Casting
From Wikipedia, the free encyclopedia
One half of a bronze mold for casting a socketed spear head
dated to the period 1400-1000 BC.
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 or part. The finished product is also called a
casting. 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 vacuumforming 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.
Casting may be used to create artistic sculptures
Lost wax casting process
The Lost Wax 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:
1. 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.
2. Mouldmaking. A mould is made of the original sculpture. Most moulds 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. To preserve the fine details on the original artwork's surface, there is
usually an inner mould made of latex or vinyl, which is supported by the plaster part
of the mould. Usually, the original artwork is destroyed during the making and initial
deconstruction of the plaster mould. This is because the originals are solid, and do not
easily bend as the plaster mould is removed. Often long, thin pieces are cut off of the
original and moulded separately. Sometimes, especially in the case of large original
(such as life-size) sculptures, many moulds are needed to recreate the original
sculpture.
3. Wax. Once the plaster and latex mould 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 mould. This may be done in several layers.
4. Removal of wax. This new, hollow wax copy of the original artwork is removed from
the mould. The artist may reuse the mould to make more wax copies, but wear and
tear on the mould limit their number. For small bronze artworks, a common number
of copies today is around 25.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. Pouring. The shell is reheated in the kiln to harden the patches, then placed cupupwards 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.
11. 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.
12. 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.
13. Patinating. The bronze is coloured to the artist's preference, using chemicals applied
to heated or cooled metal. This colouring 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 oxidisation and other effects of being on Earth for
many years.) However, many artists prefer that their bronzes have brighter, paint-like
colours. Patinas are generally less opaque than paint, which allows the lustre 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 oxidisation,
and the wax layer slows this down somewhat.
On the left is an example of a lost-wax process mold, and
on the right is the resulting bronze sculpture.
The lost-wax 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 engine blocks. The model in this case is made of polystyrene foam, which is then
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 shape. The metal is then
poured in, and the heat of the metal vaporizes the foam as the metal enters the mold.
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Flash animation of lost-wax casting
Other casting processes used in creating artworks
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.
Cuttlefish casting using cuttlebone as a mould 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.
Casting in manufacturing
Casting is a process by which a fluid melt is introduced into a mold, allowed to cool in
the shape of the form, and then ejected to make a fabricated part or casing. Four main
elements are required in the process of casting: pattern, mold, cores, and the part. The
pattern, the original template from which the mold is prepared, creates a corresponding
cavity in the casting material. Cores are used to produce tunnels or holes in the finished
mold, and the part is the final output of the process.
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 casting process is subdivided into two distinct subgroups: expendable and
nonexpendable mold casting!!
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
nonreusable molds, and need gravity to help force molten fluid into casting cavities.In
this process the mould is used only once.
Sand casting
Sand casting is a means of producing rough metal castings
that are further refined by any or all of hammer peening, shot
peening, polishing, forging, plating, rough grinding, machine
grinding or machining. Sand castings not further worked by
polishing or peening 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
casting processes.
Patterns
From the design, provided by an engineer or designer, a craftsperson called a
patternmaker 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 patternmaker 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.
Cope & drag with cores in place on the
drag
Molding box and
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.
Chills
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 finergrained and harder metal at these locations. The inner diameter of an engine cylinder is
made hard by a chilling core.
Cores
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.
Two sets of castings (bronze and
aluminum) from the above sand mold
With a completed mold at the
appropriate moisture content, the box
containing the sand mold is then
positioned for filling with molten
metal—typically iron, steel, bronze,
brass, aluminum alloy, or various pot
metal alloys, which often include lead, tin, and zinc. After filling 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.
Left:- Corebox, with resulting (wire
reinforced) cores directly below. Right:Pattern (used with the 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
smoothes the rough surface.
Design requirements
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. The sprue 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.
Alternative casting methods
Sand casting for mass production has largely been superseded by other methods.

Modern mass production 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.

Starting in the early 1980s, some 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 uncontroled way, producing a great deal of smoke.
Plaster casting (of metals)
Plaster casting 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 aluminium-, zinc-, or 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.
Plaster casting (of plaster or concrete)
Plaster itself may be cast - either using single use waste molds, multiple use piece molds,
or molds made of flexible material such as latex rubber (which is in turn supported by an
exterior mold). The finished product is, unlike marble, relatively unattractive, lacking in
transparency, and so is usually painted, often in ways that give the appearance of metal.
Alternatively, the first layers cast may contain colored sand so as to give an appearance
of stone. By casting concrete, rather than plaster, it is possible to create sculptures,
fountains, or seating for outdoor use.
Shell molding
Shell molding is also similar to sand molding 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 4590 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 processing that include:
1. initially preparing a metal-matched plate
2. mixing resin and sand
3. heating pattern, usually to between 505-550 K
4. 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)
5. curing shell and baking it
6. removing investment
7. inserting cores
8. repeating for other half
9. assembling mold
10. pouring mold
11. removing casting
12. cleaning and trimming.
The sand-resin mix can be recycled by burning off the resin at high temperatures.
Investment Casting
Investment casting (lost-wax process) yields a finely detailed and accurate product, with
excellent metallurgical properties.
Polystyrene foam is also used in investment casting—see lost-foam casting.
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
production.
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.
Nonexpendable mold casting
Nonexpendable 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 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 automated.
Die-Casting
In die-casting molten metal is injected into a mould at high pressures. Set-up time for dies
is 1-2 hours, after which production rates of 20–200 pieces per hour-mould 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 mould. 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, see: die-cast toy, Matchbox
Centrifugal casting
Main article: Centrifugal casting
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 2.3-4.5 kg.
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 Casting
Main article: Continuous casting
Continuous 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 mould, which allows a 'skin' of solid metal to form over
the still-liquid centre. 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 travelling oxyacetylene torches and transferred to further
forming processes, or to a stockpile. Cast sizes can range from strip (a few millimetres
thick by about five metres 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. Metals such as steel, copper and aluminum are continuously cast, with the
largest tonnage poured being steel.
Cooling rate
The rate at which a casting cools affects its microstructure, quality, and properties.
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
microstructure 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.
Shrinkage
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.
Volumetric Shrinkage
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 voids.
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.
Linear Shrinkage
Shrinkage after solidification can be dealt with by using an oversized 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.