Last updated: · By Eugene Tkachenko
Re-drying Nylon filament fully restores tensile strength — but factory-sealed spools may arrive wet. A pull-to-failure test across four moisture conditions on Polymaker CoPA, using real RC suspension arms as the test part.
- −33.7% strength loss — soaked vs re-dried
- −21.4% strength loss — factory-sealed vs re-dried
- −15.7% strength loss — humidity-chamber wet vs re-dried
- −1.2% strength loss — 24h over-dried (effectively zero)
- 8 arms broken across 4 moisture conditions
Why Nylon Filament Moisture Matters
The standard advice is simple: wet filament ruins parts. Dry it before use and the problem disappears. But the advice skips over several questions that matter when you are printing structural parts for real loads.
First, what does "dry" actually mean in quantitative terms? Second, does drying fully restore tensile strength, or just restore surface appearance? Third, is the filament in a factory-sealed bag actually dry? And fourth, can you damage the filament by drying it too aggressively?
To answer all four, I printed eight identical RC suspension arms in Polymaker CoPA Nylon across four controlled moisture conditions, then broke them all on a rope-and-roller pull tester until something gave. Real functional parts under real load — not test coupons. The results had nowhere to hide.
Moisture effects on nylon are well-documented in materials science: a 2025 MDPI study on the influence of filament moisture on 3D printing nylon confirmed measurable tensile strength loss at moisture content above 0.5 wt%, with moisture acting as a plasticizer that forces polymer chains apart in the amorphous regions. This test applies those findings to real-world printed parts under structural load.
Test Setup and the Four Conditions
The Test Part: RC Suspension Arms
Each arm was printed identically on a Prusa MK4 with the following settings: 265°C nozzle, 40°C bed, 0.15 mm layer height, 6 perimeters, 50% infill. All arms were oriented flat on the bed. After printing, every set was annealed at 170°F (77°C) for 6 hours before testing to normalize residual stress from printing. The only variable between groups was moisture content at print time.
Sample size per condition: 2–3 arms each. See the statistical note in the results section for what this means for interpreting the data.
These are functional RC parts from an ongoing 3D printed Toyota 4Runner RC build, not standardized test coupons. The geometry creates a real stress concentration at the pivot mounting hole — the consistent failure point across every condition tested. Learn more about the 3D print settings used throughout this RC car project.
Condition 1: Factory-Sealed (Intended Baseline)
Polymaker CoPA straight from the manufacturer's sealed bag. Respooled with approximately 10 minutes of air exposure, printed inside a dry enclosure, and stored back in the original bag between sessions. This was supposed to be the control — the ceiling everything else is measured against.
Condition 2: Wet (Humidity Chamber)
The same spool conditioned for 24 hours in a controlled humidity chamber at elevated relative humidity, then printed immediately. A second batch used a spool sprayed directly with water — same result. Moisture is moisture; the source does not change the outcome.
Condition 3: Re-Dried (Standard Protocol)
The wet spool dried at 170°F (77°C) for 6 hours — the standard manufacturer recommendation for CoPA Nylon. Extrusion returned to clean, smooth, bubble-free flow with surface finish identical to the factory-fresh spool. The question: did the strength also return?
Condition 4: Over-Dried (Stress Test)
The same spool dried at 170°F for 24 continuous hours — four times the recommended duration. Expected result: brittleness, degradation, possibly unusable output. Actual result: it printed fine with no visible difference in surface quality or layer adhesion.
The Break Tester
A rope-and-roller pull rig with a digital scale. One end fixed, the other pulled until fracture. Load recorded at failure. The fixture required two upgrades mid-test — the hook bent at 38 kg and the lever shattered on the next attempt. Both were replaced. Part geometry and load direction stayed identical across all tests.
Results: All Eight Break Tests
| Condition | Sample 1 | Sample 2 | Sample 3 | Average Break Load | vs Re-dried |
|---|---|---|---|---|---|
| Over-dried (170°F / 24h) | 43 kg | 39 kg | — | 41 kg | −1.2% |
| Re-dried (170°F / 6h) — Baseline | 41 kg | 42 kg | — | 41.5 kg | 100% (baseline) |
| Wet (humidity chamber, 24h) | 34 kg | 36 kg | — | 35 kg | −15.7% |
| Factory-sealed | 32 kg | 27 kg | 39 kg | ~33 kg | −21.4% |
| Soaked (direct water spray) | 28 kg | 27 kg | — | 27.5 kg | −33.7% |
The Factory-Sealed Problem
Factory-sealed filament was designed to be the control — the ceiling. Instead, it produced the second-lowest average and the widest result variance of any group. One sample broke at 27 kg, matching the worst soaked result. Another broke at 39 kg, approaching re-dried performance. Three samples from pristine, sealed filament spanning 12 kg of failure load.
Factory-sealed Polymaker CoPA — with 10 minutes of air exposure, printed in a dry enclosure, stored in its original bag — averaged 21.4% weaker than re-dried filament from the same spool and performed worse than actively-wet filament.
Three plausible explanations:
1. Respooling Exposure
CoPA Nylon begins absorbing moisture from ambient air within minutes of exposure. The ~10 minutes of open-air handling during respooling may have been enough to measurably raise moisture content before printing started.
2. Shipped Wet
The filament may have left the factory with elevated moisture content. Sealed packaging preserves the state the filament was in at time of sealing — it does not actively dry the material. If the spool was sealed before reaching equilibrium dryness, the bag locks that moisture in.
3. Storage Degradation
Even factory-sealed bags can admit trace moisture over time, depending on seal quality and storage conditions. The age and storage history of the specific spool tested is unknown.
