Research On Part Surface Quality Improvement Of Microcellular Foaming Molding Process

Because supercritical fluid(SCF)gathers on product surface in the process of microcellular foam injection molding,it is the main cause of poor quality in microcellular foamed plastic products surface.Usually large size of the surface defects,such as surface blistering,post-blow and so on,are considered to be caused by poor process conditions.Such defects can be removed by adjusting the mold design,SCF content etc.However,small size of surface defects,such as swirl marks and silver streak,lead to higher surface roughness.And these surface defects are difficult to be removed,even can't be removed due to equipment and processlimitation.Such small defects make that microcellular foam injection molded products can't be applied on external parts production.Predecessors have studied the relationship between single bubble and roughness successfully by numerical calculation and finite element simulation under different injection process parameters.Although the size of a single bubble can be calculated by MOLDFLOW and bubblegrowing up model and then the surface roughness Ra is calculated,the bubbles movementin non-Newtonian fluid is affected by various forces,mainlyincluding outside pressure,shear stress and viscous resistance,upward force by low density etc.These forces take place rupture,aggregation phenomenon.The mathematical model can not accurately calculate the surface roughness because of the bubble groupexist.So it is hard to quantitative guide and contrast the actual product manufacturingwith single bubble grew upmodel.Therefore,the following researches are done in this paper in order to study how to improve the surface quality of the products.Firstly,based on the shear stress nucleation in microcellularfoaming processresearch,shear stress can be considered as one of the main driving force of bubble nucleation grew up and burst.By Taylor bubble deformation theory,whenthe ratio of the bubble viscosity and melt viscosity tends to zero moment(this process is a slow cooling process of melt from the surface to inside),critical rupture dimensionless coefficient is constant.When the wall shear stress increases,the big bubbles,near the solidified layer due to meet the critical rupture conditions,are quickly split into smaller bubbles.And the surface roughness is reduced.Compared with traditional injection molding process,the microcellular foam injection molded products surface roughness and bulk shear stress in low Reynolds number flow fields present an oppositerelationship trend.Therefore,this paper get the relationship between microcellular foam injection molding process and wall shear stress byanalyze the influence of process parameters to the wall shear stress through taguchi experimental design process and finite element simulation software.Then,in order to improve the surface quality of microcellular foamed injection molding products,the actual injection molding samples are injected according to Taguchi experimental design method.The surface quality of the samples are measured by the roughness measuring instrument.By the signal-to-noise ratio and analysis of variance method,theeffect of mold temperature,melt temperature,injection rate,pre-filling quantity,gas concentration on the surface quality is studied.Thus,the relationship between process parameters and surface roughness is revealed.And the contrast scheme between wall shear stress and surface roughness is gotten.Finally,the optimization processis gotten by the above relationship model of process parameters and surface roughness.The roughness measurement instrumentis used again to characterize and contrastexperimental results between optimized samples andnon-optimized ones,the scanning electron microscopy(SEM)is used to characterize the best and the worst process parameters combination by taguchi experimental section with the optimized samples.SEM results verifiedthat the better surface roughness can be achieved by the optimized microcellular foam injection molding process parameters and CAE analysis method.And surface quality of microcellular foaming injection molded products can be improved.