The printing process is the same as the offset printing process. The offset printing process transfers the graphic ink on the PS plate through the blanket, and the pad printing process transfers the graphic ink on the pad by transferring the pad. Therefore, the pad printing head and the blanket are decisive for the printing quality. The role. In the printing and printing process, the pad printing head first compresses and deforms the surface of the plastic head to pick up the ink of the intaglio graphic area, and then moves to the surface of the printing object to transfer the ink on the surface of the plastic head to the substrate by compressive deformation.
First, the characteristics of the printing plastic head should have
1. Good ink absorption and deinking
The ink-absorbing property and the deinking property are contradictory. In fact, it means that the ink layer is diffused from the high-concentration area to the low-concentration area when the ink layer is subjected to the equalization pressure. When ink is poured, the density of the ink at the pad is high, and the ink layer on the surface of the plastic head is low. When the plastic head is compressed and deformed, the concentration of the ink layer distributed between the compressed area of ​​the plastic head and the gravure is redistributed, that is, part of the ink layer. The adhesive force of the plastic head surface is attracted to the surface of the plastic head, and the other part is attracted to the surface of the gravure by the adhesive force of the gravure surface, which is expressed as the ink absorbing property of the plastic head. The contact process between the rubber head and the substrate is mainly deinking. When the rubber head is deformed by pressure, the ink on the surface is separated. A portion of the ink is transferred to the substrate and the other portion is left on the surface of the plastic head.
2. Good resilience
The printing head absorbs ink during deformation and deinks during deformation. The printing plate is flat on the steel plate. If the surface of the printing plate is also flat, it is very important to ensure that the printing plate is not deformed due to the permanent deformation of the printing pad. During the life cycle, slight changes in the print head (non-recoverable) will occur, which will have a serious impact on print quality.
3. Good solvent resistance
The print head must have the ability to resist the various solvents in the ink. Screen printing inks and pad printing inks are generally solvent-based inks with relatively high viscosity. Solvents include solvent gasoline, ethyl acetate, acetone, butyl acetate, methyl ethyl ketone (MEK), and cyclohexanone. Experiments have shown that silicone rubber The pad printing head has good resistance to the above solvents.
Second, printing the production principle of plastic head
1. The rubber vulcanization system is the basis for the rubber industry to make great progress. The pad printing head is made of the principle that the room-temperature vulcanized silicone rubber is cross-linked into a network-like structure under the action of a vulcanizing agent and the like.
Rubber is a hydrocarbon chain of macromolecule chains. Rubber with carbon atoms in the main chain is called carbon chain rubber; rubber with other atoms in the main chain and other atoms is called heterochain rubber; if the main chain contains silicon The rubber of organic groups such as (Si), tin (Sn), and boron (B) is called an elemental organic polymer. Silicone rubber is an elemental organic polymer containing an organosilicon group in the main chain. There are two kinds of silicone rubber vulcanization process, one is high temperature vulcanization, and the corresponding silicone rubber is called high temperature vulcanized silicone rubber, referred to as HTV; the other is room temperature vulcanization, and the corresponding silicone rubber is called room temperature vulcanized silicon. Rubber, referred to as RTV. Room temperature vulcanized silicone rubber is divided into two types, one is additive molding and the other is condensation.
2. Condensed type silicone rubber with terminal hydroxyl group (-OH), easy to shrink with silane containing easily hydrolyzable groups (-OCH3, -OCOCH3), room temperature vulcanization, crosslinking agent is silicic acid ethanol, catalyst is February Silicon Acid dibutyl stannate. Add-molded silicone rubber generally contains -CH=CH, an end group, and is usually crosslinked into an elastomer by a self-addition reaction using a Si-H bond-containing polysiloxane as a crosslinking agent.
3. In the protective system, the printing head is subjected to a combination of heat and repeated mechanical compression during use, resulting in a change in the molecular structure, which causes the plastic head to become viscous, cracked, discolored and gradually lose its performance. Call aging. In order to inhibit and slow down the aging process, a protective system is usually added to the formula of the rubber head.
There is also an aging phenomenon in the rubber head which is oxygen erosion. When it is eroded by 1% to 2.5% of oxygen, it cannot be used. Therefore, vacuum deaeration is one of the keys to the production of silicone heads.
Third, printing plastic head production process
1. Printing pad formula
(1) Silicone Rubber - Various Types of RTV Silicone Rubber
(2) Silicone oil - structural control agent used to improve the hardness and processability of pad printing heads
(3) Dibutyltin Dibenzoate--Accelerator
(4) Ethyl silicate - crosslinker
2. Silicone oil content
The small amount of silicone oil directly affects the hardness of the printing head, but the ratio of silicone oil to silicone rubber has an allowable range. Within this range, the physical properties of the printing head are better. The smaller this range, the smaller the compatibility of silicone oil and silicone rubber; the larger the range, the greater the compatibility. This range can be determined experimentally, that is to say that the ratio is not arbitrary, beyond which the manufactured rubber head cannot be used.
