In the manufacturing process of insulation pads, ensuring the absence of internal air bubbles is crucial for guaranteeing their electrical insulation performance and mechanical reliability. The presence of air bubbles reduces the insulation pad's withstand voltage, increases the risk of partial discharge, and can even lead to insulation breakdown during long-term use. Therefore, every step from raw material selection to molding process must be strictly controlled to eliminate the risk of air bubbles.
The purity and drying treatment of raw materials are fundamental to preventing air bubbles. The moisture content of rubber base materials (such as natural rubber and nitrile rubber) and fillers (such as carbon black and silica) must be strictly controlled. If the raw materials are highly hygroscopic, they must be dried before mixing. For example, silica can be hot-air dried at 120°C for 4 hours to reduce its moisture content to below 0.2%. Furthermore, plasticizers, vulcanizing agents, and other additives should be selected for their low volatility to avoid decomposition and gas generation during high-temperature vulcanization. For example, when aromatic oils are used as softeners, their dosage must be controlled within a reasonable range to prevent volatilization and the formation of air bubble nuclei at high temperatures.
Optimizing the mixing process can significantly reduce residual gas in the rubber compound. During the mixing process, rubber and additives need to be thoroughly mixed, but excessive shearing can lead to increased temperature and accelerated volatilization of low-molecular-weight substances. Therefore, a staged mixing method is required: the initial stage involves low-temperature mixing at 100-120℃ to avoid overheating from shearing; the temperature before discharge is controlled below 140℃ to prevent scorching of the rubber compound. Simultaneously, open-type mixing mills require 2-3 venting operations, with the gas release effect enhanced by adjusting the roll gap (1.2-1.5mm). For highly volatile systems, a negative pressure venting device can be configured to create a vacuum environment above -0.08MPa within the mixing chamber, accelerating gas escape.
Calendarization is a crucial step in eliminating air bubbles. The mixed rubber compound needs to be extruded through a three- or four-roll press to squeeze out internal air. During this process, the roll temperature needs precise control: if the temperature is too high, the rubber compound's fluidity increases, but surface oxidation is easily caused; if the temperature is too low, the fluidity is insufficient, making it difficult for gas to escape. Typically, the roller temperature is set between 80-100℃, coupled with an appropriate roller speed ratio (1:1.2-1:1.5) to ensure uniform film thickness and no air bubbles. After calendering, the film needs to be allowed to stand for a period of time to allow residual gas to further diffuse, preventing it from directly entering the vulcanization process.
The parameters of the vulcanization process directly affect the final formation of air bubbles. Vulcanization temperature, pressure, and time are key factors: excessively high temperatures can cause low-molecular-weight substances to volatilize, while excessively low temperatures can lead to incomplete vulcanization, both of which can cause air bubbles. For example, during compression molding vulcanization, the temperature is generally controlled between 140-170℃, and the pressure is adjusted to 10-20MPa depending on the gasket thickness. In the initial stage of vulcanization, the temperature needs to be increased slowly (≤3℃/min), especially in the 90-130℃ stage, to avoid localized overheating that could cause gas expansion. The vulcanization time needs to be determined based on the rubber compound formulation and gasket thickness to ensure complete cross-linking reaction and prevent under- or over-vulcanization.
Mold design and venting structure are essential for physically eliminating air bubbles. Venting grooves, typically 0.03-0.05 mm wide and 0.02-0.04 mm deep, are required on the mold parting surface to guide gas diffusion towards the mold edge. For thick products, venting pins or holes can be added to the mold cavity to expel air bubbles using the gas pressure difference during vulcanization. Furthermore, the mold temperature must be uniform, with a temperature difference controlled within ±3℃ to prevent localized high temperatures from causing premature vulcanization of the rubber compound and the formation of air bubbles.
Depressurization and degassing processes can further eliminate residual air bubbles. During vulcanization, when the rubber compound's fluidity decreases but it is not yet fully cured, brief depressurization (1-3 times) can be performed to reduce the pressure within the mold cavity, allowing the gas to expand and escape. After depressurization, repressurization is necessary to ensure the gasket shape remains stable. For complex structures or thick products, a vacuum molding process can be used: vacuuming time of 3-8 seconds, with a vacuum level not lower than -0.085 MPa, allows the rubber compound to fill the mold under negative pressure, reducing gas encapsulation.
Finished product inspection and post-processing are the last line of defense in quality control. After vulcanization, the insulation pads undergo visual inspection to remove any products with surface or internal air bubbles. Additionally, ultrasonic testing or X-ray imaging can be used to detect the presence of tiny air bubbles. For qualified products, post-processing such as trimming and polishing is required to remove burrs and flash, preventing edge air bubbles from affecting usability. Finally, the finished product must pass electrical performance tests, including withstand voltage testing and partial discharge testing, to ensure there are no insulation defects caused by air bubbles.
