Draft Angles
This is the golden rule of injection molding design. 
To allow the molded part to be ejected smoothly from the mold, all walls parallel to the opening direction must have a certain degree of inclination.

A lack of draft angle (or an insufficient angle) can result in part drag marks, "ejector white" stress marks, or even make ejection impossible.  
Typically, we recommend a draft angle of at least 1°; for textured surfaces, the angle needs to be even larger.

                                                                   

Uniform Wall Thickness
Plastic shrinks as it cools in the mold. If the part's wall thickness is uneven, thick sections cool slower than thin ones, creating internal stress. This ultimately leads to sink marks on the surface or voids inside the part, and can even cause warping. Maintaining uniform wall thickness is the cornerstone of product quality and dimensional stability.
Rib Design
To increase part strength, we usually design ribs. However, rib thickness is critical; it is generally recommended to be no more than 60% of the main wall thickness. Otherwise, sink marks will appear on the backside of the rib. Additionally, the base of the rib needs a fillet (radius) to reduce stress concentration and prevent cracking.
Avoiding Sharp Corners
Sharp corners inside a mold create stress concentrations, which can lead to mold cracking during heat treatment or use, and cause the plastic part to fracture at the corner under stress. Designing internal corners with radii significantly improves material flow and increases the strength of both the part and the mold.
Simplifying Side Actions
If a part has holes or undercuts on the side, the mold requires complex mechanisms like sliders or lifters. These mechanisms significantly increase mold cost and complexity and can become future failure points. Through clever design—such as changing a side hole to an open slot, or splitting the part into two simpler components for assembly—you can effectively avoid side actions and reduce costs.