Method for casting cast parts

Abstract

A method for casting cast parts in which a casting mould is provided. The casting mould is enclosed in a housing, forming a filling space between an inner surface section of the housing and an associated outer surface section of the casting mould. The filling space is then filled with a free-flowing filling material and molten metal is poured in the casting mould. As a consequence a binder of the mould material begins to vaporise and combust disintegrating the casting mould. During the filling of the filling space, the filling material has a minimum temperature, starting out from which the temperature of the filling material rises, to beyond a boundary temperature at which the vaporising binder ignites and combusts.

Claims

1. A method for casting a cast part, in which a molten metal is poured into a casting mould which encloses a cavity forming the cast part which is to be produced, wherein the casting mould, designed as a lost mould, includes one or more casting mould parts or cores which are formed of a mould material which includes a core sand, a binder and, optionally, one or more additives for adjusting particular properties of the mould material, the method comprising the following working steps: providing the casting mould; enclosing the casting mould in a housing forming a filling space between at least one inner surface section of the housing and an associated outer surface section of the casting mould; filling the filling space with a free-flowing filling material; and pouring molten metal into the casting mould, wherein, as a result of the pouring of the molten metal, the casting mould begins to radiate heat, the consequence of the input of heat caused by the hot molten metal, wherein, as a consequence of the input of heat caused by the molten metal, the binder of the mould material begins to vaporise and combust, so that it loses its effect and the casting mould disintegrates into fragments; wherein the filling material poured into the filling space has such a low bulk density that the filling material packing formed by the filling material following filling of the filling space is permeated by a gas flow and wherein, on filling the filling space, the filling material has a minimum temperature starting out from which the temperature of the filling material rises as a result of process heat which is generated through the heat radiated from the casting mould and through the heat released during combustion of the binder, to above a boundary temperature at which the binder evaporating from the casting mould and coming into contact with the filling material ignites and begins to combust.

2. The method according to claim 1, wherein a product of bulk density and specific heat capacity amounts to a maximum of 1 kJ/dm.sup.3K.

3. The method according to claim 1, wherein the bulk density amounts to a maximum of 4 kg/dm.sup.3.

4. The method according to claim 1, wherein the filling material possesses a maximum specific heat capacity of 1 kJ/kgK.

5. The method according to claim 1, wherein the filling material is formed of granules with an average diameter between 1.5-100 mm.

6. The method according to claim 1, wherein the temperature of the filling material during filling of the filling space is at least 500 C.

7. The method according to claim 1, wherein the boundary temperature is 700 C.

8. The method according to claim 7, wherein the gas flow is heated to a temperature above room temperature.

9. The method according to claim 1, wherein the housing has a gas inlet and an exhaust gas outlet and wherein the filling material contained in the filling space is, at least at times and in certain sections, flowed through by an oxygen-containing gas flow.

10. The method according to claim 8, wherein the gas flow is regulated depending on exhaust gas volume flow issuing from the exhaust gas outlet.

11. The method according to claim 8, wherein an exhaust gas measurement is carried out at the exhaust gas outlet and wherein the gas flow is regulated depending on a result of this measurement.

12. The method according to claim 8, wherein a partial flow of combustion gases issuing from the exhaust gas outlet is mixed with the oxygen-containing gas flow and the resulting mixture is fed into the housing.

13. The method according to claim 1, wherein the housing is equipped with a catalytic converter for decomposing toxic substances contained in combustion products of the binder.

14. The method according to claim 1, wherein the casting mould is placed on a sieve base, and wherein the fragments of the casting mould and the filling material trickle together through the sieve base, are collected and processed together and are separated from one another following processing.

