How to condition polyamide: methods, moisture content and common mistakes
How to define the moisture condition a PA6 or PA66 part needs, choose a process on technical grounds and verify that the part has been conditioned correctly.
Consider an illustrative case. A PA6 part has run through the specified programme in the conditioning system. The chamber held its temperature and humidity, and the balance shows the expected gain in mass. Yet a snap-fit hook breaks during assembly. All the readings may be correct and still fail to describe what mattered at that spot: they do not tell you whether the hook was too dry or whether geometry or processing is to blame.
Conditioning polyamide correctly means bringing a part to a moisture condition in which it meets the requirements of its application — and demonstrating that it does. PA6 and PA66 are hygroscopic: they take up water from their surroundings. The water settles in the disordered regions of the material and acts there as a plasticiser. For PA6 it has been shown that the part becomes tougher but less stiff and less strong, and that it swells; how strongly depends on the grade. By contrast, freshly moulded parts — described as “dry as moulded” (DAM) in manufacturers’ literature — contain little water.
Which condition is right therefore does not follow from the material alone. A clip needs enough flexibility to deflect, a bearing seat needs dimensional stability, a housing needs stiffness — often all at once, at different points on the same part. Neither a particular percentage nor a particular process time proves on its own that this condition has been reached: a percentage is ambiguous without a reference basis and a sampling location, and a process time describes what the equipment did, not the condition the part actually reached.
This guide shows how the requirements of a part are turned into a testable specification, a justified choice of process and a robust production process.
Contents · 10 sections
What happens during conditioning: water changes the toughness, stiffness and dimensions of polyamide
Polyamides carry amide groups along their molecular chains. These groups form hydrogen bonds — weak links that hold neighbouring chains together. Water molecules attach to the amide groups, push the chains further apart and make them more mobile. This happens in the amorphous, or disordered, regions of the semi-crystalline material, which is where the absorbed water resides.
The result is plasticisation. The glass transition temperature of the amorphous regions — the temperature range in which they change from rigid and glassy to more mobile and rubbery — falls markedly as the water content rises. That changes how the part behaves at its service temperature. For the PA6 studied, lower stiffness and strength, higher toughness and swelling have been demonstrated; manufacturer information describes increasing toughness and swelling for its polyamide grades as well, including PA66.
This gives the principle behind everything that follows: conditioning does not make a part better across the board; it shifts a trade-off. At room temperature, dry polyamide is less tough and therefore more prone to brittle fracture under impact or assembly loads. Too much moisture, on the other hand, costs — as shown for PA6 — stiffness and strength, and dimensional stability through swelling. The water uptake itself is reversible: in dry surroundings, polyamide releases water again. Some consequences are not, however — leached constituents, for example, or cracks such as those that can appear after repeated immersion in boiling water and drying.
PA6, PA66 and reinforced grades: similar direction, different extent
In our assessment, the grades behave alike in kind but differently in degree. According to manufacturer data, unreinforced PA6 absorbs more water than unreinforced PA66 in the same climate. Additives such as impact modifiers also change the uptake behaviour. Every property value therefore applies to one specific grade.
Glass fibres absorb no water themselves but add to the mass, so the moisture content referred to the whole part is lower — without the polymer matrix being any drier. In our assessment, the extent to which plasticisation of the matrix changes a reinforced part cannot be carried over from the unreinforced grade: the fibre network carries much of the stiffness, so the effect has to be assessed per grade and load case.
Drying, conditioning and annealing serve different purposes
Before injection moulding, granules must be below a permissible residual moisture content and are dried where necessary. If the residual water content is too high when the material is melted, hydrolytic degradation of the polymer chains can occur; the strength and toughness lost in this way cannot be recovered by any later step. How much residual moisture is permissible is set by the manufacturer for each grade. The finished moulding, by contrast, is conditioned so that it reaches the moisture condition its application requires.
Annealing, finally, is a heat treatment intended mainly to relieve residual stress and anticipate post-mould shrinkage; through post-crystallisation it changes the dimensions. The moisture condition of the part should be reassessed afterwards.
