TOOLED THICK COMPOSITES by ARVEN H. SAUNDERS III ...

cycle. The ability to optimize temperature for a laminate that is 3.45” thick (at the root end) is a
challenge; the highly interactive and nonlinear nature of the laminate’s thermal state with its
reaction rate made finding an optimum a relative choice based on what constraints are
imposed. The following groundrules were established:
• A maximum laminate temperature of no more than 180°C
• A maximum temperature difference within the laminate of 10°C
• A dwell at 177C for 2 hours to complete cure
Conventional industry practice for a cure cycle is to increase temperature from ambient
via an initial ramp, of perhaps 0.1 C per minute, to an intermediate temperature within a range
of 80 to 100C, followed by a dwell there of 120 minutes, then again ramping to the final cure
temperature of 350F/177C. In order to optimize the cycle time, greater ramp rates are used;
however, as it is increased, the temperature difference within the laminate increased. One of the
constraints for an acceptable flexbeam laminate in terms of strength is limiting this difference.
The purpose of the first ramp is to get the laminate to a state where pressure application can be
completed while maintaining uniform cure and viscosity within the laminate. The dwell is used
to allow the laminate cure and viscosity to become as homogeneous as possible, so that effects
of the pressure application will be consistent throughout.
For the S2/8552 flexbeam, multiple attempts were made using the model to simulate
the process. The tool temperatures were manipulated through the simulation, while the
temperatures of the exterior laminate surfaces in contact with the tool bottom, sides, and lid
followed the tool temperatures. The first attempt using common industry practice resulted in a
reasonable overall cure time of about 14.5 hours, but with excessive temperatures
(unacceptable maximum temperature of approximately 216C) and exotherm. After all, this is a
very thick laminate, and the energy to be released during cure is formidable. Note that the
following figures, beginning with Figure 4.3, do not show the time in the cycle for cool-down
following the dwell at the final temp of 177C.
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