尼龙的尺寸不稳定，是一个被说过太多遍、也被误解太多遍的话题。
"尼龙会吸水，所以尺寸会变"——这句话没错，但它只讲了一半。
真正需要分清的是两件事：
① 吸湿膨胀。 水分子进入分子链之间，把链撑开，件变大。这个过程是缓慢的、和湿度相关的。
② 热膨胀。 温度升高，分子链运动加剧，件变大。这个过程是快速的、和温度相关的。
两者的时间尺度、影响因素、解法都不一样。 混着谈，结论一定是错的。
一、吸水对尼龙到底做了什么
水分子进入尼龙，不只是"占地方"，还会：
① 尺寸变大。 吸湿膨胀是各向同性的（不像玻纤取向那样分方向）。
② 力学性能变化。 吸水会降低刚性和强度，但提高韧性（起增塑作用）。这就是"调湿"能提升冲击性能的原因。
③ 电气性能下降。 水分会显著降低体积电阻与 CTI。这是电气件必须看调湿态数据的原因。
④ 玻璃化转变温度下降。 吸水后材料在更低温度就会变软。
所以吸水不只有"尺寸"这一个后果。 选型时只看尺寸，会漏掉电气和力学的变化。
低吸水这个需求，最常被精密件客户用一句话点破：同一副模具，七月打的件和一月打的件，装不进同一个工装。
有个做传感器外壳的客户，夏天送货装配顺畅，入冬之后陆续反馈过盈偏紧。把两批件放恒温房回温再量，差了两个丝上下。原因不复杂，PA66 吸水后膨胀，干态湿态尺寸不一样。
后来他们把图纸改成标注湿态尺寸，材料换成低吸水体系，问题翻篇。
客户的总结很精辟：塑料的尺寸是活的，图纸要跟上它的活法。
这句话后来成了我们讲吸水问题的开场白。尺寸不稳定很少是材料坏了，多数是设计的基准没有跟材料属性对上。
二、常见尼龙的吸水率分档
| 材料 | 平衡吸水率（典型） | 档次 | 说明 |
|---|
| PA46 | 约 12-14% | 高 | 最高，精密件基本排除 |
| PA6 | 约 2.5-3% | 中高 | 通用料，尺寸需管理 |
| PA66 | 约 2-2.5% | 中 | 比 PA6 略好 |
| PA610 | 约 1.5% | 中低 | 生物基长碳链 |
| PA1010 | 约 1.3-1.5% | 中低 | 生物基 |
| PA612 | 约 1.2% | 低 | 尺寸稳定好 |
| PA11 | 约 0.8% | 低 | 长碳链 |
| PA12 | 约 0.7% | 最低（脂肪族） | 长碳链代表 |
| PA9T | 约 2-3% | 低 | 半芳香族里最低 |
| PA6T | 约 4-6% | 中低 | 半芳香族，高于 PA9T |
注意半芳香族那一栏的对比：PA6T 与 PA9T 同属高温尼龙，但吸水率差了将近一倍。 这就是为什么精密连接器在高湿环境下更倾向 PA9T。
选择路径：
要极低吸水 + 常温场合 → PA12（脂肪族长碳链里最低）
要极低吸水 + 高温场合 → PA9T（半芳香族里最低）
要低吸水 + 综合平衡 → PA612 / PA610 / PA11
对吸水不敏感 → PA6 / PA66，成本最友好
三、尺寸变化的两种来源，分开算
热膨胀的系数，尼龙通常在 5-10 ×10⁻⁵ /℃（未增强），增强后会明显降低（玻纤能压到 2-3 ×10⁻⁵ /℃ 量级）。
吸湿膨胀的系数，取决于材料的吸水量和膨胀行为。粗略地说，每吸收 1% 的水，尺寸变化在 0.2-0.3% 量级（不同体系有差异）。
举个例子感受一下量级：
一个 100mm 长的 PA66 件，从干燥态吸湿到 2.5% 的平衡态，尺寸可能增加 0.5mm 以上。
这个量级，对 ±0.05mm 精度的件来说，是完全不可接受的。
所以判断的第一步是：先算清楚，你的件在工作环境下的吸湿量是多少、对应的尺寸变化是多少。 很多"尺寸超差"的争论，是从没算过这笔账开始的。
四、三条路线，三种代价
路线一：换材料（最直接，也最贵）
用吸水率更低的牌号（PA12、PA612、PA9T 等）。效果确定，但材料成本上升明显，而且可能带来其他性能的变化（如强度、耐温）。
路线二：改结构（常常最经济）
给吸湿膨胀留出空间。 具体做法：
