Vacuum forming technique

The main reason for extruding polystyrene is to prepare high-impact polystyrene sheet. Such sheet can be formed without difficulty by vacuum forming techniques. In principle the process consists of clamping the sheet above the mould, heating it so that it softens and becomes rubbery and then applying a vacuum to draw out the air between the mould and the sheet so that the sheet takes up the contours of the mould. 

Vacuum forming technique, in synthetic heart valves,... 

Deformation of a polymer in the rubber state—of importance in vacuum forming, pressure forming and warm forging techniques. 

The main applications for CPVC arise from the fact that the material has a softening point of about 100% and very good chemical resistance. Particular interest has been shown in waste and soil systems which may pass hot water effluents. Calendered sheet may be vacuum formed for uses where hot filling techniques are employed, for example in jam packing. 

The reaction of Na with Hg to form an amalgam that can then be separated from the NajO for oxygen analysis has been compared with the vacuum distillation technique . Ion-exchange techniques in which the sample is dissolved in a suitable solvent and the resulting separation of elements is achieved by an ion-exchange resin is less common. This technique is particularly suited to separating the volatile impurities such as K, Rb and Co. ... 

Finally, it may be pointed out that none of the rare metals can be smelted directly from the ore. The concentrate must first be converted to a pure chemical compound which is utilized as the raw material for the production of the metal. The refractory rare metals are often obtained in the form of a powder or sponge. They are consolidated and refined by powder metallurgy techniques or by arc melting or by electron beam melting. In fact, the current refractory rare metals technology has been crucially dependent on the development of vacuum metallurgical techniques and processes. 

Vacuum based techniques are capable of producing new materials or new forms of existing materials either in bulk form or as surface layers. There is a wide range of different techniques and applications, only some of which have been covered here. It is clear from the literature that much needs to be done in both the fundamental and applied aspects of such work. For example, relatively little is known about the effects caused by the arrival of clusters or charged particles at a growing film. 

The primary methodologies for forming thin-film materials with atomic level control are molecular beam epitaxy (MBE) [4-9], vapor phase epitaxy (VPE) [10-12], and a number of derivative vacuum based techniques [13]. These methods depend on controlling the flux of reactants and the temperature of the substrate and reactants. 

Explosives which are produced in flaked form (like TNT) lose moisture during the process of flaking (See under "Drum, Flaker ) Methods used for drying expls in Germany during WWII were described by Glucklich (Ref 6). Holland-Merten (Ref 15) described vacuum drying techniques used in explosives industry... 

The synthetically useful dianions [M3(CO)u]2- were first isolated by Shore and co-workers as the Ca2+ (M=Ru) and the K+ (M=Os) salts by the reduction of M3(CO)12 using alkali metal benzophenone solutions in THF.1 [Ru3(CO) J2 reacts with Ru3(CO) 2 to form the higher nuclearity clusters [Ru4(CO)13]2- and [Ru6(CO)i8]2- but the triruthenium anion can be obtained in high purity by slowly adding triruthenium dodecacarbonyl to an excess of reducing solution using vacuum-line techniques.2 Vacuum-line syntheses of both dianions have been described in detail.1... 

A small bubble test was adopted that allowed us to evaluate performance and appearance after vacuum forming. This technique requires a small sheet sample which is heated under specified vacuum forming conditions and then blown into a bubble of predetermined size. Now our engineers were able to predict the appearance of the final vacuum formed part during a laboratory or plant sheet extrusion run. 

The first step in sample preparation is the deposition of a thin metal film on an insulating substrate (e.g. a glass microscope slide). This base electrode is deposited by conventional vacuum deposition techniques with the electrode geometry defined by a shadow mask. Next, this electrode is oxidized either by exposing the film to room air or oxygen, or by establishing an oxygen plasma within the vacuum chamber. In the case of Al-electrodes, a remarkably uniform oxide layer is formed, typically 1-2 nm thick. The oxide film may then be dosed with the compound of interest this is achieved in one of three ways. 

An instrumental form of multiple development is referred to as programmed multiple development. The sorbent is heat dried between each development. A instrumental variation is referred to as automatic multiple development. In this case, the mobile phase is removed from the developing chamber and the sorbent dried under vacuum. These techniques have been reviewed recently [27,45]. 

Sheet forming is carried out on a sheet, in most cases extruded, sometimes calendered. The sheet is first heated to above its softening temperature, then formed and thereafter cooled. The technique most frequently applied is vacuum forming the force required for deformation is brought about by a vacuum below the heated sheet, which sucks the sheet onto the mould. In its simplest form, this process is sketched in Figure 11.28. Heating is mostly achieved with infra-red radiation sources, cooling with compressed air or water sprays. 

The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6% from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area to a point where the roughness factor approaches unity. 

One of the most important of these new experimental tools has been the development and application of the vacuum microbalance technique in which the sensitive microbalance operates directly in the vacuum or reaction system. The success of the method depends upon the coordination of a number of different experimental as well as theoretical disciplines. Thus, from an experimental point of view precise weighing techniques on properly prepared specimens must be coordinated with high vacuum techniques and the use of ceramic materials at high temperatures. From a theoretical viewpoint thermodynamic calculations must be made for all of the reactions involved and the results interpreted in terms of diffusion process for gas-solid reactions in which a film is formed or the gas diffuses into the solid, or in terms of the absolute reaction rate theory or its equivalent for gas reactions on solids including catalytic reactions and the combustion of fuels. 

There are different forming techniques that use this combination. Air flow and air pressure is used to preform a heated sheet prior to the final pull-down onto the cavity using vacuum. This is a takeoff of combining pressure forming and vacuum forming. 

For many routine reactions involving Grignard reagents in diethyl ether, the solvent vapour forms a protective blanket over the solution (cf. Ref. [5]), and drying tubes are adequate to exclude moisture. Nevertheless, the use of closed systems with inert atmospheres is recommended even for preparative work, and for physicochemical studies, where traces of alkoxides may have significant effects, it may be necessary to employ vacuum line techniques, rigorously purified atmospheres in glove boxes, and the like. The cheapest form of inert atmosphere is white spot ... 

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