Patent Application
20060086480
The invention relates to the field of the production of casting moulds. The invention relates, in particular, to the production of shell moulds by applying one or more layers to a pre-fabricated pattern.
Ceramic shell moulds are often used in casting methods such as, for example, investment casting. These shell moulds are produced by applying one or more layers to a pattern of the subsequent cast part. The individual layers applied to the pattern contain a slip and a material, generally a granular material such as sand, applied to the slip. In the first layer applied to the pattern, the addition of the granular material may also be dispensed with. Once one layer has been applied, it is dried before the following layer may be applied or—after the final layer has dried—the pattern may be removed. As a result of the successive application of the individual layers, a shell surrounding the pattern is gradually produced.
Once the final layer has been applied, the pattern is removed from the shell and the shell is then burnt. The pattern may be removed from the shell in various ways. If, for example, it is a pattern made of wax, the wax is removed by means of melting-off. If, on the other hand, the pattern is made of a thermoplastic material, the plastics material has to be burnt out from the shell.
The drying of the individual layers applied to the pattern conventionally takes place at ambient temperature, wherein care is taken that the water contained in a newly applied layer is removed promptly but not instantaneously. Drying usually takes place at approximately 21 to 23° C. and at a relative air moisture content of more than approximately 40%. In order to shorten the drying process, it is recommended to expose the respective layer to be dried to an air flow. The air flow assists the removal of the moisture as it evaporates.
One drawback of the conventional drying method is the comparatively long drying period, usually of three to more than ten hours per layer. This is caused by the low diffusion gradient within the layer applied last. Even if the drying period is significantly extended, the residual moisture in the applied layers may not be reduced as required. Especially in the lower zones of the layer applied last, the remaining moisture tends more to diffuse back into the adjacent layer, which has presumably dried, than to evaporate.
For the aforementioned reasons, a certain residual moisture is always contained in the shell, even after the final drying process. This residual moisture hinders and restricts the desired irreversible bonding of the colloids contained in the slip. Moreover, in the case of reversible colloid bonds, moisture (for example, from the atmosphere) which acts after the end of the drying process can impair the composite construction of the shell by detaching the reversible bonds.
If there is insufficient irreversible colloid bonding, there is the risk, during melt-off or burning-out of the pattern, that the material of the pattern, as a result of its heat-induced expansion, will break the shell. The less complete the irreversible colloid bonding is, the greater is this risk. Although this risk may be reduced in that the shell mould is subjected to a thermal shock (for example, in a high-pressure steam autoclave), the water vapour thereby used causes the shell mould to be penetrated again by moisture, with correspondingly negative effects on the strength of the mould.
In order to assist the drying process, GB 2 350 810 A proposes to admix water-insoluble organic fibres to the slip. The admixing of organic fibres has a positive effect on the drying period and also allows the residual moisture to be reduced. These positive influences are caused by the capillary effect of the admixed fibres, which assists the removal and the evaporation of the moisture. The fibre composite structure also produces a more uniform layer construction and allows the layer thickness to be increased.
Despite the positive effects of the admixing of organic fibres, the drying period of individual layers is often still too long. In the case of multilayered shells, in particular, it is therefore almost impossible to produce a shell mould ready for casting in a single day. This may be acceptable in the case of industrial applications, in which shell moulds are produced continuously; however, in a large number of other applications, such as the production of prototypes, it would appear desirable to reduce the production period for an individual shell mould.
The object of the invention is to specify a method and a system for the more rapid production of a shell mould.
The invention provides a method for producing a shell mould (in particular for investment casting) that involves the steps of providing a pattern, forming a shell surrounding the pattern, by applying at least one aqueous layer to the pattern and by carrying out, layer by layer, at least one drying process, and removing the pattern from the shell, the drying process being carried out above a temperature of 25° C. and assisted by infrared light radiation.
The layer applied to the pattern may be a layer containing an incombustible slip. The layer may also contain an incombustible granular material. However, according to a preferred variant of the invention, at least the first layer, which is applied directly to the pattern, does not contain any granular material. The slip may contain an incombustible, liquid binder such as, for example, an aqueous silica sol. The slip may also comprise an incombustible powder.
According to a first variant of the invention, in the case of a multilayered shell construction, each individual layer is subjected to a drying process according to the invention. According to a second variant of the invention, individual layers are either not dried (or in any case not dried completely) or else dried at a temperature of 25° C. or less and/or without infrared light radiation.
