Method and apparatus for hardening mold parts made of sand for making metal castings

An account is given of a method and apparatus for hardening mold parts, such as mold outer parts or mold cores for making metal castings, by using compressed air for forcing catalyst through the sand of the mold parts for reaction with a binding agent mixed with the sand.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention has to do with a method for hardening mold parts, 
that is to say mold cores and hollow mold parts, made of sand with the 
addition of a binding agent or binding agents able to be hardened by a 
catalyst, and used for making metal castings. 
2. Prior Art 
In this respect the starting point of the invention is a method, whose 
development I was responsible for, in the case of which a certain amount 
of a liquid catalyst is put in the mold part with the help of compressed 
air, with which the catalyst is mixed, and then the mold part is cleared 
or rinsed with catalyst-free compressed air. 
It is important to make certain that there is an even distribution of the 
mist, produced by mixing catalyst and compressed air, throughout the mold 
parts, without drops of catalyst being retained on only some of the grains 
of sand and not transported fully into the remote portions, that is to say 
all portions of the mold part. For this purpose I made a suggestion of 
using a heating system for heating the mist evenly before leaving the 
mixing zone so that the catalyst is changed into a gas. For this reason it 
is a compressed air-catalyst gas mixture which goes into the mold part. 
This is something which promotes a quicker movement of the catalyst 
through the mold part material and, for this reason, a quicker hardening 
of the mold parts. 
However, in comparison, such a manner of operation requires a great amount 
of energy, because the mist has to be heated up, generally speaking, from 
the temperature of the compressed air, which is generally between 
0.degree. and 20.degree. C., to the temperature for changing the catalyst 
into its gaseous form, this being necessary every time the mist is forced 
through to the mold parts. 
The starting temperature is normally towards the lower end of the given 
temperature range, because the compressed air is caused to give up 
moisture by forcing it through refrigeration dryers. If it is to be 
heated, it is naturally necessary for heat to be used for the compressed 
air in this respect. Furthermore, in this method the heating only takes 
place during a short time interval in the working step in question so that 
it is not possible to make certain that in fact there is a complete change 
of the catalyst into the gas form. 
Objectives 
One purpose of the invention is that of making certain the catalyst 
completely changes into its gaseous form and accomplishes this with very 
much smaller amounts of energy. 
For effecting this and other purposes in the invention, the compressed air 
is heated in a normal way for the working run and the liquid catalyst is 
changed into its gaseous form in a shut-off chamber during the time 
interval between the working runs in the expanded but still hot air 
remaining from that which had been used for clearing. 
So, in the invention the time interval between the working runs is used for 
changing the catalyst into its gaseous form and, because of the repeated 
motion of hot compressed air through the gas-producing and/or mixing zone, 
the zone is kept unchangingly at an increased temperature level so that it 
is no longer necessary for heating to take place from low temperatures to 
the gas-producing temperature, this resulting in the use of less energy. 
The use of heated compressed air for the hardening of such cores or molds, 
that is to say mold parts, forms part of a suggestion for example as 
disclosed in the German specification (Auslegeschrift ) 2,546,032. However 
in that case use is not made of liquid catalyst and in fact the catalyst 
together with the heated compressed air in the form of a supporting 
gas-catalyst mixture is put into contact with the material to be 
conditioned. This old method, however, has the shortcoming that the amount 
of catalyst is measured by fixing the opening times of the valves and, 
furthermore, is dependent on the presure and temperature. In the method of 
my present invention, on the other hand, the catalyst may be put in very 
exact amounts using a measuring or metering pump. In order in the old 
method to have, even roughly, a sufficient amount of catalyst it was 
necessary to make use of catalyst in amounts which were in fact greater 
than needed, because it was only possible by this means to make certain 
that the minimum amount of catalyst necessary did in fact make its way 
into the mold part every time. However the amounts which were more than 
the amount in fact needed were released into the outside air, affecting 
this air. 
An apparatus for using the method of the invention may have a pressure line 
joined with a mixing chamber, an outlet line joined with the mixing 
chamber for the liquid catalyst, a heating system and control valves, for 
controlling the admission of compressed air and of the catalyst to the 
mixing chamber in step with the working runs, characterised in that the 
heating system is joined with the inlet of the mixing chamber and between 
the mixing chamber and the mold part, that is to say the core or outer 
mold part, there is a further shutoff valve. 