A controlled follow-up — measuring factory-sealed filament directly out of the bag with a moisture meter, then testing immediately with zero air exposure — would isolate whether the issue is handling or the factory state itself. That test has not been run yet. Until it is, the most defensible position is to treat every spool as unknown moisture state regardless of packaging.
Does Over-Drying Damage Nylon Filament?
At 170°F, extending drying from 6 hours to 24 hours caused no measurable strength loss in this test. The highest single break result in the entire experiment — 43 kg — came from an over-dried arm. The group average of 41 kg was statistically identical to re-dried at 41.5 kg. A 0.5 kg difference across parts that failed anywhere between 27 and 43 kg is noise, not signal.
This does not mean Nylon is immune to thermal degradation at all temperatures and durations. At high enough temperatures or sufficiently long durations, any polymer will degrade. But within the range of typical filament dryer settings — up to 80°C (176°F) — over-drying appears to be a non-issue for CoPA Nylon. If you forget a spool in the dryer overnight, the data suggests you did not ruin it.
What Wet Filament Actually Does to a Part
Water trapped inside Nylon filament boils at printing temperatures — around 250°C for CoPA. The steam has nowhere to go except into the melt, where it forms bubbles that freeze into the part during cooling. Under macro photography, wet prints show thousands of microscopic voids distributed across the cross-section and surface.
These voids scatter light, which is why wet Nylon prints appear slightly lighter or more matte than dry ones. The more reliable diagnostic is surface texture: wet prints feel rough and slightly porous; dry prints are smooth and dense.
Structurally, the voids reduce the effective cross-section bearing load. The part looks solid but is internally compromised — more void space, less plastic. There is no reliable external inspection that reveals how severe the void content is. The only control point is moisture at the source, before printing.
For a deeper look at print quality variables that affect structural parts, see the complete 3D print settings guide for RC car parts and the overview of which parts in this build are most load-critical.
Practical Drying Protocol for Nylon
Based on this test, the effective drying protocol for CoPA Nylon is:
- Temperature: 170°F (77°C) — matches Polymaker's recommendation for CoPA
- Duration: 6 hours minimum; up to 24 hours appears safe at this temperature with no measurable degradation
- Timing: Dry immediately before printing — even 10 minutes of air exposure after drying can begin moisture reabsorption
- During printing: Use a sealed enclosure or dry box; do not leave the spool on an open holder between sessions
- Factory bags: Treat sealed factory bags as unknown moisture state — dry before any structural application regardless of packaging
Dry your filament every time — not because wet filament is the worst possible outcome, but because you cannot know what is in a factory bag. The data shows factory-sealed can perform as poorly as actively wet filament, at −21.4% vs re-dried.
If you are printing structural parts like these suspension arms for an RC build, see the beginner's guide to 3D printing an RC car for the full material and settings workflow, and how to remove layer lines for surface finishing techniques that also improve mechanical properties.
Strength by Condition — Normalized Summary
All values normalized to re-dried (170°F / 6h) = 100%:
| Condition | Avg Break Load | Relative Strength | Loss vs Re-dried |
|---|---|---|---|
| Over-dried (170°F / 24h) | 41 kg | 98.8% | −1.2% — no practical difference |
| Re-dried (170°F / 6h) | 41.5 kg | 100% (baseline) | — |
| Wet (humidity chamber, 24h) | 35 kg | 84.3% | −15.7% |
| Factory-sealed | ~33 kg | 78.6% | −21.4% |
| Soaked (direct water spray) | 27.5 kg | 66.3% | −33.7% |
Test material: Polymaker CoPA Nylon. Test part: RC suspension arm printed on Prusa MK4 at 265°C / 0.15 mm layer height / 6 perimeters / 50% infill, oriented flat. Load applied axially through pivot mounting hole. n=2–3 per condition; see statistical note above.
Frequently Asked Questions
Does drying filament restore strength after it gets wet?
Yes. Re-drying Nylon filament at 170°F (77°C) for 6 hours fully restores tensile strength. In pull-to-failure testing, re-dried Polymaker CoPA arms broke at 41–42 kg — statistically identical to the theoretical maximum achievable in this test. Strength loss from moisture is fully reversible with proper drying.
Does over-drying damage Nylon filament?
No — not at 170°F. Drying CoPA Nylon for 24 continuous hours at this temperature caused no measurable strength loss. The highest single result in the entire test (43 kg) came from an over-dried arm. Over-drying at typical dryer temperatures appears safe for Nylon.
Is factory-sealed filament actually dry?
Not necessarily. In this test, factory-sealed Polymaker CoPA produced the second-worst average strength (−21.4% vs re-dried) and the widest result variance of any group — 27 to 39 kg across three samples. The filament may have absorbed moisture after sealing or may have shipped with elevated moisture content. Always dry Nylon before printing structural parts, regardless of packaging.
How much strength does wet filament lose?
Nylon conditioned in a 24-hour humidity chamber lost 15.7% of tensile strength compared to re-dried filament. Filament soaked directly with water lost 33.7%. Even moderate moisture exposure causes measurable structural degradation in printed parts.
What temperature should I dry Nylon filament at?
170°F (77°C) for 6 hours is the standard protocol for CoPA Nylon and fully restores tensile strength in testing. Extending to 24 hours at the same temperature causes no measurable degradation. Check your specific filament's datasheet — PA6, PA12, and PA6-CF variants may have different optimal temperatures.
Why do wet filament prints look lighter or rougher?
Water in wet filament boils at printing temperatures, creating steam bubbles that freeze into the part as microscopic voids. These voids scatter light, making prints appear slightly lighter or more matte. Under macro photography they appear as thousands of small surface craters. Less plastic, more void space — the part is structurally weaker than it looks from the outside.
RC builder and 3D printing experimenter. Tests structural filament behavior, RC component design, and the gap between spec sheets and real-world performance under load. Follow the builds on YouTube @itkacher.