TECHNICAL PAPER

Ceramic micro parts produced by micro injection molding: latestdevelopments

Tobias Muller • Volker Piotter • Klaus Plewa •

Markus Guttmann • Hans-Joachim Ritzhaupt-Kleissl •

Juergen Hausselt

Received: 10 August 2009 / Accepted: 9 December 2009 / Published online: 30 December 2009

� Springer-Verlag 2009

Abstract Powder injection molding is a preferred

technology for the production of micro parts or microstruc-

tured parts. Derived from the well known thermoplastic

injection molding technique it is suitable for a large-scale

production of ceramic and metallic parts without final

machining. To achieve good surface quality and control the

part size and distortions is an important goal to allow mass

production. This means that all process steps like part design

adjusted for MIM/CIM-technology, appropriate choice of

powder and binder components and injection molding simu-

lation to design the sprue are required. Concerning the injec-

tion molding itself high quality mold inserts, high-precision

injection molding with suitable molding machines like Bat-

tenfeld Microsystem50 or standard machine with special

equipment like variotherm or evacuation of the molding tool

and an adjusted debinding and sintering process have to be

available. Results of producing micro parts by powder injec-

tion molding of ceramic feedstock will be presented.

1 Introduction

The production of metallic or ceramic micro parts in medium

and high quantities can be achieved by using powder

injection molding (PIM). This technique allows the fabrica-

tion of near net shape microparts with nearly no post-pro-

cessing steps (German 1990). To provide a material suitable

for injection molding, fine ceramic or metallic powders are

compounded with a binder to a so called feedstock. By var-

iation of powders, powder content and binder composition

the feedstock can be adjusted to the requirements of micro

powder injection molding (Heldele et al. 2006).

Miniaturization of micro parts produced by micro

powder injection molding is one of the goals of research

done in the context of Sonderforschungsbereich 499

(SFB499), funded by the Deutsche Forschungsgemeins-

chaft (DFG) (SFB499 2009). Within this research program

the scientific basics throughout the whole process of pro-

ducing metallic or ceramic micro-parts by powder injection

molding and micro casting are investigated. Therefore, all

parts of the process-chain from construction via prepara-

tion of production, the production itself up to quality

management are part of these investigations.

2 Experimental setup

The use of powder injection molding for the production of

small micro parts, such as a gear wheel with an outer

diameter of down to \275 lm and a structure height of

about 360 lm requires a high quality mold insert. There-

fore, a microstructured nickel mold insert was manufac-

tured by the combination of X-ray lithography and

electroforming (LIGA technology) at the Institute

for Microstructure Technology at Forschungszentrum

Karlsruhe GmbH (Fig. 1). The full fabrication process

including precision limiting effects is described in detail in

Guttmann et al. (2005) and the influence of electroforming

parameters in Schanz and Bade (2005).

T. Muller (&) � V. Piotter � K. Plewa �H.-J. Ritzhaupt-Kleissl � J. Hausselt

Institute for Materials Research III,

Karlsruhe Institute of Technology, P.O. Box 3640,

76344 Eggenstein-Leopoldshafen, Germany

e-mail: tobias.mueller@imf.fzk.de

M. Guttmann

Institute for Microstructure Technology,

Karlsruhe Institute of Technology, P.O. Box 3640,

76344 Eggenstein-Leopoldshafen, Germany

123

Microsyst Technol (2010) 16:1419–1423

DOI 10.1007/s00542-009-0992-1

The mold insert was fabricated with standard dimen-

sions of 66 9 30 mm2, a thickness of around 5 mm, and

outer dimensions of the layout area 60 9 20 mm2. Further

specifications in order to guarantee the desired function-

ality are the tolerances of outer dimensions including

structure high (±10 lm), the hardness of the electroplated

nickel ([230 HV0.1), the flatness (\40 lm) and the

roughness of the structured front side (\30 nm) as well as

the roughness of the structure bottom (\100 nm).

Two-hundred and forty-eight different test structures

were incorporated into the mold insert like gear wheels,

sprockets (Fig. 2) and a nozzle plate in different sizes and

designs to investigate the accuracies and smallest detail

sizes replicable by micro powder injection molding

(Fig. 3). Smallest structure sizes of 7 lm and maximum

aspect ratios of 35 were realized in this mold insert.

Injection molding was carried out by using a commer-

cially available feedstock of BASF SE called Catamold

TZP-A. This feedstock consists of zirconia powder, stabi-

lized with Y2O3, and a binder based on polyacetal. The

catalytic debinding step that is necessary for this feedstock

composition was done at 110–140�C. During catalytic

debinding the binder removal proceeds from the outside to

inward thus preventing pressure build-up in the interior of

the component.

In this case the catamold feedstock was suitable due to

the design of the mold inserts at the nozzle side and at the

ejector side of the tool. The ejector side consists of a ribbed

plate and the microstructures were positioned at the nozzle

side. To demold the microstructures without defects the

feedstock has to offer low volume shrinkage and high

stiffness of the green parts. Caused by the high viscosity of

the catamold feedstock variotherm injection molding pro-

cess had to be used. Thereby the cycle time increases. This

shows the need of feedstock development for the produc-

tion of micro parts with high aspect ratios by powder

injection molding. The feedstock also has to show a high

strength to withstand the forces occurring during the

demolding process. To demold the microstructures without

ejectors placed in the structure they were supported by a

substrate plate which was molded with the same feedstock

as the microstructures during the same molding cycle. The

great disadvantage of this technique is that the micro-

structures have to be singularized after injection molding.

