While in Sand castings the molds are destroyed after solidification of castings, the molds are reused repeatedly in the permanent mold castings. This requires a mold material that has a sufficiently high melting point to withstand erosion by the liquid metal at pouring temperature, a high enough strength not to deform in repeated use, and high thermal fatigue resistance to resist premature crazing ( the formation of thermal fatigue cracks) that would leave objectionable marks on the finished castings. Finally, and ideally, it should also have low adhesion.
The material used for making molds (dies) may be cast iron, although alloy steels are most widely used. For higher-melting alloys such a brasses and ferrous alloys , the mold steel must contain large proportion of stable carbides. More recently refractory metal alloys, particularly molybdenum alloys, have found increasing application. Graphite molds can also be used for steel although only for relatively simple shapes. The resistance of the mold to the melt can be increased with refractory coatings ( mold washes ) and adhesion can be reduced by graphite, silicon, or other parting compounds.
All cast metals can be cast by permanent mold method. Zinc, copper, aluminum, lead, magnesium and tin alloys are most often cast by this process. Grey Iron castings can also be produced by this method through a thin refractory coating or lining of sodium silicate or phosphoric acid is given so as to withstand high temperature of the molten metal.
In general, castings to be produced by permanent mold methods should be relatively simple in design with fairly uniform wall thickness and without undercuts or complicated coring. Undercuts on the exterior of castings complicate the mold design, resulting in additional mold parts and increased costs. Cores, if required , are made in sand.
Permanent mold castings have some distinct advantages over the typical sand mold casting. These include closer dimensional tolerances, better surface finish, greater mechanical strength, lower percentage of rejection, and more economical production in larger quantities. Some disadvantages of permanent molds are their lack of permeabilities, the high cost of the molds, the inability of the metallic mold to yield to the contraction forces of the solidifying metal, and difficulty in removing the casting from the mold since mold cannot be broken up.
Permanent metal molds can be advantageously used for small and medium sized (up to 10 kg) nonferrous castings, but would be impractical for large castings, and metals and alloys of very high melting temperature.
SLUSH CASTING :
Slush Casting , a form of permanent mold casting, is limited to some tin-zinc, or lead-base alloys. The principle involves pouring the molten metal into a permanent mold. After the skin has frozen, the mold is turned upside down or slung to remove the metal still liquid. A thin-walled casting (shell) results, the thickness depending on the cooling effect from the mold and time of operation. In this process hollow castings can be produced without the use of cores.
Slush castings find wide application in the production of such products as toys, ornaments, and lighting fixtures, where strength is not of prime importance and good appearance is an absolute necessity.
DIE CASTING :
Die casting is the art of rapidly producing accurately dimensioned parts by forcing molten metal under pressure into split metal dies which resemble a common type of permanent mold. Within a fraction of a second, the fluid alloy fills the entire die, including all the minute details. Because of the low temperature of the die(it is water-cooled), the casting solidifies quickly, permitting the die halves to be separated and the casting ejected. If the parts are small , several parts may be cast at one time in what is known as multiple-cavity die.
This process is particularly suitable for lead, magnesium, tin, and zinc alloys. The advantages of die casting practice lie in the possibility of obtaining castings of sufficient exactness and in the facility for casting thinner sections that cannot be produced by any other casting method.
Two main types of machines are used to produce die castings (1) The hot chamber , exemplified here by the plunger type machine, and (2) The cold chamber machine.
HOT CHAMBER DIE CASTING :
In a hot chamber submerged plunger-type machine , the plunger operates in one end of a goose neck casting which is submerged in the molten metal. With the plunger in the upper position, metal flow by gravity into this casting through holes 2 just below the plunger and the entrapped liquid metal is forced into the die 3 through the goose neck channel and in gate 4. As the plunger retracts, the channel is again filled with the right amount of molten metal. The plunger made of refractory material may be actuated manually or mechanically and hydraulically, that is by means of air pressure below 150 kg-f / cm2 (150 MN / m2). Heating 5 is continued throughout the operation to keep the molten metal sufficiently liquid. The range of alloys that can be handled is limited by the pump material.
COLD CHAMBER DIE CASTING :
In Cold Chamber Plunger Type Machine, The plunger 1 is driven by air or hydraulic pressure to force the charge into the die 3. As soon as the ladle 2 is emptied, plunger moves to the left and forces the metal into the cavity 4. After the metal solidified , the core 5 is withdrawn, and then the die is opened. Ejector 6 are employed to remove the casting automatically from the die.
The old chamber machine is ideal for metals such as aluminum alloys which cannot be cast in hot chamber machines due to the ready reactivity with molten aluminum with steel. High melting temperature alloys of the non ferrous type are also best die cast in cold chamber. pressure loosely in cold chamber machine range from 300 to 1600 kg-f / cm2. (about 29 to 157 MN / m2).
ADVANTAGES OF DIE CASTING ARE :
1. Very high rate of production is achieved.
2. Close dimensional tolerances of the order of 0.025 mm is possible.
3. Surface finish of 0.8 microns can be obtained.
4. Very thin sections of the order of 0.50 mm can be cast.
5. Fine details may be produced.
6. Longer die-life is obtained.
7. Less floor space is required.
8. Unit cost is minimum.
DISADVANTAGES OF THIS PROCESS ARE :
1. Non economical for small runs.
2. Only economical for non ferrous alloys.
3. Heavy castings cannot be cast. In fact, the maximum size is limited by the size of the dies and the capacity of the die casting machines available.
4. Cost of die and die casting equipment is high.
5. Die castings usually contains some porosity due to the entrapped air.
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