Extruded PA66 rod and cast MC nylon are both polyamide and both stocked in the same rack, which is why they get confused. They are made by completely different processes and that shows up in the part.
Cast MC nylon is polymerised in the mould. It has higher crystallinity, higher molecular weight, better dimensional stability in thick sections and very low internal stress. On a 200 mm diameter gear blank or a large wear pad, cast material machines flatter and moves less afterwards. It also has better abrasion resistance.
Extruded PA66 has tighter diameter tolerance, a more uniform surface and better availability in small sections. It is the right answer for rod under about 60 mm and for parts where dimensional consistency along the length matters more than bulk stability. Both absorb moisture — expect 1.5 to 2.5 percent dimensional growth from dry to saturated, and design clearances accordingly.
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