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An Overview of Die Casting Processes and Alumaximization
Die casting is a versatile process for producing
engineered metal parts by forcing molten metal
under high pressure into reusable steel molds.
These molds, called dies, can be designed to
produce complex shapes with a high degree of
accuracy and repeatability. Parts can be sharply
defined, with smooth or textured surfaces, and are
suitable for a wide variety of attractive and
serviceable finishes.
Die castings are among the highest volume,
mass-produced items manufactured by the
metalworking industry, and they can be found in
thousands of consumer, commercial and
industrial products. Die cast parts are important
components of products ranging from automobile
components to toys.
The earliest examples of die casting by pressure
injection - as opposed to casting by gravity pressure -
occurred in the mid-1800s. By 1892, commercial
applications included parts for phonographs and
cash registers, and mass production of many types of
parts began in the early 1900s.
The first die casting alloys were various
compositions of tin and lead, but their use declined
with the introduction of zinc and aluminum alloys in 1914. Magnesium and copper alloys
quickly followed, and by the 1930s, many of the modern alloys still in use today became
available.
The die casting process has evolved from the original low-pressure injection method to
techniques including high-pressure casting at forces exceeding 4500 pounds per square
inch squeeze casting and semi-solid die casting. These modern processes are capable of
producing high integrity, near net-shape castings with excellent surface finishes.
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Refinements continue in both the alloys used in die casting
and the process itself, expanding die casting applications into
almost every known market. Once limited to simple lead type,
today's die casters can produce castings in a variety of complex
shapes and sizes.
Die casting is an efficient, economical process offering a
broader range of shapes and components than any other
manufacturing technique. Parts have long service life and may
be designed to complement the visual appeal of the
surrounding part. Designers can gain a number of advantages
and benefits by specifying die cast parts.
High-speed Production :
Dimensional Accuracy and Stability :
Strength and Weight :
Multiple Finishing Techniques :
Simplified Assembly :
Die casting provides complex
shapes within closer tolerances than many other mass
production processes. Little or no machining is required
and thousands of identical castings can be produced
before additional tooling is required.
Die casting
produces parts that are durable and dimensionally stable,
while maintaining close tolerances. They are also heat
resistant.
Die cast parts are stronger than
plastic injection moldings having the same dimensions.
Thin wall castings are stronger and lighter than those
possible with other casting methods. Plus, because die
castings do not consist of separate parts welded or
fastened together, the strength is that of the alloy rather
than the joining process.
Die cast parts can be
produced with smooth or textured surfaces, and they are
easily plated or finished with a minimum of surface
preparation.
Die castings provide integral
fastening elements, such as bosses and studs. Holes can
be cored and made to tap drill sizes, or external threads
can be cast.
High pressure die casting is a manufacturing process in
which molten metal (aluminum) is injected with a die casting
machine under force using high speed and considerable
pressure into a steel
mold or die to form
products. Die casting
machines are typically
rated in clamping tons
equal to the amount of
pressure they can exert
on the die. Machine
sizes range from 400
tons to 4000 tons.
Regardless of their size, the only fundamental difference in die
casting machines is the method used to inject molten metal into
a die. The two methods are hot chamber or cold chamber. A
complete die casting cycle can vary from less than one second
for small components weighing less than an ounce, to two-to-
three minutes for a casting of several pounds, making die
casting the fastest technique available for producing precise
non-ferrous metal parts. Because of the excellent dimensional
accuracy and the smooth surfaces, most high pressure die
castings require no machining except the removal of flash
around the edge and possible drilling and tapping holes. High
pressure die casting production is fast and inexpensive relative
to other casting processes.
There are several aluminum alloys with different
mechanical properties and chemical breakdowns. Aluminium
is used in 80-90% of the high pressure die casting alloys
available in the world today. In many cases aluminum high
pressure die casting can replace steel, increasing strength and
reducing part weight. high pressure die casting parts are
produced in small sizes of less than 30 gms up to large sizes.
This equipment consists of two vertical platens on which
bolsters are located which hold the die halves. One platen is
fixed and the other can move so that the die can be opened and
closed. A measured amount of metal is poured into the shot
sleeve and then introduced into the mould cavity using a
hydraulically-driven piston. Once the metal has solidified, the
die is opened and the casting removed.
In this process, special precautions must be taken to avoid
too many gas inclusions which cause blistering during
subsequent heat-treatment or welding of the casting product.
Both the machine and its dies are very expensive, and for this
reason pressure die casting is economical only for high-volume
production.