The result is that the whole zone between the shutoff valve or the inlet of 
compressed air to the mixing chamber and a shutoff valve between the 
mixing chamber and the core mold part may be used as an evaporation 
chamber for the liquid catalyst, and this evaporation chamber is kept all 
the time at a temperature which makes evaporation possible and relatively 
higher temperatures may be used during the time intervals between the 
separate working steps for substitute evaporation. The heating system in 
the form of a heating chamber may, in this respect be made greater in 
size, by using insert blocks so that a generally larger heat reserve is 
available for the gas-forming operation. Because, in this zone, heating no 
longer has to take place from the compressed air temperature to the 
gas-forming temperature, the energy demand is dependent only on the energy 
needed for keeping the heat reserve at the desired working temperature in 
question. To prevent heat going from this evaporation zone to the 
volumetric displacement pump and, for preventing the changing of the 
catalyst into its gaseous form on its way into this gas-forming zone, it 
is best, although not completely necessary, to provide cooling between the 
pump and the gas-forming zone. As far as possible, this should be done 
immediately adjacent the gas-forming zone. This cooling may be effected by 
a pipe system through which the cool or cold compressed air is forced.

DETAILED ACCOUNT OF INVENTION 
In the diagram of FIG. 1 the horizontal axis represents time t, while the 
vertical axis represents pressure P. 
At the zero point the working run or operating cycle is started. It has a 
working step or phase T and a rest step or phase .theta.. The top curve or 
line is representative of the pressure behavior of the inlet compressed 
air. Below it, the decrease in the quantity of the catalyst in gaseous 
form in the working step is arrow, while the lowermost curve is 
representative of the takeup of the catalsyt and the introduction of the 
catalyst. 
At the time zero, that is to say O, the inlet compressed air has a pressure 
value A of, for example, 2 bar, which is controlled by a valve system to 
be detailed later. In the working step the pressure may undergo an 
increase of for, example, up to 6 bar, a value which is attained at B. At 
the time O at the pressure A there is exists catalyst in gaseous form in 
the mixing zone, which zone is made up of the heating system and the 
mixing chamber. This catalyst is forced out during time H by the admission 
of compressed air and is reduced to an amount equal to zero at the point 
C. Starting at the point C in time, the hardening is stopped at point H' 
and the clearing cycle is started in the clearing time S, which is 
terminated at the point D in time. At this point in time, the system is 
shut off from the compressed air inlet or line so that the pressure of the 
compressed air may be expended completely and finally the pressure will be 
at a normal value at E the same as the outside pressure. So the time T of 
a working step or phase comes to an end and the rest step .theta. is 
started. 
Furthermore at the time O the takeup pump for the liquid catalyst will have 
been started and it continues working till the point F. As is to be seen, 
the position of this point F is dependent on the amount of catalyst to be 
used. On getting to the point F the pump is stopped, something which is 
made clear by the broken-line curve running parallel to the time axis. 
This curve extends past the time point E as far as the point G in time and 
after this time point the pump will be forcing the liquid catalyst into 
the part of the apparatus made up of the heating system and the mixing 
chamber, as is made clear by the curve extending as far as the point H in 
time. For this reason the complete working step will have been ended and 
it will be started again after the rest step or phase .theta. at another 
starting point O. The working run is, for this reason, made up of the 
working step of time T and the rest step of time .theta., it being 
specially important to the invention that the rest time .theta. is used 
for the forcing in of the liquid catalyst and, at the same time, 
evaporation of the liquid catalyst, which is on hand in its vapor or 
gaseous condition by the start of the next working step time T. It is 
naturally necessary for the rest time, in cases in which, for example, the 
apparatus is not used overnight, to be bridged over by turning on the 
heating even before the start of the next working step time T in order, at 
the start of the working step time, to have on hand catalyst in its vapor 
or gaseous form. However this starting up does not cause any change in the 
general operation or theory of the apparatus, that is to say, the 
compressed air released into the mixing chamber will have available a 
vapor or gaseous catalyst for transport into the core and into the outer 
mold part. 
In the apparatus to be seen in FIG. 2 reference no. 1 identifies the 
connection with the compressed air line, which for example has a pressure 
of 6 bars. Reference no. 2 identifies a water, oil and dirt trap for 
making certain that the apparatus is only used with clean compressed air. 
The incoming compressed air goes to the automatic control valve 3, which is 
operated by a part of the apparatus to be detailed later. From the 
automatic control valve 3 the compressed air goes to the shut off valve 4 
by which it is possible for the unit, made up of the heating system and 
the mixing chamber, to be shut off or isolated from the compressed air 
line. 
At 5 there is a safety check valve while reference number 6 identifies an 
overpressure valve acting as a safety valve of normal design and so 
needing no detailed account. 
