As the plastic solidifies in the injection mould, it freezes from the outside (near the mould surface) toward the inside. In thick sections this results in inward pulling stresses (due to contraction) that can cause sink marks in the outer surfaces of the part.
In addition, because thinner sections will freeze faster than thicker sections there is also the possibility of stresses building up between thick & thin sections, resulting in part warpage. So in the design of parts to be injection moulded, it is a good idea to maintain consistent wall thickness and avoid thick areas whenever possible.
Thicker and non-uniform wall thicknesses can often result in sinks in the material due to the same solidification physics described above. The use of thinner, uniform wall thicknesses helps to avoid sink.
Warpage due to stresses in step transitions between wall thicknesses can be improved through the use of a ramp. The use of gussets can be helpful to provide support in corners to avoid warping.
See the Protomold Design Guide for other helpful design information.