The surface quality of liquid silicone rubber products directly affects appearance and customer acceptance. Flow marks, sink marks, and uneven gloss are three common types of surface defects, with causes primarily related to melt flow conditions and mold temperature control.

Flow Marks: Cooling Traces at the Melt Front

Flow marks appear as water-ripple-like traces or localized gloss inconsistencies on the product surface. The root cause is that the melt front contacts the cooler mold wall during filling, causing a rapid temperature drop and reduced fluidity, which forms ripple-like traces under the pushing action of subsequent melt.

Process-level countermeasures include appropriately increasing mold temperature to enhance melt fluidity, and increasing injection pressure and holding pressure to promote uniform melt spreading. At the raw material level, selecting vinyl silicone oil with a narrow molecular weight distribution and moderate fluidity, as well as optimizing the molar ratio of vinyl to silicon-hydrogen groups, helps improve leveling performance.

Sink Marks: Insufficient Holding Pressure and Uneven Cooling

Sink marks typically appear in thick-wall areas as surface depressions. Their cause is volumetric shrinkage of the product during cooling, with holding pressure insufficient to compensate for this shrinkage. The shrinkage rate of LSR is generally between 2% and 4%, depending on formulation and vulcanization conditions.

The key to resolving sink marks lies in applying sufficient holding pressure and ensuring that holding time covers the stage before gate sealing. Adopting a "slow-fast-slow" multi-stage injection speed profile also helps reduce sink marks: passing through the gate quickly, then filling at a uniform rate, and decelerating at the end to allow the melt time to compact.

Uneven Gloss: The Combined Effect of Vulcanization and Mold Surface

Uneven surface gloss on products may be caused by differences in vulcanization degree or mold surface condition. Areas with poor vulcanization often exhibit lower gloss; rough mold surfaces or localized contamination will also be reflected in product appearance.

Improvement directions include: extending vulcanization time or increasing mold temperature to ensure adequate vulcanization; grinding the mold and applying hard chrome plating; and controlling the amount and uniformity of mold release agent application.

Comprehensive Process Control Recommendations

Surface defects are often the result of multiple factors acting in combination. When troubleshooting, it is recommended to first fix key parameters such as mold temperature, injection speed, and holding pressure, then change variables one at a time and record the effects. For new molds or new products, conducting mold flow analysis can predict filling behavior and potential surface defect risks before cutting the mold.

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