Material Engineering Guide
Nylon Density by Grade: PA6, PA66, PA12, and Cast Nylon
Density is one of the first numbers engineers ask us about when they specify a polyamide part. It drives weight, strength-to-weight ratio, dimensional behavior, and how a component machines on the shop floor. This guide compares nylon density across PA6, PA66, PA12, and cast nylon, and explains how moisture and grade selection change the picture in real service conditions.
What Is Nylon and Why Density Matters
Nylon is the common trade name for the polyamide family of semi-crystalline thermoplastics. The repeating amide groups that give the material its name also explain its toughness, abrasion resistance, and ability to absorb and release moisture. PA6, PA66, PA12, and cast nylon are all polyamides, but they behave differently because their molecular backbones, crystallinity, and moisture affinity differ.
Density — usually expressed as specific gravity in g/cm³ — is a compact way to capture several things at once. It tells us how much mass a given volume of material carries, which directly affects part weight, inertia in moving assemblies, and the strength-to-weight ratio of a design. For machined components, density also influences chip load, cutting forces, and how a part feels in the hand during inspection.
Density is not just a number on a data sheet. It is the fastest proxy for how a nylon grade will behave in weight-sensitive, moisture-exposed, or wear-critical service.
Why it matters
When a customer sends a drawing for a machined nylon bushing or gear, our first material conversation often starts with density and moisture. Those two numbers predict weight, fit, and long-term dimensional stability more reliably than almost any other single data point.
Nylon Density Comparison Table
The table below summarizes typical density values for the four grades we machine most often, along with moisture absorption and general service characteristics. Values are representative ranges for unfilled, unmodified grades; glass-filled and other compounded versions will differ.
| Grade | Typical Density (g/cm³) | Moisture Absorption | General Character |
|---|---|---|---|
| PA6 | ~1.13 – 1.15 | Higher | Tough, wear-resistant, absorbs moisture |
| PA66 | ~1.14 – 1.15 | Moderate to high | Higher crystallinity, stiffer, better wear |
| PA12 | ~1.01 – 1.02 | Low | Lightest, dimensionally stable, low water uptake |
| Cast Nylon | ~1.15 – 1.17 | Moderate to high | High molecular weight, large parts, good machinability |
PA12 stands out as the lightest of the four, while cast nylon and the standard grades sit close together in the 1.13–1.17 range. The bigger practical difference is not density alone but how each grade handles moisture and crystallinity.
PA6 Nylon Density and Properties
PA6 is the workhorse polyamide for machined parts. Its typical density falls around 1.13 to 1.15 g/cm³ in unfilled form. It offers a good balance of toughness, impact resistance, and abrasion resistance, which is why it shows up so often in gears, rollers, and wear strips.
The trade-off is moisture. PA6 absorbs water from the air, and that uptake changes both dimensions and mechanical behavior. A part that machines to a tight tolerance today can grow slightly after it equilibrates in a humid environment. We always flag this when a customer specifies tight tolerances on a PA6 part that will see humidity.
PA6 Applications and Limitations
PA6 is a strong fit for gears, automotive components, bushings, and general wear parts where toughness matters more than absolute dimensional stability. It machines well and is widely available in rod and plate, which keeps lead times short.
Where PA6 struggles is in continuously wet or high-humidity service combined with tight tolerances. In those cases we typically recommend PA12 or a moisture-conditioned PA6 approach, depending on the load and cost targets.
Watch point
If a PA6 part must hold a tight tolerance in a humid plant, specify the moisture condition at which the dimension is critical, or plan a small allowance. This single conversation prevents most field fit problems we see.
PA66 Nylon Density and Properties
PA66 sits at a similar density to PA6, typically around 1.14 to 1.15 g/cm³. What sets it apart is higher crystallinity, which generally translates to greater stiffness, better wear resistance, and a higher melting point. For parts that see heat and sliding contact, PA66 often outperforms PA6.
Like PA6, PA66 absorbs moisture, though the rate and effect can differ with grade and conditioning. It machines cleanly and is a common choice for mechanical components that need a bit more thermal headroom than PA6 provides.
PA66 vs PA6: Key Differences
The most common question we get is simply: what is the difference between PA6 and PA66? The short answer is crystallinity and thermal performance. PA66 is more crystalline, which makes it stiffer and more wear-resistant, and it holds properties at higher temperatures. PA6 is generally tougher and more impact-resistant, and it is often easier to source and machine economically.
PA6 strengths
Higher impact toughness, excellent abrasion resistance, broad availability, and generally lower cost. A practical default for gears, rollers, and general wear parts.
