Pressure consolidation

The method makes use of the principle that a constant ratio of induced stress. s in the stored contents to the consolidating pressure p exists. Thus, for any hopper design for which th.eff curve is available, the shear-tester results can be potted, and the point where/= s is located. Since the distance at which this occurs above the hopper vertex is also known, these values become the hopper dimensions at that point. 

Models of the intimate contact process that have appeared in the literature are commonly composed of three parts or submodels. The first submodel is used to describe the variation in the tow heights (surface waviness or roughness) across the width of the prepreg or towpreg. The second submodel, which is used to predict the elimination of spatial gaps and the establishment of intimate contact at the ply interfaces, relates the consolidation pressure to the rate of deformation of the resin impregnated fiber tow and resin flow at ply surface. Finally, the third submodel is the constitutive relationship for the resin or resin-saturated tow, which gives the shear viscosity as a function of temperature and shear rate. 

By closely examining the layout in Figure 7.7, a representative volume element can be selected for study, as shown in Figure 7.8. Region A, which comprises 25 percent of the area of the representative volume element, is in initial contact. When the consolidation pressure is applied, the deformation is initiated in region A, and the resin flows in the x-y plane to fill the gaps in regions B, C, and D. 

Figure 7.18 shows the consolidation time versus consolidation pressure for unidirectional and cross-ply interply interfaces. As can be seen from both figures, the consolidation time decreases rapidly as pressure increases. Above 1000 kPa for T300/P1700 and 500 kPa for APC-2, increases in the consolidation pressure do not significantly decrease consolidation time. 

Figure 7.18 Consolidation time (tc) versus consolidation pressure for T300/P1700 and APC-2 Batch II prepregs. (a) Unidirectional lay-up. (b) Cross-ply lay-up...

Cohesive strength The consolidation of powder may result in arching and ratholing within transfer equipment. These behaviors are related to the cohesive strength of the powder, which is a function of the applied consolidation pressure. Cohesive strength of a powder can be measured accurately by a direct shear method. The most universally accepted method is described in ASTM standard D 6128-00 (3). 

By measuring the force required to shear a bed of powder that is under various vertical loads, a relationship describing the cohesive strength of the powder as a function of the consolidating pressure can be developed (4). This relationship, known as a flow function, FF, can be analyzed to determine the minimum outlet diameters for bins to prevent arching and ratholing. 

G(< )t) is also a derived function and is given by Figure 9. /c(ct1), the unconfined yield strength of the material, is determined by the flow function (FF) at the actual consolidating pressure al. The consolidation pressure al is a function of the head or height of powder above the outlet of the bin, as given by Janssen s equation ... 

In order to characterize this bonding tendency, the flow function of a material must be determined. Data on flow function can be generated in a testing laboratory by measuring the cohesive strength of the bulk solid as a function of consolidation pressure applied to it. Such strength is directly related to the ability of the material to form arches and ratholes in bins and hoppers. 

Compressibility. The bulk density of a solid is an essential value used in the analysis of its flow properties, such as when calculating mass flow hopper angles, opening sizes, bin loads, etc. Loose and/or packed density values are not sufficient. Bulk solids exhibit a range of densities that vary as a function of consolidating pressure. This range of densities, called the compressibility of the solid, can often be expressed on a log—log plot as a line or relationship. 

In addition to this relation, the factors of interest concerning the problems of process technology are the porosity, e, prevailing during steady state flow and the proper compressive strength, f, as fimctions of the consolidating pressure. For this it is useful to choose the larger... 

The application of pressure to consolidate the laminate would normally be made at the end of the 122"C hold cycle. Within limits, it is important to apply consolidation pressure in the proper viscosity range to insure adequate compaction of the laminate and the desired resin to fiber ratio. 

Time, temperature, viscosity models are another useful element in understanding the science of thermoset resin processing. They perform a number of valuable functions such as defining the proper point for consolidation pressure application in FRP lamin-... 

IBCs are often used to store the blend, during which time the flowability may get worse as the blend is subjected to consolidation pressures due to its own weight during storage at rest. In addition, IBCs may be used to move the blend from one process step to another, during which time the blend may be subjected to vibration that may adversely affect flowability. Therefore, it is important to determine what consolidation pressures will act on the powder as it is stored and transferred in an IBC. 

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