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)