In the manufacturing of long hollow plastic parts with closed ends,like test tubes,unevenly advancing melt front around the cores results in core deflection,which becomes a pervasive problem especially when the parts are thin-walled.Therefore.injection molding thin-wailed parts with such cores is challenging since this deflection not only causes uneven wall thickness.but also itself accelerates melt flow behind the deflection.In this study,an effective 3D numerical approach is developed to provide a convenient tool for practitioners to predict core deflection,and a single dimensionless group called core deflectability is introduced in the following discussion.To validate our simulation,we compare our core deflection predictions with a recent analytical solution(Giaeomin and Hade,2005).