JP2006312193A - Precision casting process and precision-cast product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a precision casting process for inexpensively producing a material as cast having satisfactory surface properties, and to provide a precision-cast product. <P>SOLUTION: The precision casting process is provided for producing the precision-cast product 10 by pouring molten metal nto a cavity of a shell mold and performing casting. The precision casting process is characterized in that when the pouring is performed using the molten metal having high reactivity with the shell mold, the shell mold is arranged in a pressure vessel, thereafter, the inside of the pressure vessel is evacuated, subsequently, inert gas or gaseous nitrogen is introduced into the pressure vessel to control the pressure in the atmosphere to 1 Pa to the atmospheric pressure (about 0.1 MPa), and while the atmospheric pressure is held at the state, the molten metal is poured into the cavity. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a precision casting method and a precision cast product using a disappearing model such as lost wax.

  When producing a cast product having a complicated shape and requiring high dimensional accuracy, a precision casting method (investment casting method) using a disappearing model such as lost wax is used.

In the precision casting method, a molten metal (alloy molten metal) is usually melted and cast in a melting furnace in an air atmosphere or a vacuum atmosphere. Here, the casting of the molten metal is performed in a vacuum atmosphere in order to prevent the cast product from being oxidized by oxygen contained in the atmosphere and the generation of oxides during the melting of the molten metal raw material. At this time, even if the inside of the melting furnace is evacuated using a vacuum pump, it does not become a complete vacuum, but a gas of about 1 Pa to ^{10 −10} Pa remains. This gas is the atmosphere and has the same oxygen partial pressure as the atmosphere.

  Ni-base super alloys and Co-base super alloys, which are one of the alloys manufactured by precision casting, contain active elements such as Al, Ti, or Hf. , May react with the component of the mold (oxide). By the former reaction, a reaction product is generated on the surface layer of the casting. The reaction product produced by the latter reaction is incorporated into the surface layer of the casting and integrated. Further, when the reactant is taken into the surface layer of the casting, a part of the cavity surface layer fixed to the reactant may be peeled off and taken together with the reactant. Cast products in which reactants and release materials are incorporated into the surface layer have poor surface properties (rough surface roughness), and therefore surface finishing treatment must be performed by chemical milling or mechanical cutting. As a result, there is a problem that the number of manufacturing steps of the cast product increases, leading to an increase in product cost and a decrease in productivity.

  Therefore, in order to improve the surface properties of the cast product (as cast material) in an as-cast state manufactured by a precision casting method, there is a method of forming a mold using a disappearing model having substantially spherical filler particles (for example, , See Patent Document 1).

JP-A-10-166105

  By the way, the method described in Patent Document 1 described above requires a process for producing filler fine particles, and the filler fine particles are difficult to reuse, leading to an increase in product cost and a decrease in productivity. Cannot be solved sufficiently.

  In addition, in the Ni-base superalloy and the Co-base superalloy, a C component may be included in order to strengthen the grain boundary of the cast product. Since the metal melt of such a Ni-base superalloy or Co-base superalloy has a low viscosity due to the inclusion of the C component, the metal melt is likely to be inserted (bite into) the cavity surface of the mold. As a result, the surface properties of the cast product are deteriorated, the contact area between the molten metal and the mold is further increased, and there is a problem that the reaction product described above is more easily generated.

  Further, since the casting mold for precision casting is dewaxed using steam, it has a porous structure having fine voids so that the passage of steam is improved. The larger the gap between the fine gaps, the better the passage of steam. However, in this case, there is a problem that the molten metal is easily inserted into the fine gap, the surface property of the cast product is deteriorated, and the reaction product described above is more easily generated. On the other hand, when the gap between the fine gaps is reduced (the cavity surface is made dense), the surface properties of the cast product are improved. However, in this case, there is a problem that cracks (dewaxing cracks) occur in the mold at the time of dewaxing, and the life of the ceramic slurry that is a constituent material of the mold is shortened.

  An object of the present invention, which was created in view of the above circumstances, is to provide a precision casting method and a precision cast product that can manufacture an as-cast material having a good surface property at low cost.

