Here's a brief overview of how copper plates are used in the printing process.
The design to be printed is transferred onto a copper plate. This can be done through various methods, including direct engraving or by using light-sensitive coatings that are exposed to the design and then etched.
The exposed areas of the copper plate are chemically etched away, leaving the design in relief (raised) on the plate. The depth of the etching determines the amount of ink the plate can hold.
The plate is inked, and the ink adheres to the raised areas of the plate where the design is located.
The inked copper plate is pressed onto the printing surface, typically a substrate like paper or, in the case of packaging, various types of flexible materials. The pressure transfers the ink from the plate to the substrate, creating the printed image.
The printed material needs time to dry or cure, depending on the type of ink used.
Copper plates have been historically used in letterpress printing, a method where the raised surface of the plate directly contacts the printing surface. However, as technology has advanced, other plate materials like polymers have become more common due to their cost-effectiveness and ease of use.
In the context of packaging bag printing, flexography and rotogravure are more common methods today. These methods often use polymer plates or cylinders rather than copper plates. Flexography, for example, utilizes flexible photopolymer plates mounted on printing cylinders. Rotogravure involves engraved cylinders (which can be made from copper or other materials) and is often used for high-volume printing on flexible materials like plastic films.
It's worth noting that while copper plates may be less common in modern packaging printing, the term may still be used colloquially to refer to the printing plates used in traditional methods or to emphasize the historical craftsmanship associated with certain types of printing.