Molding
and Casting
ROGER HAYWARD
Source:
Procedures in Experimental Physics
by John
Strong
PROCEDURE in molding
and casting metals has changed very little since the beginning of history.
There have been changes in the attendant mechanism, but the essential
methods are the same as those used by prehistoric man. Because the casting
of metals seemed a wonderful thing to those who did not practice the art,
those who did guarded jealously the details of their craft, lest others
find out how easy it was.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-01.jpg)
Fig. 1. Lost-wax
casting. Making the model.
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The object of this
chapter is to acquaint the reader with a general working knowledge of
the subject. Few laboratories are equipped with facilities for handling
molten metals, so that the experimenter will often be obliged to have
casts made by commercial foundries; but they are usually equipped for
him to make his own patterns from which the casts are made. He will find
it economical to do so, for the cost of having the patterns made may be
many times the cost of the casts. In order to construct his patterns with
intelligence, he should have an understanding of current foundry practice.
The lost-wax method.
The methods of casting metals fall into two classes: one, the "cire perdue,"
or "lost wax," and the other, "sand casting." The lost-wax method consists
in burying a wax model, or pattern, in sand and fire clay, and then burning
the wax out, leaving the mold ready to receive the metal. This method
is employed in sculpture, and in dentistry for the casting of gold teeth,
inlays, and so forth, when only one cast is wanted, for obviously there
can be only one cast made from a single pattern. Hence the name "lost
wax." The method is useful for making small irregular objects. It is particularly
good for casting small parts in gold, silver, or platinum, for ordinarily
the experimenter is loath to work such objects from solid metal. He finds
it a nuisance to save the chips and filings that are too valuable to lose.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-02.jpg)
Fig.
2. Proper method for mixing plaster of Paris.
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The advantage of
the lost-wax method is that the pattern may be of any imaginable complexity.
Undercutting or the lack of it is of no importance. The only problem is
in the arrangement of gates and risers for admitting the metal and for
allowing the air to escape. In the case of hollow sculpture the core is
usually supported in the mold by rods of the same metal as that of which
the statue is to be cast.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-03.jpg)
Fig. 3. Lost-wax
casting. Preparing the model for the mold.
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The wax used is a
mixture of beeswax and paraffin. This mixture can. be procured already
prepared at a dental-supply house, or it can be made up readily, for the
proportions are not critical. The wax is first softened by being heated
until it is pliable; it is then roughly formed with the hands; and after
that it is carved into the desired shape (Fig. 1). The carving may be
done with any sharp instrument. If the operator cuts away too much, he
may replace it by picking up a piece of wax with a pair of tweezers, softening
the wax in the flame, and then touching the tweezers to the spot that
is to be filled in. The tweezers act much as a ruling pen. Fig. 1 shows
the making of a mandrel for winding the spiral tungsten coils used in
evaporating metals. This object is taken as an example because the double
spiral thread that is required is very difficult to cut on a lathe. As
will be seen from the illustration, it can easily be modeled in wax.
Small models are
held on the tip of a small rod. To remove the model from the rod, a piece
of hot metal is held against the rod until it becomes warm enough for
the wax at the tip to melt and allow the model to drop off. A hotwire
tool, such as is described in the chapter on quartz-fiber technique, would
be a useful one for this purpose. When the model is finished, it is stuck
on the end of a tapered metal pin, which serves to support the model while
the mold is being made, and which, when removed, leaves a channel, or
sprue, through which the metal can flow. The object illustrated in Fig.
1 is shown as being modeled directly on the tapered rod that also serves
to form the sprue. Since the wax alone would hardly be strong enough to
stand the winding of the wires that form the threads, the pin is shown
as extending to the tip of the model–a procedure peculiar, of course,
to the example illustrated.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-04.jpg)
Fig.