The process for drying an individual layer may take place at a substantially constant temperature or at a varying temperature. The drying process may be carried out at a temperature above 28° C. or above 30° C., and expediently in a temperature range of up to approximately 45° C. A temperature range from approximately 36° C. to approximately 42° C. is preferred.
If a plurality of layers are applied to the pattern, the (maximum) drying temperature may vary from layer to layer. The maximum drying temperature may thus substantially increase from layer to layer. As a result of the cooling associated with the evaporation of the moisture, it is possible to select the maximum drying temperature (ambient temperature) during the drying process such that it is above a temperature at which the pattern might lose its dimensional stability. The maximum drying temperature may therefore be at least approximately 5° C. (preferably at least approximately 8° C. or 10° C.) above the temperature at which a decrease in the stability of the pattern might start.
During the drying process, a relative rotation may take place between the coated pattern and at least one infrared light source. This relative rotation takes place, for example, at a speed between 0.5 and 8 rpm, preferably between 1.5 and 4 rpm.
The drying process may also be assisted by means of a stream of a gaseous medium such as air. The streaming rate of the gaseous medium is, for example, approximately 0.5 to approximately 8 m/s, and preferably between approximately 1 and approximately 5 m/s. The drying process may also be assisted in that the ambient moisture content is less than 35% or less than 30%. According to a preferred variant of the invention, the ambient air moisture content is less than approximately 20% or less than approximately 10%.
The method according to the invention allows the drying period to be shortened. The process for drying a single layer may thus take less than one hour, preferably approximately 25 to 45 min. If three or more layers are applied to the pattern, the drying period for at least some of the layers applied after the first layer may be varied. The drying period of the second and/or the third layer and/or the fourth layer may therefore be selected so as to be longer than the drying period of the other layers and, in particular, of the following layers.
The drying period may be adjusted as a function of a desired degree of dryness. According to a first variant, a plurality of layers are applied to the pattern and the individual drying process is in each case carried out until complete drying of the layer applied last has been achieved. Complete drying may be assumed if, for example, the residual moisture content of a layer is less than approximately 60% and preferably between approximately 55 and approximately 40%. According to a second variant, some layers, a plurality of layers or all of the layers are dried only partially.
The pattern used for producing the shell may be made of various materials (for example, of wax or of a thermoplastic material such as ABS). In the case of a wax pattern, the melting-off from the dried shell may take place at a temperature of more than approximately 140°, preferably at approximately 150°.
The method according to the invention is suitable for a large number of highly diverse applications. Owing to the short drying period, the method is thus particularly suitable, for example, for the production of prototypes by means of investment casting (i.e. for the production of some or a few cast parts). However, the method is also suitable for industrial batch processes (using, for example, a conveying device configured as a chain conveyor).
In addition to the above-mentioned method, the invention also includes a system for producing a shell mould. The system contains a slipping device for applying a slip layer to a pattern and a drying device for drying the slip layer applied to the pattern, the drying device comprising a drying chamber and at least one infrared light source arranged in the drying chamber, wherein a temperature of more than 25° C. may be adjusted in the drying chamber. A suitable adjustment or control means, which ensures (for example, in a program-controlled manner) that the desired drying temperature or the desired drying temperature characteristic and the subsequent drying temperature are adhered to, may be provided for adjusting the drying temperature.
The heat energy required for achieving the drying temperature may be supplied by the infrared light source. In this case, the infrared light source may act as a means for heating the drying gas (for example, air). For adjusting the desired drying temperature, the energy consumption of the infrared light source may be monitored in a suitable manner. Additionally or alternatively, it is conceivable to provide a separate cooling means. The cooling means may, for example, be configured such that it allows the supply of a cooling gas into the drying chamber. It would also be conceivable to provide an additional heating means, separately from the infrared light source.
The system may comprise a means for rotating the coated pattern with respect to the at least one infrared light source. Such relative rotation between the coated pattern and the infrared light source ensures more uniform surface heating and therefore improves the layer quality. A sanding device for sanding the slip layer applied to the pattern may also be provided. The sanding device is configured for applying granular material (not necessarily sand) to the slip layer in a manner known per se.
