/film-setter to compensate.
The Apogee Prepress Web growth option will compensate for:
- Mis-registration of printed media as a result of
- Growth (printed media stretches/fans out during printing process)
- Reduction (printed media dries/shrinks during printing process).
- Web growth in both the X and Y axis independently.
- Uniform web growth; where constant scaling percentages are applied to the image.
- Non-uniform web growth; where scaling percentages are not constant across the image and various areas of the image are scaled differently.
- Specific webs in a multiple-web press configuration.