CA2189400A1 - Process and device for coating foundary molds and/or foundary cores with powder - Google Patents

Abstract

For coating foundry molds and/or foundry cores (2) with a powder to form a barrier against the casting material, the powder is mixed with a granular mineral base material or sand, and then the foundry mold and/or foundry core (2) is washed with the powder from this mixture (3) for coating. The mixture (3) is loosened and fluidized for this purpose, so that the powder components adhering to the grains of the base material or sand are transferred to the mold or the core (2) and are rubbed into its surface.

Description

PATENT
Attorney Docket No. 9003-79 (A 96 512 M US) TITLE
Process And Device For Coating Foundry Molds And/Or Foundry Cores With Powder BACKGROUND OF THE INVENTION

The invention concerns a process for coating foundry molds and/or foundry cores made of a fireproof component, especially sand and preferably a liquid binding component, with a powder to create a barrier between the mold or the core and the casting material against gases penetrating into the hardening casting.
The invention furthermore concerns a device for implementing the process with a receptacle for the foundry mold(s) and/or foundry core(s) and a mixture of a granular base material and powder.
The foundry industry works with molds and cores which must meet a series of technological requirements in order to be able to withstand the casting material which consists of liquid metal or i-ron. The foundry forms and/or cores (referred to herein more generally as casting molds or casting cores) consist of a fireproof component, mostly sand with a suitable granularity, for example quartz, zirconium, chromite-sand, etc., and at least one liquid binding component which brings about the solidity of the shaped mold or core by chemical hardening. Modern binders for this purpose are manufactured on the basis of a synthetic resin. Their hardening takes place with the aid of a suitable, usually organic hardener. When the mold is filled with metallic melts, hydrocarbons of the solidified binder polymers on the metal/mold or core boundary are partially decomposed, whereby gases arise.
These gases should not be allowed to remain in the metallic casting, because its mechanical properties will be diminished by this. Further sources of gas in the mold or core are condensed alcohols, solvents and/or water.
For this reason, it is already known to counteract the penetration of such gasses into the hardening casting by applying coatings to the surfaces of the casting mold and/or core which will contact the metallic casting.
Furthermore, spreading a powder to form a protective layer on molds and cores is known from DE 27 24 705. Of course, there exists in this regard the problem of obtaining a good adhesion, for which reason a vacuum must simultaneously be generated in the mold or the core, so that the powder can be sucked into the mold or the core. This represents a considerable expenditure, whereby it is moreover difficult and hardly conceivable to place a mold or even a core under low pressure in such a manner that a sprinkled powder can be sucked evenly into the interior.
Charging particles triboelectrically (by friction), grounding the substrate, and bringing the charged particles into contact with the grounded substrate are known from EP 0 024 067 for coating substrates which in use come into contact with molten metals. Chargeable particles are thus needed for such a coating and a corresponding electrotechnical expenditure must be assumed.

SUMMARY OF THE INVENTION
There thus exists the object of creating a process and a device as well of the type mentioned in the beginning, with which a powder for creating a barrier '_ ~ 1 89400 between the mold or the core on the one hand and the casting material on the other can be applied to the mold or the core in a sufficiently durable and stable manner, without having to place these parts to be coated with powder under vacuum, and without having to triboelectrically charge the particles serving as a coating.
To accomplish this objective, the process according to the invention is characterized by the powder being mixed with a granular mineral base material or sand, and the powder from this mixture of dry, granular mineral base material and powder being flowed around the casting mold and/or core to coat it.
Thus, a powder is not simply sprinkled or spread on the parts to be coated, but a mixture of this powder with a granular bulk material, preferably sand, is produced in a surprising manner, and the parts to be coated are washed with it. This leads to the result that powder adhering to the grains of the granular base material mixture are intensively transferred to the casting form or core and are carried into its surface, so that the surface pores are largely closed, and the roughness of these parts is also considerably diminished.
By closing the surface pores, the escape of gas from the mold or the core is prevented during the casting process in the first hardening phase. The granular base material or sand grains belonging to the mixture do not remain stuck on the mold or the core, since they are dry, so that they serve to a certain extent as carriers for the powder, on the one hand, and as mechanical aids in coating and carrying in, on the other.
It is particularly appropriate for the process of washing the mold or the core if the mixture of base material and powder is loosened up into a mass which can ' 2 1 89400 flow in the vicinity of the casting mold(s) and/or the casting core(s). It is known E~E se that granular and/or pulverized masses or materials can be so loosened up that they acquire a flow behavior similar to a liquid.
It is especially beneficial if the base material-powder or sand-powder mixture is loosened up by vibrations or sound or a carrier medium, especially compressed air, or a combination of these. Cores or molds situated in the vicinity of the base material-powder mixture are then washed around by this loosening up of this mixture, so that the powder from the mixture can be at least partially transferred to the surface of the mold or the core and rubbed and carried into the surface owing to the granular components of the base mixture. The desired permanent coating with powder results in this way, without the molds or cores having to be exposed to a low pressure, which would have to act from the surface into the interior in order to carry the powder into the surface.
It is especially favorable that the powder which covers the grains of the base material or sand of the mixture is transferred by friction to the surface of the casting mold or casting core, and especially carried into the surface pores. This leads at the same time to a smoothing of the surface so that even the structure of the mold or core is improved, and the exactness of the subsequent cast piece can be elevated. This working in of powder components into the pores of the casting molds and/or cores yields the desired barriers against released gases.
A productive, space-saving, energy-saving, and technologically beneficial procedure for treating the surfaces of casting molds and cores results above all with the combination of individual or several of the procedural .

