ADVANTAGES OF GREEN SAND MOLDING PROCESS

USING AERATION TECHNOLOGY

Hiroyasu MAKINO, Shuichi TSUZUKI, Minoru HIRATA

SINTOKOGIO, LTD., AICHI, JAPAN

ABSTRACT

Demands for near-net-shape iron castings become much stronger in order to

save resource and energy consumption. In order to make a high quality mold to

satisfy such demands, a molding process using low air pressure “Aeration” sand

filling, which can make the uniformly dense molds with lower energy

consumption, is becoming popular worldwide. In actual casting production, it has

been proven that the aeration technology provides several advantages such as,

excellent sand filling, high strength and uniformly dense mold, superior energy

saving, low noise level, less pattern wear, etc. This study provides some

valuable information for green sand molding process.

KEYWORDS

Green Sand Molding, Aeration Sand Filling, Computer Simulation, Friability,

Sensor

1. INTRODUCTION

World casting production in 2012 exceeded 100 million tons. Over 80 % of

production was iron castings and made with sand molds. Green sand molding

technology plays an important role in the iron casting production. Demands for

near-net-shape iron castings become much stronger in order to save resource

and energy consumption. In order to make a high quality mold to satisfy such

demands, it is critical to choose the most optimum green sand molding process

which achieves making uniformly dense molds with lower energy

consumption1-8.

Recently, a molding process using low air pressure “Aeration” sand filling1, which

can make uniformly dense molds with lower energy consumption, is becoming

popular worldwide. This molding process consists of the following two steps.

1) Aeration sand filling: Green sand under aeration fills the flask smoothly and

gently. Accordingly, the bridge-forming phenomenon, which often happens at

some regions with complicated pattern geometry and at the entrance of a deep

pocket, is minimized.

2) High pressure squeezing: Sand molds are finally compacted by high pressure

squeeze. The segment squeeze head is preset to contour the pattern geometry

for flask tight molding machine. Accordingly, the mold back is formed flat after

squeezing.

In actual casting production, it has been proven that the aeration technology

provides several advantages such as, excellent sand filling, high strength and

uniformly dense mold, superior energy saving, low noise level, less pattern wear,

etc.

In this paper, the green sand molding process was studied with experiments and

numerical simulation4-8. A mathematical model of aeration sand filling based on

two-phase flow continuous model was applied to actual production scale mold.

Successively, a finite element method was applied for squeezing. It was clarified

that the aerated sand flows smoothly by using a high speed video camera and a

developed sand detecting sensor. Furthermore, it had been found that aeration

sand filling can make less friable and stronger mold14-17 which was evaluated by

AFS (American Foundry Society) friability test and modified cone-jolt test. This

study provides some valuable information for green sand molding process.

2. GREEN SAND MOLDING PROCESS WITH AERATION TECHNOLOGY

The developed green sand molding process consists of two steps, namely,

“aeration sand filling” and “high pressure squeeze”. Both of two steps are

evaluated by fundamental experiment and computer simulation.

2.1. MATHEMAICAL MODEL FOR AERATION SAND FILLING

The green sand filling with air flow is very complicate behavior. In order to

analyze the behavior of molding sand during molding, a computer simulation is

performed for aeration sand filling. In the mathematical model5-8 for aeration

filling processes, the Euler two-phase flow model has been taken as the basis of

design. In this model, both gas phase and solid phase have been assumed as

continuum, i.e., they have been treated as air flow and sand flow respectively.

In the governing equations of mathematical model, the continuity equations and

momentum equations of gas phase and solid phase have been treated

separately. All of the sand particles are considered to be identical and

characterized by a mean diameter and a mean density. To describe the collision

between different sand particles, a typical kinetic theory model for multiphase

flow is built based on the Gidaspow9-10. The mass and momentum balance

equations for the gas phase and solid phase are solved by FDM (Finite

Difference Method). In these equations, the dependent variables are the gas

volume fraction, the solid volume fraction, the gas density, the velocity vectors of

the gas and sand flow.

