Lost Foam Casting Technology

Is the lost foam process the future of metal casting?


1. Overview of the Evaporative Pattern Casting Process
a. History of the Evaporative Pattern Casting Process
b. Types of Evaporative Pattern Casting Process
c. Lost foam casting, Full-Mold casting & ceramic shell process

2. Processing of Polystyrene Pattern - White Side
a. Polystyrene Pattern Molding
b. Polystyrene Pattern assembly
c. Polystyrene Pattern coating
d. Ceramic Shell

3. Metal Casting - Black Side
a. Sand Filling and Compaction
b. Pouring
c. Cleaning And Finishing

4. Evaporative Pattern Casting V/s. traditional casting processes
a. Evaporative Pattern Casting Process V/s Green Sand Process
b. Evaporative Pattern Casting Process V/s No Bake Process
c. Evaporative Pattern Casting Process V/s Lost Wax Process

5. Summary of the Evaporative Pattern Casting Process
a. Evaporative Pattern Casting Process and the Environment
b. Evaporative Pattern Casting Process capability
c. Advantages
d. Disadvantages

6. Evaporative Pattern Casting Process & India

















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1. Overview of the Evaporative Pattern Casting Process

a.
History of the Evaporative Pattern Casting Process
b.
Types of Evaporative Pattern Casting Process
c.
Lost foam casting, Full-Mold casting & ceramic shell process

a. History of the Evaporative Pattern Casting Process

Evaporative Pattern Casting Process is a class of casting processes that use
pattern materials that evaporate during the pour, which means there is no need
to remove the pattern material from the mold before casting.

The first patent for an evaporative-pattern casting process was filed in April 1956,
by H.F. Shroyer. He patented the use of foam patterns embedded in traditional
green sand for metal casting. In his patent, a pattern was machined from a block
of expanded polystyrene (EPS), and supported by bonded sand during pouring.

In 1964, M.C. Flemmings used un-bonded sand for the process. The first North
American foundry to use evaporative-pattern casting was the Robinson Foundry
at Alexander City, Alabama. General motors first product using these processes
was the 4.3L, V-6 diesel cylinder head, which were made in 1981 at Massena,
New York.

b. Types of Evaporative Pattern Casting Process

Evaporative Pattern Casting Process




EPS Polystyrene Foam (Thermocole)




Lost Foam Process
Full Mold Process
Ceramic Shell
- foamcast(R)



Molded Foam

Machined Foam

Molded Foam
Patterns

Patterns


Patterns




c. Lost foam casting, Full-Mold casting & ceramic shell process:

Lost-foam casting (LFC) is a type of evaporative-pattern casting process that is
similar to investment casting except foam is used for the pattern instead of wax.
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This process takes advantage of the low boiling point of foam to simplify the
investment casting process by removing the need to melt the wax out of the mold.

Full-mold casting is a evaporative-pattern casting process which is a
combination of sand casting and lost-foam casting. It uses a expanded
polystyrene foam pattern which is then surrounded by sand, much like sand
casting. The metal is then poured directly into the mold, which vaporizes the
foam upon contact.
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Ceramic Shell Casting - foamcast(R) is a hybrid method between lost-foam and
investment or lost-wax castings. The expendable pattern is made from
Expandable Polystyrene (Thermocole), but unlike lost foam, the pattern is
removed from the mold cavity during firing of the ceramic that surrounds the
pattern. foamcast(R) is more like the investment casting process, where the
disposable pattern material is removed from the mold shape before the
introduction of the molten metal. The process is developed for steel castings,
especially low-carbon steels as pattern removal before pouring eliminates carbon
introduction in the metal as Expanded Polystyrene (Thermocole) is mainly
comprised of carbon. The process is valuable for large parts.

