b. Vacuum Bagging Theory. When vacuum is
(3) Water is removed quicker from wet com-
applied to a vacuum bagged repair, air is removed
ponents. When vacuum is applied, water
from inside the bag to create a vacuum. To provide
vaporizes at temperatures below 212 °F
a path to draw off the air initially inside the bag,
and the vapor is drawn out. This offers a
layers of iberglass cloth or similar non-contaminating
great advantage over drying at tempera-
materials known as breather plies are placed inside
tures above 212 °F; since the water is only
the bag. The following paragraphs discuss how it
vaporized (boiled) where the vacuum exists,
there is no chance of entrapped boiling
water blowing out the honeycomb core.
(1) Porosity is tiny air bubbles that are
(4) Small parts may be envelope bagged (i.e.,
entrapped in the repair. When vacuum is
enclosing the entire part in the bag). Larger
applied, the air bubbles are drawn out of the
parts with localized repairs can be bagged
repair plies into the bleeder and/or breather.
by sealing the surface completely around
Reducing the porosity increases the quality
the repair areas with sealing tape and apply-
of the repair. Air leaks in a vacuum bag of
ing the bagging material to the sealing tape.
greater than 1 in Hg/min introduce air and
When using an oven to cure repairs, sand-
negatively affect porosity.
wich skin panels must be entirely vacuum
bagged to prevent delamination and con-
(2) Compaction, or clamping pressure, is
toured parts must be restrained with tooling
applied by normal atmospheric pressure
to prevent warpage.
when the vacuum is applied. At sea level
warpage occur when the cure temperature
with maximum vacuum on a standard day
nears the cure temperature of the original
(ideal), you can apply 14.7 psi of pressure;
manufactured part or the cure temperature
that would be the equivalent of stacking
1470 pounds of weights onto a 10 x 10 inch
6-18d for envelope bagging procedures.
repair. In reality, your pressures will vary
between 10 and 15 psi.
(5) Strive to maintain the maximum vacuum
possible. When a lack of system speciic