Gears are classified by their AGMA (American Gear Mfg Ass.) rating, AGMA 9 are easily achieved with injection molding and I have done AGMA 12 with injection molding. The AGMA rating uses a steel master against the plastic gear and measures the true run out of the driven gear as the two are rotated. This is important to understand as I have done gears for high speed laser printers and an AGMA 9 gear will cause resonance at high speed and a AGMA 12 will not. If the gear is running at a low speed then an AGMA 8 or 9 is more than adequate. The tooling for bevel gears need not be so expensive, the design is the critical piece because that will determine how round you can make them. It’s not really that complex but you don’t design a mold for an AGMA 12 like you would an AGMA 8 and the cost is what changes.
Getting to a true high class AGMA mesh in a molded gear and maintaining that over a production run is difficult. I have found that very few mold makers worldwide can achieve this. This can be said for any gear class but the higher the class the more rare. HP is the worlds number one high-class gear user, finding mold makers competent in making gears that meet these standards is an ongoing challenge.
I think there is a trend to treat this as a molding or tooling issue when the engineering aspect of the mesh is really what needs to be addressed. Couple this with the tool and process engineering as all three are interlocked and cannot be separated.
Finding a knowledgeable gear engineer to define your mesh is the best place to start. Gear engineering is not trivial and not something you can just do with a “text book” understanding. This is an engineering specialty, especially concerning molded gears as their manufacturing issues and behavior in use is very different than machined gears. I know many premier gear providers who use such outside consultants to do their meshes though this is done behind the scenes. I recommendation is, still, to use a qualified gear engineer to provide you with the mesh you need.