4. Lost-wax casting. Making the mold.
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If the object is
to be cast in lead, type metal, babbitt, tin, pewter, solder, or other
such metals, the mold may be made of plaster of Paris. If it is to be
of gold, silver, copper, brass, or other metals having a higher melting
point, the regular dental investment material procurable at a dental supply
house should be used. This material is mixed with water and sets in about
a minute. It should be mixed in the same way as plaster of Paris (Fig.
2): A bowl is partly filled with water, and the investment is sprinkled
into the water with the fingers until the level of the investment, which
spreads out under the water, reaches the level of the water. Do not stir
until all the investment has been added. If there is a little pile standing
above the water in the center of the bowl, wait until it is wet, and then
stir gently to free any trapped bubbles. (Before starting the mold, oil
the pin lightly to prevent the investment from sticking to it.) With a
soft brush, paint the outside of the model with the investment and water
mixture (Fig. 3). Immediately hold the object in the mold container, and
pour in the rest of the investment. Continue to hold the object until
the investment is set (Fig. 4). The mold container should be an iron sleeve.
A section of pipe faced-off on the bottom in the lathe will do. Dentists
use a ring which tapers 5 to
10 , the small end being the
bottom. The taper serves to prevent the entire mold from being forced
out of the ring and destroyed while the metal is being forced in. This
point will appear obvious in a later paragraph, which describes the -process
of pouring the metal.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-05.jpg)
Fig. 5. Lost-wax
casting. Making the sprue
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As soon as the investment
has set, pull out the supporting rod, and around the hole, or sprue, carve
a small funnel to receive the metal (Fig. 5). Be careful to remove any
bits of material that fall down the sprue. Place the whole thing in a
ring stand and heat with a Bunsen burner. Heat until the whole mold is
red hot, to make sure that the last traces of wax and moisture are driven
off (Fig. 6).
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-06.jpg)
Fig.
6. Lost-wax casting. Burning out the wax.
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The mold is now completed
and ready to receive the metal. There remains the problem of getting the
metal to flow into the mold. When the sprues are tiny and the masses of
metal small, the surface tension may prevent the metal from flowing. Also,
the trapped air in the mold will be a real obstacle. Dentists use a vacuum
method for getting the air out of the mold. A metal disk with a hole in
the center communicates with a small tank by means of a pipe. The pipe
has a stopcock, which is kept closed until the metal is melted. A small
hand pump is attached to the tank for reducing the air pressure to about
one-half to one-fourth atmospheric pressure. The mold is placed on the
disk, and the metal is melted in the funnel on the top of the mold with
a blast lamp. When the metal is completely molten, the cock is opened,
and the air in the mold is drawn out through its porosity, allowing the
metal to flow in. Of course, air leaks around the cast, but that does
not matter, since the object is to reduce the air pressure in the mold
for only a few seconds. This method will make the finest of casts.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-07.jpg)
Fig. 7. Lost-wax
casting. Vacuum method for filling the mold.
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Fig. 7 shows a setup
of the type described above, which can easily be assembled with the aid
of a bicycle pump and a few accessories.
Another method is
to force the metal into the mold with steam. The metal is melted in the
top of the mold as before. When it has melted, a large piece of moist
clay is quickly pressed down onto the mold. The steam generated will force
the metal into the mold. When this method is used, the mold should be
placed on a perforated plate to allow the air to escape at the bottom
(Fig. 8).
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-08.jpg)
Fig.
8. Lost-wax casting. Steam method for filling the mold.
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Still another method
introduces the metal into the mold by means of a centrifuge, which need
be only a simple device consisting of a bar pivoted to rotate in a horizontal
plane. A spring is arranged to rotate the arm, and a movable trigger acts
as a stop to prevent the rotation until the metal is molten. The mold
is placed in a holder at the end of the arm with its sprue facing the
pivot. The crucible, a small trough of firebrick or any other suitable
material, is mounted on the arm with the end of the trough adjacent to
the sprue. The procedure is to cock the spring, place the mold in the
holder, place the metal in the trough, and heat it with a hand torch.
When the metal is ready, the trigger is pulled, and the whole arm spins
on the pivot, the centrifugal force carrying the metal into the mold.
