Effects of PEG molecular weights on rheological behavior of alumina injection molding feedstocks
Introduction
Injection molding for ceramic feedstocks containing binders that impart flow during processing is an attractive method for near-net and complex shape production of engineering products [1], [2], [3], [4], [5]. The feedstocks that consists of the ceramic powder and together with the binders are first injected into a shape forming mold cavity. Subsequently, the binders are removed before sintering the ceramic.
The mold injection stage, which determines the shape of the green part, represents a critical step in the PIM process. Mold filling with the PIM feedstock is dependent on viscous flow of the mixture into the die cavity, and this requires specific rheological characteristics [6], [7]. The steady flow of melt feedstock and the filling of the mold is a key feature for successful molding.
Initially, for successful molding, the ceramic–binder mixtures should have either Bingham or pseudoplastic flow characteristics, since increasing shear rates produce lower viscosities that will assist mold filling [8]. However, for the fluid that shows quasi-plastic behavior with a yield stress τy, it should obey the expression:Here τ and γ represent the shear stress and the shear rate, respectively. K is the consistency coefficient which is a constant related to the flow characteristics of a real fluid, and n is the power-law exponent which is defined as flow behavior index of a fluid. For Bingham or pseudoplastic like flow behavior n<1, for a Newtonian fluid n=1. If n>1 the mixtures will be dilatant and result in the separation of the binders from the ceramic powders.
It is clear that the binder is the key that provides the rheological properties and determines whether the resulting feedstock can be injection molded without introducing defects. Many different powder and binder systems have been used for PIM. The most widely applied system is polymer/wax system such as mixture of polypropylene, paraffin wax, and stearic acid (SA) or polyethylene [9], [10], [11]. The advantages claimed for waxes are the low viscosity and superior wetting characteristics. Wax is the major portion of the binder to lower the viscosity of feedstocks. The thermoplastic polymer such as polypropylene or polyethylene, acts as a backbone material to give the strength of system in the green state and offers the general rheological properties required. SA is commonly used as a surfactant that helps the binder wet the powder particles and promotes flow.
Many problems are associated with using these polymer/wax systems such as low green strength and very narrow processing windows [12]. Most of the organic solvents frequently adopted in solvent debinding are flammable, carcinogenic and environmentally unacceptable. Additionally, the green compacts soften in these debinding solvents and expensive machined debinding substrates are necessary if any shape retention is to be achieved [13].
From these considerations and an environmental point of view, polyethylene glycols (PEGs) are used to eliminate the use of unsound solvents to modify the pattern of debinding in alumina injection molding, as they are soluble in water [12], [14]. The unique characteristics of PEG is water soluble and thermoplastic. In addition, they are very safe chemicals and are used quite extensively in food industry; permission was obtained from the local water authorities to dump the water/PEG containing solvents into the drain after debinding [12].
In view of the literatures, many publications have reported useful results concerning the influence of binder composition on the rheological properties of wax-based feedstock [15], [16], [17]. However, the rheological behavior of alumina injection molding feedstock with PEG binder system has not been extensively studied. Owing to the PEG molecular weight has considerable effect on the debinding behavior of the specimen according to our previous investigation [18], the main objective of this research is to characterize in detail the effects of PEG molecular weights on rheological behavior of alumina injection molding feedstock. The flow properties of all feedstocks were measured in a capillary viscometer over a wide range of shear rate and working temperature. The corresponding rheological parameters such as fluidity, pseudoplasticity, and flow energy were determined or generated from the measured viscosity data and the influence of PEG molecular weight on these parameters was analyzed statistically and discussed. These studies will help establish an understanding of the advantages of PEG on rheology for the injection molding feedstock.
Section snippets
Experimental
Commercial purity alumina powder (Japan, Showa Denko, AL-160-SG4) was used in this study. The average particle size was 0.6 μm and specific surface area was 0.41 m2 kg−1.
A multi-component binder system was used to prepare the alumina feedstock. The binders used in the experiment are listed in Table 1. To understand the effect of PEG molecular weight on rheological behavior of alumina injection molding feedstocks, different PEG molecular weights range from 1K to 20K were used, respectively, as the
Yield stress
Fig. 2 shows the shear stresses of the alumina feedstocks with different PEG molecular weights at 90 °C for different shear rates, where shear stress increased with increasing shear rate. The results indicate that τ versus γ plot shows a linear relationship with correlation coefficients greater than 0.96 for all the mixtures. Therefore, these mixtures show Bingham-like behavior, which can be expressed by the following equation [19]:where μp is the plastic viscosity. The yield stress τy
Conclusion
The rheological behavior of alumina–PEG injection-molded feedstocks has been investigated in terms of PEG molecular weights and working temperatures (50–130 °C), over a wide range of shear rate. The results reveals that the feedstock containing high molecular weight PEG as the major binder leads to a higher yield stress and shear stress. The relative viscosity of feedstocks with PEG binder system was small which shows good fluidity and the viscosity was decreased with increasing both temperature
Acknowledgements
The authors wish to thank the National Science Council of Taiwan, ROC, for their support under the project (NSC-87-2216-E-006-042).
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