15. The method according to claim 1, wherein, following disintegration of the casting mould, the cast part passes through a heat treatment during which it is cooled in a controlled manner according to a specified cooling curve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail in the following with reference to a drawing representing, in diagrammatic form, an exemplary embodiment, wherein:

(2) FIG. 1 shows a flow chart representing the process according to the invention;

(3) FIGS. 2-8 show a thermoreactor in different phases of the performance of the method according to the invention, in each case viewed as a section along its longitudinal axis;

(4) FIG. 9 shows the thermoreactor opened for removal of the cast parts in a view corresponding to FIGS. 2-8;

(5) FIG. 10 shows an apparatus for cooling down a cast part;

(6) FIG. 11 shows the finished cast part;

(7) FIG. 12 shows a collecting pan of the thermoreactor in a view corresponding to FIGS. 2-8;

(8) FIG. 13 shows a crushing mill for regenerating core sand in a section transverse to its longitudinal axis;

(9) FIG. 14 shows a casting mould for casting a cast part in a view corresponding to FIGS. 2-8;

(10) FIG. 15 shows a storage hopper filled with filling material in a view corresponding to FIGS. 2-8; and

(11) FIG. 16 is a graph showing the relationship between the concentration of toxic substances in the filling space and the temperature of the granulate as a function of time during the inventive method.

DESCRIPTION OF THE INVENTION

(12) FIG. 1 shows in diagrammatic form the cycle involved in carrying out the method according to the invention. This starts with casting mould parts and cores made of mould material which consists of a mixture of new, unused core sand, for example quartz sand, and a conventional binder, for example a commercially available cold box-binder. New filling material, for example ceramic granulate with an average grain size of 1.5-25 mm, is also used which, for its first use, must be heated to the required minimum temperature, for example 500 C., before it can be used. These starting materials can later be reused in the cycle, as explained below.

(13) The thermoreactor T, represented in different phases of the method according to the invention in FIGS. 2-8, has a sieve plate 1, on which a casting mould 2 prepared for pouring a cast iron melt is placed. The casting mould 2 is intended for the manufacture by casting of a cast part G, which in the present example is an engine block for an internal combustion engine of a commercial vehicle.

(14) The casting mould 2 is assembled in a conventional manner as a core package consisting of a plurality of outer cores or mould parts arranged on the outside and casting cores arranged on the inside. In addition, the casting mould 2 can include components consisting of steel or other indestructible materials. These include for example chills and similar which are arranged in the casting mould 2 in order to achieve a controlled solidification of the cast part G through an accelerated solidification of the melt coming into contact with the chill.

(15) The casting mould 2 delimits from the environment U a mould cavity 3 into which the cast iron melt is poured in order to form the cast part G. The iron melt thereby flows into the mould cavity 3 via a gate system, which for reasons of clarity is not shown here.

(16) The cores and mould parts of the casting mould 2 are manufactured, in a conventional manner using the cold box method, from a conventional mould material consisting of a mixture of a commercially available core sand, a commercially available organic binder and optionally added additives, which for example serve the purpose of allowing better wetting of the grains of the core sand through the binder. The casting cores and mould parts of the casting mould 2 are formed from the mould material. The obtained casting cores and mould parts are then gassed with a reaction gas in order to harden the binder through a chemical reaction and thus lend the cores and mould parts the necessary rigidity.

(17) The edge of the sieve plate 1 is supported on a peripheral edge shoulder 4 of a collecting pan 5. A sealing element 6 is integrated in the peripheral contact surface of the edge shoulder 4.

(18) Once the casting mould 2 is positioned on the sieve plate 1, an enclosure 7, which is also part of the thermoreactor T, is placed on the peripheral edge shoulder 4 of the collecting pan 5. The enclosure 7 is designed in the form of a hood and encases the casting mould 2 on its outer peripheral surfaces 8. The periphery of the space bounded by the enclosure 7 is over-dimensioned in comparison with the periphery of the casting mould 2, so that after the enclosure 7 is placed on the sieve base 1 a filling space 10 is formed between the outer peripheral surface of the casting mould 2 and the inner surface 9 of the enclosure 7. The enclosure rests on the sealing element 6 with its edge associated with the collecting pan 5, so that a tight seal of the filling space 10 with respect to the environment U is guaranteed. The enclosure consists of a thermally insulating material, which can consist of several layers, of which one layer guarantees the necessary stability of form of the enclosure 7 and another layer guarantees thermal insulation. On its upper side, the enclosure 7 surrounds a large opening 11 via which the casting mould 2 can be filled with cast iron melt and the filling space 10 with filling material F (FIG. 3).

