JP2018183805A - Evaporative pattern casting process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an evaporative pattern casting process capable of casting a mold having no seizure at an internal cavity.SOLUTION: Provided is an evaporative pattern casting process where a mold obtained by coating the surface of a foam pattern with a mold wash is buried into casting sand, thereafter, the molten metal of cast iron is poured into the mold, and the foam pattern is disappeared and is substituted with the molten metal to cast a casting of cast iron provided with an internal cavity, the time in which the mold wash is exposed to a high temperature of 1,100°C or higher upon the casting is controlled to 1,300 s, preferably to 1,200 s.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a vanishing model casting method for casting a cast iron casting having an internal cavity.

  In the disappearance model casting method, a mold made by applying a coating agent to the surface of the foam model is buried in the casting sand, and then the molten metal is poured into the mold to eliminate the foam model and replace it with the molten metal. In this method, the casting is cast. This vanishing model casting method is a method suitable for casting a casting having an internal cavity having a complicated shape (a shape having a high degree of freedom) with a near net shape (a shape close to the final shape).

  In the vanishing model casting method, the shape of the mold at the time of molding and casting is maintained by the coating agent applied to the surface of the foam model including the inner cavities and the inner surfaces of the punched holes. However, the coating agent is subjected to a heat load from the molten metal and various external forces (such as a molten metal static pressure and a dynamic pressure due to the molten metal flow) during casting. If the coating agent cannot withstand the heat load and external force described above, the coating agent will be damaged, and the molten metal will ooze out into the casting sand filled in the internal cavities and core holes, causing the casting sand and casting to melt. Casting defects called “seizure” occur.

  Therefore, Patent Document 1 discloses a vanishing model coating agent composition capable of suppressing seizure. Patent Document 2 discloses a vanishing model casting method in which the casting time during casting is set according to the modulus of the model (model volume / model surface area). Patent Document 3 discloses a method for producing a casting in which a molten metal having a temperature in the range of 1250 ° C. to 1330 ° C. is poured from a mold gate.

JP 2016-120517 A JP 2011-110577 A JP 2006-175492 A

  However, even when the techniques of Patent Documents 1 to 3 are employed, it is difficult to predict the occurrence of seizure in advance when casting a casting having an internal cavity having a complicated shape. In Patent Document 1, the shape of the internal cavity is a simple shape having a rectangular cross section, and cannot be applied to an internal cavity having a complicated shape. In patent document 2, the casting time is grasped macroscopically and cannot be applied to the local heat load situation of the internal cavity and the risk prediction of occurrence of seizure.

  Therefore, when casting a casting having an internal cavity having a complicated shape, an appropriate casting condition is usually obtained by performing trial manufacture several times. However, if further design changes are required at the prototype stage, this leads to an increase in lead time at the development stage. Therefore, when casting a casting having an internal cavity having a complicated shape, a technique that can clearly determine in advance at the design stage a place where the risk of occurrence of seizure is high is desired.

  An object of the present invention is to provide a vanishing model casting method capable of casting a casting in which an internal cavity is not seized.

  In the present invention, after a mold formed by applying a coating agent on the surface of a foam model is buried in casting sand, a molten cast iron is poured into the mold, and the foam model is eliminated to replace the melt. Thus, in the vanishing model casting method for casting a cast iron casting having an internal cavity, the time during which the coating agent is exposed to a high temperature of 1100 ° C. or more during casting is 1300 seconds or less.

  According to the present invention, during casting of a cast iron casting with an internal cavity, the time during which the coating agent applied to the surface of the foam model is exposed to a high temperature of 1100 ° C. or higher is set to 1300 seconds or shorter. When casting cast iron, the temperature of the molten cast iron is 1100 ° C. or higher, and seizure occurs at the site of the internal cavity where the time during which the mold is exposed to the high temperature is longer than 1300 seconds. Therefore, the occurrence of seizure in the internal cavity can be avoided by setting the time during which the coating agent is exposed to a high temperature of 1100 ° C. or higher to 1300 seconds or less. Thereby, it is possible to cast a casting in which the internal cavity is not seized.

It is a figure which shows the initial shape of a representative position (1). It is a figure which shows the initial shape of representative position (2) and (3). It is a figure which shows the shape of the representative position (1) of the casting A at the time of evaluation. It is a figure which shows the shape of the representative position (2) of the casting A at the time of evaluation. It is a figure which shows the shape of the representative position (1) of the casting B at the time of evaluation. It is a figure which shows the shape of the representative position (3) of the casting B at the time of evaluation. It is a figure which shows the relationship between the thermal load parameter | index of a coating type agent, and a seizing determination result.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

(Disappearance model casting method)
In the disappearance model casting method according to the embodiment of the present invention, a mold formed by applying a coating agent on the surface of a foamed model is buried in casting sand (dry sand), and then molten cast iron is poured into the mold to foam. This is a method for casting a cast iron casting having an internal cavity by eliminating the model and replacing it with molten metal. This disappearance model casting method is a method suitable for casting a casting having an internal cavity having a complicated shape (a shape having a high degree of freedom) with a near net shape (a shape close to the final shape).