Continuity equation of the gas phase:

0

ggggg V

t

(1)

Momentum equation of the gas phase:

gsgggggggg VVVVV

t

(2)

The gravity force of air is neglected because it is very small compared with the

stress of air.

Continuity equation of the solid phase:

0

sssss V

t

(3)

Momentum equation of the solid phase:

sssssss VVVt

gVV sssgs (4)

where, is the Hamilton Operator, g is the volume fraction of gas phase,

s is the volume fraction of solid phase, namely,

1 sg . g is the density

of gas phase, s is the density of pure sand particle, gV is the velocity vector

of gas flow, sV is the velocity vector of sand flow, is the drag coefficient, g

is the stress tensor of gas flow which is expressed by Sinclair’s model11, s is

the stress tensor of solid phase which is expressed by Miller’s model10, g is the

gravity acceleration.

2.2. MATHEMATICAL MODEL FOR SQUEEZE

In the squeeze molding simulation, a commercial software, which is based on

FEM (Finite Element Method), is applied. The nonlinear elastic material model is

used for squeeze molding simulation. There are three types of nonlinearity

existing simultaneously in a normal compaction process.

1) Mechanical response nonlinearity: The strain-stress curves obtained

experimentally for four green sands with different density7-8 are shown in Fig.

1. It seems that the stress increases exponentially with strain, and a large

deformation of 30%-40% can be obtained upon the completion of a compact

process.

2) Structural nonlinearity: Molding sand is composed of discrete silica sand

grains covered with bentonite/water layer and air voids, as schematically

shown in Fig. 2. For such material, the internal friction angle and cohesion are

two major factors, which are usually used to represent the features of material.

Subject to an external force, the air voids existing among molding sand are

squeezed out. Consequently, the deformation of mold sand is plastic.

3) Contact nonlinearity: External friction which exists between sand and flask

influences the compaction effect significantly.

Figure 1 : The stress-strain curve of squeezing process.

Figure 2 : The composition of molding sand.

2.3. EXPERIMENTAL AND SIMULATION CONDITION

Experiment and computer simulation are performed for flask-less molding

process. Figure 3-A) shows geometrical model for the present study. The

dimension of the flask is 608mm×510mm×260mm. Pattern plate has three

sleeves with inner diameter of 70mm and height of 110 mm. There are vents at

the bottom of the sleeve. The direction of sand flow is nearly perpendicular to the

opening of the sleeve. Figure 3-B) shows a symmetrical half part for molding

process.

A) Integrated geometrical model B) Symmetrical half part

Figure 3 : Geometrical model of molding process.

silica sandbentonite/water

air

2.4. EXPERIMENTAL AND SMULATION RESULTS

The experimental and simulation results for aeration sand filling are shown in Fig.

4. In this case, it is quite difficult to fill the horizontal sleeve completely. However,

molding sand flow smoothly into the three sleeves by the effect of low pressure

aeration. The simulation behavior is very similar to the experimental one.

Experiment

Simulation

time 0.05s 0.15s 0.25s 0.45s 0.80s

Figure 4 : Experimental and simulation results for aeration sand filling process.

Figure 5 shows the mold strength distribution experiment and simulation after

squeezing. The experimental results are measured using actual production

scale flask-less molding machine. The simulation results are very similar to the

experimental one. However, the experimental value at the corner is higher than

the simulated one because the actual flask has tapered corner.

Experiment Simulation

Parting

plane

Cross

section

Figure 5: Mold strength distribution experiment and simulation after squeezing.

Figure 6 shows the mold strength distribution experiment and simulation after

squeezing for actual brake drum pattern. The mold strength on the parting line is

almost unique in the both cases.

Figure 6 : Mold strength distribution experiment and simulation after squeeze for

brake drum pattern .

3. ANALYZE OF SAND FILLING BEHAVIOR WITH SAND DETECTION SENSOR12

It is essential to know sand filling behavior and bulk density after sand filling.

Generally, it is impossible to measure the bulk density inside mold during sand

filling. Therefore, a sand sensor has been developed in order to detect the sand

filling behavior.

