Introduction

1.1 INTRODUCTION: High pressure cold chamber die casting is a casting process in which metal of ferrous and non-ferrous alloys in molten state is forced by plunger under high pressure into the mould cavity which is reusable (i.e. die). Complex and intrinsic shapes can be produced repeatedly with the help of a well designed mould [1]. Cheapest parts can also be produced with this casting process and is best suited for the mass production. Many parts like engine housings, transmission cases, engine heads, laptop housings, mobile housings etc. can be manufactured very precisely from this casting process. Roughly, the temperature of metal in molten state is maintained near around 6950C.

1.2 HISTORY: Earlier casting processes were used to cast various metals other than aluminium. Casting processes were generally practiced from more than 5500 years ago when metals were melted and casted using different mould shapes made up of various soft minerals and clay. As with time, the casting methods developed and more complex mould shapes evolved which lead to the casting of various naturally occurring metals such as silver, gold, copper, etc. During this phase, the moulds were made from the mixture of sand and clay which were used to cast weapons and tools. Iron and steel castings were also produced by similar methods [2]. During 17th century, the aluminium with excellent casting properties, led to the adoption of existing casting methods at that time for producing more extrinsic shaped products with much ease as compared to Iron and steel metals. Also, it led to the evolution of lead-based printed type casting using Iron and steel dies. In the early 1820s, casting by pressure injection method evolved to meet the increasing demand of printed type castings [2]. In 1849, first manually operated machine was patented by Sturgis for printed type casting. This machine led to the injection of molten metal into the cavity of die using a crank lever which is purely mechanical [3]. During the next 20 years, hydraulic and pneumatic system were introduced into the machines which led to the exclusion of manually operated casting machines as it was limited to printed type castings. The introduction of hydraulic and pneumatic systems led to the production of bicycle parts, consumer durable parts and phonographs. The employment of Iron and steel mould for aluminium castings led to the commercialization of aluminium alloy casting between 1870 and 1900 [2]. With the development of other shapes, it led to the decline in the production of Lead based alloys by 1914. In 1914, the introduction of zinc and aluminium alloy castings became successful.  Finally in 1920, the progress in aluminium alloy casting technique led to the development of first cold chamber die casting process. During this century, the three emerging market including electrification industry, automotive industry and aerospace industry declared aluminium as alternate material because of its appearance, light weight and other excellent casting properties [2, 3].

1.3 Die Casting Process: Permanent die casting processes are the one which are best suited for continuous productions with large quantities at low costs [4]. In permanent die casting manufacturing process, permanent hot rolled tool steel moulds are used known as dies. The main difference between the different die casting processes is that of introducing the molten metal into the die cavity to get required casting. Two distinct die casting processes are as follows: Gravity die casting process, Pressure die casting process

1.3.1 Gravity Die Casting Process: It is the casting process similar to sand casting with a difference that in this process a permanent mould made up of metal is used to obtain the required casting and this process is also known as metallic mould casting or permanent die casting. In this casting process, as like sand casting the molten metal from graphite crucible is fed into the mould cavity under gravitational influence [4]. Both ferrous as well as non-ferrous alloys including copper alloys, zinc alloys and cast iron alloys can be used as the raw material to produce casting with required mechanical properties using this process with casting weight ranging from one and half kilograms to 50 kilograms [5].

1.3.2 Pressure Die Casting Process: Generally in pressure die casting process, the molten metal is fed into the mould cavity under pressure of piston and plunger arrangement provided in the machine. The Molten metal is fed under high pressure into the metal die with two halves as shown in Fig.1.3. As shown in figure the mould or die consist of two halves among which one is movable and other is stationary. Complex shapes are produced using high pressure die casting processes with the help of reusable moulds known as die. The castings produced under high pressure die casting process are near net shape and sized castings which require negligible machining or surface finishing. Mostly these processes are used with non ferrous alloy castings such as aluminium alloy, magnesium alloys, zinc alloys or other non ferrous alloys. But, the classification of pressure die casting processes is based on the system of injection of molten metal is either submerged in molten metal or away from molten metal. The size of the machine is based on the tons of pressure that machine exerts on the die to clamp it and hold the molten metal under pressure ranging from 400 Tons to 4000 Tons [1, 6]. The two most commonly known pressure die casting processes are: Hot Chamber High Pressure Die Casting Process, Cold Chamber High Pressure Die casting Process

