IMPORTANT DIE CASTING VARIABLES

The important casting variable that influence the final component formed are

• The velocity of the metal entering the cavity.
• The injection pressure.
• Gate section.
• The die casting alloy.
• The die.

Velocity of the metal entering the cavity:

The injection velocity is an important factor. The last part of the metal required to fill the cavity completely has to enter the cavity completely has to enter the cavity before the first part of the metal entering the cavity has solidified. The velocity of the metal at the gating point can be influenced by:

(a)Gating section

(b)Plungerspeed

(c) Injection pressure

(d) Specific gravity of the metal.

Injection pressure:

The injection pressure has got some influence on the velocity of the metal entering the cavity. Increased injection pressure results in increased pressure tightness. A combination of thick gating and high injection pressure will improve the structural soundness of the casting this increases the overall mechanical properties of the casting.

Gate section:

The time to fill a casting mainly depends on the product of gate area and the speed of the molten metal at gate. A casting can be completely filled at the same time by a larger gate with same time by a larger gate with slower injection speed or by a thin gate with proportionately higher injection speed. The position of the gate also decides the distance the metal has to travel in the die. A longer length of metal flow results in loss of velocity due to friction and loss of metal temperature due to chilling on the die walls.

Die casting alloy:

Some die casting alloys have got better casting properties than the other alloys. An important factor for the cast ability is the solidification interval. The greater the difference between solids point and liquid point more difficult is to cast this alloy. Such alloys requite faster injection speeds and higher pressures.

The die casting die:

The die temp is one of the most important casting variables. Metal injected in to a cold mould will rapidly cool and result in improper filling and poor surface finish. Further the die will be subjected to high heat stresses which results in early failure of the die. The temperature range with in which the die should be maintained depends on the metal temperature, volume of component and size of die. The thermal conductivity of die steel is also an important factor.

Die Casting Process Cycle

The process cycle for die casting consists of five main stages, which are explained below. The total cycle time is very short, typically between 2 seconds and 1 minute.

Clamping

- The first step is the preparation and clamping of the two halves of the die. Each die half is first cleaned from the previous injection and then lubricated to facilitate the ejection of the next part. The lubrication time increases with part size, as well as the number of cavities and side-cores. Also, lubrication may not be required after each cycle, but after 2 or 3 cycles, depending upon the material. After lubrication, the two die halves, which are attached inside the die casting machine, are closed and securely clamped together. Sufficient force must be applied to the die to keep it securely closed while the metal is injected. The time required to close and clamp the die is dependent upon the machine - larger machines (those with greater clamping forces) will require more time. This time can be estimated from the dry cycle time of the machine.

Injection

- The molten metal, which is maintained at a set temperature in the furnace, is next transferred into a chamber where it can be injected into the die. The method of transferring the molten metal is dependent upon the type of die casting machine, whether a hot chamber or cold chamber machine is being used. The difference in this equipment will be detailed in the next section. Once transferred, the molten metal is injected at high pressures into the die. Typical injection pressure ranges from 1,000 to 20,000 psi. This pressure holds the molten metal in the dies during solidification. The amount of metal that is injected into the die is referred to as the shot. The injection time is the time required for the molten metal to fill all of the channels and cavities in the die. This time is very short, typically less than 0.1 seconds, in order to prevent early solidification of any one part of the metal. The proper injection time can be determined by the thermodynamic properties of the material, as well as the wall thickness of the casting. A greater wall thickness will require a longer injection time. In the case where a cold chamber die casting machine is being used, the injection time must also include the time to manually ladle the molten metal into the shot chamber.

Cooling

- The molten metal that is injected into the die will begin to cool and solidify once it enters the die cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The die can not be opened until the cooling time has elapsed and the casting is solidified. The cooling time can be estimated from several thermodynamic properties of the metal, the maximum wall thickness of the casting, and the complexity of the die. A greater wall thickness will require a longer cooling time. The geometric complexity of the die also requires a longer cooling time because the additional resistance to the flow of heat.