3.1. SAND DETECTION SENSOR FOR SAND FILLING

Purpose of developing sand detection sensor is to know when green sand

arrives at pattern plate, especially, in the deep pocket. A conventional

technology for sand detection is measuring sand contact pressure. The green

sand behavior during squeeze has been evaluated by a sensor developed by J.

Bast et al.13. However, the sand contact pressure during aeration sand filling is

too weak to detect the pressure with this sensor. Furthermore, the pressure

sensor measures not only sand contact pressure but also air pressure. Recently,

a new sand detection sensor has been developed by electrical technology to

detect sand contact pressure during aeration sand filling.

View Experiment Simulation

Axonometric

drawing -

Parting

plane

Figure 7 shows schematic illustration of developed sand detection sensor which

consists of electrodes and amplifiers. When sand particles enter between

electrodes, the electric circuit is closed. The voltage is amplified because the

conductivity of the green sand is extremely low.

Figure 7 : Schematic illustration of developed sand detection sensor.

3.2. EXPERIMENTAL CONDITIONS

Actual production scale molding machine is used to verify this developed sensor.

Experimental conditions are shown in Table 1. The flask size is 508 x 610 mm.

Flat pattern and sleeve pattern are used to evaluate the sand filling behavior.

The sleeve pattern has three sleeves along the center. Inner diameter of sleeve

is 70 mm. Heights of sleeve is 70, 90 and 110 mm. Schematic illustration of

sleeve pattern is shown in Fig. 8. Aeration pressure during sand filling is

between 0.08 and 0.12 MPa. Squeeze pressure is between 0.8 and 1.0 MPa.

The green sand is adjusted to 35 % compactability.

Table 1 : Experimental conditions.

Flask Size 508 x 610 mm

Test Pattern Flat

Sleeve (Ø70 x H70, 90, 110 mm)

Aeration Pressure 0.08-0.12 MPa

Squeeze Pressure 0.8-1.0 MPa

Compactability 35 %

Figure 8 : Schematic illustration of test pattern with sleeves.

3.3. EXPERIMENTAL RESULTS

Figure 9 shows a green sand mold made by actual production scale molding

machine in case of 110mm height sleeve pattern.

Figure 9 : Green sand mold made by actual production scale molding machine.

Figure 10 shows measured results of No. 3 sand detection sensor and aeration

pressure. Aeration pressure is 0.12 MPa in this condition. This No. 3 sand

detection sensor is located at the bottom of No. 3 sleeve which is lower position

during sand molding. Before sand filling, the sand detection sensor shows 9

Volts. After aeration sand filling starts, the green sand particles contact the

electrodes and electrical circuit is closed. Therefore, the sand detection sensor

shows 2.5 Volts. However, the sand detection sensor shows 9 Volts again after

sand particles stop because a contact force between the green sand particles

and electrodes is very weak. Then, the sand detection sensor shows 2.5 Volts

again during squeeze. It is clarified that this developed sand detection sensor

can detect the green sand particles quickly.

Figure 10 : Measured result of No. 3 sand detection sensor and aeration

pressure curve.

3.4. SAND ARRIVAL TIMES

The green sand filling behavior is evaluated by the developed sand detection

sensor. Three sand detection sensors are used for aeration sand filling

experiment. No. 1, No. 2 and No.3 sand detection sensor is attached at the

bottom of upper sleeve (No. 1), middle sleeve (No. 2) and lower sleeve (No. 3),

respectively.

Figure 11 shows measured results of sand detection sensors which are located

at the bottom of three sleeves. Sand arrival times at each sleeve are measured

by sand detection sensors. The aeration sand filling behavior is observed with

video camera and shown in Fig. 4. The green sand arrives at the bottom of lower

sleeve (No.3 ) at around 0.25 sec which is very close to measured result by the

sand detection sensor. Then, the green sand arrives at the bottom of middle

sleeve (No.2 ) at around 0.5 sec and the sand filling is completed at 0.8 sec.

These observed results are also similar to measured resuts by the sand

detection sensors shown in Fig. 11. This result shows the sand filling behavior

can be evaluated by the developed sand detection sensor.

Figure 11: Measured results of sand detection sensors at the bottom of sleeves.

