Choosing between PA VS POM for injection molded components depends on factors such as strength, wear resistance, dimensional stability, moisture absorption, temperature performance, and the specific requirements of the final application.
Choosing between polyamide (PA, commonly called nylon) and polyoxymethylene (POM, commonly called acetal or by the trade name Delrin) is one of the most consequential material decisions in plastic injection molding. Both are semi-crystalline engineering thermoplastics prized for mechanical performance, but they behave very differently once a part leaves the mold. PA offers high tensile strength, toughness, and resistance to fuels and hydrocarbons, while POM delivers dimensional stability, low friction, and fatigue resistance that PA cannot match.
This guide compares the two materials across mechanical properties, moisture behavior, chemical resistance, processing parameters, tolerance capability, cost, and application fit, so engineers and product teams can select the right resin for a specific injection molded component rather than defaulting to whichever material was used last time.
The decision between these two engineering resins is rarely academic. A gear housing molded in the wrong material can wear out prematurely, a snap-fit latch can lose retention force after a season of humidity exposure, and a fuel-line connector chosen for its stiffness rather than its chemical compatibility can fail catastrophically in the field. Because PA and POM are both semi-crystalline thermoplastics with overlapping stiffness and strength ranges, it is easy to treat them as interchangeable general-purpose engineering plastics.
In practice, their differences in moisture behavior, crystallization rate, friction coefficient, and chemical sensitivity make them suited to distinctly different design intents, and understanding those differences early in the design cycle avoids costly tooling rework later.
What Is PA (Nylon) in Injection Molding?
Polyamide is a family of semi-crystalline thermoplastics formed through condensation polymerization of amide monomers. In injection molding, PA is valued for a rare combination of stiffness, toughness, abrasion resistance, and resistance to oils, greases, and many solvents. The amide linkages in the polymer backbone give PA its strength, but those same polar groups make the material hygroscopic, meaning it readily absorbs atmospheric moisture both before and after molding. This single characteristic drives most of the practical differences between PA and POM in production environments.
Key Grades of PA Used in Injection Molding
Nylon 6 and Nylon 66 are the two dominant grades in industrial injection molding. Nylon 66 has a higher melting point and greater rigidity, while Nylon 6 offers slightly better impact toughness and easier processing at lower melt temperatures. Both are frequently reinforced with glass fiber at loadings of 15% to 50% to increase stiffness, reduce creep, and improve dimensional stability under load. Unreinforced, unfilled PA is used where flexibility and impact absorption matter more than rigidity, such as cable ties, flexible clips, and living-hinge components. Mineral-filled and impact-modified grades extend the material into structural housings, gears, and automotive under-hood parts.
What Is POM (Acetal) in Injection Molding?
Polyoxymethylene is a highly crystalline engineering thermoplastic built from repeating formaldehyde-derived units. POM is best known for its exceptional stiffness-to-strength ratio, low coefficient of friction, and outstanding fatigue and creep resistance. Unlike PA, POM absorbs very little moisture, which makes molded parts dimensionally predictable straight out of the mold and throughout their service life. These properties make acetal the default choice for precision mechanisms, snap-fit assemblies, and parts that slide or rotate against another surface without external lubrication.
Homopolymer vs Copolymer POM
POM homopolymer offers the highest tensile strength, stiffness, and fatigue endurance of the two subtypes, but it is more sensitive to thermal degradation during processing and more vulnerable to attack by strong acids and alkalis. POM copolymer trades a modest reduction in mechanical properties for improved chemical resistance, better long-term thermal stability, and a wider, more forgiving processing window. For injection molded components exposed to hot water, mild acids, or repeated thermal cycling, copolymer POM is generally the safer specification.
PA vs POM: Core Material Properties Compared
The table below summarizes baseline mechanical and physical properties for unfilled, general-purpose grades of each material. Reinforced or specialty compounds will shift these numbers, but the relative pattern between PA and POM remains consistent across most commercial grades.
