Nylon Part Dimensions After Printing: Moisture, Conditioning, and Machining Allowance

The Short Answer

A nylon part does not keep the dimensions it had when it came off the printer. Polyamides absorb water from the air until they reach equilibrium with it, and as they do they grow slightly, get tougher and lose stiffness. PA6 and PA66 take up three to four times as much moisture as PA12 or PA11 at room conditions, so they move more. For critical features, decide which state the drawing describes (dry as printed, or conditioned), leave material on the features you will machine, and finish them after the part has settled.

Why Nylon Grows and Softens

Arkema, which makes PA11 and PA12 base polymers, explains the difference by chain chemistry: polyamides with more frequent amide groups are more polar and more hydrophilic, which is why short-chain PA6 and PA66 absorb more water than long-chain PA11 and PA12. BASF, which makes the Ultramid PA6 and PA66 polymers, states the consequences: parts absorb water depending on humidity, time, temperature and wall thickness, their dimensions increase slightly, and impact resistance, elongation at break and creep rise while strength, stiffness and hardness fall. Absorption is very slow at room temperature and much faster when warm. For the grades themselves, see nylon 3D printing and PA6 vs PA66 vs PA12 vs PA11.

How Much Each Grade Absorbs

Molding-resin datasheets are the most consistent reference, because they report moisture at the standard atmosphere of 23 °C and 50% relative humidity. The values below are the manufacturers' datasheet values for the grades named. Printed filament made from a similar polymer will not match them exactly, but the ranking holds.

What That Means for Size

Arkema's immersion comparison shows the effect on dimensions: after 25 weeks in 20 °C water, Rilsan PA11 changed length by 0.2–0.5%, while PA6 changed by 2.2–2.7%. BASF gives a rule for its PA6 and PA66: with the water evenly distributed through the part, length increases by about 0.2–0.3% for each percent of water absorbed, and glass-fiber grades change by less than 0.1% along the fiber direction. Hypothetical example: on BASF's molding figures, an unfilled PA6 feature 100 mm long going from fully dry to the 3.0% equilibrium at 23 °C/50% RH would grow by roughly 0.6–0.9 mm. That is arithmetic on published resin data, not a measurement of a printed part, but it is several times the ±0.2 mm typical FDM guidance, which is why a PA6 feature needs a stated moisture condition. Filament makers report the same split: Polymaker gives an equilibrium water absorption of 3.3% for its Fiberon PA6-CF20 and 1.5% for its Fiberon PA12-CF10 (the latter measured at 70% RH and 23 °C).

What Moisture Does to Printed Nylon's Properties

Polymaker tests both of those carbon-fiber nylons twice: dry, and wet after 48 hours in 60 °C water. The wet state is much harsher than room air, so read it as the direction and relative size of the effect, not as service values. These are the manufacturer's datasheet values for printed specimens annealed at 100 °C for 16 hours, X-Y direction.

Fiber Helps, but Does Not Stop It

The PA6 composite is far stiffer when dry, then loses about 70% of its tensile modulus when saturated while its notched impact strength more than triples. The PA12 composite barely moves. Molding grades show the same pattern under the gentler 23 °C/50% RH conditioning: Arkema lists Rilsamid AMNO TLD (PA12) at a tensile modulus of 1,450 MPa dry and 1,170 MPa conditioned. Fiber limits growth along the fibers, as BASF's glass-fiber figure shows, but the nylon around it still absorbs water. A PA6-CF part still needs a stated moisture condition.

Dry as Printed vs Conditioned

Resin datasheets report nylon properties twice. Arkema labels them DRY, meaning dry as molded, and COND, meaning conditioned. Printed parts have the same two states: dry as printed, and conditioned to equilibrium with the air they will live in. BASF notes that its PA6 and PA66 parts reach their best impact strength and largely constant dimensions only after absorbing moisture. Neither state is wrong, but the drawing has to say which one it means. The standards give you the vocabulary:

