Pressure and pressing

The ether is removed by distillation and the product distilled under reduced pressure (Note 7). There is obtained 215 g. (84 per cent of the theoretical amount) of trichloroethyl alcohol boiling at 94—97°/125 mm. and melting at 16-17° (Note 8). A purer compound can be obtained by refractionation under reduced pressure and pressing out the crystals on a cooled porous plate. Pure trichloroethyl alcohol has a melting point of 190 (Note 9). 

Small variations in feed properties can have a pronounced effect on maximum pressure and press performance. Roll presses are scaled on the basis of constant maximum pressure. The required roll loading increases approximately with the square root of increasing roll diameter or gap width. 

Molybdenum is generally a satisfactory metal for use in contact with molybdenum disulphide, and there have been several reports of satisfactory performance of composites with molybdenum. Koval chenko and Yulyugin investigated the effects of temperature, pressure and pressing time on the density of hot-pressed compacts of molybdenum disulphide and molybdenum. They found that full compaction was only obtained at temperatures over 1100°C at 40 MPa and... 

Hot pressing is an effective and simple way for assembling electrodes and PEMs to achieve good interfacial contacts between them. Hot-pressing conditions, such as temperature, pressure, and pressing duration, influence the performance and durability of the resulting MEA [70-74]. A study [72] showed that the combination of temperature, pressure, and time should be optimized to achieve a high-performance MEA. 

Molding, traditional or under pressure, and pressing, unidirectional or isostatic, is part of the ceramist s range of tools for obtaining the final form of the piece to be made. 

Pour the resulting dark reddish-brown liquid into 500 ml. of water to which 17 ml. of saturated sodium bisulphite solution has been added (the latter to remove the excess of bromine). Steam distil the resulting mixture (Fig. II, 41,1) , collect the first portion of the distillate, which contains a little unchanged nitrobenzene, separately. Collect about 4 litres of distillate. Filter the yellow crystalline solid at the pump, and press well to remove the adhering liquid. The resulting crude m-bromonitrobenzene, m.p. 51-52°, weighs 110 g. If required pure, distil under reduced pressure (Fig. II, 19, 1) and collect the fraction of b.p. 117-118°/9 mm. it then melts at 56° and the recovery is about 85 per cent. 

Phenol—formaldehyde (PF) was the first of the synthetic adhesives developed. By combining phenol with formaldehyde, which has exceptional cross-linking abiHties with many chemicals and materials, and a small amount of sodium hydroxide, a resin was obtained. The first resins soHdified as they cooled, and it was discovered that if it was ground to a powder with a small amount of additional formaldehyde and the appHcation of more heat, the mixture would Hquify and then convert to a permanently hard material. Upon combination of the powdered resin mixture with a filler material such as wood flour, the result then being placed in a mold and pressed under heat and pressure, a hard, durable, black plastic material was found to result. For many years these resulting products were called BakeHte, the trade name of the inventor. BakeHte products are still produced today, but this use accounts for only a small portion of the PF resins used. 

As succeeding panels are laid-up, a stack of panels is formed. If the panels are to be cold-pressed, an uncommon procedure in modern manufacturing, the stack will be high enough to fit into the cold-press. The stack is roUed into the press, the press is closed under hydraulic pressure and the bonding pressure, 1035—1205 (ca 150—175 psi) is maintained for the time requited to form a bond. This time could vary from 30 to 120 minutes, depending on temperature and the adhesive formulation used. 

Laminated Strand Products. The most recent developments in the family of wood-based composites are a group of laminated strand products, made with strands oriented in the long direction of the product and marketed as stmctural composite lumber. One product is made with long, narrow strips of softwood veneer. The strips or strands are about 2.5 x 13 x 600 mm (0.1 x 0.5 x 24 in.), coated with a PRE adhesive, and pressed under heat and pressure into large blocks. After the resin is cured the blocks are resawn and planed into lumber dimension stock. 

The beater additive process starts with a very dilute aqueous slurry of fibrous nitrocellulose, kraft process woodpulp, and a stabilizer such as diphenylamine in a felting tank. A solution of resin such as poly(vinyl acetate) is added to the slurry of these components. The next step, felting, involves use of a fine metal screen in the shape of the inner dimensions of the final molded part. The screen is lowered into the slurry. A vacuum is appHed which causes the fibrous materials to be deposited on the form. The form is pulled out after a required thickness of felt is deposited, and the wet, low density felt removed from the form. The felt is then molded in a matched metal mold by the appHcation of heat and pressure which serves to remove moisture, set the resin, and press the fibers into near final shape (180—182). 

D. M. Newitt, The Design of High Pressure Plant and the Properties of Fluids at High Pressures, Clarenden Press, Oxford, 1940. 

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. 

With a sufficiently long press cycle, a state of complete cure is reached. At this point, the laminate is cooled in the press, under pressure, and removed for finishing operations. If the press is opened at a temperature above the boiling point of trapped volatiles, vaporization occurs causing interlaminar blistering which mins the laminate. 

Ziac foil coated with a conductive, pressure-sensitive adhesive is used for repair of other ziac coatings or for imparting corrosion resistance at field sites. The 0.08-mm ziac tape or sheet has a 0.025-mm coaductive adhesive. The laminate is cut to size and pressed tightly to activate the adhesive. Conductive tape can be wrapped around pipe, especially around welds or connections. The corrosion resistance of this material is iatermediate between galvanized or thermally sprayed coatings and zinc-filled paints (21,50). 

Cones usually are pressed. These are easier to ignite than cast cones. Pressure used for cone formation is considerably lower than that used for candle formation to prevent excessive densification. The preformed cone can be added to the mold before pressing or casting the body of the candle, being held ia place by the resulting bond. Alternatively, a cavity is pressed iato the formed candle, and cone material is added and pressed iato place. The bond formed by this method is weak but generally sufficient for most purposes. 

The disk refiner, a newer development used for fine paper grades, includes one or two rotary disks and two or four working surfaces. The surfaces are pressed together uniformly by hydraulic pressure and guided by high precision, heavy-duty bearing systems. Stock usuaUy is fed through the center of one plate and leaves between the plates at their circumferences. 

Because of low injection pressure, some cost savings are possible in mold and press constmction. Mol ding cycles are somewhat longer than for injection molding. The part must be cooled in the mold long enough to be able to resist swelling from internal gas pressure. In stmctural foam parts there is almost a total absence of sink marks, even in the case of unequal section thickness. Stmctural foam has replaced wood, concrete, sohd plastics, and metals in a variety of appHcations. 

Cold-isostatic-pressing foUowed by vacuum sintering or HIP is also used to manufacture smaller intricate shapes. In this instance beryUium powder is loaded into shaped mbber bags and pressed isostaticaUy in a pressure chamber up to 410 MPa (60,000 psi). After the pressing operation the mbber bag is stripped from the part which is then vacuum sintered to about 99% of theoretical density at about 1200°C. If full theoretical density is required, the sintered part may be simply given a HIP cycle because there is no open porosity after vacuum sintering. In a similar manner, conventional axial cold-pressing... 

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