For automating the production of shell moulds, a conveying device, which moves the pattern (back and forth in the case of a multilayered construction) between the slipping device and the drying device, may be provided. The conveying device may also ensure that the pattern is conveyed to or from the sanding device. The conveyance direction is expediently selected such that the slipping device is located before the sanding device, and the sanding device before the drying device.
Further advantages and details of the invention will become apparent from the following description of preferred embodiments and from the drawings, in which:
A system 10 according to the invention for producing shell moulds will be described below, initially with reference to
For the mode of operation of the system 10, it is immaterial whether the pattern 20 has not yet been coated or has already been provided with one or more layers. For the purposes of illustration, the pattern 20 is shown in
In the case of the embodiment illustrated in
A pattern 20 received by the conveying device 18 may be supplied to the slip vat 14, the sand drum 12 or the drying device 16, as desired. The conveying device 18 itself comprises a receiver head 22, which is movable along an x-axis and a y-axis, for receiving the pattern 20. The receiver head 22 is rotatable about two axes extending perpendicularly to each other, as indicated in
In conventional use of the system 10, the pattern 20 is initially immersed into the slip vat 14, and the slipped pattern 20 (in particular if it is the first slip layer) then either immediately dried in the drying device 16 or else firstly sanded in the sand drum 12, and only then transferred into the drying device 16.
The drying device 16 also comprises an air-conditioning device 38 for cooling intake air. The air-conditioning device 38 issues warm discharged air and supplies cooling air to an air drier 40. This circumstance is illustrated by two arrows. The air drier 40, which is based on an absorption drying principle, introduces dry supply air into the drying chamber 30 and issues moist discharged air to the environment. This circumstance is also illustrated by two arrows. As may be seen in
The air-conditioning device 38 is activated in such a way that the desired drying temperature is set in the drying chamber 30. The air-conditioning device 38 therefore counteracts the heating, which originates from the infrared light sources 34, of the drying air. A separate heating means may, if necessary, be provided in addition to the infrared light sources 34 (for example, the air-conditioning device 38 could also be configured to supply warm air to the air drier 40). Unlike in
It should be noted at this point that the arrangements, illustrated in
Various specimens of ceramic moulds were produced and tested, using wax patterns, by means of the system described with reference to
The wax pattern used had a cubic shape, in which a blind hole having a diameter of 20 mm and a depth of 20 mm was formed. The temperature and moisture surface values specified in the following tables were measured during the drying process inside this blind hole.
A first pattern tree was provided with a total of six layers (or—in the dried state—coats), wherein the first layer was not subjected to sanding. Each individual layer was dried completely in a separate drying process. The individual drying processes were carried out at a streaming rate of approximately 1.5 m/s, with constant infrared light radiation. The maximum drying temperature gradually increased from layer to layer. A drying process was considered to have been completed once the residual moisture content measured on the surface was less than approximately 55%. During the drying process, the specimen was rotated with respect to the infrared light sources at a rotational speed of approximately 2.5 rpm. The moisture content of the air in the drying chamber was gradually reduced. Care was taken that the air moisture content was, as far as possible, always less than approximately 20% and the temperature always above approximately 30° C.
The total process time and the individual drying parameters and surface conditions for each layer in one of the initial tests (in which the moisture content in the air in the drying chamber was still comparatively high) may be inferred from the following table. The drying parameters and surface conditions were measured two to five times for each drying process.
As may be seen from Table 1, the total processing time of all six layers was 3 hours and 35 minutes. The drying time alone was approximately 3 hours and 15 minutes. The first layer (without sand) was dried for 10 minutes, the second layer had reached a surface residual moisture content of approximately less than 55% after approximately
20 minutes. The corresponding drying period for the third layer was approximately 30 minutes, for the fourth layer approximately 58 minutes, for the fifth layer approximately 45 minutes, and for the sixth layer approximately 30 minutes.
The thickness structure of the first ceramic mould specimen may be inferred from the following table:
According to this table, in the slip/sanding composition used, there was an average layer structure of 0.8 mm per coat.
The following table shows the drying parameters and surface conditions for a further ceramic mould specimen having seven coats. The streaming rate in the drying chamber was approximately 1.5 to 2.0 m/s.
The following two tables show corresponding measurements taken on two identical ceramic mould specimens, each comprising eight layers and with drying at a streaming rate between 2 and 4 m/s. The viscosity of the slip used in these specimens was approximately 38 s.
