Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS AND DEVICE FOR PRODUCING MOLDS OR CORES FOR
FOUNDRY PURPOSES
The invention relates to a process for producing molds
or cores for foundry purposes from a mixture of foundry
sand and binder, in which the foundry sand and binder
are mixed and are introduced into a mold or core die,
and the binder is then set, imparting the required
strength to the mold or core.
Such a procedure for the production of cores or molds
for foundry purposes is known. The majority of cases
which are currently in use employ organic binders,
which result in good setting but generate gases during
the casting operation as a result of their combustion,
which can lead to the formation of voids in the cast
workpiece which is formed. Furthermore, in particular
cores which are not sufficiently dimensionally stable
at elevated temperature when a foundry sand mixture of
this type is used expand. Also, cleaning of the dies
for the foundry molds or cores is a complex operation
on account of the relatively high likelihood of the
organic binders sticking.
It is to be considered particularly unfavorable that in
particular cores in which the sand mixture contains an
organic binder can only be removed from the finished
casting with considerable difficulty and with a high
mechanical or thermal outlay.
Therefore, it has also already been proposed, during
production of molds or cores for foundry purposes, to
admix inorganic binders, specifically water-glass, to
the sand. Although this makes it possible to
substantially avoid the evolution of environmentally
harmful gases, in this case too demolding and removal
in particular of the cores from the finished casting is
a difficult and complex operation.
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Therefore, the invention is based on the object of
providing a process of the type described in the
introduction, and also a device, which make it possible
to produce foundry molds and/or cores which are
intended to be highly dimensionally stable and strong
even during the casting operation yet can nevertheless
be removed from the finished casting in a simple way.
To achieve this object, the process as defined in the
introduction is characterized in that magnesium sulfate
is dissolved and/or dispersed in water and mixed as
binder with the foundry sand and is then shot or
introduced into the mold or core die, and in that the
water is then heated inside the mold or core die and is
at least partially evaporated and expelled from the
mold or core die.
Tests have shown that a process of this type makes it
possible to produce a stable core or a stable mold,
with the melting point being increased greatly by the
binder selected and the expulsion of water and if
appropriate at least some water of crystallization from
hydrated magnesium sulfate, so that a foundry mold or
core of this type is able to resist and withstand even
the high temperatures of the casting material without
harmful gases being released. In this context, the
invention exploits the fact that the expulsion of water
of crystallization leads to a chemical change in the
materials properties of the special binder,
specifically of the magnesium sulfate.
It has been found that foundry molds or cores of this
type, after the cast workpiece has been cooled, can be
flushed out using water, since the special binder then
again seeks to take up water of crystallization so that
it is chemically transformed back into a soluble
substance which can be dissolved using a flushing or
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cleaning water of this type and can thereby be removed
very easily even from a complicated casting without
there being any need for mechanical vibrations or
similar outlay. It may even be sufficient for the
finished casting simply to be immersed in a water bath.
It has been found that immersion of this nature for
just half a minute can be sufficient to dissolve and
flush out even a complicated core. Moreover, thereafter
the foundry sand is available for reuse in virtually
unchanged form and does not need any complex cleaning
and treatment, since there is no need for any organic
residues to be removed.
The advantages which can be achieved by the invention
therefore relate firstly to the foundry mold or cores
and the properties thereof during the casting
operation, in which harmful gases are not released, and
secondly, at a later stage, to the cleaning of the
finished casting, which is greatly simplified.
It is expedient if the mixture of foundry sand and a
dispersion and/or solution of magnesium sulfate in
water is heated inside the mold or core die by means of
microwave and/or infrared radiators. This represents a
particularly simple form of heating for expelling the
water and if appropriate at least some of the water of
crystallization. In this context, microwaves can be
used in a very targeted fashion and penetrate even into
the very middle of relatively large cores.
A particularly advantageous procedure can consist in
the mixture of foundry sand and a dispersion and/or
solution of magnesium sulfate in water being heated
inside the mold or core die by the application of an
electric voltage to the at least partially electrically
conductive parts, which are insulated from one another,
of the separable mold or core dies. Electrical energy
is available virtually wherever molds or cores are
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produced, and consequently the heating for expulsion of
the water from the mold or the core can be carried out
in a correspondingly simple way.