配合公差按吸湿平衡态设计，不按干燥态
改用浮动配合，避免刚性约束
在关键配合面做局部补偿
把定位基准从"尺寸"改成"结构特征"
这条路线不换料，不动成本，但需要设计端配合。 在很多项目里，它的收益是最大的。
路线三：控环境与后处理（针对特定场景）
调湿处理：让件在出厂前就吸到接近使用状态的含水量
环境控制：件在低湿环境使用，吸湿量本来就小
退火：消除内应力，减少后收缩
调湿是最值得展开的一条，因为它同时改善尺寸和韧性。
五、调湿怎么做
原理：让件在受控的温湿度环境中放置一段时间，吸湿到接近平衡。
典型条件：高温高湿环境放置数天（例如 70℃ / 60%RH 量级，具体按材料和目标含水率定）。
三个作用：
1. 尺寸提前稳定——件在出厂前就完成了主要的膨胀
2. 韧性提升——水分的增塑作用让冲击性能大幅改善
3. 减少后续漂移——装到设备上之后的变化幅度变小
什么时候需要调湿：
精密配合件
齿轮、需要冲击韧性的件
需要长期尺寸稳定的件
什么时候不需要：
尺寸要求宽松的结构件
本身就要在干燥环境使用的件
吸水率本来就极低的材料（PA12 等，效果不明显）
六、加工与测量要点
① 干燥要彻底。 加工前必须干燥（残余水分会水解）。这与"使用中会吸湿"不矛盾——加工时不能有水，使用中难免有水，两个阶段的要求不同。
② 测量要在稳定态。 刚从模具出来的件尺寸最小，测出来的数值没有装配参考价值。
③ 建立统一的测量口径。 尺寸验证要注明"干燥态"还是"调湿态"。双方口径不一致，是最常见的扯皮来源。
④ 模温影响内应力与后收缩。 模温控制均匀，能减少后收缩的不确定性。
⑤ 后收缩要留时间观察。 有些件在成型后几天到几周仍有尺寸变化，要按足够长的周期复测。
七、五个常见的坑
坑 1：把吸湿膨胀当成热膨胀。
两者的时间尺度和解法完全不同。
坑 2：拿刚下线的尺寸去装配。
件还没吸湿到平衡，尺寸还会变。
坑 3：只换材料，不改公差。
换了低吸水料，公差仍按干燥态定义，问题会以另一种形式回来。
坑 4：忘了电气性能也受吸水影响。
CTI、体积电阻在调湿态会明显下降，电气件必须看调湿态数据。
坑 5：忽略"吸水会让件变韧"。
干燥态的件更脆。做冲击验证时，状态没定清楚，结论就不可比。
八、边界声明
| 工况 | 建议 |
|---|
| 精密件（±0.05mm 级）+ 潮湿环境 | 换低吸水材料（PA12 / PA612 / PA9T） |
| 精密件 + 干燥环境 | 可保留通用料，重点做调湿与公差设计 |
| 高温 + 精密 + 潮湿 | PA9T（半芳香族里吸水最低） |
| 常温 + 精密 | PA12 / PA612 |
| 尺寸宽松的结构件 | 不必换料，PA6 / PA66 够用 |
| 电气件（高 CTI 要求） | 必须按调湿态数据选材 |
| 需要同时改善韧性 | 调湿处理，一举两得 |
行业里的一条实感：尺寸超差的争论里，我们遇到最多的不是料选错了，是测量时机不一致。 一个精密件项目，客户反馈"装配装不进去"，把件寄过来测量，尺寸在公差的下限——而客户收到货再测，尺寸在上限。双方数据都真实，差的是件在运输和存放中吸了湿。 后来做了两件事：出厂前统一调湿到目标含水率，并在图纸上把"调湿态尺寸"写成明确要求。 争议立刻消失。 尼龙的尺寸问题，一半是材料特性，一半是口径问题。 把口径讲清楚，成本最低、见效最快。
一次调湿处理救回的一批件
起点是批精密齿轮用 PA66 打完直接装配，客户反馈运转噪音偏大。
潜伏期没人往吸水上想，先查了齿轮精度与装配，都对。
爆发在批量复检：把一批件按标准调湿处理之后再测，中心距变化把侧隙顶没了。原因找到：干态装配的齿轮，吸水后尺寸涨，侧隙被吃掉。
结算三步：装配前统一调湿、图纸按湿态公差标注、验收条件写明环境。噪音投诉随后清零。
这单常被我们拿来说明一件事：吸水不是缺陷，是材料属性。设计把属性接住了，问题就不存在。
低吸水需求的追问，三条排顺序。
追问一：公差是谁给的？ 干态湿态先统一基准，再谈材料。