Which moisture content is right: no universal target, but a testable specification
The question of the “right” moisture content is the obvious one — and the answer is inconvenient: there is no value that applies to every PA6 or PA66 part. Materials differ in grade, reinforcement and additives. Parts make different demands. And the same figure means different things depending on the reference basis and the test method.
It helps to keep three states apart. The equilibrium moisture content always refers to a particular climate, for example the standard atmosphere 23/50 (23 °C, 50 % relative humidity). Water saturation describes uptake during immersion in water; it is considerably higher than the equilibrium moisture content in the standard atmosphere.
The target condition, on the other hand, is a decision: the condition a specific part needs for its function. It may lie close to an equilibrium moisture content, but it does not have to. Full water saturation is not a general prerequisite for a functional part.
Water absorption values determined to ISO 62 apply to test specimens of defined dimensions under defined test conditions. Such values do not tell you what condition your part needs — that follows from the material specification and the mechanical and functional requirements.
A percentage only becomes a specification with a reference basis
Even a well-founded target value is only unambiguous once it is clear what it refers to. ISO 62 expresses water absorption as the change in mass as a percentage of the mass after initial drying, and provides for reference to the mass after re-drying in certain cases. Even within one standard, the reference mass is therefore a stated parameter, not something to be assumed. In practice there are further variants: referred to the moist total mass, to the dry part mass or — for reinforced grades — to the polymer matrix.
What a technically robust conditioning specification should contain
| Item | What to define | Why it cannot be left out |
|---|---|---|
| Material | manufacturer and grade designation, reinforcement, fillers, additives | uptake behaviour and properties apply per grade |
| Part requirements | mechanical, functional, dimensional and other requirements, e.g. assembly forces, snap-fit action, tolerances, electrical properties | they determine which condition is needed |
| Moisture target | target value or range with tolerance — or explicitly a property target instead of a moisture value | without a tolerance, no result can be assessed |
| Reference basis | referred to moist total mass, dry part mass or matrix — and whether an absolute water content or an increase over a documented initial condition is meant | otherwise the same figure means different things |
| Sampling location | whole part or a defined location, e.g. a function-critical feature | a whole-part value is an average |
| Test method | e.g. weighing whole parts against a documented initial condition, or water content determination to ISO 15512 | methods measure different quantities |
| Test conditions | time after conditioning, climate until testing, sample preparation | the condition keeps changing after the process |
| Acceptance criteria | what counts as met, sample size, documentation, procedure for deviations | only then does a measurement become a decision |
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Where a customer or standard specification exists, it is binding. Insights into local moisture conditions or function-critical features do not override it unilaterally. They may form the basis for agreeing a change with the customer — documented and backed by trials.
How moisture is distributed in the part: from the outside in — and not everywhere at once
Water enters the part by diffusion: from the surface inwards, driven by the difference between the surroundings and the interior of the material. How fast this happens depends strongly on temperature. At elevated temperature uptake is much faster; at room temperature and low humidity it can take a very long time for equilibrium with the surroundings to be reached. Wall thickness, moisture supply and the initial condition also play a part.
Wall thickness does not act linearly. For Fickian diffusion through a flat plate with constant conditions at its surfaces — the model ISO 62 uses to evaluate diffusion coefficients — thickness enters the time scale as a square. Under these model assumptions, a plate twice as thick takes roughly four times as long to reach the same fraction of its final uptake.
Whether a particular polyamide follows this idealised behaviour depends on the material, the temperature and the moisture range; ISO 62 itself sets limits on the simple Fickian evaluation. Real parts with ribs, edges and varying wall thicknesses are not flat plates either. For them, the factor is therefore not a conversion rule. The model does explain why a conditioning time can never be stated without the wall thickness.
While uptake is in progress, moisture is unevenly distributed — higher near the surface than inside; how pronounced the differences are depends on material, wall thickness, temperature and duration. In PA6 test specimens, such moisture gradients have been made visible by X-ray computed tomography and compared with a diffusion calculation; this cannot be transferred directly to production parts with a different geometry and history. The authors also point out that the water content of PA6 is usually specified as an integral value — an average across the whole part.
If a part remains under constant conditions for long enough, the moisture largely evens out.
