Recent Advances in CIM Technology

In this article the PIM (Powder Injection Moulding) technology is described in brief. After that the benefits and advantages were analyzed and summarized. Ceramic injection moulding (CIM) process was analyzed in more detail: CIMalumina, CIM-zirconia and CIM ferrites as the most common technical ceramics in CIM ceramic parts production, medical applications and accessories in chemical laboratories, and cores in electronic inductive components. After that our results for CIM barium hexaferrite and piezo ceramics (barium titanate) are given. The main powder characteristics, the shrinkage and density and the main electrical characteristics of the sintered samples were compared for the isostatically pressed PM (powder metallurgy) and CIM formed samples. SEM fractographs of CIM and PM samples are given for CIM green parts, debinded (white) parts and sintered parts, and PM green parts and sintered parts. The results obtained were compared to literature data before they were applied in ceramic components production.


Introduction
PIM (powder injection moulding) is a net-shaping process, which enables the production of parts of complex shapes in highly automatised production processes.PIM allows the fabrication of unique geometric structures that are difficult to produce with other metal-working technologies.PIM combines the techniques of plastic injection moulding and powder metallurgy, including sintering.The main process consists of four steps: such as feedstock preparation, injection moulding (green samples forming), debinding (binder removing) procedure and the sintering process.
PIM is a metal and ceramic shaping procedure, using a feedstock of composite granulate.The feedstock is prepared by mixing metal or ceramic powders with plastificators such as wax, thermoplastics, silicone, agar-agar etc. Melted plastificators enable the powder to be injection moulded in the prepared casting tool (green parts).The green parts are then debinded thermally or by a solvent and finally sintered.
In this work, important factors for PIM applications such as type of material, particle size, size and sample geometry, and tool casting complexity are discussed.Additional attention was paid to the sintering of PIM samples: linear shrinkage, weight loss, density, microcracks and microstructure development.Differences between uni-axial die compacted and PIM shaped samples in the sintering regime were also analysed.
Examples from literature and practice are given for metal and ceramic powders used for new PIM shapes.Based on the introduced data, PIM application and its evolution is analyzed through the introduction of new materials and geometries suitable for technical parts like electronic components and ceramic sensors.

CIM (Ceramic Injection Moulding)
Ceramic injection moulding (CIM) uses ceramic powders.Materials like alumina, zirconia, titania, ferrites, yittria etc. are used.The feedstock is made of fine powders, binders and additives as described in the introduction above.The debinding process is similar to MIM, but the sintering process depends on the type of ceramics and their electronic properties.Linear shrinkage reaches 15-20 %, retaining the complex shape.Close tolerances can be obtained with good process control, and there is no need for mechanical finishing, which is a pronounced advantage in particular for ceramics which are extremely difficult to machine.CIM was applied in ceramic -mechanical parts, sensors, and artificial bones in medicine etc. Injection moulding of ceramics is a new and innovative process that provides cost effective solutions for design engineers.It provides ceramic components with complex geometries for mass production.The benefits of CIM can be summarised as: • Providing unique, economic solutions to increasingly stringent material and product design requirements • Excellent batch to batch repeatability and process capabilities achieving a tolerance smaller than ±0.3 % • High surface finish quality without the need for additional finishing processes • Accommodates extremely complex geometric components • Superior material performance, high hardness and mechanical strength, wear, corrosion and weathering resistant, dimensionally stab., high working temperature and good electrical insulation • Also used for metallised applications CIM applications can be found in aerospace (mechanical parts, sensors and actuators), communications, automotive (mechanical parts), electronic (sensors and actuators), chemical (valves, membranes), medical (artificial bones), oil & gas exploration (sensors, valves), etc.