追问二：服役环境的湿度范围？ 沿海与内陆、雨季与旱季都要进范围，范围定了才能算尺寸预算。
追问三：能不能改结构消化？ 加筋、留补偿间隙常常比换料便宜，先算结构账。
延伸判断（领域普适）
这四条不只针对 PA12 / PA66 / PA610 / 各种合金，是低吸水改性族共用的延伸判断。
判断一：吸水率低不等于尺寸不变。吸水率从 1.5% 降到 0.8%，尺寸变化是 0.3%，差异看似不大，但精密公差件（如 0.05mm 公差的齿轮）这就是 5 倍的累积偏差。低吸水只是入门级。
判断二：玻纤定向会让尺寸稳定往"各向异性"走。玻纤越多，流动方向与垂直方向的收缩差越大。这在高精密尺寸件上是个反向问题：玻纤含量越高，"尺寸稳定"在某一向上表现越好，其他方向越差。做精密件不要把玻纤当万能解。
判断三：尺寸稳定不等于"性能稳定"。尺寸好，但模量、强度、耐温、蠕变等其他指标未必同时好。所以精密电子件并不都是"低吸水"，还要看具体的工况需要。"低吸水"是必要条件，不是充分条件。
判断四：真正的"零吸水"塑料不存在。即便是 PVDF、PPS 这类材料，长期在高温水下也会有微量水分进入。所以任何"低吸水"的承诺都有条件——看具体测试条件。如果没有给出 1000 小时、2000 小时、5000 小时的浸泡数据，承诺本身没有意义。
这四条用得上，是因为"尺寸稳定的尼龙"听起来像"一种料"——其实是多种思路（低吸水树脂 + 玻纤定向 + 矿物填充 + 合金化）。选哪一种，看具体件的尺寸公差与长期可靠性要求。
判断一：尺寸变化有两个来源，分开算。 吸水膨胀与热胀冷缩方向可能相反，叠加后到底变大变小，要按工况算，别拍脑袋。
判断二：低吸水的代价心里要有数。 换长碳链或半芳香族，价格与加工条件都会动，省下的售后与多出的料价要放在一张表上。
判断三：验证顺序是调湿、测量、装配。 判断信号：同一批件干态湿态各量一次，差值超过公差三分之一，设计端必须动作。
收尾补三个辨析。
低吸水与低收缩是两件事。 吸水影响的是服役中的尺寸，收缩影响的是脱模后的尺寸，模具补偿针对后者，选材针对前者，混在一起讨论会绕远路。
调湿不是泡水。 标准调湿有温度与湿度条件，拿开水煮件把尺寸"煮到位"，会引入新的内应力，属于常见土办法里的坑。
低吸水体系的焊接与嵌件要复核。 换料之后熔接线强度与嵌件握力会变，精密件换低吸水料，这两项要重新验证。
最后一句给设计端：把干态、湿态两行尺寸都标进图纸，供应商报价时才不会各自理解。一行备注，省掉的是几个月的扯皮。
补一个终端经验：精密件的图纸会签，把"测量前的环境状态"写进检验规范，与尺寸公差同等级对待。检验条件统一了，尺寸争议就少了一大半。
收尾前放一张三问三答。
| 高频问题 | 一句话回答 |
|---|
| 冬夏尺寸差多少？ | PA66 可到两个丝以上，按壁厚算 |
| 换低吸水料贵多少？ | 把省下的调湿工序一起算总账 |
| 一定要换料吗？ | 不一定，调湿与结构补偿常常更便宜 |
| 图纸要标什么？ | 干态湿态两行尺寸加环境条件 |
再补一个反向案例，说说低吸水不是精度问题的全部答案。
有个精密件项目把 PA66 换成低吸水体系，尺寸投诉没停，反而多了一条。排查发现装配车间的温度波动比湿度影响更大，热胀的贡献被吸水问题盖住了。补了恒温存放与测量规范之后，投诉归零，低吸水料反而显得多余。
尺寸是温度、湿度、时间三个变量的函数，只处理其中一个，剩下两个会替它背锅。
这一单之后，我们把先归因再动手做成了一张排查卡：先测热、再测湿、后称重，三步走完再决定换不换料。卡很小，作用不小，精密件的选型决策大多输在归因不清上。
排查卡的三步顺序有讲究：先测热是因为温度响应快、当天出结果，再测湿是因为调湿要几天，最后称重是因为要积累磨损或失水的数据。从快到慢排，归因路径最短。
有个客户把这张卡贴在测量室，车间反馈尺寸异常先走三步，走完再来找供应商，一半的异常在第二步就有了答案。流程跑顺之后，他们换料的决定从拍脑袋变成了看数据，采购的底气就是这么攒出来的。
结语
