Plastic gears have moved far beyond toys and office equipment. They now sit in car actuators, medical devices and industrial controls, and buyers increasingly ask whether they can replace metal. The answer is not "plastic is cheaper" — it is whether the operating conditions match what each material can actually hold. A gear specified on price alone will fail in six months; one specified on load, speed and temperature will run for years.
Plastic wins where the load is light and noise matters. Nylon (PA66) and acetal (POM) are self-lubricating, so they run quietly without grease, and they absorb vibration better than steel. Injection moulding makes them cheap at volume, and they weigh roughly one seventh of steel. PEEK pushes the temperature ceiling to around 250°C and is chemically resistant, but it is expensive and harder to mould. These strengths make plastic the default for office equipment, home appliances and light automotive actuators.
The limits of plastic are the limits of its operating window. Nylon absorbs moisture and swells, which changes tooth geometry; continuous use above 80–100°C softens most engineering plastics; and under high torque the teeth creep and permanently deform. Precision is also capped — a moulded plastic gear typically reaches IT8–IT10, while a ground steel gear reaches IT5–IT7. If the drive needs backlash below 0.1mm or runs continuously under load, plastic alone is usually not enough.
Metal gears are still the default for anything serious. Steel can be carburised and hardened to 58–62 HRC, ground to tight tolerances, and run at high pitch line speeds for tens of thousands of hours. Cast iron handles shock and is easy to machine, while bronze and brass are chosen for worm gears because they wear well against a steel worm. The trade-off is weight, noise and the need for lubrication — metal gears almost always run in an oil bath or with grease.
The practical decision rule is simple. If torque is above roughly 5 Nm, pitch line speed above 8 m/s, or continuous temperature above 100°C, choose metal. If the drive is light, low speed, noise-sensitive and cost-sensitive, plastic is usually the better answer. For the middle ground — moderate load with a noise target — a composite gear with a steel hub and a plastic or fibre-reinforced tooth ring gives most of the strength with much of the quietness.
At our factory we machine steel, cast iron, brass and bronze gears to IT5–IT7, and we work with buyers who are evaluating a plastic-to-metal switch. Send us the torque, speed, temperature and duty cycle of your drive, and we will tell you which material fits — and where a composite design might save cost without losing life.