Ch3 Special Cast Proc by hd3hIc57

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                           CHAPTER 3
                  SPECIAL CASTING PROCESSES
3.1 INTRODUCTION

 The process used for making a casting depends on;
 - the quantity to be produced,
 - the metal to be cast,
 - the complexity of the part.
 Sand molds are single-purpose molds, and are completely destroyed after the metal
 has solidified.
 Quite obvious the use of a permanent mold effect considerable saving in labor cost.

 A summary of the various special casting methods, which will be discussed in this
    chapter, is as follows:
 A. Casting in metallic molds
 B. Centrifugal casting
 C. Precision or investment casting
 D. Continuous casting
 E. Shell molding

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3.2 METAL MOLD CASTING PROCESSES

Permanent molds must be made of metals capable of withstanding high
temperatures.

Because of their high cost they are recommended only when many casting are
to be produced.

Although permanent molds are impractical for large castings and alloys of high
melting temperatures, they can be used advantageously for small and medium-
sized castings that are manufactured in large quantities.

Die Casting: Die casting is a process in which molten metal is forced by
pressure into a metal mold known as a ‘die’.

The usual pressure is 100 to 125 atm.
It is the most widely used of any of the permanent mold processes.
There are two methods employed:
                                      1. Cold-chamber Die Casting
                                      2. Hot-chamber Die Casting

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(1) Cold-chamber Die Casting:

Material to be cast is molten outside the machine.
Used for materials having high melting temperature Tm> 550°C, i.e. brass,
aluminum, and magnesium.

(2) Hot-chamber Die Casting:
Materials to be cast is molten inside the machine.
Used for materials having low melting temperature Tm< 550°C, i.e. zinc, tin, and
lead.

(The principal distinction between the two is determined by the location of the melting
pot.

In the hot-chamber pot is included in the machine; and
In the cold-chamber pot is separate from the machine,

metal is introduced to injection cylinder by other means.)



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             Piston
                           Parting                                      Parting
                           line                                         line

                                         Ejector                                     Ejector
                                         pins                                        pins
                                      Cooling       Piston
                                                                                  Cooling
                                      water                                       water
                                      circulation                                 circulation



Melting crucible      Mold (Die)                                   Mold (Die)
      HOT CHAMBER                                      COLD CHAMBER


                                   Fig. 3.1 Types of Die Casting




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             Schematics of hot-chamber die-casting

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             Schematics of cold-chamber die-casting



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             Die Casting Cavities




                Die Casting Products




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Advantages of die casting:

1. The process is rapid (since both dies and cores are permanent)

2. The surface quality is very good. (The smooth surface improves
   appearance and reduces work required for other operations.)

3. Dimensional tolerances are very good comparing to sand casting. (The
   size is so accurately controlled that little or no machining is necessary.)

4. The scrap loss is low since sprue, runners, and gates can be remelted.




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- The optimum production quantity ranges from 1,000 to 200,000 pieces.
- Max. Weight of brass die casting is about 2 kg, but aluminum die castings of
  over 50 kg are common.
- Small to medium size castings can be made at a cycle rate of 100 to 800 die
  fillings per hour.
- The life of dies depends on the metal cast and may range from 10,000 fillings
  for brass to several million if zinc is used.
- If the part is big and complex, a single-cavity mold is used. If the parts are
  small and quantity is large, multiple-cavity die can be used.

   Optimum no. of parts: 1000-200,000
   Weight of part:       Brass about 2 kg

                          Aluminum about 50 kg
   Die life:              about 10,000 filling for Brass
                          Several millions for filling Zinc


Other methods of metal mold castings: low-pressure permanent mold casting,
gravity permanent mold casting, slush casting and pressed casting.

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3.3 DIE-CASTING ALLOYS

A relatively wide range of nonferrous alloys can be die-cast. The principal base
metals used are ZINC, ALUMUNIUM, MAGNESIUM, COPPER, LEAD and TIN. They
are classified in two groups:
1. Low-temperature alloys (casting temp. below 5500C),
2. High-temperature alloys

Zinc-Base Alloys : Over 75% of die castings are produced from zinc-base alloys.
Melting point is around 4000C. So, they are cast by Hot-chamber die casting.

           ASTM Number         Al            Cu          Mg        Zn
           AG40A(XXIII)        4.1        0.1 max        0.04   Remainder
               AC41A(XXV)      4.1          1.0          0.04   Remainder
Aluminum improves mechanical properties.
Copper improves tensile strength and ductility.
Magnesium makes casting stable in microstructure.

