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1、Casting and its processes 1.1 IntroductionManufacturing process can be classified into two general groups known as primary and secondary processes. The primary processes, namely, casting, forging, cold heading, cold extruding, brake forming, and so on, are those that convert raw material into shapes
2、. These forming methods include both hot and cold working processes and, in general, will still require further finishing operations in order to obtain an end product.The secondary processes are those that bring the part to the dimensions and surface finish specified. One may also include in the sec
3、ondary processes such operations as heat treating and surface treatments for corrosion resistance, hardness, and appearance. Secondary processes are planning , turning ,milling, drilling , boring , reaming , broaching , grinding , honing ,lapping , polishing , and special methods of metal removal su
4、ch as electrochemical machining. Operations like screw cutting, tapping, thread milling, gear cutting, and so on ,are secondary processes that are merely adaptations of one of the aforementioned processes that are merely adaption of one of the aforementioned processes. A complete discussion of these
5、 procedures is beyond the scope of this text. Therefore, we will confine ourselves to succinct descriptions of the most prominent techniques.1.2 Castings Castings are identified by the type of mold or the force required toy fill the mold. Molds are either permanent or expendable. The pattern of sand
6、, shell, and plaster molds, however, can be used repeatedly for making new molds. As the metal cools, it takes the shape of the cavity. The basic casting methods are described below. Sand castings.1. The green sand process is one in which most, bonded sands packed around a wood or metal pattern. The
7、 pattern is then removed and molten metal is poured into the cavity .When the metal solidifies, the mold is broken and the casting is removed . Almost any metal can be used, with virtually no limit as to the size or shape of the part . The method permits casting complex components at a low tooling c
8、ost and is the most direct route from pattern to casting . Some machining is always necessary with the green sand process, and large castings have rough surface finish .Close tolerances are difficult to achieve, and long, thin projections should not be cast. It is possible , however , to design for
9、bosses, undercuts and inserts .The minimum core hole diameter advisable are aluminum,3| 161|4 in. ;and the minimum section thicknesses advisable are aluminum ,3|16 in .;cooper , 3|32 in .; iron ,3|32 in .; magnesium,5|332 in ;and steel , 1|41|2 in . 2. The dry sand process is similar to the green sa
10、nd process except that core boxes are used in place of patterns .This method is usually limited to smaller parts than method 1. The sand is bonded with a setting binder, and the core is then oven baked at 300400 to remove the moisture. 3.The carben dioxide sand process is one in which the sand molds
11、 are bonded whit sodium silicate solutions and set by forcing carbon dioxide gas through the sand .This type of mold is strong and permits the production of better dimensionally controlled castings than either method 1 or 2 . 4 .The coresand mold process is one where the molds are put together compl
12、etely from oven baked cores set with organic binders such as oil or dextrines. Shell Mold Casting . This is a process where the molds are made by coating the sand with a thermosetting plastic .The mold is then supported on the outsaid by sand or shot, and molten metal is poured .When the metal has s
13、olidified , the mold is broken away from the finished casting. Shell molding produces casting with smooth surfaces, uniform grain structure ,high dimensional accuracy , rapid production rate and minimal amount of finishing operations . The minimum section thickniss castable is 1|161|4 in .but sectio
14、n differences where the maximum minimum ratio is greater than 5 to 1 should be avoided. Bosses, undercuts and inserts are easily cast and the minimum cored hole diameter is 1|81|4 in .The method is relatively costly is limited to few metals. Full-Mold Casting . A process in which a green sand or col
15、d-setting resin bonded is packed around a foamed plastic pattern (for example ,polystyrene ).The plastic pattern is vaporized with the pouring of the molten metal into the mold .An improved casting surface can be obtained by putting a refractory type of coating on the pattern surface before sand pac
16、king .The pattern can be one piece or several pieces , depending upon the complexity of the part to be cast . The plastic patterns are handled easily . They do not require any draft ,and produce no flash on the casting . For small quantity production this method can be expensive, depending upon the size and complexity of the casting .The minimum recommended section thickness is 0 .1 in .,and the maximum section thickness is unlimited .Bosses, undercuts