(19) In order to fill the filling space 10 with a filling material F in the form of a granulate, heated to a temperature Tmin of at least 500 C., a storage hopper V is positioned above the opening 11 from which the hot filling material F is then allowed to trickle into the filling space 10 via a distribution system 12 (FIG. 4).

(20) When the filling process is completed, the material packing filled into the filling space 10 can be compressed if necessary. A cover 13 is then placed on the opening 11, which also has an opening 14 via which the cast iron melt can be filled into the casting mould 2 (FIG. 5).

(21) The cast iron melt is then poured into the casting mould 2 (FIG. 6).

(22) Meanwhile, oxygen-containing ambient air can enter the filling space 10 via a gas inlet 15 moulded into the lower edge region of the enclosure 7. Ambient air which enters the collecting pan 5 via an access 16 is also sucked into the filling space 10 via the sieve base 1 (FIG. 7).

(23) The desired destruction of the casting mould 2 which commences with the pouring of the cast iron melt and the associated demoulding of the cast part G takes place in two phases.

(24) In the first phase, solvent in the binder evaporates. The solvent emitted from the casting mould 2 in vapour form reaches a concentration in the filling space 10 at which it automatically ignites and burns off. As a result of the heat thus released, the granular filling material F, which has been brought to a temperature Tmin of approx. 500 C. is heated to above the boundary temperature Tbound of 700 C. until its temperature reaches the maximum temperature Tmax of approximately 900 C.

(25) When the concentration of the binder components evaporating from the casting mould 2 is no longer sufficient for an autonomous combustion, the filling material heated in this way assumes the function of a heat accumulator, through which the temperature of the casting mould 2 and that in the filling space 10 is maintained at a level above a temperature Tbound of 700 C. In this way, the combustion of the binder components and other potential toxic substances issuing from the casting mould 2 continues until no more binder evaporates from the casting mould 2. As a result of the high temperature prevailing within the filling space 10, the vaporous substances which may still be issuing from the casting mould 2 are oxidised or otherwise rendered harmless.

(26) The oxygen-containing gas flows S1, S2 formed of ambient air which enter the filling space 10 of the enclosure 7 via the gas inlet 15 and the sieve base 1 also contribute to the completeness of the combustion of the gases issuing from the casting mould 2.

(27) Since the bulk density of the filling material F is so low that a good gas permeability of the filling material packing present in the filling space 10 is guaranteed even following compaction, a good intermixture of the gases issuing from the casting mould 2 with the gas flows S1, S2 supplying oxygen for their combustion is guaranteed. At the same time the filling material packing in the filling space 10 supports the casting mould 2 on its peripheral surfaces and in this way prevents the cast iron melt from breaking through.

(28) The flow of the gases issuing from the casting mould 2 through the filling material F causes a good intermixture with the infed gas flow S1, S2, a longer process time and a good reactivity. The casting mould 2 is heated up both through the combustion of the binder system and the heat input through the metal poured into the casting mould 2, as well as through the pre-heated filling material F. As a consequence, the binder system holding together the mould parts and cores of the casting mould 2 is virtually completely destroyed. The mould parts and cores then disintegrate into fragments B or individual grains of sand.

(29) The fragments B and the loose sand fall through the sieve base 1 into the collecting pan 5 and are collected there. Depending on the progress of the destruction of the casting mould 2, the sieve base 1 can thereby be opened so that filling material F also falls into the collecting pan 5 (FIG. 8).

