Analysis of the Role of Pressure Transmission in Vulcanizing Machines
Pressure transmission is one of the core links for the vulcanizing machine to achieve high-quality vulcanization of rubber products, directly affecting the bonding strength between rubber and reinforcing materials, the density of the products and the uniformity of vulcanization. Its mechanism of action involves mechanical force transmission, regulation of rubber rheological behavior and optimization of vulcanization reaction kinetics. The following is an analysis of the physical process, key roles and optimization directions of pressure conduction.
First, the physical process of pressure conduction
Pressure sources and transmission paths
Pressure source: The vulcanizing machine provides initial pressure (typically 10-30 MPa, depending on the type of product) through a hydraulic system (such as a cylinder) or a mechanical pressurization device (such as a screw).
Transmission path: The pressure is transferred from the pressurizing device to the heating plate, and then evenly acts on the rubber product through the mold or vulcanization medium (such as vulcanized fabric).
Key link: The pressure transmission needs to overcome the deformation resistance of the heating plate, the mold and the rubber itself to ensure that the final pressure acting on the rubber meets the process requirements.
The uniformity of pressure distribution
Influencing factors: The flatness of the heating plate, mold design, rubber thickness and the stiffness of the reinforcing material (such as steel wire rope, canvas).
Case: During the vulcanization of conveyor belts, if the heating plate is locally deformed or the mold gap is uneven, it will lead to pressure distribution deviation (such as high center pressure and low edge pressure), causing uneven vulcanization of rubber or bonding failure.
Second, the core role of pressure transmission
Promote the adhesion between rubber and reinforcing materials
Physical penetration: Pressure forces rubber to penetrate the pores or fiber gaps of the reinforcing material, forming mechanical interlocking. For instance, in the vulcanization of steel cord conveyor belts, pressure causes the rubber to wrap around the surface of the steel cord, enhancing the peel strength.
Chemical effect: Pressure promotes the contact between the active groups on the surface of the rubber and the reinforcing material, accelerates the formation of cross-linking bonds during the vulcanization reaction, and enhances the interfacial adhesion.
Remove bubbles and impurities
Gas escape: During the vulcanization process of rubber, low-molecular-weight volatile substances (such as vulcanization by-products) may be released. Pressure can compress the volume of rubber, forcing the gas to be discharged through the exhaust holes of the mold.
Impurity compaction: Pressure compacts the particulate impurities in rubber (such as carbon black aggregates), reducing internal defects in the product and enhancing the surface finish.
Control the thickness and density of rubber vulcanization
Thickness consistency: Pressure restricts the flow space of rubber to ensure uniform thickness of the product after vulcanization. For instance, during the vulcanization of thick rubber products, if the pressure is insufficient, the rubber may expand, resulting in an excess thickness.
Density optimization: Pressure increases the contact probability between rubber molecular chains, enhances the crosslinking density, and thereby improves the physical properties such as hardness and wear resistance of the product.
Regulate the kinetics of the vulcanization reaction
Reduced activation energy: Pressure can change the conformation of rubber molecules, reduce the activation energy of the vulcanization reaction, and accelerate the crosslinking reaction rate. For instance, under high-pressure conditions (such as 20 MPa), the vulcanization speed of natural rubber may increase by 10% to 15%.
Reaction path optimization: Pressure inhibits the isomerization or breakage of cross-linked bonds, promotes the formation of a more stable cross-linked structure, and extends the service life of the product.
Third, common problems and impacts of pressure conduction
Insufficient pressure
Manifestations: Low bonding strength between rubber and reinforcing material, excessive thickness of the product, and residual bubbles.
Case: During tire vulcanization, if the sidewall pressure is insufficient, it may cause the steel cord and rubber to delaminate, leading to premature tire damage.
Uneven pressure distribution
Performance: Insufficient or excessive vulcanization of the product in some areas, resulting in inconsistent performance.
Detection method: Through ultrasonic flaw detection or hardness testing, density differences caused by uneven pressure can be detected.
Pressure fluctuation
Performance: Sudden pressure changes during the vulcanization process (such as leakage in the hydraulic system) cause fluctuations in the crosslinking density of rubber.
Consequence: The performance of the products is unstable, such as the strength of the conveyor belt joints fluctuating.
Fourth, the optimization direction of pressure transmission
Improvement of the heating plate and mold design
High rigidity structure: The heating plate is made of thick-walled steel plates or alloy materials to reduce deformation. For instance, increasing the thickness of the heating plate from 30 mm to 50 mm can reduce the pressure transmission loss by 15% to 20%.
Flow channel optimization: Design flow guide channels or exhaust holes in the mold to ensure uniform pressure transmission and facilitate gas discharge.
Upgrade of the pressure control system
Closed-loop control: The vulcanization pressure is monitored in real time by a pressure sensor, and the output of the hydraulic system is dynamically adjusted in combination with the PID algorithm to keep the pressure fluctuation within ±0.5 MPa.
Preloading and holding pressure process: Preloading (such as 5 MPa) is applied at the initial stage of vulcanization to expel gas, and then the pressure is raised to the working pressure for holding pressure to prevent rubber rupture caused by pressure shock.
Collaborative optimization of rubber formulations and processes
Fluidity regulation: By adjusting the filling system of rubber (such as the amount of carbon black) or the type of plasticizer, the fluidity of rubber under high pressure is improved and internal stress is reduced.
Vulcanization temperature matching: Increasing the vulcanization temperature can reduce the viscosity of rubber and decrease the pressure requirement. For instance, raising the vulcanization temperature from 150℃ to 160℃ may reduce the required pressure by 5% to 8%.
New pressure conduction technology
Isostatic pressing technology: Isotropic pressure is applied around rubber products (such as the fluid pressure in a rubber vulcanizing tank) to completely eliminate the pressure gradient.
Ultrasonic assistance: By means of ultrasonic vibration, the viscosity of rubber is reduced, the pressure requirement is decreased, and at the same time, the gas discharge is promoted.
Fifth, industry application cases of pressure transmission
Vulcanization of conveyor belts
Requirement: Ensure the bonding strength between the steel wire rope core and the rubber.
Key points of pressure control:
At the initial stage of vulcanization, apply a pressure of 10 MPa to compact the rubber, and then increase the pressure to 15-20 MPa to maintain it, ensuring that the steel wire rope is completely wrapped.
The design of the mold exhaust holes avoids gas residue caused by pressure.
Tire vulcanization
Requirement: Balance the pressure distribution on the tread, sidewall and bead.
Key points of pressure control:
When using a double-mold vulcanizing machine, the pressure of the upper and lower molds is independently controlled to compensate for the difference in thermal expansion of the molds.
The mold runner is optimized through finite element analysis (FEA) to ensure uniform pressure on the sidewall of the tire.
Vulcanization of seals
Requirement: Control the permanent compression set of rubber.
Key points of pressure control:
The vulcanization pressure needs to be adjusted according to the hardness of the rubber (for example, 15 MPa is required for Shore A 70 rubber and 20 MPa for Shore A 90 rubber).
During the pressure-holding stage, release the pressure slowly to avoid dimensional shrinkage caused by stress relaxation.