Pad printing process as belongs to the indirect way of printing and offset printing technology.Offset printing technology transfers by rubber cloth on the PS version graphic printing ink, and pad printing technology transfers by pad printing rubber head pad printing graphic area on the gravure printing ink, therefore, as for rubber pad printing head and rubber cloth printing quality plays a decisive role.Pad printing in the printing process, pad printing rubber head by compression deformation and make the rubber head surface dip in gravure by the ink, and then move to the surface of substrates by compression deformation and make the glue on the surface of the head transfer ink to the substrates.
A, pad printing rubber head should have the features
1.Good absorbency and deinking
Absorbency and deinking is a pair of contradictions, when in fact it means that the ink layer in the equilibrium pressure from high concentration to low concentration of diffusion.Remove when dipped in ink, gravure printing ink concentration is high, the rubber head surface ink film concentration is low, head of compression deformation, the distribution between rubber head compression zone and gravure ink layer density will redistribute, which is part of the ink layer attract stick by glue head surface adhesion force to the surface of a glue head, another part of the gravure surface adhesion force attract residues in gravure surface, then show the glue head absorbency.Rubber head process of contact with the substrates are mainly for deinking sex.Header compression deformation, the ink on the surface of the separation.Part of the ink transfer onto substrates, the other part in rubber head surface.
2.Good resilience
Rubber pad printing ink absorption in the deformation, and deinking in deformation.Pad printing graphic show flat steel version, if the surface is flat substrates, so if you want to ensure that by not in the rubber pad printing head of permanent deformation and deformation, rubber pad printing head has good resilience is crucial.In the life of the mid-term, rubber pad printing head appear tiny deformation (irreversible), will bring serious influence printing quality.
3.Good solvent resistance
Rubber pad printing head must have the ability to resist the corrosion of various solvents in ink.Screen printing ink and printing ink is usually larger solvent base ink viscosity, the solvent is solvent gasoline, ethyl acetate, acetone, butyl acetate, methyl ethyl ketone, vinegar, butanone, cyclohexanone, etc., the experiment proved that silicone rubber pad printing head to the solvent has good resistance to resistance.
Second, the production principle of rubber pad printing head
1.Of rubber vulcanization system, is the foundation of the rubber industry made great progress.Rubber pad printing head use room temperature vulcanization silicone rubber vulcanizing agent in the couplet net structure under the action of the principle of the elastomer.
Rubber is some macromolecular chain hydrocarbons, there will be only on the main chain of carbon atoms rubber rubber carbon chain;In addition to the carbon atoms on the main chain contains other atoms of rubber miscellaneous rubber chain;If the main chain containing silicon (Si), tin (Sn), boron (B) atoms of organic groups such as rubber, organic polymer said elements.Silicone rubber is on the main chain element organic polymer containing silicone group.There are two kinds of silicone rubber vulcanization process, is a kind of high temperature vulcanization, and the matching of silicone rubber is called high temperature vulcanization silicone rubber, hereinafter referred to as HTV.Another is room temperature vulcanization, and at the same time of the silicone rubber is called room temperature vulcanized silicone rubber, RTV for short.Room temperature vulcanized silicone rubber is divided into two kinds, one kind is forming, the other is a condensation type.
2.Condensation type silicone rubber and have end hydroxyl (OH), and easy hydrolysis group (- OCH3, - OCOCH3) of silane will, room temperature vulcanizing, crosslinking agent is ethanol, silicate tin silicate acid salt catalyst is February.Plus silicone rubber molding generally contain - CH = CH, end group, usually with polysiloxane containing the Si - H keys as crosslinking agent on the couplet elastomer addition reaction.
3.Protective system, rubber pad printing head in use process, the comprehensive function will be affected by heat and repeated mechanical compression, lead to molecular structure change, so that the rubber head, cracks, discoloration, apparent sticky gradually lost the use performance, this phenomenon is called aging.To inhibit and slow down the aging speed, usually added in the glue formula of protective system.
Head and a phenomenon of aging is oxygen erosion, when it was 1% ~ 2.Can't use after 5% oxygen erosion.Therefore, vacuum oxygen is one of the key to making the silicone head.
