Guide to Metal Casting
Table of Contents
Summary: Casting is a process by which a material is introduced into a mold while
it is liquid, allowed to solidify in the shape inside the mold, and
then removed producing a fabricated object, part, or casing. Casting is
often used for creating one or more copies of an original piece of
sculptural (three-dimensional) artwork. It is also used extensively in
the automobile manufacture industry, such as the casting of engine
blocks or cylinder heads, or vacuum-forming of plastics and in the lost
core process. The process, particularly when performed with molten
metals, is also called founding.
Casting may be used to form hot, liquid metals or meltable
plastics (called thermoplastics), or various materials that cold set
after mixing of components such as certain plastic resins (e.g. epoxy),
water setting materials such as concrete or plaster, and materials that
become liquid or paste when moist such as clay, which when dry enough to
be rigid is removed from the mold, further dried, and fired in a kiln.
Substitution is always a factor
in deciding whether other techniques should be used instead of casting.
Alternatives include parts that can be stamped out on a punch press
or deep-drawn, items that can be manufactured by extrusion or by cold-bending,
and parts that can be made from highly active metals.
The metal casting process is subdivided
into two distinct subgroups, non-expendable and expendable mold casting.
- Expendable mold casting is a generic
classification that includes sand, plastic, shell, and investment (lost-wax
technique) moldings. All of these involve the use of temporary and non-reusable
molds, and need gravity to help force molten fluid into casting cavities. In
this process the mold is used only once.
Expendable Mold Processes
Expendable Mold Casting Processes Uses Non-reusable Temporary Metal Casting Molds
- Non-expendable mold casting
differs from expendable processes in that the mold need not be reformed after
each production cycle. This technique includes at least four different methods:
permanent, die, centrifugal, and continuous casting.
Permanent and Other Mold Processes
Many of the metal casting methods are described below.
Recent Metal Casting History
Modern mass production metal casting methods can produce
thin but accurate molds—superficially resembling paper mache, such as is
used in egg cartons, but that is refractory in nature—that are then
supported by some means—such as dry sand surrounded by a box—during the
casting process. Due to the higher accuracy it is possible to make thinner
and hence lighter castings—extra metal does not have to be present to
allow for variations in the molds—these thin-mold casting methods have
been used since the 1960s in the manufacture of cast-iron engine blocks and
cylinder heads for automotive applications.
Various automotive mechanical components are
now frequently made of aluminum, which for appropriately shaped components
may be made either by sand casting or by die casting, the latter an accurate process that greatly reduces finishing
and machining costs. While the material and the processing setup is more
expensive than the use of iron this is one of the most straightforward
ways to reduce weight in a vehicle, important as a contributor to both
fuel economy and acceleration performance.
Lost Foam Process
Starting in the early 1980s, some metal castings
such as automotive engine blocks have been made using a sand casting
technique conceptually similar to the lost wax process, known as the lost foam process. In this process, the pattern is
made of polystyrene foam, which the sand is packed around,
leaving the foam in place. When the metal is poured into the mold, the
heat of the metal vaporizes the foam a short distance away from the
surface of the metal, leaving the molding cavity into which the metal
flows. The lost-foam process supports the sand much better than
conventional sand casting, allowing greater flexibility in the design of
the cast parts, with less need for machining to finish the casting. This
technique was developed for the clay mold casting of abstract art pieces
and was first adopted for large quantity commercial production by the Saturn company. Unfortunately, this process
burns plastic in an uncontrolled way, producing a great deal of smoke.
Lost Foam Process Video Demonstration
Metal Casting At Home and Supplies
There are several kits available that will give hobbyists a head start in enjoying metal casting at home. A good place to start a search is Ebay which has multiple sellers offering kits and supplies.
Amazon has a more limited offering, but is worth a quick look.
For toy mold starter kits visit the Dunken Company. The company offers many types of kits and starter molds.
Types of Metal Casting
Lost Wax Casting Process
The Lost Wax metal casting process is
an ancient practice that is still in widespread use today. The process varies
from foundry to foundry, but the steps which are usually used in casting small
bronze sculptures in a modern bronze foundry are as follows:
- Sculpting: An artist creates an original
artwork from wax, clay, or another
material. Wax and oil-based clay are often preferred because these materials
retain their softness.
