Wall thickness is referred to the thickness of the part produced. Wall thickness depends upon the strength required. Plastics are poor conductor of heat and thicker section take time to cool and are costly to manufacture. Since there is a temperature gradient with in the part which led to development of thermal stresses which causes shrinkage, warpage etc. so wall thickness should be optimized considering these factors. Two parts with similar geometry but different wall thickness have different filling and cooling time since the minimum cooling time (Tc) is directly proportional to the square of the wall thickness (t) of the part.
A sudden change in the cross section causes the turbulence in the flow of the melt and this turbulence is responsible for the air entrapment and mold deterioration. Therefor it is advisable to have a gradual transition between changing sections.
Thicker section will take more time to cool than thinner section this will cause the development of thermal stresses in the part and part will get deformed
If thicker sections are unavoidable then provide radius at the edges or a slight chamfer, provide a core or provide ribs.