1. The formula of the pad printing head
(1) Silicone rubber-various types of room temperature vulcanized silicone rubber
(2) Silicone oil-structure control agent used to improve the hardness and processing performance of pad printing rubber heads
(3) Dibutyl tin moongear acid-accelerator
(4) Ethyl silicate——crosslinking agent
2. Silicone oil content
The content of silicone oil directly affects the hardness of the pad printing head, but the ratio of silicone oil to silicone rubber has an allowable range. Within this range, the pad printing head has better physical properties. The smaller the range, the smaller the compatibility of silicone oil and silicone rubber; the larger the range, the greater the compatibility. This range can be determined through experiments, that is to say, this ratio is not arbitrary. If it exceeds this ratio, the rubber head produced cannot be used.
3. The content of vulcanizing agent
The content of the vulcanizing agent is generally controlled within 5% by mass. If the content of the vulcanizing agent is too small, the cross-linking of the silicone rubber will be insufficient, and the vulcanized rubber will contain raw rubber components, which will be sticky; if the content of the vulcanizing agent is too large, the rubber head will become It is brittle and has low tear strength. To improve the tear resistance of the silicone head, an appropriate amount of reinforcing agent such as black and white carbon black can be added to the rubber formula.
4. crafting process
(1) Preparation of glue solution
According to the ratio of each component in the formula,
First calculate the volume of the rubber head to be made,
Then roughly calculate the mass of each component,
And pour them into fixed containers.
Fully stir the mixed glue liquid under a mixer to make it evenly mixed to form a stable and consistent liquid.
Place the mixed liquid under a vacuum pump and vacuumize. Water vapor will be generated during the vulcanization process of silicone rubber, and due to the high viscosity of the silicone rubber mixture, air is easily mixed during stirring. In order to prevent the gas from mixing into the elastomer, it is necessary to vacuumize in advance.
(4) Make plastic head model
Corresponding plastic heads should be designed for different substrate shapes and areas. The design of the rubber head should consider the exhaust function and the expected deformation treatment. The designed plastic head is then handed over to the machining department to make an 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 must 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
The nature of the pad printing rubber head
Hardness characterizes the rubber head's ability to resist compression deformation. In printing, the softer rubber head can obtain greater deformation and is suitable for printing on substrates with large curvature; while the harder rubber head has less deformation and small dot expansion during printing, which is suitable for flat prints or tone prints Printing. To adjust the hardness of the rubber head, in addition to adjusting the content of silicone oil, the key is to select the correct softener.
2. Tear strength
Rubber tears usually occur from defects or microcracks, and then progress to fracture. The tearing strength is often related to the crosslinking density, and the tearing strength is high at an appropriate crosslinking density. If the content of cross-linking agent is too much, the tear strength is often reduced.
3. Fatigue damage resistance
The phenomenon that the structure and performance of the pad printing rubber head undergo repeated dynamic compression is called "fatigue". With the continuation of the fatigue process, the silicone rubber becomes relaxed, the surface is cracked, cracks appear, and slowly expand to complete rupture, which is called "fatigue failure".
The fatigue process of silicone rubber includes two stages: crack formation and crack expansion. Under normal circumstances, the use of rubber or combined vulcanization methods can be used to improve the fatigue resistance of silicone rubber.
4. Resistant to organic solvents
Pad printing rubber heads often come into contact with ink and solvents, and the solvent resistance will directly affect their 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 silicone rubber. Experiments have proved that except for benzene and xylene which 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; the greater the difference in 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 parameter
Silicone rubber 7.3
Ethyl acetate 9.10
It can be seen from Table 1 that the solubility parameters of silicone rubber are close to that of toluene, xylene, and benzene, but are far from methyl ethyl ketone, acetone, and ethanol.
For mixed solvents, such as: benzene 35%, toluene 15%, acetone 10%, and methyl ethyl ketone 40%, it will have a greater impact on the silicone rubber. When the silicone rubber is immersed in it, the quality will change. Therefore, avoid frequent contact between the rubber head and the above mixed solvent.
5. Spray frost
After the pad printing rubber head is stored for a certain period of time, blooming occurs, that is, the accelerator, anti-aging agent, and filler in the compounding agent migrate from the inside of the rubber compound to the surface and precipitate out. In addition to affecting the appearance quality of the rubber head, frosting also impairs the use effect. The frosting of the rubber head is mainly caused by the excessive amount of silicone oil or accelerator. The control method is to strictly follow the formula, and the content of silicone oil cannot be blindly increased to reduce the hardness. Randomly increasing or reducing the amount of vulcanizing agent can also lead to oversulfur or insufficient vulcanization.