Flexography Printing Knowledge

Flexography is much like letterpress in that it is also a relief process with the image areas of the plate raised above the non-image areas. The process is widely used for the printing of packaging for the food industry. The plates and the types of inks used for flexography makes it very suitable for printing on the non-porous substrates required for food packaging.

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.

The soft and flexible material used for flexographic plates makes it more difficult and challenging to accurately mount the plates on the press. Not only must the plate be aligned properly to the cylinder so that the print looks square and is correct on the substrate, it must also be registered with other plates that may be part of the same press run. A pin register system is used to aid in accurately mounting the plate to the press cylinder. Holes punched into the plates are matched with pins to achieve registration. There are also television monitored mounting machines which show enlarged areas of dots on both ends of the plate cylinder and enlarged dots on the plate. Using a monitor, the close-up view of the dots are lined up to achieve perfect registration. When the registration is achieved, double sided tape is used to mount the plate to the press cylinder.

Mounting the flexible plate material on the press may cause it to stretch slightly. The image on the plate may become distorted causing poor results with the finished product. For example, a square may look more like a rectangle. To compensate for distortion, the original image is distorted. A mathematical formula is used to figure out the amount of stretch that will occur on a plate based on the plate thickness and the repeat length of the cylinder. Software programs are available to apply distortion to computer generated art.

Flexography - PrintingTips.com