Cures pressure applied during

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. 

During the temperature ramp period, pressure is applied. How much pressure is applied depends on the adhesive and the type of assembly. Honeycomb assemblies are limited by the compression strength of the honeycomb core, so cure pressure is typically limited to 50 psi for aluminum core of standard density. Metal to metal assemblies can withstand higher pressures and usually have fewer bondline voids when cured at higher pressures. Metal-to-metal assemblies bonded with standard modified epoxies are cured at 90 psi. 

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...

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... 

Bondline thickness. The control of bondline thickness is essential for high-power devices. A thin bondline is required since thickness is directly proportional to the junction-to-case thermal resistance, Qjq (see Ch. 2). Specially formulated solvent-based hybrid adhesives can produce uniform bondline thicknesses of one mil or less. Small hard spacers, acting as shims, added to paste adhesive formulations, have also been used to control thickness.The use ofpreforms instead of paste adhesives also assures reproducible thicknesses provided uniform pressure and temperature are applied during curing. 

In films or structural tape, the carrier is usually porous and the adhesive saturates the carrier. Glass, polyester, and nylon fabric are common carriers for supported B-staged adhesive films. With fabric constructions, (1) the carrier provides for a method of applying the adhesive (2) it may also may act as reinforcement for the adhesive and (3) it can provide an internal shim to control the thickness of the adhesive regardless of the amount of pressure applied to the joint during the time of curing. 

They are available in different viscosities, reaction speeds, etc to suit a multiplicity of uses. However, they may not be used with metal without special primers, they almost invariably require at least one surface to be porous and will not fill large gaps successfully. Because of their high water content, pressure must be applied during curing. They form strong joints with wood - the adhesive s strength is... 

When pressure is applied during curing, the process... 

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