MOLD DESIGNING METHOD AND MOLD

Abstract

In a mold designing, a deformation amount of a casting from a solidification start time to a cooling end time may be obtained by deformation analysis software, and a deformation amount of a mold from a pouring time to the solidification start time may be obtained by deformation analysis software. When a cavity shape is designed based on the obtained deformation amounts of the mold and the casting, the precision of a near net shape may be further improved by reflecting the mold cavity shape at the solidification start time to the mold design, and a lack in dimension of a casting product after the casting process may be prevented.

Description

TECHNICAL FIELD

The present invention relates to a mold designing method of designing a mold cavity shape and a mold.

BACKGROUND ART

As a mold that casts a casting, a sand mold is generally used. In a case where a casting having a complex shape is casted, the mold includes a main mold and a core. As a casting sand forming the sand mold, a quartz sand is frequently used. In general, a binder such as resin is kneaded with the quartz sand in order to improve the moldability.

In a casting industry, there has been an attempt to achieve a so-called near net shape in which a shape of a casting product to be casted becomes similar to a shape of a finished product in order to reduce the processing allowance of the casting after the casting process. In such a near net shape casting, the mold cavity shape is designed by estimating a thermal contraction amount called a contraction rule of the casting. For example, in a case where a casting material is gray cast iron or spheroidal graphite cast iron, a contraction rule of about 0/1000 to 15/1000 is estimated. Due to such a change amount, the processing allowance changes in the range of 0 to 3 mm when the casting having a length dimension or an outer diameter dimension of 200 mm or more is casted. For this reason, it is difficult to cast the casting of which the processing allowance is smaller than 3 mm as one of reference conditions for the near net shape.

In order to improve the precision of the near net shape casting, there is proposed a method other than the empirical mold designing method using the contraction rule of the related art. For example, there is proposed a mold designing method that calculates a contraction and a thermal deformation when a casting is solidified and cooled according to a finite element method as one of numerical analysis methods and determines a casting model shape, that is, a mold cavity shape based on the calculation result (for example, see Patent Document 1).

In the method disclosed in Patent Document 1, the contraction and the thermal deformation of the casting estimated by calculating the temperatures of the casting and the mold when the casting and the mold are solidified and cooled by the finite element method and calculating the thermal stress and the deformation based on the calculated result are fed back to the design of the mold cavity shape. Further, in the analysis of the thermal stress and the deformation of the casting, the deformation resistance of the mold and the physical boundary condition of the boundary surface between the casting and the mold are considered.

Furthermore, an internal static pressure called float-up is exerted on the mold into which the molten metal is poured, and a deformation is caused by the static pressure. There are proposed several methods of suppressing the deformation of the mold caused by the static pressure (for example, see Patent Document 2). Such methods all suppress the deformation by restraining the outer surface of the mold, but do not quantify the deformation of the mold caused by the static pressure.

CITATION LIST

Patent Document

Patent Document 1: JP 11-320025 A

Patent Document 2: JP 2001-259798 A

SUMMARY OF THE INVENTION

In the mold designing method disclosed in Patent Document 1, the contraction and the thermal deformation of the casing when the casting is solidified and cooled are considered. However, in the casting process, a case may be supposed in which the mold is thermally deformed from the high-temperature molten metal pouring time to the solidification start time and the mold cavity shape at the solidification start time changes due to the thermal deformation. For this reason, in a case where only the contraction and the thermal deformation of the casting are considered, a problem arises in that the design precision of the cavity shape may not be sufficiently ensured. Further, since the cavity may be narrowed by the thermal deformation of the mold due to the molten metal, the processing allowance of the casting product after the casting process becomes a minus value, and hence there is a concern that a defective product may be obtained due to the lack in dimension.

Therefore, an object of the present invention is to design a mold cavity shape without a lack in dimension of a casting product after a casting process by further improving the precision of a near net shape casting.

In order to solve the above-described problems, the present invention provides a mold designing method of designing a cavity shape of a mold that casts a casting by pouring molten metal thereinto based on a numerical analysis, including: numerically analyzing a deformation of a mold caused by heat from a molten metal pouring time to a solidification start time so as to obtain a mold cavity shape change amount from the pouring time to the solidification start time; numerically analyzing a deformation of the casting caused when the casting is solidified and cooled from the solidification start time to a cooling end time so as to obtain a casting shape change amount from the solidification start time to the cooling end time; and designing the mold cavity shape based on the mold cavity shape change amount and the casting shape change amount.

