15-10-2012, 03:19 PM
Manufacturing Processes
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BASIC STEPS IN MAKING SAND CASTINGS
The basic steps involved in making sand castings are:
1. Patternmaking. Patterns are required to make molds. The mold is
made by packing molding sand around the pattern. The mold is usually
made in two parts so that the pattern can be withdrawn. In horizontal
molding, the top half is called the cope, and the bottom half is
called the drag. In vertical molding, the leading half of the mold is
called the swing, and the back half is called the ram. When the pattern
is withdrawn from the molding material (sand or other), the imprint of
the pattern provides the cavity when the mold parts are brought
together. The mold cavity, together with any internal cores (see
below) as required, is ultimately filled with molten metal to form the
casting.
2. If the casting is to be hollow, additional patterns, referred to as
core boxes, are needed to shape the sand forms, or cores, that are placed
in the mold cavity to form the interior surfaces and sometimes the external
surfaces as well of the casting. Thus the void between the mold and
core eventually becomes the casting.
3. Molding is the operation necessary to prepare a mold for receiving
the metal. It consists of ramming sand around the pattern placed in a
support, or flask, removing the pattern, setting cores in place, and creating
the gating/feeding system to direct the metal into the mold cavity
created by the pattern, either by cutting it into the mold by hand or by
including it on the pattern, which is most commonly used.
4. Melting and pouring are the processes of preparing molten metal of
the proper composition and temperature and pouring this into the mold
from transfer ladles.
MOLDING EQUIPMENT AND MECHANIZATION
Flasks may be filled with sand by hand shoveling, gravity feed from
overhead hoppers, continuous belt feeding from a bin, sand slingers,
and, for large molds, by an overhead crane equipped with a grab bucket.
Hand ramming is the simplest method of compacting sand. To
increase the rate, pneumatic rammers are used. The method is slow, the
sand is rammed in layers, and it is difficult to gain uniform density.
More uniform results and higher production rates are obtained by
squeezing machines. Hand-operated squeezers were limited to small
molds and are obsolete; air-operated machines permit an increase in the
allowable size of molds as well as in the production rate. These
machines are suitable for shallow molds. Squeezer molding machines
produce greatest sand density at the top of the flask and softest near the
parting line of pattern. Air-operated machines are also applied in vertical
molding processes using flaskless molds. Horizontal impact molding
sends shock waves through the sand to pack the grains tightly.
In jolt molding machines the pattern is placed on a platen attached to
the top of an air cylinder. After the table is raised, a quick-release port
opens, and the piston, platen, and mold drop free against the top of the
cylinder or striking pads. The impact packs the sand. The densities produced
by this machine are greatest next to the parting line of the pattern
and softest near the top of the flask. This procedure can be used for any
flask that can be rammed on a molding machine. As a separate unit, it
is used primarily for medium and large work. Where plain jolt machines
are used on large work, it is usual to ram the top of the flask manually
with an air hammer
Mechanization of Sand Preparation
In addition to the various types of molding machines, the modern foundry
makes use of a variety of equipment to handle the sand and castings.
Sand Preparation and Handling Sand is prepared in mullers, which
serve to mix the sand, bonding agent, and water. Aerators are used in
conjunction to loosen the sand to make it more amenable to molding.
Sand cutters that operate over a heap on the foundry floor may be used
instead of mullers. Delivery of the sand to the molding floor may be by
means of dump or scoop trucks or by belt conveyors. At the molding
floor the molds may be placed on the floor or delivered by conveyors to
a pouring station. After pouring, the castings are removed from the flasks
and adhering sand at a shakeout station. This may be a mechanically
operated jolting device that shakes the loose sand from flask and casting.
The used sand, in turn, is returned to the storage bins by belt conveyor
or other means. Small castings may be poured by using stackmolding
methods. In this case, each flask has a drag cavity molded in its upper
surface and a cope cavity in its lower surface. These are stacked one on
the other to a suitable height and poured from a common sprue.
There is an almost infinite variety of equipment and methods available
to the foundry, ranging from simple, work-saving devices to completely
mechanized units, including completely automatic molding
machines. Because of this wide selection available, the degree to which
a foundry can be mechanized depends almost entirely on the economics
of the operations, rather than the availability or lack of availability
of a particular piece of equipment.
Core Sands and Core Binders
Green sand cores are made from standard molding-sand mixtures, sometimes
strengthened by adding a binder, such as dextrin, which hardens
the surface. Cores of this type are very fragile and are usually made
with an arbor or wires on the inside to facilitate handling. Their collapsibility
is useful to prevent hot tearing of the casting.
Dry sand cores are made from silica sand and a binder (usually oil)
which hardens under the action of heat. The amount of oil used should
be the minimum which will produce the necessary core strength.
Core binders are either organic, such as core oil, which are destroyed
under heat, or inorganic, which are not destroyed.
CLEANING AND INSPECTION
Tumbling barrels consist of a power-driven drum in which the castings
are tumbled in contact with hard iron stars or balls. Their impact
removes the sand and scale.
In air-blast cleaning units, compressed air forces silica sand or chilled
iron shot into violent contact with the castings, which are tumbled in a
barrel, rotated on a table, or passed between multiple orifices on a conveyor.
Large rooms are sometimes utilized, with an operator directing
the nozzle. These machines are equipped with hoppers and elevators to
return the sand or shot to the magazine. Dust-collecting systems are
required.
In centrifugal-blast cleaning units, a rotating impeller is used to impart
the necessary velocity to the chilled iron shot or abrasive grit. The velocities
are not so high as with air, but the volume of abrasive is much
greater. The construction is otherwise similar to the air blast machine.
Water in large volume at pressures of 250 to 600 lb/in2 is used to
remove sand and cores from medium and large castings.
High-pressure water and sand cleaning (Hydroblast) employs high pressure
water mixed with molding sand which has been washed off the
casting. A sand classifier is incorporated in the sand reclamation
system.
CASTING DESIGN
Design for the best utilization of metal in the cast form requires a knowledge
of metal solidification characteristics, foundry practices, and the
metallurgy of the metal being used. Metals exhibit certain peculiarities in
the formation of solid metal during freezing and also undergo shrinkage
in the liquid state during the freezing process and after freezing, and the
casting must be designed to take these factors into consideration.
Knowledge concerning the freezing process will also be of assistance in
determining the fluidity of the metal, its resistance to hot tearing, and its
tendency to evolve dissolved gases. For economy in production, casting
design should take into consideration those factors in molding and coring
that will lead to the simplest procedures. Elimination of expensive cores,
irregular parting lines, and deep drafts in the casting can often be accomplished
with a slight modification of the original design. Combination of
the foregoing factors with the selection of the right metal for the job is
important in casting design. Consultation between the design engineer
and personnel at the foundry will result in well-designed castings and
cost-effective foundry procedures. Initial guidance may be had from the
several references cited and from updated professional literature, which
abounds in the technical journals. Trade literature, as represented by the
publications issued by the various generic associations, will be useful in
assessing potential problems with specific casting designs. Generally,
time is well spent in these endeavors before an actual design concept is
reduced to a set of dimensional drawings and/or specifications.