Production of the hottest pad printing adhesive he

Production of pad printing adhesive head

pad printing process is an indirect printing method like offset printing process. The offset printing process transfers the graphic ink on the PS plate through the blanket, while the pad printing process transfers the graphic ink on the gravure plate through the pad printing rubber head. Therefore, the pad printing rubber head, like the blanket, plays a decisive role in the printing quality. In the process of pad printing, the pad printing rubber head first dips the ink in the gravure graphic area on the surface of the rubber head through compression deformation, and then moves to the surface of the substrate. Through compression deformation, the ink on the surface of the rubber head is transferred to the substrate

first, the characteristics of pad printing adhesive head

1. Good ink absorption and deinking

ink absorption and deinking are a pair of contradictions. In fact, it means the diffusion mode of the ink layer from the high concentration area to the low concentration area when under balanced pressure. When dipping ink, the ink concentration at the gravure of pad printing is high, and the ink layer concentration on the surface of the adhesive head is low. When the adhesive head is compressed and deformed, the ink layer concentration distributed between the compression area of the adhesive head and the gravure will be redistributed, that is, part of the ink layer will stick to the surface of the adhesive head by the adhesive force on the surface of the adhesive head, and the other part will remain on the gravure surface by the adhesive force on the surface of the gravure head. At this time, it shows the ink absorption of the adhesive head. The contact process between the rubber head and the substrate is mainly deinking. When the rubber head is compressed and deformed, the ink on the surface separates. Part of the ink is transferred to the substrate, and the other part remains on the surface of the rubber head

2. Good resilience

pad printing rubber head absorbs ink in deformation and deinks in deformation. The pictures and texts on the pad printing steel plate are flat. If the surface of the substrate is also flat, it is very important to ensure that the pictures and texts are not deformed due to the permanent deformation of the pad printing head. During the service life, the pad printing adhesive head has small deformation (unrecoverable), which will seriously affect the printing quality

3. Good solvent resistance

the pad printing adhesive head must have the ability to resist the corrosion of various solvents in the ink. Printing inks and pad printing inks are usually solvent-based inks with high viscosity, and their solvents include solvent gasoline, ethyl acetate, acetone, butyl acetate, methyl ethyl ketone (butanone), cyclohexanone, etc. experiments have proved that silicone rubber pad printing rubber head has good resistance to the above solvents

II. Manufacturing principle of pad printing rubber head

1. The vulcanization system of rubber is the basis for the rubber industry to make great progress. The pad printing rubber head is made by using the principle that room temperature vulcanized silicone rubber is crosslinked into an elastomer with a shape structure under the action of vulcanizing agent

rubber is some hydrocarbons with macromolecular chains. Rubber with only carbon atoms on the main chain is called carbon chain rubber; Rubber containing other atoms besides carbon atoms in the main chain is called heterochain rubber; If the main chain contains organic groups of silicon (SI), tin (SN), boron (b) and other atoms, it is called elemental organic polymer. Silicone rubber is an elemental organic polymer containing silicone groups in the main chain. There are two vulcanization processes for silicone rubber. One is high-temperature vulcanization, and the corresponding silicone rubber is called high-temperature vulcanized silicone rubber, abbreviated as HTV; The other is room temperature vulcanization. The corresponding silicone rubber is called room temperature vulcanized silicone rubber, or RTV for short. RTV silicone rubber is divided into two types, one is addition molding, the other is condensation type

2. Condensed silicone rubber has terminal hydroxyl (- OH), which is easy to contract with silane containing easily hydrolyzable groups (- OCH3, - OCH3), and is vulcanized at room temperature. The crosslinking agent is ethyl silicate, and the catalyst is dibutyltinate silicate. The addition silicone rubber generally contains -ch = ch, end group, and is usually crosslinked into elastomer by self addition reaction with polysiloxane containing Si-H bond as crosslinking agent

3. protective system: during the use of pad printing adhesive head, it will be subjected to the combined effect of heat and repeated mechanical compression, resulting in changes in molecular structure, which will make the adhesive head sticky, cracked, discolored, and gradually lose its use performance. This phenomenon is called aging. In order to inhibit and slow down the aging rate, protective system is usually added to the formula of rubber head

another aging phenomenon of rubber head is oxygen erosion. It cannot be used when it is corroded by 1% - 2.5% oxygen. Therefore, vacuumizing and deoxidizing is one of the keys to making silicone head

III Manufacturing process of pad printing rubber head

1. formula of pad printing rubber head

(1) silicone rubber - various types of room temperature vulcanized silicone rubber

(2) silicone oil - structural control agent used to improve the hardness and processability of pad printing rubber head

(3) dibutyltin sulfate - promoter

(4) ethyl silicate - crosslinker

2. The content of silicone oil

directly affects the hardness of pad printing rubber head, However, the ratio of silicone oil to silicone rubber has an allowable range. Within this range, the physical performance of pad printing rubber head is better. The smaller the range, the smaller the compatibility between silicone oil and silicone rubber; The larger the range, the greater the compatibility. This range can be determined by experiments, that is to say, this ratio is not arbitrary. Beyond this ratio, the produced rubber head cannot be used