Ejection

- After the predetermined cooling time has passed, the die halves can be opened and an ejection mechanism can push the casting out of the die cavity. The time to open the die can be estimated from the dry cycle time of the machine and the ejection time is determined by the size of the casting's envelope and should include time for the casting to fall free of the die. The ejection mechanism must apply some force to eject the part because during cooling the part shrinks and adheres to the die. Once the casting is ejected, the die can be clamped shut for the next injection.

Trimming

- During cooling, the material in the channels of the die will solidify attached to the casting. This excess material, along with any flash that has occurred, must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting's envelope. The scrap material that results from this trimming is either discarded or can be reused in the die casting process. Recycled material may need to be reconditioned to the proper chemical composition before it can be combined with non-recycled metal and reused in the die casting process.

Types of Dies

SINGLE CAVITY DIE: As the name indicates here number of cavities is only one , that means the produces only one component per shot.

Single cavity dies are used when:

a) With the available die casting machine only single cavity can be accommodated with

respect to locking force, shot height & die size.

b) Production is less, which does not necessitates a multiple cavity die.

c) Component requires center gating.

d) Complexity of component is such that it requires side cores in many directions,

which may not permit the use of more than one cavity.

Advantages:

a) Die cost is less.

b) Design and manufacturing is simple.

c) Die is smaller in size, which makes possible the use of smaller capacity machines.

Disadvantages:

a) Unit cost per casting is more.

b) Production rate is less.

c) Edge gated components on hot chamber machines will cause imbalance or they are placed on one side of the injection centerline.

MULTIPLE CAVITY DIE: Here the number of cavity is more than one & all cavities produce identical components.

Multiple cavity dies are used when:

a) The production quantity is more.

b) The available machine can accommodate more than one cavity.

c) It becomes economically feasible to go for multiple cavity dies, with respect to initial investment & unit price per component.

Advantages:

a) Production rate is high.

b) Unit price per component is less.

c) Edge gated component can be placed symmetrically around the injection centerline.

Disadvantages:

a) Initial investment for die is more.

b) With the increase in number if cavities the design and manufacturing becomes more

difficult.

c) With the increase in number of cavities the feed balancing and thermal balancing

becomes more complicated.

These require larger machines, which reduces the number of shots per hour, this because the operating speeds of larger capacity machines is less that of smaller

UNIT DIE: Unit dies are separate small dies that are inserted in a single master holding die, The master die is a fixed member whereas the unit dies are interchanged to produce different components, Normally unit dies consists of ejector and cover halves most of the unit dies are single cavity dies.

a) The production quantity is very less.

b) The investment made on die is less.

Advantages:

a) Different components can be produced in single die, with the change of inserts.

b) Unit cost per component reduces marginally.

Disadvantages:

a) Design becomes more complicated.

b) Very close machining between unit dies and master dies are required.

c) It is difficult to achieve proper operating conditions for the casting made.

In case of unit dies the inserts are normally changed while the master die is clamped on the machine.

COMBINATION DIE: Here also the number of cavities is more than one but difference between combination dies is that in case of combination dies the cavities produce two or more different type of components. Normally almost all of the combination dies are “FAMILY DIES” in which parts caste is the components of single end-product assembly.

These types of dies are used when:

The end product requires number of components, which can be cast in a single die.

Advantages:

a) All the casting part, which goes in to the final assembly, is caste in single die.

b) Investment on die is less.

Disadvantages:

a) If the components to be caste differ much in shape & size, then they will pose a lot of design and manufacturing problems.

b) These types of components may cause lot of feed balancing and thermal balancing problems.

c) Difference in size and shape of the components will cause imbalance.

d) The die as to be operated under the conditions which is suitable for most difficult to

cast component in that group, because of this the other small components which could have been caste at higher speeds are now have to be caste at slow speeds.

e) If one of the components in the group is more prone to scrap, Then the total scrap rate

of the castings produced increases, This is because all the components in that group is a defective one, Then the god components produced in that group are also good as scrap as the assembly cannot have defective component. In such cases it will be more economical to have separate dies for each casting or at least a separate die for more troublesome casting.

Die Construction

Dies, or die casting 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, cores or other sections to produce 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.

Advantages of Metal Alloys available for die casting

Each of the metal alloys available for die casting offer particular advantages for the completed part, like:

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.