The main disadvantage with this method is that the press only  applies pressure in one direction, and hence, it is difficult to  make complex-shaped (e.g., beads, closed corners) parts or  parts with legs that approach vertical. Since the temperature  of the die set need not be cycled with each part, rapid forming  times of between 10 minutes and 2 hours are achievable with  press forming.  A thermocouple (TC) is a thermoelectric device used to  accurately measure temperatures. It may be connected to  a simple temperature reading device, or connected to a hot  bonder, oven, or other type of controller that regulates the  amount of heat. TCs consist of a wire with two leads of  dissimilar metals that are joined at one end. Heating the  joint produces an electric current, which is converted to a  temperature reading with a TC monitor. Select the type of  wire (J or K) and the type of connector that are compatible  with the local temperature monitoring equipment (hot bonder,  oven, autoclave, etc.). TC wire is available with different  types of insulation; check the manufacturer’s product data  sheets to ensure the insulation withstands the highest cure  temperature. Teflon-insulated wire is generally good for  390 °F and lower cures; Kapton-insulated wire should be  used for higher temperatures. Thermocouple placement is the key in obtaining proper  cure temperatures throughout the repair. In general, the  thermocouples used for temperature control should be placed  as close as possible to the repair material without causing it  to become embedded in the repair or producing indentations  in the repair. They should also be placed in strategic hot or  cold locations to ensure the materials are adequately cured  but not exposed to excessively high temperatures that could  degrade the material structural properties. The thermocouples  should be placed as close as practical to the area that needs  to be monitored. The following steps should be taken when  using thermocouples: Thermal variations in the repair area occur for many reasons.  Primary among these are material type, material thickness,  and underlying structure in the repair zone. For these reasons,  it is important to know the structural composition of the  area to be repaired. Substructure existing in the repair zone  conducts heat away from the repair area, resulting in a cold  spot directly above the structure. Thin skins heat quickly  and can easily be overheated. Thick skin sections absorb  heat slowly and take longer to reach soak temperature. The  thermal survey identifies these problem areas and allows the  technician to develop the heat and insulation setup required  for even heating of the repair area. In order to achieve maximum structural bonded composite  repair, it is essential to cure these materials within the  recommended temperature range. Failure to cure at the correct  temperatures can produce weak patches and/or bonding  surfaces and can result in a repair failure during service. A  thermal survey should be performed prior to installing the  repair to ensure proper and uniform temperatures can be  achieved. The thermal survey determines the heating and  insulation requirements, as well as TC locations for the repair  area. The thermal survey is especially useful for determining  the methods of heating (hot air modules, heat lamps, heat  blanket method and monitoring requirements in cases where  heat sinks (substructure for instance) exist in the repair area).  It should be performed for all types of heating methods to  preclude insufficient, excessive, or uneven heating of the  repair area.  •  Place flash tape below and above the thermocouple  tips to protect them from resin flash and to protect the  control unit from electrical shorts.  •  Do not place the thermocouple under the vacuum  port as the pressure may damage the lead and cause  erroneous readings to occur. •  Do not place thermocouple wires adjacent to or  crossing the heat blanket power cord to prevent  erroneous temperature readings caused by magnetic  flux lines. •  Do not place any control thermocouple beyond the  heat blanket’s two-inch overlap of the repair to prevent  the controller from trying to compensate for the lower  temperature.  •  Always leave slack in the thermocouple wire under the  vacuum bag to prevent the thermocouple from being  pulled away from the area to be monitored as vacuum  is applied. Thermal Survey of Repair Area Thermocouples Temperature Variations in Repair Zone • Never use fewer than three thermocouples to monitor  a heating cycle.  •  If bonding a precured patch, place the thermocouple  near the center of the patch. •  A control thermocouple may be centered over a  low-temperature (200 °F or lower) co-cured patch as  long as it is placed on top of a thin metallic sheet to  prevent a thermocouple indentation onto the patch.  This may allow for a more accurate control of the  patch temperature. •  The thermocouples installed around the perimeter  of the repair patch should be placed approximately  0.5-inch away from the edge of the adhesive line.  7-25 Thermocouple Placement