In order to achieve the above object, the precision casting method according to the present invention is a method of casting a molten metal in a cavity of a shell mold to produce a precision cast product,
When casting using a molten metal that is highly reactive with the shell mold,
After placing the shell mold in the pressure vessel, the inside of the pressure vessel is evacuated, and then the atmosphere pressure is adjusted from 1 Pa to atmospheric pressure (about 0.1 MPa) by introducing an inert gas or nitrogen gas into the pressure vessel. Then, the molten metal is cast into the cavity while maintaining the atmospheric pressure in that state.

  Here, the molten metal having high reactivity with the shell mold is preferably a Ni-base superalloy or a Co-base superalloy containing an element having high activity against oxygen.

Further, the precision casting method according to the present invention is a method of casting a molten metal into a cavity of a shell mold to perform casting, and manufacturing a precision casting product,
When casting using the above shell mold that is porous and has good air permeability,
After placing the shell mold in the pressure vessel, the inside of the pressure vessel is evacuated, and then the atmosphere pressure is adjusted from 1 Pa to atmospheric pressure (about 0.1 MPa) by introducing an inert gas or nitrogen gas into the pressure vessel. Then, the molten metal is cast into the cavity while maintaining the atmospheric pressure in that state.

Furthermore, the precision casting method according to the present invention is a method of casting a molten metal into a cavity of a shell mold to perform casting, and manufacturing a precision casting product,
When casting using the above molten metal with low viscosity,
After placing the shell mold in the pressure vessel, the inside of the pressure vessel is evacuated, and then the atmosphere pressure is adjusted from 1 Pa to atmospheric pressure (about 0.1 MPa) by introducing an inert gas or nitrogen gas into the pressure vessel. Then, the molten metal is cast into the cavity while maintaining the atmospheric pressure in that state.

  Here, the molten metal having a low viscosity is preferably a Ni-base superalloy or a Co-base superalloy containing a C component for strengthening grain boundaries.

  It is preferable to produce a precision casting of a unidirectionally solidified alloy by unidirectionally solidifying the molten metal. Moreover, it is preferable to produce a precision casting of a single crystal alloy by growing a single crystal of the molten metal.

  On the other hand, the precision cast product according to the present invention is manufactured by the precision casting method described above, and the average surface roughness Ra of the as-cast material is 3.2 μm or less.

On the other hand, the precision casting apparatus according to the present invention is a precision casting apparatus for producing a precision casting product by casting a molten metal into a mold cavity.
A melting furnace for arranging a mold and melting a metal to form a molten metal;
Evacuation means for evacuating the melting furnace;
Gas supply means for supplying an inert gas or nitrogen gas into the melting furnace;
Control means for controlling the supply of inert gas or nitrogen gas;
It is equipped with.

  Here, the mold is preferably a shell mold.

  According to the present invention, an excellent effect that an as-cast material having a good surface property can be obtained is exhibited.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a preferred embodiment of the invention will be described with reference to the accompanying drawings.

(First embodiment)
The precision casting method according to a preferred embodiment of the present invention is an effective method when casting is performed using a molten metal having high reactivity with a shell mold.

  Specifically, first, a wax mold having the same shape and size as the target precision casting product is prepared in advance. After the initial layer slurry is applied around the wax shape, the initial layer stucco is applied and then dried to form the initial layer slurry film. Subsequently, after the rear layer slurry is applied around the first layer slurry film, the rear layer stucco is applied and then dried to form the rear layer slurry film. The formation process of the back layer slurry film is repeatedly performed as appropriate, whereby the film thickness of the back layer slurry film is controlled to a desired thickness. Examples of the method for attaching the slurry include an immersion method, a spraying method, and a coating method, and the immersion method is preferable. Then, after performing wax-type dewaxing using steam, a shell mold is obtained by performing a baking treatment.

The initial layer slurry is, for example, a mixture of 2 to 4 kg of filler to 1 kg of 20 to 40% colloidal silica binder. The filler is mainly composed of alumina (Al ₂ O ₃ ) and mullite (Al ₂ O ₃ · SiO ₂ ), and contains oxides such as silica, zirconia, zircon, ceria at a ratio of 20% by weight or less. Can be mentioned. Further, as the first layer stucco (refractory particles to be sprinkled and adhered to the surface of the first layer slurry), for example, alumina of about # 60 to 160 mesh can be used, but the particle size and material are not particularly limited. The constituent materials of the rear layer slurry and the rear layer stucco are not particularly limited, and any material can be used as long as it is conventionally used as the slurry and stucco for the shell mold.

