In the last article, we covered the most basic principles of screen printing and the methods used in the platemaking process.Today we’re going to talk in detail about one of the most common methods of making plates in modern silk screen technology.
We have a general understanding of the entire screen printing process, stretch screen – cleaning – drying – coating photosensitive adhesive – drying – exposure – development – drying – repair – curing treatment.After this series of processes, in the use of screen, we need to develop the place can be through the ink, and other places are not through the ink, this is the principle of screen printing.And I’ve always wondered how this is done, so I seriously studied the principle of photosensitive glue.
Early plate making with photosensitive materials are chromate as photosensitive agents, the egg fold, gelatin, chrome protein and other water-soluble natural polymer materials as film materials used after mixing.However, since the development of water soluble synthetic polymer polyvinyl alcohol, it has been used as a special film-forming material.
1. Photographic principle of dichromate.The 6-valent chromium ion in dichromate, which is reduced to 3-valent chromium after absorbing ultraviolet light, has been known for a long time.After the wide application of water soluble polymer polyvinyl alcohol (PVA) in plate-making materials, ammonia dichromate has been added to PVA as a photosensitive agent for a long time in screen plate-making.
As for the photochemical reaction of dichromate, the current general theory is that when the 6-valent chromium ion is photoreduced to 3-valent chromium ion, if it coexists with PVA, part of its hydroxyl group is oxidized, and the water-solubility of its PVA is therefore reduced.At the same time, due to the non-shared electrons of the generated 3-valent chromium ion, the hydroxyl group of polyvinyl alcohol was formed and coordinated, and the insoluble water of PVA was further deepened through the cross-linking reaction, resulting in the photocurable colloids.
2. Photosensitive principle of diazo resin.Dichromate has become more and more difficult to act as a photosensitizer due to public hazards, and diazo resins have been developed.Diazo resin has been widely used as photocatalyst for offset printing.And used for screen plate, because of the mixing of hydrophilic polyvinyl alcohol, photosensitive layer of diazo root with water decomposition, so it is difficult to use diazo resin to produce screen PS version.
Regarding the photohardening and insolubility in water of diazo resin polyvinyl alcohol emulsions, it is clear that the cross-linking reaction of polyvinyl alcohol molecules.Diazo resin molecules, the smallest n within 2 ~ 3, if n is 2, diazo dimer resin with polyvinyl alcohol light crosslinking of condensation, the lost of the polyvinyl alcohol hydroxyl is less, this to lose its hydrophilicity, although there is no obvious effect, but the molecular chain structure of poly (vinyl alcohol) caused by crosslinking becomes a three-dimensional reticular structure, is the light after hardening water insoluble has a great influence.
3. Photosensitive principle of acryolates.Acrylate acrylonitrile base, as a photosensitive resin photosensitive base is very important, its high sensitivity, good preservation, can be used as a screen PS version of photosensitive materials.Acrylate monomer or low polymer is a photosensitive resin formed by condensation with various polyacrylates. When used as a photosensitive material for screen printing, it needs to be combined with water developable polyvinyl alcohol.The highly sensitive pentaerythritol tripropylene ester has three light functional groups bonded by double bonds, showing excellent light hardening, and is insoluble in water after hardening.As a result, mixing with water-soluble polyvinyl alcohol becomes difficult.The soluble organic solvent polyethylene with long saponification value and acrylic ester are mixed and dissolved together, which can be made into photosensitive material for screen PS plate.
Of course, after understanding the principle of photosensitive adhesive, we should also see in the actual situation, screen printing industry to photosensitive products requirements.
1. Requirements for photosensitive materials for silk-screen printing.Silk-screen printing plate on the photosensitive material requirements are: plate good, such as easy to coating;Has the appropriate photosensitive spectrum range, generally in 340~440mm, the photosensitive wavelength is too long, plate making operation and printing plate storage needs to be in strict darkroom conditions, the wavelength is too short, the choice of light source, personnel protection will become more difficult;High sensitivity, can achieve the purpose of energy saving, rapid plate making;Good development performance, high resolution;Good stability, easy storage, reduce waste;Economy, health, non – toxic, non – pollution.
2. Requirements for photosensitive materials in screen printing.Screen printing on the photosensitive material requirements are: photosensitive material formed plate film should adapt to different types of ink performance requirements;Has a considerable resistance to printing, can withstand a considerable number of scraping board scraping pressure;Good combination with the screen, printing does not produce a film failure;Easy stripping, conducive to the screen plate material regeneration.