Flow compression forming

Permeability is normally determined using linear flow in the incompressible or compressible form, depending on whether a liquid or gas is used as the flowing fluid. The volumetric flowrate Q (or Q ,) is determined at several pressure drops. Q (or Q ,) is plotted versus the average pressure p . The slope of this line will yield the fluid conductivity K or, if the fluid density and viscosity are known, it provides the intrinsic permeability k. For gases, the fluid conductivity depends on pressure, so that... 

Also used is the injection-compression cycle where after a preforming stroke for the cover stock, the carrier material is injected in a partially open mold (Chapter 4). By closing the gap the part is formed and laminated. The mold corresponds to a back injection mold. The method has similarities with melt flow compression molding. 

Tablets are made by compressing a powder in a die. Under pressure the powder fuses and is ejected as a tablet. The problem of tabletting is to produce a powder that will flow freely when poured into the die but bonds satisfactorily when compressed. Some materials are available in a directly compressible form. One example is sorbitol, which goes some way to explain the popularity of pressed sorbitol tablets with manufacturers. If directly compressible materials are not being used then the material must be granulated before being tabletted.

The true direct compression process as described earlier almost invariably applies to formulations containing potent active ingredients and where the direct compression properties derive from the diluent. A few substances do possess adequate flow and cohesive properties without the need for pretreatment. These are usually crystalline inorganic salts such as sodium chloride and potassium chloride. Direct compression forms of less potent active ingredients are available e.g., paracetamol and ascorbic acid. These can be directly compressed into tablets, perhaps after the addition of a lubricant. However, such substances are more accurately described as pre-granulated, in that the granulation process—either wet granulation or precompression—has been carried out by the excipient manufacturer. 

Mannitol is an isomer of sorbitol. Like the latter, it has a negative heat of solution which makes it a useful excipient for chewable tablets and lozenges. It is less hygroscopic than sorbitol and has about one-tenth of the solubility in water. Similarly to sorbitol, several polymorphic forms are available which differ in their ability to form tablets. " However, unmodified mannitol cannot be used for direct compression because of poor flow and binding properties. Directly compressible forms are available in a range of particle sizes which are reported to produce excellent tablets. 

This is the flow permeability form, corresponding to the Darcy relationship for flow through porous media. (Alternately, the flow permeability relationship may be developed in terms of the liquid compressibility, as also previously derived.) Here, of course, Pj > Py. 

Depending on the formulation and how the pressure equation is written for compressible or incompressible flows, many forms are available to approximate

control volume to perform the mass balance in each cell [9,12],... 

Compressible ID stagnation-point flow analysis forms the basis of the equation system presented below. It was found that the prediction of the effect of internal mass transfer limitations in the catalytic washcoat of the SFR configuration is crucial to derive microkinetic data from SFR experiments (Karadeniz, 2014 Karadeniz et al., 2013) our model is extended to include the diffusion limitations due to a porous layer. It should be noted that the CFiEMKIN code has no abihty to account for internal mass transport in the catalytic coating. 

A second way of taking the spectra of polymeric samples in transmission is the heat and pressure method. This method works by compressing a polymer and heating it to above its glass transition temperature, where it will soften and flow to form a thin film. A diagram of how the technique works is shown in Figure 4.15. 

Foams that ate relatively stable on experimentally accessible time scales can be considered a form of matter but defy classification as either soHd, Hquid, or vapor. They are sol id-1 ike in being able to support shear elastically they are Hquid-like in being able to flow and deform into arbitrary shapes and they are vapor-like in being highly compressible. The theology of foams is thus both complex and unique, and makes possible a variety of important appHcations. Many features of foam theology can be understood in terms of its microscopic stmcture and its response to macroscopically imposed forces. 

During the press operation, which is actually a form of compression mol ding, the resin-treated laminate pHes are heated under pressure and the resins cured. The initial heating phases cause the resin to melt and flow into voids in the reinforcing ply and bond the individual pHes together. The appHed heat simultaneously causes the resin to polymerize and eventually to cross-link or gel. Therefore, resin viscosity reaches a minimum during the press cycle. This is the point at which the curing process becomes dominant over the melt flow process. Dynamic mechanical and dielectric analyses (11) are excellent tools for study of this behavior. 

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