JP5839582B2 - Mold design method and mold - Google Patents

Description

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

  A sand mold is generally used as a casting mold for castings. When casting a casting having a complicated shape, the casting mold may be composed of a main mold and a core. Silica sand is often used for the foundry sand forming the sand mold, and usually a binder such as resin is mixed in order to improve moldability.

  In the casting industry, in order to reduce the processing cost of a cast product after casting, so-called near net shaping, in which a cast product shape to be cast is brought close to a final product shape, is progressing. In casting of such a near net shape, the cavity shape of the mold is designed in consideration of the amount of heat shrinkage of the casting called an elongate scale. For example, when the cast material is gray cast iron or spheroidal graphite cast iron, an elongation scale of about 0/1000 to 15/1000 is expected, but since there is such a fluctuation range, the length dimension and the outer diameter dimension are expected. When casting a large casting with a thickness of 200 mm or more, the machining allowance changes in the range of 0 to 3 mm or more, and it is difficult to cast a casting with a machining allowance of less than 3 mm, which is one guideline for near net shaping. It has become.

  In order to improve the accuracy of near-net shape casting, instead of the empirical mold design method using the conventional stretch scale, shrinkage and thermal deformation during casting solidification / cooling is a numerical analysis method. There has been proposed a mold design method for calculating by a certain finite element method and determining the shape of the casting model, that is, the cavity shape of the mold based on the calculation result (see, for example, Patent Document 1).

  In the method described in Patent Document 1, the shrinkage and thermal deformation of the casting are predicted by performing the temperature calculation of the casting and the mold at the time of solidification / cooling and the thermal stress / deformation calculation based on the temperature calculation by the finite element method. This is fed back to the design of the cavity shape of the mold. In addition, in the thermal stress and deformation analysis of the casting, the deformation resistance of the mold and the dynamic boundary condition between the casting and the mold interface are taken into consideration.

  Note that it is known that a static pressure called tension is applied from the inside to the casting mold during pouring, and deformation due to the static pressure occurs. Several means for suppressing deformation have been proposed (see, for example, Patent Document 2). All of these means restrain the deformation by restraining the outer surface side of the mold, and do not quantify the deformation of the mold due to static pressure.

Japanese Patent Laid-Open No. 11-320025 JP 2001-259798 A

  The mold design method described in Patent Document 1 takes into account the shrinkage and thermal deformation of the casting during solidification / cooling. In the casting process, the mold is inserted between the pouring and the start of solidification with a high-temperature molten metal. It is assumed that the mold cavity shape changes at the start of solidification due to thermal deformation. For this reason, there is a problem that the design accuracy of the cavity shape cannot be sufficiently ensured only by considering the shrinkage and thermal deformation of the casting. Moreover, since the mold is thermally deformed by the molten metal, it is expected that the cavity is narrowed, so that the processing cost of the cast product after casting becomes negative, which may result in a defective product with insufficient dimensions.

  Therefore, an object of the present invention is to design the cavity shape of the mold so that the accuracy of near-net shape casting is further improved and the cast product after casting does not have insufficient dimensions.

  In order to solve the above problems, the present invention provides a mold design method for designing a cavity shape of a mold for casting a cast metal by casting a molten metal based on numerical analysis, and then solidification starts after the molten metal is poured. Numerical analysis of the deformation due to heat of the mold up to the time of pouring to the start of solidification, and numerical analysis of deformation due to solidification and cooling from the start of solidification to the end of cooling And determining the shape change amount of the casting from the start of solidification to the end of cooling, and designing the mold cavity shape based on the obtained shape change amount of the mold cavity and the shape change amount of the casting. Adopted.

  That is, not only the amount of shape change from the start of solidification when the outer shell of the casting is formed to the end of cooling until the casting becomes room temperature, but also the amount of shape change of the mold cavity from pouring to the start of solidification is obtained by numerical analysis. By designing the mold cavity shape based on the mold cavity shape change amount and casting shape change amount, the mold cavity shape at the start of solidification is reflected in the mold design, and the accuracy of the near net shape is further improved. In addition, the cast product after casting was made not to be short of dimensions.

  When the length dimension or outer diameter dimension of the casting is 200 mm or more, the machining allowance of the cast product can be more effectively reduced, and the machining allowance can be less than 3 mm.