- Mold Making: A mold is made of the
Most molds are at least two pieces, and a shim with keys is placed between the
two halves during construction so that the mould can be put back together
accurately. Most moulds of small sculptures are made from plaster, but can
also be made of fiberglass or other materials.
the fine details on the original artwork's surface, there is usually an inner
mold made of latex or vinyl, which is
supported by the plaster part of the mold. Usually, the original artwork is
destroyed during the making and initial deconstruction of the plaster mold.
This is because the originals are solid, and do not easily bend as the plaster
mold is removed.
Often long, thin pieces are cut off of the original and
molded separately. Sometimes, especially in the case of large original (such
as life-size) sculptures, many molds are needed to recreate the original sculpture.
- Wax: Once the plaster and latex mold is
finished, molten wax is poured into it and swished around until an even
coating, usually about 1/4 inches think, covers the entire inner surface of the
mold. This may be done in several layers.
- Removal of wax: This new, hollow wax copy
of the original artwork is removed from the mold. The artist may reuse the
mold to make more wax copies, but wear and tear on the mold limit their
number. For small bronze artworks, a common number of copies today is around
- Chasing: Each hollow wax copy is then
"chased": a heated metal tool is used to rub out all the marks which
show the "parting line" or "flashing" where the pieces of
the mould came together. Wax pieces that were moulded separately can be heated
and attached; foundries often use "registration marks" to indicate
exactly where they go.
- Spruing: Once the wax copy looks just like
the original artwork, it is "sprued" with a treelike structure of wax
that will eventually provide paths for molten bronze to flow. The
carefully-planned spruing usually begins at the top with a wax "cup,"
which is attached by wax cylinders to various points on the wax copy.
- Slurry: A "sprued" wax copy is
dipped into a ceramic slurry, then into a mixture of powdered clay and sand.
This is allowed to dry, and the process is repeated until a half-inch thick or
thicker surface covers the entire piece. Only the inside of the cup is not
coated, and the cup's flat top serves as the base upon which the piece stands
during this process.
- Burnout: The ceramic-coated piece is
placed cup-down in a kiln,
whose heat hardens the ceramic coatings into a shell, and the wax melts and
runs out. The melted wax can be recovered and reused, although often it is
simply combusted by the burnout process. Now all that remains of the original
artwork is the negative space, formerly occupied by the wax, inside the
hardened ceramic shell. The feeder and vent tubes and cup are now hollow, also.
- Testing: The ceramic shell is allowed to
cool, then is tested to see if water will flow through the feeder and vent
tubes as necessary. Cracks or leaks can be patched with thick ceramic paste. To
test the thickness, holes can be drilled into the shell, then patched.
- Pouring: The shell is reheated in the kiln
to harden the patches, then placed cup-upwards into a tub filled with sand.
Bronze is melted in a crucible in a furnace, then poured carefully into the
shell. If the shell were not hot, the temperature difference would shatter it.
The bronze-filled shells are allowed to cool.
- Release: The shell is hammered or
sand-blasted away, releasing the rough bronze. The spruing, which are also
faithfully recreated in metal, are cut off, to be reused in another casting.
- Metal-chasing: Just as the wax copies were
"chased," the bronze copies are worked until the telltale signs of
casting are removed, and the sculptures again look like the original artwork.
Pits left by air bubbles in the molten bronze are filled, and the stubs of
spruing filed down and polished.
- Patinating: The bronze is colored to the
artist's preference, using chemicals applied to heated or cooled metal. This
coloring is called patina, and is often green, black, white or brownish to
simulate the surfaces of ancient bronze sculptures. (Ancient bronzes gained
their patinas from oxidation and other effects of being on Earth for many
years.) However, many artists prefer that their bronzes have brighter,
paint-like colors. Patinas are generally less opaque than paint, which allows
the luster of the metal to show through. After the patina is applied, a coating
of wax is usually applied to protect the surface. Some patinas change colour
over time because of oxidation, and the wax layer slows this down somewhat.
