Degassing is crucial during the processing of high-hardness liquid silicone rubber (LSR) to eliminate trapped air and prevent defects such as air bubbles, voids, or incomplete curing. Below are the typical degassing methods used for high-hardness LSR casting:
1. Vacuum Degassing
Process: The LSR mixture is placed in a vacuum chamber, which removes air bubbles by reducing the surrounding pressure.
Steps:
Mix the two-part LSR (base and catalyst).
Place the mixed LSR in a vacuum chamber.
Apply vacuum pressure to reduce the air pressure, causing bubbles to rise and burst.
Maintain vacuum until all bubbles are gone (usually a few minutes).
Advantages:
Most reliable and commonly used method.
Effective for high-viscosity LSR.
Considerations:
Requires specialized vacuum equipment.
Careful monitoring is needed to avoid over-expansion of the material under vacuum.
2. Centrifugal Degassing
Process: The mixed LSR is spun at high speeds in a centrifuge to force air bubbles out by centrifugal force.
Advantages:
Effective for small batches of LSR.
Faster than vacuum degassing for certain applications.
Considerations:
Limited to smaller quantities due to equipment size constraints.
May not work well for extremely high-viscosity LSR.
3. Pressure Degassing (Pressure Casting)
Process: After pouring the LSR into the mold, the mold is placed in a pressure chamber, and high pressure is applied to collapse air bubbles.
Advantages:
Works well for molds with fine details or trapped air pockets.
Can be combined with vacuum degassing for optimal results.
Considerations:
Requires specialized pressure chambers.
Does not remove bubbles from the material before pouring; it compresses them instead.
4. Manual Degassing (Surface-Level Techniques)
Process: For low-viscosity LSR, manual techniques like stirring slowly or pouring the material from a height to break bubbles.
Advantages:
Simple and cost-effective for small-scale projects.
Considerations:
Less effective for high-hardness or high-viscosity LSR.
Limited to non-critical applications.
5. Heated Degassing
Process: Slightly warming the LSR mixture reduces its viscosity, allowing trapped air to escape more easily before curing.
Advantages:
Helps with degassing thick or high-viscosity LSR.
Considerations:
Must be done carefully to avoid premature curing or affecting the material properties.
6. Combination Methods
Example: Use vacuum degassing first to remove most air bubbles and then apply pressure casting to eliminate any remaining trapped air in the mold.
Advantages:
Ensures the highest-quality results for critical applications.
Considerations:
Requires both vacuum and pressure equipment, increasing cost and complexity.
Recommendations
For High-Hardness LSR:
Vacuum degassing is typically the most effective and widely used method.
Combine vacuum degassing with pressure casting for precision molds or intricate details.
For Small-Scale or Low-Budget Projects:
Use manual degassing or heated degassing with care.
By choosing the right method, you can ensure that your high-hardness LSR castings are free of defects and meet the required quality standards.