CIM Alumina
Aluminium oxide and zirconium oxide are ceramics with high mechanical hardness, high electrical resistivity and thermal conductivity, but low thermal expansion.They have good strength and stiffness, good wear resistance, good corrosion resistance, good thermal stability, low dielectric constant and loss tangent, low weight etc.This is very suitable for use in technical ceramic products, electronic components, and medical products.CIM alumina and zirconia exhibit properties close to the pressed and sintered samples [1,2,3] (see Tab. I).Calcined alumina powder was selected for the investigation with purity >99.7 %, green density 3.97 g/cm 2  Debinding methods include: 1. Thermal elimination of organic components, 2. Debinding by supercritical carbon dioxide (temperatures above 330 K and pressures near 300 bar), 3. Catalytical debinding process as commonly used for binders.Recent investigations on CIM of alumina and zirconia are numerous: effect of powder treatment on injection moulded zirconia [4], binder removal from injection moulded zirconia ceramics [5], viscosity of powder injection moulding feedstock and optimization of binder volume concentration [6], differential sintering in ceramic injection moulding: particle orientation effect [7], influence of surfactant on rheological behaviours of injection-moulded alumina suspension [8], sintering of nano-sized yttria stabilized zirconia process by powder injection moulding [9], feedstock aids micro PIM parts production [10], and novel alumina/cyanoacrylate green ceramic [11].

CIM Ferrites
Soft magnetic ferrites of spinel type AB 2 O 4 (B=Fe, Co, Ni; A= Mn-Zn, Ni-Zn and Mg-Zn) are widely used in electronics: transformers, choke coils, EMI filters, antennas and microwave waveguides.The production of Mn-Zn ferrite ceramics by injection moulding [12] was enabled by using binders such as: combinations of polypropylene, microcrystalline wax and stearic acid.After debinding processes similar to those for MIM described above, the ferrite samples were sintered at 1280-1320 °C/1-4 h in nitrogen atmosphere.The achieved results are given in Tab.II [13].
Tab. II.The main properties of injection moulded sintered Mn-Zn ferrites on toroids.Hard ferrites of M type (magnetoplumbite) known as Ba and Sr hexaferrite MFe 12 O 19 (M=Pb,Ba,Sr) are used as permanent magnets.CIM is used currently in manufacturing of complex anisotropic hard ferrite shapes [14].The feedstock was prepared by mixing Ba and Sr ferrite powder with polypropylene/polyethylene-glycol.After CIM shaping of hard ferrite samples in the magnetic field for orientation of the particles, the binder was removed in two steps -extracting and thermal debinding.CIM hard ferrite samples were then sintered for 1 h at optimised conditions.The results obtained are given in Tab.III.CIM hard ferrite anisotropy was obtained under a magnetic field of 632 kA/m [15].The results obtained for magnetocrystalline grain orientation are given in Fig. 3. Anisotropy is also investigated for injection moulded and pressed polymer bonded magnets: magnetic and structural properties of Ba M-type ferrite composite powders [16], optimization of ceramic magnets anisotrope processing [17], of SiO 2 and CaO additions on the microstructure and magnetic properties of sintered Sr-hexaferrite [18], orientation of c-axis of Sr-ferrite particles in rubber magnets [19].CIM soft and hard ferrite products (magnetic cores) for different applications are given in Fig. 4.

CIM PZT Materials
Piezo ceramics are applied in the industry of electronic components such as chip capacitors, filters, sensors and actuators.The main electrical parameters (resonant frequency and their tolerance) are connected to ceramic device dimensions and electrode surface value and their arrangement [21][22][23][24].That is the main reason why piezo devices are planar with thick film electrodes and why they are known as laser trimmed devices.Moreover their electrical characteristics depend on chemical composition, heat treatment, microstructure and dopants [25][26][27][28][29][30][31].Furthermore their mechanical and electrical characteristics depend on the ratio of their main constituents (PZT-Pb(Zr, Ti)O 3 , BLT-(Bi, La) 4 Ti3O 12, BT-BaTiO 3 ) (see Tab. IV and V) [32,33].Hence the piezo devices are produced by classical procedures such as tape casting, isostatic pressing of powder; polymer bonded types and very rarely by PIM technology [32].The ratio of PZT-binder for the feedstock was 74:26 % vol .Polymers used for binder aimed for the injection of PZT ceramics are very common: paraffin-wax 65 %, (ethylene vinyl acetate, polyvinyl alcohol) 35 %.PIM green samples of PZT were thermally debinded and sintered from 950-1250 °C, depending on chemical composition.The green PZT micro components and morphologies of PZT powders are given in Fig. V.The initial PZT powder was submicronic.Investigation of the influence of thermal treatment on the morphologies, dielectric and ferroelectric properties of PZT-based ceramics is continued [34].

