CHAPTER 4
PART 3
DIE CASTING
Ass.Pr.Dr. Nguyen Ngoc Ha
1. INTRODUCTION
• A permanent mold casting process in which
molten metal is injected into mold cavity
under high pressure
• Pressure is maintained during solidification,
then mold is opened and part is removed
• Molds in this casting operation are called dies;
hence the name die casting
• Use of high pressure to force metal into die
cavity is what distinguishes this from other
permanent mold processes
Die Casting Pro
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ducts
1.1. Advantages and Limitations of
Die Casting
• Advantages:
– Economical for large production quantities
– Good dimensional accuracy and surface finish
– Thin sections are possible
– Rapid cooling provides small grain size and good
strength to casting
• Disadvantages:
– Generally limited to metals with low metal points
– Part geometry must allow removal from die cavity
1.2. Common Characteristcs
• Tolerances:
Al and Mg .002/in.
Zinc .0015/in.
Brass .001/in.
Add .001 to .015
across parting line
depending on size
• Surface Finish: 32-63RMS
• Minimum Draft
Requirements:
Al & Mg: 1° to 3°
Zinc: 1/2° to 2°
Brass: 2° to 5°
• Normal Minimum Section
Thickness:
Al & Mg: .03 Small Parts:
.06 Medium Parts
Zinc: .03 Small Parts: .045
Medium Parts
Brass: .025 Small Parts:
.040 Medium Parts
• Ordering Quantities:
Usually 2,500 and up.
• Normal Lead Time:
Samples: 12-20 weeks
Production: ASAP after
approval.
1.2. Common Characteristcs
• Wide range of shapes
• Lower melting point alloys
• High mold costs – large quantity product
• High production rates with short cycle times
• Extra dies required for trimming flash and
runners
1.2. Common Characteristcs
• Product weight <0.01 to 50 kg
• Good surface finish, low porosity, capable of high
complexity parts
• Excellent dimensional accuracy +/- 0.001
tolerance
• Takes several weeks before first product output
• Produces 2-200 parts/mold hour
• Requires approximately 10,000 parts for use
1.3. Method Application
• High-melting-point alloys of aluminum,
magnesium, and copper are commonly cast by
this method, although other metals (including
ferrous metals) can also be cast in this
manner. Molten-metal temperatures start at
about 600 ˚C for aluminum and magnesium
alloys and increase considerably for copper-
base and iron-base alloys.
2. DIE-CASTING ALLOYS
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
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
2. DIE-CASTING ALLOYS
• 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.
2. DIE-CASTING ALLOYS
ASTM
Number
Cu Si Mg Al Uses
S12A&B -- 12 -- Remainder Large Intricate castings
S5C -- 5 -- Rem. Gen. Purpose
G8A 3 -- 8 Rem. High strength, res. corro.
SG100A&
B
-- 9.5 0.5 Rem.
Gen. Purpose, Good Property,
excellent casting charac.
SC84B 3.5 9 -- Rem.
Good machinability and
castability
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.
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.
2. DIE-CASTING ALLOYS
ASTM
Number
Cu Si Sn Pb Zn Uses
Z30A 57 min -- 1.5 1.5 30 min Yellow brass, good machinability
Z5331A 65 1 -- -- Rem.
Gen. Purpose casting, Corr. res.,
castability
Z5144A 81 4 -- -- Rem.
High strength, hardness, wear
resistance-but most difficult to mold
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.
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.
2. DIE-CASTING ALLOYS
ASTM
Number Al Zn Mn Si Cu Ni Mg
B94 9 0.5 0.13 0.5 0.3 0.03 Rem.
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.
Properties and Applications of Die-Casting
Alloys
3. MOLDS FOR DIE CASTING
3.1. Introduction
• Usually made of tool steel, mold steel, or
maraging steel
• Tungsten and molybdenum (good refractory
qualities) used to die cast steel and cast iron
• Ejector pins required to remove part from die
when it opens
• Lubricants must be sprayed into cavities to
prevent sticking
3.1. Introduction
• Die Materials:
- Hot-worked tools steels
- Mold steels
- Maraging Steels
- Refractory Metals
Properties:
1. High hot strength
2. High temperature wear resistance
3.2. Mold Materials
• For Al, Zn, Mg Alloys:
- 40Cr5MoV1Si (SKD 61)
- 40Cr5MoVSi (SKD 6)
• For Cu Alloys:
- 30Cr3Mo3V
- 30Cr2W8V (SKD4, SKD5)
- 40Cr8W2
• For steel: Mo, Mo-W Alloys
3.3. Types of Cavities in Die-Casting Die
Various types of cavities in a die-casting die. Source:
Courtesy of American Die Casting Institute.
3.3. Types of Cavities in Die-Casting Die
• Die-casting dies, Fig., may be made single-
cavity dies, multiple-cavity dies (with several
identical cavities), combination-cavity dies
(with several different cavities), or unit dies
(simple small dies that can be combined in two
or more units in a master holding die).