Zinc alloys are widely used in: automotive industry, washing machines,
   refrigerators, business machines, etc.
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Aluminum-Base Alloys : They are used due to their lightness in mass and
   resistance to corrosion. Compared to zinc alloys they are more difficult to cast
   (melting point around 5500C). Since molten aluminum will attack steel if kept in
   continuous contact with it, the cold-chamber process generally is used.
      ASTM
                  Cu     Si    Mg        Al                    Uses
     Number
     S12A&B        --   12      --   Remainder        Large Intricate castings
       S5C         --    5      --     Rem.                Gen. Purpose
       G8A         3     --     8      Rem.          High strength, res. corro.
                                                   Gen. Purpose, Good Property,
   SG100A&B        --   9.5    0.5     Rem.
                                                     excellent casting charac.
                                                      Good machinability and
     SC84B        3.5    9      --     Rem.
                                                           castability
  Principal elements used as alloys with aluminum are SILICON, COPPER, and
      MAGNESIUM.
  Silicon increases hardness and corrosion resisting properties, copper increases
      mechanical properties, magnesium increases lightness and resistance to
      impact. They are generally used in aerospace industry and production of
      pistons.
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Copper-Base Alloys : Die casting of brass and bronze have presented a
greater problem due to their high casting temperature. Temperatures are around
870 to 10400C, which need heat resisting die material. Copper alloys are cold
chamber die-cast.

   ASTM
                Cu       Si      Sn     Pb      Zn                 Uses
  Number
                                                             Yellow brass, good
    Z30A       57 min    --      1.5    1.5   30 min
                                                                machinability
                                                        Gen. Purpose casting, Corr.
  Z5331A         65      1        --     --    Rem.
                                                              res., castability
                                                          High strength, hardness,
  Z5144A         81      4        --     --    Rem.       wear resistance-but most
                                                              difficult to mold

Copper-base alloys have extensive use in miscellaneous hardware, electric-
machinery parts, small gears, marine, automotive and aircraft fittings, chemical
apparatus, and numerous other small parts.
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Magnesium-Base Alloys : It is alloyed principally with ALUMINUM, but may
contain small amounts of SILICON, MANGANESE, ZINC, COPPER, and
NICKEL. They have the lowest density. Their casting temperatures are around
670-7000C, so, cold chamber die casting is suitable.

   ASTM Number       Al     Zn         Mn     Si      Cu     Ni       Mg
         B94         9      0.5     0.13     0.5      0.3   0.03      Rem.




                                                       Copper parts

 Zinc parts          Aluminium parts




                                  Magnesium parts
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•   Die Materials:
-   Hot-worked tools steels
-   Mold steels
-   Maraging Steels
-   Refractory Metals

Properties:
1. High hot strength
2. High temperature wear resistance

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3. 4. CENTRIFUGAL CASTING (Savurma Döküm)

Centrifugal casting is the process of rotating a mold while the metal solidifies so
as to utilize centrifugal force to position the metal in the mold.

Castings of symmetrical shape lend themselves particularly to this method,
although many other types of casting can be produced.

                                              Guide rollers




                              Flask
        Bottom rollers                                                             Carrier
               Fig. 3.2 Centrifugal casting machine for casting steel or cast iron pipe



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Setup for true horizontal centrifugal casting
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Centrifugal casting is often more economical than other methods.

Cores in cylindrical shapes and risers or feed-heads are both eliminated.

The castings have a dense metal structure with all impurities forced back to the
centre where frequently they can be machined out.

Piston rings weighing 50-100 grams to paper mill rolls weighing over 42 tons have
been cast in this manner.

Aluminum engine block uses centrifugally cast iron liners.

In some alloys, the heavier elements tend to be separated from the base metal,
known as “gravity segregation”.

The metal is forced against the walls on the mold with a centrifugal force of
approximately 70 g, which is a force 70 times that of the force of gravity alone on
the casting. Forces as high as 150 g have been used but are seldom found
necessary unless very thick-walled cylinders are being cast.


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Semi-centrifugal casting




 - In this method, centrifugal force is used to produce solid castings rather than
 tubular parts.
 - Density of the metal in the final casting is greater in the outer sections than at the
 center of rotation.
 - The process is used on parts in which the center of the casting is machined away,
 such as wheels and pulleys.

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3.5. PRECISION OR INVESTMENT CASTING (Hassas Döküm)

Precision or investment casting employed techniques that enable very smooth
highly accurate castings to be made from both ferrous and non-ferrous alloys.

The process is useful in casting unmachinable alloys and radioactive metals.

There are a number of processes employed, but all incorporate a sand, ceramic,
plaster (alçı), or plastic shell made from an accurate pattern into which metal is
poured.


Advantages of investment techniques are;

(1)   intricate forms with undercuts can be cast;
(2)   a very smooth surface is obtained with no parting line;
(3)   dimensional accuracy is good;
(4)   unmachinable parts can be cast;
(5)   may replace die casting for short runs.

Main disadvantage is that it is expensive and not suitable for big parts.
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Patterns are produced from wax (mum) or plastics which are subsequently melted
from the mold, leaving a cavity having all the details of the original pattern.

Present practice requires that a replica of the part to be cast made from steel or
brass.

From this replica a bismuth or lead-alloy split mold is made.

After wax is poured into the mold and solidification takes place, the mold is opened
and the wax pattern removed.




