Influence of powder filing on viscosity of thermoplastic composites for injection moulding

Andrzej Skalski, Ryszard Pilawka, Dionizy Biało

Quarterly No. 3, 2009 pages 214-218

DOI:

keywords: viscosity, metal powders, injection moulding, fillers, thermoplastic compositions

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abstract Micro injection moulding process allows to manufacture very small elements (micro parts) with dimensions in the tens or hundreds micrometers range. In this process very fine powders are mixed with special, complex binder featuring thermoplastic properties. Obtained mass (feedstock for injection) is injected to the cavity of the micro mould. Next the binder is removed and shapes undergo sintering. Selected problems related to micro injection are presented in this paper - how composition and filling powder affect feedstock viscosity and, consequently, feedstock behaviour during injection. Iron and stainless steel powders with granularity of 1.4 and 16 μm were used in this study. The complex binder system was based on paraffin, wax and polyethylene. Powder-binder feedstock was prepared in 2Z type mixer at temperature of 125C during 1 hour. Volume fraction of powder in the feedstock Vp was 45, 50, 55 and 60% vol. The investigations of feedstock viscosity were carried out using the ARES rheometer, Rheometric Scientific, at the following values of the measurement parameters: the diameter of plates 25 mm, thickness between plates 1.5 mm, shear rate  up to 100 s−1 and temperature 80140C. The study has shown that binder viscosity depends on shear rate and temperature range. The higher the temperature and shear rate were, the lower was binder viscosity. Similar dependence was obtained for all the feedstocks tested. The study has revealed the nonnewtonian characteristics of these materials. It is worth to note that volume fraction of the powder Vp in the feedstock and powder granularity also affect viscosity. The feedstock viscosity was much higher then the binder viscosity and increased with the in-crease of Vp. The smaller the metal powder particles were, the higher was the feedstock viscosity. The highest viscosity value was obtained for 1 μm powder. The authors attempted to find out how the viscosity affected the micro channel filling by feeds-tock. For this purpose special testing form with micro channels was designed. The test was carried out for micro channel with 0.21 mm2 cross-selection. It was found that the length of micro channel filling i.e. the distance on which the feedstock flowed into the micro channel, was inversely proportional to the feedstock viscosity.