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Monitoring of metal powder-binder mixing process by eddy-current sensor
Journal of
ELSEVIER
Journal of Materials Processing Technology 48 (1995) 765-770
Materials Processing Technology
M o n i t o r i n g o f M e t a l P o w d e r - B i n d e r Mixing Process by E d d y - C u r r e n t S e n s o r S. Miyazawa, Y. Usui, H. Yoshida and Y. Murakoshi Mechanical Engineering Laboratory, 1-2 Namiki, Tsukuba-shi, 305 Japan Metal Injection Molding (MIM) is a technology by which metal products of high Jensity and complex shape can be mass-produced in a cost-effective manner. The raw input to the MIM process is a compound consisting of metal powder and thermoplastic binder. The distribution of metal powder throughout the compound must be uniform if high-quality products are to be obtained. This paper describes a new method for monitoring the metal powder distribution using an eddy-current sensor. Experimental sensor readings were gathered using two different compounds, each with a different binding agent. Analysis of the data showed clearly that the sensor output was proportional not to the powder to binder ratio at the sensor location, as originally expected, but rather to the metal powder contents per unit volume. This phenomenon may have arisen from the fact that the density of the compound varies with the powder to binder ratio. A series of sensor readings taken at regular time intervals was used to monitor the overall distribution of metal powder within the compound. The merit of this technique is its reliance on a simple and inexpensive sensor probe. The drawback is that the sensor measurements are affected by the mixer's metal rotor and by temperature drift. The effects of temperature drift can be compensated by monitoring the temperature of the compound. The interference of the metal rotor was an obstacle to in-process monitoring in the present research. In the future, in-process measurements may be achieved by changing the shape of the mixing container from a "bowl" to a circular channel, in which the sensor will be able to measure the compound at a safe distance from the rotor. 1. INTRODUCTION Metal Injection Molding (MIM) is a technology by which metal products of high density and complex shape can be mass-produced in a costeffective manner [1-5]. The raw input to the MIM process is a compound consisting of metal powder and thermoplastic binder. The distribution of metal powder throughout the compound must be uniform if high-quality products are to be obtained. Otherwise, variations in shrinkage rate may cause cracks to occur during the sintering process. Therefore, it is very important to monitor the mixing condition, the overall distribution of metal powder within the compound, throughout the mixing process. Two techniques have been developed for monitoring the mixing condition, one based on mixing torque [6] [7] and the other on the strength
of light reflections [8]. The mixing torque method has been used to monitor the mixing condition for polymer wax/ceramic powder compounds. As the powder surface becomes covered with wax, the frictional force decreases, leading to changes in the mixing torque. This method is best suited to measurement of the mixture's macroscopic properties. The light reflection method has been applied to compounds consisting of white aluminum oxide and black silicon carbide. Light reflection may be a less sensitive index to the MIM mixing condition than mixing torque, since the binder is colorless and transparent. This research was based on a conjecture that the eddy-current loss would be proportional to the metal powder to binder ratio when the sensor's high frequency oscillation coil was put into the compound. If this were indeed the case, the mixing condition could be monitored using a simple and
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0924-0136(94)01719-H
S. Miyazawa et al. / Journal of Materials Processing Technology 48 (1995) 765-770
766
inexpensive sensor probe. On the other hand, the effects of sensor temperature drift at high compound temperatures would have to be investigated.
(V) Q5 I
2 ?
2. M E A S U R E M E N T M E T H O D
Sens,
Figure 1 shows the experimental setup used to take sensor measurements. The eddy-current sensor used in the experiment was a typical commercial variety having four components: a coil (sensor probe, diameter: 5.4mm), an oscillator, a detector, and a linearlizer. The eddy-current circuit oscillates at 300kHz when there is no metal near the probe. When metal approaches the probe, an eddy-current is induced in the metal, causing the impedance of the probe to increase and the oscillating amplitude of the eddy-current circuit to decrease. An offset adjustor and a reversing amplifier were added to the sensor unit to make its output proportional to the eddy-current in the metal.