Sculpture and Mold
Lost-Wax Process Mold (right) and Bronze Sculpture Made With Mold (left)
Sculpture by:David Ascalon
The lost-wax metal casting process can also be
used with any material that can burn, melt, or evaporate to leave a mold
cavity. Some automobile manufacturers use a lost-foam technique to make
blocks. The model in this case is made of polystyrene foam, which is
placed into a casting flask, consisting of a cope and drag, which is
then filled with casting sand. The foam supports the sand, allowing
shapes to be made which would
not be possible if the process had to rely on the sand alone to hold its
The metal is then poured in, and the heat of the metal vaporizes the
the metal enters the mold.
The Pouring of Molten Bronze into Sculpture Mold
Sculpture being created at Laran Bronze during the casting the statue "Keys to CommunityJames Penniston Sculpture
Picture of Pouring Iron into Sand Casting Mold
Sand-casting is mainly used for
casting flat, relief-like sculptures. Aluminum is one material which is
commonly used in sand-casting. The process starts with a tub filled with sand.
The sand is wetted, and an object is pressed into the wet sand, or the sculptor
uses his hands or tools to make the desired design in the sand, which is then
dried. Molten aluminum is carefully poured into the depression and left to
cool. Then the artist may choose to continue refining the object by
"chasing" it or leave it with the roughened surface that is
characteristic of sand-cast objects.
In manufacturing, sand casting is used to produce rough metal castings that are refined by one or more of these processes:
- machine grinding
- rough grinding
- shot peening
- hammer peening
Sand castings not further worked by peening or polishing are readily recognized by the sand-like texture imparted by the mold.
As the accuracy of the casting is limited by imperfections in the mold making
process there will be extra material to be removed by grinding or machining,
more than is required by other more accurate metal casting processes.
Sand Casting Process Video
From the design, provided by an
engineer or designer, a crafts person called a pattern maker produces a
master of the object to be produced, often using wood. As the metal to be cast
will shrink somewhat between the time it first solidifies and the time it is
cool the master must be made slightly larger than the finished product. To
simplify the making of the pattern the pattern maker will use an appropriately
scaled oversize ruler—called a shrink rule—specific to the type of metal
to be cast. Additional paths for the entrance of metal—the sprue —and
the exiting of gas—the riser —are
added to the pattern.
Molding and Box Materials
A multi-part molding box (known
as a casting flask, sometimes referred to as the cope and drag) is prepared to receive the pattern. Molding boxes are made in
segments that may be latched to each other and to end closures. For a simple
object—flat on one side—the lower portion of the box, closed at the bottom,
will be filled with prepared casting sand or green sand—a
slightly moist mixture of sand and clay. The sand is packed in through a
vibratory process called ramming and, in this case, periodically screeded
level. The surface of the sand may then be stabilized with a sizing compound.
The pattern is placed on the sand and another molding box segment is added.
Additional sand is rammed over and around the pattern.
Finally a cover is
placed on the box and it is turned and unlatched, so that the halves of the
mold may be parted and the pattern with its sprue and vent patterns removed.
Additional sizing may be added and any defects introduced by the removal of the
pattern are corrected. The box is closed again. This forms a "green"
mold which must be dried to receive the hot metal. If the mold is not
sufficiently dried a steam explosion can occur that can throw molten metal
about. In some cases, the sand may be oiled instead of moistened, which makes
possible casting without waiting for the sand to dry. Sand may also be bonded by
chemical binders, such as furane resins or amine-hardened resins.
If it is desired to have most of
the—iron or steel—casting in a tough, ductile, state but with a few surfaces
hard, it is possible to introduce, into the mold, metal plates—chills—where
the metal is to be hardened. The associated, local, rapid, cooling will form a
finer-grained and harder metal at these locations. The inner diameter of an
engine cylinder is made hard by a chilling core.
To produce cavities within the
casting—such as for liquid cooling in engine blocks and cylinder heads —negative forms are used to produce cores. Usually sand-molded,
cores are inserted into the casting box after removal of the pattern. Whenever
possible, designs are made that avoid the use of cores, due to the additional
set-up time and thus greater cost.
