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Micro MIM approaches mass production
special feature
Micro MIM approaches
mass
production Micro MIM is approaching the point where series production is a real possibility, but that raises questions of quality assurance. German researchers have been at the forefront of micro moulding technologies right from the start... ncreasing interest in smaller parts and miniaturisation led to the development of the micro metal injection moulding technique (Micro MIM). In addition to the micro moulding of plastics, metal micro parts provide important functions through their high strength, temperature stability or magnetic characteristics. A team from Fraunhofer IFAM in Bremen looked at aspects of series production and the quality assurance questions that come with it. Based on the application of finer powders compared to the standard MIM
I
process and with the help of new binder formulations, mouldable feedstocks of many metallic materials have been tested in the last few years [1-3]. Microstructured surfaces with features down to 20 µm as well as miniaturised parts smaller than 1 mm have been manufactured successfully. Because of the fine powders debinding and sintering have to be performed carefully. Spaces between particles are extremely small and can promote crack formation during rapid debinding. Densification on the other hand is easier because of higher
sintering activity present. The use of very fine powders is possible but it has to be kept in mind that more binder material is necessary to cover the particle surfaces, which can have negative effects on shrinkage control. The detail of a fluidic system made of ultrafine copper powder with a mean particle size lower than 1 µm is shown in Figure 1. Channels 10 µm wide were reproduced and sintered with high accuracy. Figure 6 shows a micro reactor made out of 316L stainless steel with a photo-catalytic TiO2 coating used for water purification. The functional surface consists of 26 channels each with a width of 300 µm. Small but smooth
Figure 1. Detail of a micro fluidic device.
16
MPR December 2005
Mean particle size of the powder used was about 3.5 µm which was sintered to 97 per cent of the theoretical density. The surface roughness in the channels was measured to about 0.5 µm Ra. The system was covered with a glass plate and equipped with feeding pipes when in use. Examples for miniaturised products are illustrated in Figure 2. The stainless steel gears and impellers shown reached a density of 97 per cent of theoretical. The surface roughness of about 1 µm Ra was reached with an unpolished mould. These values can be improved while using a polished surface as moulding tool.
0026-0657/05 ©2005 Elsevier Ltd. All rights reserved.
measurement and one sensor for feedstock temperature. One thousand micro step test parts were produced using a 316L feedstock based on very fine powders of about 3 µm mean particle diameter. Measurements of cavity pressure during the production cycle should give an indication for quality assurance of this part. Figure 4 shows the kind of records obtained. Without changing processing parameters there is a visible variation in pressure for the different parts. One hundred parts were weighed and compared with the respective cavity pressure (see Figure 5). When looking at the trend for both parameters there is a clear dependence related to the respective part. This indicates that slight variations in injection pressure directly influence part quality. Similar results were found for green density and width and length of the green micro steps. Figure 2. Miniature parts out of 316L (in co-operation with the Scholz company of D-Kronach)
At Fraunhofer IFAM different machine concepts have been used to manufacture parts with sizes down to 400 µm. Standard injection moulding machines with changes only in respect to screw diameter can easily and cost-effectively be made available for micro MIM. Micro part manufacturing is possible with this concept but the ratio of gate system to real part volume increases a lot compared to standard MIM. In addition, the control accuracy of screw movement reaches its limit due to the low shot volume. For some feedstocks this might be tolerable, but using a sensitive material part quality could be influenced. Special micro moulding devices already exist where the injection system combines a small screw for plasticisation and an injection piston for dosage. Very short cycle times compared to standard injection
moulding machines can be achieved with this concept. But it has also been seen that feedstock inhomogeneity and variation of dosing volume directly influence cavity pressure. Such special micro moulding devices are more expensive but they can be equipped with integrated handling and optical quality control modules. The results presented here were achieved using the special micro moulding concept. Test parts shaped like micro steps and micro tensile bars (see Figure 3) were manufactured in series. The micro step piece consists of five steps varying in height from 0.3 mm to 3 mm. The length of the complete part is 25 mm and the width is 3 mm as a green part. The test geometry of the tensile specimen has a functional area of 13.5x1x1 mm in the green state. The mould for the micro step was equipped with three sensors for cavity pressure
New approaches needed Table 1 gives the mean values and standard deviations of green density, weight, width and length for the 100 parts investigated to show their contribution to quality and reproducibility. This Table illustrates a further aspect while defining quality for such small parts. Most measuring systems are not suitable for tiny parts and up to now there are no standardised procedures. Deviation of density, especially, is thought to be caused by inaccurate measuring due to the small volume of the samples. New ways of density determination instead of the Archimedes principle have to be found. The lack of suitable systems is a general problem for those producing micro parts and trying to ensure their quality. Tests using sintered specimens as depicted in Figure 3 were performed to estimate the micro tensile mechanical properties of 316L. As there is no standardised test
Figure 3. Test part called micro step (left); micro tensile bars green and sintered (right).