Core Material Property Comparison: PA vs POM (Unfilled Grades)
| Property | PA (Nylon 6/66) | POM (Acetal) |
|---|---|---|
| Tensile strength (dry, as molded) | 70–85 MPa | 60–70 MPa |
| Flexural modulus | 2.5–3.3 GPa | 2.6–3.1 GPa |
| Moisture absorption (24 hr, 23°C) | 1.3–3.0% | 0.2–0.5% |
| Coefficient of friction (dry, self-mated) | 0.20–0.35 | 0.15–0.35 |
| Continuous service temperature | 80–120°C | 85–105°C |
| Mold shrinkage (unfilled) | 0.7–1.5% | 1.8–2.5% |
| Fatigue resistance | Moderate | Excellent |
| UV resistance (unstabilized) | Poor to moderate | Poor to moderate |
These figures make clear that neither material is universally superior. PA tends to win on raw tensile strength and impact toughness when dry, while POM wins on dimensional consistency, fatigue life, and predictable performance in humid or wet environments.
Mechanical Performance Differences
Tensile Strength and Rigidity
In its as-molded, dry state, PA typically outperforms POM in tensile strength and elongation at break, particularly in glass-reinforced grades used for structural brackets and housings. However, PA’s strength drops noticeably as it absorbs ambient moisture, a phenomenon often called moisture conditioning. A nylon part can lose 30% to 50% of its dry tensile strength once it reaches equilibrium moisture content, while simultaneously gaining toughness and impact resistance. POM does not exhibit this trade-off; its mechanical properties remain nearly constant regardless of ambient humidity, which simplifies design validation for parts that must perform consistently across seasons or climates.
Impact Resistance
Dry-as-molded PA is relatively brittle, but once conditioned with atmospheric moisture it becomes notably tougher and more resistant to sudden impact loading. This is why many nylon snap-fit and living-hinge designs are intentionally allowed to condition before functional testing. POM, by contrast, offers moderate and stable impact resistance regardless of environmental exposure, but it does not reach the peak toughness that conditioned PA can achieve. For components subject to drop testing, repeated flexing, or shock loading in outdoor or humid environments, conditioned PA is frequently the stronger performer.
Fatigue and Wear Resistance
Where POM decisively separates itself from PA is in repetitive-motion applications. Its low coefficient of friction and high resistance to creep under cyclic loading make it the standard material for gears, cams, rollers, and snap-fit latches that engage and disengage thousands of times over a product’s service life. PA can be compounded with internal lubricants such as molybdenum disulfide or PTFE to improve its tribological performance, but even lubricated nylon grades generally fall short of acetal’s inherent wear characteristics in dry-running assemblies.
Wear, Friction, and Fatigue Behavior in Repetitive-Motion Parts
| Characteristic | PA (Unlubricated) | POM (Homopolymer) |
|---|---|---|
| Self-lubricating behavior | Limited | Strong |
| Wear rate against steel | Moderate to high | Low |
| Fatigue endurance limit (relative) | Moderate | High |
| Snap-fit cycle life (typical) | Hundreds to low thousands | Thousands to tens of thousands |
| Recommended for gear teeth | With internal lubricant only | Yes, without additives |
Tip: If a part design involves repeated snap-fit engagement, sliding contact, or gear meshing without external lubrication, default to POM unless a specific chemical or thermal requirement rules it out. Switching a fatigue-critical part from PA to POM is one of the most common design corrections made after early field failures.
Moisture Absorption and Dimensional Stability
Moisture behavior is the single biggest practical distinction between these two materials. PA is hygroscopic and will continue absorbing moisture from the air for weeks or months after molding until it reaches equilibrium with its environment, which can shift part dimensions by several thousandths of an inch depending on wall thickness and part geometry.
Designers working with PA must decide whether critical dimensions should be measured and toleranced in the dry-as-molded state or the moisture-conditioned state, since the two can differ meaningfully on precision assemblies. POM absorbs so little moisture that this distinction is largely irrelevant; a POM part measured immediately after molding will be dimensionally close to that same part measured a year later in a humid warehouse.
Tip: When specifying tolerances on a nylon component, always state whether the dimension applies to the dry-as-molded condition or the moisture-conditioned condition. Omitting this detail is a frequent source of disputes between design teams and molders during first-article inspection.
Chemical Resistance Comparison
Chemical exposure often decides the material question before mechanical properties even enter the conversation. PA holds up well against fuels, oils, greases, and many aromatic and aliphatic hydrocarbons, which is why it dominates automotive fuel-system and under-hood applications. It is vulnerable, however, to strong acids and oxidizing agents, and prolonged exposure to certain alcohols and glycols can cause swelling. POM resists a broad range of solvents, fuels, and weak bases well, and its low moisture uptake helps it resist hydrolysis better than PA in many aqueous environments, but it is notably sensitive to strong acids, which can attack the acetal backbone and cause chain scission.
