Rib is a frequently used feature in plastic injection molded parts design for the purpose to enhance the strength of the plastic parts, typical rib design see below figure.

The thickness and location is essential of the rib design. Usually, ribs should be designed with a thickness of 1/2 of the wall thickness to avoid a thick section at the base of the wall, which would cause sink marks on the part surface. Ribs are usually spaced at a distance at least twice the wall thickness to allow enough steel between the ribs for adequate cooling.

Consider the mold making and injection molding, some of the other issues need to be taken into consideration:

Draft
Typically 1 degree draft is required for rib de-molding, it requires more if the rib is textured.

Length of the rib
The length of the rib should be limited to 3 times of the wall thickness, if it’s too much, the end of the rib could be dangerously thin, thin area is difficult to fill during injection molding.

Add a radius
A radius 0.3 -0.5 times of the wall thickness at the base of the rib could reduce stress concentration effects.

Mold flow
A good rib design should shorten the mold flow length, especially on the occasion of complex parts, proper connection of ribs would enhance the strength and also allow better mold flow condition.

Venting
Mold design should consider adequate vents when the parts is complex, thin wall ribs at the tip make the injection filling difficult and short shot could occur when the mold flow length is too long, the mold vents could be designed usually at the parting surface because the possible mold flow ends are deep inside of the cavities. Usually split inserts are design to allow air released from slim space between assembled inserts.