3. Curing agent content
The content of vulcanizing agent is generally controlled within 5% of the mass ratio. If the content of the vulcanizing agent is less than 5%, the silicone rubber cross-linking is insufficient, and the vulcanizate will contain the raw rubber component, showing stickiness; if the vulcanizing agent content is too large, the rubber head becomes excessive. Brittle, low tear strength. To improve the tear strength of the silicone head, add a suitable amount of reinforcing agent such as black and white carbon in the rubber formulation.
4. crafting process
(1) Preparation of glue
According to the proportion of each component in the formula, the volume of the rubber head to be produced is first calculated, and then the mass of each component is roughly calculated and poured into a fixed container.
(2) Stirring
The mixing glue is fully stirred under the mixer to make it evenly mixed to form a stable and consistent color liquid.
(3) Vacuuming
The mixed liquid was placed under a vacuum pump and vacuumed. During the vulcanization of silicone rubber, water vapor will be generated. Since the viscosity of the silicone rubber mixture is large, air is easily mixed during stirring. In order to prevent gas from being mixed into the elastic body, it is indispensable to perform vacuum processing in advance.
(4) Making rubber head model
Different plastic heads should be designed for different shapes and areas of substrates. The design of the plastic head should take into account the exhaust function and the intended deformation process. The designed plastic head is then submitted to the machining department for making the aluminum alloy model, and then the mother mold is cast with the unsaturated polyester resin. The master mold is an indispensable tool for casting the finished plastic head. Before casting, apply a release agent to the inner wall of the master mold to facilitate mold release.
(5) Sulfidation conditions
Room temperature 25°C
Gel time 2h
Curing time 8h
Fourth, the nature of printing plastic head
1. hardness
Hardness characterizes the ability of the rubber head to resist compression deformation. In printing, the softer plastic head can get larger deformation, suitable for printing on substrates with large curvature, while the harder plastic head is less deformed, and the dot enlargement during printing is also small, which is suitable for flat printing or tonal printing Printing. Adjusting the hardness of the rubber head, in addition to adjusting the silicone oil content, the key is to select the correct softener.
2. Tearing strength
Tear of rubber usually occurs from defects or microcracks and then develops to fracture. The tear strength is often related to the crosslinking density, and the tear strength is maximized at the appropriate crosslink density. Excessive cross-linker content tends to reduce tear strength.
3. Fatigue damage
The phenomenon that the structure and performance of the printing rubber head is changed under the action of repeated dynamic compression is called "fatigue". As the fatigue process continues, the silicone rubber becomes loose, the surface cracks, cracks occur, and slowly expands to complete rupture, which is called "fatigue damage."
The fatigue process of silicone rubber includes two stages of crack generation and crack propagation. Under normal circumstances, rubber can be used together with or combined vulcanization to improve the fatigue resistance of silicone rubber.
4. Resistant to organic solvents
Printing ink heads are often exposed to inks and solvents. How solvent resistance will directly affect the service life.
How the interaction between the rubber and the solvent can be expressed by the solubility of the solvent to the silicone rubber. The more similar the chemical structure, the stronger the solvent's ability to dissolve the silicone rubber. Experiments have shown that except benzene and xylene can cause some damage to silicone rubber, other solvents will not affect the silicone rubber.
The solubility of silicone rubber in organic solvents can be expressed by the solubility parameter. The closer the solubility parameters are, the stronger the dissolving power is; the larger the solubility parameter is, the weaker the dissolving ability is. 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 parameter
Silicone Rubber
7.3
Toluene
8.97
Xylene
8.83 to 9.03
Ethyl acetate
9.10
benzene
9.22
Butanone
9.56
acetone
9.74
Ethanol
12.97
As can be seen from Table 1, the solubility parameters of silicone rubber are similar to those of toluene, xylene, and benzene, but they are far from those of butanone, acetone, and ethanol.
For mixed solvents, such as: benzene 35%, toluene 15%, acetone 10%, butanone 40%, have a greater impact on the silicone rubber, immersed in the silicone rubber, the quality changes. Therefore, frequent contact of the rubber head with the above-mentioned mixed solvent should be avoided.
5. Spraying
After the pad printing head is stored for a certain period of time, the phenomenon of blooming occurs, that is, the accelerating agent, anti-aging agent, and filler in the compounding agent migrate from the interior of the rubber compound to the surface and precipitate out. In addition to affecting the appearance of the plastic head, the application of spray cream is also detrimental to the effectiveness of the application. Gel head cream is mainly caused by the excessive use of silicone oil or accelerator. The control method is strictly according to the formula. The content of silicone oil can not be blindly increased to reduce the hardness. Increasing or reducing the amount of vulcanizing agent arbitrarily may also result in oversulfurization or insufficient vulcanization.