  Even when the casting has a large diameter portion and a small diameter portion in the axial direction, the machining allowance of the cast product can be more effectively reduced. Can be reduced. That is, the portion of the mold that forms the small-diameter portion has a greater degree of narrowing of the cavity due to thermal deformation of the mold than the portion of the mold that forms the large-diameter portion.

  By considering the temperature dependence of the physical properties of the mold material used for the numerical analysis of the deformation of the mold due to heat, the thermal deformation of the mold can be numerically analyzed with higher accuracy. In addition, as a physical-property value of this casting_mold | template material, a linear expansion coefficient, a Young's modulus, etc. are mentioned.

  The accuracy of the near net shape can be further improved by calculating the shape change amount of the mold cavity in consideration of the deformation of the mold due to the static pressure of the molten metal poured into the mold.

  The mold design method according to the present invention includes not only the amount of shape change from the start of solidification when the outer shell of the casting is formed to the end of cooling until the casting becomes room temperature, but also the shape change amount of the mold cavity from the pouring to the start of solidification. Since the cavity shape is designed based on the amount of mold cavity shape change and casting shape change obtained by numerical analysis, the near net shape accuracy is further improved and the cast product after casting Can be prevented from being insufficiently dimensioned.

A longitudinal sectional view showing an example of a mold to which the mold design methods of the first and second embodiments are applied. The front view which shows the casting product cast with the casting_mold | template of FIG. The flowchart which shows the procedure of the numerical analysis in the casting_mold | template design method of 1st Embodiment. (A), (b) is a thermal expansion diagram of the mold material and the casting material used in the first and second embodiments, respectively. (A), (b) is a stress-strain curve of the mold material and the casting material used in the first and second embodiments, respectively. The graph which shows the deformation | transformation amount of the radial direction of the casting_mold | template and casting calculated | required by the numerical analysis of 1st Embodiment (A), (b) is the graph which expands and shows a part of cross-sectional shape of the mold cavity designed based on the deformation | transformation amount of the casting_mold | template and casting of FIG. 6, respectively. The flowchart which shows the procedure of the numerical analysis in the casting_mold | template design method of 2nd Embodiment. 1 is a longitudinal sectional view for explaining the static pressure acting on the mold of FIG. A graph showing the amount of deformation in the radial direction of the mold and casting obtained by the numerical analysis of the second embodiment

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a mold 1 to which the mold design methods of the first and second embodiments are applied, and FIG. 2 shows a screw compressor rotor 11 as a cast product cast by the mold 1. The screw compressor rotor 11 is made of spheroidal graphite cast iron (JIS; FCD500), has a total axial length of 860 mm, and the screw portion 11a having a large diameter portion and a small diameter portion in the axial direction has a length. The dimensions are 460 mm and the outer diameter is 240 mm.

  The mold 1 is composed of a main mold 1a and a core 1b for casting the screw portion 11a. Silica sand in which a resin as a binder is mixed is used for any foundry sand. In the mold 1, a cavity 2 for casting a screw compressor rotor 11 is formed in a vertical direction, and a hot water pouring part 3 is provided above the cavity 2, and a pouring part 4 into which molten metal is poured is poured. A runner 5 for guiding the molten metal to the cavity 2 is provided.

  FIG. 3 shows a numerical analysis procedure in the mold design method of the first embodiment. This numerical analysis software includes a casting analysis software 21 for calculating the flow and solidification of the molten metal and a heat transfer calculation for the entire system, a deformation analysis software 22 for calculating the deformation of the mold, and a deformation analysis software for calculating the deformation of the casting. 23. Here, the casting analysis software 21 is a finite element method calculation software JSCAST (trade name; manufactured by Qualica), and each deformation analysis software 22 and 23 is a finite element method calculation software ABAQUS (trade name; manufactured by Simria). ) Is used. Note that these calculation softwares are not limited to those of the finite element method, and calculation software such as a difference method may be used.

First, the casting analysis software 21 includes a casting method (mold shape, casting shape, pouring temperature, pouring amount, pouring speed), thermal characteristics of the mold material (density, specific heat, thermal conductivity), and thermal characteristics of the casting material. (Density, specific heat, thermal conductivity, solidus temperature, liquidus temperature, latent heat of solidification) and heat boundary conditions (heat transfer coefficient between mold and casting, heat transfer coefficient between mold and atmosphere, atmosphere temperature) input as input data, together with calculating the temperature distribution and the solid fraction of the cast of the mold and the casting at the elapsed each time, and calculates the coagulation starting time T _S of the molten metal. Here, the entire surface temperature of the casting is less solidus temperature (1140 ° C.), the time at which it is the outer shell of the casting and the coagulation starting time T _S.