2. Processing of Polystyrene Pattern - White Side

a. Polystyrene Pattern Molding
b. Polystyrene Pattern assembly
c. Polystyrene Pattern coating
d. Ceramic Shell

a. Polystyrene Pattern Molding:

Unlike other thermoplastic processes, the production of Polystyrene Foam
(Thermocole) Patterns requires that the raw materials be pre-conditioned prior to
their final "tooled" moulding process. The raw material (also known as
"expandable polystyrene" or "bead") has a spherical shape and is similar to sugar
in appearance.

The conversion process is carried out in three stages:

Pre-expansion
The tiny spherical polystyrene beads are expanded to about 40 times their
original size using a small quantity of pentane (typically 5% by weight) as a
blowing agent. This process involves the heating of beads, using a flow of steam,
which causes the blowing agent to boil and thus a honeycomb of closed cells is
formed.

Maturing
As the material cools the pentane liquefies and a partial vacuum is formed inside
the bead. The beads are returned to a holding tank for approximately twelve
hours to allow the pressure differential to equalize, giving a stabilised granule.

Final Molding
In this final stage the pre-expanded stabilized beads are reheated with steam in a
mould that molds a pattern that essentially is identical to the casting. The final
expansion takes place and the beads fuse to give a shaped pattern. In this final
form the Polystyrene Foam (Thermocole) Pattern is made up of 98% air.

The Machines and Tools
The patterns are formed in aluminium mould tools. These are generally of male
and female form, with the shape between the two halves of the mould being the
shape, of the casting.
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The mould tool is fitted into a molding machine, which has the facility to
introduce steam from behind each half of the tool. The steam is introduced
through small slotted vents, which have been machined into the mould tool when
it was manufactured.



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b. Polystyrene Pattern Assembly:

After the patterns are dried they are ready for assembly. In this process the
pattern is assembled by gluing Polystyrene foam runners and risers in case of a
single large pattern. In case of smaller patterns, multiple patterns are a
assembled by gluing Polystyrene foam runners and risers in a "tree" or a "cluster"
connecting to the patterns to the main polystyrene foam "sprue" which is the
central column.

c. Polystyrene Pattern Coating:

Ceramic shell molds are made from a mixture of graded refractory fillers that are
blended to a slurry consistency. A ceramic shell builds around the polystyrene
foam pattern assembly as it is repeatedly dipped in the slurry.

d. Ceramic Shell

This mold is heated to a high temperature in a heat treatment furnace until the
polystyrene foam evaporates. Simultaneously during this process the ceramic
shell is sintered and becomes rigid.

3. Metal Casting - Black Side

a. Sand Filling and Compaction
b. Pouring
c. Cleaning And Finishing

a. Sand Filling and Compaction:

Sand Filling
One of the important tricks in making consistently reliable "lost foam," or
evaporative pattern castings, is engineering the filling of the flask with sand. The
foam cluster must be totally surrounded and structurally supported by sand
without being distorted or causing dimensional changes. Sand fill around the
foam can be accomplished in several ways, two of which, fill tubes and rain fill.

Compaction
Proper compaction of the dry sand is an essential element of a successful lost
foam system. The sand must be totally compacted in all areas of the flask to
correctly position and support the foam cluster for dimensionally stable castings
to be produced consistently. Vibration is now utilized to compact the sand.
Compaction tables for foam pattern castings having horizontal or vertical-axis
movement.

Sand fill and compaction are important keys to success of the process and must
not be taken lightly.

b. Pouring:

Once the compaction of the sand is complete the flask is ready for pouring. The
pouring can be done either manually or by automatic pouring using a bottom
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pouring ladle.

c. Cleaning and Finishing:

After the casting has solidified, it is transferred to the shakeout area and
proceeds through cleaning and finishing operations like other casting processes.
The shakeout process is easy with un-bonded sand.