Figure 12 shows comparison of sand arrival times at the bottom of sleeves in

each sleeve height. Sleeve heights are 110, 90 and 70mm. Sleeve height 0 mm

in Fig. 12 means that flat pattern is used for sand filling experiment. The sand

arrival times at the bottom of lower sleeve (No.3) are about 0.2 sec. in all

conditions. Then, the green sand arrives at the bottom of middle sleeve (No. 2)

between 0.4 and 0.7 sec. Finally, the sand filling is finished less than 1.0 sec. in

all conditions. Therefore, the behavior of aeration sand filling is not much

affected by pattern shape. Namely, the green sand flows very smoothly by

aeration sand filling.

Figure 12: Comparison of sand arrival times at the bottom of sleeves in each

sleeve height.

4. GREEN SAND PROPERTY AFTER AERATION SAND FILLING

The green sand properties after aeration sand filling are evaluated by the AFS

standard sand tests. Sub-round Michigan Lake sand is used for fundamental

experiments. Schematic illustration of fundamental aeration sand filling

equipment is shown in Figure 13. Figure 14 shows experimental pattern plate

arrangement with AFS specimen tubes. Conventional gravity sand filling into

AFS specimen tube is also performed to compare the aeration sand filling.

Figure 13 : Schematic illustration of fundamental aeration sand filling equipment.

Green Sand Tank

Aeration Filter

Air Inlet Air Inlet

Air Inlet

Air Inlet

Air Outlet

Air Outlet

Pressure Sensors

A) AFS Specimen Tubes

Mounted in Plate

B) Half-Section Sleeve

on Acrylic Cover

Figure 14 : Experimental pattern plate arrangement with AFS specimen tubes.

4.1. FRIABILITY

Experimental results of green sand property after aeration sand filling and

conventional gravity sand filling are shown in Table 2. The properties of green

sand after aeration sand filling are very close to the ones after gravity sand filling

at each compactability except friability14. Figure 15 shows friability after aeration

sand filling and gravity sand filling using several type of sand grain such as,

sub-round Michigan Lake sand, round grain sand, olivine sand, proppant

(synthetic) sand and chromite sand. The friability after aeration sand filling is

lower than the one after gravity sand filling at each sand grain. In generally,

friability is higher at lower compactability. However, the friability after aeration

sand filling at lower compactability is less than 10%. This result means that the

green sand mold17 after aeration sand filling reduces casting defects despite of

low compactability.

Table 2 : Properties of green sand after sand filling.

AFS Test Aeration Filling Gravity Filling

Compactability (%)

30

(0.2)

35

(0.2)

40

(1.0)

30

(1.0)

35

(1.5)

40

(1.0)

Bulk Density (g/cc)

0.99

(0.01)

0.99

(0.01)

0.89

(0.02)

0.98

(0.02)

0.89

(0.01)

0.88

(0.04)

Moisture content (%)

2.5

(0.02)

2.5

(0.01)

2.5

(0.01)

1.9

(0.14)

2.5

(0.05)

2.7

(0.10)

Specimen weight (g)

160

(1.0)

158

(1.0)

158

(0.5)

158

(1.0)

158

(1.5)

152

(0.5)

Permeability (#)

220

(1.0)

223

(1.5)

228

(1.2)

228

(2.0)

233

(1.0)

261

(1.0)

Splitting Strength (psi)

3.4

(0.12)

5.4

(0.20)

5.1

(0.10)

3.9

(0.05)

5.4

(0.10)

5.0

(0.09)

Compression Strength (psi)

17

(1.2)

24

(1.0)

25

(1.2)

21

(0.2)

24

(0.5)

24

(0.1)

Mold Hardness (#)

93

(1.5)

94

(1.0)

97

(0.5)

92

(1.0)

94

(1.2)

96

(1.1)

Friability (%)

9

(1.0)

7

(0.5)

5

(0.2)

31

(3.0)

17

(2.0)

10

(1.0)

Note: Italicized numbers in parenthesis indicate standard

deviations.

Figure 15 : Friability after aeration sand filling and gravity sand filling.