• ISO 291 defines standard atmospheres for conditioning and testing plastics, and says 23 °C/50% RH shall be used when obtaining comparative data on polymers.
• ISO 1110 describes accelerated conditioning of polyamide test specimens. Its scope says the moisture content reached is close to the equilibrium at 23 °C/50% RH, while mechanical values can differ slightly from those after conditioning in that atmosphere. BASF recommends ISO 1110 conditioning, at 70 °C and 62% RH, as a gentle method.
• Time grows quickly with wall thickness. For flat Ultramid B (PA6) sheets at 70 °C and 62% RH, BASF's table gives about 10 hours at 1 mm, 48 hours at 2 mm and 120 hours at 4 mm to reach the 23 °C/50% RH equilibrium. Room air takes far longer.
• Warm water is faster but, BASF warns, can leave stains and deposits and can warp thin parts with internal stresses. BASF's Forward AM unit gives printed PA6 parts the same choice: a conditioning oven controls humidity and temperature precisely, while a water bath is quicker but may slightly change color.
• Forward AM recommends conditioning printed PA6 parts to reduce the risk of damaging fragile parts in use and transport, and notes that PA6 parts need more care than PA12.

Anneal Before You Measure

Some nylon filament makers recommend annealing, and it changes dimensions. Polymaker recommends annealing Fiberon PA6-CF20 and PA12-CF10 at 100 °C for 16 hours after printing. In its datasheet shrinkage test on a 40 mm cube printed at 30% infill, annealing reduced the PA12-CF10 cube's height from 39.79 mm to 39.48 mm, while the PA6-CF20 cube changed by hundredths of a millimeter. The same test printed a 10 mm hole at 9.73 mm in PA6-CF20 and 9.91 mm in PA12-CF10. BASF describes the mechanism for molded Ultramid: annealing causes post-crystallization, which raises density and stiffness but brings slight post-shrinkage and sometimes a little warpage. If the grade's datasheet calls for annealing, plan it before conditioning and before any final machining or inspection. The grades above are examples with published data; the grade on your quote may differ, so check its own datasheet.

Leave Stock Where It Matters

A material that keeps moving after it is made cannot hold a bearing fit straight off the printer, so use the approach machinists use for plastics that change after processing: rough, stabilize, then finish. Mitsubishi Chemical Group's guide to machining plastic parts recommends rough machining, annealing and then finish machining with a very light cut for close-tolerance parts, and leaving enough oversize when pre-machining to reach final size after annealing. For printed nylon, the stabilizing steps are annealing (when the grade calls for it) and conditioning. A workable sequence:

• Model critical features with extra material: bores undersize, bearing seats and sealing faces proud of their final surface. How much depends on size, grade and process, so treat it as a decision to make with the review, not a fixed number.
• Print the part. Standard parts ship as printed, with support material removed.
• Anneal if the grade's datasheet calls for it.
• Condition to the state the drawing specifies, or let the part reach equilibrium with its service environment.
• Finish machine the critical features with light cuts.
• Measure at the stated moisture condition and temperature, and record both.

Features That Can Stay as Printed

Most of a nylon part does not need this treatment. Clearance holes, outer profiles, ribs and cosmetic faces can stay as printed, within the typical FDM guidance of ±0.2 mm, which depends on geometry and is not a guarantee. Our tolerances and accuracy guide covers what drives printed accuracy. Reserve allowance and finish machining for features that locate, seal or run: bores, bearing seats, dowel holes and sealing faces.

How to Write It on the Drawing

• State the inspection condition: dry as printed (and within what time of printing), conditioned to equilibrium at 23 °C/50% RH per ISO 291, or accelerated per ISO 1110.
• State the measurement temperature.
• Name the grade, not just nylon: PA6, PA12, PA11 or a fiber-filled grade.
• Mark which features are machined after printing and which stay as printed.
• Set running clearances for the conditioned state if the part will live in humid air.
• If parts are inspected after shipping, say who conditions them and when.

Choosing a Grade When a Dimension Must Hold

If the fit matters more than toughness, start with PA12 or PA11: their low uptake keeps both dimensions and stiffness steadier. Choose PA6 when toughness and impact resistance matter and the design tolerates growth, or condition it and finish critical features afterward. PA6-CF gives the highest dry stiffness of these but still changes with moisture, as the Polymaker data shows, while PA12-CF pairs fiber stiffness with a low-uptake base. Every grade is selectable on the quote page, and each part can have its own material.