3. Content of vulcanizing agent

the content of vulcanizing agent is generally controlled within 5% mass ratio. If it is too small, the silicone rubber crosslinking is insufficient, and the vulcanizate will contain raw rubber components, which is sticky; If the content of vulcanizing agent is too large, the rubber head becomes brittle and the tear strength is low. To improve the tear strength of the silicone head, add an appropriate amount of reinforcing agents such as black, white carbon black, etc. to the rubber formula

4. Technological process

(1) prepare glue solution

according to the proportion of each component in the formula, first calculate the volume of the glue head to be made, then roughly calculate the mass of each component, and pour it into a fixed container respectively

(2) stirring

fully stir the mixed glue liquid under the mixer to make it mix evenly to form a stable liquid with consistent color

(3) vacuum

place the mixed liquid under the vacuum pump and vacuum it. Water vapor will be produced in the vulcanization process of silicone rubber, and because the viscosity of silicone rubber mixture is large, it is easy to be mixed when stirring. However, the use of plastic materials has only begun in the 1980s. In order to prevent gas from mixing into the elastomer, it is necessary to vacuum in advance

fast and slow (4) make glue head model

design corresponding glue heads for different substrate shapes and areas. The design of rubber head should consider the exhaust function and expected deformation treatment. The designed rubber head is then handed over to the machining department to make the aluminum alloy model, and then the master mold is cast with unsaturated polyester resin. The master mold is the necessary tool for casting the finished rubber head. Before casting, the mold release agent should be evenly coated on the inner wall of the master mold to facilitate mold release

(5) vulcanization conditions

Room temperature 25 ℃

gel time 2H

curing time 8h

IV. properties of pad printing rubber head

1. Hardness

hardness represents the ability of rubber head to resist compression deformation. In printing, the softer rubber head can obtain large deformation, which is suitable for printing substrate with large curvature; The hard rubber head has small deformation and small printing time point expansion, which is suitable for the printing of plane printing or gradient printing. To adjust the hardness of the rubber head, in addition to adjusting the content of silicone oil, the key is to select the right softener

2. Tear strength

the tear of rubber usually occurs from defects or microcracks, and then develops to fracture. The tear strength is often related to the crosslinking density. Under the appropriate crosslinking density, the tear strength is the largest. However, if the content of cross-linking agent is too high, the tear strength tends to decrease

3. Fatigue resistance

the phenomenon that the structure and performance of pad printing adhesive head change under repeated dynamic compression is called "fatigue". With the continuation of the fatigue process, the silicone rubber becomes loose, the surface cracks, cracks appear, and slowly expand to complete fracture, which is called "fatigue failure"

the fatigue process of silicone rubber includes two stages: crack formation and crack propagation. Generally, the fatigue resistance of silicone rubber can be improved by vulcanization or combination of rubber

4. resistance to organic solvents

pad printing adhesive heads are often in contact with inks and solvents, and how solvent resistance performance will directly affect their service life

how the interaction between rubber and solvent can be expressed by the solubility of solvent on silicone rubber. The more similar the chemical structure, the stronger the solvent's solubility in silicone rubber. The test of foam granulator shows that except benzene and xylene can cause certain harm to silicone rubber, other solvents will not affect silicone rubber

the solubility of silicone rubber in organic solvents can be expressed by solubility parameters. The closer the solubility parameters are, the stronger the solubility is; The greater the difference of solubility parameters, the weaker the solubility. Table 1 shows the solubility parameters of silicone rubber and several solvents

Table 1 solubility parameters of silicone rubber and several solvents

chemical name solubility parameters

silicone rubber 7.3

toluene 8.97

xylene 8.83 ~ 9.03

ethyl acetate 9.10

benzene 9.22

butanone 9.56

acetone 9.74

ethanol 12.97

it can be seen from table 1 that the solubility parameters of silicone rubber are relatively close to toluene, xylene and benzene, and and with butanone, acetone Ethanol and so on are quite different

for mixed solvents, such as benzene 35%, toluene 15%, acetone 10%, butanone 40%, it has a great impact on silicone rubber. Immersing silicone rubber in it will change the quality. Therefore, frequent contact between the rubber head and the above mixed solvent should be avoided

5. Frost spraying

after a certain period of storage, the pad printing rubber head will appear frost spraying, that is, the accelerator, anti-aging agent and filler in the mixture will migrate from the inside of the rubber to the surface and precipitate. Spraying cream not only affects the appearance quality of the rubber head, but also impairs the use effect. The spray of rubber head is mainly caused by the excessive dosage of silicone oil or accelerator. The control method is to strictly follow the formula, and it is not allowed to blindly increase the content of silicone oil to reduce the hardness, etc. Arbitrarily increasing or reducing the amount of vulcanizing agent will also lead to excessive sulfur or insufficient vulcanization

information source: Minnan printing

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