Microengineering of Metals and Ceramics

24 11 Micro Metal Injection Molding
11.8
References
1 R. Gerling, F.-P. Schimansky, Adv. Eng.
Mater. 2001, 6, 387–390.
2 R. Cornwall, http://www.immnet.com, 2003.
3 V. Piotter, W. Bauer, T. Benzler, A. Emde,
J. Microsyst. Technol. 2001, 3, 99–102.
4 A. Rota, T.V. Duong, T. Hartwig, J. Microsyst.
Technol. 2002, 7, 225–228.
5 A.M. Morales, R. Pitchumani, A. K. Gutmann,
Proc. SPIE 2003, 4979, 430–439.
6 Z.Y. Liu, N.H. Loh, S. B. Tor, K. A. Khor,
Y. Murakoshi, R. Maeda, T. Shimizu, J.
Process. Technol. 2002, 127, 165–168.
7 K. Nishiyabu, S. Matsuzaki, S. Tanaka,
V. Piotter, in: Proc. of PM2TEC 2003, Las
Vegas; SPIE: Bellingham, WA, 2003.
8 A. Rota, T. V. Duong, T. Hartwig, J. Microsyst.
Technol. 2002, 8, 323–325.
9 J.H.H. TerMaat, J. Ebenhöch, H.-J. Sterzel,
Ceram. Mater. Components Engines
1992, 4, 544–551.
10 M. Blömacher, D. Weinand, Keram. Z.
1992, 44, 827–830.
11 M. Blömacher, D. Weinand, Met. Powder
Rep. 1992, 5, 43–49.
12 http://www.zschimmer-schwarz.de  Keramik
Elutec, 2004.
13 http://www.clariant.com  Product  Licomont,
2004.
14 R.M. German, in: Powder Injection Molding;
Princeton, NJ: Metal Powder Industries
Federation, 1990.
15 V.M.B. Moloney,. D. Parris, M.J. Edirisinghe,
J. Am. Ceram. Soc. 1995, 78,
3225–3232.
16 A. Smolders, J. Sleurs, J. Eur. Ceram.
Soc. 1997, 17, 171–175.
17 http://www.basf.de/basf/img/produkte/chemikalien/
catamold/Produktblatt_Catamold_
e.pdf, 2004.
18 T. Zhang, J. R. G. Evans, J. Eur. Ceram.
Soc. 1989, 5, 165–172.
19 P. Suri, S.V. Atre, R.M. German, J. P. de
Souza, Mater. Sci. Eng. A 2003, 356, 337–
344.
20 A. Bose, Powder Metall. 2003, 46, 121–126.
21 C.M. Wang, in: Proceedings of PM2TEC
Conference 2001; 2001, pp. 8-180–8-192.
22 B. C. Mutsuddy, Proc. Br. Ceram. Soc./
Fabric. Sci. 1983, 3, 117–137.
23 D. Baird, D. Collias, in: Polymer Processing
– Principles and Design; New York:
Wiley, 1998.
24 J. H.Song, J. R.G. Evans, J. Rheol. 1996,
40, 131–152.
25 V. Piotter, Th. Gietzelt, L. Merz, R. Ruprecht,
J. Hausselt, PM Sci. Technol. Briefs
2002, 4, 18–23.
26 L. Merz, S. Rath, V. Piotter, R. Ruprecht,
J. Hausselt, J. Microsyst. Technol. 2004,
10, 202–204.
27 R. Mueller, Landolt-Börnstein New Series
VIII/2A1; 2003, Chapter 6.
28 M. Koizumi (ed.), Proceedings of the Third
International Conference, Osaka, 1991; 1991.
29 L. Delaey, in: HIP’93, Proceedings of the
International Conference on Hot Isostatic
Pressing, Antwerp, 21–23 April 1993;
1993.
30 T. L. King, L. Qingfa, in: Processing and
Fabrication of Advanced Materials VI; Singapore:
Institute of Materials, 1998,
pp. 1465–1476.
31 D.M. Won, S.-W. Kim, Y.D. Kim, I.-H.
Moon, in: Proceedings of PM2TEC Conference
2001; 2001, pp. 8-173–8-179.
32 J. R. Alcock, Met. Powder Rep. 1999, June,
30–34.
33 L.-K. Tan, R. Baumgartner, R.M. German,
in: Advances in Powder Metallurgy
and Particulate Materials – 2001, compiled
by W. B. Elsen and S. Kassam;
Princeton, NJ: Metal Powder Industries
Federation, Vol. 4, pp. 191–198.
34 D.F. Heaney, P. Suri, R.M. German,
J. Mater. Sci., 2003, 38, 4869–4874.
35 P. Suri, D. F. Heaney, R.M. German,
J. Mater. Sci., 2003, 38, 4875–4881.
36 J. L. Johnson, L.-K. Tan, P. Suri, R.M.
German, JOM – J. Min. Met. Mater. Sci.,
2003, 55, 30–34.
37 A. Rota, in: Advances in Powder Metallurgy
and Particulate Materials – 2002,
compiled by V. Arnhold, C.-L. Chu, W. F.
Jandeska, Jr. and H.I. Sanderow; Princeton,
NJ: Metal Powder Industries Federation,
2002, Vol. 10, pp. 49–57.
38 V. Piotter, in: Proc. of Injection Moulding
2003 Conference; Copenhagen: Hexagon
Holding, 2003, p. 4.
