Small Scale Foundries for Developing Published by: Intermediate ...

equire support on sand or metal formers
until they are baked.
Other natural organic materials are used
alone or in conjunction with hardening oils
and include starch, flour, molasses, or sugar
derivatives. Many types of flour when mixed
with water provide a suitable medium for
bonding sand to produce cores. A disadvantage
is that starch-bonded cores tend
to produce large quantities of gas on
pouring, Frequently a mixture of starch and
oil is used to give a combination between a
workable sand before curing and a hard and
strong core after heating. It is possible to
purchase specially prepared core binders
and oils. Some of these oils are based on
mineral oils and on fish oils as well as on
natural vegetable oils. Ir is worth carrying
out experiments with locally available oils as
these may provide more economical corebinders
than purchased, specially produced,
proprietory materials. A typical mixture
might contain 2% oil, 1 YZ % starch, and 2%
to 3% of water. Such cores should be cured
by baking at 250°C for approximately 45
minutes in an oven.
These oil-bonded sand mixtures may also
be used for moulding when particularly
strong moulds are needed.
The mixing and preparation of coremaking
sand must be carefully carried out. It is
usually unsatisfactory to attempt to mix
sand by hand, and a smail sand mixer of the
type used for mouiding sand, or even a
concrete mixer type, should be used. The
composition of core sand mixes must be
carefully controlled either by weighing or by
measuring the ingredients. Cores which are
cured by heat or by the passage of CO, gas
must be made under controlled conditions.
CO2 process sand hardens slowly in the air,
so that to avoid waste it should be mixed in
small batches as it is required.
Mixers must be cleaned after use to avoid
contaminating other batches.
Coremaking
Cores may be made in metal, wooden or
plastic core boxes. These core boxes are part
of the pattern equipment for the castings.
The simplest method of making cores is to
ram the sand into the core box with a
wooden rammer. Many cores may need
reinforcement with wire or nails in order to
provide internal support. Coremaking for
complicated shapes is a skilled process
requiring several months of training,
although simple cores can be made after a
few days or weeks of practice.
An alternative method of producing cores
is to blow the sand into the core boxes. Core
blowing machines can be bought which are
suitable for both hot and coid core making
processes. A range of machines is available
from simple manually-operated blowers up
to fully automatic equipment for the production
of intricate cores on a large scale.
Such coremaking machines require compressed
air, power or gas services and maintenance,
and can usually only be justified
for repetition castings in conjunction with
mechanised mould production.
Cores that have been bonded with oil,
starch or some resins must be cured before
use. Core stoves may be fired by oil, by gas,
by coke, wood or other suitable fuels. It is
important that air should be allows3 to
circulate within the stove since the curing
process is by oxidation as well as by the
application of heat. It is necessary for the
core stove to have reasonably accurate
temperature control.
Core Assembly Moulding
Coremaking methods are sometimes used to
produce complete moulds. The mixer filler
or air-set moulding process described above
is one such example, but most coremaking
processes can be used for moulding with the
advantages of flexibility, rigid moulds, easy
stripping, and absence of the need for moulding
boxes. Moulds made from assemblies of
cores have to be securely clamped and sealed
together before pouring the metal.
Coremaking sands are usually more
expensive than moulding sands, so coremoulding
is only used when there are
definite technical advantages.
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