低吸水尼龙选型，四句话：
先分清吸湿和受热——两种膨胀，两套解法。
算出实际吸湿量——先知道变化有多大，再决定要不要换料。
优先改结构与公差——常常比换料更经济。
统一测量口径——干燥态还是调湿态，要写在图纸上。
换材料是最后一张牌，不是第一张。 这句话，在尺寸问题上尤其值钱。
The dimensional instability of nylon is a topic that has been discussed too many times and also misunderstood too many times.
"Nylon absorbs water, so its size changes" — this sentence is not wrong, but it only tells half the story.
What really needs to be distinguished are two things:
① Moisture absorption and swelling. Water molecules enter between the molecular chains, pushing the chains apart and causing the piece to enlarge. This process is slow and related to humidity.
② Thermal expansion. As the temperature rises, the movement of molecular chains intensifies, causing the part to increase in size. This process is fast and related to temperature.
The time scales, influencing factors, and solutions of the two are all different. Mixing them together will definitely lead to incorrect conclusions.
1. What does water absorption actually do to nylon?
When water molecules enter nylon, they not only 'take up space,' but also:
① The size increases. Hygroscopic expansion is isotropic (unlike the directional orientation of glass fibers).
② Changes in mechanical properties. Water absorption can reduce stiffness and strength, but increase toughness (acting as a plasticizer). This is why 'moisture conditioning' can improve impact performance.
③ Deterioration of electrical performance. Moisture can significantly reduce volume resistivity and CTI. This is the reason why electrical components must refer to humidity-conditioned data.
④ The glass transition temperature decreases. After absorbing water, the material softens at a lower temperature.
