Technical Paper: PIM International, Vol.3 No. 4 December 2009, pages 60-65, 2741 words

Authors: Seokyoung Ahn [1], Seong-Taek Chung [2], Seong Jin Park [3], and Randall M. German [4]

[1] Rapid Response Manufacturing Center, The University of Texas-Pan American, 1201 W University Dr., Edinburg, TX 78539, USA
[2] CetaTech, Inc., TIC 296-3, Seonjin-Ri, Sacheon-Si, Kyongnam, 664-953, Korea
[3] Mechanical Engineering, POSTECH, San 31, Hyoja-Dong, Pohang, Kyongbuk, 790-784, Korea
[4] College of Engineering, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1326, USA

Abstract

Simulations used for plastics have been applied to PIM, but the high solid content often makes for differences. Several situations demonstrate the problems, such as slip phenomena, powder-binder separation, high inertia effects and rapid heat loss. Thus, the simulations to analyse the PIM process build from the success demonstrated in plastics, but adapt those concepts for filling, packing, and cooling stages. Demonstration of usefulness and optimisation for moulding defects, balanced filling, delivery system and design of experiments is discussed.

Introduction

A typical injection moulded component has a thickness much smaller than the overall largest dimension. A typical wall thickness is in 1- 3 mm range, while the longest dimension might range near 25 mm with an overall mass near 10 g. There is much variation, but these values offer a glimpse at the typical components [1,2]. In moulding such components, the molten powder-binder feedstock mixture is highly viscous. As a result, the Reynolds number is low and the flow is modelled as a creeping flow with lubrication, as treated with the Hele-Shaw formulation. With the Hele-Shaw model, the continuity and momentum equations for the melt flow in the injection moulding cavity are merged into a single Poisson equation in terms of the pressure and fluidity. Computer simulation is usually based on a 2.5D approach because of the thin wall and axial symmetry. But the Hele-Shaw model has its limitations and cannot accurately describe 3D flow behaviour in the melt front, which is called fountain flow, and special problems arise with thick parts with sudden thickness changes, which cause race-track flow.

Today, several 3D computer aided engineering simulations exist that successfully predict conventional plastic advancement and pressure variation with changes in component design and forming parameters [3]. For 3D PIM simulation, Hwang and Kwon [4] developed a filling simulation with slip using an adaptive mesh refinement technique to capture the large deformation of the free surfaces, but this is computationally intensive [4-7], so further research is moving toward simplified solution routes [3].....

Further sections of this paper include:

- Part I. Development of a micron-size MIM powder
- Experimental material and procedure
- Powder characterisation
- Binder system and mixing
- Results and discussion
- Behaviour of powders in the feedstocks
- Tap density as criterion for MIM powders
- Ability of Powder 3 to be held in different binder systems
- Part II. Complete evaluation of PA-FN08
- Introduction
- Experimental
- PA-FN08 technical characteristics
- F-FN08 feedstock preparation and injection moulding
- Debinding and sintering conditions
- Results and discussion
- Green parts
- Sintered parts
- Mechanical properties
- Microstructure evaluation
- Conclusion
- Acknowledgement

Figures and Tables:

Fig. 1 SEM pictures of powders: Powder 0 top left, Powder 1 top right, Powder 2 lower left, Powder 3 lower right

Fig. 2 Evolution of critical solid loading for the four powders

Fig. 3 Torque as a function of solid loading for four different powders

Fig. 4 Link between critical solid loading and tap density

Fig. 5 Critical solid loading as a function of paraffin wt%, for Powder 3

Fig. 6 SEM of the PA-FN08 powder

Fig. 7 Particule size distribution (laser diffraction) of PA-FN08 powder

Fig. 8 Viscosity versus shear for F-FN08

Fig. 9 MIM tensile bar dimensions (ISO2740)

Fig. 10 Optimised sintering cycle for F-FN08

Fig. 11 Weight and density of the sintered parts

Fig. 12 Linear shrinkage rate from green to sintered state

Fig. 13 Porosity evaluation: top row, PA-FN08 (left 100µm, right 20µm). Bottom row Iron-Nickel 8% mix, (left 100µm, right 20µm)

Fig. 14 Microstructures after nital etching. Top row PA-FN08 after nital etching (left 100µm, right 20µm). Bottom row Iron-Nickel 8% mix after nital etching (left 100µm, right 20µm)

Table 1 Physical analysis of the four powders

Table 2 Chemical and physical characteristics of PA-FN08 powder

Table 3 Moulding parameters

Table 4 Weight and dimensions of green parts

Table 5 Chemical composition of the sintered parts after sintering

Table 6 Dimensions of the sintered parts

Table 7 Mechanical properties of the sintered parts

Table 8 Comparison of porosity