For this method the bottom of the mold should be made especially strong;
otherwise it may give way and the molten metal be thrown about the room.
This method is used by commercial jewelers as well as by some dentists.
Fig. 9 shows a centrifuge of the type described. For casting the object
illustrated in Fig. 1, this method should probably be used, since the
top of the mold would hardly be large enough to contain the required amount
of metal.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-09.jpg)
Fig.
9. Lost-wax casting. Centrifuge method for filling the mold.
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In any of the foregoing
methods metals such as gold, silver, copper, brass, and so forth, should
be liberally sprinkled with borax as they are fused. This treatment prevents
oxides from forming. Metals such as lead, babbitt, solder, and so forth,
may be kept covered with powdered charcoal for the same purpose.
Patterns for sand
casting. Sand casting from permanent patterns of metal or wood is
the method commonly employed for all manner of mechanical parts, irrespective
of size or of metal. Die casting is the only other method of importance,
but it is restricted to commercial work in which the otherwise prohibitive
cost of the metal dies is absorbed in the tremendous quantity of casts
to be produced. In making the patterns for sand casting, the first step
is to make a careful drawing of the object (Fig. 10). Over this drawing
a second one should be made (Fig. 11), which is the pattern drawing. It
can be made on thin paper. For future convenience it is well to dimension
the pattern drawing copiously.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-10.jpg)
Fig.
10. Sand casting. Mechanical drawing for the polar-axis mounting
for a small telescope.
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As an object for
the illustrations that demonstrate sand casting, a polar axis for a telescope
has been chosen. The axis is designed to have babbitt bearings. Such a
design eliminates the necessity for chucking the whole axis in a lathe
and boring it–a job to be accomplished only in a very large lathe.
Figs. 10 and 12 show drawings of the parts required, and Fig. 24 shows
the completed mounting.
In planning the pattern,
the first thing that must be determined is the plane of division of the
mold. This plane should pass through the object in such a way that both
parts of the pattern can be drawn from the sand mold. The plane must intersect
the object in such a way that all points on the object, when they are
projected normally to the plane, will fall on or within the intersection.
It is to this plane that all questions of "draft" are referred. If there
is no single plane that will fulfill the requirements, one must be chosen
which will come the nearest possible, and the parts that do not conform
will have to be accomplished with cores.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-11.jpg)
Fig.
11. Sand casting. Mechanical drawing of the pattern.
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The pattern will
be divided along the plane of division unless the plane happens to coincide
with one surface of the object, in which case the pattern will be in one
piece (Fig. 12) Since the usual cases are those in which the pattern must
be divided into two halves, we will describe the procedure for a two-piece
pattern. The designer need not be limited to the two-piece pattern, but
the multi-piece pattern is so seldom employed that it is not within the
scope of this chapter to treat of it. The reader is referred to books
on commercial founding.
Since parallel-sided
objects can not be withdrawn from sand without the friction of the sides
destroying the walls, it is necessary to taper the sides slightly. This
taper is called "draft," and it may be as little as 1/2
for fine commercial work. In most work, however, 3
is regarded as the proper draft angle. Round objects need no draft when
the plane of separation passes through the axis. When the requirements
of the object are such that there can be no draft on one of the faces,
that is, when that face must be at a right angle with the plane of separation,
then the draft on all opposite faces should be doubled (Fig. 11).
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-12.jpg)
Fig.
11. Sand casting.
Drawing of the cap for the polar-axis.
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If there is a projection
on one of the pieces of the pattern that does not touch the dividing plane,
then between it and the dividing plane must be provided a separate block
which can be removed after that half of the mold is made. This block is
called the "false core," and the volume which it occupies in that half
of the mold will be replaced by an equal volume of sand in the other half
of the mold. The false core must also have draft. It will be seen later
in the description of the process for making a mold that false cores must
be used on only one piece of the mold, and that this must be the piece
which has the holes to receive the pegs that hold the parts of the pattern
in alignment. Since this is the part that is molded first, it must be
capable of lying flat, with the plane of separation in contact with a
table. If false cores are required on the other piece of the mold, they
must be regarded as true cores. The blocks must be fixed in place and
a core box made for them as will be described later in this chapter.
