Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
1Optimum Mold Structure Design Technology Utilizing
Sand Mold Additive Manufacturing

SATOSHI NAKATSUKA*1 YOICHI TAJITSU*2

KANG JINWU*3

The material characteristics of sand castings are affected by the cooling velocity duringcasting. Conventionally, the cooling velocity could not be increased because of the limited moldstructures, and manufacturing of high strength and large/thinned castings had its limit. On theother hand, additive manufacturing technologies have been improved in recent years, and it isbecoming possible to manufacture molds with a high degree of freedom in shape. By utilizing thesetechnologies, Mitsubishi Heavy Industries, Ltd. (MHI) developed a mold design technology thatallowed production of castings having characteristics that could not be obtained in the past. This
report describes the details and application effects of the developed technology.

|1. Introduction
In recent years, additive manufacturing technologies have been improved and the degree offreedom in mold shape has been increasing. Figure 1 shows the outline of the mold manufacturing
process using the additive manufacturing technology. First, sand, which is a mold material, isspread by one layer on the table and a binder is printed to the parts to be solidified. Next, the table
is lowered by one layer and a new sand layer is spread. These steps are repeated to laminate sandlayers. After the completion of the lamination, sand in the non-binder printed portions is removed
from the molding body and a mold is completed. By this process, a sand mold having a shape that
could not be easily formed by the conventional method can be made.

Figure 1

Outline of the sand mold additive manufacturing process(1)It is considered that when this technology is used, it becomes possible not only to prevent
casting defects but also to control the cooling velocity during casting by manufacturing a mold
*1

Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Research Manager, Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi Heavy Industries,*3

Associate Professor, School of Materials Science & Engineering, Tsinghua University.

Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
2having an optimal shape according to the casting shape, realizing increased strength and thinning ofcastings. MHI did not have a mold design technology for that purpose. In addition, although theadditive manufacturing technology has the above-described characteristics and further eliminates
the process of making wooden molds, significantly reducing the lead time, it has not been fully
utilized because it requires special sand having a high unit cost, and so on.
Accordingly, we developed a mold design technology for increasing the cooling velocity ofcastings while reducing mold damage and casting defects. We also conducted prototype tests forcastings and evaluated the effect of reducing the special sand and the possibility of increasing thehardness of castings when the additive manufacturing molds are applied.

|2. Development of mold design technology
MHI has developed the FT-Hollow software as a design tool in collaboration with Tsinghua
University. Figure 2 shows the flow chart of the mold design. First, a 3D shape model representing
a product shape and the parts designed for casting such as a feed head and a sprue runner areprepared. Next, the kind of material and the major thicknesses of a casting to be produced are set,
and according to the settings, an initial design model for the mold shape is output with reference tothe mold thickness data shown in Figure 3. Figure 3 shows the relationship between the casting
thickness and the required mold thickness for cast iron and aluminum alloy as an example. The
mold temperature which varies by the casting thickness is obtained through analysis and therequired mold thickness is determined from the actual measured values of the varying moldstrength which varies by the mold temperature. The mold strength is affected by the solidus
temperature in addition to the specific gravity of the casting material.
When the actual casting conditions are input for the output initial design model, the presenceor absence of mold damage during a casting test is evaluated based on the hydrostatic pressure, thebuoyancy and the calculated residual strength of the mold obtained from the heat-transfer analysisresult. The mold strength is temperature dependent as shown in Figure 4, and as the temperature
becomes higher, the strength tends to become lower. Therefore, the mold strength is evaluated inconsideration of the temperature change during a casting test and if necessary, readjustments suchas increase of the mold thickness are conducted so that the mold strength can be secured. When thisdesign flow is repeated, a thin-walled mold that is not damaged during a casting test can bedesigned.

