User login

Film Positives and Accurate Image Translation

(December 2002) posted on Sun Jan 05, 2003

Coudray examine the different types of imagesetters and offers advice for getting consistent, high-quality output.


By Mark A. Coudray

The first thing we need to know is how well the base material will support and maintain the desired image size. Polyester, the most common base material, is susceptible to shrinkage and damage from heat. Both toner- and thermal-based film-generation processes subject polyester to substantial heat, and if the substrate is too thin or has not been heat stabilized, it can change size. The more heat the material is subjected to, the greater the chance of distortion. Shrinkage problems occur primarily when very heavy reverse or negative graphics are being imaged.

Problems also occur when imaging a mix of high- and low-coverage areas. In these situations, the material may develop localized misregistration due to uneven shrinkage. Imagesetter manufacturers are very much aware of shrinkage problems, and many have developed their own solutions. Before selecting a system, it's important to make sure it can address this issue.

Drive systems

The second concern is distortion of the substrate or image caused by the film-transport system on the imagesetting device. Two types of transport systems are employed on imagesetters: capstan drives, which use mechanized spindles to move a continuous roll of film past the device's imaging heads, and drums, which hold a sheet of film in a fixed position against the drum surface. The film is imaged in this fixed position as an imaging mirror moves across the film plane.

Capstan-drive systems All four types of imagesetting devices utilize capstan drives (or similar friction-feeding mechanical rollers in the case of laser printers). In fact, capstan-drive imagesetters are, by far, the most common because they are much more economical than drum-based systems. They offer size advantages in both width and length of output.

Many capstan systems are very sophisticated, providing internal feedback loops and the ability to accurately track material position. On large-format imagesetters, manufacturers typically quote the degree of error their devices exhibit over a given length of material.

Older machines, and those requiring minimal investment, are the most prone to drive-based distortion problems. They need to be carefully analyzed before a purchasing decision is made. The most dependable devices have warranties that address this area specifically.


Terms:

Did you enjoy this article? Click here to subscribe to the magazine.