If there is to be
a hole through the object, bosses must be put on the pattern wherever
the hole comes through the surface of the object. These bosses are to
form s6ckets in the mold for carrying the ends of the core, thereby preventing
its floating in the molten metal. The bosses must have draft angles to
conform to the rest of the pattern. Separate drawings should be made of
the cores, the drawings to include the bosses just mentioned (Fig. 18).
A core is any piece
of sand or mold material that is molded separately and inserted in a mold,
thereby forming a hole or cavity in the finished cast. If the core comes
to the surface of the pattern at only one place, the boss to receive it
must be long enough to support the core as a cantilever. Should this prove
impractical, projections may be made on the core which will bear against
the inside of the mold and carry the core. These projections will leave
holes in the finished cast which will have to be plugged. Still another
way is to use metal supports to carry the core. The supports are in the
form of pins with broad heads and crooked shanks, and they are pressed
into the sand by the molder. They weld with the metal of the cast. It
is well to remember that not only must the core be supported against gravity
in the empty mold, but it must also be restrained from floating in the
molten metal when the mold is filled.
Since nearly all
metals shrink in solidifying, patterns have to be made enough larger to
compensate. The amount of shrinkage varies with the metal. A list of shrinkage
scales is given in Table I.
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This
shrinkage is the amount that must be added per foot to a pattern. For
convenience, shrink rules may be purchased that are the correct amount
oversize but are calibrated as ordinary rules.
For patterns that
are to be cast many times, it is usual to reproduce the original wooden
pattern in aluminum or some other metal, and to use this metal pattern
for making the molds. In such case the original wooden pattern must be
made large enough to allow for shrinkage of both metals. The allowance
should be the sum of the shrinkages of both metals. Metal patterns are
better for repeated use, since commercial foundries are quite rough on
the wooden ones. Furthermore, several metal patterns may be reproduced
from the first wooden one, and from these many molds may be made at a
time. Thereby the cost of production is reduced.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-13.jpg)
Fig. 13. Sand
casting. Method of using wooden dowels to hold the two halves of
the pattern in alignment
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The rate of solidification
of metal in the mold is a function of the thickness of the metal. Since
the greater part of the shrinkage occurs at the instant of solidification,
it is apparent that unequal thicknesses of metal in the same pattern win
cause a certain amount of distortion and warping. For this reason patterns
are usually designed to have as uniform a thickness throughout as possible.
Of course, the warping may be unimportant, since small projecting lugs
on large casts will be carried by large masses of metal. Nevertheless,
care should be exercised in designing the patterns to preserve the equality
of thickness. Metals cooling from the molten state do not necessarily
shrink evenly as the temperature falls. White cast iron, for instance,
shrinks a while, then expands a little, and then continues shrinking.
Gray iron expands twice and phosphorous iron three times. Thus, in casts
of uneven thickness one part may be shrinking while another is expanding,
with the result that the casts will be under stresses. In fact, the casts
may be broken by the stresses. The hand wheels that are used to tighten
the brakes on freight cars are made with spiral spokes, because the uneven
cooling of the thick hub and the thin rim sets up stresses in the spokes
which might break them if they were straight.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-14.jpg)
Fig.
14. Sand casting. Method of using patented dowels.
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Pure metal, such
as aluminum, copper, tin, or zinc, may be cast in almost any thickness.
The case is different with the alloys, especially with those alloys in
which there is a great difference between the melting points of the constituent
metals. The alloys cooled slowly have time to grow large crystals. In
fact, some of the constituent metals may crystallize out, so that thick
alloy castings may be weak and full of crystal pockets. The best thickness
for alloy castings seems to be between 3/16 and 5/16 inch. Even 1/8 inch
is not too thin. All metals, pure or alloyed, must be cast in a thickness
sufficient for the metal to reach all parts of the mold before it sets.