First, the characteristics of the printing plastic head should have
1. Good ink absorption and deinking
The ink-absorbing property and the deinking property are contradictory. In fact, it means that the ink layer is diffused from the high-concentration area to the low-concentration area when the ink layer is subjected to the equalization pressure. When ink is poured, the density of the ink at the pad is high, and the ink layer on the surface of the plastic head is low. When the plastic head is compressed and deformed, the concentration of the ink layer distributed between the compressed area of ​​the plastic head and the gravure is redistributed, that is, part of the ink layer. The adhesive force of the plastic head surface is attracted to the surface of the plastic head, and the other part is attracted to the surface of the gravure by the adhesive force of the gravure surface, which is expressed as the ink absorbing property of the plastic head. The contact process between the rubber head and the substrate is mainly deinking. When the rubber head is deformed by pressure, the ink on the surface is separated. A portion of the ink is transferred to the substrate and the other portion is left on the surface of the plastic head.
2. Good resilience
The printing head absorbs ink during deformation and deinks during deformation. The printing plate is flat on the steel plate. If the surface of the printing plate is also flat, it is very important to ensure that the printing plate is not deformed due to the permanent deformation of the printing pad. During the life cycle, slight changes in the print head (non-recoverable) will occur, which will have a serious impact on print quality.
3. Good solvent resistance
The print head must have the ability to resist the various solvents in the ink. Screen printing inks and pad printing inks are generally solvent-based inks with relatively high viscosity. Solvents include solvent gasoline, ethyl acetate, acetone, butyl acetate, methyl ethyl ketone (MEK), and cyclohexanone. Experiments have shown that silicone rubber The pad printing head has good resistance to the above solvents.
Second, printing the production principle of plastic head
1. The rubber vulcanization system is the basis for the rubber industry to make great progress. The pad printing head is made of the principle that the room-temperature vulcanized silicone rubber is cross-linked into a network-like structure under the action of a vulcanizing agent and the like.
Rubber is a hydrocarbon chain of macromolecule chains. Rubber with carbon atoms in the main chain is called carbon chain rubber; rubber with other atoms in the main chain and other atoms is called heterochain rubber; if the main chain contains silicon The rubber of organic groups such as (Si), tin (Sn), and boron (B) is called an elemental organic polymer. Silicone rubber is an elemental organic polymer containing an organosilicon group in the main chain. There are two kinds of silicone rubber vulcanization process, one is high temperature vulcanization, and the corresponding silicone rubber is called high temperature vulcanized silicone rubber, referred to as HTV; the other is room temperature vulcanization, and the corresponding silicone rubber is called room temperature vulcanized silicon. Rubber, referred to as RTV. Room temperature vulcanized silicone rubber is divided into two types, one is additive molding and the other is condensation.
2. Condensed type silicone rubber with terminal hydroxyl group (-OH), easy to shrink with silane containing easily hydrolyzable groups (-OCH3, -OCOCH3), room temperature vulcanization, crosslinking agent is silicic acid ethanol, catalyst is February Silicon Acid dibutyl stannate. Add-molded silicone rubber generally contains -CH=CH, an end group, and is usually crosslinked into an elastomer by a self-addition reaction using a Si-H bond-containing polysiloxane as a crosslinking agent.
3. In the protective system, the printing head is subjected to a combination of heat and repeated mechanical compression during use, resulting in a change in the molecular structure, which causes the plastic head to become viscous, cracked, discolored and gradually lose its performance. Call aging. In order to inhibit and slow down the aging process, a protective system is usually added to the formula of the rubber head.
There is also an aging phenomenon in the rubber head which is oxygen erosion. When it is eroded by 1% to 2.5% of oxygen, it cannot be used. Therefore, vacuum deaeration is one of the keys to the production of silicone heads.
Third, printing plastic head production process
1. Printing pad formula
(1) Silicone Rubber - Various Types of RTV Silicone Rubber
(2) Silicone oil - structural control agent used to improve the hardness and processability of pad printing heads
(3) Dibutyltin Dibenzoate--Accelerator
(4) Ethyl silicate - crosslinker
2. Silicone oil content
The small amount of silicone oil directly affects the hardness of the printing head, but the ratio of silicone oil to silicone rubber has an allowable range. Within this range, the physical properties of the printing head are better. The smaller this range, the smaller the compatibility of silicone oil and silicone rubber; the larger the range, the greater the compatibility. This range can be determined experimentally, that is to say that the ratio is not arbitrary, beyond which the manufactured rubber head cannot be used.