Next, after the shell mold is placed in a pressure vessel (melting furnace), the inside of the pressure vessel is evacuated using an exhaust means (for example, a vacuum pump). Thereafter, an inert gas (or nitrogen gas) is supplied into the pressure vessel from a gas supply line in the gas supply means. At this time, the atmospheric pressure is increased to 1 Pa to atmospheric pressure (about 0.1 MPa (1 × 10 ⁵ Pa)), preferably 1 Pa to 1 × 10 ⁴ Pa, more preferably 1 × 10 ³ Pa to 1 × 10 ⁴ Pa. The supply amount of the inert gas is controlled using the control means so as to be adjusted.

Here, in the precision casting method according to the present embodiment, the greatest feature is that the atmospheric pressure is adjusted to 1 Pa to atmospheric pressure. This is because if the atmospheric pressure is less than 1 Pa, the effect of reducing the oxygen partial pressure due to the supply of the inert gas (or nitrogen gas) cannot be obtained, and the gas pressure described later in the second embodiment cannot be obtained. Further, when the atmospheric pressure exceeds atmospheric pressure, the hot water circumference (hot water flow) is deteriorated, and the precision castability is deteriorated. The atmospheric pressure can be freely adjusted by adjusting the amount of inert gas (or nitrogen gas) introduced. As the inert gas, either Ar gas or He gas may be used, but inexpensive Ar gas is preferable. The degree of vacuum during evacuation is, for example, 1 × 10 ⁻⁴ Pa to 100 Pa, preferably 1 × 10 ⁻¹ Pa.

  Next, a molten metal having high reactivity with the shell mold is poured into the cavity of the pre-heated shell mold while maintaining the atmospheric pressure of the inert gas (or nitrogen gas) at 1 Pa to atmospheric pressure. Casting is performed. Examples of the metal having high reactivity with the shell mold include a Ni-base superalloy or a Co-base superalloy containing an element (Al, Ti, Hf, etc.) having high activity against oxygen. For example, as a Ni-base superalloy containing a relatively large amount of Hf, Rene'142, CM247LC, PWA1426 (all registered trademarks) for unidirectionally solidified alloys, Rene'125, Inco713C, MarM247 (all registered) for ordinary casting alloys Trademark). Examples of the Co-base superalloy containing a relatively large amount of Al include AiResist 213 and AiResist 215 (both are registered trademarks). As a method for casting a molten metal, it is possible to apply pouring, centrifugal casting, suction casting (low pressure casting) and the like.

  Next, the molten metal is solidified by cooling the mold, and casting is completed. Thereby, a cast body is formed in the shell mold. Thereafter, the shell mold is immersed in a hot alkaline bath or the like to dissolve and remove the shell, that is, the precision casting mold, and the mold is released. As a result, an as-cast material (precision cast product) is obtained. As the mold release, a physical method (for example, blast cleaning) may be used in addition to a chemical method using a high-temperature alkaline bath. As the blast cleaning, any of sand blasting, shot blasting, and water jet (high-pressure water spraying) may be used. Further, shakeout may be used as a physical method other than blast cleaning.

  In the precision casting method according to the present embodiment, casting is performed not in a normal vacuum atmosphere but in an inert gas atmosphere (hereinafter referred to as an inert gas decompression atmosphere) of 1 Pa to atmospheric pressure. When this inert gas decompression atmosphere is compared with the vacuum atmosphere, the oxygen partial pressure is relatively lower in the inert gas decompression atmosphere.

  For this reason, at the time of casting of the precision casting method according to the present embodiment, the reaction between oxygen (residual air) and the molten metal is suppressed, and the reaction between the cavity surface layer and the molten metal is suppressed. As a result, the generation of the reaction product itself is suppressed, and there is almost no possibility that the reaction product is generated on the surface layer of the cast product or that the reaction product is taken into the surface layer of the cast product. Further, since the reaction between the cavity surface layer and the molten metal is suppressed, there is almost no possibility that a part of the cavity surface layer is peeled off by the reactant.