~C)ff-setshiflor~[ Amplifier }-~-Output
t Linearlizer l~
Detector ~[
Oscillator
Sensorprobe ~ ' (Co11)
Fig.1 Experimentalsetup. The m e a s u r e m e n t range of the sensor was established using the following technique: several sensor readings were taken with the sensor probe at various distances from a metallic ball (SUS304).
--[ Fig.2 Measurementrange. The relationship between the sensor output and the sensor probe to metal ball distance is shown in Figure 2. The measurement range of the sensor probe used in the experiment was taken to be a hemisphere of 6-8mm radius about the sensor probe. The eddy-current sensor was used to monitor the mixing condition for two c o m p o u n d s , each consisting of metal powder (SUS304, particle diameter:6~n) and a binding agent (grease or thermoplastic binder). The grease/metal powder c o m p o u n d could be m e a s u r e d over a large temperature range. However, the thermoplastic MIM compound (1800g of metal powder and 200g of thermoplastic binder) could be measured only at a high temperature, since at room temperature it was difficult to insert the sensor probe. This presented a problem, since the zero points of the sensor output depended upon the temperature in an unknown manner. Before data was collected for the MIM compound, the grease/metal powder compound was used to determine the relationship between the compound temperature and the sensor's zero point. This having been accomplished, sensor readings
S. M~azawa et aL / Journal o f Materials Processing Technology 48 (1995) 765-770 were taken on the MIM compound and postprocessed to normalize for the effects of temperature drift. Both of the compounds were measured in the same manner. The metal powder and the thermoplastic binder were placed in the mixer, heated to a temperature of 80 "C and mixed for one hour. In-process measurement was not possible, since during the mixing process, the sensor output would have been affected by the motion of the metal rotor in the mixer. The mixing condition was monitored out-process in the current research. When the mixing condition was measured by the sensor probe, the rotor had to be stopped and the compound moved to a measuring cylinder, where sensor readings could be taken without interference from the rotor. The readings were taken at oneminute intervals.
767
sensor output increases with K, reaching its peak value at K=.9 before decreasing rapidly. Clearly, the eddy-current sensor cannot be used to measure the weight fraction of powder directly. The complicating factor is the density of the compound, which varies with the weight fraction of powder. The bold line in Figure 4 shows the relationship between the weight fraction of powder and the actual compound density, which is given by D=(Wl+W2)/V (2) where V is the volume of the compound. The broken line represents the "ideal density" of the compound, which was derived from the density of the metal (8.0g/cm3) and the density of the binder (0.95g/cm3). When K < .9, the actual density remains near the ideal value, but the two densities diverge sharply when K >0.9. s
3. RESULTS AND DISCUSSION Figure 3 shows the relationship between the sensor output for the grease/metal powder compound and weight fraction of powder, which is defined to be K=W1/(WI+W2) (1) where W1 and W2 are the weights of the metal powder and the binding agent, respectively. The
3
•~
0 7
/\
6 5
>4 Q.
3
0 2
/S
1
/
/
0: o
10 20 30 40 50 60 70 80 90 100 Weight fraction of powder (%) Fig.3 Relationship between weight fraction of powder and sensor output.
I/d j
2
0
10 20 30 40 50 60 70 80 90 100 Weight fraction of powder (%) Fig.4 Relationship between weight fraction of powder and density.
Figure 5 shows the relationship between the sensor output and the weight per unit volume of the metal powder in the compound, which is depends upon the weight fraction of powder and density as follows: W1/V=KxD (3) The sensor output increases in proportion to the weight per unit volume of metal powder in the compound. The mixing condition may be determined from the relationship between W1/V and the sensor output pictured in Figure 5.
S. M~azawa et aL / Journal of Materials Processing Technology 48 (1995) 765-770
768
7
temperature. The effects of temperature drift on the sensor output in the MIM compound readings were corrected through the use of a thermocoupler to monitor the compound temperature. Figure 7 shows the relationship between the sensor output and the weight fraction of powder of the MIM compound, given by K in eq.(1). As with the metal powder/grease compound, the sensor output increases with K, reaching a maximum value at 1{=.9 and dropping sharply thereafter.