The electrically conductive core/mold, which consists
of a mixture of foundry sand and a dispersion and/or
solution of magnesium sulfate in water, may, in a
simple and expedient way, be used as an electrical
resistor of a resistance heating means and can be
heated by means of an electric voltage applied to it
and the current which flows as a result. This means
that the heat is formed directly where the water is to
be expelled.
The electric voltage can be applied to electrodes which
make contact with the core/mold, and the at least
partially electrically conductive parts, which are
insulated from one another, of the separable mold or
core dies can be used for this purpose. The internal
cavities of these dies, which receive the mold or core
to be formed, therefore make contact, as electrodes,
with the mold or core and provide the corresponding
heating, since the mold or core is electrically
conductive on account of the wet conditions or moisture
and the remaining constituents.
It is particularly expedient if an AC voltage is
applied as the electric voltage. In this case, a
pulsed, in particular square-wave voltage can be
applied as the electric voltage. A suitable AC voltage
can make use of the reactive properties of the sand
mixture in the core or mold to heat the latter.
Particularly good results can in this case be achieved
with the pulsed and in particular square-wave voltages.
As a result, it is possible in particular to control
the introduction of power by varying the pulse width of
the electric voltage. The voltage can therefore be
selected to be controllable and in particular to be
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greater than 1000V or greater than 1500 V, in order to
achieve correspondingly rapid and powerful heating.
Thorough drying within a short time can be achieved if
an AC voltage with a frequency of over 1000 Hz, for
example of 3000 Hz or more, is selected. Since the
entire core box or the core or mold die is used as
electrode surface, the energy can be transmitted very
quickly and effectively, and therefore the
corresponding core or mold can be dried within an
extremely short time.
It may be expedient if the water which has been
evaporated as a result of heating is expelled from the
die by means of a gaseous medium, such as nitrogen
and/or carbon dioxide and/or air, it being possible for
this gaseous medium, which is used to expel the
evaporated water, to be transported through the die and
therefore through the foundry mold or core which has
been formed, by means of pressure or by suction and
pressure reduction. In particular air is available in
virtually unlimited quantities and can be used without
problems to expel water vapor from the die.
In this case, it is advantageous if the water vapor
which is produced by the heating in the die is expelled
using hot gas. This makes it possible to prevent the
water vapor which is to be expelled from potentially
condensing again prematurely, or means that slightly
less heating of this water vapor may even be sufficient
to allow it then to be expelled from the die as far as
possible.
An expedient configuration of the process may consist
in the fact that magnesium sulfate without water of
crystallization or with at least one mole of water of
crystallization mixed with magnesium sulfate with more
than one mole of water of crystallization, if
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appropriate with up to seven mol of water of
crystallization, is dissolved and/or dispersed in water
and mixed as binder with the foundry sand, and that the
water and some of the water of crystallization are
evaporated by heating and then expelled.
This surprisingly makes it possible to reduce the
quantity of water which is to be expelled. This is
because the magnesium sulfate which does not contain
any water of crystallization or contains only a small
amount of water of crystallization can, during heating,
take up such water of crystallization from the
magnesium sulfate which contains more (polyhydrate)
water of crystallization, so that corresponding
crystals are formed inside the foundry mold or the
core, leading to corresponding strengthening without
the water of crystallization of the entire mixture
having to be completely expelled.
It is known that magnesium sulfate which does not
contain any water of crystallization or contains only a
small amount of, in particular just one mole of, water
of crystallization, when combined with magnesium
sulfate containing more than one mole of water of
crystallization, when they are reacted with one another
with heating, causes the corresponding crystals to
become interlaced, which in the application according
to the invention contributes to the formation of an
extremely strong core or a correspondingly strong mold.
An alternative or additional option for reducing the
quantity of water or water vapor to be expelled during
the process according to the invention may consist in a
highly or more highly concentrated solution of
magnesium sulfate with or without at least one mole of
water of crystallization being mixed with a
hydrocolloid, and this mixture being used as binder.
The addition of hydrocolloid may make it possible to
achieve higher salt concentrations in what is in
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relative terms a small quantity of dispersion and/or
dissolution water, so that a correspondingly reduced
amount of water has to be expelled.
A further configuration of the process may consist in
more magnesium sulfate being mixed with the quantity of
dissolution water which is predetermined for a defined
quantity of foundry sand than is required to produce a
saturated solution, and in some of the magnesium
sulfate being dispersed in the solution and mixed with
the foundry sand as a dispersion. This makes it
possible for the maximum possible amount of magnesium
sulfate to be introduced as binder into the foundry
sand and for the quantity of dissolution water required
to be kept as low as possible, so that subsequently it
is also the case that a correspondingly small amount of
water vapor has to be expelled. At the same time, the
advantages during subsequent removal of foundry sand
residues on the casting with the aid of a simple water
rinse or immersion in water are retained.