metal-powder.net
December 2005 MPR
17
procedure for micro components, specimen geometry was chosen in a way to obtain a relation of initial sample length to cross section of 10 during the tests. This corresponds to a “downsized” standard specimen for conventional tensile testing. Prior to tensile testing, the density of samples sintered at 1150°C was measured. Values between 7.74 g/cm3 and 7.84 g/cm3 (97.0 per cent to 98.1 per cent relative density) were found and again show a deviation which is assumed to be high due to the measuring system. Density achieved
Figure 4. Cavity pressure depending on respective part.
Nevertheless, the values obtained indicate that a good density was achieved in the sintering procedure. Figure 8 shows the results of 10 tensile tests carried out for 316L material sintered at 1150°C. The curves show good reproducibility of the yield stress (270-278 MPa) and ultimate tensile stress (601-625 MPa). These values are in the range of data found for conventional 316L MIM material. However, very high values of elastic and plastic elongation (up to 7 per cent and 79 per cent, respectively) were recorded. Elongation of the sample suspension or displacement of the specimens probably caused these results, but further research on this is necessary. Pressure control
Figure 5. Green weight and cavity pressure depending on respective part.
Similar results concerning reproducibility were obtained for sintering temperatures of 1080°C and 1300°C as well as for tests performed on 17-4PH specimens. Detailed evaluation of the data obtained is currently in progress. In terms of quality assurance the control of injection pressure and therefore homogeneity of the feedstock used is of great importance for micro part production. The data achieved while investigating green parts have also been compared to the sintered values. The measurements of the sintered parts confirm the prior reported
Acknowledgement THE authors like to thank Krämer Engineering of Rendsburg, Germany, for their support in mould design and mould manufacturing for the Micro MIM process. Figure 6. Micro reactor for water purification.
18
MPR December 2005
page 20
metal-powder.net
The authors THIS feature is taken from Micro Metal Injection Moulding - Quality Assurance in Series Production, a paper by Astrid Rota, Philipp Imgrund, Lutz Kramer, Ralf Meyer and Janne Haack of Fraunhofer IFAM in Bremen, and given at the European Powder Metallurgy Association’s EuroPM2005 Conference and Exhibition in Prague. Figure 7. Series production of ear implant. Left: green parts; Right: sintered part.
