Polyester plates are a crucial component in modern printing, offering a balance of quality and cost-effectiveness. In this article, we explore what polyester plates are and how they work in an offset lithographic printing press. Whether you are a printing professional looking to upgrade your equipment or simply curious about advancements in printing technology, this article will provide a comprehensive overview of polyester plates and their role in printing press machines.
What is a Polyester Plate?
A polyester plate is a type of printing plate made from a durable polyester film. These plates are used primarily in offset printing and are known for their cost-effectiveness and ease of use. Unlike traditional metal plates, polyester plates are lighter and more flexible, making them easier to handle and ideal for short to medium print runs.
Polyester plates are created using a photographic process where the image to be printed is transferred onto the plate using a laser or UV light. The exposed areas of the plate are chemically treated to create hydrophobic (ink-receptive) and hydrophilic (water-receptive) regions, which are essential for the offset printing process. This ensures that only the desired image areas attract ink while the non-image areas repel ink and accept water, allowing for high-quality, consistent printing.
How Does Polyester Plate Work in a Printing Press?
Polyester plate lithography is a user-friendly, accessible, and safe printmaking method. Originally designed as a cost-effective alternative to aluminum plate offset lithography in commercial printing, it has since been embraced by the printmaking community. Since its introduction in the late 1990s, polyester plate lithography has gained popularity among artists and educators who experiment with lithography and produce small to medium-sized editions of lithographic prints.
Processing and inking of the plate should be done in advance. A polyester plate is semi-durable and can produce over 20 quality prints before the image deteriorates and fills in, although this number can vary based on several factors.
- Sponge gum Arabic onto the polyester plate. Apply a small amount thinly and evenly with cheesecloth or a barely damp, clean sponge. DO NOT BUFF. This step processes the plate, so do not skip it.
- Let the plate dry.
- Dampen the plate with water and attach it to the plate backer using high-quality clear packaging tape.
- Keep the plate damp and ink your image until it is fully inked.
- Take proof onto newsprint.
- Ink the image and print it onto good paper. You can place your paper face down on top of the polyester plate or remove the plate from the plate backer, lay your print down, then position the polyester plate on top of it and cover it with newsprint.
The process is more straightforward than conventional lithography because the plate does not require chemical processing, such as etching with nitric acid. The most common issue encountered when printing with a polyester plate is “scumming,” where the plate picks up ink in unwanted areas. The solution is to use a wiping solution fortified with a small amount of gum Arabic and a sprinkling of citric acid. The gum arabic makes the plate more receptive to ink, while the citric acid helps prevent scumming, ensuring a clean print.
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