The invention describes a bonding material for casting mold, which
comprises from about 0.5 to about 20 mass % plant-derived lecithin,
plant oil or a mixture of lecithin and plant oil, from about 0.1
to about 5.5 mass % starch, starch derivative, sodium carboxymethylcellulose
or a mixture thereof, from about 0.3 to about 4.5 mass % of sodium
carbonate, 0.05 to 1.25 mass % of a boron compound, and the balance
to 100 % being bentonite.
1. Bonding material for casting mold, which comprises from about
0.5 to about 20 mass % a first material selected from the group
consisting of plant-derived lecithin, plant oil or a mixture thereof
from about 0.1 to about 5.5 mass % a second material selected from
the group consisting of starch, starch derivative, sodium carboxymethylcellulose
or mixtures thereof from about 0.3 to about 4.5 mass % of sodium
carbonate; from about 0.05 to about 1.25 mass % of a boron compound,
and the balance to 100% of the bonding material being bentonite.
2. The bonding material for casting mold of claim 1, wherein said
plant-derived lecithin is derived from the group consisting of sunflower,
soy or mixtures thereof, and said plant oil is selected from the
group consisting of sunflower oil, sou oil or mixtures thereof.
3. The bonding material for casting mold of claim 1, wherein said
where the second material is selected from the group consisting
of wheat starch, corn starch, dextrine, sodium carboxymethylcellulose
or mixtures thereof.
4. The bonding material for casting mold of claim 1, wherein said
boron compound is selected from the group consisting of B.sub.2
O.sub.3, borax or mixtures thereof.
5. A process for refining continuously recirculated sand free of
coal dust in a process of casting in sand molds, which comprises
mixing the recirculated moldmaking sand with the binding material
of claim 1, adjusting its humidity content to a value of from about
3.0 to about 5.8%, and forming the mixture into a casting mold and/or
core for a casting mold.
6. The process of claim 5, wherein said bonding composition is
added to the sand either as a dry powder, or as an aqueous suspension
containing from about 30 to about 39 mass % solids.
FIELD OF THE INVENTION
The present invention relates to a process for preparing improved
bonding material to be used for coal dust-free refining, for molding
in bentonite-bonded molds and mold core formation.
BACKGROUND OF THE INVENTION
Molding mixtures containing quartz sand, clay, bentonite, coal
dust and water are widely known in the preparation of casting molds.
Coal dust is used in bentonite-containing molding mixtures as glossy
carbon carrier for improving the surface quality of the casting,
and also as an additive for reducing the burning on of the sand
onto the cast object.
After the casting in bentonite containing molds containing coal
dust additive and of cores new sand, it is known to add bentonite,
coal dust and water to the removed, then in continuous recirculation
reused sand mixture for supplementing the burned out coal dust,
and the amount of the degraded bentonite. This is done by separate
measuring and adding methods to refine the molding sand.
In addition to the aforementioned known advantages of using coal
dust in casting, there are also a number of disadvantageous physical
chemical and colloidal effects.
During our investigations we have found that the desirable solidity
of sand mixtures that are reused after casting in circulation the
bentonite containing molds and cores containing coal dust additive,
can be readjusted only by using much greater amounts of bentonite
The reason for this is that due to unfavorable chemical reaction,
the Na-cation exchange necessary in the coal dust additive containing
molding sand for the formation of the maximal working capacity of
the bentonite added while refining, can be formed only to a very
small degree. This is so, on the one hand because of the time dependence
of the reaction rate of the ion exchange, on the other hand because
of the lack of suitable pH of the reused molding sand and of the
We have found that the aforementioned essential and important preconditions
cannot be endures by the traditional refining method due to the
continuous addition of the coal dust additive, and due to its physical
and especially dangerous chemical characteristics, that are so different
from those of bentonite.
Coaldust cannot be suitably homogenized during refining, during
the short mechanical stirring either by the added bentonite or by
the reused molding sand due to the greater specific gravity and
hydrophobic characteristics of the bentonite. In the casing mold
and in cores prepared from molding sands the desired constant strength
of material cannot be obtained.
The main problem is probably due to the especially disadvantageous
physical, chemical and colloidal properties of the especially disadvantageous
combustion by-products of the coal dust to bentonite.
Due to the heat in casting, not only the advantageous glossy carbon
will be formed from the coal dust but at the same time fused slag
and powdered ash are also formed in very significant amounts, of
about 18-35 mass % as solid combustion by-products.
Their presence in the casting molds with bentonite binding and
in the cores is very disadvantageous, because the fused slag contains
substantial amounts of refractory granule attached to the surface
of the cast object by which the required time and energy involved
in cleaning the surface of the cast object is significantly increased.
The presence of powdered sand is also disadvantageous because the
molding sand becomes continuously and quickly turned to dust. Thus,
its ability to be formed deteriorates, its water requirement increases,
along with its permeability and technological strength, increasing
the amount of casting rejects.
