Choosing the wrong engineering plastic can lead to part failure, costly redesigns, and frustrating production delays. You have two excellent options, Nylon (PA66) and POM (Delrin), but they are not interchangeable. A gear that wears out too fast or a clip that snaps under pressure can ruin a product’s reputation. Making the wrong choice here wastes valuable time and money. Understanding their key differences will help you select the suitable material for your specific needs, every single time.
PA66 and POM are both widely used engineering plastics, but they behave very differently. PA66 is strong and tough, especially with glass fiber reinforcement, while POM is valued for low friction, wear resistance and dimensional stability in many mechanical parts.
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POM is a candidate for low-friction mechanisms, while PA66 may suit tough or reinforced structural parts. Both families include grades used for snap features; allowable strain, humidity, temperature and fatigue testing decide the choice. Do not select a living hinge from the family name alone.
What is the Difference Between Delrin and POM Plastic?
You constantly see “POM” and “Delrin” on spec sheets and in technical discussions, but are they actually the same thing? Using the wrong term can create confusion with your suppliers and even lead to ordering an incorrect material grade. This seemingly small mistake can cause big problems in production and part performance. Let’s clear this up right now so you generally know exactly what you are working with and can specify your materials with precision.
POM is the polymer-family name for polyoxymethylene (acetal). Delrin® is a brand of acetal homopolymer; other homopolymer and copolymer grades are available. Specify the manufacturer, exact grade, additives and approved equivalent rather than treating every POM as Delrin.
Homopolymer vs. Copolymer
The choice between POM-H and POM-C often comes down to a trade-off between mechanical performance and chemical or thermal stability.
| Feature | POM Homopolymer (e.g., Delrin®) | POM Copolymer (e.g., Ticona Celcon®) |
|---|---|---|
| Stiffness & Strength | Higher tensile strength and rigidity. | Slightly lower, but still very stiff. |
| Creep Resistance | Excellent, better for long-term loads. | Very good, but slightly less than POM-H. |
| Centerline Porosity | Can be an issue in thick-walled parts. | Generally less prone to porosity. |
| Chemical Resistance | Good, but less resistant to hot water/alkalis. | Excellent, especially against hot water/alkalis. |
| Thermal Stability | Good, but lower long-term thermal stability. | Better long-term thermal stability. |
In short, if your design requires the absolute maximum stiffness and strength, and it won’t be in a chemically harsh environment, a POM-H like Delrin is a fantastic choice. If your part needs to survive exposure to hot water, steam, or certain chemicals, or if you’re molding a very thick part where porosity could be a problem, a POM-C is often the safer and more reliable option.
What is the Core Difference Between PA66 and POM Material?
You need a part that is strong and durable, but PA66 and POM offer very different kinds of strength. Choosing a material based on a simple “strong” label can lead to unexpected and catastrophic failures in the field. A part might be incredibly stiff but shatter on impact, or it might be tough but too flexible for its job. Let’s put these two materials side-by-side to compare their most important core properties and understand their unique strengths.
The most critical difference between PA66 and POM lies in how they interact with moisture. PA66 is hygroscopic, meaning it readily absorbs water from the environment. This absorption makes it significantly tougher and more impact-resistant, but it also causes it to swell and lose dimensional stability. In contrast, POM absorbs very little water, giving it excellent dimensional stability, consistent stiffness, and predictable properties in both wet and dry conditions. This is the single most important distinction for any designer to understand.
Material Properties at a Glance
This is the kind of chart I keep handy when I’m starting a new project.
| Feature | Nylon 66 (PA66) | POM (Acetal/Delrin®) |
|---|---|---|
| Toughness & Impact | Excellent, especially when moisture-conditioned. | Good, but more rigid and can be notch-sensitive. |
| Stiffness & Hardness | Depends on grade, reinforcement and dry/conditioned state. | Compare the selected grade under the same conditions. |
| Friction & Wear | Good, naturally lubricious. | Excellent, one of the best plastics for low friction. |
| Moisture Absorption | Specify equilibrium/test conditions and the selected grade. | Typically lower than unfilled PA66; use the same test basis. |
| Dimensional Stability | Moisture conditioning and thermal expansion must be allowed for. | Often lower moisture sensitivity; still affected by shrinkage, load and temperature. |
| Operating Temp. | Use exact grade data, load, duration and test method. | Use exact grade data, load, duration and test method. |
| Chemical Resistance | Good against oils, greases, and fuels. | Excellent against solvents, fuels, and alcohols. |
The Critical Role of Moisture
For PA66, water acts as a plasticizer. When the material is bone-dry, it’s quite rigid but also more brittle. As it absorbs moisture, the polymer chains can move more freely, which dramatically increases its toughness and impact strength. This is why PA66 is often “conditioned” with moisture after molding to achieve its optimal properties. However, this same process causes the material to swell and lose some of its stiffness and tensile strength. For a designer, this means you must account for these dimensional changes in your design if the part will operate in varying humidity.
POM usually absorbs less moisture than unfilled PA66, but its dimensions still respond to temperature, moisture, molding shrinkage, stress and time. Evaluate the actual tolerance under the specified conditioning and load; no resin holds every tolerance in every environment.
So, Which is Better for Your Application, Nylon or Delrin?