Then, the deformation analysis software 22, the temperature distribution of the mold at the time 0 to T _S calculated in cast analysis software 21, the linear expansion coefficient and Young's modulus are physical properties of the mold material obtained separately input is, the amount of deformation due to heat of the mold which occurs during the time 0 to T _S is calculated. In parallel with this, the deformation analysis software 23 gives the casting temperature distribution calculated from the time T _S calculated by the casting analysis software 21 to the end of cooling, that is, until the casting reaches room temperature, and the casting material obtained separately. The linear expansion coefficient and Young's modulus, which are physical property values, are input, and the amount of deformation of the casting that occurs between time T _S and the end of cooling is calculated. Finally, the calculated amount of deformation due to solidification and cooling of the mold and casting is added to the cavity shape of the mold that was initially input, and the cavity shape is designed.

  FIGS. 4A and 4B are known thermal expansion diagrams for silica sand mixed with resin as the mold material and FCD 500 as the casting material, respectively. Tables 1 (a) and 1 (b) show the linear expansion rates of resin-mixed quartz sand and FCD500 at typical temperatures obtained from the thermal expansion diagrams of FIGS. 4 (a) and 4 (b), respectively. This is used as input data for the analysis software 22 and 23.

  FIGS. 5A and 5B are a stress-strain curve of the mold material obtained by the compression test at each test temperature and a stress-strain curve of the casting material obtained by the tensile test, respectively. Tables 2 (a) and 2 (b) show the Young's modulus of resin-mixed quartz sand and FCD500 at typical temperatures obtained from the stress-strain curves in FIGS. 5 (a) and 5 (b), respectively. This is used as input data for the software 22 and 23.

  FIG. 6 shows the radial deformation amount (for the radius) of the mold and the casting at each axial coordinate determined by the numerical analysis of the first embodiment. In FIG. 6, the value which added the deformation | transformation amount of a casting_mold | template and a casting is also shown, and a casting product shape is also shown collectively. In addition, this casting product shape anticipates a machining allowance of 2 mm. From this numerical analysis result, the amount of deformation of the mold becomes a negative value that narrows the cavity, and in the part where the screw part of the casting is cast, it is narrower at the valley part (small diameter part) of the screw than at the peak part (large diameter part) of the screw. The amount increases. Further, the amount of deformation of the casting also becomes a negative value that the diameter is reduced, and in the screw portion, the amount of diameter reduction is larger in the mountain portion (large diameter portion) than in the valley portion (small diameter portion). As a result, the value obtained by adding the deformation amount of the mold and the casting is a substantially constant negative value (shrinkage value) of about 3 mm for the radius and 6 mm for the diameter in the screw equivalent portion. In addition, although illustration is abbreviate | omitted, in this numerical analysis, the three-dimensional deformation amount of a casting_mold | template and a casting is calculated | required, and these axial deformation amounts also become a negative value.

  7 (a) and 7 (b) expand the cavity cross-sectional shape of the embodiment in which the mold is designed based on the value obtained by adding the deformation amount of the mold and the casting of FIG. Show. In each figure, as a comparative example, a cavity cross-sectional shape designed in consideration of only the deformation amount of the casting from the start of solidification to the end of cooling is also shown. In the comparative example in which the amount of deformation of the mold from pouring to the start of solidification is not taken into account, the shrinkage amount at the valley portion of the screw portion shown in FIG. 7B is expected to be small, and the cavity diameter at the valley portion is implemented. Designed smaller than the example. For this reason, the machining allowance of 2 mm is interrupted at the valley of the screw part, and the cast product may be a defective product with insufficient dimensions. On the other hand, in the example considering the amount of deformation of the mold from pouring to the start of solidification, the amount of shrinkage of the screw part is expected to be almost constant in both the crest and trough, and the casting product is in the trough. There is no risk of a defective product with insufficient dimensions, and the machining allowance can be reduced to increase the accuracy of the near net shape.