  The vanishing model casting method includes a melting step of melting cast iron to form a molten metal, a molding step of molding a foam model, and a coating step of applying a coating agent to the surface of the foam model to form a mold. Yes. Furthermore, the disappearing model casting method is a molding process in which the mold is filled in the casting sand and filled with the casting sand to every corner of the casting mold, and the molten cast iron (molten metal) is poured into the casting mold. It has a casting step for melting and replacing the molten metal, a cooling step for cooling the molten metal poured into the mold to form a casting, and a separation step for separating the casting from the casting sand.

As cast iron to be melted, gray cast iron (JIS-FC250), spheroidal graphite cast iron (JIS-FCD450), or the like can be used. In addition, as the foam model, a foam resin such as polystyrene foam can be used. As the coating agent, a silica-based aggregate coating agent or the like can be used. Further, as the sand, “silica sand” containing SiO ₂ as a main component, zircon sand, chromite sand, synthetic ceramic sand and the like can be used. In addition, you may add a binder and a hardening | curing agent to foundry sand.

  A coating agent film is formed on the surface layer of the foam model by applying a coating agent to the surface of the foam model and drying it appropriately. The film of the coating agent contains a refractory aggregate and a resin binder for forming the film. When the film of the coating agent is exposed to the molten metal during casting, the thermal decomposition of the resin binder proceeds inside the coating agent, and the strength of the coating agent itself is reduced. When the thermal decomposition of the resin binder is completed, the film of the coating agent is supported only by the bonding force between the aggregates, and has almost no strength. Therefore, the coating agent film is easily damaged.

  Therefore, when casting cast iron castings with internal cavities, the local thermal load applied to the coating material during casting is determined in advance according to the shape of the internal cavities, and fired as the coating material is damaged. If the threshold value of the heat load state when sticking occurs is clarified, it becomes possible to avoid seizure in the stage before casting by changing the shape of the internal cavity.

(Heat load index)
Generally, the melting point of cast iron is in the range of 1100 to 1200 ° C., although it depends on the molten metal component. When casting a cast iron casting, the temperature of the molten cast iron is 1100 ° C. or higher. The casting experiment and numerical analysis (casting simulation) reveal that there is a correlation between the site where the coating film is exposed to the molten metal at a high temperature and the location where seizure occurs. Became.

  In particular, it has been clarified that seizure occurs in the internal cavity at a site where the time during which the coating agent film is exposed to a high temperature of 1100 ° C. or more is longer than 1300 seconds. A sample of the coating agent film prepared separately was subjected to a heat exposure test that applied a similar heat load, and when the internal state of the coating agent was investigated, the resin binder was completely pyrolyzed and the coating mold The agent itself has almost no strength, and it was confirmed that the coating agent was very easily damaged.

  In areas where seizure is likely to occur in the internal cavity, a device to ensure that the sand is filled with casting sand (preserve the shape with back sand as much as possible) or to apply to the surface of the internal cavity. Additional measures such as increasing the number of times the mold is applied are required. As another technique for suppressing seizure, it is conceivable to reduce the thermal load on this part by reducing the thickness around the part where seizure is likely to occur.

  For castings where seizure occurs locally in the internal cavity, using a foam model with the same internal cavity shape and a reduced overall size, casting under the same conditions, the site where seizure occurred In a corresponding portion, the time during which the coating agent film was exposed to a high temperature of 1100 ° C. or higher was 1300 seconds or shorter, and seizure could be avoided.

  In this way, by setting the time during which the film of the coating agent applied to the surface of the foam model is exposed to a high temperature of 1100 ° C. or more as a thermal load index (seizure determination index), regardless of the shape of the internal cavity, It becomes possible to predict the risk of seizure in advance. Then, by setting the shape design or casting conditions of the casting so that this index is 1300 seconds or less, it is possible to cast a casting in which the internal cavity is not seized.

  Here, as a heat load index, it is conceivable to evaluate by “time until solidification of casting surface layer is completed”, but the solidification completion point is determined from the cooling curve near the casting surface layer obtained by casting simulation or actual measurement. It is difficult to determine exactly. Therefore, in order to be able to determine the threshold value of the index clearly and simply, “time during which the coating agent was exposed to a high temperature of 1100 ° C. or higher” was adopted as the thermal load index.

(Scoring evaluation)
Next, the cast was cast by the vanishing model casting method to evaluate seizure, and the thermal load applied to the coating agent film was evaluated by casting simulation.

  First, two types of castings (casting A and casting B), in which the gray cast iron was used as the molten metal and the internal cavities had the same shape and different overall sizes, were cast as prototypes by the disappearance model casting method. Here, the casting A is larger in size than the casting B. And two types of castings after casting were cut | disconnected, respectively, and the occurrence condition of the seizure in the representative positions (1)-(3) in the internal cavity was investigated. The initial shape of the representative position (1) is shown in FIG. 1A. Further, the initial shapes of the representative positions (2) and (3) are shown in FIG. 1B. The representative positions (2) and (3) have the same initial shape, but the positions in the internal cavity are different.