CIM Ba Hexaferrite (isotropic)
Selected SEM micrographs of PIM and PM Ba hexaferrite are shown in fig.6.The microstructure of PIM Ba hexaferrite green samples made by melt feedstock injected into the mould is shown in fig.6 (a); particles around 1.6 µm can be seen together with binder.After the debinding process, when most of the binder was removed by solvent, the same particles can be seen very clearly as shown in fig.6 (b).The sintered microstructure of PIM samples is achieved by sintering at 1250 °C/2h in air.The fracture surface of sintered PM Ba hexaferrite samples, achieved at the same sintering profile as for PIM samples, is shown in fig.6 (d).A small gear, our first isotropic Magneto-PIM product of Ba hexaferrite is shown in fig.6 (e  CIM Ba hexaferrite (isotropic) results (SEM Fig. 6 and Tab.VI) are quite close to the best results of known hard ferrite producers [35,36].Our PIM and PM samples (Tab.VI) do not differ much in density (5.05 vs. 5.11 g/cm 3 , respectively), and they have similar magnetic properties.We have not yet produced PIM anisotropic samples (PIM shaping in applied magnetic field) to compare the results with anisotropic results given in Tab. 3, which is to be done in the near future.

CIM PZT Piezo Ceramic
Selected SEM micrographs of PIM and PM PZT piezo ceramic are shown in Fig. 7.The microstructure of PIM PZT piezo ceramic green samples made by melt feedstock injected into the mould is shown in fig.7 (a); particles around 4 µm can be seen together with binder.After the debinding process, when most of the binder was removed by solvent, the same  The CIM PZT results (SEM in Fig. 7, Tab.VII) are very close to the PZT data given in Fig. 5 and Tab.IV and available literature data, [32,33].The PIM and PM density results (Tab.VII) are quite close, implying small differences in piezo electric properties as well.Our main product, micro resonator tube made of PZT ceramics and shaped by µ-PIM, emits acoustic signals in the frequency range of the human ear.The piezo properties attained are comparable to those given in the literature [21,28,34].Our experiments are continuing.

Conclusion
Initially, our interest in PIM was to develop some metallic mechanical parts with high complexity for industrial applications such as micro gears, heat sinks, fly wheel, lock part cutting tool mechanical parts through using the MIM procedure.Later, experiments were started with ceramics and then sensor materials: ferrites and PZT ceramics using the CIM procedure.It was a difficult task to control the functional properties of ceramics by controlling micro-structural development.
Comparison between PIM and PM (powder metallurgy) samples is a common method in research of materials, which the authors have also applied in their experiments.The values to be compared are usually particle average grain size, density and porosity, shrinkage, hardness, brittleness and the main functional properties of the materials.
Previous results for PM technology was fundamental, thus CIM samples were followed by PM samples compacted from powder (dry pressed).Two optimisations in pressures and sintering temperatures were done in parallel to ensure simultaneous analysis of both (PM and PIM) optimum to achieve the optimum mechanical and electrical, thermal, magnetic and piezo properties.In all PIM experiments the feedstock was prepared using a solvent binder system (wax, thermoplast and additives).
CIM samples have the same starting powder particle size (1.6 and 4 µm on average for Ba hexaferrite and PZT piezo ceramics) as such manufactured by PM but differ in pressure applied: PM pressures are also several times higher.It is well known that pressing (compacting) gives the first significant contacts between particles, which enhances subsequent sintering; nevertheless, the PIM route yields excellent results even without this compacting step.
Of course, direct comparison of products obtained by PIM and PM press-and-sinter techniques, respectively, is difficult, due to the strong effect of the manufacturing process on the final properties, but in any case it can be concluded that both for metallic and ceramic materials, powder injection moulding is attractive for combining complex 3D geometries with excellent material properties.

Fig. 5
Fig. 5 Green PZT micro components (a), depth of green PZT micro component (b) and micrograph of PZT powder particles (c).

Ceramics d 50 [µm] Density [g/cm 3 ] Density [% TD] R max [µm]
Sr hexaferrite with variation of sintering temperature T sint , BB r denotes the remanent induction, H c the coercive force, and BH the hysteresis energy density.

Tab. VII. CIM
and PM samples sintered in air