Indirect extrusion

Ammonia has deleterious effects on brain function by direct and indirect mechanisms. Concentrations of ammonia in the 1-2 mmol/1 range, equivalent to those reported in the brain in liver failure, impair postsynaptic inhibition in cerebral cortex and brainstem by a direct effect on Cl extrusion from the postsynaptic neuron. Millimolar concentrations of ammonia also inhibit excitatory neurotransmission. Synaptic transmission from Schaffer collaterals to CA1 hippocampal neurons is reversibly depressed by 1 mmol/1 ammonia, and the firing of CA1 neurons by iontophoretic application of glutamate is inhibited by 2 mmol/1 ammonia [10],... 

Indigoid soluble dyes, 7 373t Indigo vat dye, 9 181 Indirect-arc furnaces, 12 297—298 Indirect coal liquefaction, 6 858-867 Indirect cooler evaporators, 21 537 Indirect extrusion, copper, 7 693 Indirect food additives, 12 29, 34 categories of, 12 31 Indirect-gap semiconductors, 14 837 ... 

Extrusion can fall into one of two categories direct extrusion and indirect extrusion (Figure 7.17). In direct extrusion a metal billet is placed into a container and driven though a die by a ram, usually attached to a hydraulic press. In indirect extrusion the billet is again placed in a container, but the container end is closed. A hollow ram, carrying a die of the desired shape, is forced into the billet, with the extrusion exiting through the ram Indirect extrusion may occur as a result of die movement or container movement. Because there is no movement between the container wall and billet surface, frictional forces are lower in this method, which reduces the force needed to extrade. There are, however, some limitations on the load that can be applied by the hollow ram. 

The principal variables that govern the amount of force required for extrusion are the type of extrusion (direct or indirect), the ratio of input to output areas, the working temperature, the speed of deformation, and the friction between the billet and the die and container walls. At elevated temperatures the material is easier to deform, however, the surface of the billet will tend to oxidize more. This oxidized surface may be carried along shear bands to the interior of the extrusions, resulting in internal oxide stringers. Lubricant potentially becomes... 

Figure 7.17 (a) Direct extrusion process where billet moves against a stationary die by a moving ram (b) indirect extrusion, where the billet is fixed in a container and the die is at the end of a hollow, moving ram. 

After discussing the generation and quantitation of the potential term of the proton circuit, we shall now turn to the proton current, and examine the factors which control the flux of protons around the circuit. Although it is not possible to determine the proton current under steady-state conditions directly, the parameter may be calculated indirectly from the respiratory rate and the stoicheiometry of proton extrusion by the respiratory chain. It is outside the scope of this chapter to discuss the contentious issue of the proton stoicheiometries of the complexes, but the important feature is that, unless the complexities of variable stoicheiometry are invoked, respiration and proton current vary in parallel. 

Low-viscosity soybean briquetting adhesives are generally applied by spray. Paper and softboard laminating glues are usually applied by curtain coater, knife, or indirect roller. High-viscosity plywood and lumber assembly formulations are fairly well limited to application by spreader roll or extrusion. For very small assembly jobs, soybean glue can easily be applied by brush. 

Concentrations of ammonia equivalent to those reported in the brain in HE are known to cause deleterious effects on the CNS functions by both direct and indirect mechanisms (Szerb and Butterworth, 1992, review). Direct effects of the NH/ ion on both inhibitory and excitatory neurotransmission have been reported. Millimolar concentrations of ammonia impair postsynaptic inhibition in the brain by inactivation of the extrusion of Cl" from neurons (Raabe, 1987). This inactivation of Cl extrusion abolishes the concentration gradient for Cl across the neuronal membrane. Consequently, the opening of CT channels by the inhibitory neurotransmitter no longer takes place and the inhibitory postsynaptic potential (IPSP) is abolished. It has been demonstrated that brain ammonia concentrations as low as 0.5 mM may exert this adverse effect on inhibitory neurotransmission. 

Bar extmsion is a process in which a block of metal (billet) is forced to flow by compression through a tool (die) opening of a smaller cross-sectional area than that of the original billet. There are two basic types of bar extrusion direct and indirect. The most important and common method used is the direct extrusion (Lange 1988). Figure 1 shows the principle of direct bar extrusion where the billet is placed in the container and pushed through the die by moving a ram towards the die (Saha 2000). 

In indirect bar extrusion, the die at the front end of the hoUow ram moves relative to the container, but there is no relative displacement between the billet and the container as shown in Fig. 2. Therefore, indirect extrusirm is characterized by the absence of friction between the billet surface and the container. The absence of friction leads to reduced axial extrusion forces compared to direct extrusion. Therefore, indirect extmsion is used for the forming of heavy extradable alloys. But the major disadvantage of the method is the strong limited die design due to the required hollow ram geometry. 

Due to its surface texture and its anisotropic mechanical behavior, extrusion processes are not reconnnended for tooling, for neither Prototype nor Indirect or Direct Tooling. 

There are a number of variations of the extrusion process, two common methods being direct extrusion and indirect PrBSMiira extrusion. 

The indirect-extrusion process does have limitations, however. 

Oose tolerances difficult to achieve openings must be indirection of extrusion limited to shapes of uniform cross section (alcng length). 

The thia[ll]annulene (5) reacts with maleic anhydride at room temperature to give the dihydrothiepin (7) as a 2 1 mixture of the exo and endo-adducts (Scheme 2). The intermediate adduct (6) iexo- and e/u o-mixture) was not isolated but its presence was demonstrated by n.m.r. spectroscopy. Dehydrogenation of the dihydrothiepin (7) was accompanied by sulphur extrusion and led eventually to the ester (9). The presence of the thiepin (8) as an intermediate was presumed on the basis of indirect evidence. 

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