(30) In order to achieve optimal combustion of the gases issuing from the casting mould 2 and for the regeneration of the core sand already in the enclosure, the temperatures of filling material F and the gases flowing into the filling space 10 are, optimally, in each case well above 700 C. For this purpose, the conditions within the thermoreactor T are such that the regeneration process and the exhaust gas treatment proceed independently of plant availability. Determining and set values are the start temperature of the filling material F, the oxygen-containing gas flows S1, S2 flowing in via the gas inlet 15 and the intake 16 and the casting mould 2 itself.

(31) The progress of the destruction of the casting mould 2 and the progress of solidification of the cast iron melt poured into the casting mould 2 are matched to one another such that the cast part G is sufficiently solidified when the disintegration of the casting mould 2 begins.

(32) Once the casting mould 2 has substantially disintegrated completely, the collecting pan 5 with the mould material-filling material mixture contained therein is separated from the sieve base 1 and the enclosure 7 is also removed from the sieve base 1. The largely de-sanded cast part G is now freely accessible and can be cooled down in a controlled manner in a tunnel-like space 17 provided for this purpose (FIG. 10). On being removed, the cast part G is at a high temperature at which the austenite transformation has not yet been completed and a rapid cooling would lead to internal stresses and thus to cracks. For this reason, the cast part G is cooled down slowly in a cooling tunnel 17 according to the annealing curves for stress-free annealing. The supply of cooling air is so dimensioned that the cooling profile is achieved on a product-specific basis.

(33) The still-hot mixture of filling material F, core sand and fragments B contained in the collecting pan 5 is intensively mixed in a crushing mill 18, which can for example be a rotary mill, and mixed with sufficient oxidation air so that any binder residues which may still be present subsequently combust. In this process stage, the filling material F can also be separated from the core sand and both passed to a separate cooling stage. Such a regeneration reliably guarantees complete combustion of the binder system and in addition, through mechanical friction, prepares the core sand surface for a good adhesion of the binder for re-use as core sand.

(34) The obtained core sand is cooled virtually to room temperature and, following separation of the fractions, once again processed into casting mould parts or casting cores for a new casting mould 2.

(35) The filling material F is in contrast cooled to the intended starting temperature Tmin and, as part of the cycle, filled into the storage hopper V for renewed filling of the filling space 10.

(36) The quantity of the combustion air introduced into the filling space 10 as gas flows S1, S2 is regulated by means of mechanically adjustable flaps or slide valves with which the opening cross sections of the gas inlet 15 and of the intake 16 can be adjusted. The relevant adjustment can first be determined through the quantity of air stoichiometrically necessary for combustion of the binder system and then finely adjusted by means of measurements of CO, NOx and O2 at the exhaust gas outlet 19, formed in this case by the opening 14 of the cover 13 which is moulded into the cover 13 and via which the exhaust gases produced in the filling space 10 are extracted from the enclosure 7.

(37) As can be seen from FIG. 16, a high toxic substance concentration represented by the curve Ktox is reached in the filling space 10 immediately following pouring, through the evaporation of the solvent from the binder system of the casting mould 2 and the other vaporous emissions from the casting mould 2, which would combust autonomously even at room temperatures. The boundary Kbound, from which a toxic substance concentration is reached which is combustible at room temperature, is indicated in FIG. 16 through the dotted line. However, due to the high minimum temperature Tmin of 500 C., which prevails in the filling space 10 due to the hot filling material F which has been introduced there, the combustion of the gases entering the filling space 10 from the casting mould 2 already begins at a significantly lower concentration (see FIG. 16).

(38) As a result of the combustion within the granulate in phase 1, the granulate heats up and after a short time its temperature Tfill exceeds the boundary temperature Tbound of 700 C., at which, given a sufficient oxygen content, organic substances are known to oxidise and thus combust autonomously. The curve of the temperature Tfill is shown in FIG. 16 as a broken line.

(39) This phase (phase 1) of intensive combustion of the binder evaporating from the casting mould 2 continues until the concentration Ktox of the combustible gases escaping into the filling space 10 from the casting mould 2, substantially formed by the evaporating binder, reduces to such an extent that no further combustion would take place at room temperature.

