Transcript Slide 1

INVESTMENT CASTING
Also called LOST WAX PROCESS
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Die for casting wax pattern made with
allowances for wax and metal.
Pattern and gating systems made of
wax (bee wax, aera wax, paraffin) or
plastic (polystyrene) by injecting -in
molten condition - into the metal die
PRECOATING- The pattern dipped
in a slurry of refractory material (fine
325 mesh silica &binders, water, ethyl
silicate, acids), and sprinkled with
silica sand
This pattern with initial coating dried,
coated repeatedly to increase
thickness
- used during 4000-3000 BC
• The one piece mould is dried
• DEWAXING- Inverted and
heated to 900C -1750 C for 12
hours
• Wax melts. Can be reclaimed
and reused.
• Mould fired to 6500C-10500C
for about 4 hours
• POURING- Metal poured,
allowed to solidify
• Mould broken, casting taken
out
INVESTMENT CASTING- SEQUENCES
INVESTMENT CASTING
(LOST WAX PROCESS)
MAKE WAX (or Polystyrene)
PATTERN
TRIM
POSITION AFTER repeated SLURRY
COATING (very fine silica,
binders, ethyl silicate and acids)
ON WAX STEM AS TREE
For 1g to 35 kg, intricate shapes,
ferrous & non ferrous metals
PATTERN INVESTED WITH THE
REFRACTORY MATERIAL
ONE PIECE MOLD DRIED IN AIR,
HEATED TO 90-175 C,
KEEP IN FLASK FILL WITH
INVESTMENT MATERIAL
MELT OUT THE WAX BY
INVERTING,MOLD FIRED TO 6501050 C FOR 4 hrs
CAVITY MADE, FILLED WITH
MOLTEN MATERIAL, CASTING
REMOVED.
Plus and Minus points
• Very good dimensional
accuracy
• No or very little finishing
• Intricate and thin shapes
possible
• About 40 kg parts cast
• Both for ferrous and
nonferrous alloys
• Suited for mechanization
• Careful handling
needed,as the patterns are
not strong.
• Close control of process
needed
• Labour and material costs
high, but high melting
point alloys cast with
good surface finish &
close tolerances.
• Eg: gears, cams, valves,
ratchets, turbine blades,
electrical & electronic
components etc.
INTRODUCTION
• Investment casting, often called lost wax casting, is
regarded as a precision casting process to fabricate nearnet-shaped metal parts from almost any alloy. Although its
history lies to a great extent in the production of art, the
most common use of investment casting in more recent
history has been the production of components requiring
complex, often thin-wall castings. A complete description of
the process is complex. But, the sequential steps of the
investment casting process are as below, with emphasis on
casting from rapid prototyping patterns.
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Fig: 1- Investment casting process
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• The investment casting process begins with fabrication of a
sacrificial pattern with the same basic geometrical shape as
the finished cast part
• Patterns are normally made of investment casting wax that
is injected into a metal wax injection die. Fabricating the
injection die is a costlier process and can require several
months of lead time.
• Once a wax pattern is produced, it is assembled with other
wax components to form a metal delivery system, called
the gate and runner system. The entire wax assembly is
then dipped in a ceramic slurry, covered with a sand
stucco, and allowed to dry. The dipping and stuccoing
process is repeated until a shell of ~6-8 mm (1/4-3/8 in) is
applied.
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Fig. 2- Investment casting process - dewaxing
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• Once the ceramic has dried, the entire assembly is placed in a
steam autoclave to remove most of the wax.
• After autoclaving, the remaining amount of wax that soaked
into the ceramic shell is burned out in a furnace. At this point,
all of the residual pattern and gating material is removed, and
the ceramic mold remains.
• The mold is then preheated to a specific temperature and filled
with molten metal, creating the metal casting. Once the casting
has cooled sufficiently, the mold shell is chipped away from the
casting.
• Next, the gates and runners are cut from the casting, and final
post-processing (sandblasting, machining) is done to finish the
casting.