  The evaluation results are shown in Table 1. 2A shows the shape of the representative position (1) of the casting A at the time of evaluation, and FIG. 2B shows the shape of the representative position (2) of the casting A at the time of evaluation. Moreover, the shape of the representative position (1) of the casting B at the time of evaluation is shown in FIG. 3A, and the shape of the representative position (3) of the casting B at the time of evaluation is shown in FIG. 3B.

  At the representative position (1) of the casting A where seizure occurred and the representative position (2) of the casting A, it was confirmed that the cast sand filled inside was seized at the place where the cavity should be originally formed. it can.

  Next, when two types of castings (casting A and casting B) were cast, the thermal load condition received by the coating agent film at the representative positions (1) to (3) was evaluated by casting simulation. As the evaluation index, the time (seconds) during which the coating agent was exposed to a high temperature of 1100 ° C. or higher was used. The results are shown in Table 2.

  Moreover, about representative position (2), when two types of castings (casting A, casting B) were prototyped twice by the disappearance model casting method, temperature measurement was performed, and the coating agent was 1100 ° C. or higher. By measuring the time of exposure to high temperatures, the heat load index based on the actual measurement results was obtained. The results are also shown in Table 2.

  Here, for temperature measurement, a thermocouple placed between the foam model and the coating agent film is attached to the surface of the foam model, and the temperature of the casting surface layer of the coating agent is measured with the thermocouple. did.

  When the thermal load state in Table 2 and the seizure occurrence state in Table 1 were compared, the time at which the coating agent film was exposed to a high temperature of 1100 ° C. or higher was as long as 1700 seconds or longer. 1) and the seizure occurs at the representative position (2) of the casting A, and it can be seen that there is a correlation between the occurrence of seizure and the thermal load applied to the coating agent film.

  FIG. 4 shows the relationship between the thermal load index of the current coating agent and the seizure determination result. In FIG. 4, A- (1) means the representative position (1) of the casting A. The same applies to A- (2) and the like. Moreover, in FIG. 4, a numerical analysis result is an evaluation result by casting simulation. Further, the actual measurement (1) is an actual measurement result measured during the first trial casting, and the actual measurement (2) is an actual measurement result measured during the second trial casting.

  As a result of obtaining the threshold value for occurrence of seizure defects using the present thermal load index, if the time during which the coating agent is exposed to a high temperature of 1100 ° C. or higher is 1300 seconds or less (preferably 1200 seconds or less), It can be seen that the occurrence of seizure in the internal cavity can be avoided. Here, 1300 seconds were derived from the numerical analysis results, and 1200 seconds were derived from the actual measurement results.

  From the above results, the time for which the film of the coating agent applied to the surface of the foamed model is exposed to a high temperature of 1100 ° C. or higher is used as a thermal load index (seizure determination index), regardless of the shape of the internal cavity. It can be seen that the risk of seizure can be predicted in advance. Then, it can be seen that a casting having no seizure in the internal cavity can be cast by setting the shape design or casting conditions of the casting so that the index is 1300 seconds or less.

(effect)
As described above, according to the vanishing model casting method according to the present embodiment, the casting agent applied to the surface of the foam model is exposed to a high temperature of 1100 ° C. or higher when casting a cast iron casting having an internal cavity. Time to be set to 1300 seconds or less. When casting cast iron, the temperature of the molten cast iron is 1100 ° C. or higher, and seizure occurs at the site of the internal cavity where the time during which the mold is exposed to the high temperature is longer than 1300 seconds. Therefore, the occurrence of seizure in the internal cavity can be avoided by setting the time during which the coating agent is exposed to a high temperature of 1100 ° C. or higher to 1300 seconds or less. Thereby, it is possible to cast a casting in which the internal cavity is not seized.

  In addition, by setting the time during which the coating agent is exposed to a high temperature of 1100 ° C. or more to 1200 seconds or less, the occurrence of seizure in the internal cavity can be suitably avoided.

  The embodiment of the present invention has been described above, but only specific examples are illustrated, and the present invention is not particularly limited, and the specific configuration and the like can be appropriately changed in design. Further, the actions and effects described in the embodiments of the invention only list the most preferable actions and effects resulting from the present invention, and the actions and effects according to the present invention are described in the embodiments of the present invention. It is not limited to what was done.

Claims (2)

After embedding a mold formed by applying a coating agent on the surface of the foam model in casting sand, pouring a molten cast iron into the mold, and disappearing the foam model to replace the molten metal, In the vanishing model casting method for casting a cast iron casting with a cavity,
A disappearance model casting method characterized in that the time during which the coating agent is exposed to a high temperature of 1100 ° C. or more during casting is 1300 seconds or less.   2. The disappearance model casting method according to claim 1, wherein a time during which the coating agent is exposed to a high temperature of 1100 ° C. or more during casting is 1200 seconds or less.