3. Curing agent content
The content of vulcanizing agent is generally controlled within 5% of the mass ratio. If the content of the vulcanizing agent is less than 5%, the silicone rubber cross-linking is insufficient, and the vulcanizate will contain the raw rubber component, showing stickiness; if the vulcanizing agent content is too large, the rubber head becomes excessive. Brittle, low tear strength. To improve the tear strength of the silicone head, add a suitable amount of reinforcing agent such as black and white carbon in the rubber formulation.
4. crafting process
(1) Preparation of glue
According to the proportion of each component in the formula, the volume of the rubber head to be produced is first calculated, and then the mass of each component is roughly calculated and poured into a fixed container.
(2) Stirring
The mixing glue is fully stirred under the mixer to make it evenly mixed to form a stable and consistent color liquid.
(3) Vacuuming
The mixed liquid was placed under a vacuum pump and vacuumed. During the vulcanization of silicone rubber, water vapor will be generated. Since the viscosity of the silicone rubber mixture is large, air is easily mixed during stirring. In order to prevent gas from being mixed into the elastic body, it is indispensable to perform vacuum processing in advance.
(4) Making rubber head model
Different plastic heads should be designed for different shapes and areas of substrates. The design of the plastic head should take into account the exhaust function and the intended deformation process. The designed plastic head is then submitted to the machining department for making the aluminum alloy model, and then the mother mold is cast with the unsaturated polyester resin. The master mold is an indispensable tool for casting the finished plastic head. Before casting, apply a release agent to the inner wall of the master mold to facilitate mold release.
(5) Sulfidation conditions
Room temperature 25°C
Gel time 2h
Curing time 8h
Fourth, the nature of printing plastic head
1. hardness
Hardness characterizes the ability of the rubber head to resist compression deformation. In printing, the softer plastic head can get larger deformation, suitable for printing on substrates with large curvature, while the harder plastic head is less deformed, and the dot enlargement during printing is also small, which is suitable for flat printing or tonal printing Printing. Adjusting the hardness of the rubber head, in addition to adjusting the silicone oil content, the key is to select the correct softener.
2. Tearing strength
Tear of rubber usually occurs from defects or microcracks and then develops to fracture. The tear strength is often related to the crosslinking density, and the tear strength is maximized at the appropriate crosslink density. Excessive cross-linker content tends to reduce tear strength.
3. Fatigue damage
The phenomenon that the structure and performance of the printing rubber head is changed under the action of repeated dynamic compression is called "fatigue". As the fatigue process continues, the silicone rubber becomes loose, the surface cracks, cracks occur, and slowly expands to complete rupture, which is called "fatigue damage."
The fatigue process of silicone rubber includes two stages of crack generation and crack propagation. Under normal circumstances, rubber can be used together with or combined vulcanization to improve the fatigue resistance of silicone rubber.
4. Resistant to organic solvents
Printing ink heads are often exposed to inks and solvents. How solvent resistance will directly affect the service life.
How the interaction between the rubber and the solvent can be expressed by the solubility of the solvent to the silicone rubber. The more similar the chemical structure, the stronger the solvent's ability to dissolve the silicone rubber. Experiments have shown that except benzene and xylene can cause some damage to silicone rubber, other solvents will not affect the silicone rubber.
The solubility of silicone rubber in organic solvents can be expressed by the solubility parameter. The closer the solubility parameters are, the stronger the dissolving power is; the larger the solubility parameter is, the weaker the dissolving ability is. 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 parameter
Silicone Rubber
7.3
Toluene
8.97
Xylene
8.83 to 9.03
Ethyl acetate
9.10
benzene
9.22
Butanone
9.56
acetone
9.74
Ethanol
12.97
As can be seen from Table 1, the solubility parameters of silicone rubber are similar to those of toluene, xylene, and benzene, but they are far from those of butanone, acetone, and ethanol.
For mixed solvents, such as: benzene 35%, toluene 15%, acetone 10%, butanone 40%, have a greater impact on the silicone rubber, immersed in the silicone rubber, the quality changes. Therefore, frequent contact of the rubber head with the above-mentioned mixed solvent should be avoided.
5. Spraying
After the pad printing head is stored for a certain period of time, the phenomenon of blooming occurs, that is, the accelerating agent, anti-aging agent, and filler in the compounding agent migrate from the interior of the rubber compound to the surface and precipitate out. In addition to affecting the appearance of the plastic head, the application of spray cream is also detrimental to the effectiveness of the application. Gel head cream is mainly caused by the excessive use of silicone oil or accelerator. The control method is strictly according to the formula. The content of silicone oil can not be blindly increased to reduce the hardness. Increasing or reducing the amount of vulcanizing agent arbitrarily may also result in oversulfurization or insufficient vulcanization.
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