So water absorption does not only have 'size' as a consequence. If you only look at size when selecting, you will miss changes in electrical and mechanical properties.
The demand for low water absorption is most often summed up by precision parts customers with one sentence: parts made with the same mold in July and parts made in January cannot fit into the same fixture.
There is a customer who makes sensor housings. During summer, deliveries and assembly went smoothly, but after winter, they gradually reported interference fit being too tight. After placing two batches in a constant temperature room to return to room temperature and measuring again, the difference was about two threads. The reason is not complicated: PA66 absorbs moisture and expands, so the dimensions are different in dry and wet states.
Later, they changed the drawings to indicate wet dimensions and switched the materials to a low water absorption system, and the problem was resolved.
The client's summary is very incisive: the dimensions of the plastic are alive, and the drawings need to keep up with its way of being alive.
This sentence later became our opening line when discussing water absorption issues. Dimensional instability is rarely due to material defects; most of the time it is because the design reference didn’t align with the material properties.
2. Water Absorption Grades of Common Nylons
| Material | Balanced Water Absorption (Typical) | grade; class; quality | Explanation |
|---|
| PA46 | About 12-14% | Tall | Top, precision parts basically excluded |
| PA6 | About 2.5-3% | Medium and high school | General materials, size needs to be managed |
| PA66 | About 2-2.5% | middle | Slightly better than PA6 |
| PA610 | About 1.5% | Medium-low | Bio-based long carbon chain |
| PA1010 | About 1.3-1.5% | Medium-low | Bio-based |
| PA612 | About 1.2% | Low | Good dimensional stability |
| PA11 | About 0.8% | Low | long carbon chain |
| PA12 | About 0.7% | Minimum (aliphatic) | Long carbon chain representative |
| PA9T | About 2-3% | Low | The lowest among semi-aromatic |
| PA6T | About 4-6% | Medium-low | Semi-aromatic, higher than PA9T |
Pay attention to the comparison in the semi-aromatic column: PA6T and PA9T are both high-temperature nylons, but the water absorption rate differs by nearly a factor of two. This is why precision connectors tend to prefer PA9T in high-humidity environments.
Select Path:
For extremely low water absorption, at room temperature → PA12 (lowest among aliphatic long carbon chains)
Needs extremely low water absorption; for high-temperature situations → PA9T (the lowest among semi-aromatic types)
Low water absorption, comprehensive balance → PA612 / PA610 / PA11
Not sensitive to water absorption → PA6 / PA66, most cost-friendly
3. Two sources of size variation, calculated separately
Coefficient of thermal expansion: Nylon is usually 5-10 ×10⁻⁵ /℃ (unreinforced), and it decreases significantly after reinforcement (glass fiber can reduce it to the order of 2-3 ×10⁻⁵ /℃).
The coefficient of hygroscopic expansion depends on the material's water absorption and swelling behavior. Roughly speaking, for every 1% of water absorbed, the dimensional change is on the order of 0.2-0.3% (varying for different systems).
Let's take an example to get a sense of the scale:
A 100mm long PA66 part may increase in size by more than 0.5mm when it absorbs moisture from a dry state to a 2.5% equilibrium state.
This magnitude is completely unacceptable for parts with a ±0.05mm tolerance.
So the first step in judgment is: first calculate clearly how much moisture your piece absorbs in the working environment and what the corresponding size change is. Many disputes over 'dimension deviations' start from never having done this calculation.
Four, three routes, three kinds of costs
Route 1: Change materials (the most direct, and also the most expensive)
Use grades with lower water absorption (PA12, PA612, PA9T, etc.). The effect is certain, but the material cost increases significantly, and it may also bring changes in other properties (such as strength and temperature resistance).
Route 2: Change the structure (often the most economical)
Allow space for moisture absorption and expansion. Specific approach:
Designed according to moisture absorption equilibrium rather than the dry state, in conjunction with tolerance.