Table 1: Standard deviation of density, weight, width and length of 100 green parts. Density [g/cm3] Weight [mg] Length [mm] Width [mm] Mean value
4.69
439.66
24.82
3.02
Standard deviation
0.08
0.35
0.01
0.01
1.65%
0.08%
0.05%
0.46%
Standard deviation in %
results. Similar dependencies were found for the sintered parts. To apply the results achieved so far, stapes used as ear implants were manufactured in series. 316L stainless steel was chosen for the first experiments, but titanium will later be of especial interest. A series of about 1000 parts was manufactured and the cavity pressure report also showed considerable pressure variation. Figure 7 illustrates green parts and one sintered part in comparison with a one cent
piece. Investigations of 100 green parts via micro balance showed a mean weight of 9.38 mg for each part with a deviation of ± 0.29 per cent. The results for the sintered parts underline these evaluations. The mould for this part was manufactured by micro EDM in order to obtain good surface quality and tight tolerances. The measured mould quality of about 1 µm Ra could be transferred completely to the sintered part. Micro MIM is approaching production maturity. Reproducibility as well as
quality assurance are important parameters for successful series production of micro parts. The results obtained show that there is an influence on part density, weight and size due to variations in injection pressure. For micro parts this influence cannot be neglected if the tight tolerances required are to be retained and it is assumed that feedstock homogeneity can improve this effort. However, says the Fraunhofer team, there is a lack of suitable and standardised measuring systems for micro parts.
References [1] A C Rota, New Features in Material Issues for Metallic Micro Components by MIM, Proceedings of PM2TEC, Orlando, Florida, USA, 2002, pp 10-49 - 10-57 [2] A Rota, P Imgrund, F Petzoldt, Micro MIM - a production process for micro components with enhanced material properties, Proceedings of Powder Metallurgy World Congress, Vienna, 2004, Vol 1, 467-472 [3] V Piotter et al, PIM enters Microsystems Technology; P/M Science & Technology Briefs, vol. 4, no. 1, 2002, pp 18-23 [4] Ph Imgrund, A Rota, Multifunctional Microparts by Metal Injection Molding, Proceedings of Micro System Technologies, München, October 7-8, 2003, pp 218-225 [5] A Rota, P Imgrund, F Petzoldt, Micro Moulding of Material Combinations to Avoid Assembly, Proceedings of Euro PM2003, Valencia, Spain, Vol. 3, pp 143-148
Figure 8. Stress-strain diagram of ten 316L micro tensile test specimens sintered at 1150°C.
20
MPR December 2005
metal-powder.net
Micro MIM approaches
mass
production Micro MIM is approaching the point where series production is a real possibility, but that raises questions of quality assurance. German researchers have been at the forefront of micro moulding technologies right from the start... ncreasing interest in smaller parts and miniaturisation led to the development of the micro metal injection moulding technique (Micro MIM). In addition to the micro moulding of plastics, metal micro parts provide important functions through their high strength, temperature stability or magnetic characteristics. A team from Fraunhofer IFAM in Bremen looked at aspects of series production and the quality assurance questions that come with it. Based on the application of finer powders compared to the standard MIM
I
process and with the help of new binder formulations, mouldable feedstocks of many metallic materials have been tested in the last few years [1-3]. Microstructured surfaces with features down to 20 µm as well as miniaturised parts smaller than 1 mm have been manufactured successfully. Because of the fine powders debinding and sintering have to be performed carefully. Spaces between particles are extremely small and can promote crack formation during rapid debinding. Densification on the other hand is easier because of higher
sintering activity present. The use of very fine powders is possible but it has to be kept in mind that more binder material is necessary to cover the particle surfaces, which can have negative effects on shrinkage control. The detail of a fluidic system made of ultrafine copper powder with a mean particle size lower than 1 µm is shown in Figure 1. Channels 10 µm wide were reproduced and sintered with high accuracy. Figure 6 shows a micro reactor made out of 316L stainless steel with a photo-catalytic TiO2 coating used for water purification. The functional surface consists of 26 channels each with a width of 300 µm. Small but smooth
Figure 1. Detail of a micro fluidic device.
16
MPR December 2005
Mean particle size of the powder used was about 3.5 µm which was sintered to 97 per cent of the theoretical density. The surface roughness in the channels was measured to about 0.5 µm Ra. The system was covered with a glass plate and equipped with feeding pipes when in use. Examples for miniaturised products are illustrated in Figure 2. The stainless steel gears and impellers shown reached a density of 97 per cent of theoretical. The surface roughness of about 1 µm Ra was reached with an unpolished mould. These values can be improved while using a polished surface as moulding tool.
