221 改性尼龙与PBT怎么选
从一副点火线圈的投诉讲起
前年，重庆一家做汽车电子的厂收到主机厂的整改通知：某个用 PA66-GF30 做的点火线圈护套，在南方的梅雨季里电气性能漂移，高压泄漏电流超标。设计的时候干态测试全数通过，一到高温高湿的实车环境就露馅。
整改方案换成了 PBT-GF30，尺寸链重新核算、模具局部修改，三个月后问题闭环。多花的模具费和验证费，比当初直接选对材料贵了五倍不止。
有意思的是，同一家厂的另一款卡扣支架，用 PA 做得妥妥帖帖——卡扣要韧性，PBT 的缺口敏感性在那里反而成了短板。这一对材料的分工，就藏在"吸湿"两个字里。 这一篇把 PA 和 PBT 的账算细：差在哪、各自的领地在哪、重叠区怎么判。
最本质的差别：吸湿
PA66 吸水率 2.5-3%，PBT 只有 0.1%。这一个数字决定了后面所有的差异：PA 吸湿后尺寸涨、强度降、介电性能下降；PBT 几乎不受湿度影响，尺寸和电性能稳定。
这是选材时要问的第一个问题：这个件的使用环境湿度大不大、尺寸精度要求高不高。
力学性能的对比
PA66 的拉伸强度、冲击韧性、耐磨性都优于 PBT。PA66 未增强拉伸 85 MPa、缺口冲击 6 kJ/m²；PBT 未增强拉伸 55 MPa、缺口冲击 4 kJ/m²。
但 PBT 玻纤增强后性能提升明显，PBT-GF30 拉伸可达 130 MPa。PA 的优势在韧性，PBT 增强后能追上强度。
耐热对比
PA66 的 HDT（1.8 MPa）约 90℃，增强后 250℃；PBT 的 HDT 约 60℃，增强后 210℃。PA66 在耐热上明显占优。但 PBT 有一项优势——PBT 的长期使用温度与短期耐温差距小，而 PA 在湿热环境下性能衰减更明显。所以高温 + 干燥选 PA，高温 + 潮湿要仔细算。
电气性能是 PBT 的主场
PBT 的介电强度、体积电阻率受湿度影响极小，CTI 也普遍更高。这就是为什么连接器、线圈骨架、电容器外壳这些电气件普遍选 PBT。PA 在潮湿环境下介电性能下降明显——吸湿 2.5% 后介电强度可能降 25%。电气件选材，PBT 是更稳妥的选择。
耐化学与耐水解
PBT 的耐水解性差于 PA——PBT 的酯键在高温水中会水解，这是 PBT 的致命短板。所以 PBT 不能用于长期接触热水的场合（如水泵、水表、洗碗机）。PA 虽然吸湿，但酰胺键比酯键耐水解，加抗水解剂后可用于热水环境。这一条是反直觉的——很多人以为 PBT 不吸水就耐水，其实相反。
加工性对比
PBT 的结晶速度快、流动性好、成型周期短，加工性优于 PA。PBT 模温 60-80℃ 即可，PA66 要 80-120℃。但 PBT 有一个麻烦——PBT 在高温下容易降解，加工温度不能超过 260℃，且必须充分干燥（含水率 < 0.03%，比 PA 要求更严）。PBT 干燥不到位会严重降解。
价格和应用的现实
PBT 价格一般略低于 PA66，是 PA66 的主要竞争对手之一。典型分工：PBT 主攻电气电子（连接器、骨架、开关）；PA 主攻结构受力件（齿轮、轴承、结构支架）。
重叠区域（如汽车电气件）就要按上面几条逐项对比。记住一句话：要尺寸和电气选 PBT，要强度和韧性选 PA。
工程实测：4 条强制测试
测试1：吸水率。PA66 2.5-3%，PBT 0.1%——最本质差别。
测试2：介电。PBT 吸湿后介电强度几乎不变，PA66 下降 25%——电气件选 PBT。
测试3：耐水解。PBT 在 80℃ 热水 1000 h 强度保持 45%，抗水解 PA66 保持 85%——热水选 PA。
测试4：耐热。PA66-GF30 HDT 250℃，PBT-GF30 210℃——高温选 PA。
边界声明
| 工况 | 推荐材料 |
|---|
| 电气电子件 | PBT |
| 结构受力件 | PA66 |
| 精密尺寸件 | PBT |
| 长期接触热水 | 抗水解 PA66（PBT 不行） |
| 高温干燥环境 | PA66 |
工程备忘
PA vs PBT：要尺寸和电气选 PBT，要强度和韧性选 PA。但 PBT 不耐热水水解，这一点反直觉。
实战案例：常见踩坑与正解
踩坑一：尼龙与PBT只比强度就下结论。选材对比要看短板——PA 的短板是吸水和耐酸，PBT 的短板是耐热和冲击，金属的短板是重量和成本。正解：列一张短板对照表，看哪家的短板在这个工况下不致命。
踩坑二：以塑代金属时直接按金属件的形状做塑料件。正解：塑料和金属的设计逻辑不同，塑料靠加强筋和壁厚分布，金属靠截面惯性矩，必须重新设计。踩坑三：换了材料不重算成本。
材料便宜了但壁厚要加厚，或者后处理工序增加，总成本可能反而更高。正解：算整件成本，不算单公斤价格。
延伸判断：最容易被漏掉的隐性变量
尼龙与PBT的量产事故里，有一半不是料选错了，是隐性变量没控住。
第一个变量是含水率。PA 系材料出厂含水率、干燥条件、注塑前的存放时间，三者共同决定实际含水率，含水率不对，强度和外观都会变。
