Use rounded corners.
Add draft to walls to allow easy ejection.

When the design does not allow for additional structures to improve strength, consider using a stronger material, such as glass fiber filled plastic.

Consider specifying a fire retardant material when necessary.

A small rough spot will appear at the gate; a small line will occur at the parting line; and a round mark will occur at ejector pins.

Consider specifying where to place the gate and parting line and what surface finish to use - polished, matte, textured. 

Since a seam between two halves of a box is difficult to fully hide, consider making the joint pronounced - to make it look like it is decorative.

Contoured parts warp less than flat parts.
Review existing injection molding components for additional ideas and techniques.
See additional tips and the material selection table linked in the sidebar 

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