Metal Casting Processes

Metal
die casting is used casting pieces that have complex shapes and no dimensional
tolerance. This technique provides to produce a large number of casting pieces
which have same shapes and consist of two or more parts.In addition to this,
great numbers of cast parts in the same way 
can be produced many times thanks to metal die casting.

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In
this casting technology, the produced molten metal is shaped by pouring
permanent (usually metal) molds. The liquid metal is filled into the mold with
influence of gravity. While simple cores comprised of metals, complex shaped
cores are composed from sand mixture or plaster. The method used for
non-metallic cores is called “semi-permanent die casting”.

Mould
cavity is coated with refractor materials in order to expand life-span of mold
and it is also easier to remove the part from the core. Special quality cast
iron or steel is usually used as the mold material in order to have the
refractoriness required by the cast metal. Also,bronze is used to cast metals
that melt in low temperatures.

Permanent Mold Casting Methods

In expendable-mold casting, a new mold requires
for each new casting. However, in permanent-mold casting,mold is used again in
many times. 

Method Types:

·        
Die
Casting

·        
Pressure
Die Casting

·        
Squeeze Casting

·        
Centrifugal
Casting

§ 
Particle
shapes are restricted in permanent-mold casting.

§ 
Permanent
mold casting is more economical in mass production.

§ 
Permanent
molds are expensive, production and design of them is difficult.

§ 
In
permanent molds, rate of solidification is high so fine grained parts are
produced.

§ 

Metals which have high
melting temperatures can not cast in permanent molds.

Die Casting

Thanks to die casting, between 3000 and 10000 pieces of
materials based on iron can be casted. Moreover, about 100000 pieces can be
casted  from materials that melt at low
temperatures like aluminum.

Permanent moulds are comprised of two or more parts which
can open and close.

After the mold is closed, the liquid metal is poured into
the gap.Then, the mold is opened and the part is removed ,after the
solidification.

These operations can be made by hands or machines through
automation.

Achievements of these methods depends on continuous working
temperature of the mold. Section thickness of molds is chosen between 18-50 mm.

Examples of parts produced by the die casting method include
refrigerated compressor housings, hydraulic brake cylinders, connecting rods,
toys and kitchen utensils.

Advantages of parts that are produced by die casting

·        
Mechanical properties of parts which are
produced by die casting are better through fine grained interior.

·        
The rapid formation of solidification and
cooling makes it possible to obtain a more frequent and fine structure.

·        
Tensile stress and yield stress increase.

·        
Greater hardness and toughness are achieved.

·        
Casting time shortens.

·        
Critical dimensional tolerances are provided.

·        
Complex parts can be produced.

·        
Surface quality of part is good and expenditures
of surface cleaning is low.

·        

Die casting is an economic application for mass production.

Pressure Die Casting

Liquid metal is filled into the metal mold under high
pressure in this method. Too much liquid metal can be filled very fast through
high pressure. The pressure is applied until the solidification is completed
and then the mold is opened and the process is completed by removing the piece
from the mold by pusher rods. This method makes it possible to cast very
complicated shaped parts.

Generally, the melting temperature of material to be cast is
below 1000 ºC. Parts that was obtained from casting do not require to
machining. Also,surfaces of parts are fine grained and high strength as a
result of cooling fast. If production is more than 5000 parts, this method can
be used since the molds are expensive. Cast iron, carbon steel, alloy steel or
nonferrous metals can be used as a mold material. If temperature of pouring
metal is high, alloy steel molds are used; if it is low, carbon steel molds are
used. Generally, molds have two parts and pusher rods in order to remove the
part. Molds are cooled to provide constant temperature in them.Thus, life span
of molds is increased as well as fast cooling is ensured during solidification.

Advantages of the method

·        
Parts that have complex shapes and small
dimensions can be casted with pressure die casting method.

·        
Rate of production is high.

·        
Additional finishing operations are not required
since dimensional tolerance and surface quality is high.

·        
The mechanical properties of the fine-grained
interior, which is the result of rapid cooling, are good.

Limits of
the method

·        
Only small parts can be produced.