Function-critical features matter — and so does the rest of the part
For process design this is central. Thin-walled functional features such as snap-fit hooks, latches or clips can, under suitable conditions, reach a required moisture condition earlier than a thick base. For a specific part this is not automatically proven; geometry, the way the feature joins the body and the process conditions all play a part.
It does not follow, however, that only these features count. The base can have requirements of its own — dimensional stability, warpage, stiffness or its later behaviour in the service climate. Which area needs which condition is a question of the part requirements, not of diffusion.
Nor is the distribution frozen once the process ends. Manufacturer information describes excess surface moisture being released quickly and the remaining water content evening out within the part over time. At the same time, the part continues to exchange water with its surroundings. Storage, packaging and transport are therefore part of the process design.
Which conditioning processes exist: six principles, two levers
All processes rely on the same physics. They differ in how they supply water — as humidity, as steam or as liquid water — and at what temperature. Storage in the production hall is not a process in the strict sense: it also leads to moisture uptake, but in a fluctuating climate and therefore with only limited reproducibility.
Three terms are often confused. A steam atmosphere is an atmosphere with a high water-vapour content; it need be neither saturated nor pressurised. Steam exposure means that steam is introduced into a space or vessel. A pressurised process exists only when the vessel is operated above ambient pressure — and with pure saturated steam, the temperature rises with the pressure. What matters for the part is therefore not the name of the process but the temperature and moisture supply that actually act on it.
Process principles at a glance
| Process | Basic principle | Relevant selection criteria | Limits to consider |
|---|---|---|---|
| Controlled humid atmosphere (climate chamber or room) | air at controlled temperature and relative humidity | the material approaches the equilibrium of the set climate; if that lies above the target condition, time control is needed; no contact with liquid water | slow at low temperature; uniformity depends on air flow and loading |
| Unheated humid atmosphere | closed space with very high humidity at ambient temperature, used during storage or transport time | Conditioning at nearly 100 % relative humidity without additional heating or active steam introduction. In the regular process, the parts remain surface-dry and stain-free. | Conditioning time depends in particular on the material, part geometry and ambient temperature. The method accordingly requires longer process times and is particularly suited to applications without high time pressure. |
| Heated steam or humid atmosphere (chamber, cabinet) | parts, often in their transport containers, in a heated, vapour-rich atmosphere; overpressure not required | faster uptake than at room temperature; large quantities per batch | assess thermal load; check uniformity within the load; condensate on surfaces |
| Pressurised steam exposure | steam is admitted to a closed vessel operated above ambient pressure | high temperature and high moisture supply at the same time | with pure saturated steam, temperature rises with pressure: check heat resistance, warpage and surface; pressure-vessel requirements |
| Vacuum steam process | pressure reduction in a closed chamber, combined with steam | set the chamber atmosphere in a targeted way; programme-controlled sequence | it has not been demonstrated that the pressure reduction accelerates diffusion in polyamide; demonstrate the effect per system and part |
| Water bath | parts are immersed in cold or heated water | simple to implement; used in research to condition specimens | parts come out wet; water-soluble constituents can be leached out; a target below water saturation requires precise time control |
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The table describes process principles independently of any manufacturer. Selection criteria and limits are largely editorial technical assessments — not measurement results and not equipment data. Process names used by equipment manufacturers are product designations and do not automatically correspond to a row of this table.
No process is the best for every part
No process is inherently superior. Which one fits follows from the process design in the next section.
According to its website, Condisystems supplies systems for the saturated steam, vacuum, steam atmosphere and No-Energy processes; an overview is available under conditioning systems. How these systems work in detail is not described in this guide.
How to design a conditioning process: five steps to a robust production process
Robust process design starts not with the process, but with the part. Only once it is clear what the part must be able to do can you judge which condition is needed and how to reach it.
Step 1: Define the part requirements
Which properties must the part have after conditioning — and when: at assembly, on delivery, in service? This covers mechanical requirements such as snap-fit forces or impact loads, dimensional ones such as tolerances, and others such as electrical properties or surface quality. Also record where on the part they apply and which specification is binding.
Step 2: Analyse material and geometry
Polyamide grade, compound, reinforcement and additives determine the uptake behaviour. The geometry gives the wall thicknesses, function-critical features and thick sections. The initial moisture content matters just as much: how long and how the parts are stored between demoulding and conditioning.