J
6
5 ,,/
>4
~3
7"
2
.-'" 1
2
3
4
7
5
Weight of powder per unit volume (g/cm 3)
6
Fig.5 Relationship between metal powder contents and sensor output.
5 >~, 4
The first part of the experiment established that the sensor output bears a linear relationship to W1/V in the case of the metal powder/grease compound. Before this result could be extended to the MIM compound, however, the problem of temperature drift in the sensor had to be addressed. An oil bath was used to examine the temperature drift of the eddy-current sensor. Figure 6 shows that the drift from 20 to 100"C was clearly proportional to the
/,
/
o. 3
2
J
1 O, 0
&-E
i,/
~2
i~/I" 10 20 30 40 50 60 70 80 90 100 Weightfraction ofpowder(%) Fig.7 Relationship between weight fraction of powder and output.
4
3
0
l\
J
,/
~'3 -'E a
/
,/ Y
2
N
0
0 2o
30
40
50
60
70
80
Temperature ( °C ) Fig.6 Zero drift for temperature.
90
100
0
10 20 30 40 50 60 70 80 90 100 Weightfraction ofpowder(%) Fig. 8 Relationship between weight fraction of powder and density.
S. M(yazawa et aL / Journal o f Materials Processing Technology 48 (1995) 765-770
7
7
6
6
5
t/
7
1
0
f 0
j
N 3 0 2
l 2
3
0,
4
2
3
4
5
Weight of powder per unit volume (g/cm s) Fig .10 Relationship between metal powder contents and sensor output.
Fig.9 Relationship between metal powder contents and sensor output.
MIM compound. Clearly, the sensor output is proportional to Wl/V, the weight per unit volume of the metal powder in the compound, regardless of the binding agent selected. The maximum sensor output for the grease binding agent was 5.9V; for the thermoplastic binder, 6.4V. The density of both compounds remained low during the first stages of the mixing process, because
f 1
Weight of powder per unit volume (g/cm3)
Figure 8 shows the relationship between K and the compound density, which was calculated using eq.(2). The broken line represents the ideal compound density, which is derived from the metal powder density (8.0g/cm3) and the binder density (1.0g/cm3). Here, the difference between the actual and ideal values is smaller than that of the metal powder/grease compound (Figure 4). Figure 9 shows the relationship between the sensor output and the weight per unit volume WI/V of the metal powder in the compound, which was calculated using eq.(3). As before, the sensor output bears a linear relationship to W 1/V. The experimental results for the two different types of compound are summarized in Figure 10, where the symbol • denotes the metal powder• grease compound and the symbol • denotes the
t~
,x"
1
1
f
g4
J
2
,z" z"
5
J
~3 0
769
the metal powder and the binding agent (grease or MIM binder) were still separate. With time, the metal powder and the binding agent became well mixed and the density of the compounds increased, explaining the increase in the sensor output. In both cases, the density reached its peak value at K=.9, and decreased markedly thereafter. This behavior stemmed from the many pores which form at high temperatures in compounds having a high 7 6
5
g4 Q.
"53 0 2
o o
Io
20 30 40 Time (min)
50
60
Fig.11 Monitoring result in mixing process.
70
770
S. M~,azawa et aL / Journal of Materials Processing Technology 48 (1995) 765-770
proportion of metal powder, due to low liquidity. Figure 11 shows the MIM sensor output as a function of time, based on a series of 60 sensor readings taken at one-minute intervals. The output rises sharply at the beginning of the mixing process, increasing more gradually thereafter and reaching its peak value 60 minutes later. The continuous increase in the sensor values is a result of the compound's density, which is at a minimum at the beginning of the mixing process, when the metal powder and binder are separate, and at a maximum at the end of the process, when the metal powder and binder have been thoroughly mixed.