The foundry sand can be mixed with the dispersed or
dissolved binder in a weight ratio of from 97:3 to
approximately 80:20.
It is expedient if approximately 100 parts by weight of
foundry sand are mixed with approximately 3 parts by
weight to approximately 20 parts by weight of dispersed
or dissolved binder, i.e. dissolved magnesium sulfate
and/or without water of crystallization. An optimized
procedure may involve mixing approximately 5 to
10 parts by weight of binder in dispersed or dissolved
form with approximately 100 parts by weight of sand.
Tests have shown that this leads to strong cores or
foundry molds which are able to successfully withstand
the casting operation and in which as little water as
possible has to be expelled from the die.
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The invention also relates to a device for producing
foundry molds or cores, having at least one heating
device for setting purposes, wherein the device for
producing foundry molds may be a molding machine and
the device for producing cores may be a core-shooting
machine. This device may be characterized in that at
least one microwave generator is installed as heating
device on the molding machine or on the core-shooting
machine, and in that at least one microwave antenna
which is or can be coupled to the microwave generator
via a waveguide, is arranged in the region of the mold
die for the foundry mold or for the core or cores. A
feed opening of a gas purge hood which is known per se
can in this case be used for expulsion of gases and/or
of heated water vapor.
At this point, it should be noted that the mold die for
the foundry mold or for the core may also be a multi
cavity die in which, by way of example, a plurality of
cores are molded and/or heated simultaneously.
The device according to the invention may therefore
advantageously be formed substantially by a known
molding machine or core-shooting machine, which has
been additionally equipped with a heating device,
specifically with a microwave generator and a microwave
antenna. Moreover, it is advantageous if the inlet
openings for the foundry sand mixed with binder can be
used to expel gases or heated water vapor, so that
overall an inexpensive device is available. Even
existing core-shooting machines or molding machines may
if appropriate be retrofitted in order to allow the
advantageous invention and in particular the process
according to the invention to be employed therewith.
It is expedient if, as a result of the device being set
to the gas purge operation for the expulsion of gases
or of water vapor, the microwave generator can
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the core is made from a mixture of foundry sand and a
binder which is a dispersed or dissolved magnesium
sulfate. In this case, the device can be set to the gas
purge operation for expulsion of the water vapor formed
during the heating operations, as has already been
mentioned above.
The overall advantageous result is that the molding
machine or core-shooting machine and the actual mold
dies can remain virtually unchanged, since the existing
ventilation systems can also be employed in the device
according to the invention and can be used for the
expulsion of the heated and evaporated dissolution
water in accordance with the invention. It is merely
necessary to additionally install an antenna for the
microwave, for example on the gas feed hood. In this
case, of course, the dies are to be made from materials
which are suitable for microwaves. This device having a
heating device designed as a microwave generator and an
antenna may, however, also be used for the production
of molds or cores in which a different binder than the
abovementioned dispersed or dissolved magnesium sulfate
is used and a heating operation is required to set the
binder.
Another possibility which is worthy of protection
provides a device for producing foundry molds or cores,
having at least one heating device for setting
purposes, wherein the device for producing foundry
molds is a molding machine and the device for producing
cores is a core-shooting machine, into which machine a
mold or core die can be or is inserted. In this device,
it is possible for the heating device provided to be an
electrical resistance heating means, in which the
electrically conductive core or the mold forms the
electrical resistor, and the mold or core dies, which
are composed of a plurality of parts in order to allow
a mold or core to be removed, may be at least partially
electrically conductive and may be insulated from one
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another at the locations where they are in contact with
one another, and the parts of the dies may in each case
have at least one electrical terminal for application
of an electric voltage for the resistance heating
device.
In this way, the molds or cores, which initially
contain dissolution water and/or water of
crystallization and are electrically conductive on
account of the further constituents which they contain,
can be electrically heated in what is in design terms a
very simple way in order for water to be expelled. The
moist core or the moist mold constitutes an impedance,
resulting in electrical conductivity. The voltage
applied thereto can therefore be used for drying
purposes.