                                                                          Fig.          3.3
                                                                          Processes      of
      Replica   Split pattern   Wax pattern
                                                                          Precision      or
                                                                          Investment
                                              Wax Tree   Plaster Coated   Casting

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Several patterns are usually assembled (Wax Tree) with necessary gates and risers
by heating the contact surfaces (wax welding) with a hot wire.

This cluster is molded by silica sand, plaster or ceramic slurries.

After the mold material gets strength, the mold is placed upside down and heated in
an oven for several hours to melt out the wax and to dry the mold.

The casting can be produced by gravity, vacuum, pressure, or centrifugal casting.

When the solidification finished, mold is broken away and gates and risers are cut-
off.




                                                                           Fig. 3.3
       Replica   Split pattern   Wax pattern
                                                                           Processes of
                                                                           Precision or
                                               Wax Tree   Plaster Coated   Investment
                                                                           Casting

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                                         Figure

                                         Schematic
                                         illustration of
                                         investment
                                         casting, (lostwax
                                         process).

                                         Castings by this
                                         method can be
                                         made with very
                                         fine detail and
                                         from a variety of
                                         metals.




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Examples




Investment casting of an integrally cast
rotor for a gas turbine.                         Crosssection and microstructure
(a) Wax pattern assembly.                        of two rotors:
(b) Ceramic shell around wax pattern.            (top) investment-cast;
(c) Wax is melted out and the mold is filled,    (bottom) conventionally cast.
    under     a    vacuum,   with    molten
    superalloy.
(d) The cast rotor, produced to net or near-
    net shape.

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  3. 6. CONTINUOUS CASTING )

Continuous          casting                                             Laddle
consists    of     pouring
molten metal into one
                                         Liquid metal
end of a metal mold
open at both ends,
cooling    rapidly,    and
extracting    the     solid
product in a continuous
length from the other
end.                                                       Mold ( , , ,...)
                                                           Water cooling jets
This is done with copper,
brass,            bronze,          Guide
aluminum, and to a                 rollers
growing extent, cast iron
and steel.                                                         Rolling mills

                                  Fig. 3.4 A continuous casting rolling process

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Continuous casting is done for a number of purposes.

It is suitable for any shapes of uniform cross-section: round,
square, rectangular, hexagonal, fluted, gear toothed, and
many other forms; solid or hollow.

A growing use is to produce blooms, billets, and slabs for
rolling structural shapes. This is cheaper than rolling from
ingots.

A bloom has a square cross section with a minimum size of
15 by 15 cm.

A billet is smaller than a bloom and may have any square
section from 4 cm up to the size of a bloom.

Slabs may be rolled from either an ingot or a bloom. They
have a rectangular cross-sectional area with a minimum
width of 25 cm and a minimum thickness of 4 cm. The width
is always three or more times the thicknesses, which may be
as much as 40 cm. Plates, skelp and thin strips are rolled
from slab.
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Continuous casting offers several advantages:

1. It yields 10% or more over rolling from ingots. Ingots have an appreciable
   amount of porous end, which returns back to furnace. This waste is eliminated
   in continuous casting.

2. A hollow center occurs from shrinkage in continuous casting but it is welded shut
    after four rolling passes. Continuous cast structure is more uniform and dense.

3. Physical properties and surface finishes are comparable to those obtained in
   other metal mold processes.

Continuous casting is essentially automatic, and unit labor cost is low. Dies or
  molds are made of copper or graphite.




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 3. 7. SHELL MOLDING
Shell moulding is a casting process in which the mold is a thin shell (typically 9mm)
made of sand held together by a thermosetting resin binder.




 Steps in shell moulding: (1) a match-plate or cope-and-drag metal pattern is heated and placed over a box
 containing sand mixed with thermosetting resin;
 (2) Box is inverted so that sand and resin fall onto the hot pattern, causinga layer of the mixture to partially
 cure on the surface to form a hard shell;
 (3) Box is repositioned so that loose, uncured particles drop away;
 (4) Sand and shell is heated in oven for several minutes to complete curing
 (5) Shell mold is stripped from the pattern
 (6) Two halves of the shell mold are assembled, supported by sand or metal shot in a box,
 (7) The finished casting is removed
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 Advantages:
 1. Good surface finish (up to 2.5
    mm)
 2. Good dimensional accuracy
    (±0.25 mm)
 3. Suitable for mass production

 Disadvantages:
 Expensive metal pattern
Area of application:                                Two halves of a shell mold pattern
Mass production of steel casting of less than 10 kg




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3. 8. Expandable-pattern casting (Lost foam Process)

 This process uses a polystyrene pattern, which evaporates upon contact with
 molten metal to form a cavity for the casting; this process is also known as lost-
 foam casting.

 It has become one important casting process for ferrous and nonferrous
 metals, particularly for the automative industry.




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3. 9. Casting Techniques for single-crystal components




Methods of casting turbine
blades: (a) directional
solidification; (b) method to
produce a single-crystal blade;
and (c) a single-crystal blade with
the constriction portion still
attached.



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