Unlike Liquid Silicone Rubber (LSR) injection molding, the processing of traditional solid silicone is closer to that of plastics or rubber. Its core lies in a process of “preparing the material first, then heating and pressurizing to mold.”
Material Preparation and Mixing
Raw Material Form: Solid silicone is typically supplied as a raw compound. It is a solid sheet or strip with high viscoelasticity, already pre-filled with certain fillers (such as silica).
Mixing: Before entering the molding process, the compound usually needs further mixing on an open mill or in an internal mixer. Here, vulcanizing agents (typically peroxides), color masterbatches, and other additives are added and uniformly blended.
Pre-forming
The mixed compound is cut or punched into blanks (commonly called “preforms”) similar in shape and weight to the mold cavity. This step is crucial to ensure the compound fills the mold cavity evenly, reducing internal stress and the risk of short shots.
Compression Molding
This is the most core and traditional molding process.
Process: The precisely weighed preform is placed into a pre-heated metal mold. After closing the mold, high pressure is applied (usually via a platen vulcanizing press). Under high temperature (typically 170°C – 180°C) and high pressure, the compound softens, flows, and fills the entire mold cavity. Simultaneously, the peroxide vulcanizing agent decomposes, initiating the cross-linking reaction (vulcanization). After a certain dwell time, the final shape and properties are formed.
Characteristics: Relatively low equipment investment, very suitable for small to medium batch production of relatively simple parts.
Post-Processing – Secondary Vulcanization
Parts after compression molding usually require secondary vulcanization (also called post-curing).
Purpose: The parts are placed in a high-temperature oven for several hours. This step aims to decompose and volatilize the by-products generated from the peroxide decomposition during molding (which can cause odor or affect biocompatibility) and to make the cross-linking reaction more complete. This stabilizes the part’s properties and improves heat resistance and compression set.
Comparison: Traditional Solid Silicone vs. Liquid Silicone Rubber Molding Processes
| Characteristic | Traditional Solid Silicone (HCR) Compression Molding | Liquid Silicone Rubber (LSR) Injection Molding |
|---|---|---|
| Raw Material Form | High-viscosity solid/thick sheet | Low-viscosity liquid (A/B components) |
| Processing Method | Compression: Prepare material first, then heat and pressurize | Injection: Meter, mix, and inject simultaneously |
| Automation Level | Lower, often relies on manual operation | Very high, fully enclosed, automated continuous production |
| Production Efficiency | Lower, long cycle time, includes secondary vulcanization | Very high, short cycle time, requires little or no secondary vulcanization |
| Molding Cycle Time | Relatively long (several minutes) | Very short (tens of seconds or less) |
| Waste Situation | Has flash and runner scrap, waste is recyclable/reusable | Minimal flash, cold runner no waste, hot runner minimal waste |
| Part Complexity | Suitable for relatively simple parts | Suitable for complex, thin-walled, precision parts |
| Raw Material Storage | Limited shelf life, potential for pre-vulcanization | Stable at room temperature, long shelf life |