However, this matter is not too important, for a good molder can arrange
his gates to insure that the molten metal reaches all parts of the mold
before it solidifies.
Patterns are usually
made of wood–white pine, sugar pine, or mahogany. The wood should
be clear and well seasoned. If the pattern is in two pieces (not including
false cores), they must be pinned together in such a way that they can
be separated and reassembled by the molder. In making the pattern, it
is well first to make the division part with its pins. Wooden dowels may
be used, or special pins and sockets, which can be purchased from a hardware
dealer (Figs. 13 and 14).
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-15.jpg)
Fig. 15. Sand
casting. Rapping plates.
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In general, the construction
of patterns follows the usual practice in carpentry and cabinetmaking.
The parts may be glued together at will, and all the tricks of joinery
may be employed, provided the finished result is the desired shape. The
exterior surfaces of the pattern should be carefully smoothed. Any roughnesses
or irregularities in the surfaces, particularly in the draft surfaces,
will mean that the molder will have to rap the pattern vigorously to free
it from the mold, with the result that the mold, and consequently the
cast, will be larger than designed.
Each piece of pattern,
including the false cores, should be provided with a rapping plate, which
is set in flush with the parting face (Fig. 15). These plates can be purchased
from a hardware dealer. They provide a hole for rapping and a hole for
a lifting screw, or handle, which the molder will use in withdrawing the
pattern from the sand.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-16.jpg)
Fig.
16. Sand casting. Method of applying wax fillets.
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In making casts it
has been found that sharp, internal corners and dihedral angles are a
source of trouble. The sharp edges of the mold may crumble, or strains
in the metal may cause the cast to break at these corners. To avoid such
trouble, it is the practice to round the corners, and the corners so rounded
are called "fillets." They may be carved in the pattern, but it is easier
to make them of wax. The wax can be purchased at hardware stores. It comes
on spools or in strips in the form of an extruded ribbon of the proper
profile. Before the ribbon is used, the pattern must be shellacked. The
ribbon, cut into convenient lengths, is rubbed into place with a hot tool
that has a ball-shaped end (Fig. 16). The fillets are sold in sizes according
to the radius desired, and the tools can be made or purchased to correspond.
After the fillets are finished, the whole pattern is again shellacked.
In patterns that are to be used only once the fillets may be made of plastiline
or plasticine, which can be purchased at an artists' supply house. Only
the hardest grade should be used. It will stick quite well to the shellacked
pattern, and may be modeled with the fingers or with wooden modeling tools.
It should be shellacked when it is finished. For larger work, leather
fillets can be in many sizes. They are glued directly to the wooden pattern,
finished with sandpaper, and shellacked.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-17.jpg)
Fig. 17. Sand
casting. The finished pattern.
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If there are no cores,
clear shellac is used over the whole pattern; but if there are cores,
the projections which correspond to the ends of the cores are left in
the clear shellac finish or are painted red, while the rest of the pattern
is finished with shellac and lampblack. The shellac and lampblack are
usually mixed to give a dead black finish. The mixture is liberally thinned
with alcohol. The reason for this color distinction is that it tells the
molder where the cores belong (Fig. 17).
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-18.jpg)
Fig.
18. Sand casting. Mechanical drawing of the core.
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If the pattern has
cores, the next problem is to make the core boxes. These are essentially
wooden molds, in which the cores are cast. The requirements for draft
and shrinkage are the same as for the patterns. If the core requires a
two-piece mold, as for cylindrical shapes and the like, the plane of division
need have no relation to the plane of division of the original pattern.
There is no need to pin the two sides of the core box together, for the
two halves of the core are made separately- and are stuck together after
they are baked. When the two halves are alike, it is necessary to make
a box for only one side. If simple cylindrical cores are required, it
is unnecessary to make the boxes at all, for most foundries have stock
boxes, or even stock cores. In general, core boxes are carved from solid
wood, and they are frequently more complex than the pattern itself (Fig.