3.3. Types of Cavities in Die-Casting Die
• Typically, the ratio of die weight to part weight is
1000 to 1. Thus the die for a casting weighing 2 kg
will weigh about 2000 kg. Dies are usually made
of hot-work die steel or mold steels. Die-wear
increases with the temperature of the molten
metal. Heat checking of dies (surface cracking
from repeated heating and cooling of the die) can
be a problem. When die materials are selected
and maintained properly, dies may last more than
half a million shots before any significant die
wear takes place.
Example of a Die Casting Mold
Example of a Die Casting Mold
4. DIE CASTING MACHINES
• Designed to hold and accurately close two
mold halves and keep them closed while
liquid metal is forced into cavity
• Two main types:
1. Hot-chamber machine
2. Cold-chamber machine
4.1. Hot-Chamber Die Casting
Metal is melted in a container, and a piston
injects liquid metal under high pressure into
the die
• High production rates - 500 parts per hour not
uncommon
• Applications limited to low melting-point
metals that do not chemically attack plunger
and other mechanical components
• Casting metals: zinc, tin, lead, and magnesium
4.1. Hot-Chamber Die Casting
Cycle in Hot Chamber Casting
Cycle in hot-chamber casting:
(1) with die closed and plunger withdrawn, molten
metal flows into the chamber
Cycle in Hot Chamber Casting
Cycle in hot-chamber casting:
(2) plunger forces metal in chamber to flow into die,
maintaining pressure during cooling and solidification
800-ton hot-chamber die-casting machine
4.2. Cold-Chamber Die Casting Machine
• Molten metal is poured into unheated chamber
from external melting container, and a piston
injects metal under high pressure into die cavity
• High production but not usually as fast as
hot-chamber machines because of pouring step
• Casting metals: aluminum, brass, and magnesium
alloys
• Advantages of hot-chamber process favor its use
on low melting-point alloys (zinc, tin, lead)
4.2. Cold-Chamber Die Casting Machine
Schematic illustration of the cold-chamber die-casting process. These machines
are large compared to the size of the casting, because high forces are required to
keep the two halves of the dies closed under pressure
Cycle in Cold Chamber Casting
Cycle in cold-chamber casting:
(1) with die closed and ram withdrawn, molten metal
is poured into the chamber
Cycle in Cold Chamber Casting
Cycle in cold-chamber casting:
(2) ram forces metal to flow into die, maintaining pressure
during cooling and solidification
4.3. Process Capabilities and Machine
Selection
• Machines are rated according to the clamping
force needed to keep the dies closed.
• Capacities range: 25 to 3000 tons.
• Other factors involved in the selection of die-
casting machines are die size, piston stroke, shot
pressure and cost.
• Ratio of die weight to part weight is 1000 to 1
• Dies are usually made of hot-work die steels or
mold steels.
4.3. Process Capabilities and Machine
Selection
• Die design includes draft to allow removal of the
casting.
• Die casting has the capability for rapid production
of strong, high-quality parts with complex shapes.
• It also produces good dimensional accuracy and
surface details, (net-shape forming).
• Because of the high pressures involved, walls as
thin as 0.38 mm are produced. Ejector marks
remain.
4.3. Process Capabilities and Machine
Selection
• Because the molten metal chills rapidly at the
die walls, the casting has a fine-grained, hard
skin with higher strength.
• Strength-to-weight ratio of die-cast parts
increases with decreasing wall thickness.
• Components such as pins, shafts, and
threaded fasteners can be die cast integrally
Called insert molding.
• For good interfacial strength, inserts may be
knurled, grooved, or splined.
4.3. Process Capabilities and Machine
Selection
• In selecting insert materials, the possibility of
galvanic corrosion should be taken into account.
• If galvanic corrosion is a potential problem, the
insert can be insulated, plated, or surface-
treated.
• Equipment costs, particularly the cost of dies, are
somewhat high, but labor costs are generally low.
• Die casting is economical for large production
runs.
• Lubricants (parting agents) often are applied as
thin coatings on die surfaces. The usually are
water-based lubricants with graphite.
4.3. Process Capabilities and Machine
Selection – Insert Molding
Grooved bolts cast into a part, after the molten
metal solidifies, they become a part of the
product
5. DESIGN CONSIDERATIONS IN DIE
CASTING
5.1. Construction of casting parts
5.1. Construction of casting parts
5.1. Construction of casting parts
5.2. Gating System
5.2.1. Direct Gating System
5.2.2. Inner Gating System
5.2.3. Outer Gating System
5.3. Gating System Design
a) Poor; b) Better; c) The Best
5.4. Vent
5.5. Washing Canals
6. LOW PRESSURE CASTING
• The basic process is shown in Fig.
- In basic permanent and slush casting processes, metal in
cavity is poured under gravity. However, in low-pressure
casting, the metal is forced into cavity under low pressure (0.1
MPa) of air.
Advantages
- Clean molten metal
from the center of ladle
(cup) is introduced into
the cavity.
- Reduced- gas porosity,
oxidation defects,
improvement in
mechanical properties
Products
7. VACUUM PERMANENT-MOLD
CASTING
• This is a variation of low-pressure permanent
casting
• Instead of rising molten into the cavity
through air pressure, vacuum in cavity is
created which caused the molten metal to rise
in the cavity from metal pool.
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