(40) However, as already described, due to the high filling material temperature of more than 700 C., this oxidation or combustion is continued in the following phase 2, wherein which the heat thereby released is sufficient to further increase the temperature of the filling material 10 until the maximum temperature Tmax is reached. The filling material 10 remains at this temperature until the decomposition process of the casting mould 2 is so advanced that no further significant vapour emissions take place, the casting mould 2 disintegrates into small parts and the mould material remnants fall into the pan 5. However, as long as combustion processes take place in the filling space 10, so much heat is still thereby generated that the filling material F remains over a sufficiently long period within a range the upper limit of which is the temperature Tmax and the lower limit the temperature Tbound.

(41) Thus, according to the invention, through the selection of the temperature at which the filling material F is filled into the filling space 10, the time at which the boundary temperature Tbound of 700 C. is exceeded is so defined that this is achieved before, as a result of low toxic substance concentrations Ktox, the process of combustion in the filling space 10 no longer reliably takes place with the necessary intensity. The still highly heated filling material F then ensures that the decomposition and residual combustion of the gases still issuing from the casting mould 2 takes place, even if the concentration of combustible gases present in the filling space, considered in themselves, would be too low for this at temperatures below the temperature Tbound.

(42) It has been proved that, with the evaporating and combustible substances contained in the casting mould 2, so much chemical energy is available for a combustion that filling material temperatures of well above 1,000 C. could be achieved. However, in this case the cooling of the cast piece would be drawn out over a long time, so that long process times would be necessary. This too can be determined through the start temperature with which the filling material F is filled into the filling space 10. Too sharp a rise in temperature can also be prevented through an increase in the gas flows S1, S2, in this case acting as cooling air.

(43) In choosing the filling material F, which is for example ceramic filling material, it is ensured that the individual grains of the filling material F possess a high compressive strength in order to absorb the compressive forces occurring during casting and to minimise friction losses as far as possible during circulation. A further selection criterion is a low heat capacity in combination with the bulk density of the filling material F, in order, from phase 1, to achieve a temperature rise above the 700 C. as quickly as possible. A formation of nitrogen oxide is largely prevented through the oxidation in the bulk material with an adjusted supply of combustion air and relatively low temperature.

(44) Since according to the invention the output exhaust gases substantially heat up the filling material packing even in the first phase, a temperature profile results within the packing which guarantees clean combustion. Due to the thermal convection flow created in the filling space 10, the combustion air flows upwards in a vertical direction and, due to the pronounced vapour formation in the first phase, the emission of the gaseous toxic substances from the casting mould 2 into the filling material packing takes place in a horizontal direction. The intersection of the gas flows within the filling material F guarantees a good intermixture.

(45) In the region above the casting mould 2, the gas flows then follow the same direction and can post-combust sufficiently in the hottest region of the exhaust gas conduit in the combustion space between the cover 13 and filling material F before exiting from the exhaust gas outlet 19 above the pouring funnel.

(46) In an example calculation, the thermal energy Qa released through the cooling of the melt and the combustion of the binder as well as the thermal energy Qb required for the heating of the filling material as well as the heating of the core sand of the casting mould are determined on the basis of the parameters and material values stated in Table 1 for a process according to the invention.

(47) It has thereby been assumed that, as melt, a grey cast iron melt is poured into a casting mould the mould parts and cores of which are manufactured, using the conventional cold box method, of mould material which consists of conventional core sand, i.e. quartz sand, and a binder which is also commercially available for this purpose.

(48) Moreover, for the purpose of simplification it has been assumed that the cast metal gives off its heat to the casting mould and the filling material after casting and that the chemical energy latent in the binder used is completely available for heating of the filling material in the form of combustion heat.

(49) The fusion heat Hfus which needs to be conducted away in order to solidify the melt is then calculated according to the formula
Hfus=m.sub.melthfus1/1000 MJ/kJ

(50) thus, in the present example
Hfus=170kg96kJ/kg1/1000 MJ/kJ=16.3MJ.