  FIG. 8 shows a numerical analysis procedure in the mold design method of the second embodiment. The basic procedure of this numerical analysis is the same as that of the first embodiment, and is caused by the static pressure of the molten metal poured into the deformation analysis software 22 for performing deformation calculation of the mold 1 shown in FIG. The difference is that the deformation of the casting mold 1 is taken into consideration. Others are the same as those of the first embodiment, and the screw compressor rotor 11 as a cast product shown in FIG. 2 is formed of FCD500, and silica sand mixed with resin is used for foundry sand.

That is, in the second embodiment, as shown in FIG. 9, the deformation due to the static pressure p acting on the mold 1 from the inside at each part of the cavity 2 is taken into consideration. When the depth from the molten metal surface A is z, the density of the molten metal is ρ, and the gravitational acceleration is 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 p (z) of each part obtained by the equation (1) is added perpendicularly to each contact point on the cavity surface of the finite element method model used in the deformation analysis software 22.

  FIG. 10 shows a value obtained by adding the deformation amount (radius) in the radial direction of the mold and the casting at each axial coordinate obtained in the numerical analysis of the second embodiment, and the deformation amount of these mold and casting. Show. FIG. 10 also shows the shape of the cast product with a machining allowance of 2 mm. In addition, the axial coordinate has the origin at the deepest position of the casting product in the mold, and the depth z from the molten metal surface becomes shallower as the axial coordinate becomes larger. Further, the deformation amount of the mold and the casting and the added value thereof in FIG. 10 are shown in an enlarged scale than FIG. 6 showing the numerical analysis result of the first embodiment.

  In FIG. 10 showing the numerical analysis result of the second embodiment in consideration of the deformation of the mold due to the static pressure p, the deformation amount of the mold is on the plus side compared to FIG. 6 showing the numerical analysis result of the first embodiment. There is a shift, and this shift amount is larger in a region where the axial coordinate is smaller and the depth z from the molten metal surface is deeper. Therefore, in FIG. 6, the deformation amount of the mold is a negative value that narrows the cavity in almost all portions where the screw portion is cast, whereas in FIG. 10, each peak portion (large diameter portion) of the screw ), It can be seen that it is a positive value to expand the cavity. Note that the amount of deformation of the casting is the same as that of the first embodiment. As a result, the absolute value of the negative value (shrinkage value) of the value obtained by adding the deformation amount of the mold and the casting at the screw equivalent portion is smaller than that in FIG. Net shape accuracy can be further increased.

  In the embodiment described above, the cast product to be cast is a rotor for a spheroidal graphite cast iron screw compressor. However, the mold design method and the mold according to the present invention are limited to casting of a spheroidal graphite cast iron casting. It can also be used for casting gray iron or steel, and can also be used for casting non-ferrous metals such as aluminum. Also, the casting product is not limited to the rotor for the screw compressor, and in particular, a casting product having a large dimension with a length dimension and an outer diameter dimension of 200 mm or more, and a casting having a large diameter part and a small diameter part in the axial direction. Suitable for casting of products.

DESCRIPTION OF SYMBOLS 1 Mold 1a Main mold 1b Core 2 Cavity 3 Pressurizing part 4 Pouring part 5 Pouring part 5 Runner 11 Screw compressor rotor 11a Screw part 21 Casting analysis software 22, 23 Deformation analysis software

Claims (5)

  In a mold design method for designing a cavity shape of a mold for casting a molten metal by casting based on numerical analysis, numerical analysis is performed for deformation due to heat of the mold from when the molten metal is poured to when solidification starts. The shape of the casting from the start of solidification to the end of cooling is determined by numerically analyzing the deformation due to solidification and cooling from the start of solidification to the end of cooling. A mold design method characterized by obtaining a change amount and designing the mold cavity shape based on the obtained mold cavity shape change amount and casting shape change amount.   The mold design method according to claim 1, wherein a length or an outer diameter of the casting is 200 mm or more.   The mold design method according to claim 1 or 2, wherein the casting has a large diameter portion and a small diameter portion in the axial direction.   The mold design method according to any one of claims 1 to 3, wherein temperature dependence is taken into consideration for a physical property value of a mold material used for numerical analysis of deformation due to heat of the mold.   The mold design method according to any one of claims 1 to 4, wherein a shape change amount of the mold cavity is obtained in consideration of deformation of the mold caused by a static pressure of a molten metal poured into the mold.

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