Thousands of high pressure die casting parts can be
produced in a single day with the right die casting tooling and
proper high pressure die casting part design. Production of
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quantities of 20,000 to 30,000 high pressure die casting parts a
week in some cases. Most of the casting manufacturers are
capable to design or work with buyer's designer to develop
high volume high pressure die casting tooling.
Cold Chamber Plunger TipPlunger Rod
Plunger Rod Colpler
Shot Cylinder Rod
Shot Control Valve
Shot Cylinder
Shot Accum
IntensifierAccum
High pressure die casting (HPDC) is a widely used
manufacturing process for mass production of components of
aluminium and magnesium alloys, such as automotive
transmission housings and gearbox parts. Molten metal is
injected at high speed (50 to 100 metres/sec) and under very
high pressures into a die through a complex gate and runner
system. The geometrical complexity of the die leads to
strongly three-dimensional fluid flow. Within the die cavity,
jetting and splashing results in liquid droplet and possibly
atomised spray formation. Crucial to the production of
homogeneous cast components with minimal entrapped voids
is the order in which the various parts of the die fill and the
positioning of the gas exits. This is determined by the design of
the gate configuration and the geometry of the die.
The geometry of the die,
the gate, the runner and the
cylindrical shot sleeve
considered in the present
study is shown in Figure.
The fluid initially fills the
cylindrical column and is
pushed downward by a
plunger at the top of the fluid
moving at 15 m s-1. The fluid has a density =1000 kg m-3 and a
dynamic viscosity = 0.08 Pa s. The Reynolds number at the
gate is about 2700, with reference to the gate height of 5 mm.
(Simulations have also been performed for Reynolds numbers
of 500 and 2.7x104.) In these simulations, a resolution of 1
particle/mm was used giving a total of 292,931 particles.
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Hold molten metal in the shape of the desired casting.
Provide a means for molten metal to get to a space where it
will be held to the desired shape.
Remove heat from the molten metal and to allow the metal
to solidify.
To provide for the removal of the casting.
Two perspective views of the filling pattern at different
times are shown in Fig. 2. The first frame at 4 ms shows the
system after the runner has been filled and the fluid is just
entering the die. The second frame at 6 ms shows the fluid
entering the vertical cylindrical section. These frames indicate
that the leading material consists of fast moving fragments and
droplets generated by splashing as the fluid flows around the
distinct features of the die cavity. The final two frames, at times
8 and 10 ms, show the fluid converging into the slotted section
of the die. Despite the geometrical symmetry of the die, the
flow is observed to be asymmetric. In addition, the jetting of
fluid with a high velocity through the gate gives rise to a small
unfilled cavity on each side of the die; this cavity remains one
of the last regions to be filled.
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The extreme complexity of the filling of this relatively
simple die geometry illustrates the severe demands imposed on
a numerical method by HPDC simulation.
Die Casting vs. Plastic Molding : Die casting produces stronger parts with closer tolerances that have greater stability and durability. Die cast parts have greater resistance to temperature extremes and superior electrical properties.
Die Casting vs. Sand Casting : Die casting produces parts with thinner walls, closer dimensional limits and smoother surfaces. Production is faster and labor costs per casting are lower. Finishing costs are also less.
Die Casting vs. Permanent Mold : Die casting offers the same advantages versus permanent molding as it does compared with sand casting.
Die Casting vs. Forging : Die casting produces more complex shapes with closer tolerances, thinner walls and lower finishing costs. Cast coring holes are not available with forging.
Die Casting vs. Stamping : Die casting produces complex shapes with variations possible in section thickness. One casting may replace several stampings, resulting in reduced assembly time.
Die Casting vs. Screw Machine Products : Die casting produces shapes that are difficult or impossible from bar or tubular stock, while maintaining tolerances without tooling adjustments. Die casting requires fewer operations and reduces waste and scrap.
Each of the metal alloys available for die casting offer
particular advantages for the completed part.
Zinc - The easiest alloy to cast, it offers high ductility, high
impact strength and is easily plated. Zinc is economical for
small parts, has a low melting point and promotes long die
life.
Aluminum - This alloy is lightweight, while possessing
high dimensional stability for complex shapes and thin
walls. Aluminum has good corrosion resistance and
mechanical properties, high thermal and electrical
conductivity, as well as strength at high temperatures.
Magnesium - The easiest alloy to machine, magnesium has
an excellent strength-to-weight ratio and is the lightest
alloy commonly die cast.
Copper - This alloy possesses high hardness, high
corrosion resistance and the highest mechanical properties
of alloys cast. It offers excellent wear resistance and
dimensional stability, with strength approaching that of
steel parts.
Lead and Tin - These alloys offer high density and are
capable of producing parts with extremely close
dimensions. They are also used for special forms of
corrosion resistance.
