Description

Flexography is much like letterpress in that it is as well as a relief

technique with the picture areas of the plate raised above the non-

image areas. The technique is widely used for the printing of ackaging for the food industry. The plates and the categories of inks

used for flexography makes it suitable for printing on the non-porous

substrates necessary 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 picture areas to

produce a printed picture. The print quality is slightly better than

conventional letterpress but it still doesn't accomplish the quality

of offset or gravure printing. The rubber plates are used for printing

milk cartons, corrugated cartons, bags, and envelopes.

Flexographic Plates

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 generate the matrix board. From

the matrix, lots 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 generate 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 generate a right reading picture

on the printed surface. Because duplicate plates can be made from the

matrix, the technique is called a duplicate platemaking technique.

Photopolymer plates can hold finer detail than rubber plates and they

are more long lasting. They are faster to produce because it is not

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.

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 generate a flexo photopolymer plate, a wrong

reading negative of the picture is placed on the plate material and

exposed to ultra-violet light. The UV light causes the picture 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 whole plate to additional ultra-

violet light to complete the curing method.

Mounting Flexographic Plates

Plate Stretch

The soft and flexible material used for flexographic plates makes it

more difficult and challenging to exactly 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

even be registered with other plates that may be part of the same

press run. A pin register method is used to aid in exactly mounting

the plate to the press cylinder. Holes punched in to the plates are

matched with pins to accomplish registration. There's also tv

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 accomplish

ideal 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 picture on the plate may become distorted

causing poor results with the completed product. For example, a square

may look more like a rectangle. To compensate for distortion, the

original picture 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. Application

programs are available to apply distortion to computer generated art.