That is, in the present invention, not only the shape change amount from the solidification start time at which the outer shell of the casting is formed to the cooling end time at which the temperature of the casting becomes a normal temperature, but also the mold cavity shape change amount from the pouring time to the solidification start time are also obtained by the numerical analysis. Then, when the mold cavity shape is designed based on the mold cavity shape change amount and the casting shape change amount, the mold cavity shape at the solidification start time may be reflected to the mold design. Accordingly, the precision of the near net shape may be further improved, and hence a lack in dimension of the casting product after the casting process may be prevented.

When a length dimension or an outer diameter dimension of the casting is 200 mm or more, the processing allowance of the casting product may be further effectively reduced, and hence the processing allowance may be set to be smaller than 3 mm.

Even when the casting includes a large-diameter portion and a small-diameter portion in the axial direction, the processing allowance of the casting product may be further effectively reduced. In particular, the processing allowance may be reduced so that a lack in dimension of the small-diameter portion does not occur. This is because a cavity narrowing degree caused by the thermal deformation of the mold in a portion of the mold forming the small-diameter portion is larger than that of a portion of the mold forming the large-diameter portion.

When temperature dependency of a physical property value of a mold material used in the numerical analysis of the deformation of the mold caused by the heat is considered, the thermal deformation of the mold may be more precisely numerically analyzed. Furthermore, as the physical property value of the mold material, a linear expansion coefficient and a Young's modulus may be exemplified.

When the mold cavity shape change amount is obtained by adding the deformation of the mold caused by a static pressure of the molten metal poured into the mold, the precision of the near net shape may be further improved.

Further, in the present invention, in the mold that casts the casting by pouring the molten metal thereinto, the cavity shape may be designed by any one of the mold designing methods.

According to the mold designing method of the present invention, not only the shape change amount from the solidification start time at which the outer shell of the casting is formed to the cooling end time at which the temperature of the casting becomes a normal temperature, but also the mold cavity shape change amount from the pouring time to the solidification start time are also obtained by the numerical analysis. Since the cavity shape is designed based on the mold cavity shape change amount and the casting shape change amount, the precision of the near net shape may be further improved, and hence a lack in dimension of the casting product after the casting process may be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional view illustrating an example of a mold that employs mold designing methods of first and second embodiments.

FIG. 2 is a front view illustrating a casting product that is casted by the mold of FIG. 1.

FIG. 3 is a flowchart illustrating a sequence of a numerical analysis of the mold designing method of the first embodiment.

FIGS. 4A and 4B are thermal expansion line maps of a mold material and a casting material respectively used in the first and second embodiments.

FIGS. 5A and 5B are stress-strain curves of the mold material and the casting material respectively used in the first and second embodiments.

FIG. 6 is a graph illustrating the radial deformation amounts of the casting and the mold obtained by the numerical analysis of the first embodiment.

FIGS. 7A and 7B are partially enlarged graphs of a cross-sectional shape of a mold cavity that is designed based on the deformation amounts of the mold and the casting of FIG. 6.

FIG. 8 is a flowchart illustrating a sequence of a numerical analysis of the mold designing method of the second embodiment.

FIG. 9 is a longitudinal sectional view illustrating a static pressure exerted on the mold of FIG. 1.

FIG. 10 is a graph illustrating the radial deformation amounts of the casting and the mold obtained by the numerical analysis of the second embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 illustrates a mold 1 that employs mold designing methods of first and second embodiments. FIG. 2 illustrates a screw compressing rotor 11 as a casting product that is casted by the mold 1. The screw compressing rotor 11 is formed of spheroidal graphite cast iron (JIS; FCD500), and the entire axial length thereof is 860 mm. A screw portion 11a that includes a large-diameter portion and a small-diameter portion in the axial direction is formed so that the length dimension is 460 mm and the outer diameter dimension is 240 mm.

The mold 1 includes a main mold 1a and a core 1b that casts the screw portion 11a, and a quartz sand kneaded with a resin as a binder is used as any casting sand. In the mold 1, a cavity 2 that casts the screw compressing rotor 11 is formed in the longitudinal direction. The mold is provided with a riser portion 3 which is provided above the cavity 2, a pouring portion 4 into which the molten metal is poured, and a runner 5 that leads the molten metal to the cavity 2.