such as the thermocoupler used in the present research. Interference from the metal rotor was an obstacle to on-line sensor measurement in the current research. In the future, this problem may be avoided by changing the mixing container from a "bowl" shape to a circular channel, in which the sensor will be able to measure the compound at a safe distance from the rotor. ACKNOWLEDGEMENTS Thanks to the engineers of Pacific Metals and Adeka Fine Chemical who provided the materials. REFERENCES
4. CONCLUSIONS [1] G.R.Loosemore and A.Haywad, Powder The goal of this research was to measure the mixing condition of MIM compound using an eddycurrent sensor. Two compounds with different binding agents (grease and thermoplastic binder) were used to experimentally determine the relationship between the compounds metal contents
Metallurgy, 35, 2 (1992) 90. [2] I.E.Pinwill, F.Ahmad, P.S.AIIan
and the sensor output. Analysis of data revealed that the sensor output was proportional not to the powder to binder ratio, as originally expected, but rather to the metal powder contents per unit volume, the reason being that the density of the compound varied with the powder to binder ratio. Moreover, the mixture condition could be monitored using regular sensor readings throughout the mixture process. The merit of this technique is that it enables the mixing condition to be monitored with a simple and inexpensive sensor. The drawback is that the sensor is affected both by changes in temperature and by the motion of the metal rotor. The temperature drift can be corrected with the help of temperature sensor
P.J.James, Powder Metallurgy, 31, 2 (1988) 106. I'4] I.E.Pinwill, M.J.Edirisnghe and M.J. Bevis,
and
M.J.Bevis, Powder Metallurgy, 35, 2 (1992) 107. [3] M.T.Martyn, D.A.Issit, B.Haworth and
Powder Metallurgy, 35, 2 (1992) 113. 1'5] N.H.Loh, K.F.Hens and R.M.German, Proc. 3rd Int. Conf. High Tech., Chiba (1992) 47. 1'6] K.Terashita, K.Miyaminami, T.Konishi and J.Yoshida, Materials, Japan, 30 (1981) 873. 1'7] M.Sato, T.Fujimoto and K.Miyaminami, Proc. 23rd Conf. Powder Tech., Nagoya, 4 (1985) 11. 1'8] M.Sato, Y.Deguchi, H.Tsumura and K.Miyaminami, J. Soc. Powder Tech., Japan, 24, 11 (1987) 707.
ELSEVIER
Journal of Materials Processing Technology 48 (1995) 765-770
Materials Processing Technology
M o n i t o r i n g o f M e t a l P o w d e r - B i n d e r Mixing Process by E d d y - C u r r e n t S e n s o r S. Miyazawa, Y. Usui, H. Yoshida and Y. Murakoshi Mechanical Engineering Laboratory, 1-2 Namiki, Tsukuba-shi, 305 Japan Metal Injection Molding (MIM) is a technology by which metal products of high Jensity and complex shape can be mass-produced in a cost-effective manner. The raw input to the MIM process is a compound consisting of metal powder and thermoplastic binder. The distribution of metal powder throughout the compound must be uniform if high-quality products are to be obtained. This paper describes a new method for monitoring the metal powder distribution using an eddy-current sensor. Experimental sensor readings were gathered using two different compounds, each with a different binding agent. Analysis of the data showed clearly that the sensor output was proportional not to the powder to binder ratio at the sensor location, as originally expected, but rather to the metal powder contents per unit volume. This phenomenon may have arisen from the fact that the density of the compound varies with the powder to binder ratio. A series of sensor readings taken at regular time intervals was used to monitor the overall distribution of metal powder within the compound. The merit of this technique is its reliance on a simple and inexpensive sensor probe. The drawback is that the sensor measurements are affected by the mixer's metal rotor and by temperature drift. The effects of temperature drift can be compensated by monitoring the temperature of the compound. The interference of the metal rotor was an obstacle to in-process monitoring in the present research. In the future, in-process measurements may be achieved by changing the shape of the mixing container from a "bowl" to a circular channel, in which the sensor will be able to measure the compound at a safe distance from the rotor. 1. INTRODUCTION Metal Injection Molding (MIM) is a technology by which metal products of high density and complex shape can be mass-produced in a costeffective manner [1-5]. The raw input to the MIM process is a compound consisting of metal powder and thermoplastic binder. The distribution of metal powder throughout the compound must be uniform if high-quality products are to be obtained. Otherwise, variations in shrinkage rate may cause cracks to occur during the sintering process. Therefore, it is very important to monitor the mixing condition, the overall distribution of metal powder within the compound, throughout the mixing process. Two techniques have been developed for monitoring the mixing condition, one based on mixing torque [6] [7] and the other on the strength