39 G. Örlygsson, G. Finnah, V. Piotter, U.
Kaufmann, R. Ruprecht, J. Hausselt, in:
Advances in Powder Metallurgy and Particulate
Materials – 2004; Princeton, NJ: Metal
Powder Industries Federation, in press.
Abstract
Ceramic injection molding is a shaping technology which is well suited for the
manufacture of complex-shaped ceramic parts with small dimensions. The great
freedom of design and the high degree of automation permit a near net shape
and high-volume production and make injection molding attractive for the manufacture
of microparts or microstructured devices. The application of injection
molding for the manufacture of ceramic microparts requires the development
of feedstocks with high flowability and sufficient green strength. In addition to
an adapted tooling technology, an enhanced machine controlling is also necessary
for the successful shaping of the sensitive features. Owing to high machine
and tooling costs, injection molding is normally associated with large-scale production
only. However, the broad spectrum of binder systems also provides the
opportunity to manufacture small production lots or even a small number of
prototypes in an economic way.
Keywords
ceramic injection molding; CIM; HPIM; LPIM; feedstock; microsystems technologies;
microparts
12.1 Introduction 326
12.2 Basic Requirements for Ceramic Powders and Binder Systems 328
12.3 High-pressure Ceramic Injection Molding (HPIM) 330
12.3.1 Feedstocks for HPIM 330
12.3.2 Binders for MicroHPIM (HPIM) 331
12.3.3 Compounding the Feedstock 331
12.3.4 Rheology 333
12.3.5 Molding Process 336
12.3.6 Thermal Treatment 338
12.3.7 Example of a Typical HPIM Production Cycle 340
325
12
Microceramic Injection Molding
W. Bauer, J. Hausselt, L. Merz, M. Müller, G. Örlygsson, S. Rath,
Materials Research III (IMF III), Forschungszentrum Karlsruhe, Germany
Advanced Micro and Nanosystems Vol. 3. Microengineering of Metals and Ceramics.
Edited by H. Baltes, O. Brand, G. K. Fedder, C. Hierold, J. Korvink, O. Tabata
Copyright © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 3-527-31208-0
12.3.8 Special Development: Two-component MicroCIM 342
12.4 Low-pressure Injection Molding (LPIM) 344
12.4.1 Characteristic Features of LPIM 344
12.4.2 Feedstocks for LPIM 345
12.4.3 Compounding the Feedstock 346
12.4.4 Rheology 347
12.4.5 Machines, Tooling 348
12.4.6 Thermal Treatment 350
12.4.7 Examples 353
12.5 Conclusions 355
12.6 References 355
12.1
Introduction
Injection molding has been used in ceramic manufacture since about 1930 [1,
2]. The driving force was the demand for a large number of spark plugs for
automobiles. As the spark plug insulators had a fairly complex shape, which
was difficult to manufacture by other methods, a new shaping technique had to
be developed to fulfil the demand. Another push came in the 1960s, when injection
molding became established in the foundry industry as a routine shaping
method for the manufacture of ceramic cores for aircraft turbines. In the
1950s, injection molding was developed for the production of ceramic thread
guides. The rapidly growing synthetic fiber and textile industry required a large
number of complex-shaped guides which were able to resist the high-speed
threads without wear.
During the last 20 years, the interest in ceramic injection molding (CIM) has
accelerated as the demands of parts design have approached the limits of conventional
powder processing routes. Nowadays CIM is a standard shaping method
for the mass production of small ceramic parts with complex shapes. Examples,
in addition to the already mentioned thread guides, are cutting tools and
nozzles for wire bonding machines. However, some frequently presented examples
such as turbocharger rotors or turbine wheels [3] are still on a prototype basis
and have not yet reached the stage of industrial application as high costs often
put up a commercial barrier for the manufacture of large parts [4].
In contrast, CIM plays a commanding role in the manufacture of small or
miniaturized devices. These parts are often characterized by a complex geometry
which cannot be accomplished by traditional ceramic shaping methods.
Shaped by dry pressing or casting, the strength of the unfired (‘green’) compact
is low. During the demolding step, i.e. the removal of the part from the mold,
there is a high risk that fine details of low-strength materials will be damaged.
Mechanical tooling of ceramics is always an expensive issue, even for small
work pieces. Owing to the brittle nature of the material, machining can introduce
flaws which may lead to failure in use. In contrast, CIM offers a near-net-
326 12 Microceramic Injection Molding
shape process for the manufacture of complex-shaped or miniaturized parts
with dimensional accuracy. It requires little or no final machining and the high
binder content of the injection molded bodies offers sufficient green strength
for a damage-free demolding procedure. Injection molding can be fully automated
with proper process control, allowing the economic production of large
series of parts. The most crucial process step in injection molding, the debinding
step, is less problematic for microparts owing to their small wall thickness.
However, one has to keep in mind that owing to the thermal process steps,
meeting tolerance issues can be problematic. If a microstructure 200 m in
width shows a variation of only 2 m, the tolerance is 1%. In this respect, the
debinding and sintering processes can have a large impact on the functionality
of the final parts.
As already described in Chapters 10 and 11, the powder injection molding
technique comprises three processing steps: feedstock preparation, the molding
process itself and the subsequent thermal processing, i.e. debinding and sintering.
This is valid for all variants of powder injection molding, micro and macro,
and also metal or ceramic injection molding. In this section, the specific features
of microceramic injection molding (CIM) will be described.
