An engine block with aluminium and magnesium die castings.

Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mould cavity. The mold cavity is created using two hardened tool steel dies which are heat ressistant and have been machined into shape and work.The two dies meet along the parting line , each cavity is machined to have the opposite shape of each other. There are small vents cut along the edges to allow the air to go out during the process. If the air is not allowed to go out, it produces blow holes in the castings. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter and tin based alloys.

Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

Process

The following are the four steps in traditional die casting, also known as high-pressure die casting,[4] these are also the basis for any of the die casting variations:

1. Die preparation,2. Filling

3. Ejection

4. Shakeout

1. The dies are prepared by spraying the mold cavity with lubricant. The lubricant both helps control the temp of the die and it also assist in the removal of the casting

2. The dies are then closed and molten metal is injected into the dies under high pressure (1500 – 25400 psi).As metal flows under high pressure in pressure die casting, within a fraction of second the liquid alloys fills the entire die including

all minute cavities, therefore intricate parts can be produced succesfully. Once the mold cavity is filled, the pressure is maintained until the casting solidifies.

3. The dies are then opened and the shot (shots are different from castings because there can be multiple cavities in a die, yielding multiple castings per shot) is ejected by the ejector pins.

4. The shakeout involves separating the scrap, which includes the gate, runners, sprues and flash, from the shot. This is often done using a special trim die in a power press or hydraulic press. Other methods of shaking out include sawing and grinding. A less labor-intensive method is to tumble shots if gates are thin and easily broken; separation of gates from finished parts must follow. This scrap is recycled by remelting it.[9] The yield is approximately 67%

Gravity Die casting

In gravity die casting no external force is applied other than the head of the liquid.A feeder is incorporated so that when the level of metal in the feeder is above that in the highest point of the casting, head of liquid in feeder forces the metal into all parts of the mould cavity. The feeder is cut off after solidification. It is usually used for shorter production runs.

Casting materials : The main die casting alloys are: zinc, aluminium, magnesium, copper, lead, and tin; although uncommon, ferrous die casting is also possible.[5] Specific die casting alloys include: ZAMAK; zinc aluminium; aluminium to, e.g. The Aluminum Association (AA)Depending upon the melting temp of the alloys there suitability for die casting they are classified as high MP ( above 500 deg C) and low MP( Below 500 deg C)Under the low temp category come Zn, Sn, Pb based alloys and these have advantages of lower die maintainance cost and lower cost of production.Under high temp category comes Al and copper based alloys . these alloys create lot of difficulties in die castings because of destructive effects of high temperature.The erosion of dies increases with temp and at high temp the molten alloy has solvent action on the machine part and dies.Al has more pronounced effect. Zn has emerged as the materials that possess best overall properties for die casting.With Zn we can make castings weighing as high as 20 Kgs and as thin as 1,25 wall thickness.

Zinc : the easiest alloy to cast; high ductility; high impact strength; easily plated; economical for small parts; promotes long die life.

Aluminium : lightweight; high dimensional stability for complex shapes and thin walls; good corrosion resistance; good mechanical properties; high thermal and electrical conductivity; retains strength at high temperatures.

Magnesium : the easiest alloy to machine; excellent strength-to-weight ratio; lightest alloy commonly die cast.

Copper : high hardness; high corrosion resistance; highest mechanical properties of alloys die cast; excellent wear resistance; excellent dimensional stability; strength approaching that of steel parts.

Lead and tin: high density; extremely close dimensional accuracy; used for special forms of corrosion resistance. Such alloys are not used in foodservice applications for public health reasons.

Maximum weight limits for aluminium, brass, magnesium, and zinc castings are approximately 70 pounds (32 kg), 10 lb (4.5 kg), 44 lb (20 kg), and 75 lb (34 kg), respectively.[7]

The material used defines the minimum section thickness and minimum draft required for a casting as outlined in the table below. The thickest section should be less than 13 mm (0.5 in), but can be greater.

Die casting machines :

A die casting machine is a heavy rigid machine which supports the die halves, one moving one into allignmnet with other. It must have sufiicient strength to lock and hold the dies together under considerable pressure, so that no leakage occurs at the parting line. The machine should resist the injection pressure.Very little deflection or streching can be allowed in any part of the machine.

The necessary parts that a die casting machine should have are : 1. Die cast machanism for opening and closing the dies.2. Mens of forcing the metal in the die.3. device to keep the movable die half pressed untill the solidification of the molten

metal takes place.4. Arrangement for automatic insertion and removal of cores from die casting5. Ejector pins to eject casting from the die.

There are three types of die casting machines: 1. Hot chamber die casting machine:

The metal melting unit forms an integral part of the machine.It mainly consists of a hot chamber and a goose neck shaped metal container made of CI.This machine is mainly used for low MP alloys like Zn etc. Metals and alloys having High MP and metals having affinity for iron cann’t be cast by this machine which othervice attack the dies and damage the machine.

