Outside of general design criteria, concerning shape, form and material type, it is beneficial to understand the following items, as well:
What is the final function of the part?
How does it interact with adjacent items in the assembly?
What are the stresses applied when it is in use?
How is the material interacting with the mold within the forming process?
The function of the part should always play a role in the design of a mold. Designing a molding process that will hold tolerances within a few thousandths of an inch adds unnecessary costs when tolerances of hundredths of an inch will not affect the functionality of the final part.