This was done by mill cutting and grinding the substrate

plate in green part state. The singularized micro parts were

then debinded as mentioned above and sintered at 1,500�C

for 1 h.

2.1 Injection molding results

The injection molded and sintered parts reached theoretical

densities of about 99%, which suits the statements of the

data sheet provided by BASF AG. The microstructures

were investigated by using a Zeiss Supra 55 scanning

electron microscope and showed a medium grain size of

about 200 nm and very few pores (Fig. 4).

Earlier experiments showed that small gear wheels with

an outer diameter of about 560 lm can be fabricated byFig. 1 LIGA mold insert for SFB499 with different test structures

Fig. 2 SEM image of gear wheels and sprockets

Fig. 3 SEM image of nozzle plate in LIGA in LIGA mold insert

mold insert

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powder injection molding (Merz 2004). In this work the

minimum outer diameter of gear wheels could be reduced

to 275 lm, which equals a reduction to about half of the

size that was realized before (Fig. 5).

In addition to reducing the size of single components the

replication of very small structural details was one of the

main goals of the work presented in this paper. Smallest

details with very high aspect ratios can be found in the

nozzle plate (Fig. 6). The trenches of this nozzle plate were

limited to a size of about 20–25 lm in earlier experiments

(Merz 2004). By using the new LIGA mold inserts trenches

with a minimum width of 7 lm could be replicated

(Fig. 7).

The SEM investigations showed a lot of flaws on the

components. Some structures could not be replicated and

others showed fractures on the surface of the microstruc-

ture. Therefore, the mold insert and green parts were

compared with SEM characterization. It was found that

most of the defects found at the microstructures can be

explained by flaws on the corresponding parts of the LIGA

mold insert as well as tensions caused by shrinkage during

cooling of the mold insert before ejecting the molded parts.

To reduce the exhibited stresses injection molding with an

insert plate shall be conducted. In this case only small areas

of the mold insert will be gated so that a reduction of the

molded area leads to a reduction of volume shrinkage

which should reduce shear stresses acting on the micro

parts during the cooling step.

To get more information of the actual part sizes in

comparison to the dimensions of the used mold insert and

green parts, topographic measurements were made by

using a chromatic confocal white light measuring device

MicroProf of FRT GmbH. Results of this measurements

are presented in Table 1 and in Fig. 8.

Fig. 4 SEM image of microstructure made of sintered ZrO2

Fig. 5 Gear wheel made of ZrO2, made of sintered ZrO2 (outer

diameter 275 lm)

Fig. 6 SEM image of nozzle plate made of sintered ZrO2

Fig. 7 Detail of trench in nozzle plate; made of sintered ZrO2 Trench

width is about 7 lm

Microsyst Technol (2010) 16:1419–1423 1421

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Roughnesses of the mold insert surface are the best

because of the polishing done. Roughness in the ground of

the nickel structures is relatively larger because it repre-

sents the polished PMMA resist sheets used in the LIGA

process. Roughness of the structured side walls could not

be influenced, because it depends on the quality of the gold

absorbers on the working mask in the LIGA process

(Guttmann et al. 2005). The sintered part shows Ra values

from 0.17–0.19 lm which corresponds to the grain size

that can be seen in SEM images. Roughness of the green

body is slightly better because of the compensating influ-

ence of the binder.

Topographic measurements were also used to determine

the structure height of the nozzle plate. The mold inserts

show heights of about 383 lm, green parts show step

heights around 362 lm and sintered parts of about 277 lm.

This equals a sinter shrinkage rate of 24%, which is in the

range given by the datasheet for the material used for the

experiments.

3 Summary and outlook

In this work it is shown that the production of ceramic

micro parts with very small details and high aspect ratios is

possible by using the micro powder injection molding

technology (PIM). Structure details smaller than 10 lm

and aspect ratios [30 could be replicated by using high

quality mold inserts produced by LIGA technique. How-

ever, there are still some problems to be solved in order to

be able to provide a stable process for manufacturing micro

parts with high qualities.

Therefore, further material and process development is

going on. These experiments will mainly deal with creating

a higher strength of the injection molding green parts to

avoid damaging the micro-structures during the demolding

step.

To avoid the process step of milling away the supporting

structure it is planned to establish a tool technology that

enables a direct molding of singular structures in a mold

insert produced by LIGA technology.

Acknowledgments We would like to thank all our colleagues at the

Forschungszentrum Karlsruhe (FZK) for their helpful support.

A special thank for financial support is dedicated to the Deutsche

Forschungsgemeinschaft (DFG, German Research Foundation) in

context of SFB499.

References

German RM (1990) Powder injection molding. MPIF, Princeton

Guttmann M, Schulz J, Saile V (2005) Lithographic fabrication of

mold inserts. In: Baltes H, Brand O, Fedder GK, Hierold C,

Korvink JG, Tabata O (eds) Advanced micro and nanosystems,

vol 3. Microengineering of metals and ceramics. Wiley-VCH

Verlag GmbH & Co. KGaA, Weinheim, pp 187–219

Heldele R, Schulz M, Kauzlaric D, Korvink G, Haußelt J (2006)

Micro powder injection molding: process characterization and

Fig. 8 Topographic illustration of a different nozzle plates and b detail of a single nozzle plate with a structure height of 360 lm

Table 1 Topographic measurements of structure height and roughness

Measured parts Structure height (lm) Ra structure surface (lm) Ra structure ground (lm)

LIGA mold insert 384 0.02 0.17

Green part 363 0.09 0.14

Sintered part 277 0.19 0.17

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