Two variants of ceramic injection molding exist, the industrially established
high-pressure injection molding (HPIM) and the less common low-pressure injection
molding (LPIM). In most HPIM processes, thermoplastic polymers are
used as the binder component, whereas for LPIM paraffin- or wax-based binders
solely are used [5]. In Table 12-1, a comparison between HPIM and LPIM is
shown, concerning the characteristic differences in the shaping process. In spite
of the different process parameters, both methods generate ceramic microparts
with comparable properties (Table 12-2). As HPIM generally requires relatively
high investments for machine equipment and tooling, this technique is usually
applied for large-scale production running with production numbers >10 000
pieces. For product development or for smaller production numbers, the LPIM
technique is an attractive alternative, because of the markedly lower equipment
and tooling costs. Both variants are described below with respect to their capability
for the manufacture of three-dimensional ceramic microparts.
12.1 Introduction 327
Table 12-1 Comparison between HPIM and LPIM: typical shaping properties
Property HPIM LPIM
Major binder component Thermoplastic polymers Paraffin, waxes
Viscosity range (Pa s) 100–1000 1–20
Injection pressure (MPa) >50 0.1–1
Processing temperature (C) 120–200 70–90
Green strength Medium Low
Tool costs High Low
Tool wear High Low
Units Mass production Prototypes and small series
12.2
Basic Requirements for Ceramic Powders and Binder Systems
Ceramic injection molding is a versatile shaping method, as almost every ceramic
material is available in powder form. To be optimally suited for CIM, the
powder should feature a number of characteristics and properties. In general, a
decrease in particle size results in a higher viscosity [7] and a lower moldable
solid content of the powder–polymer mixture [8]. For that reason, often relatively
coarse powders with a mean particle size >1 m are preferred for injection
molding. However, in the case of microparts, submicron powders have to be
used to ensure precise replication of the microscopic details and a good surface
quality, and also to achieve homogeneous properties on a micrometer scale.
Fine powders also exhibit high sintering activity, allowing for a good densification
and providing a high strength of the naturally fragile ceramic bodies. Ceramic
powders with a mean particle size in the region of 0.5 m are preferred for
CIM. Such powders allow the replication of details in the range of a few micrometers
with sufficient quality.
A general finding is that, irrespective of the mean powder size, the powder
moldability decreases as the particle size distribution narrows (Fig. 12-1). Broad
or multimodal size distributions are, therefore, preferred to narrow distributions
to decrease the viscosity [4]. With bimodal powder mixtures, the packing density
can be increased, especially when the particles are very different in size. For a
high packing density, the particle shapes should be spherical or rounded or at
least equiaxial. On the other hand, problems can exist for these useful powder
characteristics. For example, a broad particle size distribution can result in extensive
grain growth [9] and spherical particles reduce the strength of the after
binder removal compacts, whereas in the presence of irregular particles the
handling is eased as the green strength is higher [3].
Owing to adhesion forces between the particles, agglomeration is natural in
ceramic powders. In particular, fine powders, which are obligatory for the shaping
of microparts, show an increased tendency to build agglomerates because of
their high specific surface area. As agglomerates may cause cracks during the
debinding or sintering steps and strength-limiting defects in the final sintered
328 12 Microceramic Injection Molding
Table 12-2 Comparison between HPIM and LPIM: properties of
injection molded zirconia microparts [6]
Property HPIM LPIM
Green density (vol.%) 45–55 50–60
Maximum bending strength, 0 (MPa) 2100 3000
Weibull exponent, m 3.5–10 5.6–11.1
Finest details (m) >2 >2
Aspect ratio >10 possible >10 possible
Surface roughness, Ra (nm) <500 <500
product [10], they must be destroyed in the feedstock compounding process and
the resulting particles must be distributed homogeneously throughout the organic
matrix. However, experiments showed that even with very intensive mixing
of the compound not all agglomerates will be disintegrated [11]. Nevertheless,
a powder that is nearly free of hard agglomerates is essential for the preparation
of a feedstock for the fabrication of microparts.
The lack of plasticity is characteristic of non-clay ceramic powders. However,
plasticity is essential for the injection molding process, so the powders have to
be blended with an organic vehicle to build a compound called the feedstock.
The organic components should not only allow flowability for the filling of the
mold, but also provide the green strength for defect-free ejection of the solidified
body from the cavity. Furthermore, organic media are used to ensure defect-
free solidification in the mold and fast and defect-free removal from the
powder compact, e.g. by pyrolysis or dissolution [12].
The organic binder system usually consists of several organic components.
The general range of the feedstock properties is determined by a major binder
component which is normally a thermoplastic polymer or a wax-like organic.
New binder systems which are based on water-soluble organics promise benefits
for the debinding process. Their low green strength, however, prohibits application
to fragile microparts [5]. Minor components, such as plasticizers, are added
to modify the flow behavior. Dispersants improve the wetting of the particles
and stabilize the dispersion against reagglomeration. The amount of binder depends
on the powder characteristics and covers a range from 15 to 50 vol.% of
the feedstock [13]. In general, it is desirable to attain a high solid content in the
mixture while maintaining a low feedstock viscosity. A solid content well below
50 vol.% causes high shrinkage during the debinding and sintering process,
leading to undesirable results such as poor dimensional control or cracking of
the part.
12.2 Basic Requirements for Ceramic Powders and Binder Systems 329
Fig. 12-1 Viscosity of a dispersion
of particles in a liquid decreases
as the ratio R of large
to small particle size increases.