Sand Mold Metal Castings
Sand Mold Metal Castings: Bronze (right) and Aluminum (left)
With a completed mold at the
appropriate moisture content, the box containing the sand mold is then
positioned for filling with molten metal—typically:
- pot metal alloys
- aluminum alloy
with liquid metal the box is set aside until the metal is sufficiently cool to
be strong. The sand is then removed revealing a rough casting that, in the case
of iron or steel, may still be glowing red. When casting with metals like iron
or lead, which are significantly heavier than the casting sand, the casting
flask is often covered with a heavy plate to prevent a problem known as floating
the mold. Floating the mold occurs when the pressure of the metal pushes
the sand above the mold cavity out of shape, causing the casting to fail.
Corebox Sand Metal Casting Examples
Corebox with resulting cores (reinforced by wire) resulting cores below. Left pattern (used with core) and the resulting casting below (the wires are from the remains of the core)
After casting, the cores are
broken up by rods or shot and removed from the casting. The metal from the
sprue and risers is cut from the rough casting. Various heat treatments may be applied to relieve stresses from the initial cooling and
to add hardness—in the case of steel or iron, by quenching in water or oil. The
casting may be further strengthened by surface compression treatment—like shot peening —that adds resistance to tensile cracking and smooths the rough
The part to be made and its
pattern must be designed to accommodate each stage of the process, as it must
be possible to remove the pattern without disturbing the molding sand and to
have proper locations to receive and position the cores. A slight taper, known as
draft, must be used on surfaces perpendicular to the parting line, in order to
be able to remove the pattern from the mold. This requirement also applies to cores,
as they must be removed from the core box in which they are formed.
and risers must be arranged to allow a proper flow of metal and gasses within
the mold in order to avoid an incomplete casting. Should a piece of core or
mold become dislodged it may be embedded in the final casting, forming a sand
pit, which may render the casting unusable. Gas pockets can cause internal
voids. These may be immediately visible or may only be revealed after extensive
machining has been performed. For critical applications, or where the cost of
wasted effort is a factor, non-destructive testing methods may be applied
before further work is performed.
Decorative Use of Patterns
Old wood-patterns, once used to
make molds for casting machine parts, are sought out and collected by some for
use as interior decorations.
Cuttlefish casting using
cuttlebone as a mold is a traditional casting method used by jewelers and silversmiths for small objects, especially in taking a copy from a metal original. The fine
grain of the calcium carbonate cuttlebone offers good
definition, although it imparts a characteristic surface texture to the cast.
is similar to sand molding except that plaster is substituted for sand. Plaster
compound is actually composed of 70-80% gypsum and 20-30%
strengthener and water. Generally, the form takes less than a week to prepare,
after which a production rate of 1-10 units/hr-mold is achieved with a
capability to pour items as massive as 45 kg and as small as 30 g with very
high surface resolution and fine tolerances.
Once used and cracked away,
normal plaster cannot easily be recast. Plaster casting is normally used for
nonferrous metals such as:
- copper-based alloys
It cannot be used to cast ferrous material because sulfur in gypsum
slowly reacts with iron. Prior to mold preparation the pattern is sprayed with
a thin film of parting compound to prevent the mold from sticking to the
pattern. The unit is shaken so plaster fills the small cavities around the
pattern. The form is removed after the plaster sets.
Plaster casting represents a step
up in sophistication and required skill. The automatic functions easily are
handed over to robots, yet the higher-precision pattern designs required demand
even higher levels of direct human assistance.
Shell molding is also similar to
the sand molding metal casting process except that a mixture of sand and 3-6% resin holds the
grains together. Set-up and production of shell mold patterns takes weeks,
after which an output of 5-50 pieces/hr-mold is attainable. Aluminium and
magnesium products average about 13.5 kg as a normal limit, but it is possible
to cast items in the 45-90 kg range. Shell mold walling varies from 3-10 mm
thick, depending on the forming time of the resin.
There are a dozen different
stages in shell mold metal casting processing that include:
- initially preparing a metal-matched plate
- mixing resin and sand
- heating pattern, usually to between 505-550 K
- inverting the pattern (the sand is at one end
of a box and the pattern at the other, and the box is inverted for a time
determined by the desired thickness of the mill)
- curing shell and baking it
- removing investment
- inserting cores
- repeating for other half
- assembling mold
- pouring mold
- removing casting
- cleaning and trimming.
