For precision LSR products, dimensional stability is a core quality indicator. Dimensional fluctuations within the same batch exceeding tolerance not only cause assembly difficulties but may also result in batch scrapping. Dimensional issues in LSR involve multiple aspects including vulcanization control, metering accuracy, and mold design.

Degree of Vulcanization: The Decisive Factor in Shrinkage Rate

When LSR is heated and vulcanized in the mold, its dimensions undergo a process from expansion to contraction. Due to the high thermal expansion coefficient of LSR, it expands when heated and undergoes slight shrinkage upon cooling, typically with an additional 2% to 4% shrinkage after demolding. The higher the vulcanization temperature, the greater the shrinkage rate generally is; inadequate vulcanization leads to continued shrinkage afterward, with dimensions changing over time.

The key control point is to strictly stabilize vulcanization temperature (controlled within ±1°C) and time to ensure the crosslinking reaction is complete and consistent. For products requiring secondary vulcanization, an additional 0.5% to 0.7% shrinkage must be factored into mold design considerations.

Metering Accuracy: Ensuring Injection Volume Consistency

The A/B components of LSR are precisely mixed at a 1:1 ratio, and the stability of injection volume directly affects product weight and dimensions. Inaccurate metering leads to underfilling or overfilling of some cavities, manifesting as undersized or oversized dimensions.

Regularly calibrating the metering system and checking the sealing integrity of feed lines and the working condition of the static mixer are fundamental to maintaining metering accuracy. The reliability of "shut-off pins" or "needle valves" in the cold runner system is also critical, as they control the accurate metering of material in each cycle.

Mold Design: Coordination of Cold Runner and Thermal Expansion

LSR injection molding typically employs a cold runner system, keeping the material in a low-temperature flowable state before it enters the hot mold cavity. Cold runner design must balance two requirements: maintaining material fluidity to avoid premature curing, while ensuring precise and controllable injection volume. Closed cold runner systems control flow through needle valves, achieving precise metering at lower injection pressures; open systems utilize high shear rates for flow shut-off.

Mold temperature uniformity is equally important for dimensional stability. Local temperature differences lead to inconsistent vulcanization degrees, which in turn cause warpage and dimensional deviations. It is recommended to use multi-point mold temperature control and regularly check the working condition of heating rods.

Comprehensive Control Strategy

The core of ensuring dimensional stability in LSR products lies in "strictly locking down curing process parameters." Once parameters such as mold temperature, pressure, injection volume, and vulcanization time are confirmed, they should not be arbitrarily changed. Establishing a well-developed work instruction process, measuring and recording the dimensions of the first article, and conducting regular subsequent sampling comparisons are effective means of maintaining batch consistency.

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