PA66 strengths
Higher crystallinity, greater stiffness, better wear resistance, and better retention of properties at elevated temperatures. A good upgrade when heat and sliding wear dominate.
Density-wise the two are nearly identical, so weight is rarely the deciding factor between them. Choose based on temperature, wear, and stiffness requirements instead.
PA12 Nylon Density and Properties
PA12 is the lightest of the standard polyamides, with a typical density near 1.01 to 1.02 g/cm³. That lower density comes from a longer, more aliphatic carbon chain, which also gives PA12 its standout property: low moisture absorption.
Because it takes up very little water, PA12 holds dimensions far better than PA6 or PA66 in humid or wet environments. It is also well suited to molding and additive manufacturing, which is why it appears frequently in 3D printing filaments and precision molded parts. For machined components, PA12 is our go-to when dimensional stability outweighs raw stiffness.
PA12 Applications in Moisture-Sensitive Environments
PA12 shines in fuel lines, pneumatic fittings, precision components, and any part that must stay dimensionally stable through humidity cycling. It is also a common choice for lightweight parts where every gram counts.
Selection tip
If a customer's parts live in a humid environment and the tolerance is tighter than about ±0.05 mm, PA12 is usually the first grade we recommend. The lower water uptake protects the fit over time.
Cast Nylon Density and Properties
Cast nylon is produced by polymerizing monomer directly in a mold, which yields a higher molecular weight than most extruded grades. Its density typically lands around 1.15 to 1.17 g/cm³. The higher molecular weight gives cast nylon excellent toughness, good machinability, and the ability to be produced in large, thick sections that would be impractical to extrude.
Cast nylon is a strong candidate for large gears, sheaves, wear pads, and replacement parts for heavy machinery. It machines cleanly and holds detail well, though like other nylons it still absorbs moisture.
Cast Nylon vs Extruded Grades
Compared with extruded PA6 and PA66 stock shapes, cast nylon offers a slightly higher density, higher molecular weight, and better performance in thick sections. Extruded grades tend to be more economical in small and medium cross-sections and are available in a wider range of standard sizes.
| Aspect | Cast Nylon | Extruded PA6 / PA66 |
|---|---|---|
| Typical density | ~1.15 – 1.17 g/cm³ | ~1.13 – 1.15 g/cm³ |
| Molecular weight | Higher | Lower |
| Large sections | Excellent | Limited |
| Cost profile | Higher for small parts | More economical in small sizes |
How to Choose the Right Nylon Grade by Density and Service Conditions
Matching a grade to a project is a short, structured process. We walk through these steps with every customer before we cut material.
-
1
Define the service environment
Humidity, temperature, chemicals, and load direction all narrow the choice quickly.
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2
Check dimensional stability needs
Tight tolerances in humid air point to PA12; looser tolerances open up PA6, PA66, and cast nylon.
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3
Weigh wear and temperature
Sliding contact and heat favor PA66; large wear parts often favor cast nylon.
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4
Confirm size and cost
Small parts usually favor extruded PA6 or PA66; very large sections favor cast nylon.
Practical note
Density differences between PA6, PA66, and cast nylon are small enough that they rarely decide a project on weight alone. Moisture behavior and crystallinity usually drive the final choice.
At PlasticCNCPro we machine nylon and other engineering polymers every day, and we are happy to review a drawing or sample and recommend a grade before you commit to production.
Frequently Asked Questions
What is the density of PA6 nylon?
Unfilled PA6 typically has a density of about 1.13 to 1.15 g/cm³. Glass-filled and other compounded versions will be higher depending on the filler content.
What is the density of nylon PA12?
PA12 is the lightest of the common polyamides, with a typical density near 1.01 to 1.02 g/cm³. Its low moisture absorption makes it a strong choice for dimensionally critical parts.
What is the density of cast nylon?
Cast nylon typically falls around 1.15 to 1.17 g/cm³. The slightly higher density reflects its higher molecular weight and suitability for large, thick sections.
What are the key differences between nylon PA6 and PA66?
PA66 has higher crystallinity, which gives it greater stiffness, better wear resistance, and better performance at elevated temperatures. PA6 is generally tougher and more impact-resistant. Their densities are nearly identical, so the choice usually comes down to temperature, wear, and stiffness.
Talk to Us About Your Nylon Part
We at PlasticCNCPro are a Dongguan-based manufacturer specializing in precision CNC machining of plastics and engineering polymers. From PA6 gears and PA66 wear components to PA12 precision parts and large cast nylon replacements, we help global B2B customers move from drawing or sample to reliable production parts. Send us your 2D drawing, 3D model, material requirement, quantity, tolerance, and application details, and our team will review the design, material, and manufacturability and come back with a quotation and production recommendation.