0026-0657/05 ©2005 Elsevier Ltd. All rights reserved.
measurement and one sensor for feedstock temperature. One thousand micro step test parts were produced using a 316L feedstock based on very fine powders of about 3 µm mean particle diameter. Measurements of cavity pressure during the production cycle should give an indication for quality assurance of this part. Figure 4 shows the kind of records obtained. Without changing processing parameters there is a visible variation in pressure for the different parts. One hundred parts were weighed and compared with the respective cavity pressure (see Figure 5). When looking at the trend for both parameters there is a clear dependence related to the respective part. This indicates that slight variations in injection pressure directly influence part quality. Similar results were found for green density and width and length of the green micro steps. Figure 2. Miniature parts out of 316L (in co-operation with the Scholz company of D-Kronach)
At Fraunhofer IFAM different machine concepts have been used to manufacture parts with sizes down to 400 µm. Standard injection moulding machines with changes only in respect to screw diameter can easily and cost-effectively be made available for micro MIM. Micro part manufacturing is possible with this concept but the ratio of gate system to real part volume increases a lot compared to standard MIM. In addition, the control accuracy of screw movement reaches its limit due to the low shot volume. For some feedstocks this might be tolerable, but using a sensitive material part quality could be influenced. Special micro moulding devices already exist where the injection system combines a small screw for plasticisation and an injection piston for dosage. Very short cycle times compared to standard injection
moulding machines can be achieved with this concept. But it has also been seen that feedstock inhomogeneity and variation of dosing volume directly influence cavity pressure. Such special micro moulding devices are more expensive but they can be equipped with integrated handling and optical quality control modules. The results presented here were achieved using the special micro moulding concept. Test parts shaped like micro steps and micro tensile bars (see Figure 3) were manufactured in series. The micro step piece consists of five steps varying in height from 0.3 mm to 3 mm. The length of the complete part is 25 mm and the width is 3 mm as a green part. The test geometry of the tensile specimen has a functional area of 13.5x1x1 mm in the green state. The mould for the micro step was equipped with three sensors for cavity pressure
New approaches needed Table 1 gives the mean values and standard deviations of green density, weight, width and length for the 100 parts investigated to show their contribution to quality and reproducibility. This Table illustrates a further aspect while defining quality for such small parts. Most measuring systems are not suitable for tiny parts and up to now there are no standardised procedures. Deviation of density, especially, is thought to be caused by inaccurate measuring due to the small volume of the samples. New ways of density determination instead of the Archimedes principle have to be found. The lack of suitable systems is a general problem for those producing micro parts and trying to ensure their quality. Tests using sintered specimens as depicted in Figure 3 were performed to estimate the micro tensile mechanical properties of 316L. As there is no standardised test
Figure 3. Test part called micro step (left); micro tensile bars green and sintered (right).
metal-powder.net
December 2005 MPR
17
procedure for micro components, specimen geometry was chosen in a way to obtain a relation of initial sample length to cross section of 10 during the tests. This corresponds to a “downsized” standard specimen for conventional tensile testing. Prior to tensile testing, the density of samples sintered at 1150°C was measured. Values between 7.74 g/cm3 and 7.84 g/cm3 (97.0 per cent to 98.1 per cent relative density) were found and again show a deviation which is assumed to be high due to the measuring system. Density achieved
Figure 4. Cavity pressure depending on respective part.
Nevertheless, the values obtained indicate that a good density was achieved in the sintering procedure. Figure 8 shows the results of 10 tensile tests carried out for 316L material sintered at 1150°C. The curves show good reproducibility of the yield stress (270-278 MPa) and ultimate tensile stress (601-625 MPa). These values are in the range of data found for conventional 316L MIM material. However, very high values of elastic and plastic elongation (up to 7 per cent and 79 per cent, respectively) were recorded. Elongation of the sample suspension or displacement of the specimens probably caused these results, but further research on this is necessary. Pressure control
Figure 5. Green weight and cavity pressure depending on respective part.
