Casting to shape

Figure 7.7 Solidification history tor a cast-to-shape component. McGraw-Hill Encyclopedia of Science and Technology, Vol. 11, 8th ed., p. 35. Copyright 1997 by McGraw-Hill.

Bulk Polymerization. This involves only monomer, initiator, and perhaps chain-transfer agent. It gives the greatest polymer yield per unit of reactor volume and a very pure polymer. However, in large-scale batch form, it must be run slowly or in continuous form with a lot of heat-transfer area per unit of conversion to avoid mnaway. Objects are conveniendy cast to shape using batch bulk polymerization. Poly(methyl methacrylate) glazing sheets are produced by batch bulk polymerization between glass plates. They are also made by continuous bulk polymerization between polished stainless steel... 

The specimens used in these tests were standard ASTM "dogbone" specimens cast to shape (ASTM D638-72, Type I Specimen). Apart from the specimens that were tensile tested, evaluations were also made of thermal shock behavior. 

Grey irons are brittle materials, have a high compressive strength between three and four times the tensile strength, and can be easily cast to shape and easily machined. 

Objects may be conveniently cast to shape. If the polymer is the one that is crosslinked in the synthesis reaction, the only way of obtaining such objects short of machining from larger blocks. 

Many proprietary methods have been developed for casting and shaping DADC, especially for lenses. In one method DADC containing 3.5% diisopropyl percarbonate is prepolymerized by warming to a symp of viscosity 40—60 mm /s (=cSt) (15). Polymerization is continued in a lens for 18 h at 90°C followed by annealing at 120°C. 

Fusion-Cast Shapes. Refractory compositions are arc-melted and cast into shapes, eg, glass-tank flux blocks as large as 305 X 610 X 1219 mm. After casting and annealing, the blocks are accurately diamond ground to ensure a precise fit. 

These super-alloys are remarkable materials. They resist creep so well that they can be used at 850°C - and since they melt at 1280°C, this is 0.72 of their (absolute) melting point. They are so hard that they cannot be machined easily by normal methods, and must be precision-cast to their final shape. This is done by investment casting a precise wax model of the blade is embedded in an alumina paste which is then fired the wax bums out leaving an accurate mould from which one blade can be made by pouring liquid super-alloy into it (Fig. 20.4). Because the blades have to be made by this one-off method, they are expensive. One blade costs about UK 250 or US 375, of which only UK 20 (US 30) is materials the total cost of a rotor of 102 blades is UK 25,000 or US 38,000. 

Subtracting matter, by cutting, chipping, or carving, for example, provides a way to shape solid materials into objects. Many objects are not shaped by the subtraction of matter, however, but are either cast or wrought cast objects are shaped when the material used to make them, such as glass, plaster of Paris, or bronze, is in a fluid condition. Wrought objects, on the... 

In open-mold casting, the shape of the desired object is carved as a "negative" depression or hollow into a suitable bulk of material that withstands the temperature required for the process wood is often used for cold casting molds stone, sand, and dry mud, for hot casting. The fluid casting material is then poured into the carved depression, where it is left to set and from which, when solid, it is finally removed. 

By adding relatively large amounts of carbon (several weight per cent) to iron, it was found in China (500 BCE) that large and complex cast-iron shapes could be made readily. Through chill-casting, cast-iron becomes very hard (albeit brittle) this gives it considerable wear resistance. 

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