Flash, also known as spew or burrs, is one of the most common defects in liquid silicone rubber injection molding. When thin layers of silicone extend beyond the designed contour at parting surfaces, ejector pin holes, or slider gaps, it not only increases the labor cost of deflashing but may also affect assembly precision and product appearance. The causes of flash involve three dimensions—mold precision, process parameters, and material characteristics—requiring systematic troubleshooting.

Mold Factors: The Primary Cause of Flash Formation

The machining precision and condition of the parting surface are the first line of defense determining whether flash occurs. Insufficient flatness, wear damage, or residual foreign matter on the parting surface will prevent the upper and lower molds from fully conforming, providing a pathway for silicone overflow. For molds in long-term use, wear at the parting surface edges warrants particular attention and requires regular inspection and timely repair.

Vent groove design is equally critical. In LSR injection molding, vent grooves serve to expel air from the cavity, but their depth must be precisely controlled. Industry technical documentation indicates that LSR vent groove depth should typically be controlled between 0.005 and 0.015 mm. Too shallow results in poor venting, while too deep directly becomes a spew pathway. This precision requirement is far higher than that for conventional thermoplastic injection molding vent groove design.

The fit clearance of moving components such as ejector pins and sliders is another easily overlooked spew pathway. LSR has extremely low viscosity and strong penetrability, and the clearance of moving components should typically be controlled below 0.02 mm; otherwise, silicone will squeeze out through these gaps.

Process Factors: The Balance Between Pressure and Temperature

Excessively high injection pressure is the most direct process cause of flash. Excessive injection pressure forces silicone into all small gaps, including areas that would not spew under normal pressure. Insufficient clamping force also causes problems—when the in-mold pressure generated by injection pressure exceeds the clamping force, the mold is slightly forced open, creating a dynamic spew pathway.

Excessively high material temperature and mold temperature significantly reduce silicone viscosity, making it easier to penetrate small gaps. Under the premise of ensuring vulcanization quality, appropriately lowering material temperature and mold temperature helps reduce flash.

Material Factors and Troubleshooting Recommendations

When selecting LSR grades with excessively low viscosity, the material itself has extremely strong penetrability, and the requirements for mold sealing performance increase accordingly. If the same mold uses materials of different viscosities, flash behavior may differ significantly.

When troubleshooting flash issues, it is recommended to proceed in the following order: first clean and inspect the mold parting surface, confirming no damage or foreign matter; then reduce injection pressure and speed, observing whether flash improves; next increase clamping force; finally fine-tune material temperature and mold temperature. If none of the above adjustments are effective, mold repair or material grade replacement should be considered.

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