Oven forming

Drape forming (also called oven forming) is carried out commercially on small and large parts where the plastics material is pre-heated in an oven to the forming temperature, placed in a mould and held in place while it cools. Applications include windscreens, seat backs and bottoms, covers and doors. 

Frequently the water or other solvent is so firmly held that it cannot be completely removed in a vacuum desiccator at the ordinary temperature. These substances are dried in a vacuum oven at a higher temperature. A convenient laboratory form of vacuum oven is the so-called... 

Other vapor introduction systems are discussed in Parts B and C (Chapters 16 and 17) because, although liquids and solids are ultimately introduced to the plasma flame as vapors, these samples are usually prepared differently from naturally gaseous ones. For example, electrothermal (oven) or laser heating of solids and liquids to form vapors is used extensively to get the samples into the plasma flame. At one extreme with very volatile liquids, no heating is necessary, but, at the other extreme, very high temperatures are needed to vaporize a sample. For convenience, the electrothermal and laser devices are discussed in Part C (Chapter 17) rather than here. 

For GC, the injector is most frequently a small heated space attached to the start of the column. A sample of the mixture to be analyzed is injected into this space by use of a syringe, which pierces a rubber septum. The injector needs to be hot enough to immediately vaporize the sample, which is then swept onto the head of the column by the mobile gas phase. Generally, the injector is kept at a temperature 50 C higher than is the column oven. Variants on this principle are in use, in particular the split/splitless injector. This injector can be used in a splitless mode, in which the entire injected sample goes onto the column, or in a split mode, in which only part of the sample goes onto the column, the remainder vented to atmosphere. For other less usual forms of injector, a specialist book on GC should be consulted. 

Specific gravity is direcdy related to the bulk density of waste fuels prepared ia a variety of ways. Solid oven-dry (OD) wood, for example, has a typical bulk density of 48.1 kg/m (301b/ft ). In coarse hogged form, eg, <1.9-cm minor dimension, this bulk density declines to about24kg/m (151b/ft ). In pulverized form, at a particle size <0.16 cm, this bulk density declines to 16—19 kg/m (10—12 Ib/ft ). Similar relationships hold for municipal waste, agricultural wastes, and related fuels. 

The original process of heating coal in rounded heaps, the hearth process, remained the principal method of coke production for over a century, although an improved oven in the form of a beehive was developed in the Durham-Newcastie area of England in about 1759 (2,26,28). These processes lacked the capabiHty to collect the volatile products, both Hquids and gases. It was not until the mid-nineteenth century, with the introduction of indirectiy heated slot ovens, that it became possible to collect the Hquid and gaseous products for further use. 

Some power tubes can be operated without the need for a protective ferrite isolator. One example is the cooker magnetron (700 W) used in modern microwave ovens (57). At higher power levels, such as 25 kW, it is more common to employ a protective ferrite device, particularly in the form of a circulator (58), as shown in Figure 3. This results in a power loss equivalent to a few percentage points in system efficiency. The ferrite circulator prevents reflected power from returning to the power tube and instead directs it into an auxiHary dummy load. The pulling of tube frequency is thus minimised. 

Rotational Molding. Large containers and some toys are manufactured with a specialized technique caHed rotational mol ding. A rotational mol ding machine contains several large metal molds that can be rotated in two perpendicular planes. A load of fine LLDPE particles is introduced into each mold, and the mold assembly is transferred into an oven. Inside the oven, heated molds rotate at speeds ranging from 10 to 40 rpm. The polymer powder melts and is uniformly distributed on the internal surface of each mold. After the container is formed, the mold assembly is removed from the oven and cooled at which point the plastic containers are removed. 

Foundry Resins. In the foundry industry, phenoHc resins are used as the binder for sand in the manufacture of sheU molds and cores. The two mating halves are joined by clamps or a bonding agent to form a sheU mold into which the molten metal is poured for castings. The sheU is formed by depositing a resin—sand mix on a hot metal pattern plate. After a certain period the pattern is inverted and the excess resin sand is removed. The sand particles are bonded by an oven cure, and the sheU is ejected from the pattern plate. 

Rotational Molding. Hodow articles and large, complex shapes are made by rotational mol ding, usuady from polyethylene powder of relatively low viscosity (57—59). The resin is in the form of a fine powder. A measured quantity is placed inside an aluminum mold and the mold is heated in an oven and rotated at low speed. The resin sinters and fuses, coating the inside of the mold. The mold is then cooled by water spray and the part solidifies, dupHcating the inside of the mold. 

In the semidirect process, (Fig. 23) the taw coke oven gas is cooled to condense tar and ammonia Hquor. The heavy layer, tar phase, is pumped to storage and the aqueous layer containing free and fixed ammonia is subsequendy processed in a stiH operation. Free ammonia is that which is in a form which readily dissociates by heat. Fixed ammonia is in a form which requites the presence of an alkaH, such as milk of lime, to effect the ammonia release. 

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