Hot presses

The first industrial hardboard was developed by W. Mason in the mid-1920s he found that a mat of wet fiber pressed in a hot press would produce a self-bonded flat panel with good strength, durabiUty, and stabiUty. The product was patented in 1928, trademarked as Masonite, and commercial production began. Over time several other processes for producing hardboards have been developed from modifications of the original process. Brief descriptions of these processes foUow and a flow chart of the process is shown in Figure 5. 

M. H. Leipold, "Hot Pressing," ia F. F. Y. Wang, ed.. Treatise on Materials Science and Technology, Vol. 9 (Ceramic Fabrication Processes), Academic Press, New York, 1976. 

Fig. 2. Strength as a function of temperature for representative SiC stmctural ceramics A, sintered (Y2O2 added) , hot-pressed (2% AI2O2) sintered...

Reaction-bond ed Sintered Sintered beta alpha Hot-pressed (AI2O3)" Sintered... 

Fig. 3. Stress mpture behavior in air at 1200°C for SiC stmctural ceramics —hot-pressed -, reaction-bonded , sintered alpha —sintered beta. To...

Fig. 4. Thermal diffusivity of silicon-based stmctural ceramics (a) reaction-bonded SiC (b) hot-pressed and sintered SiC (c) hot-pressed (1% MgO,...

Using hot-pressing, shaping and densiftcation occur in a single process step. The temperatures are in the range of 1650—1800°C and appHed pressures are from 30—40 MPa (4000—6000 psi) (45), resulting in parts of high quaHty. This method is limited to simple shapes and low production volumes, however, and the process may also impart anisotropic characteristics to the material (46). 

Property Reaction-bonded Sintered Hot-pressed isostaticaHy Hot-pressed ... 

Magnesium fluoride optical crystals are made by hot-pressing (14) high quaUty MgF2 powder. The optical quaUty powder is made by the NH4HF2... 

Hot pressing with a smooth plate has an advantage in smoothing the grain, and the heat can be used to cure the resin of the finish. The hot pressing is anticipated in the design of the finish system and in the choice of the resins by the finish manufacturer. 

Hot pressing to produce substantial texture and magnetic anisotropy via plastic deformation is accompHshed by a process referred to as... 

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. 

Hot pressing produces compacts that have superior properties, mainly because of higher density and finer grain size. Closer dimensional tolerances than can be obtained with pressing at room temperature are also possible. Hot pressing is used only where the higher cost can be justified. It has been usehil in producing reactive materials. One use is the combination of P/M and composites to produce hot-pressed parts that are fiber reinforced. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Silicon Nitride. SiUcon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, siUcon nitride parts. a-SiUcon nitride, used in the manufacture of Si N intended for hot pressing, can be obtained by nitriding Si powder in an atmosphere of H2, N2, and NH. Reaction conditions, eg, temperature, time, and atmosphere, have to be controlled closely. Special additions, such as Fe202 to the precursor material, act as catalysts for the formation of predorninately a-Si N. SiUcon nitride is ball-milled to a very fine powder and is purified by acid leaching. SiUcon nitride can be hot pressed to full density by adding 1—5% MgO. 

Annual production of powdered BN is ca 180—200 metric tons per year and its cost is 50—250/kg, depending on purity and density. The price of cubic boron nitride is similar to that of synthetic diamond bort. Hot-pressed, dense BN parts are 3—10 times more expensive than reaction-sintered parts. 

Annual production of sihcon nitride is ca 100—200 t. Utility-grade sihcon nitride costs 4—5/kg in 100- to 500-kg quantities. The reaction-sintered parts are sold for 120 to 300/kg, depending on complexity of shape. Hot-pressed, fully dense Si N parts are priced 5—10 times higher than reaction-sintered parts. 

Sihcon nitride is one of the few nonmetaUic nitrides that is able to form alloys with other refractory compounds. Numerous soHd solutions of P-Si N and AI2O2 have gained technical interest. Many companies have begun to mass produce reaction-sintered and hot-pressed Si N parts. 

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