Bubbles and voids are among the most common defects in liquid silicone rubber (LSR) injection molding. The presence of bubbles inside or on the surface of molded parts not only affects appearance but may also impair sealing performance and mechanical strength. Troubleshooting this issue requires systematic analysis from three dimensions: raw materials, mold, and process.

Raw Material Dimension: Control of Degassing and Conveying

The A/B components of LSR are highly prone to entraining air during mixing and conveying. If degassing is inadequate, air bubbles will be carried directly into the mold, ultimately forming voids in the molded parts. Industry technical documentation recommends continuous vacuum degassing of A/B material drums, with vacuum levels controlled at -0.08 to -0.1 MPa. Degassing time generally requires 5–10 minutes, and the material should be used as soon as possible after degassing to avoid air re-dissolving into the material during prolonged standing.

The sealing integrity of the feed lines is equally critical. Any minor leak point can become a pathway for air to enter the system. When bubbles appear in molded parts, the connection points and seal condition of the metering and mixing system should be checked first.

Mold Dimension: The Core Role of Venting Design

Poor mold venting is the primary cause of bubble formation. During LSR injection, air within the cavity must be smoothly expelled. Technical documentation indicates that vent grooves should be positioned at melt convergence points and at the product's terminal ends, with depth typically controlled between 0.05 and 0.1 mm. For deep-cavity products or complex geometries, adding ejector pin vents or designing vent inserts is recommended.

Vacuum-assisted venting systems can further enhance venting effectiveness, but it should be noted that vacuum effectiveness depends on coordinated control of parting surface sealing, vent positioning, and runner balance.

Process Dimension: Balancing Injection Speed and Curing Window

Injection speed settings require weighing two mutually conflicting factors. Injecting too fast will entrain air and create turbulence—the ideal flow state for LSR is laminar flow; injecting too slow may cause curing to begin before filling is complete, sealing off venting pathways.

Excessively fast curing can also exacerbate bubble problems. If the surface layer cures first and seals off the venting channels, internal volatiles or residual gases will be unable to escape. Adjusting vulcanization temperature and time to avoid overly rapid curing is key to resolving this type of bubble issue.

Systematic Troubleshooting Recommendations

For persistent bubble problems, it is recommended to follow the sequence of "first confirm the pattern, then investigate materials, then check the mold, and finally adjust the process." If bubbles are concentrated in a specific cavity, prioritize checking that cavity's venting and runners; if bubbles appear across all cavities, start troubleshooting from the feed system and injection parameters.

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