Metal Injection Molding or Casting

For low multiples and proof-of concept, I'd seriously consider direct additive manufacturing, using selective laser sintering, straight from 3D CAD data. Glass-filled and carbon fiber-filled nylon are just two of the material possibilities. Others include PEEK and many metals, with the option for selective laser (or electron beam) melting. On the plus side, no tooling costs; on the negative side, each part is going to be a little pricey. If you have CAD models already, you can get automatic, instant quotes from Materialize and other bureaux. If you make a few that satisfy your needs and impress your potential customer base, you can certainly consider up-scaling using vacuum cast polyurethane in silicone rubber molds, the latter made by casting against a master (could be one of your SLS parts even). Much depends on complexity and size also. If "human-size", the parts (I believe) would not be feasible in metal injection molding, which tends to be limited to much smaller components and would incur very expensive tooling and high part costs. I'd be very nervous of an approach that uses a single material type for gears AND shell, as these elements require very, very different property balances. Acetal for gears is great, but you simply wouldn't consider this for a shell molding.

Regarding ribbing, this option is always limited to line-of-draw (unless the expense of moving cores can be tolerated and I guess not so here). Also regarding ribbing, the extra rigidity can actually mean a POORER impact performance, due to the shock waves and stresses not being dissipated as well. It may seem counter-intuitive, but thicker sections can also be more prone to impact failure than more flexible thinner sections. Have you already machined, as planned? If so, did you machine a master, for casting the mold from, or vice versa?

Specific decisions for the best approach (material, process, design detailing) can only be made with a detailed knowledge of all the constraints (structural, dimensional and financial) and all the must-have, nice-to-have and absolute must-not-have features and attributes. Is there not a web address where we can get more detailed information? Another process (actually, a hybrid process) came to mind and this might suit the combination of requirements that I think you have. This is offered by a couple of machine manufacturers. You'd need to contact the machinery manufacturers direct for contact details of processors offering this service. A thin skin (can be vacuum-formed) is normally inserted in the relatively low-cost mold, to give high-quality surface. The robotic head dispenses PUR foam with reinforcing fibers - that are chopped in the head also - into the open mold. Once the mold is closed, the "B" surface is formed, complete with any ribbing and fixing points, etc. However, the total thickness will tend to be significantly well above the 5mm maximum that you have defined. Rigidity is very impressive. Impact strength will depend on surface material and density/reinforcement variables in the backing material.