Chemical and Environmental Resistance Comparison
| Exposure Environment | PA Resistance | POM Resistance |
|---|---|---|
| Gasoline and diesel fuel | Good to excellent | Good |
| Motor oils and greases | Excellent | Good |
| Strong acids | Poor | Poor |
| Weak bases/alkalis | Good | Good to excellent |
| Hot water/steam | Fair (hydrolysis risk) | Good (copolymer preferred) |
| Alcohols and glycols | Fair (may swell) | Good |
| UV / outdoor exposure | Requires stabilization | Requires stabilization |
Tip: Neither PA nor POM is inherently UV-stable. Any component destined for prolonged outdoor exposure should specify a UV-stabilized grade of either resin rather than relying on the base polymer’s natural resistance.
Processing Parameters for PA vs POM
Both materials require disciplined process control, but the risks differ sharply. PA’s central processing challenge is moisture management before molding; POM’s central challenge is thermal stability during molding.
Typical Injection Molding Processing Parameters
| Parameter | PA (Nylon 6/66) | POM (Acetal) |
|---|---|---|
| Melt temperature | 230–290°C | 190–230°C |
| Mold temperature | 60–90°C | 60–120°C |
| Pre-drying requirement | Mandatory, below 0.2% moisture | Generally not required |
| Sensitivity to overheating | Moderate | High (formaldehyde off-gassing risk) |
| Typical shrinkage rate | 0.7–1.5% | 1.8–2.5% |
| Cycle time (relative) | Moderate | Fast, due to rapid crystallization |
Drying Requirements
PA pellets absorb atmospheric moisture rapidly, even within minutes of being exposed to open air, and molding wet resin causes hydrolytic degradation, splay, and a measurable loss of mechanical properties in the finished part. Dew-point desiccant dryers running for four to six hours at 80°C are standard practice before molding. POM does not require this step under normal conditions, which simplifies material handling and reduces the risk of moisture-related defects on the shop floor.
Shrinkage and Warping Considerations
POM’s higher crystallinity gives it a higher and more direction-dependent shrinkage rate than unfilled PA, which can translate into greater warping risk on parts with uneven wall thickness or long unsupported flat sections. Mold designers compensate with balanced gating, uniform wall thickness, and, where needed, glass or mineral reinforcement to reduce shrinkage variability. PA’s lower baseline shrinkage is more forgiving on complex geometries, though its own shrinkage rate can vary considerably between the dry and moisture-conditioned states.
Tip: Because POM crystallizes quickly and shrinks anisotropically along the flow direction, always validate wall thickness uniformity and gate placement with mold flow analysis before cutting steel, particularly on parts with ribs or bosses that intersect the main flow path.
Tolerance and Precision Molding
For tight-tolerance components such as gears, connectors, and precision housings, POM is generally the more predictable material to hold to close dimensional targets over the life of the product, precisely because it does not absorb moisture and does not undergo the dry-to-conditioned property shift that affects PA. That said, PA with proper moisture management and glass reinforcement can also hold tight tolerances, and its greater toughness makes it a better choice where a precision part must also survive impact loading. The right choice depends on whether the critical failure mode is dimensional drift or mechanical shock.
Repeatability across production lots is another factor worth considering separately from single-part tolerance capability. Because POM’s shrinkage behavior is less sensitive to ambient conditions, tooling calibrated for POM tends to produce consistent dimensions from the first shot of a run to the last, even across shifts or seasonal humidity changes in the molding facility. PA tooling calibrated for a specific dry or conditioned target can drift slightly if incoming resin moisture content is not tightly controlled between lots, which is why disciplined drying protocols matter as much for dimensional consistency as they do for mechanical performance.
Reinforced and Modified Grades
Neither PA nor POM is limited to its unfilled, general-purpose form, and much of the real-world selection decision actually happens at the grade level rather than the polymer-family level. Glass-fiber-reinforced PA, typically compounded at 15% to 50% glass loading, substantially increases stiffness, reduces creep under sustained load, and improves dimensional stability, though it also increases brittleness and can accelerate wear on any mating steel or plastic surface.