The figure shows R
values from 1 (monomodal) to
nearly infinity. Reprinted from
[4], Copyright 1995, with permission
from Kluwer
12.3
High-pressure Ceramic Injection Molding (HPIM)
12.3.1
Feedstocks for HPIM
Several commercial binders and ready-to-mold feedstocks are already available
for the production of CIM parts. The most common binder systems are Licomont
(Clariant, Gersthofen, Germany) and Siliplast (Zschimmer & Schwarz,
Lahnstein, Germany). These binders are mixed with the ceramic powder to
form the feedstock. Both binders are also available as ready to mold feedstocks
(e.g. Inmafeed based on Licomont or Elutec based on Siliplast). Another established
feedstock system is Catamold (BASF, Ludwigshafen, Germany). All feedstocks
are available for many common ceramic and metal materials, designed
for a broad range of applications and fitting most requirements of macroscopic
fabrication. The binders Licomont and Siliplast are based on polyolefin wax and
polyvinylalcohol, respectively. The concept of these binders is solvent debinding,
either completely (Siliplast) or partly (Licomont), using e.g. water or alcohol.
The remaining organic components necessary to maintain the stability of the
debindered part, the so-called brown part, are removed during sintering. In contrast
to the binders Licomont and Siliplast, the Catamold system is debindered
catalytically. Thereby the main component polyoxymethylene (POM) is decomposed
in a furnace using nitric acid as catalyst. Owing to the naturally higher
strength of POM, the green stability of molded parts is higher than for parts
made using Licomont- or Siliplast-based feedstocks. However, the Catamold
feedstock shows high viscosity even at higher temperatures, making form filling
of fine details difficult. Commercial binders and feedstocks are designed for
general use and for the production of parts in the centimeter or millimeter
range. They are optimized for short cycle times and easy post-processing, e.g. a
fast debinding procedure. However, for the production of microparts, the use of
these systems is limited. For microparts made by PIM, parameters such as
homogeneous green density and easy defect-free demolding are crucial owing to
the much higher surface-to-volume ratio. The smaller the produced parts are,
the greater are the demands on tolerances and surface quality. To comply with
these requirements, the use of specially designed binders and feedstocks with
fine-sized powders is essential. Therefore, it is necessary to use specially designed
binders consisting of appropriate thermoplastic components, waxes and
additives providing, e.g., better flow behavior or higher powder–binder interaction
[14]. Subsequently these binders have to be mixed together with suitable
fine-sized powders to obtain a feedstock that fulfils the requirements for the
production of microparts.
330 12 Microceramic Injection Molding
12.3.2
Binders for MicroHPIM (HPIM)
The basics of the binders used for the development and production of CIM
feedstocks are similar to those for the binders for MIM feedstocks. For the production
of microparts or micropatterned parts, they contain a significant
amount of at least one thermoplastic component providing sufficient green
strength for damage-free demolding of the green parts. Similarly to MIM, thermoplastics
such as polyoxymethylene (POM), polyethylene (PE), polypropylene
(PP), polyamide (PA) and polyvinylalcohol (PVA) or modified versions of these
components are used as thermoplasts. They are often combined with waxes and
further additives to form a binder system usually consisting of at least three,
and often up to five, components [13]. Binders consisting of several components
are preferred to realize a stepwise debinding process in order to avoid high
pressure or stress in the green compacts during debinding. The waxes are used
to reduce viscosity to improve the mold filling [15] and to provide better debinding
behavior [8, 16]. Further details of these components are described in Section
3.2.2.1. The main difference in binder composition compared with MIM is
the application of additives, so-called surfactants or dispersants, for better powder–
binder interaction. Binders are based on hydrocarbons and are, therefore,
non-polar. The oxide ceramic powders are polar and reactive to water, building
hydroxides on the surface [17, 18]. This leads to a poor interaction between powder
and binder when pure thermoplastic polymers or waxes are used. Better
wetting of the polar surface of ceramic powders with the non-polar binder can
be achieved by using surfactants. Relevant molecules are based on non-polar hydrocarbon
chains (chain length often C16 or C18) carrying a functional polar
group such as carboxylic acids (e.g. stearic acid [13, 19, 20]), amines, esters and
ethers. The interaction of the hydrophilic end of a surfactant with the polar sites
of the powder results in a powder surface with hydrocarbon molecules, that can
be mixed better with the non-polar thermoplastic- and/or wax-based binder
components leading to more homogeneous compounds. The use of additives is
even more crucial when fine ceramic powders with particle size down to the
submicron range are processed. Owing to the about one order of magnitude
smaller particle size compared with metal powders, ceramic powders usually
have a much higher specific surface area interacting with the surfactant and the
other binder components during the mixing process.
12.3.3
Compounding the Feedstock
Mixing of powder and binder is an essential step in the CIM process, as the
feedstock properties have a large influence on the following steps of injection
molding, debinding and sintering. Especially for the shaping of microparts, the
CIM feedstock must have a low viscosity at higher shear rates, even for a small
powder particle size which is necessary for complete filling of the small cavity
12.3 High-pressure Ceramic Injection Molding (HPIM) 331
details of the mold. A high powder content is preferred for the reduction of the
shrinkage during sintering to increase the geometric accuracy. As with higher
powder content the feedstock viscosity increases, a good CIM feedstock is always
a compromise between high powder content and sufficient flowability. Besides,
the compounding method has to ensure good homogeneity of the mix to
avoid problems regarding injection molding or during post-processing. For mixing
CIM feedstocks, there are different types of machines available. Kneaders
are used for development purposes, making small batches of material, whereas
twin screw extruders or shear roll compactors are continuously operating machines
commonly used for commercial feedstock production on a large scale.