FIG. 3 illustrates a sequence of a numerical analysis of the mold designing method of the first embodiment. Numerical analysis software includes casting analysis software 21 that calculates the flow and the solidification of the molten metal and the heat transfer of the entire system, deformation analysis software 22 that calculates the deformation of the mold, and deformation analysis software 23 that calculates the deformation of the casting. Here, calculation software (trade name: JSCAST, manufactured by QUALICA Corporation) of a finite element method is used in the casting analysis software 21, and calculation software (trade name: ABAQUS, manufactured by SIMULIA Corporation) of a finite element method is used in each of the deformation analysis software 22 and the deformation analysis software 23. Furthermore, this calculation software is not limited to the finite element method, and may be calculation software of a difference method or the like.

First, a casting plan (a mold shape, a casting shape, a pouring temperature, a pouring amount, and a pouring speed), thermal characteristics (a density, a specific heat, and thermal conductivity) of a mold material, thermal characteristics (a density, a specific heat, thermal conductivity, a solidus temperature, a liquidus temperature, and a coagulation latent heat) of a casting material, and a thermal boundary condition (a heat transfer coefficient between the mold and the casting, a heat transfer coefficient between the mold and the atmosphere, and an atmosphere temperature) are input as input data to the casting analysis software 21. Then, the casting analysis software 21 calculates the temperature distribution of the mold and the casting and the solid phase rate of the casting at each elapse time and calculates a molten metal solidification start time Ts. Here, the time at which the temperature of the entire surface of the casting becomes a solidus temperature (1140° C.) or less and the outer shell of the casting is formed is set as the solidification start time Ts.

Next, the temperature distribution of the mold at the time 0 to the time T_S calculated by the casting analysis software 21 and the linear expansion coefficient and the Young's modulus separately obtained as the physical property value of the mold material are input to the deformation analysis software 22, thereby calculating the deformation amount generated by the heat of the mold between the time 0 to the time T_S. Also, the temperature distribution of the casting from the time Ts calculated by the casting analysis software 21 to the cooling end time, that is, the temperature of the casting becomes a normal temperature and the linear expansion coefficient and the Young's modulus separately obtained as the physical property value of the casting material are input to the deformation analysis software 23. Then, the casting deformation amount generated from the time T_S to the cooling end time is calculated. Finally, the calculated deformation amounts of the mold and the casting caused by solidifying and cooling the mold and the casting are added to the initially input mold cavity shape, and then the cavity shape is designed.

FIGS. 4A and 4B are respectively known thermal expansion line maps for the quartz sand kneaded with resin as the mold material and FCD500 as the casting material. Tables 1A and 1B are respectively the representative linear expansion coefficients of the quartz sand kneaded with resin and the FCD500 obtained from the thermal expansion line maps of FIGS. 4A and 4B, and are used as input data to the deformation analysis software 22 and the deformation analysis software 23.

TABLE 1
TEMPERATURE
(° C.)
(a) QUARTZ SAND KNEADED WITH RESIN
     LINEAR EXPANSION COEFFICIENT
     (BASED ON 0° C.)
200  1.01 × 10−5
400  1.17 × 10−5
482  1.33 × 10−5
523  1.45 × 10−5
565  1.83 × 10−5
581  2.05 × 10−5
600  2.09 × 10−5
700  1.83 × 10−5
1282 9.93 × 10−6
1323 1.20 × 10−5
(b) FCD500
     LINEAR EXPANSION COEFFICIENT
     (BASED ON 28° C.)
50   1.12 × 10−5
100  1.15 × 10−5
150  1.19 × 10−5
200  1.24 × 10−5
250  1.27 × 10−5
300  1.28 × 10−5
350  1.29 × 10−5
400  1.31 × 10−5
450  1.33 × 10−5
500  1.35 × 10−5
550  1.36 × 10−5
600  1.38 × 10−5
650  1.39 × 10−5
700  1.40 × 10−5
750  1.20 × 10−5
800  1.27 × 10−5
850  1.37 × 10−5
900  1.47 × 10−5
950  1.59 × 10−5
1000 1.72 × 10−5
1050 1.85 × 10−5
1100 1.96 × 10−5

FIG. 5A is a stress-strain curve of the mold material obtained from the compression test at each test temperature, and FIG. 5B is a stress-strain curve of the casting material obtained from the tensile test at each test temperature. Tables 2A and 2B are respectively the representative Young's modulus of the quartz sand kneaded with resin and the FCD500 at each temperature obtained from the stress-strain curves of FIGS. 5A and 5B, and are used as input data to the deformation analysis software 22 and the deformation analysis software 23.

