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.
A practical selection route for photoinitiator-related projects
When technical buyers or formulators screen photoinitiators, the most useful decision frame is usually cure quality plus application fit: which package cures reliably, keeps appearance acceptable, and still works under the lamp, film thickness, and substrate conditions of the actual process.
- Match the package to the lamp first: mercury lamps, UV LEDs, and visible-light systems can rank the same photoinitiators very differently.
- Check depth cure and surface cure separately: a film that feels dry on top can still be weak underneath.
- Balance yellowing with reactivity: the strongest deep-cure route is not always the best commercial choice if color or migration risk becomes unacceptable.
- Use the final formula as the benchmark: pigment load, monomer package, and film thickness can all change the apparent ranking of the same initiator.
Recommended product references
- CHLUMINIT TPO-L: A strong low-yellowing reference for LED-oriented UV systems.
- CHLUMINIT 819: Useful when a formulation needs stronger absorption and deeper cure support.
- CHLUMINIT 1173: A practical comparison point for classic short-wave UV initiation.
- CHLUMINIT ITX: A useful long-wave support route in many printing-ink packages.
FAQ for buyers and formulators
Why are blended photoinitiator packages so common?
Because one product may control yellowing or lamp fit well while another improves cure depth or line-speed performance, so the full package is often stronger than any single grade.
Should incomplete cure always be solved by adding more initiator?
Not automatically. The real limitation may be the lamp, film thickness, pigment shading, or the rest of the reactive system rather than simple under-dosage.
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Quick answer: For practical formulation work, photoinitiator screening starts with the light source and film build, then checks yellowing, adhesion, and cure completeness under real production conditions.
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