Theory is great, but how do you apply it to the actual part on your screen? You’re looking at your CAD model, wondering if that gear needs more precision or if that clip needs more flex to survive. Making the wrong call means going back to the drawing board, wasting time and resources. Let’s move past the data sheets and look at real-world examples to make this critical choice simple and clear.
Neither material is universally better. Consider POM for sliding and precision mechanisms, and PA66 for suitable tough or reinforced components. Grade, reinforcement, moisture state, sustained load and mating material determine performance; both require application testing.
Use function to narrow the candidates, then check actual grade data and samples. A sliding mechanism needs friction, wear and clearance evidence; a flexible latch needs strain, retention and fatigue evidence. Neither test is replaced by a rule-of-thumb success percentage.
When to Choose POM (Delrin®)
Think of POM as the precision machinist of the plastics world. Its strengths are ideal for parts that are part of a larger mechanical assembly.
- Precision Gears: POM’s high stiffness and incredible dimensional stability ensure that the gear tooth profile remains exact. This prevents backlash, ensures smooth power transmission, and helps support reliability over millions of cycles.
- Bearings and Bushings: POM’s naturally low coefficient of friction and excellent wear resistance make it a suitable replacement for metal in many bearing applications. It runs quietly and often requires no external lubrication.
- Fuel System Components: In automotive applications, parts are exposed to gasoline and other solvents. POM’s excellent chemical resistance and stability in these fluids make it the go-to material for fuel pump housings, caps, and float components.
- Medical Device Components: A qualified POM grade may suit a dosing mechanism, but dimensional consistency, extractables, contact conditions, wear and device-level validation remain necessary. A material family does not guarantee dosing accuracy.
When to Choose Nylon (PA66)
Think of Nylon as the tough brawler. It’s built to take a punch, bend under pressure, and keep going.
- Snap-Fit Latches and Clips: PA66 and POM can both be considered. Compare allowable strain, root radius, repeated engagement, retention force and environmental conditioning. A POM snap-fit is not inherently too rigid to work.
- Power Tool Housings: The housing of a drill or saw needs to protect the internal components from drops and rough handling on a job site. PA66’s superior impact strength absorbs that energy, preventing the housing from cracking or shattering.
- Wheels and Rollers: For applications like caster wheels or conveyor rollers, PA66’s toughness and load-bearing capacity are key. It can handle high loads and resist abrasion from rolling contact.
- Cable Ties and Fasteners: The flexibility and high tensile strength of PA66 make it the widely used material for cable ties, with grade and conditioning requirements. It can be pulled tight without snapping and holds its grip reliably.
PA66: Strength With Moisture Considerations
Nylon PA66 can provide high strength, heat resistance and fatigue performance. Glass-filled PA66 is common for structural parts, brackets and under-hood components. The key limitation is moisture absorption, which can change dimensions and mechanical properties after molding.
POM: Low Friction and Dimensional Stability
POM, often known by the Delrin trade name, is used for gears, bushings, clips, sliding parts and precision mechanisms. It offers low friction and good wear performance, but designers must consider creep, chemical exposure and safe processing conditions.
Mold Design and Processing Differences
PA66 often needs careful drying and shrinkage review, especially with glass-filled grades. POM requires controlled processing to avoid degradation and gas issues. Gate design, venting, cooling and steel selection should match the chosen material instead of treating both as generic engineering plastics.
PA66 vs POM: Buyer Review Checklist
- Choose PA66 for strength, heat and fatigue performance when moisture effects are acceptable.
- Choose POM for low friction, wear and precision sliding applications.
- Review moisture conditioning for nylon dimensions and properties.
- Control POM processing temperature and venting carefully.
- Validate wear, creep and dimensional behavior under real use conditions.
FAQ
Is POM more dimensionally stable than PA66?
Often yes, because PA66 absorbs more moisture, but final stability depends on grade, filler, geometry and environment.
Which is better for gears, PA66 or POM?
POM is often preferred for low-friction gears, while PA66 may be chosen when higher toughness or heat performance is needed.
A Qualification Plan for a Gear, Bearing or Latch
Request comparable data for the exact grades: dry and conditioned dimensions, modulus at service temperature, creep under sustained load, friction/wear against the real mating surface and fatigue at the intended strain. Filled and unfilled PA66 should not occupy one comparison column without identification.
For a gear, agree backlash and noise after humidity/temperature conditioning, the loading profile and lubrication policy. For a latch, measure insertion and retention force before and after repeated use; inspect the root and weld-line region. State whether the dimension is measured as molded or after conditioning so the supplier can propose a meaningful shrinkage and inspection plan.
Delrin’s creep and stress-relaxation guide explains why short-term strength does not predict long-term deflection. Its snap-fit/spring example also shows why POM should not be dismissed as unsuitable for flexing features. For a material and tooling review, provide the load, service environment, mating component, drawing and accepted wear or retention limits.
Conclusion
The choice between Nylon (PA66) and POM (Delrin) is not about finding a “better” material, but about selecting the “right” one for the job. POM delivers precision, stiffness, and stability for intricate mechanical parts. Nylon provides unmatched toughness, flexibility, and impact resistance for structural and dynamic components. Understanding this fundamental difference empowers you to design more robust, reliable parts and truly master your material selection process, getting your molding right every time.