split geometry in swept boss

Hi Everyone,

I am getting an error while doing a swept boss/base option. I have a 3D sketch I want to give a rectangular cross-section with dimension 3x 2 cm. But while giving this value the surface of the geometry is showing many split lines. but if i am giving a comparatively small cross-section (1 x 1 cm) the split lines are disappearing. 

Please help me to remove the split lines in the geometry with 3 x 2 cm.

SolidworksModeling And Assemblies