Journal of Mechanical Science and Technology 27 (6) (2013) 1815~1824
www.springerlink.com/content/1738(494x
DOI 10.1007/s12206(013(0432(0
Determination of the flow stress and thermal properties of ceramic powder
feedstock in ceramic injection molding
†
Jihoon Hwang
1
, Sunchul Choi
2
, Seokmoo Hong
2
and Naksoo Kim
1,*
1
Department of Mechanical Engineering, Sogang University, Seoul, 121742, Korea
2
Global Production Technology Center, Samsung Electronics, Suwon, Korea
(Manuscript Received August 15, 2012; Revised December 17, 2012; Accepted January 2, 2013)
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To simulate numerically the material behavior of a ceramic powder feedstock that consist of a two(phase mixture of zirconia powder
and polymer binder, a material model is needed that incorporates the change in volume fraction and temperature dependency of viscosity.
Heat transfer occurs between the feedstock and the mold during ceramic injection molding (CIM). The feedstock is heavily influenced by
thermal properties such as thermal conductivity and specific heat. In this study, three models are proposed to explain the material and
thermal properties: a rigid(plastic flow stress model that is dependent on volume fraction and viscosity, a thermal conductivity model,
and a specific heat model as a function of temperature. The material parameters in each model are obtained by using the optimization
method. Error functions are defined as the differences between the experimental measurements and numerical simulation results. The
parameters are determined by minimizing the error functions. The confirmation simulation for each model is conducted by applying
cases that are not directly used in the optimization. The results of the confirmation simulation tend to follow the experimental results well,
with correlation coefficients exceeding 0.92. The numerical simulation of the CIM process with the determined parameters is compared
with the flow behavior of an actual CIM process. Simulation results, such as flow pattern and direction, are in good agreement with the
measured feedstock behavior. Therefore, the method for determining the material parameters of the proposed models is feasible.
Keywords: Ceramic injection molding; Ceramic powder feedstock; Rigid(plastic flow stress model; Thermal properties; Optimization
Ceramic injection molding (CIM) is a forming process used
to inject a mixture of ceramic powder and polymer binder into
a mold at 135°C to 200°C. CIM is a complicated process that
includes several steps such as kneading, injection molding, de(
binding, and sintering. CIM is an efficient process for the
high(precision manufacture of large quantities of thin and
complex 3D parts. Defects such as weld lines are formed in
injection molding when the flow direction of the feedstock
separates into two flow lines and then recombines into a single
line. High(pressure is required in the CIM process for the in(
jection of feedstocks into molds because feedstocks have high
viscosities and are sensitive to the changes in volume fraction
and temperature. The flow of feedstock is sensitive to tem(
perature changes caused by heat transfer, whereas thermal
properties such as thermal conductivity and specific heat de(
pend on changes in temperature. Thus, defining process condi(
tions, such as injection temperature, gateway location, and
injection pressure, is difficult. The design of CIM process
conditions generally depends on trial and error rather than on a
systematic approach. Poor process conditions cause bending
and twisting failures after the sintering process of CIM prod(
ucts. Thus, establishing a numerical tool is important to simu(
late CIM processes, predict feedstock behavior, and determine
fragile locations during and after the CIM process. Previous
studies have developed simulations for CIM processes. Kwak
and Seo (2009) predicted the weld line location and flow pat(
tern in a CIM process by using a commercial injection mold(
ing simulation software (3D(TIMON) and found that changes
in sintering shrinkage depend on flow direction [1]. Soykan
and Karakas (2001) studied a new method in which a ceramic
powder and binder can be uniformly mixed to produce a ho(
mogeneous distribution of the volume fraction after injection
molding [2]. Wu and Wei (2004) modeled the rheological
behavior with respect to the homogeneity of the feedstock by
using the Casson model to obtain the relationship between
shear stress and viscosity [3]. Lee et al. (2000) compared a
finite element method (FEM) simulation that applies sintering
and grain growth models based on diffusion creep with ex(
perimental results to analyze the sintering process systemati(
cally [4]. Luo et al. (2006) studied the influence of powder
characteristics such as particle size in a CIM process [5].
*
Corresponding author. Tel.: +82 2 705 8635, Fax.: +82 2 712 0799
E(mail address: nskim@sogang.ac.kr
†
Recommended by Associate Editor Dae(Cheol Ko
© KSME & Springer 2013