steps described previously, which process is in a position to replace the previously customary coating of layers or sprinkling with powder, without a loss in quality in the casting. In fact, the powder can be conveyed into the surface pores so that these are closed to the greatest extent, thus forming a good barrier against escaping gases at least during the casting and first hardening phase, whereby at the same time the surface roughness of the mold or core is reduced, and the quality of the casting can also be improved thereby. The washing of the mold or the core with the mixture of base material or sand and powder can at the same time also take place, for example in the manner of a fluidized bed without problems.
It is especially economically beneficial if the mixture consisting of base material or sand and powder is used several times (i.e., recycled), and powder is again admixed with this mixture from time to time. With the previously described process, the base material or sand partially deposits the powder adhering to it on the mold or core so that the powder proportion in the mixture thus gradually diminishes. The base material or sand remains unchanged, and consequently only powder needs to be admixed to the mixture from time to time in order to be able to coat further molds or cores with it.
The device for implementing this process already mentioned at the beginning is chiefly characterized by vibrators and/or sources of sound and/or a compressed air system being attached to the receptacle for the mold(s) and/or cores(s) in order to loosen up and fluidize the base material-powder mixture situated in the receptacle together with one or several molds and/or cores. It is thus sufficient to accommodate molds or cores together with the base material-powder mixture in a suitable receptacle, and then to loosen the mixture up with 2 1 8940~
. ~

vibrators, sound sources and/or compressed air situated on the receptacle such that this mixture in a sense flows around the parts to be coated, so that the powder components from this mixture can be worked into the surface of the forms or cores in the manner already described.
The receptacle can have a bottom on which the mixture directly lies and on which the casting mold and or the casting core can be laid. This provides an especially simple container, in which the washing of the molds or cores with the base material-powder mixture can take place.
A modified specific embodiment can consist of installing in the receptacle a gas-permeable plate, a net or the like above the bottom for laying the mold and/or core upon it, or mounting a supporting device for the molds or cores on the bottom of the receptacle. In this way, the mold and/or core can be positioned within the container somewhat higher than the bottom, so that an even better washing with powder, and therewith a better coating is made possible.
A compressed air dryer, and if necessary a pressure reducer, can be installed in the compressed air supply to the receptacle. In this way, it is assured that the compressed air serving to fluidize or to create a fluidized bed within the receptacle contains the requisite dryness, so that the base material-powder mixture is not impeded by moisture in being set in motion or loosened up.

BRIEF DESCRIPTION OF THE
SEVERAL VIEWS OF THE DRAWINGS
The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in 2~ 89400 conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments of a device for carrying out the process of the invention, which are presently preferred.
It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:
Fig. 1 is a longitudinal section of a device in the starting position for coating casting molds and/or casting cores with a powder having a receptacle for this purpose, and for a mixture of a granular base material and powder, wherein vibrators, sound sources and a compressed air system are connected to the receptacle for loosening up the mixture containing the powder;
Fig. 2 is a longitudinal section in accordance with Fig. 1 during processing of the casting molds or cores with the base material-powder mixture;
Fig. 3 is a modified specific embodiment in which a gas-permeable plate is mounted within the receptacle for laying the molds and/or cores upon it above a first plate of the receptacle, again in the starting position;
Fig. 4 is the receptacle in accordance with Fig.

