The printing plate originally used for making flexographic printing was rubber, which is still favored by many print processors and is far superior to the polymer printing plate in terms of printing quality. Current natural and synthetic die-casting rubber plates have different uses in narrow web printing, wide web printing, and corrugated printing.

Rubber began to be used in printing in the late 19th century, and it was only at the beginning of the 20th century that molded or composite plates were used.

Rubber plate features
Both natural and synthetic rubber compounds actually differ in hardness, rheological properties, chemical resistance and elasticity. The performance of the two plates is different, so care must be taken when using water, alcohol, and ink. Butyl rubber or vinyl propylene rubber can handle most of the ketones and some aromatic compounds.

With different printed substrate materials, better wear resistance can be achieved. With different plate surface materials, better ink transferability or better chemical compatibility can also be obtained.

The plate can use iron or steel as the base, suitable for magnetic cylinders, can be used as a base material with flexible controllable material, and can also be die-casting plates according to the required thickness, and is not limited by the thickness of the substrate.

The most commonly used flexographic printing plate is the general rubber, without any restrictions on the substrate, and is suitable for all the plate cylinders, and the phenomena of mushroom surface, concave surface or hard edge will not occur when the plate is mounted. This type of plate is well suited for ink and solvent variations, so it has great solvent/chemical compatibility, and it is also well suited for use with compressible adhesive backings and compressible backing materials.

The commonly molded rubber plates occupy a very important position in the flexographic printing industry and have great flexibility in thickness, size, chemical compatibility, and surface properties. Loading is also very easy. Plates do not break and are not easily damaged.

Compared with the photopolymer plate, the rubber plate has a wide range of solvent adaptability. The molded rubber plate does not generate static electricity and does not attract or accumulate surface impurities or foreign materials. In general, they have more ink-transfer, making the ink on the ground thicker. In the flexographic printing industry, there are more than 20 types of ink solvents and thinners used, and die-casting rubber plates can use 17 of them, while polymer plates are rarely used.

Rubber type and use

The flexographic printing market has only one genuine natural rubber plate and three synthetic rubber plates, each with its own hardness, ink and solvent resistance, and softness. The rubber plate can be made in a variety of thicknesses, 0.047-0.250 inches thick, and 0.047-0.125 inches thick. This thickness plate is commonly used in narrow web and wide web printing, cardboard, and pre-printed. Printed cardboard printing, such as printing paper, plastic film, cellophane, foil, uneven labels, multilayer bags, linen, folding cartons, etc. Thicker plates are 0.125-0.250 inches and are commonly used for combination corrugated board printing.

The four types of rubber currently used for plate making are: natural rubber, nitrile rubber, vinyl resins, and vinyl propylene (EP). In natural rubber compounds, there are six different grades and types, varying in hardness from the lightest Shore A20 degrees to the hardest Shore A 60 degrees. The thickness of these rubber compounds ranges from 0.047 to 0.250 inches. According to the actual situation, it is recommended to use Shore A20-50 degrees, printing A, B, C and E channel type corrugated cardboard; with Shore A50-60 degrees, printing paper, film, wax paper, foil, cardboard, cellophane, etc. Wait. Ethanol based inks, water glycol based inks, glycerol inks, and ethanol/acetic inks were used.
Natural rubber composite printing plates have super-resistance, excellent ink transfer properties, and unusual flexural properties when printed on rough and uneven substrates.

Synthetic rubber - nitrile rubber, vinyl resin and vinyl propylene, they are at A20-90 degrees Shore A.
Nitrile rubber: Shore A20-90 degrees. Recommended use: printing corrugated liners, business forms, books, and multilayer bags. With oil-based inks, the solvents are aliphatic hydrocarbons, benzene, petroleum, naphtha, kerosene, and mineral oil. The thickness range is 0.047-0.250 inches.

Ethylene: Shore A 50 degrees, recommended use: printing film, wax paper, etc. The thickness range is 0.047-0.250 inch. The ink uses fast-drying solvent inks, such as high gloss latent solvent inks, oil inks, ethanol inks, acetic acid inks, and water-based inks. Tricolor printing has good ink transfer properties.
Vinyl Propylene: Shore A hardness of 50 degrees, thickness 0.047-0.250 inches, acetic acid ink and ketone ketone ink are recommended.

Production plate

Regardless of the actual rubber material used to produce the plate, all rubber plates are processed in the same basic way. First, the color separation is performed, screen printing is performed to generate a photographic negative, and then the hot stamping is performed using a die plate made from a metal engraving, a photopolymer master or a hard rubber master/engraved rubber master, and a phenolic molding powder. Molded plates, and finally, hot pressed composite unvulcanized rubber plate material, made into a printing plate.
The above production process sounds easy, but in order to produce a printing plate with low cost, high printing quality, and high production speed, it is necessary to follow some basic principles: using good materials, using appropriate equipment and methods for processing. The consistent uniformity of the plate thickness directly depends on the die casting equipment, but the most important one is the professional die casting technician or plate maker.

In most cases, the printer has requirements for the thickness and tolerance of the rubber plate. If the printing plate used for corrugated board printing has a thickness of 0.250 inches, the tolerance should not exceed ±0.002 inches. The fully automatic vulcanizers on the market today are capable of molding the largest plates with a tolerance of ±0.001 inch and a minimum plate tolerance of ±0.0005 inch. If there are too many variations in plate thickness, there are two ways to correct it:

(1) Correct the mold.
(2) Grind the back of the rubber plate with a precision rotating automatic grinder to achieve the required thickness tolerance.

Correct handling and cleaning before printing plates can increase the service life of printing plates. The treatment proposal is as follows:

(1) Before depositing, clean the ink and solvent with a soft brush as much as possible.
(2) When cleaning, be sure to use a compatible solvent with the plate.
In any case, the plate maker should be asked to provide some precautions related to the preservation of the contents, because the existence and improper preservation of the plates is of critical importance to the useful life of the plates and the quality of the printing. E.g:
(1) The temperature of the plate storage should not exceed 38°C. The cooler the better, it should be far away from the ozone source, the motor driver and the corona treatment device.
(2) Prior to storage, the plates must be cleaned and dried.
(3) Prevent the plate from being exposed to sunlight and ultraviolet light, otherwise it will harden and crack.
(4) To prevent overlap with the printing plate, if it cannot be avoided, it should be used to line the printing plate between the printing plates.
(5) The plate is stored on a roller or placed in an arc, which increases the surface tension and makes it more sensitive to ozone attack. If it is necessary to store plates in this way, it is best to protect them with black materials. If the plates are in an arc shape, they cannot be stored flat.

The printing plate storage method described above can be applied to any flexographic printing plate, rubber or polymer printing plate.

At present, there are many users of the rubber plate, especially in the fields of flexible packaging printing and corrugated printing. The advantages of the rubber version are good ink transfer performance, safe plate making process, recyclability, and no environmental pollution.

Source: Packaging Community

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