Die casting mould integrates such know how and technology as
casting, mould cooling, and further, cast finishing methods.
Die cast products are used in various field, including areas
related to automotive engines, house ware, toys, life style tools. In the future, die cast products will need to be
ever more precise, durable, and economical with shorter delivery times. The key
to these improvements is mould design, including casting methods, and casting
technology.
Design of Die Casting
It has been well established that in a simple die casting
die the injected stream travels across the cavity and impinges on opposite
wall. It then spreads out along the
cavity wall and finally backfills the entire cavity. In complex dies or those
with cores, the flowing mass is more turbulent. This concept of cavity filling
is basic to the proper design of these dies.
Gate Location and Size
For a given casting there are many workable gating designs,
each more or less approaching the prevailing conditions. In general, a gate is
usually placed on an edge from which it can be cleanly sheared, or where its
trimming will not impair the casting surface. But gate location also affects
the character of the metal flow, hence porosity and surface finish.
One authority advocates a gate at the smallest cross section
of the die cavity so that the surface of the casting increases with increasing
distance from the gate. This technique creates a better heat balance within the
die ; the hotter metal contacts the die surfaces of smaller area and transfers
less heat to the die. As the metal flows farther, it cool and transfers less
per unit area to the die. Thus, the
total heat input to the die block is more uniform throughout the cavity.
Vents
Porosity is most likely to occur where vents are too small
or so poorly located that they became blocked by the inflowing metal. To
prevent this, some vents should be located as close to the gate as possible to
allow air to escape from that part of the cavity which fills last.
Test have shown that venting has an important bearing on the
injection speed and casting quality. They indicated that the speed of the
entering jet is seldom equal to that of a free jet. When the vents were sealed,
the jet velocity was reduced by 50%, and increased injection pressure will not
increase filling time unless adequate is provided.
Vents should not exceed about 0.008 in. thickness if fins
are to be avoid, but commercial practice uses vents up to 0.016 in. deep.
Ejection pin, slides, insert clearances, and the parting line all add to the
total cavity venting are. Some designers use a vent area of at least 50% of the
ingate area.
Runner
Runner design and locations can be varied to meet the needs
of conservation or dissipation of heat to maintain thermal equilibrium and appropriate
die temperature gradients, Shapes of runner cross sections vary from round to
trapezoidal to elliptical; from wide and flat to narrow and thick. If the heat
transfer ate of the runner system high, high surface area to volume ratio, the
metal is cooled somewhat by the time it reaches the gate. This is advantage for
a casting of thick cross section but is poor practice for a thin walled casting
because the metal will lose more heat traveling within the cavity, and
incomplete fusion and deep striation are likely. In such case, the heat
transfer between the runner and cavity can be altered by using a runner more
nearly circular in cross section.
In practice most die casting runners are trapezoidal, about
two times as wide as they are deep. As they approach a round or square cross
section, internal porosity is more likely to appear.
Sprue Pin
In edge gate cavities, which are typical of cold chamber
machines, the sprue is merely a small appendage to the main runner and has
little influence on metal floe or thermal equilibrium within the die . In
center gate dies for zinc, however, the sprue may have considerable influence
on the thermal balance within a die block. Heat transfer from the sprue depends
upon the contact area between the sprue pin and the cast metal. A slender sprue
pin present a small heat absorption mass and consequently, because of friction,
the sprue pin tip may reach temperatures equal to or greater than the injection
temperature of molten alloy. This impedes solidifications at the gate. Therefore,
it is better to use a well splayed sprue of large diameter in conjunction with
a blunt conical sprue pin. Thus, an
adequate cross section can be maintained with a relatively thin walled sprue.
This design allow the sprue o water cooled, so a better die block temperature
gradient can be maintained because a large proportion of heat dissipated from
the injected metal is absorbed by the
spue pin an less passes to the adjacent die cavity.
Materials
Die casting materials must be resistant to thermal shock,
softening, and erosion at elevated temperatures. Of lesser importance are heat
treatability, machinability, weldability, and resistance to heat checking. The
performance of die materials is direcly related to the injection temperature of
the molten alloy, the thermal gradients within the die, and the production
cycle. Tool steels of increasing alloy content are required as the injection
temperature of the molten alloy and the thermal gradients within the die
increase, and the production cycle becomes shorter. Dies for use with zinc can be prehardened by the
manufacturer in a range of Rc 29-34. The higher melting alloy require hot work
tool steel.
hopefully the above can make inputs,
and technical development
of the world is more excited,
thank you for your attention.
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