19).
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-19.jpg)
Fig.
19. Sand casting. The core boxes.
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The core in the illustration
is shown to be made in two pieces, because the surface in the mold which
it has to fit has draft. If the pattern had no draft on this top face,
the core could easily be in one piece.
Core boxes are made
of sugar pine or any other clear and workable wood, and they are finished
with shellac like the patterns. One point is important: Since the pattern
shows only the ends of the core, and since the core may be unsymmetrical
longitudinally, it is well to make the ends different, so that the molder
will get the core in the proper orientation.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-20.jpg)
Fig.
20. The core-box plane
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The process of making
cylindrical core boxes, as it has been practiced for years, is very interesting.
A core-box plane is used, the face of which is two surfaces at 90
to each other. The plane iron comes through the edge of these surfaces
and is sharpened to conform to them. Two parallel lines are drawn on the
surface of the stock that is to be used. The plane is made to cut away
all the wood possible without removing the two lines. The 90
angle automatically generates a semicircular groove (Fig. 20). The plane
can be used to generate conical shapes as well.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-21a.jpg)
Fig. 21. Sand
casting. Filling the drag
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Sand casting.
The procedure for making the mold is as follows: The molder first
separates the pattern and sets the part that has no pegs with its separation
plane face down on the table, putting the false cores in place. Around
this piece he puts a wooden or metal frame called a "drag." The "drag"
and the "cope" together constitute the "flask," which is a framework to
contain the sand forming the mold. The cope and the drag are open rectangular
boxes without lid or bottom. They are fitted with crossbars to help hold
the sand. There are three sockets in the rim of the drag and three corresponding
pins in the rim of the cope which permit them to be separated and reassembled
in the same relation as shown in Fig. 21(a).
The molder next sprinkles
molding sand through a riddle, or sieve, until the drag is half full.
Molding sand is a mixture of fine clear sand and a small amount of clay,
and sometimes a little powdered charcoal or graphite. It has been moistened
until a handful compressed will retain the print of the hand and will
form a fairly firm piece, but moistened not so much but that it can be
shaken through a riddle of about 1/4-inch mesh, the riddle being shaken
vigorously. Molding sand is used over and over again with only the addition
necessary to replace the inevitable losses.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-21b.jpg)
Fig. 21 (continued).
Sand casting. Removing the false core and filling the cope.
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When the drag has
been half filled, the sand is carefully tamped around the pattern. A wooden
implement is used which is about the shape and size of a dumbbell except
that one end is fiat and the other is a blunt, truncated wedge as shown
in Fig. 21(b). More sand IS added and tamped until the drag is level,
full, and firmly tamped. The sand is then perforated with a thin metal
wire to assist the escape of steam and gases that are given off when the
metal is poured in. This is illustrated in Fig. 21 (c). The drag is now
picked up bodily and inverted on the table, exposing the separation face
of the pattern. The false cores are removed as shown in Figs. 21(d) and
21(e).
The
other half of the pattern is next placed on the part already molded, the
pegs insuring alignment of the parts. Dry sand is sprinkled over all the
exposed sand of the mold as illustrated in Fig. 21(f) to prevent the two
halves of the mold from sticking together. The cope is placed on the drag.
It is filled with sand and is tamped, as was the drag, as shown in Fig.
21(g). The sand is pierced many times with a thin wire, as in the case
of the drag, in the manner illustrated in Fig. 21(h).
At a point in the
sand well clear of the pattern, a sprue is cut deep enough to reach a
little below the separation plane. It is cut with a piece of thin-walled
brass tubing which is pressed gently into the sand and removed, bringing
with it a plug of sand as shown in Fig. 21(i). The hole left is about
an inch in diameter, large enough for casts of 10 to 100 pounds. It is
well to cut the hole an inch at a time. It is not cut to communicate directly
with the pattern, for the force of the descending metal might injure the
mold. A trough is cut around the top of the hole as illustrated in Figs.