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4. Evaporative Pattern Casting V/s. traditional casting processes

a.
Evaporative Pattern Casting Process V/s Green Sand Process
b.
Evaporative Pattern Casting Process V/s No Bake Process
c.
Evaporative Pattern Casting Process V/s Lost Wax Process

a. Evaporative Pattern Casting Process V/s Green Sand Process:

Property
Green Sand Casting
Evaporative Pattern Casting
Complex Internal
Complexity determined by sand core limitations, Extensive and complex internal features
Features and Part
geometry, strength, and cost.
(as thin as 3mm), based on detail
Consolidation.
duplication and pattern assembly in foam.
Dimensional Tolerances +/- 0.030" is typical depending on part size,
+/- 0.005"-0.010" is typical depending on
complexity, and geometry
part size, complexity, and geometry.
Surface Finish
Depends on grain fineness of sand.
Depends on bead size and ceramic coating
Capabilities
grain fineness.
Feature Accuracy
Core movement and shift between mold halves
No cores or mold halves to shift and
across the parting line limit feature accuracy.
degrade feature accuracy.
Parting Line and Draft
Parting lines and draft angles are necessary for
No parting lines in the mold and minimal
Angles
molding.
draft on tools.
Environmental Costs
Sand recovery requires binder removal
Sand is binder free, so it can be easily and
rapidly recovered at low cost.
Tool Life
Wear on wood and metal tools from sand
Low wear and long life with aluminium tool
abrasion

b. Evaporative Pattern Casting Process V/s No Bake Process:

Property
No Bake
Evaporative Pattern Casting
Molding Process
Mix moulding sand
Pre-formed EPS bead a pneumatically injected in
Mould cope and drag
Pattern Mold.
Mix core sand
Mould cores
Assembly time
Approx. 15 mins. Assembly of EPS Patterns
Approx. 25 mins. Hardening of cores and molds
Process Time
Short process as the pattern is molded in one Involves setting up of the mold, cores , etc.
piece.
Knock-out & finishing Thin shell requires short vibratory knockout & Is labour intensive as grinding, welding, etc. is
shot-blasting
required

c. Evaporative Pattern Casting Process V/s Lost Wax Process:

Property
Lost Wax
Evaporative Pattern Casting
Composition of Pattern
Microcrystalline Wax
Expanded Polystyrene Foam
Density of Material used for
Density of Microcrystalline Wax Patterns Density of Expanded Polystyrene Patterns - 42
pattern
- 795 kgs./m, resulting in heavy
kgs./m sufficient for strong, sturdy,
delicate patterns.
dimensionally accurate and light patterns.
Maximum Weight of Cast Part Well Below 125 kgs.
No limitation of weight of cast part.
Pre-Heating of Ceramic Shell Required
No required, pouring is possible at room
temperature of ceramic shell.
Rejection of Shell during
The ceramic shell has a tendency to
The ceramic shell does not crack during the
process
crack during the autoclave process due
burn-off process as EPS does not expand but
to the expansion of Wax
evaporates/melts.
Methoding
Difficult and has limitations in addition to Flexible, risers are simply glued on the pattern
the problem of riser backfilling.
to suit the methoding. The feeding ability of
riser is improved with the use of exothermic
sleeves.
Ceramic Shell Thickness
Approx. 10-15mm
Approx. 5-10mm depending on the size of
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component.








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5. Summary of the Evaporative Pattern Casting Process

a. Evaporative Pattern Casting Process and the Environment
b. Evaporative Pattern Casting Process capability
c. Advantages
d. Disadvantages

a. Evaporative Pattern Casting Process and the Environment:

