UV Letterpress Ink – Photoinitiator
Letterpress is one of the earliest printing methods utilized in the printing industry, with the invention of collodion printing by Bi Sheng in the Song Dynasty, woodcut printing by Wang Zhen in the Yuan Dynasty, and lead printing by Gutenberg in the 15th century, all of which are letterpresses. In letterpress printing, the graphic portion is raised on the surface of the printing plate, and the ink is applied only to the graphic portion and then transmitted directly to the substrate. There are two types of letterpress, platform type and rotary type. The platform type has the printing plate as a flat surface and the impression roller as a cylinder, while the rotary type has both the printing plate and the impression roller as a cylinder structure.
3.6.1 Plate making for letterpress printing
The plates used for letterpress printing are now photopolymerized resin letterpress plates made from photopolymerized resin through UV exposure and development. Photopolymerized resin letterpress plates are divided into two types: liquid photopolymerized resin letterpress plates and solid photopolymerized resin letterpress plates.
(1) Solid Photopolymerized Resin Toppan Platemaking
Solid photopolymer photopolymer letterpress plate adopts solid polymer material to pre-produce the photopolymer plate, the structure of which is shown in Figure 3-18.
Figure 3-18 Structure of Solid Photopolymer Toppan Plates
The solid resin plate consists of a saturated polymer, a cross-linking agent and a photoinitiator. The saturated polymers are polyvinyl alcohol derivatives, cellulose derivatives and polyamides; the cross-linking agent is divinyl compounds; the photoinitiators are mainly benzoin ethers or anthraquinones. After the above components are mixed evenly, they are coated onto a polyester sheet base or aluminum base with an anti-halation layer, and after drying, they are made into a solid photosensitive resin letterpress plate. There is also a solid resin plate component that is co-mingled and produced by extrusion molding.
Solid photopolymeric resin letterpress plate making process:
Covering the negative → UV exposure → development → drying → post-exposure → printing plate.
After the polyethylene protective film is removed from the solid resin plate, the negative and the photopolymerized resin layer are vacuum laminated, UV exposed, the light-seeing portion crosslinked and cured to form the graphic portion, and the light-seeing portion is removed in the developing process. After drying, the second UV exposure is sufficiently applied to completely solidify the plate to improve the hardness of the plate and make it into a printing plate.
(2) Liquid Photopolymerized Resin Letterpress Platemaking
Liquid photopolymerized resin letterpress refers to the liquid state of the resin before sensitization, and the resin letterpress becomes solid after sensitization. Liquid resin version of the photopolymerization resin is mainly unsaturated polyester and polyurethane acrylate, crosslinking agent for unsaturated bis-alkenes or alkenes, photoinitiators are mainly benzoin ether class. The plate-making process for liquid photopolymerized resin letterpress plates is (see Figure 3-19):
Coating with photopolymerizing resin → UV exposure of both front and back sides → development → drying → post-exposure → printing plate
Figure 3-19 Platemaking Process for Liquid Photopolymer Letterpress Plates
Solid photopolymeric resin letterpress plates and liquid photopolymeric resin letterpress plates were widely used in the 1970s and 1990s in the printing of newspapers and magazines, labels, wrapping paper, and letterheads. However, because of the low resolution of the printing plates, low printing resistance, and poor printing quality, they were gradually replaced by offset printing and flexographic printing, and withdrew from the stage of printing history.
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