Figure 2

Flow chart of the mold designMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
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Figure 3
Casting thickness
required mold thickness

andFigure 4
Temperature dependency of moldstrength|3. Trial design of additive manufacturing sand molds
In order to evaluate the appropriateness of the design flow, we made a trial design of molds
for a casting having a simple shape. Figure 5 shows the casting shape and the designed moldshapes. Nodular graphite cast iron (FCD450) was applied as the casting material.
The block mold has a conventional mold shape. The lattice mold is designed according to theflow in Chapter 2, in which sand is eliminated from parts except the parts requiring sand in view ofstrength, and it uses less sand than the block mold. In order to conduct comparative verification in
the casting test described later, we also output an excessively thinned lattice mold which wasassumed not to resist the hydrostatic pressure and buoyant force of the molten metal.

Figure 5

Shapes of the casting and the molds for trial production

Figure 6 shows the results of the damage assessment of the designed lattice mold. We
conducted a heat-transfer analysis of the lattice mold using casting analysis software and confirmedthat most of the casting surface was solidified in about 30 minutes, and at the same timing, thedamage evaluation was conducted. As a result, it was evaluated that in the area having themaximum hydrostatic pressure, the residual strength value of the mold was high and there was nomold damage risk. We also conducted the same evaluation at other timings and confirmed that at
any timing, the residual mold strength was higher than the hydrostatic pressure and the buoyant
force and no mold damage would occur.
The sand usage reduction effect of the molds designed according to this flow is as shown in
Table 1. The usage of sand can be reduced by 62% and the mold material cost can be also reducedin the same way.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
4Figure 6 Assessment result of the casting and the mold during the casting test (about30 minutes after pouring of the molten metal)
Table 1

Sand mold weight and sand mold weight reduction rate

Kind of mold
Block mold
Lattice mold
Lattice mold (thinned)

Mold weight (kg)
665
251.6
75.6

Sand mold weight reduction rate (%)
062%
89%

|4. Casting test using additive manufacturing sand molds
We actually conducted a casting test using the output mold shapes. In order to evaluate thedifference in the cooling effect between molds having different designs, we also performed forced
cooling by air blow to a specific area of each mold. In the same way as in Chapter 3, nodular
graphite cast iron (FCD450) was applied as the casting material.
Figure 7 shows the external appearances of the molds after the pouring of the molten metal.
We confirmed that casting could be performed without spilt molten metal not only by the block
mold but by the lattice mold (the red hot part on the top face of the mold is the molten metal that
overflowed the run-off part at the time of pouring). The intended shape of casting was obtained. On
the other hand, with the excessively thinned lattice mold, there was no abnormality immediately
after the pouring of the molten metal, but a leakage of the molten metal occurred within 10 minutes
after pouring. This was a phenomenon predicted by the mold damage evaluation. It is consideredthat the temperature of the external surface of the mold increased, the binder was lost, the residual
mold strength was reduced and the mold could not resist the hydrostatic pressure of the moltenmetal. From this result, it can be said that the output lattice mold has a shape securing the required
mold strength and also reducing the sand mold weight.

Figure 7

External appearances of the molds after the pouring of the molten metal

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
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Figure 8 shows the hardness of the casting surface at the force-cooled portion on each of theblock mold and the lattice mold. No increased hardness was observed at the force-cooled portion ofthe block mold, while the lattice mold showed the tendency of increased hardness by forced
cooling and its hardness increased by 6% compared to that of the block mold. That is, we could
confirm that the increase of the cooling velocity in addition to the thinning of the mold could
increase the strength of the casting.

Figure 8 Evaluation result of the hardness ofthe forced-cooled portion|5. Conclusion
We developed a mold design technology that allowed casting without mold damage andmade efforts to increase the strength of castings produced by the additive manufacturing
technology. By appropriately thinning a mold and forming it into a lattice shape, we could
reduce the usage of expensive special sand for additive manufacturing by 62%, and by applying
external cooling by air blow or the like, we achieved the increase of the casting strength by 6%.
In the future, we will promote application of this technology to small-lot and complicatedcasting shapes which MHI Group deals with.

References(1)

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