·        
Design of mold is hard.

·        
A preliminary investment requires for casting
machine.

·        
This method is economic for mass production due
to expenditures of mold.

·        
Materials that melts at high temperatures can
not be casted with this method.

Cold Chamber Die Casting

Cold chamber die casting method is used for alloys
(copper,aluminum,magnesium alloys) which melts at high temperatures and they
are undesirable to keep in touch with working parts of machine. In this method,
desirable amount of molten metal is transposed to chamber. Then, piston fills
the molten metal into the mold compressing and pressure is applied until the
solidification process is finished. Later on, mold is separeted as in hot chamber
and part is removed by pusher rods.

 

Hot Chamber Die Casting

Metals which melt at low temperatures like tin,lead,and zinc
are casted with this method. In this method, chamber is in pot. Runner as
gooseneck provides to fill the molten metal into the metal mold under pressure.
Pressure is applied until the solidification process is finished. Then, mold is
distinguished and part is removed by pusher rods.

 Low-Pressure Die Casting

Low-pressure die casting is parted from die casting because of
injection step and applying a positive pressure to the metal in mold during
solidification. In the low pressure casting method, a pressure of 40

  is applied
differently from the pressure of 100-1000

 in hot and
cold chamber high pressure casting methods. Liquid metal that fills to the mold
from the chamber is more clean so this method is more advantageous than
conventional casting methods. Thus, gas holes and corrosion are minimized and
mechanical properties enhance.

Vacuum Permanent Mold Casting

This method resembles to low-pressure die casting on
principle. Pressure in mold is decreased through vacuum pump and pressure
difference provides that liquid metal fills to the mold. Because less
turbulence is created with this method, less gas vacancy occurs than with other
methods.As a result of this, more durable casting pieces are obtained. Furthermore,
this method generates a part which can be tolerated heat  treatment cycles.

 Squeeze
Casting

As it is seen in
the figure,liquid metal is filled into a open mold through gravitation. After the
closure of the other half of the mold, the liquid metal fills the mold
completely compressed. High
fluidity of liquid metal are not required since movement of liquid metal is low
in the mold. Thus, forged alloys ,which can not be casted with other methods,
can be casted with this method.

Volume and temperature of liquid
metal and applied pressure is important in squeeze casting. This technique can
be defined that it can produce part by applying a single forging stroke to
liquid metal.

 

Thanks to this
method, advanced properties of casting and forging operations are combined.

Squeezing after
the forging provides a homogenous casted part by breaking inclusions and
welding micro pores. The process performed on a small size automatic machine is
economical compared to other casting methods in terms of the use of all of the
molten metal resulting from the direct effect of the press, less energy cost,
removed running costs and thus lower mechanical machining cost. Excellent
mechanical properties in casting is supplied by obtaining void-free and thin
structure. Obtaining this structure is possible with squeeze casting that
involves slow and uniform filling and solidification of the liquid metal in a
closed mold under high pressure. Parts which are produced through squeeze
casting have better mechanical properties than products manufactured by other
methods.

 Centrifugal Casting

In centfifugal
casting, liquid metal is poured into molds that are revolved around an axis.
Pressure that creates through centrifugal force provides homogenous
distrubition of metal and shaping of outside of part according to inner shape
of the mold. Due to the high centrifugal force, low density sand and slag
particles, non-metallic residuals and gasses in the spilled liquid metal are
dragged towards the axis of rotation.

Consequently, this method provides
to obtain part with a clean and fine-grained surface.

 

Advantages

·        
Pipes
and other parts can be poured without using a core.

·        
Since
a non-porous and clean internal structure is obtained, the mechanical
properties of the material poured by this method are superior to other methods.

·        
Part
tolerances are narrow, surfaces are of good quality.

·        
Since
there is no runner system, the amount of material thrown away is very small.

·        
The
thin sections of the mold are easily filled.

·        
Since
feed rate of molten metal is high, casting temperature can be selected low.

Disadvantages

·        
Pour-cast
parts are limited.

·        
The
casting machine requires a high investment.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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