The most important environmental impacts during foundry are the large use of
energy and all kinds of emissions and dust.
The process has tremendous advantages for the production of castings which
require complex cored passageways, uniform wall thickness and limited or no
draft angles. As the casting is a virtual reproduction of the molded foam pattern,
dimensional control is improved over castings produced by the green-sand route
and for many features such as gasket surfaces and drilled holes. Furthermore, the
improved dimensional control means a significant reduction in, or even the
elimination of, machining. And this means a reduction in the use of coolants and
lubricants, the largest environmental impacts in the metal industry!
The process differs from other techniques primarily in the following areas:
i. there are no cores and no parting lines,
ii. dry un-bonded sand is used,
iii. there is no mold wall movement and
iv. the tooling is not subjected to foundry wear.
These differences result in the following advantages to foamcast(R) over
conventional foundry processes:
* There is less energy needed;
* There are lower emissions, the waste is solid and relatively clean, and the sand
is easily recycled;
* Complex shapes are possible and the surface is almost finished. This means
that there is less or no machining, less or no finishing and less or no assembly
needed;
* The financial advantages are that there is less labour needed & the lifetime of
tools is extremely long.
* Concerning the core: there are no related defects, shifts or fins, there is no core
removal and there is no core equipment necessary, so no handling of hazardous
core materials;
* Due to the close dimensional tolerances and the possibility of complex shapes,
the design freedom is larger;
* The employees'/workers working conditions are improved.

b. Evaporative Pattern Casting Process capability:

Casting Alloy:

Cast Iron
Grey Iron, Ferrite/Pearlite Ductile Iron, High-Nickel Ductile Iron, Hi-
Chrome White Iron
Cast Steel
Carbon Steel, Tool Steel, Hi-Manganese Steel, Heat-resistant Steel
Non-ferrous
Aluminum alloy, Bronze, Brass






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Process Capability:




Weight Size Liner
Thickness Sharpness Surface
Finish
Tolerance
Non-Ferrous,
ISO8062,
Ra3.2~6.3m;
15~2500kgs. 1200mm
CT6~9
3mm
R0.5
Ferrous,
Ra6.3~12.5m;


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c. Advantages
* Foam patterns offer the opportunity to produce large, near investment quality
castings without restrictions on section thickness and casting weight.
* Ceramic cores may be set into foam patterns to create complex internal
geometries and reducing drying times typically associated with complex parts.
* Expanded polystyrene patterns typically weigh 90% less that a wax pattern.
Shell weights using this process are 30-50% less than investment shells.
* Reduced lead times for finished products can be attained because foam patterns
are dimensionally stable at elevated temperatures. . This means that heat and
circulating air accelerate drying of ceramic in the shell building process.
* Shells are poured cold. Increased thermal gradients equal high integrity casting.
* The unique polymer binders used in the process breakdown more easily than
typical investment ceramic, making castings easier to clean.
* Foam patterns can be molded complete on the in-gate system saving assembly
labour.
* Surface finish and dimensional tolerances are close to investment cast parts.
* Heavier castings may be made depending on geometry and equipment
capabilities.

d. Advantages:

There are two main disadvantages of the Evaporative Pattern Casting Process:
i. Perhaps the greatest disadvantage of the lost foam process is the cost of the
tooling, which may prevent parts produced in lower volumes to take advantage of
the unique process capabilities and
ii. If a molding tool/die is used to create the patterns there is a substantial initial
cost of tooling.

7. Evaporative Pattern Casting Process & India

The Evaporative Pattern Casting Process was first introduced in India by Alexcon
Foamcast Ltd. at Khopoli near Pune, Maharashtra. The Company had commenced
operations, albeit on a relatively low scale of operations in 1997-98. But due to
the financial irregularities the plant was closed within a year of operations.

This some how was misinterpreted by the foundry industry in India as a failure of
the Lost Foam Casting Process. Alexcon Foamcast Ltd. failed due to financial
reasons and NOT because any process shortcomings.

Various variants of the Evaporative Pattern Casting Process today are being
successfully implemented in India by the following foundries:
* Gujarat Metal Cast Industries Ltd., Vadodara, Gujarat.
* PTC Industries Ltd., Lucknow, Uttar Pradesh.
* Kirloskar Brothers Ltd., Sangli, Maharashtra.
* Electrosteel Castings Ltd., Khardah, West Bengal.
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