1.4 Equipment for Die Casting: Die casting equipments are the machines which are employed to produce the required casting. Hot chamber die casting machine and cold chamber die casting machine are the two distinct die casting machines with distinct method of injecting the molten metal into the die cavity. Die casting machines perform different functions while their operation which are as follows: The first function is the closing of the die, The second function is of feeding the molten metal into the die cavity with required velocity and  under high pressure, The third function is of holding the molten metal inside the die cavity under high intensification pressure for some seconds, The last function is of ejecting the solidified casting out of the die with the help of ejector pins in the movable die, Along with the listed functions, sometimes the machine performs one more function of melting the metal in the integrated melting unit. As earlier discussed that the classification of pressure die casting processes is based on whether the system of injection is submerged in the integrated metal melting furnace or metal melting furnace is away from the molten metal injector. The hot chamber high pressure die casting machine and cold chamber high pressure die casting machine are of two types of pressure die casting machines [4].

1.4.1 Hot Chamber Die Casting Machine: Name of the machine itself indicates that the furnace for melting the metallic raw material is integrated to the machine. As the plunger and gooseneck remains submerged in the furnace, therefore this machine is mostly used for alloys with low melting temperatures such as tin, lead and zinc. The other metals with higher melting temperature are not casted with this machine because the pump is in direct contact with the molten metal in the furnace and if casting with such metals is carried out then it would destroy the plunger assembly as well as the pump [8]. The vital parts of hot chamber high pressure die casting machine are as follows [4]: Metal container, Furnace with heating chamber, plunger assembly for feeding the molten metal into the die cavity, two half dies with one fixed to the stationary platen and other to the movable platen, ejector pins for the removal of the casting from the die, frame of the casting machine. As listed above that the machine consists of two die halves such that the one which is fixed to the stationary platen is called cover die and it is so constructed that it aligns with the gooseneck channel. The other die fixed to movable platen is called as ejector die as it contains the ejector pins. Before the molten metal is fed into the die cavity, the two die halves are closed and clamped under a high hydraulic pressure of hydraulically powered clamping bars attached to the movable platen. Once the dies gets closed and the molten metal in the hot chamber gets sucked into the shot chamber through an opening and then the plunger gets actuated with the hydraulic power of the machine and injects the molten metal into the die. The hydraulic system keeps the plunger down under a high pressure to keep the molten metal under high intensification pressure till it gets solidifies. Once the molten metal gets solidifies, the clamping bars releases the pressure from the movable die and the movable die retracts back and the casting is taken out with the help of the ejector pins. The plunger also moves back to its initial position and made ready for a new casting [8]. Depending on the method of actuation of plunger, the hot chamber die casting is further classified into the following two kinds [4]:

Submerged plunger die casting machine: In this machine, the plunger is submerged in molten metal and is either hydraulically operated or pneumatically operated and forces the molten metal into the die under high pneumatic or hydraulic pressure. Once the casting gets solidifies inside the die cavity, the die halve open and casting is taken out with the help of ejector pins provided in the movable die.

Pneumatic die casting machine: In this machine, the molten metal is fed into the die cavity directly through the goose neck under direct air pressure. As the casting cycle begins, the goose neck is submerged into the hot chamber containing the molten metal which gets filled by gravity. After the filling of molten metal into the goose neck, it is raised to align its nozzle with the die open and connected to die opening after aligning. Once connected to the die, air under high pressure is blown which forces the molten metal in the gooseneck to fill the cavity. The air pressure is maintained till the metal inside the cavity gets solidifies. As the metal inside the cavity solidifies, the supply of compressed air is turned off.

1.4.2 Some applications of hot chamber dir casting machine: Hot chamber die casting machine are mostly compatible with: Alloys with low melting temperatures such that the molten metal should not harm the plunger assembly and the pump. Those metals which do not harm the submerged parts of the plunger assembly.

1.4.3 Cold chamber High Pressure Die Casting Process: In cold chamber

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Verlag: BookRix GmbH & Co. KG

Tag der Veröffentlichung: 16.09.2019
ISBN: 978-3-7487-1558-0

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