Because of the way 3d printing lays the plastic down, if you print in the right orientation, printed objects can actually stronger than injected while using less material.

In the last 12 months or so there have also been a number of super tough new filaments. The problem is that the cost is often related to strength :-)

PEEK for example - the genuine superman of plastics is in the order of $5-600 per kg.

Something like ninjateks Armadilo - a rigid polyurethane - is a lot cheaper and seriously tough.

Some of a printed objects strength is down to design, some is down to print orientation and some is down to material.

3d printing is as much - if not more - of an art than a science.