of light reflections [8]. The mixing torque method has been used to monitor the mixing condition for polymer wax/ceramic powder compounds. As the powder surface becomes covered with wax, the frictional force decreases, leading to changes in the mixing torque. This method is best suited to measurement of the mixture's macroscopic properties. The light reflection method has been applied to compounds consisting of white aluminum oxide and black silicon carbide. Light reflection may be a less sensitive index to the MIM mixing condition than mixing torque, since the binder is colorless and transparent. This research was based on a conjecture that the eddy-current loss would be proportional to the metal powder to binder ratio when the sensor's high frequency oscillation coil was put into the compound. If this were indeed the case, the mixing condition could be monitored using a simple and
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0924-0136(94)01719-H
S. Miyazawa et al. / Journal of Materials Processing Technology 48 (1995) 765-770
766
inexpensive sensor probe. On the other hand, the effects of sensor temperature drift at high compound temperatures would have to be investigated.
(V) Q5 I
2 ?
2. M E A S U R E M E N T M E T H O D
Sens,
Figure 1 shows the experimental setup used to take sensor measurements. The eddy-current sensor used in the experiment was a typical commercial variety having four components: a coil (sensor probe, diameter: 5.4mm), an oscillator, a detector, and a linearlizer. The eddy-current circuit oscillates at 300kHz when there is no metal near the probe. When metal approaches the probe, an eddy-current is induced in the metal, causing the impedance of the probe to increase and the oscillating amplitude of the eddy-current circuit to decrease. An offset adjustor and a reversing amplifier were added to the sensor unit to make its output proportional to the eddy-current in the metal.
~C)ff-setshiflor~[ Amplifier }-~-Output
t Linearlizer l~
Detector ~[
Oscillator
Sensorprobe ~ ' (Co11)
Fig.1 Experimentalsetup. The m e a s u r e m e n t range of the sensor was established using the following technique: several sensor readings were taken with the sensor probe at various distances from a metallic ball (SUS304).
--[ Fig.2 Measurementrange. The relationship between the sensor output and the sensor probe to metal ball distance is shown in Figure 2. The measurement range of the sensor probe used in the experiment was taken to be a hemisphere of 6-8mm radius about the sensor probe. The eddy-current sensor was used to monitor the mixing condition for two c o m p o u n d s , each consisting of metal powder (SUS304, particle diameter:6~n) and a binding agent (grease or thermoplastic binder). The grease/metal powder c o m p o u n d could be m e a s u r e d over a large temperature range. However, the thermoplastic MIM compound (1800g of metal powder and 200g of thermoplastic binder) could be measured only at a high temperature, since at room temperature it was difficult to insert the sensor probe. This presented a problem, since the zero points of the sensor output depended upon the temperature in an unknown manner. Before data was collected for the MIM compound, the grease/metal powder compound was used to determine the relationship between the compound temperature and the sensor's zero point. This having been accomplished, sensor readings
S. M~azawa et aL / Journal o f Materials Processing Technology 48 (1995) 765-770 were taken on the MIM compound and postprocessed to normalize for the effects of temperature drift. Both of the compounds were measured in the same manner. The metal powder and the thermoplastic binder were placed in the mixer, heated to a temperature of 80 "C and mixed for one hour. In-process measurement was not possible, since during the mixing process, the sensor output would have been affected by the motion of the metal rotor in the mixer. The mixing condition was monitored out-process in the current research. When the mixing condition was measured by the sensor probe, the rotor had to be stopped and the compound moved to a measuring cylinder, where sensor readings could be taken without interference from the rotor. The readings were taken at oneminute intervals.