The resistance heating device may have a voltage source
with a frequency converter for increasing the frequency
and/or a pulse former for forming a pulsed voltage.
Good results during heating can be achieved with a
pulsed voltage.
The resistance heating device may have a voltage source
and a transformer for increasing the voltage, which are
connected, via supply conductors, to the terminals on
the parts of the mold or core die. This makes it
possible to increase efficiency.
At least one part of the mold or core die may have a
plurality of electrical terminals, and switches for
alternately or optionally applying a voltage to these
terminals may be provided between the terminals and the
voltage source, so that alternately one switch is
closed and the others are open. This means that any
polarities which occur at an electrode can always be
reduced again and/or altered. The heating of a mold or
core can be carried out correspondingly uniformly, and
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even very differing contours of molds or cores of this
type can be taken into account by changing the
electrical terminals which are active at any given
time.
In the case of a mold or core die comprising more than
two parts, each part may have an electrical terminal
and electrical supply conductors, and in each case two
parts, cyclically, of a die of this type can always be
connected to the current source. Multipart dies of this
type are often required in particular for complicated
cores. Nevertheless, with the abovementioned
configuration it is possible to use the applied voltage
in each case to form a resistance heating means so that
the core is thoroughly heated.
The overall result is a process and a device which
allow mechanical production of molds or cores in
standard core-shooting machines, and wherein the mold
sand can set within about half a minute. In this
context, the process makes use of the extremely
different melting points of magnesium sulfate in its
hydrated form, on the one hand, and in its anhydrous
state, on the other hand. Specifically, magnesium
sulfate in the form of its heptahydrate has a melting
point of approximately 75° Celsius and in its anhydrous
form has a melting point of 1124° Celsius. Therefore,
by targeted removal of the chemically bonded water of
crystallization, it is possible to achieve virtually
instantaneous setting of the mold sand. Considerable
interlacing of partially hydrated and fully hydrated
magnesium sulfate is in this case a favorable property
which can also be utilized in order to obtain a very
high strength even with small quantities of magnesium
sulfate, for example 1% based on the mold sand.
The important ventilation for removal of the water of
crystallization, which was originally chemically
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bonded, after heating may if appropriate be effected
through specially arranged inlet and outlet nozzles, in
which case a superatmospheric pressure of 1 to 6 bar of
a dry gas, preferably of heated air, is expedient . The
heating may expediently be effected using microwaves,
since the quartz sand which is normally used is
"transparent" to microwave radiation, so that such
radiation can penetrate all the way through even
relatively large molds or cores. Moreover, only the
magnesium sulfate which contains water of
crystallization is heated. As soon as the water of
crystallization has escaped, this magnesium sulfate,
which is now anhydrous, is also "transparent" and no
longer presents any obstacle to the further penetration
of the microwaves.
However, the heating may expediently also be effected
by resistance heating, as has been explained above.
Therefore, the targeted removal of even the chemically
bonded water of crystallization - at least in part -
from the magnesium sulfate is essential. This leads to
very rapid setting, which is advantageous for
economically viable production. Moreover, sufficient
strength is achieved with a relatively low
concentration of magnesium sulfate. The mold parts or
cores produced in this way are dimensionally stable up
to at least 1124° Celsius and can be dissolved out of
the metal casting using a small amount of water.
If conventional binders are used, accelerated setting
is likewise possible on account of the targeted heating
by means of microwave or electrically conductive
heating.