The sand-resin mix can be recycled
by burning off the resin at high temperatures.
Investment casting (lost-wax
process) yields a finely detailed and accurate product, with excellent
Polystyrene foam is also used in
After a variable lead time,
usually weeks, 1–1000 pieces/hour-mold can be produced in the mass range
2.3–2.7 kg. Items up to 45 kg and as light as 30 g are possible for unit
The process starts by creating an
injection die to the desired specifications. This die will be used to inject wax to create
the patterns needed for investment casting. The patterns are attached to a
central wax sprue, creating an assembly, or mold. The sprue contains the fill
cup where the molten metal will be poured into the assembly.
The wax assembly is now dipped
multiple times in a ceramic slurry, depending on the shell thickness desired. A
layer of fine sand (usually zircon) is added on top of each ceramic layer. This process
will be repeated until the desired shell is created.
After the shell is created to the
specifications desired, the wax must be removed; this is normally achieved
using an autoclave.
This is where the name "lost-wax process" comes from. This leaves an
impression of the desired castings, which will be filled with metal. Before
being cast, however, the shells must be heated in a furnace so they do not
break during the casting process.
Next, the desired metal is poured
into the hot ceramic shell. The metal fills each part on the assembly, and the
central sprue cavity and fill cup. The individual parts will be removed after
the mold cools and the shell is removed. The shell is generally removed with
water-blasting, although alternate methods can be used. What remains are the
cast metal parts, but they are still attached to the sprue assembly. The
individual parts are removed by cold-break (dipping in liquid nitrogen and
breaking the parts off with hammer and chisel) or with large cutoff saws.
The last step is finishing. First
the gate, or the place where the part was connected to the sprue, must be
removed. The gate is ground off to part specifications. Parts are also
inspected to make sure they were cast properly, and if not are either fixed or
scrapped. Depending on the investment casting facility and specifications, more
finishing work can be done on-site, sub-contracted, or not done at all.
Investment casting yields
exceedingly fine quality products made of all types of metals. It has special
applications in fabricating very high-temperature metals, especially those
which cannot be cast in metal or plaster molds and those which are difficult to
machine or work.
Permanent Mold Casting
Permanent mold casting (typically
for non-ferrous metals) requires a set-up time on the order of weeks to prepare
a steel tool, after which production rates of 5-50 pieces/hr-mold are achieved
with an upper mass limit of 9 kg per iron alloy item (cf., up to 135 kg for
many nonferrous metal parts) and a lower limit of about 0.1 kg. Steel cavities
are coated with refractory wash of acetylene soot before processing
to allow easy removal of the workpiece and promote longer tool life. Permanent
molds have a life which varies depending on maintenance of after which they
require refinishing or replacement. Cast parts from a permanent mold generally
show 20% increase in tensile strength and 30% increase in elongation as
compared to the products of sand casting.
The only necessary input is the
coating applied regularly. Typically, permanent mold casting is used in forming
iron-, aluminum-, magnesium-, and copper-based alloys. The process is highly
In die-casting molten metal casting is
injected into a mold at high pressures. Set-up time for dies is 1-2 hours,
after which production rates of 20–200 pieces per hour-mold are normally
obtained. Maximum mass limits for magnesium, zinc, and aluminum parts are
roughly 4.5 kg, 18 kg, and 45 kg, respectively (though larger machines do
exist); the lower limit in all cases is about 30 g. Die injection machines are
generally large (up to 3 × 8 m) and operate at high pressures — 100 megapascals (1000 kgf/cm2) and higher, although aluminum
usually is processed at lower pressure. A well-designed unit produces over
500,000 castings during the production lifetime of a single mold. While the
dies used in the process are quite expensive, if a very large number of
castings can be produced, significant cost savings can be achieved when a
component is manufactured by die casting. The major production step is die
construction, usually a steel alloy requiring a great deal of skill and fine
tooling to prepare. Mostly non-ferrous materials are die-cast, such as aluminum,
zinc, magnesium, and copper-based alloys.
This is the process used in the
production of certain toys, notably that of model automobiles.
Centrifugal casting is both
gravity- and pressure-independent since it creates its own force feed using a
temporary sand mold held in a spinning chamber at up to 90 g (900 m/s²).
