During the processing of silicone straps, the formation of air bubbles can severely affect their appearance and performance, leading to surface defects, reduced structural strength, and even impacting wearing comfort. To avoid this problem, comprehensive control is needed in all aspects, including raw material selection, mixing process, molding control, mold design, vacuum treatment, environmental management, and post-processing, to ensure stable quality of the silicone straps.
Raw material selection is fundamental to avoiding air bubbles. Silicone straps typically use liquid silicone rubber (LSR) or high-density silicone rubber (HTV) as the main raw material, requiring assurance of purity and dryness. If the moisture content in the raw material is too high, water evaporation during vulcanization will form air bubbles. Therefore, raw materials with low moisture content must be selected, and the filler material must be dried before mixing to prevent hygroscopic materials from introducing moisture. Furthermore, additives such as colorants and vulcanizing agents must also be kept dry to prevent impurities from contaminating and causing air bubbles.
The mixing process directly affects the uniformity of the silicone. Solid silicone requires plasticizing using an open mill or internal mixer before adding vulcanizing agents and colorants for thorough mixing. If the mixing is insufficient, uneven distribution of components can easily lead to air bubbles during vulcanization due to inconsistent reactions. Liquid silicone requires mixing with the curing agent in a specific ratio using a metering pump; deviations in the mixing ratio or uneven stirring will also introduce air. Therefore, it is necessary to control the stirring speed and time during mixing, using alternating stirring or low-speed stirring to reduce air incorporation, and extending the settling time to allow air bubbles to rise and dissipate naturally.
The molding process is a crucial aspect of bubble control. Silicone straps can be molded using injection molding, compression molding, and extrusion molding. Injection molding is suitable for liquid silicone and requires controlling the injection speed and pressure to avoid turbulence that could trap air. Compression molding requires proper setting of vulcanization temperature and time; if the temperature is too high or the time is insufficient, premature curing of the surface silicone can trap internal air. Extrusion molding requires optimizing the extrusion speed and cooling method to prevent air bubbles from forming due to temperature changes. Regardless of the method, it is essential to ensure that the silicone is uniformly heated and pressurized in the mold to avoid incomplete local vulcanization.
Mold design is critical for air bubble removal. The mold must have appropriate venting channels and pores to ensure that air can escape smoothly when the mold is closed. The depth and number of venting grooves need to be adjusted according to the fluidity of the silicone. Insufficient venting will cause air to be compressed within the mold cavity, forming bubbles. Furthermore, the mold surface must be kept smooth to prevent burrs or scratches from obstructing silicone flow and causing bubbles. For complex structures, a parting mold design or the addition of overflow grooves can reduce air retention.
Vacuum treatment is an effective means of removing bubbles. Before injecting silicone into the mold, vacuum degassing is necessary to reduce ambient air pressure and allow bubbles to expand and escape. Vacuum equipment should be selected that effectively reduces air pressure, and the degassing time must be controlled to avoid leaving bubbles due to insufficient time or introducing new bubbles due to excessive time. For liquid silicone, mixing can be performed under vacuum to further reduce air ingress.
Environmental factors also affect bubble formation. The humidity, temperature, and airflow in the processing workshop must be controlled within appropriate ranges. High humidity can cause raw materials to absorb moisture, increasing the risk of bubbles; excessively low temperatures will increase the viscosity of the silicone, making it difficult to expel bubbles. Therefore, the workshop must be kept dry and well-ventilated, and the ambient temperature must be controlled during mixing and molding to ensure the silicone is in optimal processing condition.
Post-processing can further eliminate air bubbles. The molded silicone strap needs to be trimmed, deburred, and inspected for any remaining air bubbles. Small air bubbles can be eliminated through secondary vulcanization or heat treatment to shrink them away; larger air bubbles require rework or scrapping. Furthermore, surface treatments such as spray painting and laser engraving must be performed after all air bubbles have been eliminated to avoid problems caused by processing covering them up.
