摘要
采用以超临界氮气为物理发泡剂的微孔注塑发泡技术,成型聚丙烯(PP)电器盒子,定量研究2种较高注射速度(250和290 mm/s)对盒子泡孔结构、未发泡皮层厚度和翘曲的影响,根据数值模拟所得模腔厚度上温度、剪切速率、黏度和压力的分布,分析盒子泡孔结构和皮层的形成机理.结果表明,PP盒子壁内芯层形成了直径较小(小于90μm)的微孔,这主要归因于较高注射速度使模腔内熔体的压降速率和剪切速率提高,促进泡孔的成核.较高剪切应力和较低熔体温度的综合作用,使得靠近皮层处形成形状较规则的小泡孔区(直径小于20μm).研究发现,盒子侧壁沿充模方向,泡孔的密度与平均直径之间服从指数规律,皮层厚度近似线性地增加,后者归因于相应的表面温度近似线性地降低.注射速度由250 mm/s提高至290 mm/s时,盒子开口端的翘曲有所减小.研究表明较致密均匀的泡孔结构和较薄的皮层有助于减小微孔注塑制品的翘曲.
A series of microcellular polypropylene(PP)electrical boxes were fabricated by microcellular injection molding with supercritical nitrogen(Sc-N2)as the physical foaming agent.The effects of higher injection speeds(250 and 290 mm/s)on the cellular structure,unfoamed skin layer thickness,and warpage of the microcellular boxes were quantitatively investigated.According to the simulated gapwise distributions of shear rate,temperature,viscosity,and pressure of PP melt/Sc-N2 solution at the last moment of the filling stage in the molding of the PP boxes,the formation mechanisms of cellular structure and skin layer within the walls of the boxes were analyzed in detail.It was demonstrated that microcellular cells with smaller diameters(less than90μm)were developed at the core layer of the boxes.This is mainly because higher injection speeds led to higher pressure drop rate and shear rate,which thereby promoted bubble nucleation.It should be noted that tiny cell area with diameters less than 20μm and more regular shape appeared near the skin layer of the boxes.This is due to the synthetic effects of higher shear stress combined with lower melt temperature.Along the filling direction,an exponential relationship was found between density and mean diameter of the cells within box sidewall,and the skin layer thickness increased in a nearly linear manner.The latter is mainly attributed to an almost linear decrease in the corresponding surface temperature.Warpages at the open end of the boxes were decreased through increasing the injection speed from 250 mm/s to 290 mm/s.The results demonstrate that more uniform and compact cellular structure as well as thinner unfoamed skin layer is beneficial to lowering the warpage of microcellular injection molded parts.This is worthy of further investigation in the future work.
作者
郭凯强
黄汉雄
Kai-qiang Guo;Han-xiong Huang(Department of Industrial Equipment and Control Engineering,Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing,South China University of Technology,Guangzhou 510640)
出处
《高分子学报》
SCIE
CAS
CSCD
北大核心
2019年第8期850-856,共7页
Acta Polymerica Sinica
基金
广东省科技计划(项目号2017B090911009)
容桂科技计划资助项目
关键词
微孔注塑
超临界流体发泡
注射速度
聚丙烯
泡孔结构
翘曲变形
Microcellular injection molding
Supercritical fluid foaming
Injection speed
Polypropylene
Cellular structure
Warpage