5

10

15

20

25

30

30 35 40 30 35 40

Gravity Sand Filling Aeration Sand Filling

Compactability [%]

Fri

ab

ility

[%

]

Lake sand

Round Grain

Olivine

Proppant

Chromite

4.2. MODIFIED CONE JOLT TEST

The modified cone jolt test16-17 is used to compare gravity and aeration filled

green sand in ultimate strength, stiffness, and toughness (Tables 3 and 4).

Above 35% compactability, there is negligible gain in ultimate strength for the

aeration green sand filling system. Please note the authors identify a

discriminant of 5 jolts to be considered significant. Interestingly, lower

compactability levels shows lower stiffness and toughness regardless of the

green sand filling system. Though there are trends between gravity and aeration

filled green sand at various compactability levels, there are differences observed

between the filling systems.

Table 3 : Mechanical properties of the gravity filled green sand at various

compactability levels.

Compactability

(%)

Ultimate Strength

(# Jolts)

Stiffness

(Jolts/in)

Toughness

(Jolt*in)

28 14 256 0.21

35 21 333 1.48

35 (1.0 MPa) 54 760 2.05

40 29 714 0.49

Table 4 : Mechanical properties of the aeration filled green sand at various

compactability levels.

Compactability

(%)

Ultimate Strength

(# Jolts)

Stiffness

(Jolts/in)

Toughness

(Jolt*in)

28 18 286 0.30

35 29 417 1.64

35 (1.0 MPa) 61 835 2.47

40 28 667 0.55

At 35% compactability, the gravity filled sand specimen cracked while the

aeration filled sands shows no flaw as shown in Fig. 16. At 40% compactability,

the modified cone jolt test shows that gravity and aeration filled green sand

specimen are similar in ultimate strength, stiffness, and toughness. The cone jolt

specimens for gravity and aeration filled green sand show no flaw after testing.

Therefore, these specimens were friability tested. There is lower friability in the

aeration filled green sand and this is consistent.

Figure 16 : Cone jolt specimens at 35% compactability alternating aeration (no

crack) and gravity (crack).

A key observation about the difference in gravity and aeration filled green sand

specimens is identified when the squeeze pressure for the standard 2 inches by

2 inches specimen preparation is increased from 0.8 MPa to 1.0 MPa. Higher

squeeze pressure improves ultimate strength, stiffness, and toughness in the

green sand system. However, aeration filled green sand is significantly greater in

ultimate strength, stiffness, and toughness compared to the gravity filled green

sand. (Fig. 17 and Tables 3 and 4)

Figure 17 : Number of Jolts versus Compactability.

5. DEVELOPMENT OF MOLDING MACHINE1)

Various types of molding machines are developed using aeration technology

shown in Figure 18. These molding machines cover from low production to high

production and from small flask size to large flask size which satisfies demands

of foundry shops.

aeration gravity aeration gravity

10

20

30

40

50

60

70

28 35 35(1.0MPa) 40

Jo

lts

Compactability

Aeration

Gravity

1) 2) 3) 4)

Figure 18 : Actual molding machine using aeration technology.

1) tight flask molding machine

2) flask-less molding machine - two station

3) flask-less molding machine - single station

4) flask-less molding machine - small flask size(450x350mm)

5. CONCLUSION

Aeration sand filling technology in green sand molding process is evaluated from

the view of sand filling process and mold properties after sand filling. The results

are summarized as follows.

1) Aeration sand filling is numerically simulated with the two-phase flow theory

and the mathematical model is built with kinetic theory. The calculated

behavior of aeration sand filling is in agreement with the experimental results.

The final squeeze compaction process is calculated with Finite Element

Method. The mold strength can be numerically simulated. The results of

simulation are compared with the experimental results. Through these

experiment and simulation, aeration sand filling is effective for sleeve pattern

and can make highly strong and uniformly dense mold by lower air

consumption.

2) The sand detection sensor, which is developed using electrical technology,

can evaluate sand filling behavior. The behavior of aeration sand filling is not

much affected by pattern shape. Namely, the green sand flows very smoothly

by low pressure aeration.