Switch to floating support to avoid rigid constraints
Perform local compensation on the critical mating surface
Change the reference for positioning from 'dimension' to 'structural feature'
This route does not change materials or affect costs, but it requires cooperation from the design side. In many projects, it yields the greatest benefits.
Route Three: Environmental Control and Post-Treatment (for Specific Scenarios)
Humidity treatment: Let the item absorb a moisture content close to its intended use state before leaving the factory
Environmental control: The component is used in a low-humidity environment, and its moisture absorption is naturally small.
Annealing: Eliminates internal stress and reduces subsequent shrinkage
Moisture regulation is the most worthwhile aspect to develop, because it simultaneously improves both size and toughness.
5. How to adjust humidity
Principle: Place the item in a controlled temperature and humidity environment for a period of time, allowing it to absorb moisture until it reaches near equilibrium.
Typical conditions: placed in a high-temperature, high-humidity environment for several days (for example, around 70°C / 60% RH, specifically determined according to the material and target moisture content).
Three functions:
1. Size stabilized in advance — the piece has completed most of its expansion before leaving the factory
2. Toughness Improvement — The plasticizing effect of moisture significantly enhances impact performance
3. Reduce subsequent drift — the degree of change after being installed on the device becomes smaller
When is humidity adjustment needed:
Precision fitted parts
Gears and parts that require impact toughness
Parts that require long-term dimensional stability
When not needed:
Structural components with loose size requirements
Parts that are meant to be used in a dry environment
Materials with inherently very low water absorption (such as PA12, etc., the effect is not obvious)
6. Key Points of Processing and Measurement
① Drying must be thorough. It must be dried before processing (residual moisture will hydrolyze). This does not contradict 'it absorbs moisture during use'—there must be no water during processing, while water is inevitable during use; the requirements are different for the two stages.
② Measurements should be taken in a stable state. The parts just out of the mold have the smallest dimensions, and the measured values have no reference value for assembly.
③ Establish a unified measurement standard. Size verification should indicate whether it is in a 'dry state' or a 'conditioned state.' Discrepancies between parties' standards are the most common source of disputes.
④ Mold temperature affects internal stress and post-shrinkage. Uniform mold temperature control can reduce the uncertainty of post-shrinkage.
⑤ After shrinkage, time should be allowed for observation. Some parts may still undergo dimensional changes days to weeks after forming, so they should be re-measured over a sufficiently long period.
Seven, Five Common Pitfalls
Pitfall 1: Mistaking moisture expansion for thermal expansion.
The time scales and solutions of the two are completely different.
Pitfall 2: Using the dimensions of a newly offline item for assembly.
The item hasn’t absorbed moisture to reach equilibrium yet, so the size will still change.
Pitfall 3: Only change the material, without modifying the tolerance.
After switching to low water absorption material, the tolerance is still defined in the dry state, and the problem will come back in another form.
Pitfall 4: Forgetting that electrical performance is also affected by water absorption.
CTI and volume resistivity will significantly decrease in the conditioned state, so electrical components must refer to the data in the conditioned state.
Pitfall 5: Ignoring 'absorption will make the material tougher.'
Parts in a dry state are more brittle. When conducting impact tests, if the condition is not clearly defined, the conclusions cannot be compared.
