You start by mentioning IM, but then say the part is 3D printed. Is printing the permanent method of manufacture or just prototyping with the intention of molding later? Printing will limit your choice of polymer and results may not be equivalent to a molded part.

Pay attention to part design – avoid stress risers like sharp corners.

You ask for strength but also talk about rigidity. These are not the same thing for plastics. Make sure you understand your requirements.

What is the base resin? Plastics are not created equal – PEEK is a lot stronger and stiffer than HDPE for example. Just because your printer will work with it doesn’t make your plastic the right choice for the product.

Nanotubes are expensive and typically used in low loading proportions for ESD applications. Powders typically won’t help much with strength, although they can add stiffness. Torlon is not melt processible.

All that said, you can get much higher strength and stiffness with just about any plastic by adding high loadings of either glass or carbon fiber (30% CF or up to 40% GF is not uncommon). Whether your printer will handle them is a different question.