Exemplary embodiments of a device for producing molds
or cores for foundry purposes in accordance with the
invention are described in more detail below with
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reference to the drawing, in which, in some cases in
diagrammatic form:
fig. 1 shows a schematic illustration of a device for
producing foundry molds or cores, having a
microwave generator and corresponding antenna,
in the form of a core-shooting machine,
fig. 2 shows, on an enlarged scale and in even more
schematic form, a longitudinal section through
part of the shooting unit after sand has been
shot into a mold die formed as a core box and
before this core box is moved to meet a purge
hood located above it and is pressed onto the
shooting head from below, or vice versa, with
the microwave antenna for heating the core and
expelling the dissolution water being arranged
in this purge hood; the connection between the
microwave generator and this emitting antenna
is still open and can be closed automatically
when the parts are moved together or lifted and
pressed together,
fig. 3 shows an illustration corresponding to fig. 2,
with the emitting antenna arranged in the lower
region of the core box designed as a mold die,
fig. 4 shows an embodiment which has been modified
with respect to figs. 2 and 3 in a similar form
of illustration; in this case, an antenna which
is or can be coupled to the microwave generator
in the position of use for heating the shot
core is arranged in both the purge hood and the
core box,
fig. 5 shows an illustration corresponding to figs. 2
to 4 of a modified embodiment, in which
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infrared radiators for heating the shot core
are arranged in the core box,
fig. 6 shows an illustration corresponding to figs. 2
to 5 of a modified embodiment, in which the
heating device provided is an electrical
resistance heating means, in which the mold for
the electrically conductive core is likewise
electrically conductive and its parts are
insulated at the locations where they are in
contact, and an electrical terminal for a
resistance heating device is provided at each
part of the mold or core dies,
fig. 7 shows an arrangement and device corresponding
to fig. 6, in which a plurality of electrical
terminals, which can be connected up optionally
and alternately in switches, in order to avoid
polarization at one of the terminals, are
provided on one of the parts of the die for the
core,
fig. 8 shows a further modified device, in which the
core die comprises three electrically
conductive parts which are insulated from one
another and each of which has an electrical
terminal, it being possible for in each case
two of the three parts alternately to be
connected to the voltage source via switches,
and
fig. 9 shows an embodiment and arrangement
corresponding to fig. 6, in which, however, the
sequence of the voltage transformer and the
pulse former located behind the voltage source
is reversed compared to the arrangement shown
in fig. 6.
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A device which is denoted overall by 1 and is
illustrated diagrammatically and partially cut away in
fig. 1 is used to produce cores, but could also be
employed to produce foundry molds. In the exemplary
embodiment, the device is a core-shooting machine.
The cores 2 which are to be produced with it (figs. 2
to 9) - or by analogy foundry molds - are molded from a
mixture 3 of foundry sand and binder, which is a
magnesium sulfate which is dissolved in water and
preferably includes at least one mole of water of
crystallization, or alternatively is some other form of
binder, this mixture 3 of sand and binder being
introduced into a sand feed funnel 4 in a known way and
as a result being introduced into the shot head 5 of a
shooting unit denoted overall by 6. Fig. 1 also
illustrates the air boiler 7, which is essential to the
shooting operation, in partially cut-away form.
The core box 8 which is illustrated in each of figs. 2
to 9 and is assembled from a core box upper part 8a and
a core box lower part 8b in the position of use, but
could also be a correspondingly differently configured
mold die if foundry molds are to be produced, belongs
to this device 1 in the form of a core-shooting
machine. Fig. 8 shows an embodiment in which the core
box upper part 8a is in turn subdivided in order to
allow the removal of a correspondingly complicated core
after it has set.
With this device, it is possible to produce molds or
cores for foundry purposes from the mixture 3 of
foundry sand and binder, with the foundry sand and
binder initially being mixed and then being introduced
into the mold or core die - in the exemplary embodiment
the core box 8 - with the aid of the shooting unit 6.
This has already occurred in figures 2 to 9, and the
binder can then set and impart the required strength to
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the mold or core 2. The binder in this case is
magnesium sulfate preferably with at least one mole of
water of crystallization dissolved and/or dispersed in
water, and this binder is then mixed with the foundry
sand to form the mixture 3. This mixture is then
introduced or shot into the mold or core die 8.
Then, inside this die, the core box 8, the dispersing
and/or dissolution water and at least some of the water
of crystallization is evaporated by heating and
expelled from the mold or core die, i.e. from the core
box 8, by means of a gaseous medium.
To carry out this process, at least one heating device,
which is to be described in more detail below and can
be used to heat and expel the dissolution water and/or
the water of crystallization, is provided on the
molding or core-shooting machine 1.
In the exemplary embodiments shown in figs. 1 to 4, a
microwave generator 9 is installed on the core-shooting
machine 1 as heating device, and at least one microwave
antenna 10, which can be coupled, and in the position
of use is coupled, to the microwave generator 9 via a
waveguide 11, is arranged in the region of the mold
die, i.e. of the core box 8, at different locations
depending on the particular exemplary embodiment. In
the exemplary embodiment, the corresponding coupling 12
is still open, since although a core 2 has already been
shot, the core box 8 has not yet been brought to meet a
gas purge hood 13 and the heating and setting by means
of microwave have not yet been carried out.