8. Boundary Statement
| Operating condition | Suggestion |
|---|
| Precision parts (±0.05mm level) Humid environment | Switch to low water absorption materials (PA12 / PA612 / PA9T) |
| Precision parts dry environment | General materials can be retained, with focus on moisture adjustment and tolerance design. |
| High temperature Precision Humidity | PA9T (lowest water absorption among semi-aromatic types) |
| Room Temperature Precision | PA12 / PA612 |
| Loosely sized structural components | No need to change the material, PA6 / PA66 is sufficient |
| Electrical components (high CTI requirements) | Materials must be selected according to the moisture-adjusted state data. |
| Need to improve resilience at the same time | Moisture regulation treatment, killing two birds with one stone |
A real insight from the industry: in disputes over dimensional tolerances, what we encounter most is not that the wrong material was chosen, but that the measurement timing is inconsistent. In one precision parts project, the client reported that the parts 'couldn't be assembled.' When we measured the parts sent over, the dimensions were at the lower limit of the tolerance — yet when the client measured the received parts, the dimensions were at the upper limit. Both sets of data were accurate; the difference came from the parts absorbing moisture during transport and storage. Later, two measures were taken: before leaving the factory, humidity was standardized to a target moisture content, and 'moisture-conditioned dimensions' were explicitly written as a requirement on the drawings. The disputes immediately disappeared. For nylon dimensional issues, half are due to material properties and half to measurement practices. Explaining the measurement practices clearly is the lowest cost and fastest solution.
A batch of items salvaged with one humidity adjustment treatment
The starting point was assembling precision gears directly after molding with PA66, and the customer feedback indicated that the operational noise was relatively high.
During the incubation period, no one thought about going to the water intake, so we first checked the gear accuracy and assembly, and both were correct.
Outbreak occurs during batch re-inspection: after a batch of parts is conditioned with moisture according to the standard and then tested, the center distance change eliminates the backlash. The cause was found: the gears assembled in a dry state swell after absorbing water, and the backlash is consumed.
Three steps for settlement: unify humidity adjustment before assembly, mark drawings according to wet-state tolerances, and specify environmental conditions in the acceptance criteria. Noise complaints will then be cleared.
This case is often used by us to illustrate one thing: water absorption is not a defect, it is a material property. If the design accommodates the property, the problem does not exist.
Follow-up on low water absorption demand, three items in order.
Follow-up Question 1: Who provided the tolerance? Let's first unify the standard for dry and wet states, then discuss the material.
Follow-up Question 2: What is the humidity range of the service environment? Both coastal and inland areas, as well as rainy and dry seasons, need to be included in the range. The range must be determined before the size can be estimated.
Follow-up Question 3: Can the structure be modified to accommodate it? Adding reinforcement or leaving compensation gaps is often cheaper than changing the material, so calculate the structural costs first.
Extended Judgment (Domain-General)
These four points are not only for PA12 / PA66 / PA610 / various alloys, but are also extended judgments commonly used for the low water absorption modified series.
Judgment 1: Low water absorption does not equal no dimensional change. When water absorption drops from 1.5% to 0.8%, the dimensional change is 0.3%. The difference may seem small, but for precision tolerance parts (such as a gear with a 0.05mm tolerance), this is a fivefold cumulative deviation. Low water absorption is just an entry-level property.
Judgment 2: Glass fiber orientation causes dimensional stability to become 'anisotropic.' The more glass fiber there is, the greater the shrinkage difference between the flow direction and the perpendicular direction. This is a contrary issue for high-precision dimensional parts: the higher the glass fiber content, the better 'dimensional stability' in one direction, but worse in other directions. When making precision parts, do not consider glass fiber as a universal solution.
Judgment Three: Dimensional stability does not equal 'performance stability.' The dimensions may be good, but other indicators such as modulus, strength, temperature resistance, and creep may not necessarily be good at the same time. Therefore, precision electronic components are not all 'low water absorption'; it also depends on the specific working conditions. 'Low water absorption' is a necessary condition, not a sufficient one.
Judgment Four: Truly 'zero water absorption' plastics do not exist. Even materials like PVDF and PPS will absorb a small amount of water if exposed to high temperatures for a long period. Therefore, any promise of 'low water absorption' comes with conditions—it depends on the specific testing conditions. If soaking data for 1000 hours, 2000 hours, or 5000 hours is not provided, the promise itself is meaningless.
These four points are applicable because 'dimensionally stable nylon' sounds like 'a type of material'—but it actually refers to multiple approaches (low water absorption resin, oriented glass fiber, mineral filling, alloying). Which one to choose depends on the specific part's dimensional tolerances and long-term reliability requirements.