There are 2 types of arrangements machines in this type :

a. Submerged plunger type die casting machineb. Direct air pressure die casting machine.

In submerged type, The goose neck type container is always remains immersed in the metal pot.

The die is made in two parts – stationery and movable die platen The molten metal from the metal container is forced inside the die with the help

of a plunger submerged inside the container. This is operated hydraulically. The plunger moves inside the cylinder, which has a hole in its bottom. When the plunger moves up, the molten metal comes up and fills the cylinder . Then it is forced into the die cavity through the nozzle in the goose necked

container in the down stroke of the plunger. The movement of the plunger and movable die platten are synchronised such that

when the metal is being forced, the two die halves are in match. On solidification the plunger moves up and the movable die platten moves away

and the casting is removed. The pressure used is in the range of 100 – 150 kg/cm2.

In direct air pressure die casting machine, direct pr ( ~ 40 kg/cm2) is used for

forcing metal inside the die. The goose neck conatiner operated by lifting mechanism Initially is submerged in the molten metal and is filled by gravity. Then it is raised so as to bring the nozzle in contact with the die opening and is locked in that position.Compressed air is then forces metal into the die,and pressure is mainatined till solidification.When solidification is complete the goose neck is lowered down. And castings are removed by ejector pins after opening the dies and withdrawing the cores if any.

Cold chamber die casting machine : This m/c is used for cating alloys having high MP and require high pr. Ex Brass, Al and Mg.

The metal melting unit is situated outside the main machine. The metals are melted in a self contained pot in an auxilarry furnace.the molten metal is landled in a the plunger cavity next to the dies and forced into the die cavity by a hydraulically operated plunger and the pre is mainateined till solidification.these m/c can have either horizontal or verticle plunger for forcing molten metal into the die, and built very strong and rigid to withstand heavy pressures. The process of castings manily consist s of 4 steps

1) Pouring molten metal below the plunger and placing the movable platen and cores in position.

2) Forcing the molten metal into the die by means of a plunger

3) Withdrawing cores and opening dies.4) Ejecting the casting for movable die platten.

The problem of rapid oxidation of steel dies at high temp is overcomme by using better die materilas, circulating water through the pipes adjacent to the dies, and using metal in semi liquid and plastic state so that lower temperatures are encountered.The adv of cold die casting is that even semi molten metal can be Esily cast because the pr applied is very high and casting produced will be very dense and defects like porosity and blow holes are eliminated.

Vacuum die casting machine.There are chances of air getting trapped inside the dies in the above two methods there by creating the casting with blow holes etc.To overcome this problem a vaccum pump is used for craeting the vaccum in the cavity. Also a seal is used which cutts off the vaccum pipe connection to the die, after proper vaccum is attained in die so that metal does not flow from die to the vaccum pipe. Also due to vaccum the metal flows automattically and quickly in this machine.

Dies :

Two dies are used in die casting; "cover die half" and the "ejector die half".

The ejector die half

Where they meet is called the parting line.The two halves after meeting forms a shape that is simillar to the shape of the casting required.the two halves can be relatively easily and removed from each other and re bolt again .

The cover die contains the sprue (for hot-chamber machines) or shot hole (for cold-chamber machines), which allows the molten metal to flow into the dies;

This feature matches up with the injector nozzle on the hot-chamber machines or the shot chamber in the cold-chamber machines.

The ejector die contains the ejector pins and usually the runner, which is the path from the sprue or shot hole to the mold cavity.

The cover die is secured to the stationary, or front, platen of the casting machine, while the ejector die is attached to the movable platen.

Initially the molten metal enters from the stationary side . and then flows into the ejector die and clings to it on solidification.The die is then opened and casting from the movable die is removed by advancing the ejector plate into the movable part of the die .

Die surfaces should be free from tool marks .The tool marks act as notches and result in fatigue failure. Die material should be properly heat traeted and be of correct composition.

The die is made from a suitable alloy steel selection of which depends on the temp of the metal to be cast.The die materials should alos have low expansion characteristics, be wear resistant against erosion, stable against high temp and pr.and should not have any tendency of welding with castings.

In cold chamber m/c the die material used is Nickel chrome steel and for hot chamber it is hardened and tempered chrome vanadium or chrome tungsten steel.

Advantages of Die castings:

1. Requires less floor space than other castings2. It gives castings with precision dimensions which redoes

further machining

3. Thin sections and complex shapes are possible.

4. Provides good surface finish compared to other process.

5. Castings are less defective and increased soundness,

6. High yield can be obtained.

7. Less labor costs and less trig is sufficient.

8. High productivity can be achieved

9. High die life.

Limitations:

1. The cost of die and eqt. Used is high 2. There are limited scope for non froous alloys that can be used

on die casting.

3. There are many complex features that limits the application of die casting

4. The size of castings produced by this method is limited,

5. Spl skill is reqd for maintainance and supervision of the die.