Impact-modified PA grades add elastomeric phases to the polymer to boost low-temperature toughness for outdoor or automotive applications where sudden impact loading is a design concern. Internally lubricated PA, compounded with molybdenum disulfide, silicone, or PTFE, narrows the wear-performance gap with POM in sliding applications, though it rarely closes that gap entirely.
POM is similarly available in reinforced and modified forms. Glass-filled POM increases stiffness and reduces the material’s naturally higher shrinkage rate, which helps control warping on large or flat parts. PTFE-filled and silicone-filled POM grades further reduce the coefficient of friction beyond what unfilled acetal already offers, making them a common choice for high-speed sliding or rotating components where even POM’s native lubricity is not sufficient. UV-stabilized versions of both materials are widely available and should be specified by default for any part with meaningful outdoor exposure, since neither base polymer holds up well to prolonged sunlight without additive protection.
Tip: Do not assume a reinforced grade automatically outperforms an unfilled grade for a given part. Glass fiber improves stiffness and creep resistance but can reduce impact toughness and increase mold wear, so match the reinforcement level to the actual load case rather than specifying the highest glass content available.
Design Guidelines: Wall Thickness, Ribbing, and Gate Placement
Because PA and POM shrink at different rates and along different flow-dependent patterns, mold and part design should be treated as material-specific rather than applied generically across both resins. For PA components, wall thickness should stay as uniform as possible to avoid sink marks and to minimize the risk of differential moisture uptake between thick and thin sections over the part’s service life. Ribs and bosses should be kept at 50% to 60% of the nominal wall thickness to prevent visible sink on the opposite surface, and gates should be placed to minimize weld lines in load-bearing areas, since PA’s crystalline structure makes weld lines a measurable strength reduction point.
POM’s faster and more directional crystallization behavior makes gate placement and flow balance even more critical. Because POM shrinks more along the flow direction than across it, asymmetric gating can produce warping on parts with large flat surfaces or long unsupported spans. Uniform wall thickness remains important, but designers working in POM should pay particular attention to venting, since inadequate venting combined with POM’s sensitivity to overheating increases the risk of degradation byproducts forming visible surface defects near the gate. Cooling channel layout should also be balanced on both sides of the mold to prevent uneven crystallization that can distort precision features such as gear teeth or snap-fit geometry.
Tip: On precision POM parts such as gears or snap-fit closures, use mold flow simulation to check for flow-direction-dependent shrinkage before finalizing gate location. A gate placed without this analysis is one of the most common root causes of out-of-tolerance acetal parts discovered during first-article inspection.
Cost Considerations
Base resin pricing for PA and POM is broadly comparable, with Nylon 66 typically commanding a modest premium over Nylon 6 and POM homopolymer often priced slightly above POM copolymer. The larger cost driver in practice is processing overhead: PA’s mandatory drying step adds energy cost and cycle-time planning that POM avoids, while POM’s faster crystallization can shorten in-mold cooling time and improve overall cycle efficiency. Reinforced or specialty grades of either material, such as glass-filled PA or PTFE-filled POM, carry a further cost premium tied to the additive package rather than the base polymer.
Total cost of ownership should also account for downstream factors beyond piece price. PA’s sensitivity to moisture means that improperly dried or stored material can lead to scrapped shots, unplanned downtime for redrying, and inconsistent part quality that increases inspection burden. POM’s sensitivity to thermal degradation means that molders must control barrel residence time and purge schedules carefully to avoid scrap from discoloration or off-gassing, particularly on longer production runs or when switching colors. Neither material carries a decisive cost advantage on its own; the more useful comparison is which material’s processing risks are easier for a given production environment to control consistently.
Typical Applications
Where PA Excels
- Automotive fuel-system components, connectors, and under-hood housings exposed to oils and hydrocarbons
- Cable ties, clips, and flexible fasteners that rely on moisture-conditioned toughness
- Structural brackets and load-bearing housings using glass-reinforced grades
- Industrial equipment components exposed to solvents, greases, or fuels
Where POM Excels
- Precision gears, cams, and rack-and-pinion mechanisms
- Snap-fit closures, zippers, and mechanical fasteners requiring high cycle life
- Plumbing fittings, valve components, and water-contact parts
- Conveyor rollers, bushings, and bearings running without external lubrication
- Precision electronic and consumer device housings requiring tight, stable tolerances
How to Choose Between PA and POM
Selecting between PA and POM comes down to identifying the dominant failure mode the part must resist. If the part will experience repeated mechanical cycling, sliding contact, or must hold precise dimensions across a range of humidity conditions, POM is generally the stronger default. If the part must resist impact loading, survive exposure to fuels or oils, or benefit from moisture-conditioned toughness, PA is usually the better fit. Parts that combine both requirements sometimes call for reinforced or modified grades of either resin, and in borderline cases, prototyping both materials and testing under real service conditions remains the most reliable way to confirm the choice.
