CHAPTER 4
PART 5
INVESTMENT CASTING
Ass.Pr.Dr. Nguyen Ngoc Ha
1. INTRODUCTION
• A pattern made of wax is coated with a
refractory material to make mold, after which
wax is melted away prior to pouring molten
metal
• "Investment" comes from one of the less
familiar definitions of "invest" - "to cover
completely," which refers to coating of
refractory material around wax pattern
• It is a precision casting process - capable of
castings of high accuracy and intricate detail
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1.1. Process Characteristics
Size limits:
As small as 1/10 oz; usually less than 10 lb
Thickness limits:
As thin as .025 in, less than 3 in
Tolerances:
.005 in on the first inch; .002 in per additional
inch
Draft allowance:
none required
Surface finish:
50-125 µin
1.2. Process procedure
Schematic illustration of the investment
casting process
1.2. Process procedure
Steps in investment casting: (1) wax patterns are
produced, (2) several patterns are attached to a sprue
to form a pattern tree
1.2. Process procedure
Steps in investment casting: (3) the pattern tree is
coated with a thin layer of refractory material, (4) the
full mold is formed by covering the coated tree with
sufficient refractory material to make it rigid
1.2. Process procedure
Steps in investment casting: (5) the mold is held in an
inverted position and heated to melt the wax and permit it
to drip out of the cavity, (6) the mold is preheated to a high
temperature, the molten metal is poured, and it solidifies
1.2. Process procedure
Steps in investment casting: (7) the mold is broken away
from the finished casting and the parts are separated
from the sprue
Example of a Wax Injection Mold
Example of a Wax Pattern
Example of a Coated Pattern
Example of Finished Castings
1.3. Advantages
• Although the labor and material involved
make the lost-wax process costly;
1- It is suitable for casting high-melting alloys
2- With good surface finish and close tolerance.
3- Thus little or no finishing operations are
required.
4- This process is capable of producing complex
shapes
Investment Casting of a Rotor
Investment casting of an integrally cast rotor for a gas turbine.
(a) Wax pattern assembly. (b) Ceramic shell around wax
pattern. (c) Wax is melted out and the mold is filled, under a
vacuum, with molten superalloy. (d) The cast rotor, produced
to net or near-net shape. Source: Howmet Corporation
Investment and Conventionally Cast Rotors
Cross-section and microstructure of two rotors: (top)
investment-cast; (bottom) conventionally cast. Source:
Advanced Materials and Processes, October 1990, p. 25
ASM International
1.4. Disadvantages
Many processing steps are required
Relatively expensive process
1.5. Uses
• The produced parts weighing from 1 g to 35
kg, for a wide variety of ferrous and
nonferrous metals and alloys. Typical parts
made are components for office equipment
and mechanical components such as gears,
cams, valves and ratchets.
• Common metals:
Mainly aluminum, copper and steel; also used
with stainless steel, nickel, magnesium and
precious metals
Products
Products
Products
Products
2. WAX PATTERN MAKING
2.1. Materials for wax pattern
Raw Materials
• Parafin
• Stearin
• Ceresine
• Ethyl Cellulose
• Colophony
• Polystirol
Pattern Wax
• PS 50-50: 50% parefin +
50% stearin
• PS 30-70: 30% parefin +
70% stearin
• PSE: parafin + stearin +
ethyl cellulose
• PCE: parafin + ceresine
+ ethyl cellulose
2.1. Materials for wax pattern
• Colophony-polystirol-ceresine 50-30-20
• Colophony-ceresine
2.2. Wax Pattern Making
Ejecting Pattern
Pattern Assembly (Tree)
3. MOULD MAKING
3.1. Mold Materials
Refractories
• Silica (SiO2)
• Corundum (Al2O3)
• Silicate zirconium
(ZrSiO4)
• Dioxide titanium (TiO2)
• Magnesite
Grain size: Slurry coating:
<0.05mm; Stucco
coating: 0.074 – 2mm
Binders
• Ethyl Silicate
• Water Glass
3.2. Mold Making
Slurry Coating
Slucco Coating
Slucco Coating
Mold Drying
Pattern Meltout
(Autoclaved)
P= 0.6 – 0.8 MPa
4. POURING
4.1. Mold Preheating
T= 600 – 10500C
= 1 – 3 h
4.2. Pouring
4. FINISHING
4. FINISHING
4. FINISHING
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