Judgment 1: There are two sources of dimensional changes, calculated separately. Water absorption expansion and thermal expansion and contraction may be in opposite directions; after stacking, they increase or decrease in total. Calculate based on working conditions, don't just think about it.
Judgment 2: Be aware of the cost of low water absorption. Switch to long carbon chains or semi-aromatic types, price and processing conditions will change, and the saved after-sales and excess material costs should be recorded in a single table.
Judgment 3: The verification sequence is humidity control, measurement, and assembly. Judgment signal: Measure both dry and wet states once in the same batch; if the difference exceeds one-third of the tolerance, the design side must take action.
Final Supplement Three Analyses.
Low Water Absorption and Low Shrinkage Are Two Things. Water absorption affects the dimensions during service, shrinkage affects the dimensions after demolding, mold compensation targets the latter, material selection targets the former. Mixing and discussing them together will take a long way.
Humidity control is not water soaking. Standard humidity control requires temperature and humidity conditions. Boiling parts with boiling water to "cook them to the right" size introduces new internal stress, which is a common pitfall in dirt methods.
Welding and inserts with low water absorption systems need to be reviewed. After material replacement, the strength of the welding line and insert grip will change; for precision parts with low water absorption materials, these two must be re-verified.
The last sentence to the design side: Mark both dry and wet state dimensions in the drawings so suppliers won't understand each other when quoting. A single line of notes saves months of disputes.
Here's a tip: For precision parts, check the drawings and write the "pre-measurement environmental condition" into the inspection specification, treating it at the same level as dimensional tolerances. With unified inspection conditions, size disputes are reduced by more than half.
Put a three-question and three-answer sheet before wrapping up.
| High-frequency questions | One-sentence answer |
|---|
| How much is the size difference between winter and summer? | PA66 Can be up to two or more wires, calculate by wall thickness |
| How much is it more expensive to switch to low-absorbency material? | Calculate the saved humidity adjustment process together |
| Is material replacement absolutely necessary? | Not necessarily, humidity regulation and structural compensation are often cheaper . |
| What should be labeled on the drawings? | Dry and wet two-row dimensions plus environmental conditions |
Add another reverse case, explaining that low water absorption is not the whole answer to accuracy.
A precision parts project replaced PA66 with a low water absorption system, and size complaints never stopped; instead, there was an extra one. Investigation found that temperature fluctuations in the assembly workshop affected the area more than humidity, and the contribution of thermal expansion was overshadowed by water absorption issues. After adding the constant temperature storage and measurement standards, complaints were zeroed, and low water absorption materials actually seemed unnecessary.
Size is a function of temperature, humidity, and time; only one is handled, and the other two take the blame.
After this order, we made a check-up card by attributing first, then doing it ourselves: first measure heat, then measure humidity, then weigh it. After completing these three steps, decide whether to change the material. The card is small but plays a significant role; most precision parts selection decisions are due to unclear attribution.
The three-step order of the troubleshooting card is important: first measure heat because temperature responds quickly and results come out the same day; then measure humidity because humidity adjustment takes several days; and finally weigh because it accumulates wear or water loss data. Arranging from fast to slow is the shortest attribution path.
A customer posted this card in the measurement room. The workshop reported dimensional anomalies in three steps first, then contacted the supplier. Half of the abnormalities were answered in the second step. Once the process ran smoothly, their decision to change materials shifted from just guessing to looking at data. That's how confidence in procurement was built.
Conclusion
Low Water Absorption Nylon Selection, Four Sentences:
First, distinguish between moisture absorption and heating—two types of expansion, two solutions.
Calculate the actual moisture absorption amount—first know how much the change will be, then decide whether to change the material.
Prioritize modifying structure and tolerances—often more economical than material replacement.
Unified measurement aperture—whether the dry state or the wet state should be written on the drawing.
Changing materials is the last card, not the first. This statement is especially valuable when it comes to dimensions