3 during treatment of casting molds and/or cores with the powder-base material mixture.
Fig. 5 is a schematic and enlarged view of the surface of a casting mold or a core from the base material components of the core or mold sand with binding components therebetween prior to treatment with the base material-powder mixture; and Fig. 6 is a representation corresponding to Fig.
5 after coating with the powder.

DETAILED DESCRIPTION OF THE INVENTION

In the following description corresponding parts of the various devices are designated with identical reference numbers even if they deviate in their function or form.
The respective device represented in each of Figs. 1 to 4 has above all a receptacle 1 in which casting molds or cores 2 can be placed together with a base material-powder mixture 3. The base material can be sand, for example.
The receptacle 1 is provided with vibrators 4 and/or sound sources 5 and/or a compressed air system 6, with which the powder-base material mixture 3 can be brought into motion and loosened up, so that it flows around the molds and cores 2 lying on or above it first according to Fig. 1 and also in accordance with Fig. 3.
In the embodiment according to Figs. 1 and 2 a gas-permeable plate 7 is fastened over the bottom of the receptacle 1, which permits the introduction and uniform distribution of the compressed air supplied by the compressed air system 6 over the entire horizontal cross section of the receptacle 1. This plate 7 is mounted for this purpose between fastening elements 8. Its interstices or openings are smaller than the mean grain size of the mineral base material contained in the powder-base material mixture 3, so that the intervening space between the bottom and this plate'7 remains free of this mixture 3. In this connection, one sees in Fig. 1 the starting position in which the molds and cores 2 lie upon the powder-base material mixture, which for its part is piled up on the plate 7.
How the molds and cores 2 are coated with powder is depicted in Fig. 2. The mixture is loosened by turning 2 1 894~0 on the vibrators 4 or the sound sources 5. In addition, the compressed air system 6 can be turned on. The mixture 3 is thereby fluidized and loosened up practically as in a fluidized bed, so that the molds and the cores 2 are washed by it, such that the powder found in the mixture can be deposited on these parts 2.
The compressed air system 6 consists of a compressed air source 9, a compressed air dryer 10, and a pressure reducer 11, as well as conduits, valves and connections, as schematically indicated.
The individual particles and components of the powder-base material mixture 3 come into intensive motion by the loosening up and consequently flow around all contact surfaces of the casting molds or cores 2. The kinetic energy of the mixture 3 serves for creating friction and for introducing the powder into the pores of the casting molds and/or cores 2. The roughness of the casting molds and/or cores 2 is considerably reduced as a result of this treatment.
If one compares Figs. 5 and 6, one recognizes the surface of a casting mold or of a core 2 in greatly enlarged and schematic presentation before treatment in accordance with Fig. 5 and after treatment according to Fig. 6. One recognizes the base components 30 therein, which can be grains of sand, for example, and binder components 31 arranged in between.
In Fig. 6, these are now covered over and sealed on their surface with powder particles 32, whereby the powder particles 32 form a clear barrier against the outside, so that gases arising in the mold or core 2 during the casting process cannot exit for the time being, and thus cannot harm the cast workpiece.
The specific embodiment of the device according to Figs. 3 and 4 largely corresponds to that in accordance 21 894~)0 with Figs. 1 and 2. One simply notes in addition above the plate 7 a further plate 13, which is perforated and upon which the casting forms and/or cores 2 can be laid.
Consequently, these have a certain distance from the gas permeable plate 7 in the starting position and also during their processing.
The additional plate 13 rests upon damping elements 12, so that where washing around occurs by vibration, oscillations can be compensated by these - 10 damping elements 12 so that the casting forms and/or cores 2 which are situated on the plate 13 cannot be damaged by such vibrations.
The mixture 3 is also loosened up in this case after appropriate activation of vibrators and/or sound sources and/or the compressed air system, so that it flows around the molds and cores 2 and coats the surfaces with powder in the manner which has already been mentioned in connection with the first embodiment, so that a result corresponding to Fig. 6 can be obtained.
At the same time, the compressed air can be dried efficiently so that the powder in the mixture 3 is not made unusable by air moisture. The intensity of the washing with compressed air can be controlled by means of the pressure reducer 11. The duration of the washing depends upon the dimensions of the receptacle 1, on the geometry of the form or the core, on the granularity of the base material as well as of the powder, and can be varied within broad limits.
With both devices, one or several casting molds and/or casting cores 2 can therefore be coated with a powder, on the one hand in accordance with Figs. 1 and 2 and on the other hand in accordance with Fig. 3 and 4, whereby the powder is in the first place mixed with a granular mineral base material or sand so that the mixture -3 shown in the Figures is already made up. The casting mold and/or casting core 2 can subsequently be washed to coat them with the powder from this mixture 3 of dry, granular mineral base material and powder, so that the 5 larger base material grains pass on the powder adhering to them to the surface of the molds and cores 2 and to a --certain extent rub it on and into the surface. By this means, the surface is smoothed at the same time. Existing pores are closed so that the desired barrier arises, which prevents the escape of gas from the mold or the core 2, at least during the first phase of the casting process, and thereby prevents damage to the casting.
The powders which may be used in the mixtures for coating the casting molds and/or cores according to the present invention include, for example, zirconium silicate (ZrSiO4), corundum (A12O3), magnesite (MgO), mullite (3A12O3 2SiO2), olivine (Mg2 SiO4), quartz (SiO2), chromite (A12O3 Cr2O3 FeO), coke (90~ carbon), graphite (70-85~ carbon), kaolinite (Al2[(OH)4/Si2O5]), pyrophyllite (Al2[(OH)2/Si4O1o]), talcum (Mg3[(OH)2/AlSi~Ol0]), and mica (KAl2[(OH)2/AlSi3O1o]). The powder may be mixed with the base material, preferably silicate sand, at a ratio of about 1:10 parts by volume, for example. Typically, the powder will have a particle diameter in the range of about 0.02-0.06 mm, and the carrier sand (base material) will have a particle diameter in the range of about 0.16-0.3 mm. The choice of powder for a particular application will depend upon a number of factors, but above all must be compatible with the material to be cast, such as steel, steel alloys, iron, copper alloys, aluminum alloys, magnesium alloys, etc. -It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive ~ concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

