Influence Of Shear History On Morphology And Mechanical Properties Of Micro Injection Molded Parts

The polymer melt should be molded at high speed under high temperature andpressure condition for the tiny size of produces in micro injection molding. In thefilling stage, there will be a large temperature gradient formed in the contact surfaceof the melt at high temperature and the cool mold wall. The polymer melt experiencesmore complex shear history compared with conventional injection molding, whichcan change the crystalline structure of polymers and then affect the morphology ofmicro-injection produces.The internal morphology structure of the polymer melt,which determines the mechanical properties of the products, is investigated tooptimize the process parameters and produce better properties of the parts.In this research, the isotactic polypropylene material with three differentthickness (0.2mm、0.5mm and1mm)was chosen to investigate the influence of shearhistory on morphology and mechanical properties of micro injection molded partsthrough changing the injection speed. The thesis is summarized as following:1. The analysis of the flow characteristic of micro parts in micro-injection moldingwas carried out firstly. The influence of injection speed, mold temperature and melttemperature on filling length and flow phenomenon of micro injection molded partswere investigated on the micro parts with thickness of0.2mm、0.5mm and1mm. Theresults are shown that: the filling length increased as the injection speed, moldtemperature and melt temperature increased on the micro parts with thickness of0.2mm. The simulation results were consistent with the experiment results. The effectsof the micro-scale were significant in the injection process and become moresignificant when the thickness drops. It is easily to fill completely at lower processparameter of0.5mm and1mm parts. The imbalance phenomenon was analyzed in theexperiment of0.2mm parts. The influence of the form of gate on the flowcharacteristic is obvious, It is easily to fill completely with side gate but difficult forpoint gate.2. Influence of shear history on morphology of micro injection molded parts was researched secondly. The polymer material experienced different shear rate bychanging injection speed. The shear rate in different injection speed and position ofproducts was simulated by Moldflow software under the actual molding processconditions. The influence of shear time on morphology of micro parts wereinvestigated by changing the plasticizing stroke.The morphology in different positionof micro parts was carried out through polarized light microscopy (PLM). The resultsare as follows: the morphology changes with the different shear rate. The imperfectshish-kebab structure appeared in the skin-core border in the thickness direction of0.5mm and1mm parts. However, the column crystal was shown in the skin-coreborder in the thickness direction of0.2mm parts. The ratio of layered structure is notidentical in different thickness of parts. The skin-core structure has appeared in bothof the thickness of0.5mm and1mm parts. The ratio of skin layer increased as theshare time increased while the ratio of core layer decreased.3. The relationship between morphology and mechanical properties of micro partswas discussed then. The mechanical properties were tested by changing injectionspeed of three different thicknesses to analyze the relationship between morphologyand mechanical properties parts and the influence of micro-scale effects onmorphology and mechanical properties of parts. The results are shown that: theYield.stress, elongation at break, breaking strength and modulus decreased as theinjection speed increased of0.5mm and1mm parts. On the contrast, the Yield stress,breaking strength, modulus increased and elongation at break decreased as theinjection speed increased of0.2mm parts while the elongation at break decreased. Theeffects of micro-scale were more significant in the mechanical properties of microparts. The Yield stress, breaking strength, modulus increased and elongation at breakdecreased as the thickness drops. It is concluded that the strength is better as thethickness drops, but the ductility is worse4. The special morphology in the micro-injection molded parts ware analyzed last.The interval column crystal structure appeared in the convex platform while themulti-layer column crystal structure appeared in core layer under the share action.The column crystal structure changed into the spherulites crystalline and increasedalong the flow direction in0.2mm parts.