TM 1-1500-204-23-11
Figure 6-43. C-Clamp Sequence and Placement
(2) Negative Pressure. Negative pressure is
porosity present is increased. Posi-
produced using a vacuum bag. When a component
tive pressure, on the other hand, sup-
or repair area is sealed in a vacuum bag, the vacuum
presses their size. Also, many mate-
source removes air from the sealed area. This creates
rials actually foam up when exposed
a vacuum in the bag and allows atmospheric pressure
to a vacuum, some more than others.
to be exerted on the bag and repair area. The amount
The presence of voids and porosity is
of pressure obtained depends upon the vacuum seal
detrimental to the resulting strength
integrity, the vacuum source, and the altitude (the
of the repair. Consequently, some
higher the elevation, the lower the compaction level)
materials require partial vacuum lev-
at which the repair is being performed. A vacuum
els to be used when curing them to
gauge is required to be installed in the vacuum
minimize this foaming effect.
bag to monitor the vacuum level achieved at the
The second disadvantage is that the
2
repair. Typically, a minimum of 22 in Hg (mercury),
atmospheric pressure exerted on a
equivalent to approximately 10.8 psi, is used to cure
standard single vacuum bag tends to
most composite materials.
pinch off the edges around the repair
patch, thereby, trapping air and other
(a) Disadvantages of Negative Pressure.
volatiles from escaping. Alternate
Although vacuum pressure is the most feasible type
cure cycles that incorporate a debulk
pressure that can be applied to ield level repairs,
cycle, discussed in paragraph 6-19.b,
due to the level of pressure it can exert compared
and alternate vacuum bag methods,
to weights or clamps, it has several disadvantages
such as the double bag method dis-
when compared to positive pressure.
cussed in paragraph 7-11, have been
developed to compensate for this
The irst disadvantage is that vac-
1
effect.
uum or negative pressure inside the
vacuum bag adversely affects many
Third, obtaining a sealed vacuum bag
3
types of materials during processing,
can be quite a challenge on aircraft
such that the size of any voids and
as very small leaks can occur that
are dificult to correct. Any leak that
is present allows air to low through