3) Friability by aeration sand filling is lower than that of conventional gravity sand

filling, especially in case of lower compactability. Modified cone jolt test

evaluates the difference of aeration and gravity filled sand. Mechanical

properties of the aeration filled green sand mold are higher than those of the

gravity filled green sand mold, especially, at low compactability.

ACKNOWLEDGMENT

The authors gratefully acknowledge to Prof. WU Junjiao at Tsinghua University,

China for computer simulation of sand molding. They also gratefully

acknowledge Dr. Hartmut Polzin and Dr. Matthias Strehle at TU Bergakademie

Freiberg, Germany for developing sand detection sensor. They also gratefully

acknowledge Dr. Sam Ramrattan at Western Michigan University, USA for

fundamental testing of aerated green sand.

REFERENCES

1) M. Hirata, H. Makino, and Y. Hadano, “Green Sand Filling Using Aeration”

Journal of Japan Foundrymen’s Society, vol. 73 (2001) pp436-440

2) Y. Maeda, H. Nomura and K. Terashima, “Numerical Simulation on Mold

Making by Blowing Green Sands into Cavity” Proc. of the Technical Program,

Powder & Bulk Solids Conference, Chicago (1992) 149

3) H. Makino, Y. Maeda, and H. Nomura, “Force Analysis in Air-Flow Press

Moulding using the Distinct Element Method” International Journal of Cast

Metals Research, vol 10, (1997) pp171-175

4) H. Makino, M. Hirata and Y. Hadano, “Computer Simulation and Optimization

of Sand Filling Using the Distinct Element Method” WFO Technical Forum on

GIFA2003, Dusseldolf (2003) session 5.1

5) J. Wu ,H. Li, W. Li, H. Makino, T. Hanai and M. Hirata, “Computer Simulation

of Sand Molding Using Blow Filling and Aeration Filling” AFS Trans. 112

(2006) pp419-427

6) J. Wu ,H. Li, W. Li, H. Makino and M. Hirata, “Two Phase Flow Analysis of

Aeration Sand Filling for Green Sand Molding Machine” WFO International

Foundry Research 60(2008)No.1, pp20-28

7) H. Li, J. Wu, T. Huang and H. Makino, “A new numerical simulation model for

high pressure squeezing moulding” Asia Foundry Congress, China (2011)

pp25-29

8) C. Ni, Q. Zhang, T. Jing, J. Wu and H. Makino, “3D Numerical Simulation of

Aeration Sand Filling-High Pressure Squeeze Molding Method” 71th World

Foundry Congress, Spain (2014)

9) D. Gidaspow : Multiphase flow and fluidization (Academic Press, Inc.) (1994)

10) A. Miller and D. Gidaspow : AIChE J. 38 (1992) 11, 1801

11) J. L. Sinclair and R. Jackson : AIChE J. 35 (1989) 9, 1473

12) M. Strehle, H. Polzin and H. Makino, “Sensor for the Detection of

Sand-Filling-Process in Aeration Molding Machines” 71th World Foundry

Congress, Spain (2014)

13) J. Bast, A. Malashkin and T. Hahm, “Optimizing Molding Parameters for

Green Sand Compaction”, AFS Transactions, Paper 05-008 (2005)

pp309-318

14) S. N. Ramrattan, A. M. Paudel, H. Makino and M. Hirata, “Desirable Green

Sand Properties via Aeration Sand Filling,” AFS Transactions, Paper 08-128

(2008) pp493-503

15) S. Ramrattan, A. Paudel, K. Nagarajan, R. Bharadwaj, H. Makino & M. Hirata

“Test Casting Evaluation of Aeration Green Sand Molds” 69th World Foundry

Congress, China (2010) pp679-686

16) S. Ramrattan, K. Nagarajan, A. Oman, H. Makino, M. Hirata, S. Takasu and

H. Miyazaki, ” Using a Modified Cone Jolt Test to Discriminate between

Aeration and Gravity Filled Green Sands” 70th World Foundry Congress,

Mexico(2012)

17) S. Ramrattan, H. Makino, M. Hirata, S. Takasu, and H. Miyazaki “A Rational

for Determining the Benefit of Aeration Green Sands Filling” 71th World

Foundry Congress, Spain (2014)