767
sensor output increases with K, reaching its peak value at K=.9 before decreasing rapidly. Clearly, the eddy-current sensor cannot be used to measure the weight fraction of powder directly. The complicating factor is the density of the compound, which varies with the weight fraction of powder. The bold line in Figure 4 shows the relationship between the weight fraction of powder and the actual compound density, which is given by D=(Wl+W2)/V (2) where V is the volume of the compound. The broken line represents the "ideal density" of the compound, which was derived from the density of the metal (8.0g/cm3) and the density of the binder (0.95g/cm3). When K < .9, the actual density remains near the ideal value, but the two densities diverge sharply when K >0.9. s
3. RESULTS AND DISCUSSION Figure 3 shows the relationship between the sensor output for the grease/metal powder compound and weight fraction of powder, which is defined to be K=W1/(WI+W2) (1) where W1 and W2 are the weights of the metal powder and the binding agent, respectively. The
3
•~
0 7
/\
6 5
>4 Q.
3
0 2
/S
1
/
/
0: o
10 20 30 40 50 60 70 80 90 100 Weight fraction of powder (%) Fig.3 Relationship between weight fraction of powder and sensor output.
I/d j
2
0
10 20 30 40 50 60 70 80 90 100 Weight fraction of powder (%) Fig.4 Relationship between weight fraction of powder and density.
Figure 5 shows the relationship between the sensor output and the weight per unit volume of the metal powder in the compound, which is depends upon the weight fraction of powder and density as follows: W1/V=KxD (3) The sensor output increases in proportion to the weight per unit volume of metal powder in the compound. The mixing condition may be determined from the relationship between W1/V and the sensor output pictured in Figure 5.
S. M~azawa et aL / Journal of Materials Processing Technology 48 (1995) 765-770
768
7
temperature. The effects of temperature drift on the sensor output in the MIM compound readings were corrected through the use of a thermocoupler to monitor the compound temperature. Figure 7 shows the relationship between the sensor output and the weight fraction of powder of the MIM compound, given by K in eq.(1). As with the metal powder/grease compound, the sensor output increases with K, reaching a maximum value at 1{=.9 and dropping sharply thereafter.
J
6
5 ,,/
>4
~3
7"
2
.-'" 1
2
3
4
7
5
Weight of powder per unit volume (g/cm 3)
6
Fig.5 Relationship between metal powder contents and sensor output.
5 >~, 4
The first part of the experiment established that the sensor output bears a linear relationship to W1/V in the case of the metal powder/grease compound. Before this result could be extended to the MIM compound, however, the problem of temperature drift in the sensor had to be addressed. An oil bath was used to examine the temperature drift of the eddy-current sensor. Figure 6 shows that the drift from 20 to 100"C was clearly proportional to the
/,
/
o. 3
2
J
1 O, 0
&-E
i,/
~2
i~/I" 10 20 30 40 50 60 70 80 90 100 Weightfraction ofpowder(%) Fig.7 Relationship between weight fraction of powder and output.
4
3
0
l\
J
,/
~'3 -'E a
/
,/ Y
2
N
0
0 2o
30
40
50
60
70
80
Temperature ( °C ) Fig.6 Zero drift for temperature.
90
100
0
10 20 30 40 50 60 70 80 90 100 Weightfraction ofpowder(%) Fig. 8 Relationship between weight fraction of powder and density.
S. M(yazawa et aL / Journal o f Materials Processing Technology 48 (1995) 765-770
7
7
6
6
5
t/
7
1
0
f 0
j
N 3 0 2
l 2
3
0,
4
2
3
4
5
Weight of powder per unit volume (g/cm s) Fig .10 Relationship between metal powder contents and sensor output.
Fig.9 Relationship between metal powder contents and sensor output.
