Unidirectional pressing

This technique overcomes the drawbacks of unidirectional pressing and produces a charge of uniform density and homogeneity. In this method, the explosive formulation in the mold is compressed into position in two modes. In the first mode, pressure acts from top by the downward movement of the ram. In the second stage of pressing, the spacer is removed and the base plate moves upwards on application of pressure. Thus the formulation experiences equal pressure throughout the length. The L/D ratio is two in this case. 

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Lancaster studied the transfer of molybdenum disulphide and graphite to low-carbon steel discs from compacts. The compacts were formed at relatively low pressures, up to 80 MPa, but later work ° indicated that when compacts were formed at higher pressures up to 1500 MPa, there was no obvious difference in the nature of the transfer films produced from them. It can therefore be assumed that the crystallites in the compacts were mainly randomly oriented, although there is some evidence that in unidirectional pressing of a lamellar solid a relatively high degree of orientation occurs in the surface layers. 

Double pressing , that is, the densification of particulate solids by the movement of both the top and bottom punches, is used to overcome at least part of the uneven density distribution caused by unidirectional pressing. If both punches move towards each other with the same speed, exerting the same force, a mirror image of the density distributions across a neutral axis , which, in this case, is in the middle of the body, will be created (Fig. 6.7-23). If in addition, the die walls move with or to some extent in regard to the travel of the punches (withdrawal die), the location of the neutral plane... 

Another product is made with strands produced by flakers, having dimensions of about 0.8 x 13 x 300 mm (0.030 x 0.5 x 12 in.). The strands are dried and then coated with isocyanate adhesives. The strands are formed into mats with unidirectional orientation and pressed in a steam-through press. The large panels are then sawn into dimension lumber sizes. 

Hot Pressing. Hot pressing may be used either to consoHdate a powder that has poor compactabiHty at room temperature, or to combine compaction and sintering in one operation. The technique is essentially the same as described for unidirectional die compacting. The powder is heated by either heating the entire die assembly in a furnace or by induction heating. In most instances, a protective atmosphere must be suppHed. 

Applications. Boron fibers are used as unidirectional reinforcement for epoxy composites in the form of preimpregnated tape. The material is used extensively, mostly in fixed and rotary wing military aircrafts for horizontal and vertical stabilizers, mdders, longerons, wing doublers, and rotors. They are also used in sporting goods. Another application is as reinforcement for metal matrix composites, in the form of an array of fibers pressed between metal foils, the metal being aluminum in most applications. 

Fig. 17-2 The web of interactions in the atmospheric part of the global climate system. The strength of the interactions is qualitatively depicted by the thickness of the line. Bidirectional interactions have two arrowheads, unidirectional ones have only one. (From Houghton (1984), reprinted with permission from Cambridge University Press.)...

Measurement of biofilm activity can be performed based on laboratory reactor experiments or with a technique combining biofilm growth taking place in a sewer followed by measurements in laboratory scale (Raunkjaer et al., 1997 Bjerre et al., 1998). Huisman et al. (1999) developed a sewer in situ biofilm respiration chamber. It includes a DO sensor and a chamber that can be pressed onto the sewer wall. It is designed to achieve an even and unidirectional flow distribution over the entire measurement area. Pure oxygen is injected for oxygenation. 

Figure 5.85 Definition of axes in a continuous, unidirectional fiber-reinforced composite. Reprinted, by permission, from N. G. McCmm, C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, 2nd ed., p. 258. Copyright 1997 by Oxford University Press.

Pressure methods Unidirectional compaction 1. Powders compressed within die cavity into cylindrical or other simple shapes by action of punch. Tableting, powder pressing, dry pressing, hot pressing Tablet machines, powder presses. 

Pressure methods Unidirectional, piston-type compaction 1. Large briquets formed (e.g. from scrap metal turnings) by piston compression into die. Briquetting, compacting, preforming Heavy duty compacting presses... 

WJ/v, Eichenberger, General observations on metal briquetting practices using the unidirectional piston briquetting press, Proc. Inst. Briquet. Agglom. Bien. Conf., 9 (1965) 52-60. 

Fig. 2.7 The tensile stress-strain curves versus temperatures for (a) unidirectional SCS-6 fiber-reinforced hot-pressed Si3N4, (b) unidirectional SCS-6 fiber-reinforced reaction-bonded Si3N4, (c) 2-D Nicalon fabric-reinforced CVI-SiC, and (d) 3-D braided Nicalon fabric-reinforced CVI-SiC.49-52...

Fig. 5.2 Comparison of creep behavior and time-dependent change in fiber and matrix stress predicted using a 1-D concentric cylinder model (ROM model) (solid lines) and a 2-D finite element analysis (dashed lines). In both approaches it was assumed that a unidirectional creep specimen was instantaneously loaded parallel to the fibers to a constant creep stress. The analyses, which assumed a creep temperature of 1200°C, were conducted assuming 40 vol.% SCS-6 SiC fibers in a hot-pressed SijN4 matrix. The constituents were assumed to undergo steady-state creep only, with perfect interfacial bonding. For the FEM analysis, Poisson s ratio was 0.17 for the fibers and 0.27 for the matrix, (a) Total composite strain (axial), (b) composite creep rate, and (c) transient redistribution in axial stress in the fibers and matrix (the initial loading transient has been ignored). Although the fibers and matrix were assumed to exhibit only steady-state creep behavior, the transient redistribution in stress gives rise to the transient creep response shown in parts (a) and (b). After Wu et al 1...

A first step in the validation of this approach is to test simple specimens under controlled conditions and to compare predictions with measured failure load values. First lap shear geometries were examined, then an L-geometry was studied in more detail. The bond-line in these small specimens was very similar to that in the quasi-unidirectional fracture specimens as the small dimensions allow panels to be pressed uniformly after assembly (which is not the case for industrial top-hat stiffeners). 

Fig. 6.7-24 Drawings describing ways of influencingthe location ofthe neutral plane in unidirectional (upper punch) pressing by controlled die withdrawal [B.28, B.48, B.97]...

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