In all the exemplary embodiments, it is possible to see
a feed opening 14, which can be used, for example, to
introduce hot air in order to expel the heated water or
water vapor which is formed as a result of the heating
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with the aid of the heating device, i . a . the microwave
9 in the position of use.
In figures 1 to 4, the connection between microwave
generator 9 and antenna 10 is still open. Setting the
device 1 to the gas purge operation for expulsion of
the water vapor, i.e. the relative lifting motion of
the core box 1 with respect to the gas purge hood 13
and with respect to the shooting head 5 or vice versa
simultaneously allows the microwave generator 9 to be
coupled to the antenna 10 via the waveguide 11 by
virtue of the coupling 12 being closed during the
abovementioned relative movement.
Then, the heating with the aid of the microwave energy
and, at the same time or slightly afterward, the
expulsion of the water vapor which is formed can take
place.
The movement of setting to the gas purge operation can
automatically couple the microwave generator 9 to the
antenna 10, so that the entire operation can be carried
out quickly.
The path of the waveguide 11 can therefore be
interrupted, and the abovementioned coupling 12 is
provided at the location where it is interrupted, it
being possible for the antenna-side part of the
waveguide 11 optionally to be arranged and connected at
the gas purge hood 13, as shown in fig. 2, or in the
mold die or core box 8, as shown in fig. 3, or even at
both locations, as shown in fig. 4. Fig. 4 shows that
the microwave generator 9 can be coupled and is
connected, via two waveguides 11, to an antenna 10
arranged in the gas purge hood 13 and an antenna 10
arranged in the mold die or core box 8, so that the
foundry mold or the core 2 can be heated
correspondingly quickly and powerfully and the time
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required to expel the dissolution water and/or water of
crystallization can be shortened.
Fig. 5 shows a modified embodiment, in which infrared
radiators 15 are provided as the heating device at or
in the mold die, in this case in the core box 8; the
infrared radiators 15 may be provided as an alternative
to heating by means of microwave or even in addition to
heating by means of microwave, for example if an
antenna 10 as shown in fig. 2 were additionally to be
provided in the gas purge hood.
Figs. 6 to 9 in turn show modified embodiments in which
the heating device provided is an electrical resistance
heating means, in which the electrically conductive
core 2 forms the electrical resistance. The core die 8,
which for removal of a core 2 once again is composed of
two parts (figs. 6, 7 and 9) or three parts (fig. 8) ,
is in this case at least partially, or expediently
completely, electrically conductive, by virtue of it
consisting, for example of aluminum or cast iron or
steel. At the locations where they are in contact with
one another, the parts 8a and 8b are insulated from one
another, and this insulation 16 is diagrammatically
depicted in figs. 6 to 9.
It can also be seen that the parts 8a and 8b of the
dies or of the core box 8 each have an electrical
terminal 17 for application of an electric voltage for
the resistance heating device.
The core box upper part 8a and core box lower part 8b,
i.e. the parts of the core die 8, therefore belong to
the resistance heating means, wherein the core 2 forms
the actual resistor.
In the usual way, this resistance heating device has a
voltage source 19, which in the present case leads
through a three-phase network 20 to a frequency
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converter for increasing the frequency and/or a pulse
former 21 for forming a pulse voltage.
Moreover, this resistance heating device has a
transformer 22 for increasing the voltage, from which
supply conductors 23 lead to the terminals 17 on the
parts 8a and 8b of the core die 8. When the voltage is
switched on, the moist core 2 inside the die 8 acts as
a corresponding resistor or as an impedance, so that
current flows in order to dry the core. The level of
the voltage can be selected according to the thickness
of the core 2. Very intensive and effective drying is
achieved since the parts 8a and 8b act as electrodes
which bear against and make contact with the core and
to which the electric voltage is applied, these
"electrodes" 8a and 8b being isolated from one another
by the insulation 16 in order to avoid a short circuit.
The electric voltage may expediently be a sinusoidal or
pulsed, in particular square-wave voltage, with an AC
voltage of high frequency of over 1000 Hz, for example
of 3000 Hz or even above, being particularly effective.
It is also possible for the voltage to be controlled
and to be selected to be greater than 1000 V. By
changing the pulse width of the electric voltage, it is
possible to control or regulate the introduction of
power and to match it to the shape and size of a core
2, and in the case of a mold being produced in a mold
die, to the mold.
