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Sandwich Structures

Fabrication of Composite Sandwich Structure with aramid honeycomb core

When fabricating sandwich structures of honeycomb, three conditions should be met:

1. Tooling or fixtures must maintain the desired shape;

2. Uniform pressure must applied to hold the structure together during curing;

3. Uniform heating must be used to cure the resin

There are many procedures to make aramid core sandwich structures. We will outline a typical prepreg based autoclave cure procedure that will meet the above requirements:

1. Start with a clean, smooth tool plate. Note that any flaws or irregularities on the tool plate will show up on the cured laminate.

2. Tape tool plate along edges with adhesive tape 1'' wide. This keeps this aera clean and free of any release agent in preparation for the application of sealant tape.

3. Apply release agent as directed to tool surface.

4. Peel the polyethylene backing off of each prepreg layer before laying it down on the tool.

5. Lay down the first ply in the center of the tool, making sure that it is smooth and free of wrinkless distortion.

6. Lay each subsequence ply directly over the first, lining up the edges. This is important to ensure prpper fiber orientation. If the prepreg i not self-adhering, include film adhesive where and if it is called for.

7. A roller may be used to remove any trapped air bubbles and wrinkles from the layup.

8. The core is laid down in the same manner as the prepreg, with the edges lined up with the prepreg plies.

9. Subsequent prepreg plies are laid up on top of the core in the same way. Again, a roller may be used to smooth out the plies.

10. Place a layer of release film over the laminates. This layer should over-hang the laminate on each side by at least 8 to 10 cm. This release film layer allows volatiles to escape while containing resin.

11. Place supports around edges of the laminate to prevent core crush when vacuum and pressure are applied. The supports should be placed over the release film so that they do not stick to the laminate, 1/2'' metal strips (flat stock) are used here. 

12. A caul plate is placed over the laminate. This plate, like the tool plate, should be both clean and smooth.

13. Place four to six layers of breather ply material on top of the part. This allow for the movement of air and of volatiles while part is being cured.

14. Remove adhesive tape from the edges of the tool.

15. Replace with sealant tape and remove the sealant backing.

16. Insert thermocouples on top of sealant tape, thermocouples must be stripped at the point where they contact the sealant tape in order to avoid leakage. Put a piece of sealant tape over the thermocouples to secure and complete seal.

17. Place vacuum bag material over assembly/ The vacuum bag layer should be large enough to accommodate the volume of the part.

18. Press the vacuum bag material down on the thermocouples sealant tape to ensure a good seal.

19. Turn on the vacuum pump and connect the vacuum lines after checking for leaks. Pull 17 kPal of vacuum and hold for 20 to 30 minutes. Do not exceed this limit of-higher vacuum may cause the facesheet to "dimple" over the honeycombe cells. Before this 20 to 30 minutes period a leak check must be made.

20. Put the part into the autoclave and hook up the vacuum lines. Start the autoclave run.

21. After cure, the part is removed from the autoclave.

22. And the bag material, breather plies and rlease film are removed.

23. The finished part is shown.

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