Microstructure And Properties Of Micro-Structured Parts Injection Molded

Micro injection molding (MIM) as one micro forming process which has many good features, such as, applicability for many materials, applicability for complex shape, low production cost, mass production capability, and so on. The micro-structured parts fabricated by MIM will be widely applied in the fields of information and communication technology, precise instrument, aerospace, medical and biotechnology, military, and so on. The overseas scholars began to study MIM since 1986 and the investigation on MIM in our country is on initial stage. There are still many aspects about MIM need to be solved. So, it is significative on theory and application aspects to have a further study on MIM.This paper investigated the effects of micro injection molding process parameters on the forming properties of micro structures, characterized the microstructure and mechanical properties of micro-structured part and disclosed the corresponding mechanics, during injection molding polymer and ceramic micro-structured part.Mold for MIM were designed and fabricated based on analysis to the filling theory of melt in microcavities. The mold contains main structure and micro-cavities insert. Polypropylene micro-structured parts with micro pillars array were fabricated by polymer MIM. The effects of process conditions on the forming properties of micro structures were analyzed. Test results indicate that vacuum is the necessary condition for micro blind cavity and mold temperature has more obvious effect on the height of micro structures compared with the injection pressure. The polymer micro injection molding process was optimized based on above analysis.The microstructure morphology, crystal style and molecule orientation were firstly investigated by polarization microscope (PM), scanning electron microscope (SEM), laser confocal microscope (OLS), X-ray diffraction (XRD). Analysis results show that micro structures also represent"skin-core"morphology, i.e. non-crystalline skin layer, shear zone with column crystal and spherulites core. Both micro structures and macroscopical base plate containαandβphase. However, the relative proportion ofβphase in micro structures is markedly higher than that of the base plate. In addition, the orientation of micro structure is slight. The hardness and modulus of micro structures was evaluated by nano-indentation and micro hardness test. The hardness and modulus of a same micro pillar increase from core zone to shear zone and skin layer. There is no markedly difference between the hardness of different size micro pillar.Feedstock suitable for powder MIM was prepared with the paraffin wax based (PW, PP and SA) binder system. Zirconia (ZrO2) micro-structured green parts were fabricated by powder micro injection molding process. The debinding and sintering process parameters were established based on analysis of thermodynamics during debinding and sintering process. The analysis on forming properties of micro structures indicates that higher mold temperature and injection pressure is advantageous for not only the filling performance but also the increase of green part density. Increasing sinter temperature can induce the densification of micro-structured part, but result in the grown and non-homogeneous grain. The powder micro injection molding process was optimized based on above analysis.The crystal growth and densification mechanics of micro structures and base plate were discussed combining with the analysis of microstructure of ZrO2 micro-structured part. Test results indicate that micro structures and base plate have the same crystal growth and densification mechanics which is boundary diffusion. So, there is no difference of crystal type and size between micro structures and base plate. The hardness and modulus of ZrO2 micro structures were characterized by nano-indentation and micro indentation. Test results show that the hardness and modulus of different size micro structures is approximate.