Lead time varies with the application. Semi- and true-centrifugal processing
permit 30-50 pieces/hr-mold to be produced, with a practical limit for batch
processing of approximately 9000 kg total mass with a typical per-item limit of
Industrially, the centrifugal
casting of railway wheels was an early application of the method developed by German industrial
company Krupp and
this capability enabled the rapid growth of the enterprise.
Small art pieces such as jewelry
are often cast by this method using the lost wax process, as the forces enable
the rather viscous liquid metals to flow through very small passages and into
fine details such as leaves and petals. This effect is similar to the benefits
from vacuum casting, also applied to jewelry casting.
Continuous metal casting is a
refinement of the casting process for the continuous, high-volume production of
metal sections with a constant cross-section. Molten metal is poured into an
open-ended, water-cooled copper mold, which allows a 'skin' of solid metal to
form over the still-liquid center. The strand, as it is now called, is
withdrawn from the mould and passed into a chamber of rollers and water sprays;
the rollers support the thin skin of the strand while the sprays remove heat
from the strand, gradually solidifying the strand from the outside in. After
solidification, predetermined lengths of the strand are cut off by either
mechanical shears or traveling oxyacetylene torches and transferred to further
forming processes, or to a stockpile. Cast sizes can range from strip (a few millimeters thick by about five meters wide) to billets (90 to 160 mm square)
to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an
initial hot rolling process before being cut.
Continuous casting provides
better quality product as it allows finer control over the casting process,
along with the obvious advantages inherent in a continuous forming process.
Continuously cast metals such as aluminum, copper and steel are
continuously cast, with the largest tonnage poured being steel.
The rate at which a casting cools affects its microstructure, quality, and
The products of sand casting and slurry-mold processes, often large with thick walls, generally
cool slowly. This increases the metal's grain size,
creating a coarse micro-structure that lowers the strength of the casting.
Coarse grains can allow elements of an alloy to separate, which also weakens
the casting. But slower cooling keeps the casting metal liquid longer, which
allows more gases and waste metal to escape, reducing the voids and inclusions
that can weaken a casting.
Conversely, the products of die-casting and metal-mold processes generally cool more quickly, resulting in a fine
microstructure with small grain and less alloy segregation but more trapped
gases and inclusions. On the other hand, according to the Pillings Bedworth
Ratio, strength of a material is inversely proportional to the square root of
its grain size. Provided de-gassing techniques are used during molten metal
preparation, die cast products may have superior strength when compared with
equivalent sand castings.
Like nearly all materials, metal
is less dense as a liquid than a solid, and so a casting shrinks as it cools --
mostly as it solidifies, but also as the temperature of the solid material
drops. Compensation for this natural phenomena must be considered in two ways.
The shrinkage caused by
solidification can leave cavities in a casting, weakening it. Risers
provide additional material to the casting as it solidifies. The riser
(sometimes called a "feeder") is designed to solidify later than the
part of the casting to which it is attached. Thus the liquid metal in the riser
will flow into the solidifying casting and feed it until the casting is
completely solid. In the riser itself there will be a cavity showing the metal
which was fed. Risers add cost because some of their material must be removed,
by cutting away from the casting which will be shipped to the customer. They
are often necessary to produce parts which are free of internal shrinkage
Sometimes, to promote directional shrinkage. Chills must be used in the mold. A chill is any material which will conduct heat away from the
casting more rapidly that the material used for molding. Thus if silica sand is
used for molding, a chill may be made of copper, iron, aluminum, graphite,
zircon sand, chromite or any other material with the ability to remove heat
faster locally from the casting. All castings solidify with progressive solidification but in some designs a chill is used to control the
rate and sequence of solidification of the casting.
Shrinkage after solidification
can be dealt with by using an over-sized pattern designed for the relevant alloy.
Pattern makers use special "shrink rulers" to make the patterns used
by the foundry to make castings to the design size required. These rulers are 2
- 6 % oversize, depending on the material to be cast. Using such a ruler during
pattern making will ensure an oversize pattern. Thus, the mold is larger also,
and when the molten metal solidifies it will shrink and the casting will be the
size required by the design.