Third, the making craft of rubber pad printing head
1.Formula of the rubber pad printing head
(1) silicone rubber, various types of room temperature vulcanized silicone rubber string. 3
(2) silicone oil - used to improve the rubber pad printing head hardness and processing performance of the structure control agent
(3) month hang acid tin - promoter string 1
(4) ethyl silicate - crosslinking agent
2.The content of silicone oil
The content of silicone oil more little direct impact on the hardness of rubber pad printing head, but the proportion of silicone oil and silicone rubber has a permissible range, in this range, the physical properties of the rubber pad printing head better.The smaller the range, the smaller the compatibility of silicone oil and silicone rubber;The greater the extent, the greater the compatibility.The range can be determined through the experiment, that is to say, the proportion is not arbitrary, more than the proportion, make the rubber head cannot be used.
3.The content of curing agent
Vulcanizing agent content of general control within 5% mass ratio, low silicon rubber crosslinking inadequate, would contain raw rubber vulcanizates composition, expression is sticky;Curing agent content is too large, makes the rubber head become brittle, tear strength is low.To improve the silicone head tear strength can add right amount of reinforcing agent in rubber formulations such as not black and white carbon black, etc.
4.The technological process
(1) preparation of adhesive solution
According to the proportion of each component in the formula, to calculate the first to make the rubber head size, and then calculated roughly the quality of the components, and respectively in a fixed containers.
(2) stir
Blending adhesive solution under the mixer fully mixing, mixing, forming stable liquid, colour and lustre is consistent.
(3) the vacuum
Put mixed liquid in vacuum pump, vacuum processing.There will be water vapor produced in the process of silicone rubber vulcanization, and because of its high viscosity of silicone rubber mixture, stirring easily when mixed with air, so as to prevent gas mixed with elastic body, vacuum processing beforehand is necessary.
(4) to produce rubber head model
Substrates of different shape and size to design the corresponding plastic head.The design of rubber head should consider exhaust function and the expected deformation processing.Design good glue head model are then passed on to the machining department made aluminum alloy, and then with unsaturated polyester resin casting out button.Button is used to cast an essential tool for finished product rubber head.Before casting, on the inner wall of the cavities evenly coated mold release agent, mold release.
(5) curing conditions
Room temperature 25 ℃
Gel time 2 h
Cure time 8 h
Fourth, the nature of the rubber pad printing head
1.hardness
Hardness characterization of rubber head compression deformation resistance.In printing, a soft rubber head can obtain larger deformation, suitable for large curvature of substrates and printing;And a hard rubber head deformation is small, when printing dot enlargement also small, apply to the face print or order printed matter printing.Adjust the hardness of rubber head, in addition to adjust the content of silicone oil, the key is to choose the softener.
2.Tear strength
Rubber torn from defects or micro cracks usually occur, then to rupture.Tear strength is often associated with crosslink density, under the proper crosslinking density, the tear strength is the largest.But the content of crosslinking agent is too much, often tear strength decreased.
3.Resistance to fatigue damage
Rubber pad printing head under the effect of dynamic compression by repeatedly, changes in the structure and the properties of the phenomenon, referred to as the "fatigue".With the continuation of fatigue, silicone rubber is flabby, surface crack, fissure, and gradually extended to collapse, it's called "fatigue".
Fatigue process of silicone rubber contains cracks generated and crack extension two stages.In general, can use rubber and or combination of sulfide method to improve the fatigue resistance of silicone rubber.
4.Resistance to organic solvents
Rubber pad printing head often contact printing ink and solvent, solvent resistance how will directly affect the service life.
How the interaction between rubber and solvent can be said with solvent solubility of silicone rubber.The more similar chemical structure, the stronger the solvent dissolving force of silicon rubber.Experiments show that in addition to the benzene and dimethyl benzene can cause certain harm to silicone rubber, other solvents will not have impact on silicone rubber.
The size of the silicone rubber solubility in organic solvent solubility parameters available, the closer the solubility parameter, the stronger the dissolving capacity;Solubility parameter difference, the greater the ability to dissolve the weaker.Table 1 shows the silicone rubber and several kinds of solvent solubility parameters.