Rubber Plate: The traditional flexo plate is made from rubber and looks like a giant rubber stamp. The rubber plate is mounted on a press cylinder where ink passes over the raised image areas to produce a printed image. The print quality is slightly better than conventional letterpress but it still doesn't achieve the quality of offset or gravure printing. The rubber plates are used for printing milk cartons, corrugated cartons, bags, and envelopes.
In order to produce a rubber plate, several steps are necessary. An original photoengraving is etched from a magnesium base or it can be formed with polymer which is sensitive to ultra-violet light. The photoengraving is produced wrong reading and becomes the master. A matrix or mold, which is right reading, is produced from the master. Heat and pressure are used in order to create the matrix board. From the matrix, a number of rubber plates can be created. Unvulcanized rubber is placed over the matrix board followed by silicone coated cloth. Special equipment is used to apply heat and pressure which vulcanizes the rubber and molds it around the matrix to create a wrong reading rubber plate. The silicone coated cloth is used to prevent the rubber from sticking to the equipment. When mounted on the press cylinder, the wrong reading rubber plate will create a right reading image on the printed surface. Because duplicate plates can be made from the matrix, the process is called a duplicate platemaking process.
Photopolymer Plate: An alternative to the rubber plate is the photopolymer plate which has become the dominant type of plate used in flexography. In order to create a flexo photopolymer plate, a wrong reading negative of the image is placed on the plate material and exposed to ultra-violet light. The UV light causes the image areas of the plate to harden. After the exposure, the unexposed areas of the plate are washed out (usually with water) and the plated is allowed to dry. The final step is to expose the entire plate to additional ultra-violet light to complete the curing process.
Photopolymer plates are able to hold finer detail than rubber plates and they are more durable. They are faster to produce because it isn't necessary to make a master photoengraving or matrix board. They are used for printing such items as flexible films, pressure sensitive labels, and even newspapers.