Similar results concerning reproducibility were obtained for sintering temperatures of 1080°C and 1300°C as well as for tests performed on 17-4PH specimens. Detailed evaluation of the data obtained is currently in progress. In terms of quality assurance the control of injection pressure and therefore homogeneity of the feedstock used is of great importance for micro part production. The data achieved while investigating green parts have also been compared to the sintered values. The measurements of the sintered parts confirm the prior reported
Acknowledgement THE authors like to thank Krämer Engineering of Rendsburg, Germany, for their support in mould design and mould manufacturing for the Micro MIM process. Figure 6. Micro reactor for water purification.
18
MPR December 2005
page 20
metal-powder.net
The authors THIS feature is taken from Micro Metal Injection Moulding - Quality Assurance in Series Production, a paper by Astrid Rota, Philipp Imgrund, Lutz Kramer, Ralf Meyer and Janne Haack of Fraunhofer IFAM in Bremen, and given at the European Powder Metallurgy Association’s EuroPM2005 Conference and Exhibition in Prague. Figure 7. Series production of ear implant. Left: green parts; Right: sintered part.
Table 1: Standard deviation of density, weight, width and length of 100 green parts. Density [g/cm3] Weight [mg] Length [mm] Width [mm] Mean value
4.69
439.66
24.82
3.02
Standard deviation
0.08
0.35
0.01
0.01
1.65%
0.08%
0.05%
0.46%
Standard deviation in %
results. Similar dependencies were found for the sintered parts. To apply the results achieved so far, stapes used as ear implants were manufactured in series. 316L stainless steel was chosen for the first experiments, but titanium will later be of especial interest. A series of about 1000 parts was manufactured and the cavity pressure report also showed considerable pressure variation. Figure 7 illustrates green parts and one sintered part in comparison with a one cent
piece. Investigations of 100 green parts via micro balance showed a mean weight of 9.38 mg for each part with a deviation of ± 0.29 per cent. The results for the sintered parts underline these evaluations. The mould for this part was manufactured by micro EDM in order to obtain good surface quality and tight tolerances. The measured mould quality of about 1 µm Ra could be transferred completely to the sintered part. Micro MIM is approaching production maturity. Reproducibility as well as
quality assurance are important parameters for successful series production of micro parts. The results obtained show that there is an influence on part density, weight and size due to variations in injection pressure. For micro parts this influence cannot be neglected if the tight tolerances required are to be retained and it is assumed that feedstock homogeneity can improve this effort. However, says the Fraunhofer team, there is a lack of suitable and standardised measuring systems for micro parts.
References [1] A C Rota, New Features in Material Issues for Metallic Micro Components by MIM, Proceedings of PM2TEC, Orlando, Florida, USA, 2002, pp 10-49 - 10-57 [2] A Rota, P Imgrund, F Petzoldt, Micro MIM - a production process for micro components with enhanced material properties, Proceedings of Powder Metallurgy World Congress, Vienna, 2004, Vol 1, 467-472 [3] V Piotter et al, PIM enters Microsystems Technology; P/M Science & Technology Briefs, vol. 4, no. 1, 2002, pp 18-23 [4] Ph Imgrund, A Rota, Multifunctional Microparts by Metal Injection Molding, Proceedings of Micro System Technologies, München, October 7-8, 2003, pp 218-225 [5] A Rota, P Imgrund, F Petzoldt, Micro Moulding of Material Combinations to Avoid Assembly, Proceedings of Euro PM2003, Valencia, Spain, Vol. 3, pp 143-148
Figure 8. Stress-strain diagram of ten 316L micro tensile test specimens sintered at 1150°C.
20
MPR December 2005
metal-powder.net