0.090 inch. When authorized by the system speciic
procedure, testing for vacuum leaks, and conducting
technical manual, use the following procedure for the
a thermal survey. See Chapter 3 for identiication and
liquid shimming process:
selection of vacuum bagging materials. Vacuum bag
materials vary in quality and application. Ensure the
(1) Prepare those surfaces to be shimmed by
bagging materials, including the sealant or tacky tape,
thoroughly sanding and using a solvent
being used are those speciied in the system speciic
technical manual or by engineering. Consideration
6-7 for cleaning instructions..
should be given to the bagging material capability,
at the cure temperatures required, and the time at
(2) Apply a parting agent to any surfaces you
this temperature. Rehearse the repair procedures
don t want bonded; this allows for easy sep-
to discover trouble before the actual process starts.
aration of parts.
Rehearsal is important for placement of the bagging
materials and placement of the thermocouples in
(3) Mix and apply the required compound
close proximity to the patch.
according to system speciic technical man-
a. Purpose. Vacuum bags serve a twofold pur-
poses. First, they assist in the removal of entrapped
(4) Assemble parts and clamp together. Install
air from wet layups and pre-preg materials. Remov-
fasteners if applicable. Bolts and screws
ing air reduces porosity which makes the laminate
which may require removal after the shim-
stronger. Second, they apply pressure. The pressure
ming operation is complete should be coated
is created because the atmospheric pressure outside
the bag is greater than the pressure inside the bag.
The greater the difference in pressures, the greater
(5) Remove excess compound.
the compacting force. When the layup is compacted,
the different layers of iber are pushed closer together,
(6) Cure compound as required.
thus making a stronger laminate. The key to any suc-
cessful vacuum bag process is to ensure the proper
6-18. VACUUM BAGGING THEORY AND TECH-
level of vacuum is maintained, ensure a tight vacuum
NIQUES. Vacuum bag theory and techniques are
seal exists, and ensure a good air path exists from
discussed in this paragraph. Techniques include
the repair material to the vacuum source (see igure
single side vacuum bag procedure, backside seal