INVESTMENT CASTING:
Principle
Method also called
as precision investment casting. The method involves the use of expendable
Pattern with a shell of refractory material surrounded to form a casting mould.
Since the pattern made up of wax is melted out and gets destroyed. That is why
the name-"Lost wax method"
Size limits -As small as (1/10) inch but usually less than
10 lb.
Thickness limits -As thickness as 0.025 inch but less than 3
inch.
Typical tolerance -Approximately 0.005 inch.
Draft allowance -Not required.
Surface finish -50 to 125 micron
PROCEDURE:
1. Produce a master pattern
The pattern is a
modified replica of the desired product made from metal, wood, plastic, or some
other easily worked material.
2. From the master pattern, produce a master
die
This can be made
from low-melting-point metal, steel, or possibly even wood. If
low-melting-point metal is used.
3. Produce wax patterns
Patterns are made by
pouring molten wax into the master die, or injecting it under pressure, and
allowing it to harden. Plastic and frozen mercury have also been used as
pattern material.
4. Assemble the wax patterns onto common wax
sprues
The individual wax
patterns are attached to a central sprues and runner system by means of heated
tools and melted wax. In some cases, several pattern pieces may first be united
to form a complex.
5. Coat the cluster with a thin layer of
investment material
This step is usually
accomplished by dipping the cluster into a watery slurry of finely ground
refractory material.
6. Produce the final investment around the
coated cluster
After the initial
layer is formed, the cluster can be re dipped, but this time the wet ceramic is
coated with a layer of sand and allowed to dry. This process can be repeated
until the investment coating is the desired thickness (typically 5 to 15 mm)
7. Allow the investment to fully harden
8. Melt or dissolve the wax pattern to remove
it from the mould
This is generally
accomplished by placing the moulds upside down in an oven, where the wax melts
and runs out, and any residue subsequently vaporizes.
9. Preheat the mould in preparation for
pouring
Heating to 550 to
1100°C (1000 to 2000°F) ensures complete removal of the mould wax, curves the
mould to give added strength, and allows the molten metal to retain its heat
and flow more readily into all of the thin sections.
10. Pour the molten metal
Various methods,
beyond simple pouring, can be used to ensure complete filling of the mould,
especially when complex, thin sections are involved.
11. Remove the casting from the mould
This is accomplished
by breaking the mould away from the casting. Techniques include mechanical
vibration and high-pressure water.
Advantages:
i) Smoother surfaces
(1500 to 2250 µm rms).Close tolerance (of +0.003 mm/mm)
ii) High dimensional
accuracy
iii) Intricate shape
can be cast
iv) Castings do not
contain any disfiguring parting line
v) Machining
operations can be eliminated
Disadvantages:
i) Process is
relatively slow
ii) Use of cores
makes the process more difficult
iii) The process is
relatively expensive than other process
iv) Size limitation of the component part to
be cast. Majority of the castings produce weight less than 0.5 kg.
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