Step 3: Capture the production requirements
Volumes, batch size and the time available before assembly or dispatch narrow the choice of process — as do loading (bulk, small load carriers, mesh pallet boxes), handling, storage and the required throughput.
Step 4: Select and qualify the process and process window
The process conditions are set on technical grounds: temperature in relation to what the material and part can tolerate, moisture supply, duration, loading and, where needed, surface after-treatment. Qualification takes place on production parts in a realistic load, including the least favourable positions and the critical features. For trial series or as an interim solution, conditioning can also be outsourced as contract conditioning.
Step 5: Verify the results and safeguard the process
Testing is carried out against the criteria from step 1, using the methods in the verification section below. For series production, add process monitoring, evidence of reproducibility, documentation and rules for the period afterwards: packaging, storage climate, time to assembly.
If a requirement is not met, that is not a signal simply to condition for longer, but a reason to review the process design: Is the requirement right? Does the target condition fit? Does the process act on the critical feature — or does the cause lie outside moisture altogether? Whether a design focused on function-critical features saves time or energy can only be judged with comparative data on the specific part; a shorter process time does not automatically mean less energy.
Checklist: information to have ready before a conditioning process is assessed
- Material with manufacturer, grade designation, reinforcement and additives
- Drawing or photos showing wall thicknesses and critical features
- Required properties and how they are tested
- Any existing moisture specification, with reference basis and test method
- Time and storage between demoulding and conditioning
- Volumes, batch sizes, available time, loading
- Requirements for surface, cleanliness and packaging
- Current process and problems observed, if any
Common mistakes in conditioning polyamide: narrow down the cause before adjusting the process
Conditioning problems often show up not as a wrong reading but as a part that fails during assembly, testing or service. That alone does not establish moisture as the cause. Brittle fracture, for instance, can also result from notch effects, weld lines, material defects or mechanical overload — or from damage caused by moulding granules that were too moist. The table links typical observations to possible causes and a first check — as hypotheses, not diagnoses.
Typical observations and how to narrow them down
| Observation | Possible causes | Suggested checks |
|---|---|---|
| Snap-fit hooks break during assembly | functional feature too dry or dried out again; notches, sharp transitions; weld line at the hook; processing damage, e.g. through hydrolysis; overstrain during assembly | document the fracture; determine moisture on a sample from the hook area; assembly trial with reference parts; check weld line and processing |
| Parts warp | uneven shrinkage due to fibre orientation, mould temperature or changes in wall thickness; swelling superimposed on post-mould shrinkage; uneven moisture uptake or thermal load in the process | measure dimensions before and after conditioning and after storage; compare unconditioned reference parts; record the position in the load |
| Results vary between batches | different initial moisture; changing loading; different material batch; processing effects; sensor or sampling | record the initial condition per batch; fix the loading; sample several load positions; check the measuring equipment |
| The equipment reaches its set points, the part does not meet its requirements | sensor readings describe the atmosphere, not the part; duration too short for the geometry; gradient at the critical feature; requirement not determined by moisture alone | test the part, not just the equipment; trial series on production parts; re-read the requirement and specification |
| Moisture value does not match the expected function | different reference basis; average instead of a value at the critical feature; different test method; function limited by geometry or processing | align reference basis and method; sample at the functional feature; establish the link between moisture value and function |
| Properties change after storage or transport | water release or further uptake; moisture evening out within the part; climate and packaging | test at the time of use; specify storage climate and packaging; repeat testing after a defined storage period |
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Conditioning for longer or more intensively is not a cure-all. More moisture means more swelling and — as shown for PA6 — less stiffness; more temperature means more thermal load. If the cause lies in geometry or processing, conditioning will not remove it — but it can mask it, for example when greater toughness makes a weak weld line appear adequate for a while.
How to verify successful conditioning: four levels of testing, each telling you something different
Whether conditioning has succeeded can be checked at four levels. Each answers a different question; none replaces the others on its own.
Process monitoring shows what the equipment did. A hygrometer measures the humidity of the air at the sensor, not the water content of the plastic part. A Pt100 is a platinum resistance thermometer and measures temperature — neither air humidity nor part moisture. A correctly monitored process atmosphere is therefore a prerequisite, but not proof that the part meets its requirements.