21(j) and 21(k), and into this the metal will be poured.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-21c.jpg)
Fig.
21 (continued). Sand casting.
Completing the drag.
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The cope is now carefully
lifted off the drag and laid beside it, face up. A rod is inserted in
the rapping plate and rapped smartly in all directions to break the adhesion
of the sand as shown in Fig. 21(1). A handle is screwed into the rapping
plate, and the pattern is carefully withdrawn from the sand as illustrated
in Fig. 21(m).
A channel, or "gate,"
is next cut along the parting plane from the mold to the sprue. The reason
for not cutting the sprue to communicate directly with the mold is that
the sudden rush of heavy metal directly into the mold might injure it.
The horizontal gate breaks the fall of the metal as shown in Figs. 21(m)
and 21(n).
The mold should be
closely inspected for broken corners and edges that must be carefully
mended and modeled with steel molder's tools. Bits of sand that have fallen
in are brushed away with a soft brush or are blown out with a bellows.
A cloth bag half full of powdered graphite is shaken over the mold, and
frequently powdered graphite is painted on the surface of the mold with
a soft camel's-hair brush as shown in Fig. 21(o). In iron and steel castings
this coating of graphite is responsible for the very hard surface layer.
Its function is to harden the surface of the mold, and also partly to
fill the grainy surface of the sand.
The pattern is removed
from the cope as illustrated in Fig. 21 (p) in the same manner as from
the drag. If the cast is complicated or very large, a riser or several
risers are cut in the cope. They are like the sprue, except that they
are cut from the highest parts of the mold. In complicated molds they
assist in carrying off the entrapped air; in large molds they serve to
collect the slag that rises to the surface, and to provide reservoirs
for extra metal that will flow back into the mold as the metal within
cools and shrinks. This is shown in Fig. 21(q).
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-tbl-ii.jpg) |
The
mold is now ready to receive the cores and to be put together. For thin
casts in alloy the halves of the mold may be heated with a blowtorch to
drive off the water nearest the surface, which otherwise would cool the
metal before it had time to flow.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-21d.jpg)
Fig. 21 (concluded).
Sand casting.
Finishing the cope and filling the mold.
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Cores are made by
filling the core boxes with a mixture of coarse sand and a binder. The
core boxes are tamped full and leveled off with a straightedge. They are
then inverted on sheets of metal, rapped, and removed, leaving the halves
of the core, which are baked for a few hours in an oven. The baked halves
are stuck together with the mixture of which they are made, or with mucilage
or paste, and are baked again (Fig. 22). Pure silica sand which has passed
through a 50-mesh screen and has been retained on a 70-mesh screen should
be used. Many substances may be used as binders a complete list of which
the reader will find in books on founding. The ones most easily available
are given in Table II, together with the amounts to be used.
If linseed oil is
used, the cores must be baked at 425 F.
for 1-1/2 hours. For the others 350 F.
for 1-1/2 hours will do. Linseed oil makes the strongest cores, and pitch
and resin the weakest.
When the cores are
thin and fragile, they are frequently reinforced with iron wires. If the
cores are very bulky, provision should be made for conducting away the
gases that will be formed in them and that will blow the cast apart if
allowed to remain unvented. Such provision is made by laying strips of
wax in the sand as the cores are being made. When the cores are baked,
the wax is driven off, leaving holes for the gases to escape through.
The completed cores
are placed in the molds, and the flask is reassembled. The halves–the
cope and drag– should be securely clamped together, for otherwise
the metal may actually float the cope off and let out the metal as shown
in Fig. 21(t). The mold is now ready to be filled as illustrated in Figs.
21(r) and 21(s).
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-22.jpg)
Fig.
