TM 1-1500-204-23-11
(3) Pressure Ramp Rates. Ramp up and ramp-
c. Heat Transfer Theory. Heat is transferred to the
down may also apply to pressure increases and
surrounding parts and air by conduction, convection,
decreases; however, they are seldom called out in ield
radiation, and induction as detailed in the following
repairs. If a pressure ramp rate is not speciied, full
paragraphs. Heat is also lost from parts in the same
pressure is applied prior to the temperature ramp-up
ways. Heat transfer during repairs often uses a
and then released after temperature ramp-down.
combination of the types. For example, when a heat
lamp is used to repair a part, it warms the surface
of the part and/or patch by radiation. The ibers in
(4) Control Thermocouples. When the control
the surface of the patch are heated and conduct the
thermocouples are not able to be placed over the
heat to other layers of the patch. Likewise, heat is
center of the repair area but must be placed around
conducted away from the skins to any substructure
the perimeter of the repair, the temperature holds
which is touching the heated skin. Air in contact
may need to be adjusted to compensate for the
with the warm surface area of the patch or vacuum
temperature difference at the patch center. This
bag warms up and rises, thus taking heat away from
adjustment and correlation of the outer thermocouple
the repair by convection. When in a liquid state,
readings to the actual temperature at the center of
the resin matrix transfers heat throughout the resin
the repair depends greatly upon a thermal survey and
by convection. Knowledge of heat transfer principals
the type of heat blanket used, constant watt-density
may aid the mechanic in selecting a heat source
and/or mitigating heat sink problems. It is important
directions on conducting a thermal survey.
to know the following concepts as they apply to the
heat transfer theory.
(5) Cure Temperature. Some adhesives have
multiple cure cycles to choose from. For instance,
(1) Conduction. Conduction takes place when
some resins, according to the manufacturer s data
a hot surface meets a cold surface, such as when
sheet, can be cured at 200 F for 220 minutes or
a heat blanket touches the part to be repaired.
250 F for 150 minutes or 300 F for 130 minutes.
Technically, conduction is heat transfer by means
The properties of the resin are dependent on the inal
of molecular agitation within a material without any
cure temperature; therefore, when a particular cure
motion of the material as a whole. If one end of
temperature is called for in a system speciic tech-
a metal rod is at a higher temperature, energy will
nical manual, it must be followed. When you have
be transferred down the rod toward the colder end
the prerogative to choose between cure temperatures,
because the higher speed particles will collide with
you should use the highest cure temperature at which
the slower ones with a net transfer of energy to the
you can maintain a uniform temperature across the
slower ones.
repair. If a temperature range is given such as 240 to
260 F, ensure all areas of the repair are maintained
between those temperatures. When no temperature
range is given, ensure the lagging thermocouple (low-
a luid, such as within an adhesive. Convection is
est temperature) reaches the given cure temperature
heat transfer by mass motion of a luid such as air
and the leading thermocouple (highest temperature)
or water when the heated luid is caused to move
is within 20 F. If the difference between the leading
away from the source of heat, carrying energy with
and lagging thermocouples exceeds 20 F, seek engi-
it. Convection above a hot surface occurs because
neering assistance. For complex structures or where
hot air expands, becomes less dense, and rises. Hot
substructure exists in the repair area, a thermal survey
water is likewise less dense than cold water and rises,
shall be performed prior to the bonding operation and
causing convection currents which transport energy.
corrective actions taken in accordance with paragraph
Heat guns use convection to heat a repair.
6-18g to ensure a uniform cure temperature can be
maintained.
(3) Radiation. Heat from the sun is the classic
example of radiation. When a heat lamp is used in a
(6) Cure Time. Cure time is not the same as
repair, heat is radiated from the lamp to the part or
cure cycle. Cure cycle is the total times of the curing
patch surface, then conduction and convection take
process, including cold debulk, ramp up, hot debulk,
place. The amount of heat transferred depends on
cure time, and ramp-down. When a programmable
the amount of radiation absorbed by the object; dark
heat source is used, the cure cycle must be determined
colors absorb and heat quicker than light colors. The
in order to program the control unit. The cure cycle can
heat source does not touch the object being heated.
be determined using the speciied cure temperature
Technically, radiation is heat transfer by the emission
and the desired ramp rate. Unless otherwise speciied
of electromagnetic waves which carry energy away
in the repair, the cure time starts when the lagging
from the emitting object. For ordinary temperatures
thermocouple reaches the cure temperature.
region of the electromagnetic spectrum.
6-73