TABLE 2
TEMPERATURE YOUNG'S MODULUS
(° C.)      (MPa)
(a) QUARTZ SAND KNEADED WITH RESIN
20          5.46
500         4.53
800         1.97
1000        1.92
1350        1.49
(b) FCD500
24          172.0
200         167.0
400         157.0
600         141.0
800         91.6
900         74.8
1000        68.7

FIG. 6 illustrates the radial deformation amounts (in the radius) of the mold and the casting in the axial coordinates obtained by the numerical analysis of the first embodiment. FIG. 6 illustrates a value in which the deformation amounts of the mold and the casting are added to each other, and also illustrates the casting product shape. Furthermore, the processing allowance of the casting product shape is 2 mm. According to the numerical analysis result, the deformation amount of the mold becomes a minus value that narrows the cavity, and in a portion casting the screw portion in the casting, the narrowed amount of the valley portion (the small-diameter portion) of the screw is larger than that of the ridge portion (the large-diameter portion) of the screw. Further, the deformation amount of the casting also becomes a minus value that reduces the diameter, and in the screw portion, the diameter reduction amount of the ridge portion (the large-diameter portion) is larger than that of the valley portion (the small-diameter portion). As a result, in the screw corresponding portion, the value obtained by adding the deformation amounts of the mold and the casting becomes a substantially constant minus value (contraction value) of about 3 mm in the radius and about 6 mm in the diameter. Furthermore, although not shown in the drawings, in the numerical analysis, the three-dimensional deformation amounts of the mold and the casting are obtained, and the axial deformation amounts also become minus values.

EXAMPLE

FIGS. 7A and 7B respectively enlarged views of the ridge portion and the valley portion of the screw portion in the cavity sectional shape in an example in which the mold is designed based on the value obtained by adding the deformation amounts of the mold and the casting of FIG. 6. In the drawings, a comparative example is shown in which the cavity sectional shape is designed in consideration of only the deformation amount of the casting from the solidification start time to the cooling end time. In the comparative example in which the deformation amount of the mold from the pouring time to the solidification start time is not considered, the contraction amount of the valley portion of the screw portion shown in FIG. 7B is small, and the cavity diameter of the valley portion is smaller than that of the example. For this reason, a processing allowance of 2 mm is formed in the valley portion of the screw portion, and hence there is a concern that the casting product may be a defective product due to the lack in dimension. On the contrary, in the example in which the deformation amount of the mold from the pouring time to the solidification start time is considered, the contraction amount of the screw portion becomes substantially constant in both the ridge portion and the valley portion. For this reason, there is not any concern that the casting product may become a defective product due to the lack in dimension of the valley portion, and the precision of the near net shape may be improved by decreasing the processing allowance.

FIG. 8 illustrates a sequence of the numerical analysis in the mold designing method of the second embodiment. The basic sequence of the numerical analysis is substantially the same as that of the first embodiment. However, the second embodiment is different from the first embodiment in that the deformation of the mold 1 caused by the static pressure of the molten metal poured thereinto is added to the deformation analysis software 22 that calculates the deformation of the mold 1 shown in FIG. 1. The other configurations of the second embodiment are the same as those of the first embodiment. Then, the screw compressing rotor 11 as the casting product shown in FIG. 2 is formed of the FCD500, and the quartz sand kneaded with resin is used as the casting sand.

That is, in the second embodiment, as shown in FIG. 9, the deformation caused by the static pressure p exerted from the inside of the mold 1 at each position of the cavity 2 is added. When the depth from the molten metal surface A is denoted by z, the density of the molten metal is denoted by ρ, and the gravitational acceleration is denoted by g, the static pressure p(z) at the position of the depth z is expressed by Equation (1).

p(z)=ρ×g×z  (1)

The static pressure p(z) of each position obtained in Equation (1) is perpendicularly applied to each contact point of the cavity surface of the finite element method model used in the deformation analysis software 22.