The ashes of coal dust are disadvantageous not only for their mere
presence, but mainly they concentrate strongly acidic compounds
therein, which dissolve and dissociate well in the water of the
bentonite-containing molding sand. Contrary to the alkaline chemical
effect necessary for the advantageous Na-cation-exchange the pH
of the electrolyte thus becomes reduced in the direction of the
especially disadvantageous acidic pH range. The continuously increasing
hydrogen ion concentration results in an especially unfavorable
cation exchange to bentonite, whereby the swellability and working
capacity will significantly be reduced or completely ceases.
The high sulfur content of the coal dust also results in some of
the fundamental problems. As a result of the heat during casting,
not only the advantageous glossy carbon forming hydrocarbon develop
from the coal dust additive of the bentonite-containing molds and
cores, but also significant amounts of SO.sub.2 gas are formed from
the sulfur content. A large part of the SO.sub.2 is absorbed in
the molds and cores themselves, while another part of the SO.sub.2
evaporates into the air space of the casting house and causes various
labor hygienic and environmental protection problems, as well as
acts as a corrosive.
The absorption of SO.sub.2 gas is very disadvantageous in the bentonite-containing
casting molds and cores because under the conditions after the different
oxidation reactions sulfurous acid or sulfuric acid is formed, which
dissolve well in the water that is present, they dissociate well
and ensure thereby the continuous formation of acidic pH within
the casting molds and cores. In other words they increase the unfavorable
hydrogen ion concentration and thus force a disadvantageous cation
exchange to the sill bindable bentonite, as they transform it to
Thus they significantly decrease or completely eliminate the advantageous
swellability and working capacity of bentonite, resulting in a reduction
of the desired strength of casting molds and cores.
DESCRIPTION OF THE INVENTION
An objective of the invention is to eliminate the aforementioned
disadvantageous characteristics of coal dust containing large amounts
of elementary sulfur, when used for molding employing bentonite
binding according to the traditional process. By the elimination
of coal dust additive, and its replacement by the improved binding
material system of the present invention the refining method can
be carried out more simply, precisely and effectively, the amount
of bentonite necessary for readjusting the suitable strength of
the molding sand when continuously, reused in recirculation can
be reduced by 25-40%; thereby resulting in a significant reduction
in the delivery, storage and moving of materials, reduction of energy
and worktime requirements and of casting rejects, an improvement
in productivity, a facilitation of making the castings sand free
and thus an improvement of the surface quality of the cast object
Another objective of the invention is to increase the level of
labor hygiene and environmental protection and provide better protection
against corrosion. The present invention relates to an improved
binder composition for casting molds and cores from about 0.5 to
about 20 mass % of plant-derived lecithine, or plant oil, or the
mixture thereof in any desired ratio; from about 0.1 to about 5.5
mass % of starch, starch derivative of carboxymethylcellulose-sodium
or the mixture thereof in any desired ratio; from about 0.3 to about
4.5 mass % of sodium carbonate; from about 0.05 to about 1.25 mass
% of a boron compound; and bentonite to 100 mass %.
The invention also relates to a process for the use of the improved
casting binder material system according to the process in the coal
dust free refining of the bentonite-containing molding sand continuously
reused in recirculation by further processing it by a manual or
mechanical method in the preparation of casting molds and of cores,
from which SO.sub.2 gas will not be formed during molding and enabling
a substantial reduction in the use of bentonite, improving the labor
hygienic and environmental conditions of casting house, and provide
improved protection against corrosion.
The process according to the invention enables a reduction of materials,
energy and worktime, and the refining method can be carried out
in a simpler, more precise and more effective manner. The addition
of coal dust can be eliminated and the amount of bentonite necessary
for refining can significantly be reduced, by about 25-40 mass %.
Coaldust containing large amounts of sulfur is not used in accordance
with the invention as carbon carrier, additional bentonite is not
required, instead of these the binding material system of the process
of the present invention can be homogenized more quickly and more
perfectly with the molding sand.
In accordance with the present invention sulfur-free carbonaceous
compounds are employed which are compatible with the bindable NA-bentonite
in every respect, and their combustion by-products do no cause the
formation of hydrogen bentonite. They enable attainment of the alkaline
pH required for NA-bentonite formation and also of the suitable
NA-ion concentration. They also make possible the use of the perfected
casting binding material system of the present invention as stable
bentonite suspension for the coal dust free refining.
The present invention also relates to a process for preparing an
improved binder by preparing a composition of from about 0.05 to
about 20% mass of plant-derived lectine or a plant oil or the mixture
of both in any desired ratio; from about 0.1 to about 5.5 mass %
of starch or starch derivative or carboxymethylcellulose sodium
or the mixture thereof in any desired ratio; from about 0.3 to about
4.5 mass % of Na-carbonate; from about 0.5 to about 1.25 mass %
of boron compound, and bentonite to 100 mass %.
The invention thus involves the use of the foregoing improved binder
in the refining in recirculation continuously reused bentonite-containing
molding sand without employing any coal dust additive, and the processing
manually or by mechanical means with the improved bentonite binder,
into improved casting molds and cores in a manner known per se from
the aforementioned organic carbonaceous materials from which no
SO.sub.2 gas will be liberated during exposure to the heat of casting.
The improved binder for casting molds and cores of the present
invention is further exemplified in the following six examples.