From mold to product: Key control points for the entire process of precision injection molding of liquid silicone (LSR)

Precision injection molding of liquid silicone rubber (LSR) is a key technology for manufacturing highly reliable micro-seals, medical device components, and consumer electronics parts. The process is not a simple "injection molding," but a systematic engineering project that begins with mold design, permeates every process stage, and culminates in a stable finished product. This article will systematically analyze the core control points in this process, clarifying how to achieve product consistency and reliability through precise control throughout the entire process.

Liquid Silicone Rubber Mold
I. Front-end Foundation: Precision Mold Design and Manufacturing

The mold is the foundation of LSR molding, and its quality directly determines the precision and appearance of the final product.

1. Parting Surface and Venting System Design

The design of the parting surface must ensure smooth demolding of the product and minimize flash. Given the excellent flowability of LSR, the parting surface requires extremely high precision. Simultaneously, a sufficient and precise venting system must be designed. Venting channels with a depth of 0.001-0.003 inches are typically used to allow air to escape smoothly, preventing material shortages, scorching, or internal air bubbles caused by trapped air. This is especially critical for thin-walled and structurally complex products.

2. Cold Runner System and Temperature Control

To reduce material waste, LSR injection molding widely employs a cold runner system. The runner design must ensure minimal pressure loss, balanced filling, and maintain a suitable temperature (typically cooled) for the LSR within the runner during injection intervals to prevent premature vulcanization. The mold heating system must be highly uniform and stable, typically using a high-precision hot oil mold temperature controller or multi-point independently controlled heating rods to maintain the mold temperature within ±2°C of the set value. This is a prerequisite for ensuring a consistent vulcanization reaction.

3. Surface Treatment and Detail Finishing

The mold steel surfaces in contact with the LSR usually require high polishing or plating (such as nickel-PTFE plating) to achieve excellent demolding results and replicate high-gloss or specific textured product surfaces. For microstructures (such as micropores and thin ribs), precision micromachining techniques (such as micro-EDM) are required to ensure dimensional and shape accuracy.

II. Core Process: Precision Control of Injection Molding Parameters

1. Accuracy of Metering and Mixing

LSR is a two-component material. Components A and B must be continuously and consistently delivered in a 1:1 ratio using a high-precision gear pump and mixed absolutely uniformly in a static mixer. Even slight deviations in the metering ratio or uneven mixing will directly lead to poor local curing or decreased physical properties of the product. This process must be fully automated and stably monitored.

2. Dynamic Control During Injection

Temperature: The barrel must be effectively cooled (typically 20-40°C) to prevent pre-curing of the compound before injection.

Speed ​​and Pressure: Multi-stage injection control is employed. Initial high-speed injection quickly fills the main cavity, transitioning to low speed towards the end of the filling and when encapsulating inserts to ensure adequate venting and avoid air entrapment, and to protect fragile inserts. Injection pressure is typically significantly lower than that used in thermoplastic injection molding.

**Positioning and Switching:** The switching point from injection to holding pressure (VP switching) should be based on precise screw position or mold cavity pressure, which is crucial for controlling product size and weight.

3. **Scientific Approach to Vulcanization (Cure) Management:** LSR undergoes an irreversible cross-linking reaction in the hot mold. Vulcanization time is a core parameter, depending on the compound formulation, the temperature at the thickest part of the product, and the mold temperature. Insufficient time leads to undercooked products and substandard performance; excessive time reduces production efficiency. Scientific vulcanization management requires determining the optimal time based on material manufacturer data and actual process verification.

III. **Back-End Support:** Post-Molding Processing and a Comprehensive Quality System

1. **Automated Post-Processing and Cleaning:** After mold opening, products are typically removed by robotic arms or automated systems to avoid contamination from manual contact. For medical or optical components with strict cleanliness requirements, deburring, inspection, and packaging must be performed in a clean environment. Temperature and humidity control in the production environment also indirectly affects product stability.

2. End-to-End Collaboration and Data Monitoring

True "end-to-end control" relies on deep collaboration and data-driven processes at every stage. From thorough communication with customers and material suppliers during the mold design (DFM) phase, to real-time monitoring and recording of key process parameters (such as mold temperature, injection speed, holding pressure, and vulcanization time) during production, a traceable, optimizable, and replicable closed-loop system is formed. This not only ensures batch-to-batch stability but also provides a basis for rapid troubleshooting and problem-solving.

Conclusion

The success of liquid silicone precision injection molding is not dependent on the excellence of a single环节 (link/step), but rather on the deep integration of "precision molds," "stable processes," and "system control." It requires manufacturers to possess not only sophisticated processing equipment but also profound material knowledge, process understanding, and cross-stage systems engineering thinking. Only through scientific definition and strict control of every detail from mold to product can high-performance LSR precision products be continuously and stably produced, providing a solid guarantee for the reliability and market competitiveness of customers' products.

This article, provided by the Suzhou Donghao Precision Technology team, is based on our practical experience in precision mold manufacturing and liquid silicone injection molding. We focus on providing complete solutions from mold development to precision injection molding for the medical device, automotive electronics, and high-end consumer electronics industries.