Material concentrations in whatever form should be avoided for a number of reasons:
    

• They result in abrupt stiffness changes, causing stress concentrations which can lead to premature failure of a part
  
• Often it is not possible to effectively pack a material concentration. This can lead to:
  
  o         Internal stresses
  o         Warpage of a component
  o        Sink marks
  o        In extreme cases to voids in the part.
  
• Cycle time will be longer than necessary, increasing the cost of the component
  

When designing a part, also consider that a component has to be cooled when it is moulded. Any area where there is no contact of the molten plastic with the tool will be a hot spot that results in increased cycle time.
 
It is essential to ensure that the wall thickness is as uniform as possible. Thick walled sections will take longer to cool than thinner sections. As a result, the polymer chains in the thick part have more time to relax and crystallize, resulting in higher shrinkage than the adjoining thin parts. Thicker sections will continue to cool and shrink when thinner regions have already set, causing distortion and internal stresses.
 
Bear in mind that cycle time is directly proportional to the square of the wall thickness. Keeping the wall thickness down lowers raw material cost as well as processing cost!