(51) The thermal energy Qa1 released from the melt as it cools is then calculated according to the formula
Qa1=cpTm1/1000 MJ/kJHfus

(52) where, in the present example,
T=(T1T2)=(850K1500K)=650K as Qa1 =950 J/kgK650K170kg1/1000 MJ/kJ 16.3MJ
Qa1 =121 MJ.

(53) In a corresponding calculation, the thermal energy Qa2 released through the combustion of the binder contained in the mould material is calculated, according to the formula
Qa2=him.sub.Binder(1)

(54) as
Qa2=30 MJ/kg4 kg(1)=120 MJ.

(55) The total of the released thermal energy Qa=Qa1+Qa2 then amounts to 241 MJ.

(56) The thermal energy Qb1 required for the heating of the core sand of the casting mould from the temperature T1 to the temperature T2 is calculated according to the formula
Qb1=cp.sub.core sand(T2T1)encore sand

(57) as
Qb1=835 J/kgK(800K20K)255 kg=166 [MJ].

(58) Again, the thermal energy Qb2 for the heating of the core sand of the casting mould from the temperature T1 to the temperature T2 is calculated according to the formula
Qb2=cp.sub.filling material(T2T1)m.sub.filling material
as
Qb2=754 J/kgK(800K500K)125 kg=28 [MJ].

(59) The heat Qb=Qb1+Qb2 required in order to heat the core sand of the casting mould, initially still at the room temperature of 20 C., and the filling material filled with the temperature T1 of 500 C. to the final temperature T2 of 800 C. then amounts in total to Qb=166 MJ+28 MJ=194 MJ.

(60) Accordingly, with the parameters stated in Table 1, as a result of the heat input through the melt and the combustion of the binder emitted from the casting mould, an energy surplus of 47 MJ is available for heating of the filling material F and for the compensation of tolerances and losses.

(61) The determination of an energy balance achievable on pouring a grey cast iron melt reproduced in Table 1 shows that, using a conventional mould material produced on the basis of a conventional binder system and using quartz sand, a clear surplus capacity of thermal energy is present. The infed oxygen-containing gas flows S1, S2 are disregarded in this consideration, since their influence in energy terms is very slight.

(62) In Table 2, the bulk densities Sd, the specific heat capacities cp and the product P=Sdcp are stated for different bulk materials which in terms of their temperature-resistance would be fundamentally suitable for use as filling material. It can be seen that, for example, steel shot, while having a significantly lower specific heat capacity cp than a ceramic granulate of the kind referred to here, has much too high a bulk density to guarantee the gas permeability of the filling material packing provided in the filling space around the casting mould which is required according to the invention.

REFERENCE NUMBERS

(63) 1 sieve plate 2 casting mould 3 mould cavity 4 peripheral edge shoulder 5 collecting pan 6 sealing element 7 enclosure (housing) 8 peripheral surfaces of the casting mould 2 9 inner surface of the enclosure 7 10 filling space 11 opening of the enclosure 12 distribution system 13 cover 14 opening of the cover 13 15 gas inlet 16 intake 17 cooling tunnel 18 crushing mill 19 exhaust gas outlet B fragments F filling material G cast part S1, S2 oxygen-containing gas flows T thermoreactor U environment V storage hopper

(64) TABLE-US-00001 TABLE 1 Cast Filling metal material Core sand Grey Binder Material value/ Ceramic Quartz cast Cold box parameter granulate sand iron binder Unit Melting hfus 96 kJ/ enthalpy kg Heat capacity cp 754 835 950 J/kg/ at 800 C. K Calorific hi 30 MJ/ value kg Mass m 125 255 170 4 kg Input T1 500 20 1500 C. temperature Output T2 800 800 850 C. temperature

(65) TABLE-US-00002 TABLE 2 Specific heat Bulk density capacity Sd cp Filling material [kg/dm.sup.3] [J/kgK] P = Sd cp Ceramic material 0.61 754 460 Steel shot 4.20 470 1,974 Quartz sand 1.40 835 1,169