How to design plastic parts
Plastic conducts heat slowly, so in the plastic mold the product will be cool on the outside and hot on the inside. Plastic shrinks when it cools, so this temperature difference means it will shrink unevenly. This is noticeable where ribs meet the walls of the plastic part.
To greatly reduce the ugly sink marks, we try to keep the ribs at less than 2/3 the width of the walls.
Square corners are similar to ribs, so they also have a shrink problem. They can be fixed by radiusing them. A good rule of thumb is to make the outside radius about 1.5 times the wall thickness, and the inside radius would be that minus the wall thickness or 1/2 the wall thickness.
If we don't want the outside corner to be radiused, we can still do that and reduce our shrink problem. Since the corner acts like a rib, we can reduce the shrink problem by making the corner seem like a thinner rib to the plastic. We do that by adding a detail which sticks up on the inside of the corner as shown in the picture at right.
In some part shapes there is a thick section of plastic which is unavoidable. Those will not only make sink marks in the walls, but also will often create a void bubble in the part.
Some parts need lenses, where the thickness varies a great deal from location to location. There are a few ways to deal with this, to prevent severe distortion of the lens. One example is the Fresnel Lens.