Mixing of powder and binder is carried out at temperatures above the softening
point of the binder. The lowered viscosity of the binder components allows efficient
mixing with the powder (see Chapter 11.2). When fine powder particles
are used, this is often accompanied by increasing powder agglomeration. Better
feedstock homogeneity can be achieved when these powder agglomerates are
destroyed by applying sufficiently high shear rates. Because of the serrated arrangement
of the screws (Fig. 12-2), higher shear rates can be realized in an extruder,
compared with mixing in a kneader where only tangential forces are acting.
However, the shear rates that may be applied for the destruction of hard agglomerates
have to be limited to a certain level, depending on the binder constitution,
to prevent chain degradation of the binder. This can be realized with an
adapted screw configuration and appropriate adjustment of the extrusion process
itself regarding, e.g., rotation speed of the screws and overall machine loading.
For development purposes, especially for scientific investigations, a measuring
mixer with a small mixing volume is advantageous [21]. This machine allows
332 12 Microceramic Injection Molding
Fig. 12-2 Machines for feedstock preparation: kneader (W 50 EHT,
Brabender, Duisburg, Germany) and extruder (ZSE 27 HP, Leistritz,
Nürnberg, Germany)
the measurement of material temperature and applied torque necessary to
maintain the preset rotating speed of the rotors. If the loading of the machine
is kept constant, the measured torque directly correlates with the viscosity of
the investigated feedstock. The example presented in Fig. 12-3 shows the development
of torque as a function of the solid loading. Measurement for different
solid loadings was established to determine the critical loading for zirconia powders
in a commercial binder (Licomont). As expected, torque (and hence viscosity)
rises with increased solid loading [19]. A mixed feedstock is said to be homogeneous
as soon as the measured torque reaches a constant level during mixing
and when fluctuations are low, especially at the end of the mixing process. Data
show that for the given example, a homogeneous feedstock is obtained for a solid
loading of 56 vol.% powder or less.
If this powder content is slightly exceeded, the proceeding powder deagglomeration
leads to an increase of the mixing torque. A further increase in the
powder content leads to strong scattering of the measured torque introduced by
the remaining powder agglomerates. With respect to the production of microparts,
it is necessary to limit the solid loading to a value well below the critical
loading. This gives a certain reserve to guarantee automated production even
when some agglomerates still remain in the feedstock after mixing.
12.3.4
Rheology
For injection molding, especially for CIM, where molds with fine details have
to be filled, the moldability and the mold filling behavior of the feedstocks used
are crucial. For proper mold filling, the viscosity of a feedstock has to be sufficiently
low during the injection process. To ensure this demand, the measure-
12.3 High-pressure Ceramic Injection Molding (HPIM) 333
Fig. 12-3 Torques of zirconia feedstocks with different solid loadings, measured in a
kneading measuring mixer (Brabender W 50 EHT)
ment of rheological data in a high-pressure capillary rheometer (HPCR) (Rheograph
2003, Goettfert, Buchen/Odenwald, Germany) is a valuable tool for improvement
of the compound quality, e.g. binder composition or powder loading.
Thereby the viscosity of a compound is measured as a function of the applied
shear rate at a given temperature. Regarding the production of microparts, the
viscosity of the feedstock during mold filling should be as low as possible, preferably
not higher than several tens of Pa · s at shear rates of about 10 000/s or
more.
The powders used for CIM are usually finer than those used for MIM. This is
the reason why the solid loading of CIM feedstocks is lower than those of MIM
feedstocks at a comparable viscosity. Furthermore, the smaller powder particles
of the CIM feedstock with their higher specific surface area lead to higher powder–
binder interaction. Therefore, the phenomenon of powder–binder separation,
described in Chapter 11.2.5, is less critical for CIM feedstocks. In contrast
to the rheological measurement of metal feedstocks, the measurement of ceramic
feedstocks is possible at higher shear rates provided by nozzles of smaller
diameter and/or higher travel speeds of the piston. Especially when using zirconia
feedstocks with an average powder particle size of 0.35 m and solid loadings
of 50 vol.%, viscosity can be measured at shear rates of 10 000/s and
more. As an example, data for feedstocks with solid loading of 48 vol.% using
zirconia powders of different producers are shown in Fig. 12-4. Nevertheless,
the measured apparent viscosity of the feedstocks produced differs over a wide
range although the particle size and specific surface area (BET) of the different
powders are similar. This indicates that particle shape and surface chemistry interacting
with the binder are crucial for the viscosity of a feedstock, giving an
334 12 Microceramic Injection Molding
Fig. 12-4 Viscosity of zirconia feedstocks with a Licomont binder
using different zirconia powders, measured in a high-pressure
capillary rheometer (Rheograph 2003, Göttfert, Buchen/Odenwald,
Germany)
indication of the applicability of the respective type of powder regarding the fabrication
of suitable feedstocks.
Especially for the production of microparts, mold filling and damage-free demolding
are decisive steps. Hence it is necessary to adapt the feedstocks to good
moldability and high green strength. The characteristics of the viscosity to fit these
purposes are illustrated with the data presented in Fig. 12-5. The graph demonstrates
two similar zirconia feedstocks with almost identical solid loading, one
with a commercial binder (Licomont) and the other with a binder based on a thermoplastic
component and a wax. Comparison of the rheological data indicates that
both feedstocks show shear thinning behavior. However, the viscosity of the feedstock
with the commercial binder decreases less with increasing shear rate than
for the feedstock with the thermoplast/wax binder. This means that the thermoplast/
wax-based feedstock shows higher flowability and better form filling when
injected into the mold during processing at high piston/screw traveling speed,
and also better stability after mold filling due to low flowability at low shear rates.
