Localized burning or scorching is a relatively serious defect in liquid silicone rubber injection molding. Black or dark brown scorched spots appearing locally on the product not only affect appearance but may also degrade mechanical properties in that area or even render the product unusable. Scorching typically occurs in poorly vented areas or at flow ends, and its formation mechanism is closely related to the adiabatic compression of air within the cavity.

Poor Venting: The Primary Cause of Scorching

When air within the cavity cannot be smoothly expelled, this trapped air is rapidly compressed under the driving force of injection pressure. According to thermodynamic principles, gas temperature rises sharply during adiabatic compression, and localized temperatures may far exceed the decomposition temperature of silicone, causing scorching.

Insufficient vent groove depth or improper positioning is the direct cause of poor venting. The vent groove depth for LSR injection molding should typically be controlled between 0.005 and 0.015 mm, a precision requirement that demands special attention during mold machining. Vent grooves should be positioned at the end of melt flow and at the convergence points of two melt streams, as these locations are most prone to air entrapment.

For deep-cavity products or complex geometries, adding vent inserts or employing vacuum-assisted venting systems is recommended. Vacuum-assisted venting can extract air from the cavity after mold closing and before injection, fundamentally eliminating air entrapment conditions.

Mold Temperature and Injection Speed

Excessively high localized mold temperature causes silicone in that area to scorch prematurely. Using thermocouples for multi-point temperature monitoring of the mold to ensure uniform temperature distribution, with temperature variation controlled within ±2°C, helps avoid localized overheating.

When injection speed is too fast, shear heat is generated at the melt front, raising localized temperatures. This shear heating effect is particularly pronounced near gates and in narrow runner areas. Optimizing the injection speed profile to appropriately reduce speed in areas prone to scorching can reduce shear heat generation.

Material Factors and Troubleshooting Recommendations

Differences in LSR batch consistency or material contamination may also cause scorching. Slight differences in vulcanization characteristics may exist between batches. Selecting stable, reputable suppliers and implementing proper incoming material inspection are effective measures to reduce such problems.

Troubleshooting scorching issues should prioritize the venting system. Check whether vent grooves are blocked, whether depth meets design requirements, and whether positioning covers all flow ends. If the venting system is normal, then check mold temperature uniformity and injection speed settings. For molds with recurring scorching, it is recommended to use mold flow analysis software to simulate the filling process, identify air entrapment locations, and optimize venting design.

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