POM vs Nylon: Choosing the Right Resin for Gears

Why Resin Selection Matters for Gears

Gear design is a balancing act between mechanical performance, manufacturing cost, and production speed. The chosen resin dictates friction behavior, dimensional stability, and how the part survives repeated meshing cycles. Many buyers start with the part geometry and then force a material choice that does not fit the loading profile. This leads to premature wear, inconsistent cycle times, or expensive redesigns.

The two most common engineering plastics for this application are polyoxymethylene and nylon. Both are commodity engineering resins with established processing windows. The decision between them is rarely about which one is “better” in isolation. It is about matching the resin’s inherent properties to the gear’s service conditions and the plant’s production constraints.

In a typical plastic gear, the tooth profile must maintain its shape under load. If the material creeps, the backlash changes, and the drive slips. If the surface wears, the pitch diameter shifts, and the meshing ratio drifts. These are not minor defects in high-precision applications. They are functional failures.

Resin selection also affects the mold itself. A resin that is highly abrasive will eat into the polished steel of the tool. A resin that requires high temperatures for processing will age the mold steel faster than a resin that processes at lower heat. The choice of polymer, therefore, extends beyond the part to the tooling and the long-term economics of the production run.

POM vs Nylon: Core Property Comparison

The table below summarizes the primary trade-offs for engineers evaluating these two resins for gear applications.

Option	Best for	Limitations
POM	Low-friction gears, high-speed meshing, standard mechanical drives	Lower impact resistance, higher cost per kilogram, less chemical resistance
Nylon	High-impact applications, chemically aggressive environments, cost-sensitive high volume	Higher friction, requires plasticizers or oils to reduce wear, dimensional instability from moisture
Glass-filled Nylon	High-stiffness gears, load-bearing applications	Abrasive tooling wear, higher mold temperatures, poor impact resistance
Glass-filled POM	High-stiffness gears, low-maintenance environments	More expensive than unfilled POM, requires careful mold temperature control
Acetal (POM) Blend	Customized friction and stiffness profiles	Limited availability, higher cost, requires specific supplier support

POM, often referred to as acetal or polyoxymethylene, is a crystalline homopolymer. Its molecular structure creates a naturally smooth surface. This results in low friction coefficients and good self-lubrication properties. For gears that mesh under steady loads without shock, this is the dominant advantage. The material also resists creep, meaning it holds its shape under constant pressure without deforming over time.

Consider a small planetary gear set used in a camera shutter mechanism. The gears must operate in a sealed, dry environment with no external lubrication. POM performs well here because its surface is naturally smooth and it does not swell with humidity. The low friction keeps the mechanism quiet and extends the life of the drive.

Nylon, or polyamide, is also a crystalline polymer but with different structural characteristics. It has higher tensile strength and better impact resistance than unfilled POM. However, it absorbs moisture from the environment. This absorption increases its weight and changes its dimensions. For precision gears, this moisture sensitivity is a significant variable. It requires careful drying before molding and control of ambient humidity during storage.

A gear made from standard nylon will expand slightly as it absorbs moisture from the air. If the housing is made of a rigid material like glass-filled nylon or metal, the differential expansion can cause binding. The gear may stick in the housing or fail to rotate smoothly. This is a common issue in humidity-controlled environments where the nylon component swells but the housing does not.

Friction and Wear Behavior

Wear is the primary failure mode for plastic gears. The resin’s surface hardness and internal friction determine how long the part lasts. POM has a naturally low coefficient of friction. When two POM gears mesh, they generate less heat and less wear than nylon gears of the same geometry.

The low friction of POM is not just a benefit for the part. It is a benefit for the system. Less friction means less heat generation within the drive train. In compact mechanisms where heat dissipation is difficult, this can be the difference between a reliable device and a failed one.

Nylon requires modification to perform well in gear applications. Unfilled nylon has higher internal friction. It generates more heat during meshing, which accelerates wear. Most nylon gears are produced with added lubricants or as glass-filled variants. Glass-filled nylon has higher stiffness but also higher abrasiveness. The glass particles wear against the mating surface and the mold itself. This requires more frequent mold maintenance and can limit the lifespan of the tooling.

For high-speed gears, where heat generation is a concern, POM is generally preferred. The lower friction means less energy is lost to heat, and the material stays cooler during operation. For low-speed, high-torque gears, the higher stiffness of glass-filled nylon may be more appropriate, provided the abrasive wear is managed.

In a conveyor drive system, the gears may experience sudden shock loads when a heavy pallet shifts. Unfilled POM might crack or chip under this impact. Glass-filled nylon, with its higher stiffness and impact resistance, can absorb the shock without breaking. However, the abrasive nature of the glass filler means the mating surfaces will wear faster. The engineer must balance the need for strength against the cost of frequent part replacement.

Moldability and Process Control

Processing POM and nylon requires different approaches. POM has a lower melt temperature than nylon. It flows easily at lower temperatures, which reduces energy consumption and the risk of degradation. Its low shrinkage rate also makes it easier to achieve tight dimensional tolerances. This is critical for gear teeth, where profile accuracy affects meshing efficiency.

