IMPORTANT CASTING VARIABLES in die casting

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) Plunger speed

(c) Injection pressure

(d) Specific gravity of the metal.

If the plunger speed is constant the velocity of the metal will increase as the gating section is decreased or vice-versa. The velocity of the metal entering the cavity is given by

V=Ö2g p/3

Where, V = velocity of the metal entering the cavity

g= acceleration due to gravity

P=injection pressure

S.I = specific gravity of the die casting alloy.

The above formula shoes that the velocity increases as square root of injection pressure, the velocity decreases while casting heavier metals. Also the metal velocity decreases while casting intricate shaper due to lot of directional changes and friction with the die walls. Though with the time increase of injection velocity the time taken to fill the cavity decreases there are some disadvantages associated with increased the switches which results in less effective air venting die to higher velocity the friction increases and hence the wearing of due higher metal velocity also results in rough structure poor surface finish and decreased impact strength of components. Therefore in practice the lowest injection velocity which satisfactorily fills the cavity should be set.

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.