  In addition, the precision casting method according to the present embodiment is particularly effective for unidirectionally solidified alloys and single crystal alloys as well as ordinary cast alloys. This is because a unidirectionally solidified alloy or a single crystal alloy is apt to cause a reaction between the molten metal and the surface of the cavity surface because the molten metal of high temperature is brought into contact with the surface of the cavity surface for a long time. Therefore, the reaction between the cavity surface layer and the molten metal can be remarkably suppressed by producing a precision cast product of a unidirectionally solidified alloy or a single crystal alloy using the precision casting method according to the present embodiment.

  As described above, the precision cast product obtained by the precision casting method according to the present embodiment has almost no reactants or release material on the surface layer, and thus the surface properties of the as-cast material are good. For example, in the precision cast product according to the present embodiment, the average surface roughness Ra of the as-cast material is 3.2 μm or less, preferably Ra is 1.6 μm or less. Therefore, it is not necessary to subject the as-cast material to surface finishing treatment such as chemical milling or mechanical cutting (or the surface finishing is very small), and therefore the number of manufacturing steps of the cast product can be reduced. As a result, the product cost can be reduced and the productivity can be improved.

  In the precision casting method according to the present embodiment, the lower the gas pressure of the inert gas (or nitrogen gas) that is the atmosphere during casting, the smaller the amount of inert gas used and the shorter the time required for gas introduction. Therefore, a precision casting can be manufactured at a lower manufacturing cost and better productivity. On the other hand, the higher the gas pressure of the inert gas, the smaller the oxygen partial pressure, so that the reaction between the cavity surface layer and the molten metal can be further suppressed, and a precision cast product with better surface properties can be produced. Can do.

  In the precision casting method according to the present embodiment, after evacuating the pressure vessel, an inert gas (or nitrogen gas) is introduced into the pressure vessel, and the atmospheric pressure is increased to 1 Pa to atmospheric pressure and adjusted. However, the present invention is not particularly limited to this. For example, first, after introducing a sufficient amount of inert gas (or nitrogen gas) into the pressure vessel, the inside of the pressure vessel is evacuated to reduce and adjust the atmospheric pressure from 1 Pa to atmospheric pressure. .

  The precision casting method according to the present embodiment can be applied to a casting process for a rotating member or a blade member for a jet engine or a gas turbine engine, but is not particularly limited thereto. For example, the present invention can be applied to all casting processes for cast products that preferably have good surface properties.

  Next, another embodiment of the present invention will be described with reference to the accompanying drawings.

(Second Embodiment)
The precision casting method according to another preferred embodiment of the present invention is an effective method when casting is performed using a shell mold that is porous and has good air permeability.

  Specifically, a shell mold is produced in the same manner as the precision casting method according to the first embodiment described above. This shell mold has a porous structure having minute voids in the shell mold body in order to improve air permeability. For example, the void size is 10 μm or less, preferably 3 μm or less, and the porosity (ratio of voids in the shell mold body) is 30% or more, preferably 40 to 60%. The void size and void ratio can be freely controlled by adjusting the particle size of the filler constituting the slurry and the particle size of the stucco. As an example, when the filler particle size or stucco particle size is increased, the void size is increased and the void ratio is increased. Conversely, when the filler particle size or stucco particle size is decreased, the void size is decreased and the void ratio is decreased.

  Next, using this shell mold, casting is performed in an inert gas reduced pressure atmosphere in the same manner as the precision casting method according to the first embodiment.

  At this time, the gas pressure of the inert gas acts on the surface of the molten metal. The molten metal is pushed by this gas pressure, and the molten metal is inserted into the cavity surface of the shell mold. However, at the same time, an inert gas is introduced into the shell mold main body through the gap, and the gas pressure acts on the molten metal from the entire cavity surface. Therefore, the gas pressures acting on the molten metal are offset.

  As a result, insertion of the molten metal into the cavity surface of the shell mold is suppressed, and a so-called precision casting product with few insertion defects is obtained. Further, by using a shell mold having good air permeability, the shell mold is resistant to dewaxing cracking (dewaxing cracking is difficult to occur). Furthermore, since it is difficult for molten metal to be inserted, the cavity surface of the mold can be made somewhat rough (for example, the average surface roughness (Ra) is 3 to 20 μm), and the particle size of the filler can be set within a certain range. Can be enlarged. As a result, the gelation of the ceramic slurry is difficult to occur, the management of the slurry for preventing the gelation is unnecessary, and the life of the slurry can be extended.