This is helpful for maintaining the shape during solidification. These rheological
characteristics show that for HPIM the feedstocks based on the thermoplast/wax
binders are preferable to those with the commercial binder.
In addition to the measurement of the rheological data, a high-pressure capillary
rheometer is an effective tool for measuring pVT data. Thereby the dependence
of pressure (p), volume (V) and temperature (T) is measured. The significant
volume changes at specific melting temperatures induced by the different
components of the binder give the opportunity to deduce parameters for injection
molding or debinding of the green parts. Furthermore, pVT data are needed
for the simulation of the microinjection molding process regarding feedstock
flow characteristics and mold filling (described in Chapter 3). Fig. 12-6 shows a
12.3 High-pressure Ceramic Injection Molding (HPIM) 335
Fig. 12-5 Viscosity of zirconia feedstocks with thermoplast/wax
binder and Licomont measured in a high-pressure capillary
rheometer
pVT diagram in typical isobaric presentation for a feedstock produced with a
commercial binder. The specific volume at defined temperatures measured for
several pressures shows the behavior of the feedstock when pressure and temperature
are applied. For the presented zirconia feedstock the investigation
shows that the binder consists of at least two significant components. One
melts at temperatures of about 50 C and the other at around 140 C at normal
pressure. Therefore, the temperature for injection molding has to be at least
150C to ensure that the material is properly molten and the viscosity is low enough
for complete mold filling. Although the calculated diagram is drawn in
isobaric presentation, measurement is performed in the isothermal mode when
using a high-pressure capillary rheometer, as the variation of temperature is difficult
to establish. This means that the resulting volume of a sample is measured
for different applied pressures at a preset temperature. Subsequently the
temperature is varied and measurement of the volume as a function of the applied
pressures is repeated for each temperature.
12.3.5
Molding Process
In terms of machinery, there is basically no difference between metal and ceramic
PIM. Principles, requirements and further details were given in Chapter
11. Variations in the injection molding parameters are mainly a function of the
binder composition; the filler material plays only a subordinate role in spite of
the different powder particle size. Differences in the flow properties of metallic
and ceramic feedstocks can be balanced to a certain extent by adjusting the sol-
336 12 Microceramic Injection Molding
Fig. 12-6 pVT data for a zirconia feedstock with Licomont binder
id loading. Owing to the fact that the average particle size of ceramic powders
is about one order of magnitude smaller than, e.g., steel powders, the achievable
solid loading is far lower for ceramic feedstocks. As a result, a longer cooling
time of the tool may be necessary for ceramic feedstocks because of the lower
thermal conductivity of polymers and ceramics in comparison with metals.
Typically, CIM shows more shrinkage during the sintering process, and consequently
the dimensions of the molds are different to those with MIM. This
means that for sintered parts with identical sizes made from metals or ceramics,
two sets of tools and mold inserts have to be manufactured.
Up to now, microparts have usually been injection molded on a substrate plate
which is required for the demolding step. In Chapter 11.3.2, examples of microparts
made by metal injection molding (MIM) are shown which can also be manufactured
by CIM. The outer diameter of the smallest involute toothed gearwheel
is 550 m in the sintered state. The major disadvantage of the use of a substrate
plate is that the microparts have to be isolated mechanically by grinding or mill
cutting the substrate plate. This mechanical machining leads to microdefects on
the surface and to residual stresses, which influence the mechanical properties
of microparts significantly. By further tool developments, e.g. a three-plate mold
which separates the micropart from the runner during the opening movement,
this substrate plate can be omitted. Figs. 12-7–12-9 show a variety of injection
molded microsized and micropatterned ZrO2 parts. The ceramic rings are injection
molded in a fully automated process using a fan gate and automated separation
of micropart and runner system. The diameter of the hole is 160 m in the
green state. The scale of the ruler in Fig. 12-9 is in centimeters. On the left side
in the center a base plate of a planetary gear can be seen; the central planetary gear
is located on the ruler in the lower right. A tensile test bar is shown on the upper
border, surrounded by nozzle plates of a microturbine. The smallest channels of
these nozzle plates are 25 m in width.
12.3 High-pressure Ceramic Injection Molding (HPIM) 337
Fig. 12-7 Ceramic microrings
(ZrO2, green parts)
12.3.6
Thermal Treatment
The parameters for debinding are primarily dependent on the binder components,
whereas the temperature profiles and required atmospheres and pressures
for the sintering process are governed by the material. The basic princi-
338 12 Microceramic Injection Molding
Fig. 12-8 SEM image of a ceramic microring. Bore diameter: 160 m
Fig. 12-9 Various sintered ZrO2 microparts made by HPIM
ples of debinding green compacts and sintering of nearly binder-free (‘brown’)
compacts are given below.
After shaping by injection molding, the binder has to be removed and the
parts must be sintered to obtain dense compacts of the desired material. The removal
of the binder is one of the most critical steps of the process, from both
the technical and economic points of view. Debinding can be carried out by
thermal degradation, solvent extraction or catalytic decomposition of the binder
components or by a combination of these methods. Prior to the described debinding
steps, a wax or an oily component of the binder can be absorbed by
wicking in a powder bed. Owing to the fact that handling of microparts is fairly
difficult and that the parts have to be removed from the powder bed and rinsed,
the method is disadvantageous in this case. For large parts, the debinding time
depends considerably on the wall thickness. Usually this limits the economics
of the whole process to a wall thickness below 5 mm. However, for microparts
with a wall thickness significantly below 1 mm, debinding times are less critical
but still in the range 8–24 h. Here the interaction of the material with the sintering
tray and the conservation of the shape are more important than shortening
the debinding cycle. Furthermore, one run in the debinding furnace will
produce a large number of parts, so the operational capacity is very high even
with long debinding times. Exemplary temperature–time profiles for debinding
and sintering of CIM parts with and without preliminary solvent extraction are
shown in Figs. 12-10 and 12-11. A preliminary solvent extraction of some binder
components creates an open porosity in the green compact, thus reducing the
time required for the thermal debinding step and allowing higher heating rates
and shorter residence times.