22. Sand casting. Making the cores.
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For production work,
when numbers of identical casts are required, match boards are used. These
are boards having pins and sockets that correspond to those in the cope
and drag, one to fit the cope and one to fit the drag. The two halves
of the pattern are permanently fixed to the boards, properly placed to
insure correct matching of the two. The halves of the mold are then made
separately. They may even be made by separate workmen, and are not assembled
until the time for the pouring. In the case of metal patterns several
are placed on each board so that a number may be cast at a time.
The molten metal
should be poured gently into the mold. It is well to skim back the slag
and scum that form on the top of the metal in the crucible or ladle. For
the soft metals, which can be handled in iron containers, a kettle having
a spout that communicates with the bottom may be used. The floating slag
and oxides will be left in the kettle. In molds involving large masses
of metal it is common for the workmen to stir the metal in the mold. They
churn it up and down in the riser with an iron rod.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-23.jpg)
Fig.
23. Method of making a babbitt bearing.
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This action prevents
the metal in the riser from solidifying until the outer shell of the cast
has set. As the center of the cast sets, the metal in the riser descends
and prevents shrinkage pockets from being left in the top of the cast.
The escaping gases usually burn briskly, but if they burn too much at
the division of the mold, the flames should be doused with water to save
the flask from being badly charred.
Large casts are usually
left overnight to cool. Smaller ones may be immediately dug out of the
mold. In fact, it is the practice in a certain instrument company to remove
the small casts immediately from the sand and throw them red-hot into
the water. The steam generated blows away the sand and even blows out
the cores, leaving the casts virtually clean.
When the sprues and
gates and risers have been cut off, the cast is ready to be machined or
otherwise finished as shown in Fig. 21(u).
In having the casts
made it is often desirable to estimate the weight of the finished casts
before they are ordered. Such an estimation is easily made: Weigh the
pattern, allow for the cores, and multiply the result by a coefficient
which is the ratio of the weight of the pattern material to the weight
of the metal of the cast. A short list of these coefficients is given
in Table III.
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If one is making
one's own casts, enough metal must be figured to include the sprues and
risers. Foundries do not charge for this metal, since they cut it off
and use it over again.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-24.jpg)
Fig.
24. The completed polar axis for a small telescope
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Babbitt metal is
essentially a mixture of lead and tin with the addition of enough antimony
to cause the metal to expand slightly when it freezes. Many variations
of this alloy are commercially available having different properties,
some being suitable for high-speed bearings and others for bearings which
must work under heavy loads. Bearings of babbitt are usually cast in some
sort of carrier, so that the babbitt forms a bushing. These bearings are
sometimes cast as solid plugs, which are then bored and reamed to size.
More frequently they are cast in two halves with a dummy shaft in place.
For many purposes such bearings are good enough just as they are, but
for precision bearings they should be scraped to fit the spindle. A dummy
shaft should always be used, because the hot babbitt metal is apt to warp
the shaft. Fig. 23 shows the method of casting a split bearing.
![](/file/16516/Scientific American - The Amateur Scientist (Tinker's Guild)(2000).iso/amsci01/tblib/moldncasting/molding and casting-25.jpg)
Fig. 25. Cuttlebone
casting.
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Cuttlebone casting.
There remains
one other method of casting which might be found useful in the laboratory.
This is cuttlebone casting. The desirable properties of the method are
the ease and rapidity with which a cast can be produced: A mold can easily
be made and filled in half an hour. The objects to be cast should not
exceed the dimensions of 1/4 inch in thickness, 1-1/2 inches in width,
and 3 inches in length. The patterns should be of metal, since they must
be subjected to pressure. Draft angles can be very slight, or they may
be ignored altogether.
There can be no
cores. Fig. 25 shows the method quite clearly. Cuttlebones can be procured
from a pet shop or drug store for a few cents. The soft, calcareous face
is easily crushed and takes a very firm imprint of any object that is
pressed into it. Difficulty of pressing thick patterns into the cuttlebone
can be overcome by repeatedly pressing the pattern into the bone, the
crushed material being brushed out of the imprint after each operation.
The cuttlebone will stand quite high temperatures and is sufficiently
porous to allow the air in the mold to escape.
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