Tip: When a part sits on the border between the two materials, run a short accelerated-cycling or humidity-conditioning test on both PA and POM prototypes before committing to production tooling. The cost of dual prototyping is small compared to a tooling redesign after a field failure.
Common Defects and Troubleshooting
PA parts molded from insufficiently dried resin frequently show splay marks, silver streaking, and brittleness caused by hydrolytic degradation during the melt phase; the fix is almost always improved drying rather than a process parameter change. POM parts that are overheated or held too long in the barrel can degrade and release formaldehyde gas, producing surface streaking, discoloration, and a noticeable odor; molds running POM should use adequate venting and avoid excessive residence time. Both materials are prone to sink marks on thick sections and warping on unevenly cooled geometries, which is best addressed through wall thickness uniformity and balanced cooling channel design rather than by adjusting hold pressure alone.
Brittle or cracking PA parts immediately after ejection, before any moisture conditioning has occurred, are usually not a material defect but an expected characteristic of dry-as-molded nylon; if the application requires immediate toughness rather than toughness after conditioning, an impact-modified grade should be specified rather than troubleshooting the base resin. Warped POM parts often trace back to an unbalanced cooling circuit or asymmetric gating rather than a resin issue, since POM’s crystallization rate makes it considerably more sensitive to uneven mold temperatures than PA.
Black specks or brittle, discolored regions near the gate on POM parts are a strong indicator of localized overheating or excessive shear during injection, and are best resolved by reducing injection speed or barrel temperature at the nozzle rather than adjusting packing pressure. In both materials, persistent short shots or flash should be investigated as a process and tooling issue first, since neither PA nor POM has an inherent tendency toward these defects when processed within its recommended window.
Tip: If POM parts show a persistent sharp odor or surface discoloration near the gate, check barrel residence time and melt temperature first. This is one of the clearest early warning signs of thermal degradation before it becomes a visible molded-in defect.
Frequently Asked Questions
Which material is stronger, PA or POM?
It depends on the environment. Dry, as-molded PA has higher tensile strength than POM, but PA’s strength drops once it absorbs moisture. POM’s mechanical properties stay essentially constant regardless of humidity, so in wet or variable environments, POM often behaves more predictably even though its absolute strength is somewhat lower than dry PA.
Does POM absorb moisture the way nylon does?
No. POM typically absorbs only 0.2% to 0.5% moisture at equilibrium, compared to 1.3% to 3.0% or more for PA. This is the primary reason POM is chosen for precision parts that must hold tight tolerances regardless of ambient humidity, while PA is more sensitive to dimensional and mechanical shifts caused by moisture uptake.
Can PA and POM be used interchangeably for gears?
Not reliably. POM is the preferred material for most gear applications because of its low friction, high fatigue resistance, and dimensional stability under repeated loading. PA can be used for gears when internally lubricated or when the application also requires higher impact resistance, but unlubricated PA gears generally wear faster than POM gears under the same cyclic load.
Which material offers better dimensional stability?
POM offers better dimensional stability overall because of its minimal moisture absorption and more consistent shrinkage behavior across environmental conditions. PA can be dimensionally stable as well, but only if the design accounts for the shift between the dry-as-molded and moisture-conditioned states, which adds complexity to tolerance planning.
Is POM more expensive than PA?
Base resin costs for PA and POM are generally comparable, with pricing varying more by specific grade and reinforcement level than by polymer family. POM can often reduce total production cost because it skips the mandatory drying step required for PA and benefits from faster crystallization, which can shorten cycle times.
Which material is better for parts exposed to fuel or oil?
PA is generally the stronger choice for fuel and oil exposure, which is why it is widely used in automotive fuel-system components and under-hood parts. POM also resists many fuels and oils reasonably well, but PA’s long track record and broader chemical compatibility with hydrocarbons make it the more common specification for this category of application.