Weighing and water content determination answer related but different questions. Weighing whole parts before and after conditioning is non-destructive and practical on the shop floor. It first gives the change in mass of the part; with a documented initial condition, the water taken up relative to that condition can be determined from it, averaged across the whole part.
This requires controlled weighing: under ISO 62, specimens must neither absorb nor release water during weighing; they are weighed immediately after removal, surface water is removed first, and dried specimens are cooled in a desiccator. For parts, this means no moisture loss or gain between removal and weighing — for example through prolonged open cooling — and a balance whose resolution suits the expected change in mass.
Weighing only gives the absolute water content if the water content in the initial condition is also known. Other changes in mass, such as leached soluble constituents, also enter the result. The water content of a sample, by contrast, is determined by the methods of ISO 15512, for example Karl Fischer titration. To know the local water content of a snap-fit hook, you need a sample from that area; whether the hook performs its function is shown by a functional test instead.
Mechanical or functional testing — assembly trials, snap-in and release forces, impact or dimensional checks — shows whether the requirement is met in the condition tested. It is the most direct evidence.
Process qualification links the levels: once it is established how process parameters, moisture value and function relate, process monitoring and moisture measurement can carry much of the ongoing assurance in series production — within the qualified range of material, geometry and loading. They do not replace sample testing of parts, customer-specific test requirements or a fresh assessment after changes.
What the standards cover — and what they do not
What a measurement tells you — and what it does not
| Measurement | What it shows | Limitations |
|---|---|---|
| Relative humidity (hygrometer) | moisture supply of the air at the sensor | not a value for the part; differences within chamber and load; calibration |
| Process temperature (e.g. Pt100) | temperature at the sensor | neither air humidity nor part moisture; the part heats up with a delay |
| Process time, programme sequence | that the process ran as specified | no evidence of the part condition |
| Change in mass (weighing whole parts) | water taken up relative to a documented initial condition, averaged across the part; non-destructive | no absolute water content without a known initial water content; surface water and leaching affect the result, as does moisture lost or gained between removal and weighing; nothing about distribution |
| Mass loss on heating (e.g. moisture analyser) | volatile content of a sample | not water-specific, other volatiles are included; depends on the chosen temperature |
| Water content of a sample (Karl Fischer to ISO 15512) | water in the sample taken | applies only to the sampling location; method and sample preparation must suit the material and must not change the moisture; destructive |
| Mechanical or functional test | whether the property is met in the condition tested | applies to the time of testing and the sample; series production needs an established link to process and moisture values |
| Process qualification | that the process delivers the result reproducibly | applies within the qualified range; does not replace sample testing; changes to material, geometry or loading need a new assessment |
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A process that produces a good part once is not yet a production process. It is reproducible when it repeats the result across batches, loading patterns and changing ambient conditions — and when that is documented. The documentation should therefore include the process parameters per batch, the initial condition, the tests and their results, the acceptance criteria and the procedure for deviations.
Conclusion: when is polyamide correctly conditioned?
A polyamide part is correctly conditioned when it demonstrably meets the requirements of its application — at the locations where they apply, and at the time when they apply. A moisture value is an important tool for this, but it only becomes a robust specification together with material, reference basis, sampling location and test method.
Material, geometry and part function have to be considered together, because moisture moves from the outside in and can reach thin and thick sections at different times. The choice of process follows from this assessment, not from the name of the process. And a production process is only robust once it delivers its results reproducibly and this is documented.
Looking to design or improve your conditioning process?
If you have a specific application or problem — a new part, a changed customer requirement or an existing process that is not running consistently — tell us about it. For your enquiry, it helps to know the material, the part with its wall thicknesses and critical features, the production volume, the required properties and, if there is one, the current process.
Frequently asked questions
Does every PA6 or PA66 part need to be conditioned?
No. Whether conditioning is needed follows from the requirements. If the part has to withstand impact or assembly loads soon after moulding, the dry condition can be critical; if maximum stiffness is the priority, it can be desirable. Bear in mind that a part takes up moisture from its surroundings over time even without conditioning, so the dry condition does not last. The deciding factors are the material grade, the part function, the service climate and the customer’s specification.
