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.
There are four types of dies:
SINGLE CAVITY DIES: As the name indicates here number of cavities is only one as shown in fig 2.2, 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 ca
vity 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 DIES: Here the number of cavity is more than one as shown in fig 2.3 & 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 therma
l 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 DIES:
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 compo
nents, 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 DIES:
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.
c) Difference in size and shape of the components will cause imbalance.
a) 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.
b) 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.
0 comments:
Post a Comment