Immediately melted off
In the aforementioned ceramic mould specimens, the melting-off of the wax pattern took place immediately after the application and drying of the final layer. Melting-off took place in a hot cabinet preheated to 150°. The wax had completely melted off after, in each case 15 to 20 minutes. A visual inspection revealed that the specimens produced in the drying chamber could be melted off without any damage or cracks.
A ceramic mould specimen produced at the same time under conventional drying conditions (see the following table) was completely destroyed, under the selected melting-off conditions, by cracks. The comparative specimen was produced in the same manner as the above ceramic moulds, by means of repeated slipping, sanding and drying. However, drying took place under conventional drying conditions (no drying chamber was used) and without infrared light radiation, but with accelerated ambient air (1.5 m/s).
Real time in this case: 17:15; melting-off 24 hours later
As may be seen from the above table, the drying periods are significantly longer in the case of the comparative specimen than in the case of the specimens produced using the method according to the invention.
The strength of the specimens according to the invention is also significantly greater than the strength of the conventional specimens. Test strips having the dimensions 50 mm 20 mm×5 mm were produced in order to determine the bending strength of the ceramic. A silicone mould comprising a plurality of bowl-like indentations was used for producing the test strips. For applying a plurality of coats, the silicone mould was repeatedly slipped, sanded and dried. Six to eight coatings were typically provided in order to achieve a strip height of approximately 5 mm.
The test strips according to the invention were subjected (in some cases simultaneously with pattern trees) to a drying process in the drying chamber at a temperature of approximately 40° C., an air moisture content of approximately 5 to 10% and a drying period of approximately 30 min. During the drying process, the strips were irradiated with infrared light. The conventional test strips, on the other hand, were dried at ambient temperature and an air moisture content of approximately 50%. Each layer was dried until the surface moisture content was less than 60% (this typically took several hours to one day). All of the test strips were then subjected to a bending test. The 7/18 strength-testing device from Feinmechanik Ralf Kögel was used for this purpose.
The basis for the advantages according to the invention is believed to be that the ion exchange at the surface of the binder colloids is intensified at relatively high drying temperatures, and this allows strong irreversible bonding of these colloids to one another. The intensive, surface-based drying caused by the infrared light radiation also results in a higher diffusion gradient within the applied slip layer, and thus in accelerated drying. The effect of the latent heat allows the drying temperature to be increased, beyond the temperature at which the pattern used would lose its stability. This also allows accelerated drying.
Each coat layer preferably experiences complete drying, in order to bring about irreversible colloid bonding. The desired final strength of the overall shell is therefore achieved immediately after the end of the drying of the final layer. In other words, it is no longer essential to continue waiting, once the layer applied last has dried, before the melting-off/burning-out of the model and the burning of the ceramic mould may commence. Nevertheless, this recognition does not rule out carrying out a concluding, longer final drying process in specific cases.
For achieving particularly short drying times, it was expedient to reduce the air moisture content in the drying chamber. In a series of consecutive tests, the air moisture content was reduced to less than 10%, often to 2% to 8%.
The tests revealed that the first sanded layer (i.e. generally the second layer applied to the pattern) dries relatively quickly (approximately 20 min), whereas the first or the second following layer requires a longer-than-average time (up to 60 min) in order to dry completely. The drying times for the subsequent layers were typically 30 to 35 minutes. At the start of the drying process, the residual moisture content in the immersed layer often rises briefly to above 80%, then remains for a long time at 65 to approximately 70%, in order then, approximately 2 to 10 minutes (typically approximately 5 min) before the determinable end of the drying period, to drop almost spontaneously to less than 50%.
When using the method according to the invention for the production of prototypes, it is advisable to have only one slip of uniform viscosity (uniform slip) for all of the coat layers and only one grain size of the scattering material. The uniform slip results in improved wetting, while at the same time reducing the runout time to 38 s (measured using the WEX beaker). An adequate surface quality of the cast parts may be achieved by inserting an immersion layer, without sanding, at the start of the shell construction process, while keeping the initial drying process short (typically less than 15 min).
The invention has been described, by way of example, with reference to various embodiments. A person skilled in the art may make modifications and additions on the basis of his specialised knowledge. The location, the positioning and the number of the infrared light sources and the arrangement and the number of the fans may therefore, in particular, be altered.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 048 451.1 | Oct 2004 | DE | national |
