For vacuum degassing liquid silicone rubber (LSR), including high-hardness grades, the typical vacuum pressure range is:
-25 inHg to -29.9 inHg
This corresponds to 635 mmHg to 760 mmHg or approximately 85% to 99% of a full vacuum.
Key Considerations:
Pressure Range:
-25 inHg (~635 mmHg) is sufficient for degassing lower-viscosity LSR or when fewer air bubbles are present.
-29.9 inHg (~760 mmHg) (close to a full vacuum) is ideal for high-viscosity or high-hardness LSR, where trapped air is more difficult to remove.
Vacuum Chamber Setup:
A vacuum pump capable of achieving at least 29 inHg is recommended for optimal degassing.
Use a transparent chamber to monitor the degassing process (bubbling and expansion).
Time Under Vacuum:
Maintain the vacuum for 2–5 minutes or until bubbling subsides, depending on the material's viscosity and the amount of air trapped.
Watch for Over-Expansion:
LSR can expand significantly under a vacuum (2–4 times its original volume). Use a container with enough headspace to prevent overflow.
Temperature:
Slightly warming the LSR (e.g., 30–40°C) before vacuum degassing can reduce viscosity, making it easier to remove air bubbles. However, avoid excessive heat to prevent premature curing.
Recommended Equipment:
A high-quality rotary vane vacuum pump or similar pump capable of achieving 29.5–29.9 inHg.
Vacuum chamber with a lid that can withstand the reduced pressure.
By maintaining the proper vacuum pressure range (close to 29.9 inHg) and monitoring the process, you can ensure effective degassing for high-quality LSR castings.