Pressure cure cycle

These three cure systems have in common the need for a two-step cure cycle to generate the best cured properties. The first step is the appHcation of heat and pressure in a mold to shape the article (press cure). The second step is a high temperature oven cycle at atmospheric pressure to obtain the final cured properties. 

Benzoyl peroxide is most commonly used for elevated temperature curing. The peroxide is generally supplied as a paste (-50%) in a liquid such as dimethyl phthalate to reduce explosion hazards and to facilitate mixing. The curing cycle in pressure moulding processes is normally less than five minutes. 

Pressure bag molding This is a take-of to vacuum bag molding where the bag and mold is placed in a closed system and is subjected to pressure during the curing cycle. 

The mold and sample were placed in the press also preheated to 185°C and the cure cycle started immediately. Press temperature and pressure were microprocessor-controlled for consistent cure cycles. The cure cycle consisted of an initial 30 minute hold at 185°C with 1300 psi of pressure followed by 45 minutes at 221°C with 850 psi pressure and finally 15 minutes at 260°C with 300 psi pressure. The mold was removed from the press immediately following the completion of the final step and the sample removed from the mold while still hot. 

Figure 6.2 Curing cycle temperature-time profile for typical graphite-epoxy composite in a vacuum bag autoclave process. Autoclave pressure is applied during the 135°C (275°F) hold...

Figure 6.4 Effect of water on the total void pressure as the temperature increases in a typical curing cycle. Percentages are equilibrium water contents by weight in the resin phase...

For a prepreg equilibrated with moisture at a particular relative humidity, in order to prevent the potential for pure water void growth by diffusion at all times and temperatures during the curing cycle, the pressure at all points of the prepreg must satisfy the following inequality ... 

Based strictly on equilibrium considerations, bounds can be set on the stability of voids as a function of temperature and pressure. Although this type of phase map does not depict the time dependency of an actual process, it does provide a limiting scenario toward which the actual process would be heading at any point in the curing cycle. It is surprising that high void pressures are possible if sufficient moisture is present in the resin. 

The objective of the Springer KBES is twofold To ensure a high-quality part in the shortest autoclave curing cycle duration. This KBES is similar to QPA in that sensor outputs are combined with heuristics not with an analytical curing model. The rules for compaction dictate that dielectrically measured resin viscosity be held Constant during the First temperature hold in the autoclave curing run. The autoclave temperature is made to oscillate about the target hold temperature in an attempt to attain constant viscosity. Full pressure is applied from the cure cycle start. 

The second ramp portion of this cure cycle is critical from a void nucleation and growth standpoint. During this ramp, the temperature is high, the resin pressure can be near its minimum, and the volatile vapor pressure is high and rising with temperature. These are the ideal conditions for void formation and growth. 

In the early stages of the cure cycle, the hydrostatic resin pressure should be equal to the applied autoclave pressure. As resin flow occurs, the resin pressure drops. If a laminate is severely overbled, then the resin pressure could drop low enough to allow void formation. Thus, the hydrostatic resin pressure is directly dependent on the amount of resin bleeding that occurs. As the amount of bleeding increases, the fiber volume increases, resulting in an increase in the load carrying capability of the fiber bed. 

The rate of bleeding is dependent on several factors, including the permeability of the fiber bed, both vertically and horizontally, and the viscosity of the liquid resin. The permeability of the fiber bed will depend on the weave of the fabric, the fiber diameter, and the fiber volume fraction. The resin viscosity is determined by the chemistry of the resin and the thermal profile of the cure cycle. The cure cycle greatly affects resin viscosity and the flow process, both directly through the pressure application and indirectly through the effect of the thermal profile on resin viscosity. 

The second potential problem is illustrated in Figure 10.15. With only vacuum pressure applied during the initial part of the cure cycle, the hydrostatic pressure on the resin can be extremely low, even negative. This is an ideal condition for void formation and growth if allowed to persist to high enough temperatures. 

This section describes some of the tools available for intelligent development of process cycles, such as the time-temperature cycles used in curing composites. Current industrial practice is typically limited to the use of cure cycles. The cycles are based on a series of autoclave temperature and pressure states so that traditional linear, regulatory process control methods can be used. These recipes may not be the ideal method for process control of batch processes because they do not ... 

The autoclave cure of advanced composites serves as a good illustration of this sort of complex control problem. A typical autoclave cure cycle is shown in Figure 15.1. The temperature is usually increased in stages until some maximum cure temperature is reached. It is then held at maximum cure temperature until complete cure is certain. During this temperature cycle, the pressure is applied to achieve compaction. The 11 items capitalized in Figure 15.1 are all variables. It is clearly not possible to perform an exhaustive study of all variations of all of these variables with finite time and money. In addition, although the preceding template is common, it is not the only possibility. It would also be possible to heat... 

Figure 15.4 The large thermal lag between the part (PARTTC) and autoclave (AIRTC) is not linear or a constant. This cure cycle, however, was written for the autoclave temperature. Pressure is applied at minimum viscosity and compaction is good with few voids...

Hinrichs and Thuen [28] used ultrasonic attenuation to determine the proper time for pressure application during an otherwise traditional pre-established cure cycle. Because dielectric is an electrical property, it is influenced by moisture content and temperature as well as viscosity, so it may vary quantitatively. Ultrasonic measurements are also affected by other parameters (i.e., void content), but they are a mechanical measurement rather than an electric one. The ultrasonic sensors used by Hinrichs unfortunately were less reliable than the dielectric sensors. 

Choosing the correct flow period is a very important aspect in the forming and shaping process. Mooney cure curves give good data on the flow period. The flow period also depends on the flow distances in the cavities of the mould and the viscosity of the rubber stock and the repeat pressure application cycles, called "bumping" cycles on the shop floor. 

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