What is the difference between drying and conditioning?
They have opposite aims. Where necessary, granules are dried before processing, because too high a residual water content during melting can lead to hydrolytic degradation of the polymer chains. The finished moulding is conditioned so that it takes up moisture in a controlled way and reaches the condition its application requires. Annealing is a third, separate process: a heat treatment that can affect dimensions and structure.
Which moisture content is right for polyamide?
There is no generally valid value. The right target condition follows from the material specification and the part requirements, and it is only unambiguous together with the reference basis, sampling location and test method. An equilibrium moisture content from the data sheet describes the material in a particular climate, not a specification for your part.
Can a part take up too much moisture?
Yes. For PA6 it has been shown that, as the water content rises, stiffness and strength continue to fall and the part swells; for other grades, the extent should be taken from the data sheet. Dimensional tolerances, fits or stiffness requirements can be violated as a result. This is another reason why full water saturation is not a general aim.
How can I tell whether my part has been conditioned sufficiently?
Most reliably by testing the required property on the part, for example with an assembly trial or a force or dimensional check at the relevant point in time. Moisture measurements complement this: weighing whole parts shows, without destroying them, the water taken up relative to the initial condition as an average; a water content determination to ISO 15512 gives the water content of a sample. Series production also needs a qualified, monitored process.
Sources
Supported technical statements in this article are based on the following sources. Definitions and conclusions worded as assessments are not referenced separately. The standards serve different purposes — test method, conditioning of test specimens, water content determination, test atmosphere — and none of them is a specification for the industrial conditioning of a particular part. Where only the scope or an abstract was consulted, this is stated.
- 1ISO 62:2008 — Plastics — Determination of water absorption. Full text consulted. Supports: change in mass as a percentage of the mass after initial drying, alternative reference mass; weighing requirements; leaching of water-soluble constituents; the flat-plate diffusion model and its limits; limits of comparison under non-equilibrium conditions.
- 2ISO 1110:2019 — Plastics — Polyamides — Accelerated conditioning of test specimens. Supports: scope and limitations according to the published abstract; full text not consulted.
- 3ISO 15512:2019 — Plastics — Determination of water content. Supports: scope (powder, granules, finished articles) and overview of methods according to the ISO abstract and a standards catalogue; full text not consulted.
- 4ISO 291:2008 — Plastics — Standard atmospheres for conditioning and testing. Title and scope according to standards catalogues.
- 5IEC 60751:2022 — Industrial platinum resistance thermometers and platinum temperature sensors. Supports: Pt100 as a platinum resistance thermometer for temperature measurement (scope according to standards catalogues).
- 6IAPWS R7-97(2012) — Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam. Supports: the link between pressure and temperature of saturated steam (saturation line); structure according to the documentation, full text not consulted.
- 7Sambale AK, Stanko M, Emde J, Stommel M: Characterisation and FE Modelling of the Sorption and Swelling Behaviour of Polyamide 6 in Water. Polymers 2021, 13(9), 1480. doi:10.3390/polym13091480. Supports: water uptake in the amorphous phase of PA6, plasticisation, effect on stiffness, strength and toughness, swelling, reversibility, temperature dependence; one PA6 grade was studied.
- 8Sambale AK, Maisl M, Herrmann HG, Stommel M: Characterisation and Modelling of Moisture Gradients in Polyamide 6. Polymers 2021, 13(18), 3141. doi:10.3390/polym13183141. Supports: measured moisture gradients in PA6 test specimens; water content stated as an integral value.
- 9DuPont: Zytel® and Minlon® Nylon Resins — Molding Guide (05/17) GNE-A11218-00-A1216, © 2017 (manufacturer’s guide; title quoted verbatim). Supports: drying of granules because too high a residual water content in the melt leads to hydrolysis, with a grade-dependent limit; factors influencing moisture uptake; different uptake of PA6 and PA66; increase in toughness through moisture uptake; moisture evening out after conditioning; dimensional change through swelling and post-mould shrinkage; the term “dry as moulded” (DAM). Manufacturer information for its own grades.
- 10condisystems.com, accessed 20 October 2026: pages on conditioning systems (processes offered) and contract conditioning (scope of service). Manufacturer information.
