1) Part designed with incorrect wall thickness – The part should be designed with the thinnest wall section the part can tolerate (taking into account strength, functionality and other critical aspects of the design). The two risks as:

Too thin – if the wall section is too thin it may not fill correctly or it may break off in the tool.
Too thick – wall sections that are too thick take longer to cool and solidify before ejecting and may result in sink marks, warping and even cracking.
2) Sharp internal corners – Having a sharp corner at an intersection will cause stress within the part and could result in part failure. Best practice is to design corners with a radius.

3) Thick solid sections – Designing a part with thick solid section because you need the strength or rigidity is not the answer. Having a thick solid section will only result in sink marks and defective molding due to the cooling effect of the polymer after injection. Best practice is to core out the section and add ribs to strengthen that area (this will allow the molten polymer to fill the part and for cooling to be even).

4) Deep pocket with parallel sides – In order for a part to be ejected off of the core without sticking, there has to be draft on the walls. A parallel-sided part will inevitably cause molding issues and high reject rates. Adding draft will overcome these problems.