Claims (14)

1. A process for coating casting molds and/or casting cores with a powder to form a barrier between the mold and/or core and a cast workpiece against gases penetrating a hardening casting, wherein the powder is mixed with a granular base material to form a dry, granular mixture and wherein the mold and/or core is washed with the powder from the mixture to form a coating.

2. The process according to claim 1, wherein the granular base material is selected from the group consisting of mineral and sand.

3. The process according to claim 1, wherein the mixture is loosened up into a mass capable of flowing in a vicinity of the casting mold and/or casting core.

4. The process according to claim 3, wherein the mixture is loosened up by at least one of vibration, sound and a carrier medium.

5. The process according to claim 4, wherein the carrier medium is compressed air.

6. The process according to claim 1, wherein the powder covers grains of the base material in the mixture, and the powder is transferred to the surface of the mold and/or core by friction.

7. The process according to claim 6, wherein the powder is introduced into pores of the mold and/or core surface by friction.

8. The process according to claim 1, wherein the mixture is recycled, and wherein from time to time powder is admixed again to this mixture.

9. An apparatus for carrying out the process according to claim 1 comprising a receptacle for holding a casting mold and/or casting core and a mixture of a granular base material and powder, and at least one of a vibrator, a sound source, and a compressed air system attached to the receptacle for loosening up and fluidizing the mixture of granular base material and powder.

10. The apparatus according to claim 9, wherein the receptacle has a bottom on which the mixture can directly lie and upon which the casting mold and/or the casting core can be laid.

11. The apparatus according to claim 10, wherein at least one of a gas-permeable plate and a net is arranged in the receptacle above the bottom for laying the form and/or the core thereon.

12. The apparatus according to claim 10, wherein a support device for the mold and/or the cores is mounted on the bottom of the receptacle.

13. The apparatus according to claim 9 wherein the compressed air system comprises a compressed air dryer installed in a compressed air supply to the receptacle.

14. The apparatus according to claim 13, wherein the compressed air system further comprises a pressure reducer installed in the compressed supply.