MIM compound. Clearly, the sensor output is proportional to Wl/V, the weight per unit volume of the metal powder in the compound, regardless of the binding agent selected. The maximum sensor output for the grease binding agent was 5.9V; for the thermoplastic binder, 6.4V. The density of both compounds remained low during the first stages of the mixing process, because
f 1
Weight of powder per unit volume (g/cm3)
Figure 8 shows the relationship between K and the compound density, which was calculated using eq.(2). The broken line represents the ideal compound density, which is derived from the metal powder density (8.0g/cm3) and the binder density (1.0g/cm3). Here, the difference between the actual and ideal values is smaller than that of the metal powder/grease compound (Figure 4). Figure 9 shows the relationship between the sensor output and the weight per unit volume WI/V of the metal powder in the compound, which was calculated using eq.(3). As before, the sensor output bears a linear relationship to W 1/V. The experimental results for the two different types of compound are summarized in Figure 10, where the symbol • denotes the metal powder• grease compound and the symbol • denotes the
t~
,x"
1
1
f
g4
J
2
,z" z"
5
J
~3 0
769
the metal powder and the binding agent (grease or MIM binder) were still separate. With time, the metal powder and the binding agent became well mixed and the density of the compounds increased, explaining the increase in the sensor output. In both cases, the density reached its peak value at K=.9, and decreased markedly thereafter. This behavior stemmed from the many pores which form at high temperatures in compounds having a high 7 6
5
g4 Q.
"53 0 2
o o
Io
20 30 40 Time (min)
50
60
Fig.11 Monitoring result in mixing process.
70
770
S. M~,azawa et aL / Journal of Materials Processing Technology 48 (1995) 765-770
proportion of metal powder, due to low liquidity. Figure 11 shows the MIM sensor output as a function of time, based on a series of 60 sensor readings taken at one-minute intervals. The output rises sharply at the beginning of the mixing process, increasing more gradually thereafter and reaching its peak value 60 minutes later. The continuous increase in the sensor values is a result of the compound's density, which is at a minimum at the beginning of the mixing process, when the metal powder and binder are separate, and at a maximum at the end of the process, when the metal powder and binder have been thoroughly mixed.
such as the thermocoupler used in the present research. Interference from the metal rotor was an obstacle to on-line sensor measurement in the current research. In the future, this problem may be avoided by changing the mixing container from a "bowl" shape to a circular channel, in which the sensor will be able to measure the compound at a safe distance from the rotor. ACKNOWLEDGEMENTS Thanks to the engineers of Pacific Metals and Adeka Fine Chemical who provided the materials. REFERENCES
4. CONCLUSIONS [1] G.R.Loosemore and A.Haywad, Powder The goal of this research was to measure the mixing condition of MIM compound using an eddycurrent sensor. Two compounds with different binding agents (grease and thermoplastic binder) were used to experimentally determine the relationship between the compounds metal contents
Metallurgy, 35, 2 (1992) 90. [2] I.E.Pinwill, F.Ahmad, P.S.AIIan
and the sensor output. Analysis of data revealed that the sensor output was proportional not to the powder to binder ratio, as originally expected, but rather to the metal powder contents per unit volume, the reason being that the density of the compound varied with the powder to binder ratio. Moreover, the mixture condition could be monitored using regular sensor readings throughout the mixture process. The merit of this technique is that it enables the mixing condition to be monitored with a simple and inexpensive sensor. The drawback is that the sensor is affected both by changes in temperature and by the motion of the metal rotor. The temperature drift can be corrected with the help of temperature sensor
P.J.James, Powder Metallurgy, 31, 2 (1988) 106. I'4] I.E.Pinwill, M.J.Edirisnghe and M.J. Bevis,
and
M.J.Bevis, Powder Metallurgy, 35, 2 (1992) 107. [3] M.T.Martyn, D.A.Issit, B.Haworth and
Powder Metallurgy, 35, 2 (1992) 113. 1'5] N.H.Loh, K.F.Hens and R.M.German, Proc. 3rd Int. Conf. High Tech., Chiba (1992) 47. 1'6] K.Terashita, K.Miyaminami, T.Konishi and J.Yoshida, Materials, Japan, 30 (1981) 873. 1'7] M.Sato, T.Fujimoto and K.Miyaminami, Proc. 23rd Conf. Powder Tech., Nagoya, 4 (1985) 11. 1'8] M.Sato, Y.Deguchi, H.Tsumura and K.Miyaminami, J. Soc. Powder Tech., Japan, 24, 11 (1987) 707.