Feature Article Figure 4. CYCOM 5320-1 93°C initial cure and freestanding postcure to 177°C. Figure 5. CYCOM 5320-1 121°C initial cure and freestanding postcure to 177°C. It can be seen that the material effectively gels during the 177°C dwell and the Tg lags behind the cure temperature until the 177°C dwell is started. Such a cure cycle is therefore used when the entire cure takes place with the part supported in a tool and the tool in question needs to operate at 177°C. The cure duration is however considerably shorter than that of the prototype part cure cycles shown in Figures 4 and 5 and permits a production efficiency comparable to that of autoclave cured parts. As noted in the previous discussion above, efficient air and volatile extraction prior to the commencement of any cure cycle is highly desirable and may be achieved by the addition of a super-ambient dwell at 50°C - 60°C to each of the cure cycles shown in Figures 4, 5 and 6. It should be noted that for all of the scenarios considered in this discussion and the variables identified in several previous coverages on these topics, the characteristics of the prepreg and in particular viscosity, will change as the material SAMPE Journal, Volume 52, No. 4, July/August 2016 accumulates out time during layup. There is therefore an out time limitation in a given case which may be part construction specific and not purely a material characteristic. A discussion of this and related issues may be found in Reference4. There are also certain circumstances where something other than a 'standard' cure cycle may be needed. The use of ultra-high modulus fibers such as Hexcel® HM63 or Toray® M46J in unidirectional tapes produce a different pattern of residual stresses compared with that arising from the use of standard or intermediate modulus carbon fiber. These materials can exhibit microcracking after cure or subsequent thermal cycling if a standard cure cycle such as one of those shown in Figures 4, 5 or 6 is used, particularly in applications involving cryogenic service temperatures. Figure 7 shows an example of microcracking in a discriminator panel built from MTM45-1/HM63 unidirectional tape after a standard cure/postcure and thermal cycling. 11