The temperature profiles during debinding have to be controlled very accurately
to avoid distortion and the formation of cracks and bubbles. When an
open porosity has been achieved after the initial debinding stage, the polymeric
component should still bind the powder particles together while the temperature
is increasing slowly. The part should be completely free of polymer as soon
as the first sinter necks have formed. Binder decomposition has to be avoided
to prevent residues which influence the base materials and the microstructure
12.3 High-pressure Ceramic Injection Molding (HPIM) 339
Fig. 12-10 Representative temperature–
time profile for the thermal debinding
and sintering of a ceramic feedstock
(ZrO2)
of the final parts. In the case of carbon removal, this process can be assisted
and accelerated to a certain extent by flushing air through the debinding furnace.
To avoid an additional handling step combined with cooling and heating
the parts one more time, it is advantageous if the subsequent sintering process
can be carried out directly after the debinding step using the same furnace.
The sintering conditions, such as temperature profile and atmosphere, mainly
depend on the type of material processed. Batch furnaces running under inert
or oxidizing atmospheres are fairly common. More recently, semi-continuous
running beam furnaces with separated compartments for debinding and sintering
were introduced. Usually the parts experience a linear shrinkage of between
15 and 22% during the sintering process, depending on the powder loading,
the material used and the density reached.
12.3.7
Example of a Typical HPIM Production Cycle
The following example describes the complete production process for the manufacture
of microtensile test specimens, as shown in Fig. 12-12, and bending test
340 12 Microceramic Injection Molding
Fig. 12-11 Representative temperature–
time profile for the thermal decomposition
of residual binder after partial debinding
of the feedstock with a solvent,
followed by the sintering procedure
(ZrO2)
Fig. 12-12 Test specimen for the determination
of mechanical properties of
microparts
bars with a size of 0.260.263.25 mm3. For feedstock preparation, 48 vol.% of
ZrO2 powder type 3YS-E from Tosoh is mixed with 52 vol.% of the thermoplast
and wax binder components at 130C in a laboratory mixer with torque measurement
(W 50 EHT, Brabender, Duisburg, Germany) for 1 h at 30 rpm. To obtain
a moldable feedstock, the registered torque must not exceed 35 N m at the
end of the mixing period. The rheological properties of this mixture are determined
with a high-pressure capillary rheometer (Rheograph 2003, Goettfert, Buchen/
Odenwald, Germany), which provides a granulated feedstock directly after
carrying out the measurement. This feedstock is injection molded on a microinjection
molding machine (MicroSystem 50, Battenfeld, Kottingbrunn, Austria).
The temperatures of the barrel are 60–140–145–150C from the inlet to the nozzle.
The injection speed is in the range between 200 and 400 mm/s depending
on the structure of the cavity. The temperature of the tool is 60C when running
an isothermal process. When the variotherm technique (see Section
11.3.3) is applied, the mold temperature is 85 C for injection and 60 C for demolding.
The injection molded microparts are directly placed on sintering trays
by an automatic handling system. When the sintering trays are completely
filled, they are put into a closed vessel containing an organic solvent (e.g. hexane)
for the first step of the debinding process where the wax component is partially
extracted at 40 C for 8–12 h. After drying the parts for 3 h at 50 C in a
vacuum drying chamber, the thermal debinding step and the sintering process
are carried out in a sintering furnace (RHF-1700, CarboLite, Ubstadt-Weiher,
Germany). The parameters used in this example were developed and optimized
to achieve dense compacts without any cracks, bubbles or distortions and are
listed in Table 12-3. The as-fired microbending test specimens are used for the
determination of mechanical properties. Results of these investigations can be
found in Chapter 20. The mean shrinkage during sintering is 22% linear. To
achieve sufficient mechanical properties, the density should reach at least 98%
of the theoretical density (6.05 g/cm3; value given by Tosoh). The manufactured
microtensile test and bending test specimens show porosities between 0.3 and
0.5% and densities of 5.98–6.03 g/cm3. The measured bending strength is in
the range 1400–1850 MPa, whereas the value for macroscopic specimens is
900–1000 MPa (given by the powder manufacturer). The grain sizes in the sintered
bodies are well below 2 m.
12.3 High-pressure Ceramic Injection Molding (HPIM) 341
Table 12-3 Parameters for thermal debinding and sintering of ZrO2 microbending test specimens
Start temperature (C) End temperature (C) Heating rate (K/min) Hold time (min)
50 180 0.2 60
180 270 0.5 30
270 600 2.0 60
600 1500 5.0 60
1500 100 5.0
12.3.8
Special Development: Two-component MicroCIM
A brief introduction to two-component PIM and two-component MicroMIM was
given in Chapter 11.5, describing a two-component tooling, injection molding
machine and the design of a tensile test specimen with the accompanying mold
insert. The described equipment and mold insert were also employed at the Forschungszentrum
Karlsruhe, Germany, in work on two-component CIM [22,
23]. This research was concentrated on the development and molding of alumina/
titanium nitride feedstocks for electrically conductive ceramics. Using two
different electrically conductive ceramic mixtures, a simple heater was manufactured
for demonstration purposes. Apart from functioning as a heater, other
fields of use could include, for example, electrically conductive components
working under extreme mechanical, tribological or chemical conditions.
