Research On The Relationship Between Bubble Breakup Behavior On Parts Surface And Surface Quality Of Microcellular Foam Injection Molding

The surface defects have strongly limited the application range of microcellular injection molding process.According to the previous research results,the surface defects of microcellular foam injection molding products can be roughly divided into two categories: one is the larger size surface defects such as surface blistering and post-blow;the other is the small size surface defects such as silver streak and swirl mark.Existing studies have shown that the large size defects are caused by improper foaming process and unscientific mould structure design.They can be eliminated by setting proper foaming process and optimizing the mould structure.But the small size surface defects can not be eliminated completely by such ways.A lot of work have been done to eliminate silver streak and swirl mark,and a few new technological method such as gas counter pressure(GCP)and rapid heat cycle molding(RHCM)have been used to improve surface quality.However there is no clear study that indicates the control mechanism of the surface defects.According to the fact that the formation of these defects are due to the traces left by bubble breakup on the surface,the breakup behavior is the research emphasis in this paper.At the same time,considering that the shear flow effect will cause the bubble deformation and breakup,it is necessary to invest on the influence of shear flow field to the bubble nucleation process and bubble growth process.Usually,in the filling stage,shear stress gradually decrease along with process time increasement,and the viscosity of the polymer melt increases quickly.Combined with the mathematical model of the bubble breakup,when the polymer just fill into the cavity,the bubble is under the biggest breakup possibility.So this paper emphatically analyzes the bubble breakup behavior at the flow front.Firstly,based on the Taylor bubble deformation theory,the bubble breakup model of the flow front on the surface is established.With the finite element simulation method and orthogonal experiment designed,the influence of shear field to bubble breakup and the effect of process parameters on the shear field are analyzed.Thus the bubble critical breakup sizes with different flow conditions are calculated by MATLAB.It is the base to research on the relationship among bubble breakup behavior,the shear field and the surface quality.Then,the surface roughness of each experimental sample is measured.According to the orthogonal experiment design and the calculated bubble critical breakup size,the effect of process parameters on surface roughness,and the relationship between surface roughness and bubble critical breakup size are analyzed.The results show that the surface roughness and the bubble critical breakup radius are in the same order of magnitude,and they are somehow positively related.Therefore,combining the previous analysis of shear flow and bubble critical size,the relationship between process parameters,bubble breakup behavior and the surface quality is established.Finally,by summing up the above research results,two ways are brought up to improve surface quality of microcellular foam injection molding products: on the one hand,by refraining the bubble growth condition,shortening the growth time etc.can be an effective way to reduce the bubble size on part surface,as to lower the surface roughness,improve surface quality of effect.On the other hand,by enhancing shear flow effect to make bubble more likely breakup can be an effective way to lower the surface roughness,improve surface quality of effect.This theory also well illustrates the scientific and feasibility of gas counter pressure and rapid heat cycle molding.