  Also, in the precision casting method according to the present embodiment, the same effects as those of the precision casting method according to the first embodiment can be obtained, the surface properties are good, and surface finishing is unnecessary (or simple). A casting is obtained.

(Third embodiment)
The precision casting method according to another preferred embodiment of the present invention is an effective method when casting is performed using a molten metal having low viscosity.

  Specifically, a shell mold is produced in the same manner as the precision casting method according to the first embodiment described above. Using this shell mold, casting is performed under an inert gas reduced pressure atmosphere in the same manner as the precision casting method according to the first embodiment. At this time, as an example of the molten metal poured into the shell mold, there is a Ni-base superalloy or a Co-base superalloy. These alloy systems, except for a part for single crystal alloys, strengthen grain boundaries. C component for For example, Inconel 713C (registered trademark) and the like can be cited as a Ni-base superalloy for ordinary casting alloys containing a large amount of C component (for example, 0.05 wt% or more). Examples of Ni-base superalloys for unidirectionally solidified alloys containing a large amount of C component include GTD111 and PWA1426 (both are registered trademarks). Furthermore, YH61, Rene'N5, Rene'N6 (all are registered trademarks) etc. are mentioned as Ni base superalloy for single crystal alloys containing C component. Examples of the Co-base superalloy containing the C component include AiResist 213 and AiResist 215 (both are registered trademarks).

  Here, in the alloy system containing the C component, the viscosity of the molten metal is usually lowered and the wettability is improved. For this reason, the alloy-based metal melt containing the C component is easily inserted into the cavity surface of the shell mold when cast. However, in the precision casting method according to the present embodiment, casting is performed in an inert gas decompression atmosphere, and the shell mold has a porous structure. Therefore, the inert gas is introduced into the shell mold body through the voids of the shell mold, and the gas pressure acts on the molten metal from the entire cavity surface. As a result, even if the molten metal has a low viscosity, the insertion of the molten metal into the cavity surface is suppressed, and a so-called precision casting product with few insertion defects can be obtained.

  Also, in the precision casting method according to the present embodiment, the same effects as those of the precision casting methods according to the first and second embodiments can be obtained, the surface properties are good, and surface finishing is unnecessary (or simple). N) Precision castings can be obtained.

  As described above, the present invention is not limited to the above-described embodiment, and it goes without saying that various other things are assumed.

  Next, although this invention is demonstrated based on an Example, this invention is not limited to these Examples.

  About 1% of colloidal silica binder of 30% around the wax type, filler (mainly composed of alumina and mullite and containing oxides such as silica, zirconia, zircon, and ceria at a ratio of 20% by weight or less) The first layer slurry mixed at a rate of 3 kg was applied. Thereafter, an initial layer stucco made of about # 60 to 160 mesh alumina was applied and then dried to form an initial layer slurry film. Subsequently, after applying the rear layer slurry around the first layer slurry film, the rear layer stucco was applied and then dried to form the rear layer slurry film. Then, after wax-type dewaxing was performed using steam, a baking treatment was performed to produce a shell mold.

  This shell mold is set in a melting furnace, preheated to 1520 ° C, and then a molten metal (Rene'142 (registered trademark)) with high reactivity with the surface of the cavity surface of the shell mold is cast into the cavity. It is. The molten metal temperature was 1520 ° C., the same as the mold preheating temperature. Thereafter, the mold was cooled to solidify the molten metal, thereby producing a precision casting.

Here, when casting the molten metal, first, the melting furnace was evacuated to a vacuum degree of about 1 × 10 ⁻¹ Pa, and Ar gas was introduced into the melting furnace to adjust the atmospheric pressure to 0.005 MPa. Then, casting was performed under an inert gas decompression atmosphere to produce a precision cast product (as cast material) (Example). On the other hand, when casting molten metal, the inside of the melting furnace was evacuated to a vacuum of about 1 × 10 ^-2 Pa, and then cast in a vacuum atmosphere to produce a precision cast product (as cast material) ( Conventional example).

  Cross-sectional observation views of the precision castings of Examples and Conventional Examples are shown in FIGS. The left side of each of FIGS. 1 to 4 is a cross section of a precision casting, and the black part on the right is a background.