With the aim of producing two-component ceramic parts with adjustable electrical
resistivities, a mixture of insulating aluminum oxide (Al2O3) and electrically
conductive titanium nitride (TiN) was chosen [24]. Because of the almost
identical thermal expansion coefficients of the two materials, they are particularly
suitable for this purpose. Moreover, both materials are chemically inert
against one another and TiN shows metallic conductivity with a positive temperature
coefficient of resistivity. It is also important that sintering can be carried
out under a nitrogen atmosphere. Electrically conductive carbides, for example,
have to be sintered under a noble gas atmosphere.
Feedstocks were prepared containing 50–60 vol.% of powder. An exemplary
feedstock composition would be one with a volume ratio of Al2O3 to TiN of
60:40 (ATN40) and 16 wt.% binder. A feedstock of this kind contains 54.8 vol%
of the powder mixture, and, in particular, 21.9 vol.% TiN. Fig. 12-13 shows
green and sintered two-component tensile test bars, injection molded using the
mold insert shown in Fig. 11-36. The cross-section of the narrow part of the tensile
test bar is 0.480.48 mm2 in the green part and 0.410.41 mm2 in the
342 12 Microceramic Injection Molding
Fig. 12-13 Two-component tensile test bars containing two different
mixtures of Al2O3 and TiN (ATN). In each case the interface between
the mixtures lies in the middle of the narrow section of the part.
Green (top) and sintered (bottom) parts. Smallest cross-section of
the green part is 0.480.48 mm
sintered part. The interface between the two different ceramic mixtures, which
contain 35 and 40 vol.% TiN, respectively, is in the center of the narrow part of
the tensile test bars.
Fig. 12-14 a shows a two-component heater made of an ATN feedstock (green
part). The feedstock in the tip of the heater is colored with carbon black for easier
detection of the materials interface. The penetration of the feedstocks into
one another does not seem to be extensive. On the other hand, tests of the
bonding strength in sintered parts show that they do fail outside the interface
area (see Fig. 12-15 a), indicating strong interfacial bonding of the two components.
Fig. 12-14b also shows a sintered heater glowing in a functional test. An
applied voltage of 4.5 V results in a current of 3.5 A, corresponding to a power
uptake of around 16 W. Fig. 12-15b shows an EDX mapping of Ti in the bonding
area. High and low TiN concentrations are in this case separated by a rela-
12.3 High-pressure Ceramic Injection Molding (HPIM) 343
Fig. 12-14 Two-component ATN ceramic heater.
(a) green part. Dimensions: length=22 mm,
cross-section of the legs=1.01.0 mm. The
heating zone of the part (black) possesses a
lower electrical conductivity than the legs.
(b) A sintered part undergoing a functional test
as a heater
Fig. 12-15 Tensile test specimen made of two different ATN
ceramics. (a) Failure outside the boundary zone. (b) EDX
mapping (Ti) of a boundary zone in an ATN27–ATN40 tensile
test specimen. High and low TiN concentrations are clearly
separated
tively sharp line. Despite an injection molding process which allows the feedstocks
to combine in the molten state, a minimal amount of intermixing takes
place. This can certainly be attributed to the high viscosities of the feedstocks
employed.
With regard to the research described here, a substantial amount of development
work is still necessary, e.g. concerning the reproducibility in molding, debinding
and sintering. Generally, research on two-component CIM should be
focused on more fundamental topics such as the adjustment of the sintering
shrinkage of feedstocks, which is a challenge. The behavior of the feedstocks at
the boundary between two components during injection has to be investigated
in addition to the mechanical properties of two-component parts. Further miniaturization
and an expansion of the materials range would also be interesting
tasks.
12.4
Low-pressure Injection Molding (LPIM)
12.4.1
Characteristic Features of LPIM
Low-pressure injection molding (LPIM) is a variant of the powder injection
molding process (PIM), which is virtually identical with the conventional highpressure
injection molding (HPIM). However, there is one essential difference
between the two processes: the pressures employed in LPIM are in the range
0.1–1 MPa, whereas HPIM takes place at pressures of >50 MPa. This difference
arises from the use of a low-viscosity paraffin or wax instead of a high-viscosity
polymeric binder. Owing to the different binder systems, slight differences between
HPIM and LPIM also occur for the feedstock preparation, the injection
molding machinery and the debinding process. Nevertheless, the methods are
closely related as the basic principles of plastic shaping are common for both
processes.
LPIM, which is sometimes also called hot molding [25], was invented by
Gribovski in the former Soviet Union [26], where the process gained similar importance
as its high-pressure counterpart in the USA, Western Europe or Japan.
Recently, LPIM has been strongly propagated as a method for prototyping and
small series fabrication. In contrast to HPIM, with its high costs of tooling fabrication
requiring mass production for a return of investment, LPIM can work
with simple and inexpensive molds and is economic even for a small number
of parts. The good flowabilty of the low-viscosity feedstocks and the ability to
employ fine-sized powders also recommend LPIM for the injection molding of
microdimensional devices [27].
The major drawback of LPIM is the low mechanical strength of the binders
used. This can lead to rupture of patterns during the demolding of the green
compact, especially when fine particulars with high aspect ratio (height to

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