FIG. 10 illustrates the radial deformation amounts (in the radius) of the mold and the casting at each axial coordinate obtained by the numerical analysis of the second embodiment and a value obtained by adding the deformation amounts of the mold and the casting. FIG. 10 also illustrates the casting product shape of which the processing allowance is 2 mm. Furthermore, in the axial coordinate, the maximal depth position of the casting product inside the mold is set as the origin, and the depth z from the molten metal surface decreases with an increase in axial coordinate. Further, the deformation amounts of the mold and the casting and the added value thereof in FIG. 10 are shown in a scale larger than that of FIG. 6 showing the numerical analysis result of the first embodiment.

In FIG. 10 showing the numerical analysis result of the second embodiment in which the deformation of the mold caused by the static pressure p is added, the deformation amount of the mold is shifted to the plus side compared to FIG. 6 showing the numerical analysis result of the first embodiment. The shift amount increases by an area in which the depth z from the molten metal surface is deep, that is, an area in which the axial coordinate is small. For this reason, the deformation amount of the mold becomes a minus value that narrows the cavity of the substantially entire portion casting the screw portion in FIG. 6, but the deformation amount becomes a plus value that widens the cavity at each ridge portion (each large-diameter portion) of the screw in FIG. 10. Furthermore, the deformation amount of the casting is the same as that of the first embodiment. As a result, in the value obtained by adding the deformation amounts of the mold and the casting of the screw corresponding portion, the absolute value of the minus value (the contraction value) is smaller than that of FIG. 6. Accordingly, the precision of the near net shape may be further improved by more precisely estimating the contraction value.

In the above-described embodiments, the casting product to be casted is the screw compressing rotor that is formed of spheroidal graphite cast iron. However, the mold designing method and the mold according to the present invention are not limited for the casting of spheroidal graphite cast iron, and may be used for the casting of gray cast iron or steel or the casting of non-ferrous metal such as aluminum. Further, the casting product is not limited to the screw compressing rotor. In particular, the casting product may have a large dimension in which the length dimension or the outer diameter dimension is 200 mm or more or the casting product may have a large-diameter portion and a small-diameter portion in the axial direction. Such a casting product may be appropriately casted by the present invention.

While the embodiments of the present invention have been described, the present invention is not limited to the above-described embodiments, and may be modified into various forms within the limitation of claims. Priority is claimed on Japanese Patent Application No. 2011-211108, filed on Sep. 27, 2011 and Japanese Patent Application No. 2012-163293, filed on Jul. 24, 2012, the content of which is incorporated herein by reference.

EXPLANATION OF REFERENCE NUMERALS

• • 1 mold
  • 1 a main mold
  • 1 b core
  • 2 cavity
  • 3 riser portion
  • 4 pouring portion
  • 5 runner
  • 11 screw compressing rotor
  • 11 a screw portion
  • 21 casting analysis software
  • 22, 23 deformation analysis software

Claims

1. A mold designing method of designing a cavity shape of a mold that casts a casting by pouring molten metal thereinto based on a numerical analysis, comprising: numerically analyzing a deformation of a mold caused by heat from a molten metal pouring time to a solidification start time so as to obtain a mold cavity shape change amount from the pouring time to the solidification start time;numerically analyzing a deformation of the casting caused when the casting is solidified and cooled from the solidification start time to a cooling end time so as to obtain a casting shape change amount from the solidification start time to the cooling end time; anddesigning the mold cavity shape based on the mold cavity shape change amount and the casting shape change amount.

2. The mold designing method according to claim 1, wherein a length dimension or an outer diameter dimension of the casting is 200 mm or more.

3. The mold designing method according to claim 1, wherein the casting includes a large-diameter portion and a small-diameter portion in the axial direction.

4. The mold designing method according to claim 1, wherein temperature dependency of a physical property value of a mold material used in the numerical analysis of the deformation of the mold caused by the heat is considered.

5. The mold designing method according to claim 1, wherein the mold cavity shape change amount is obtained by adding the deformation of the mold caused by a static pressure of the molten metal poured into the mold.

6. A mold that casts a casting by pouring molten metal thereinto, wherein a cavity shape is designed by the mold designing method according to claim 1.