Technical Paper: PIM International, Vol.4 No.1 March 2010, pages 55-65, 5013 words

Authors: Ian Andrews [1], Kazuaki Nishiyabu [2] and Shigeo Tanaka [3]

[1] Dept. of Industrial Systems Engineering, Osaka Prefectural College of Technology, Osaka, Japan  [2] Associate Professor, Department of Mechanical Engineering, Kinki University, Osaka, Japan
[3] President, Taisei Kogyo Co, LTD., Osaka, Japan 

Abstract

Many people are familiar with computational fluid dynamics studies for plastic injection moulded parts but little study has been undertaken in the field of micro metal injection moulded parts.

Micro metal injection moulding (MIM) is a relatively new technology and many advances are progressing in production methods that increase accuracy and applicability of its use.

Computational fluid dynamics studies of such methods can assist in furthering the advancement of this technology and can provide valuable information to manufacturers and consumers alike.

A study on two micro-MIM manufactured test parts was done using Moldex 9.0 CFD and Rhinoceros 4.0 (with Moldex3D-Mesh plug-in software) Modelling software. The methods for realising some common problems unique to micro-MIM analysis were proposed and applied to two micro-size MIM parts to find suitable process conditions in this paper.

The results of this study were used to check for production viability and to improve the design of the test parts for possible future production. An ideal set of process conditions and assumptions was found for MIM CFD analysis performed in Moldex 9.0

Introduction

Computational Fluid Dynamics (CFD) is the science of determining a numerical solution to the governing equations of fluid flow whilst advancing the solution through space or time to obtain a numerical description of the complete flow field of interest. CFD analysis consists of 3 main stages; Modelling and Meshing, Pre-Processing and Post-Processing. Each stage can be further split down into smaller sections.

Modelling is the first stage that is undertaken with all CFD processes. This involves making 2D or 3D models of the geometry of the areas where the fluid flow analysis is required. For injection moulded parts, this usually consists of models of the part, its mould and the cooling arrangement with the mould. After modelling the geometry of the required areas, these areas are required to be split into many small areas called meshes. The many complicated equations involved in CFD are solved in each of the many small areas throughout the geometry and flow properties can be derived from the results.

There are many types of mesh, each suitable for various geometries or analysis methods. Pre-processing involves using the models and meshes created and inputting the desired material properties, flow properties, temperatures, process durations etc. For injection moulding analysis, the injection machine properties such as injection pressure and packing length etc are often specified. The computation methods are also set at this stage. The type of mathematical solver, desired accuracy and various assumptions and simplification can be set here. Once all the properties and conditions are set the CFD analysis can be started.

Post-processing is the final stage and is undertaken after the CFD analysis has finished obtaining a solution. In this stage the flow properties can be analysed and various images, graphs and data can be seen. This data can be used as a qualitative tool for discarding (or narrowing down the choices between), various designs. Designers and analysts can study prototypes numerically, and then test by experimentation only those which show promise......

Further sections of this paper include:

Analytical Procedure
- Modelling
- Meshing
- Pre-processing and Materials
- Post-processing
Results and Discussions
- Determining micro-analysis process conditions
- Improving accuracy of micro-analysis
Conclusions
Acknowledgments
References

Figures and Tables:

Fig. 1 The geometry of the micro-parts

Fig. 2 Project settings in the Process Wizard

Fig. 3 The initial test with cold-slug gate in the runner

Fig. 4 Weld Lines form in the circled location. This area would be weaker in comparison to the rest of the part

Fig. 5 The result summary window from Moldex 9.0

Fig. 6 Shear stress during filling. The areas on the sides of the hole show very high stresses which would lead to problems

Fig. 7 Shear rate during filling. The areas on the sides would be subject to high viscous heating amongst other problems

Fig. 8 Temperature distribution. The region in the circle is the area of no calculation

Fig. 9 Volumetric shrinkage at EoP. The imbalance leads to warpage of the part 

Fig. 10 The alternative geometry of the part, gate location and runner

Fig. 11 The Melt Flow Front and the distribution. The shear rate is still in the order of 10 times the recommended level

Fig. 12 The Shear Stress distribution. All values of shear stress fall under 0.6 MPa

Fig. 13 The Shear Rate distribution. Values range between 20000-50000/s

Fig. 14 The melt flow front. Incomplete filling can be seen indicating short-shot

Fig. 15 The Shear Stress and Shear Rate distributions. The critical areas of the part now have a value of approximately 0.4MPa and 7000-30000/s respectively

Fig. 16 The volume shrinkage distribution of the alternative geometry

Fig. 17 Shear Rate Distribution of Run 4, showing the cross-section near the gate and the relatively low shear rate

Fig. 18 Model with no central hole

Fig. 19 Melt Front Flow Time and Volumetric shrinkage at End of Packing phase

Fig. 20 Shear rate distribution at the gate location

Fig. 21 Geometry of the micro-gear part

Fig. 22 Geometry of the runner/gate locations

Fig. 23 The shear stress distributions for the micro-gear tests. Many small areas of no calculation are present

Fig. 24 The runner cap shape and location

Fig. 25 A cross-sectional view of the runner cap model and mesh elements. The cap area above the red line must be removed after manufacture. The internal tetra mesh elements can be seen

Fig. 26 The volumetric shrinkage distribution for the capped model. The shrinkage is symmetrical and more balanced than previous models

Fig. 27 The shear stress distribution for the runner-cap model with BLM in Moldex 9.0

Fig. 28 The temperature distribution for the runner cap model using tetra mesh in Moldex 9.0

Fig. 29 Showing the cross-section of the BLM mesh. Two very thin prism elements can be seen outside the tetra elements

Fig. 30 The temperature distribution for the runner cap model using BLM mesh in Moldex 9.0

Fig. 31 The warning message W1502 in the packing log file in Moldex 9.0

Fig. 32 The temperature distribution for the runner cap model using BLM mesh in Moldex 9.1

Table 1 Testing parameters