POM also has a low tendency to degrade during processing. Because it processes at lower temperatures, the risk of thermal breakdown is minimal. This makes it suitable for longer production runs without constant monitoring of melt quality. The material can be recycled and remelted without significant loss of properties, which is useful for managing offcuts and flash.

Nylon requires higher melt temperatures to achieve the same flow rates. Its higher viscosity means it needs more holding pressure and longer cooling times to achieve solidification. The moisture sensitivity adds another layer of complexity. If the nylon is not dried properly, bubbles and surface defects appear in the finished part. This often requires rework or scrap, increasing production cost.

The drying process for nylon is critical. The material must be dried to a specific moisture content before it enters the hopper. If the moisture content is too high, the gear will have voids in the teeth, reducing the load-bearing area. If the moisture content is too low, the material may become brittle. The drying time and temperature must be controlled carefully, and the material must be dried immediately before injection to prevent re-absorption of moisture from the air.

For high-volume production, the process window of POM is generally wider. This means the machine can run more consistently with less operator intervention. Nylon requires tighter control of drying, melt temperature, and holding pressure. If your plant has limited process engineering support, POM is often the safer choice for stable, high-throughput gear production.

A plant producing small plastic gears for a consumer electronic device may not have a dedicated process engineer on the floor. In this case, the process window of POM allows the machine to run unattended for longer periods. If a parameter drifts slightly, the part may still be within tolerance. With nylon, a slight drift in drying time or mold temperature can result in a batch of defective parts that must be scrapped.

Cost and Supply Chain Considerations

Material cost is a major factor in gear selection. POM is typically more expensive per kilogram than unfilled nylon. This is due to the production process and the market dynamics for these specific resins. For low-volume, high-performance gears, the material cost difference may be acceptable. For high-volume, cost-sensitive applications, nylon becomes attractive.

The cost difference is not just about the resin itself. It includes the cost of processing. POM processes at lower temperatures, which reduces the energy cost per part. Nylon requires higher temperatures and longer cycle times, which increases the energy cost and the machine time per part. Over a production run of millions of parts, these differences can be significant.

Supply chain stability also matters. Both resins are widely available, but POM may have less price volatility in some regions. Nylon is a commodity with global production, which can lead to price fluctuations based on oil prices and demand. Buyers should consider total cost of ownership, not just material price. This includes mold life, cycle time, scrap rates, and end-of-life performance.

A machine tool manufacturer producing a large number of plastic gears for a CNC machine may be sensitive to material price fluctuations. If the price of nylon rises, the cost per unit increases significantly. By switching to POM, the manufacturer may accept a higher material cost in exchange for a more stable supply chain and lower processing costs.

Application-Specific Selection

The right choice depends on the gear’s service environment. Consider the following scenarios:

1. Automotive transmission components: High-speed, continuous meshing. POM is preferred for low friction and wear resistance. Glass-filled POM may be used if higher stiffness is needed.
2. Industrial conveyor drives: Moderate speed, occasional shock loads. Nylon with added lubricants is often selected for its impact resistance and cost advantage.
3. Medical device actuators: Clean environment, low maintenance. POM is ideal due to its low friction and ease of cleaning.
4. Agricultural machinery: Harsh, dusty environments. Nylon with glass filler is common for its toughness and cost, despite higher wear.
5. Consumer electronics: Small, precise gears. POM is often chosen for its dimensional stability and low shrinkage, which ensures accurate meshing.

In the automotive sector, the gears must operate under high loads and high temperatures. POM’s resistance to heat and its low friction make it a strong candidate for these applications. Glass-filled POM is used when the stiffness requirement is higher, such as in a transmission that must handle high torque.

In industrial conveyor drives, the gears may be exposed to dust and debris. Nylon’s ability to absorb some of the shock from the conveyor belt makes it a good choice. However, the abrasive nature of the dust can accelerate wear, so the engineer must specify a harder surface or a lubricated variant.

In medical device actuators, the gears must be easy to clean and disinfect. POM’s smooth surface and resistance to many cleaning agents make it ideal for these applications. The low friction also ensures that the device operates quietly and smoothly, which is important for patient comfort.

Final Selection Criteria

When selecting between POM and nylon for gears, focus on three core questions:

• What is the expected service life? If the gear must last for years under continuous load, prioritize wear resistance. POM is usually the better choice.
• What are the environmental conditions? If the gear is exposed to chemicals, moisture, or high humidity, nylon may be more suitable. If the environment is dry and clean, POM performs better.
• What are the production constraints? If the plant needs high throughput with low process variation, POM’s easier processing is a significant advantage. If the plant has advanced drying and process control, nylon is viable.

The decision is rarely black and white. Many designs use POM for the gear itself and nylon for the housing or other structural components. The key is to match the resin’s properties to the specific mechanical and environmental demands of the part.

A final check of the design should include a review of the mating surfaces. If the gear is meshing with a metal surface, the friction and wear characteristics will be different than if it is meshing with another plastic gear. The engineer must consider the entire system, not just the individual component.