In a heating chamber 7, made more specially of aluminum, there is a duct 
made during casting of the chamber for the transport of compressed air 
through the chamber. Naturally it is possible to make use of any other 
heating system of the necessary design. Reference number 8 identifies the 
power connection, for example to electric power, for the heating chamber. 
From the heating chamber the compressed air goes into the mixing chamber 9 
and from it through a shut off valve 10 in the direction of the arrow 11 
to the core or outer mold part box. The air conduit from the compressed 
air source 1 to the shut off valve 10 forms an air passage 35. 
In the mixing chamber there is the opening of a duct 12, which runs from a 
measuring pump 14. The duct 12 has a check valve 13 in it. The measuring 
(or metering) pump 14 obtains the liquid catalyst through the supply line 
15 and a check valve 16 from the tank 17. The pump takes the liquid 
catalyst from the tank 17 forcing it, after shutting off the check valve 
16, through the check valve 13 into the mixing chamber 9. In the duct 12 I 
have placed a cooling tube 18 for making certain that the heat from the 
heating system 7 and the mixing chamber 9 is not conducted by the duct to 
the pump 14, thus stopping any premature evaporation of liquid catalyst in 
this part. A portion of the compressed air coming from the line 1 is 
branched off at 19 through the duct 20 as control air. This control air 
goes through a pressure-decreasing valve 21 and a check valve 22 to an 
air-operated control valve 23. Also through a pressure-decreasing valve 24 
and a control choke 25 the compressed air is supplied to the control valve 
23. The pressures, fixed by adjustment, may be seen at once by reading the 
pressure gages 26 and 27. If a time control clock 40 is used for working 
the control valve 28, this control valve 28 will operate, on the one hand, 
the valve 4 and on the other hand of the valve 23 so that first air at the 
lower pressure, for example 2 bars, will be passed through the automatic 
control valve 3 and the valve 4 and the check valve 5 into the line going 
to the heating chamber 7. At the same time the valve 29 is operated, which 
makes possible the movement of air through the duct 30 to the measuring 
pump 14 so that the pump's piston is moved downwards and the necessary 
amount, fixed by adjustment, to withdraw liquid catalyst from the tank 17 
into the pump. This operation is illustrated in FIG. 1 as the drop in the 
catalyst curve III between the points O and F. The speed of take-up may 
undergo adjustment with the help of a choke valve 31. At the same time as 
the operation of the valves 28 and 29, the valves 4 and 10 are opened by 
way of the lines used for this purpose. Because, in the unit made up of 
the heating chamber and the mixing chamber the necessary mixture of air 
and catalyst in gaseous form is present from the preceding working step, 
this mixture is now forced by the compressed air, coming from the 
compressed air line, through the open valves to the core box. When this is 
done the valve 3 is smoothly opened further because the control valve 24, 
which has been set at a higher pressure value, will be operating the valve 
3 more and more, through the valves 25 and 23, until a certain pressure 
value is produced, as is the case at point B in FIG. 1. 
This point is attained after the whole of the catalyst in gaseous form has 
been forced into the core on reaching the point C, and then the clearing 
time is started as well. At the end of the clearing time, that is to say 
on reaching point D in the graph of FIG. 1, the valve 28 is switched off 
by the clock 40 so that the valve 4 is shut. The valve 10 is kept open by 
the valve 29, which is controlled by a second clock 50, but up to this 
point has not been operated. The pressure of the compressed air is for 
this reason able to undergo a decrease from point D to point E along the 
curve IV. It is only at point G that the valve 29 has the added effect of 
shutting the valve 10 so that there is now available an isolated portion 
of the apparatus, into which from the point G to the point H the quantity 
of liquid catalyst, taken up in the pump 14 from the tank 17, is forced 
into the space made up of the heating chamber 7 and the mixing chamber 9. 
In this space there is heated, unexpanded compressed air, which because of 
the amount of heat in it, is responsible for changing the catalyst into 
its gaseous form at once. This gaseous form catalyst is kept in the 
portion of the apparatus between the valves 5 and 10 and is on hand for 
the next working operation, which if necessary, after a long resting time, 
will take place in quite the same way as detailed. 
It is furthermore pointed out that it is possible, by reading the pressure 
gage 32, to take note of the complete development of pressure of the 
compressed air as made clear by the curve for the compressed air of FIG. 
1. It is for this reason possible to make a change in the form of the 
curves, as may be desired, by making a further adjustment of the choke 25, 
and keeping an eye on the changes in the position of the hand of the 
pressure gage 32.