(The CAD solid model, the shell, and the pattern produced in the QuickCast
process is schematically shown)
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Fig. 3. Investment casting process –Preheating and pouring
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SHELL MOULDING-DEVELOPED IN 1940s
• THERMOSETTING RESINS •
USED AS BINDERS
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• PHENOL
FORMALDEHYDE(3% BY
WT.OF SAND)
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• 15% HEXAMETHYLENE
TETRAMINE ADDED TO
GIVE THERMOSETTING
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PROPERTY
• RESIN SETS AT ABOUT 2500 C
(1750 C- 3700 C)
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• SHELL OF 4 to 9 MM FORMS
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PATTERN HEATED TO 2500 C
CLEANED WITH COMPRESSED
AIR, PETROLEUM SPIRIT
APPLIED
PATTERN INVERTED, PLACED
IN DUMP BOX CONTAINING
SAND MIX , LOCKED
DUMP BOX INVERTED, KEPT
FOR A FEW MINUTES, (1-3 MINS)
SHELL FORMS
RE-INVERTED, SHELL FORMED
IS TRIMMED, REMOVED USING
GUIDE PIN EJECTION,
SHELL MOULDING MACHINES
USED
• ANOTHER HALF ASSEMBLED,
PATTERN MADE OF METAL
READY FOR POURING
MOUNTED ON MATCH PLATES
WITH GUIDE PINS
SHELL MOULDING - SEQUENCES
+ AND - POINTS
Disadvantages
Advantages
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Least moulding material
(about 5%)
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Quick operating cycle
( <5 minutes)
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Ready for pouring
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Excellent surface finish (inner
surface of shell smooth, less
resistance to flow of metal)
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Dimensional tolerances superb
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Very thin possible
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Defects minimum
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Good collapsibility
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Any metal can be cast
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Mechanisation of process
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Initial cost high
Metal patterns expensive
Binder expensive
Reclamation is difficult
Even though 100 kg possible,
generally limited to 10 kg
------------------------------------------When production runs high, cost of
pattern and cost of resin can be
compensated
Reduced cleaning, machining and
finishing are added advantages
Small mechanical parts – gear housings,
cylinder heads, cylinders,
connecting rods etc made by
SHELL MOULDING
V-Process
1. Pattern (with vent holes) is placed on hollow carrier
plate.
2. A heater softens the .003" to .007" plastic film.
Plastic has good elasticity and high plastic deformation
ratio.
3. Softened film drapes over the pattern with 300 to
600 mm Hg vacuum acting through the pattern vents
to draw it tightly around pattern.
4. Flask is placed on the film-coated pattern. Flask
walls are also a vacuum chamber with outlet shown.
5. Flask is filled with fine, dry unbonded sand. Slight
vibration compacts sand to maximum bulk density.
6. Sprue cup is formed and the mold surface leveled.
The back of the mold is covered with unheated plastic
film.
7. Vacuum is applied to flask. Atmospheric pressure
then hardens the sand. When the vacuum is released
on the pattern carrier plate, the mold strips easily.
8. Cope and drag assembly form a plastic-lined cavity.
During pouring, molds are kept under vacuum.
9. After cooling, the vacuum is released and freeflowing sand drops away leaving a clean casting, with
no sand lumps. Sand is cooled for reuse.
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Benefits Of Using The V-Process:
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Very Smooth Surface Finish
125-150 RMS is the norm. Cast surface of 200 or better, based on The Aluminum
Association of America STD AA-C5-E18.
Excellent Dimensional Accuracy
Typically +/-.010 up to 1 inch plus +/-.002 per additional inch. Certain details can
be held closer.
+/-.010 across the parting line.
Cored areas may require additional tolerances.
Zero Draft
Eliminates the need for machining off draft to provide clearance for mating parts
and assembly.
Provides consistent wall thickness for weight reduction and aesthetic appeal.
Allows for simple fixturing for machining and inspection.
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With wall thickness to 0.12 in., this casting
requires moderate strength, good stability
and resistance to stress-corrosion cracking
to 600F (316C).
This casting exhibits mechanical properties
at room temperature of 32-ksi tensile
strength, 24-ksi yield strength and 1.5%
elongation, while maintaining a 16-ksi
tensile strength and 4% elongation at 600F.
The component's as-cast surface finish
meets the customer's requirements, and
the invest casting process reduced the
customer's finishing and machining costs.
SEMI-PERMANENT MOLD CASTING
Semi-permanent mold is a casting process producing Aluminum alloy castings - using
re-usable metal molds and sand cores to
form internal passages within the
casting. Molds are typically arranged in two
halves - the sand cores being put into place
before the two halves are placed together.
The molten metal flows into the mold cavity
and surrounds the sand core while filling the
mold cavity. When the casting is removed
from the mold the sand core is removed
from the casting leaving an internal passage
in the casting.
The re-usable metal molds are used
time and again, but the sand cores
have to be replaced each time the
product is cast, hence the term semipermanent molding.
Semi-permanent molding affords a
very high precision quality to the
casting at a reduced price compared
to the sand casting processes.
NO BAKE CASTING
Filling a wood mold with
sand
The No-Bake Sand
Casting process consists
of sand molds created
using a wood, metal or
plastic pattern. Sand is
mixed with a urethane
binder and deposited into
a box containing the
pattern (and all necessary
formers and inserts) for
pouring.
PRODUCTS
15. TENSIONER PULLEY
Material: Gray iron
Process: Nobake sand
Casting Supplier: Wellsville Foundry, Wellsville, Ohio
This 175-lb component is used as a
brake that puts tension on a 4 ft. wide
roll of rubber feeding into a tire press.
Converted from a steel fabrication (two
ring burn-outs with spokes), the
foundry provided the end-user with a
50% cost savings.
Previously made from two steel stampings
welded together with two tube sections and
subsequently
tin-plated
for
corrosion
resistance (r), this bronze cast component (l)
now is a one-piece permanent mold casting.
The cast component (l) exhibits good
corrosion resistance (without plating or
painting), 50 ksi yield strength and 95 ksi
tensile strength.