Experienced gear molders caution design engineers to avoid common pitfalls with plastic gears. One old problem, says ABA/PGT, is misaligned mating parallel-axis (spur and helical) gear tooth surfaces. The solution is the modification of gear flanks by making them full thickness at mid-face-width and tapering them to each edge, as shown in the drawing below. This modification, extending over the full height of the gear tooth, is referred to as crowning. Other useful advice comes from UFE of Stillwater, MN. Operating temperature must be factored into designs using dissimilar materials. If a gearcase is made of metal and the gears are plastic, the distance between the gears will change because of the differing coefficients of linear thermal expansion.
For the gearing manual from ABA/PGT, visit http://rbi.ims.ca/4933-520. Plastic Gear Must Ask Questions from UFE can be found at http://rbi.ims.ca/4915-511.