Whereas in the exemplary embodiment shown in Fig. 6 two
parts 8a and 8b form the core box 8 and each have one
electrical terminal 17, the exemplary embodiment shown
in Fig. 7 reveals four electrical terminals 17 of this
type on the core box lower part 8b, these terminals
being connected in parallel, and switches 24 for
alternately or optionally applying a voltage to the
various electrical terminals 17 being provided between
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these terminals 17 and the voltage source 19, in which
case alternately one switch 24 is closed and the others
are open. This makes it possible to avoid polarization
at a connection location on the core box lower part 8b
and to heat the core 2 as uniformly as possible.
Fig. 8 shows an embodiment in which the core die 8 is
composed of more than two parts, the core box upper
part 8a for its part being subdivided into two parts,
which parts are electrically isolated from one another
by an insulation 25. This makes it possible to produce
correspondingly complicated cores 2.
Fig. 8 illustrates that each of these three parts has
an electrical terminal 17 and an electrical supply
conductor 23, which is initially composed of two
parallel sections 23a and 23b in which switches 26 are
arranged. These parallel-connected sections 23b enable
in each case two parts 8a or 8b, cyclically, of a
multiply divided die 8 of this type to be connected to
the voltage source 19 by the switches 26 being opened
and closed cyclically. Therefore, in each case only two
parts of the core box 8 are energized, cyclically, in
order for the core 2 which is present therein to be
used as a resistor and heated.
In the embodiment shown in fig. 9, which substantially
corresponds to that shown in fig. 6, it can be seen
that the order in which the pulse former 21 and the
transformer 22 are arranged may also be switched, so
that the voltage transformer 22 is provided first,
followed by the pulse former 21, in series.
In the embodiments shown in figs. 6 to 9, as in the
exemplary embodiments illustrated in figs. 2 to 5,
there is a gas purge hood 13 having a feed opening 14,
by means of which, by way of example, hot air can be
introduced in order to expel the heated water or water
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vapor which is formed as a result of the heating with
the aid of the electric voltage in the position of use.
The gas purge hood 13 can be moved in the same way as
in the exemplary embodiments described above in order
for the gas purge operation to be carried out.
A gaseous medium, for example nitrogen and/or carbon
dioxide and/or air, preferably hot air or hot gas, can
be supplied via the feed opening 14 in order to expel
the evaporated water. Therefore, the expulsion of the
evaporated water can best be effected by means of
superatmospheric pressure.
As has already been mentioned above, the mixture 3
contains, as binder, magnesium sulfate, without and/or
with one mole or if appropriate more than one mole of
water of crystallization, dissolved in water. By way of
example, it is possible to use magnesium sulfate
without any water of crystallization, with one mole of
water of crystallization and magnesium sulfate with
more than one mole of water of crystallization and/or
also mixed with a hydrocolloid, as binder. In this
context, it is particularly expedient if only magnesium
sulfate or magnesium sulfate with hydrocolloid are
used, since magnesium sulfate with water of
crystallization can be successfully dissolved and/or
dispersed in water and mixed as binder with foundry
sand, but also can subsequently be successfully
dissolved out of a cast workpiece again with the aid of
water.
According to one example of an expedient mixture of
foundry sand and dispersed or dissolved binder, it is
possible for approximately 100 parts by weight of
foundry sand to be mixed with approximately 3 parts by
weight to approximately 20 parts by weight of dissolved
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binder, in particular comprising magnesium sulfate in
dissolved form.
In this case, it is possible for approximately 100
parts by weight of sand to be mixed preferably with
approximately 5 to 10 parts by weight of binder in
dispersed or dissolved form. A correspondingly small
amount of water has to be expelled from the core box 8
by heating and using a gas, which means that the
process can be carried out correspondingly quickly.
To produce molds or cores 2 for foundry purposes, a
mixture 3 of foundry sand and binder is produced and
introduced, for example shot in a core-shooting
machine, into a mold or core die 8. A known binder or
magnesium sulfate without any water of crystallization
and/or with at least one mole or alternatively more
than one mole of water of crystallization dissolved or
dispersed in water is used as binder and mixed with the
foundry sand and introduced or shot into the mold die
or the core box 8. For setting purposes, the water and
some of the water of crystallization is then evaporated
by heating and expelled by means of a gaseous medium,
an operation which can be carried out very quickly.
After the casting operation, a core of this type or a
mold of this type consisting of foundry sand can very
quickly be dissolved out of the workpiece and flushed
out by means of water, since the magnesium sulfate
retains its ability to be dissolved.
Claims