  As shown in FIGS. 3 and 4, the precision cast product 30 of the conventional example has a reaction product or release material layer (layer thickness of about 40 to 60 μm) 41 formed on almost the entire surface. Further, on the surface layer of the precision casting 30, a trace (depth of about 60 to 100 μm) of the molten metal inserted into the cavity surface of the shell mold at the time of casting, a so-called insertion defect 42 is observed, and the average surface roughness Ra is It was as large as about 20 μm.

  On the other hand, as shown in FIGS. 1 and 2, the precision cast product 10 of the example had almost no reactant or release material layer on its surface layer. Further, no insertion defect was observed on the surface layer of the precision casting product 10, and the average surface roughness Ra was as small as about 1.2 μm.

  From the above, it has been confirmed that an as-cast material having a good surface property can be obtained by casting in an inert gas decompression atmosphere using the precision casting method according to the present invention.

It is a cross-sectional observation figure of the precision casting goods of the Example in [Example]. It is an enlarged view of the principal part A in FIG. It is a cross-sectional observation figure of the precision casting product of the prior art example in [Example]. It is an enlarged view of the principal part B in FIG.

Explanation of symbols

10 Precision castings

Claims (12)

In a method of casting a molten metal into a cavity of a shell mold and casting to produce a precision casting product,
When casting using a molten metal that is highly reactive with the shell mold,
After placing the shell mold in the pressure vessel, the inside of the pressure vessel is evacuated, and then the atmosphere pressure is adjusted from 1 Pa to atmospheric pressure (about 0.1 MPa) by introducing an inert gas or nitrogen gas into the pressure vessel. Then, a precision casting method characterized by casting a molten metal into the cavity while maintaining the atmospheric pressure in that state.   2. The precision casting method according to claim 1, wherein the molten metal having high reactivity with the shell mold is a Ni-based superalloy containing an element having high activity against oxygen.   2. The precision casting method according to claim 1, wherein the molten metal having high reactivity with the shell mold is a Co-base superalloy containing an element having high activity against oxygen. In a method of casting a molten metal into a cavity of a shell mold and casting to produce a precision casting product,
When casting using the above shell mold that is porous and has good air permeability,
After placing the shell mold in the pressure vessel, the inside of the pressure vessel is evacuated, and then the atmosphere pressure is adjusted from 1 Pa to atmospheric pressure (about 0.1 MPa) by introducing an inert gas or nitrogen gas into the pressure vessel. Then, a precision casting method characterized by casting a molten metal into the cavity while maintaining the atmospheric pressure in that state. In a method of casting a molten metal into a cavity of a shell mold and casting to produce a precision casting product,
When casting using the above molten metal with low viscosity,
After placing the shell mold in the pressure vessel, the inside of the pressure vessel is evacuated, and then the atmosphere pressure is adjusted from 1 Pa to atmospheric pressure (about 0.1 MPa) by introducing an inert gas or nitrogen gas into the pressure vessel. Then, a precision casting method characterized by casting a molten metal into the cavity while maintaining the atmospheric pressure in that state.   6. The precision casting method according to claim 5, wherein the molten metal having a low viscosity is a Ni-base superalloy containing a C component for strengthening grain boundaries.   The precision casting method according to claim 5, wherein the molten metal having a low viscosity is a Co-base superalloy containing a C component for strengthening grain boundaries.   The precision casting method according to any one of claims 1 to 7, wherein the molten metal is unidirectionally solidified to produce the precision cast product of a unidirectionally solidified alloy.   The precision casting method according to any one of claims 1 to 7, wherein the molten metal is grown as a single crystal to produce the precision cast product of a single crystal alloy.   A precision casting product produced by the precision casting method according to claim 1, wherein an average surface roughness Ra of the as-cast material is 3.2 μm or less. In precision casting equipment that manufactures precision castings by casting molten metal into the mold cavity,
A melting furnace in which the mold is disposed and the metal is melted to form the molten metal;
Evacuation means for evacuating the melting furnace;
Gas supply means for supplying an inert gas or nitrogen gas into the melting furnace;
Control means for controlling the supply of inert gas or nitrogen gas;
A precision casting apparatus characterized by comprising: The precision casting apparatus according to claim 11, wherein the mold is a shell mold.

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