High Pressure die casting molds OR Tooling, are made
from steel hardened to withstand high temperatures and
extreme pressures. There are many types of high pressure die
casting tooling from simple inexpensive inserts to complete
high pressure die casting dies that are dedicated to only one
part. Once a high pressure die casting tool is produced, the cost
to make high pressure die casting parts is very little.
Precisely, tooling are made of alloy tool steels in at least
two sections, the
fixed die half, or
cover half, and the
ejector die half, to
permit removal of
castings. Modern
dies also may have
moveable slides,
c o r e s o r o t h e r
sections to produce
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holes, threads and other desired shapes in the casting. Sprue
holes in the fixed die half allow molten metal to enter the die
and fill the cavity. The ejector half usually contains the runners
(passageways) and gates (inlets) that route molten metal to the
cavity. Dies also include locking pins to secure the two halves,
ejector pins to help remove the cast part, and openings for
coolant and lubricant. When the die casting machine closes, the
two die halves are locked and held together by the machine's
hydraulic pressure. The surface where the ejector and fixed
halves of the die meet and lock is referred to as the "die parting
line." The total projected surface area of the part being cast,
measured at the die parting line, and the pressure required of
the machine to inject metal into the die cavity governs the
clamping force of the machine.
Novaflow and Solid is a complete
mould filling and Solidification simulation
Package based upon advanced fluid flow
and heat transfer theories. The progression
of liquid metal flow is visualised in 2D or
transparent 3D and can be viewed during or
after simulation. The optimal velocity and
time settings for 1st and 2ND phase can be found out easily and
quickly. Slags and other non-metallic inclusions can be
introduced and traced.
Heat transfer is fully taken into account enabling the
Foundryman to predict and study not only flow but also
temperature changes in the metal and Die .Different alternative
positions and dimensions of cooling and heating channels can
be tested as well as cycling to a steady state temperature. Flow
and solid simulations visualise the consequences of Gating
system and Die Design. Casting Defects such as Oxide
Inclusions due to excessive turbulence, cold shuts , shrinkage
cavities and slag inclusions can be avoided by optimizing the
design of the gating and Venting system.
Gas entrapment often occurs in the high pressure die
casting (HPDC) due to the highly turbulent flow
characteristics during cavity filling. The entrapped gas forms
porosity in the castings that can result in rejects or make them
unsuitable for heat treatment. Gas entrapped in a casting is
commonly believed to originate from three main sources:
trapped air, steam and burnt lubricant.
In the cold chamber HPDC process, the shot sleeve is only
partially filled with molten metal, the fill ratio is normally in the
range of a third to a half. This means volume of air to be vented
from cavity is at least 2-3 times the volume of metal poured into
shot sleeve and it is a major source of gas. Die and plunger
lubricants can also evaporate or burn when in contact with
molten metal. Hydrogen in the metal can also be a source of gas.
However, the maximum release of hydrogen can only account
for less than 3 percent of the casting volume. A common
practice in the industry to eliminate gas entrapment is to apply a
vacuum technique during cavity filling. In the cold chamber
process, cavity filling takes place within a few to tens of
milliseconds. The effective evacuating time is only a few
seconds (plunger travel time from covering the pour hole to
change-over position). The amount of gas evacuated will
depend upon the efficiency of the vacuum system applied.
The first high volume applications of Aluminium were the heat Exchangers of Volkswagens First Generation Golf in 1974.In the years to follow, different types of heat Exchangers were developed for different purposes(cooling of Engines, Turbochargers, A/C, oil coolers and so on).Today more than 95% of heat Exchangers are made of Aluminium. Aluminium cylinder heads are found in nearly all modern cars .This is also due to excellent heat conductivity properties of the block which enables the catalyst to be effective very fast. Alumaximized car study was made for the first time in Aachen University sometime back to study how light a modern car could be without any compromise on safety by intensive use of Aluminium. The result has surprising. Starting with a reference car weight of 1229 khs the primary weight savings led to 1003 and 928 kgs car weight. Applying now the secondary weight savings ,one can finally achieve 887 and 785 kgs total weight respectively. This implies the intensive use of Aluminium will lead to cars with today's safety, comfort and environmental standards, but having a significantly lower weight and also meet tomorrow's CO2 challenges.
To meet the challenges for furure cars regarding emissions,
Safety and sustainability, Aluminium usage is predominant and
unavoidable. The history has shown that Aluminium application
have brought together Automotive and Aluminium Industries a
big step forwarding Innovative Technology and attractive
Products. The challenges of today is to produce defect free
components though the HPDC route with total control on aimed
Properties.
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(Papers presented at the seminar on -
Light Metal Casting Technology)
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