Contoured Silicone Vacuum Blanket Fabrication
The procedures outlined below are general recommendations for film bagging and oven or dry heat autoclave curing of a contoured silicone vacuum blanket.
Preparation of the lay-up tool
- The lay up tool can be a sheet metal mandrel, aluminum, or an actual production composite part. It should be inspected for smoothness and cleaned with solvent.
- Use fillets and felt to prevent bag bridging on inside angles or seams. Mylar tape can be used to secure and cover the fillets.
- The tool should be evenly coated with release agent. Silicone based release agent should not be used. Dry Teflon aerosol spray or Joy detergent solution in water work well.
- If a production composite article is used as a tool, it may be covered with a layer of Tedlar film to prevent uncured silicone transfer to the part. The film can be secured with mylar tape.
Fabrication of the silicone lay-up
- Rolls of uncured silicone should be removed from refrigerated storage and allowed to reach room temperature.
- Unroll the silicone sheet as per instructions on the roll.
- Cut the uncured sheet using a pattern to give a minimum of waste and seams.
- Lay the uncured silicone sheet into place with the exposed surface down. Starting at the top, work around and down using very gentle hand pressure to remove any air that may be trapped under the sheet. Remove the second layer of film and rub gently with a double layer of clean nylon fabric to mold the silicone into place.
- Joints or seams can be overlapped or butt jointed. An overlap is usually preferred for strength and vacuum integrity. The overlap can be skived or feathered using a small spatula.
- Additional layers can be applied to areas of extreme contours or over radii where the sheet may have been thinned. Use the same procedure to remove air that maybe trapped between layers. A hypodermic needle can be used to facilitate air removal. Smear the uncured silicone to fill in and cover the needle hole.
- When the lay-up is completed, check it to make sure there are no areas of foreign matter or seams that have not been feathered.
Film bagging of the lay-up
- Cover the entire lay-up with a layer of release cloth or film. This prevents the silicone from being embedded in the bleeder fabric during cure.
- Place a layer of breather fabric over the lay-up.
- Position any vacuum ports or thermocouple. Bag the entire lay-up with suitable nylon film and seal with sealant tape.
- Attach the tool and lay-up to a vacuum source and pull 24 to 28 inches of mercury vacuum on the lay-up. Verify the integrity of the seal with a vacuum gage.
Curing the lay-up
- Place the tool in an autoclave or oven, while maintaining vacuum pressure.
- Cure the silicone for at least 30 minutes at 300°F. Time the cure cycle after the assembly reaches 300°F. If an autoclave is used for external pressure, 60 psi is usually sufficient. For lay-ups that are thicker than 1/8 inch, cure an additional five minutes at 300°F for each extra 1/8 inch of thickness.
- Allow the lay-up to cool to room temperature before removing it from the tool. Inspect the blanket for any areas that may be thin. Add uncured silicone to the thinned areas and repeat the curing procedure.
- Remove the blanket from the tool and place it in an air circulating oven for 3 hours at 400°F. During the post cure, do not allow silicone surfaces to touch. Use clean fiberglass cloth as a separator.
- After post cure and cooling, the blanket is ready for use.