Cure cycle, time reduction

Probably the first major publication of a process model for the autoclave curing process is one by Springer and Loos [14]. Their model is still the basis, in structure if not in detail, for many autoclave cure models. There is little information about results obtained by the use of this model only instructions on how to use it for trial and error cure cycle development. Lee [16], however, used a very similar model, modified to run on a personal computer, to do a parametric study on variables affecting the autoclave cure. A cure model developed by Pursley was used by Kays in parametric studies for thick graphite epoxy laminates [18]. Quantitative data on the reduction in cure cycle time obtained by Kays was not available, but he did achieve about a 25 percent reduction in cycle time for thick laminates based on historical experience. A model developed by Dave et al. [17] was used to do parametric studies and develop general rules for the prevention of voids in composites. Although the value of this sort of information is difficult to assess, especially without production trials, there is a potential impact on rejection rates. 

The modeling of residual stress development during cure can be used to optimize the processing conditions to reduce or control residual stresses. The current process model is used next to assess the effects of several processing conditions on residual stresses. Reduced cure temperature, longer dwell times, slower cool down rate, and the use of novel cure cycles are all feasible for the reduction of residual stresses. 

Mechanical testing of the three-step cure specimens indicated that no sacrifice in properties resulted from the modification of the process cycle. The retainment of mechanical properties (transverse strength and modulus) coupled with the reduction in dimensionless curvature for the three-step cure cycles investigated provides another suitable cure cycle modification for reduction of residual stresses in composite materials. Overall processing time has not been increased beyond that specified in the MRC cycle. Thus, with no increase in process time and comparable mechanical properties, the residual stresses have been reduced by more than 20 percent in comparison to the MRC cycle baseline data. 

An example of the cure cycle optimization is the work of Thomas et al. who used a very sophisticated model, together with a rule-based optimization routine, to pick the shortest cure cycle that met a set of performance criteria [15]. Reductions in cure time using this method ranged up to 36 percent for a single complex part and from 8 percent up to 43 percent for batches of mixed parts. Rejection rates were not increased in any case, and they were actually reduced significantly for one part. This model, although transferred to a number of companies, unfortunately has had limited use because of the lack of support for the code and the cost of qualifying it on new materials. 

Curing by light irradiation can be apphed to a variety of polymers. The acrylic family is a typical example of photosensitive monomers. A well-documented review has recently been dedicated to the curing of composites by ultraviolet radiation [45], Photocuring time is much shorter than for traditional thermal curing (minutes rather than hours), leading to a significant reduction in the cycle time. 

For use in the manufacture of high production/performance epoxy prepreg and allied needs, but allowing a reduction in the cure cycle from typically 60 to 90 minutes to 15 to 20 minutes at between 113 -121. Has gel time and flow characteristics similar to standard epoxies. 

During press cure the primary diamine reacts with the acid cure site producing an amide crosslink. In the chemical environment of the AEM terpolymer backbone the amide crosslinks are reactive and are converted with time and temperature to imide crosslinks. Since the reduction of the amide to the imide needs to occur before exposing a functional part to high temperatures and stress, a post-cure is required. A separate post-cure step can minimize press-cure times, increases vulcanizate modulus, tensile strength and hardness, and generally results in compression set values <20% (70 h at 150°C). Alternative post-cure cycles are... 

The MRC cycle calls for a 182°C cure temperature. The effect of cure temperature on residual stress was investigated by curing specimens at four other cure temperatures (171, 165, 160, and 149°C) while holding the dwell time (4 hours) constant. In Figure 8.18 the dimensionless curvature for these specimens is plotted versus the cure temperature. The curvature is reduced as the cure temperature is decreased with significant reduction in curvature obtained for dwell temperatures of 165°C or less. The final curvature as predicted by the viscoelastic process model is overlaid with the experimental data in Figure 8.18 and is shown to capture the trend. 

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