Influence of injection condition on micro channel filling by thermoplastic - powder feedstocks
Andrzej Skalski, Dionizy Biało, Waldemar Wiśniewski Politechnika Warszawska, Instytut Inżynierii Precyzyjnej i Biomedycznej, ul. św. A. Boboli 8, 02-525 Warszawa, Poland
Quarterly No. 4, 2008 pages 322-326
DOI:
keywords: injection moulding, micro parts, metal powders
abstract In the last few years micro system technology has gained increasing development and new method of fabrication of micro components are needed. Micro metal injection moulding (µMIM) is the process which allows to manufacture very small elements (micro parts) with dimensions below 1 mm in large - scale production cycle, complex shape, wide range of materials and in economic way. In µMIM very fine powders are mixed with special, complex binder with thermoplastic properties. Obtained mass (feedstock for injection) is injected to the micro mould. Next the binder is removed and shapes undergo sintering. Selected problems of micro injection moulding of micro parts is presented in this paper. It deals with first step of the micro forming process. The metal powder used in this study was carbonyl iron powder with particles of 1, 4 μm and stainless steel powder with particles of 16 μm. The binder system was based on wax, polyethylene and paraffin. Powder / binder feedstock was prepared in 2Z type mixer with heating system at temperature of 125oC in 1 hour. Volume fraction of powder in the feedstock Vp was 60%. For the purpose of micro injection, special micro form was designed. Micro form was equipped in heating - cooling system and insert having 6 micro channel with cross - section ranged from 2 to 210 μm2. The following injection parameters were used: feedstock temperature of 115 and 125 oC, pressure of 60 MPa and mould temperature of 25 to 80 oC. The way of channel feeling i.e. the distance L on which the feedstock flow - in to the channel was taken as the indicator of feedstock behavior during injection. The research has shown that the principal parameter influenced on distance L is temperature of the mould Tf. The higher is Tf, the longer is L. It is worth to note that characteristic threshold value of temperature above which feeling of channel is possible occurs. For the feedstock tested it was above 70 oC. The following conclusion which may be drown from the presented research is that the higher is volume fraction of the powder in the feedstock the more difficult is micro channel feeling. It has strong connection with higher viscosity of the feedstock.