Material selection in Plastic part design

The decision to use plastic rather than wood, aluminum, or steel will have consequential

results on the shape, manufacture, cost, and end-use of the product. As such, product

designers should consider what properties are truly required in a given application when
developing plastic parts. Selection of an appropriate plastic material is vital in product
development since the properties of the plastic are closely tied not only to the product design but also the mold designs and process optimization. Inappropriate material selection may incur
severe costs and delays late in the product development cycle  related to trying
different materials and product redesigns.

 Following Plot shows you the comparison between yield stress and elastic properties of plastic parts and metal:

 

At first look it seems that plastics can't compete with other plastic except in the cost consideration.

"While plastics have low elastic modulus and yield strength compared to steel, aluminum,
titanium, magnesium, and other alternative materials, their use in structural applications can be
justified due to their low density and high strain to failure. "