Technical Paper: PIM International, Vol.1 No. 3 September 2007, pages 54-55, 881 words

Authors: L.H. Cheng and K.S. Hwang

Department of Materials Science and Engineering, National Taiwan University, 1 Roosevelt Road, Sec 4, Taipei, 106, Taiwan, ROC

Abstract

To lower the cost of metal injection moulded products, the gate and runner materials and green parts with defects are usually recycled. It is therefore necessary to understand what causes the recycled products to deteriorate. The results show that the viscosity of the 1R (recycled once) feedstock was slightly lower than that of the fresh material. However, as the number of recycle times increased, the viscosity increases while the density decreases and more defects were noticed during solvent debinding. These deteriorations were mainly caused by the increase of the melting point of the backbone binder and the oxidation of the filler or paraffin wax.

Introduction

A lot of waste materials, such as gate, runner, flash, and defect parts are produced during each step of the metal injection moulding (MIM) process. In order to lower the cost of and increase the competitiveness of the metal injection moulding process, it is necessary to reuse the waste materials effectively.

Two methods are frequently adopted by the metal injection moulding industry to recycle the waste materials. One is to add about 30 to 50wt% recycled materials to fresh feedstock. The other is use 100% recycled materials.

After recycling, there is little change in the content and characteristics of the metal powders. However, much more significant changes can occur to the binder components. It is very likely that the melting temperatures and degradation temperatures of the binder components could cause binder evaporation or deterioration of the chemical bonds during the process. This could change the binder to powder ratio and thus cause variations in the characteristics of the feedstock. An example is given by Kulkarni [1] who demonstrated that different green density, viscosity, and length were obtained in the moulded specimens when the feedstocks were recycled. As the number of recycling times increased, the discrepancies also increased.

Since very little has been reported to date on this topic, the objective of this study was thus to investigate the effect of the different number of recycle times on the degree of the deterioration of the material.

Further sections of this article include:

- Experimentation and results
- Summary
- References

Figures and Tables:

Fig. 1 Spiral flow length of the specimens of 0R, 1R, 3R, 6R and 8R (from left to right)

Fig. 2 The surface conditions of the specimen that used feedstocks recycled for 6 and 8 times after solvent debinding showing more defects in the 6R specimen

Fig. 3 The heptane colour after solvent debinding for pure soluble binder, 0R, and 8R specimens. (From left to right)

Table 1 The length and weight of the as-moulded specimens prepared using the feedstocks that were recycled

Table 2 The oxygen content of each feedstock with different recycle times

Table 3 The oxygen content (wt%) of PW and PE (wt%) before and after being held at 140ºC for 4 hours

Table 4 The melting temperature of PE before kneading and after recycling