第二个变量是模具温度。模温低 20℃，表面浮纤和熔接痕强度可能差一倍。
第三个变量是装配后的时间。装完 24 h 和装完 30 天的扭矩、尺寸、密封压缩量都不一样。
这三个变量都不写在物性表上，但都写在失效报告里。
把这三件事写成一张表发给供应商，比打十通电话有用——尼龙与PBT的选型沟通成本，基本都花在这几项反复确认上。
一张选料决策表的背后逻辑
把前面几节的内容压成一张决策表，逻辑就三行。
第一行问湿度。件在使用中会不会吸潮？发动机舱、户外、水汽环境里，PA 的吸湿是绕不开的变量——尺寸链精密的件，PA 吸湿后 0.1% 的尺寸漂移就能把公差吃掉。PBT 几乎不受影响，这是它进电子件的入场券。
第二行问介质。接触不接触热水、水乙二醇、制动液？PBT 的酯键怕热水，这一条是死穴；PA 加抗水解剂后反而能扛。很多人栽在"不吸水等于耐水"这个想当然上。
第三行问韧性。有没有卡扣、铰接、装配受力？PA 的韧性和耐磨是主场；PBT 增强后强度能追上，但缺口冲击始终矮一截，薄壁卡扣反复装配的件，PBT 容易裂口。
三行问完，选料方向基本就定了。重叠区的订单（汽车电气件、电机端盖这类电气加结构双重身份的），把三条各打分再合计，比拍脑袋换料的试错成本低得多。
PA 与 PBT 的高频问答
问：做阻燃件，这一对谁更省心？ PBT 的阻燃体系更成熟，溴系和无卤路线都有大量现成配方，加上 CTI 本底高，做连接器这类耐电压件一步到位。PA 阻燃同样成熟，但要注意吸湿对漏电起痕的叠加影响——阻燃 PA 的 CTI 要按湿态复测，干态报告不够用。
问：接触热水的件选谁？ 反直觉的答案是 PA。PBT 的酯键在高温水里持续水解，八十度热水泡一千小时强度掉一半以上；PA 加抗水解剂之后，同样的条件能保住八成五。水泵、水表、洗碗机这些位置，选 PBT 的厂后来都返工过。
问：增强比例怎么配？ 常规结构件 GF30 是两边的通用档。差异在薄壁件：PBT 流动好，薄壁位置玻纤分布更均匀；PA 薄壁容易浮纤，要配低翘曲和表面改善配方，成本会上浮一截。
问：两边的价格和交期怎么跟？ 原料链不同步——PBT 挂 PTA 和丁二醇的行情，PA 挂己内酰胺，半年里价差方向掉头是常事。量大的订单两边各锁一个季度的量，比单边押注稳当。
问：PBT 的干燥为什么要求那么苛刻？ 因为酯键在料筒里遇上水汽就直接水解断链——含水率超标一次，分子量掉的就不是一层皮，做出来的件发脆发黄、冲击数据腰斩。规矩是热风一百二十度烘足三四个小时，烘好的料开袋即用，超过半小时暴露就要回烘。PA 的干燥纪律已经够严了，PBT 还要再高一档。
问：两边模温差异，对排产有什么影响？ PBT 模温六十到八十度、周期短，PA 模温八十到一百二十度、周期长，同一副模具换料，节拍差两成是常态。两条线并行的厂要把节拍差异写进产能计划——旺季排单排错料线，交期就是这么延误的。
新料验收的四个必测项
PA 和 PBT 都要用到的验收清单，四项，照着做能挡掉绝大多数到货风险。
湿态介电复测。做电气件的，干态报告只是入场券——样条按工况吸湿处理后再测介电和 CTI，湿态数据才是实车实机的真实状态。
吸水率样条。每批留一组样条泡水称重，吸水率异常的批次背后常是配方变动，早发现早处理。
热老化对比。两组样条烘箱一百五十度放一百小时，强度保持率对不上标称值的批次挂起——热老化数据是基材品控的照妖镜。
批次粘度跟踪。连续五批的粘度值进台账，走势平稳的供应商才值得长期合作，单批合格说明不了什么。
四项动作每批不到一小时，挡住的却是动辄整批退货的批量风险——验收的成本永远比救火的成本低。
失效复盘：梅雨季里的电气盒
华东一家做户外电表的厂，电气盒盖用了阻燃 PA66-GF30，干态 CTI 报告四百多，出厂检验全数合格。头一年太平无事，第二年梅雨季连着来了一串投诉：盒盖接线端子附近出现碳化导电通道，漏电流报警。
送检复测的是湿态样品：吸湿之后表面漏电起痕指标掉了一档，端子部位的爬电距离裕量被吃光。整改换阻燃 PBT，模具不动，三周切换完成，两个雨季过去零投诉。
复盘会上算的账值得记下：换料整改花了十几万，如果当初选材时多做一项湿态 CTI 复测，成本是几千块。电气件的认证，必须按最潮湿的使用状态测——这一条后来写进了这家厂的选材流程，谁签字谁负责。
两类件的边界速记
这一对材料吵了几十年，边界其实可以速记成三句。
潮湿加电气，PBT——接线盒、线圈骨架、传感器外壳，凡是怕吸湿漂移的位置，PBT 的稳定是真金白银。受力加韧性，PA——齿轮、卡扣、支架，反复装配和冲击载荷是 PA 的主场。
热水加介质，抗水解 PA——PBT 的酯键在热水里是死穴，这一条没有商量余地。
落到实物上更直观：变速箱里的齿轮轴套用 PA，电池管理盒的绝缘罩用 PBT，发动机水室用加了抗水解剂的 PA——同一台机器上，两类料各守各的岗位，谁也别想通吃。
采购要做的事，是在重叠区把三句速记展开成实测数据。速记帮你省掉八成的争论，剩下两成交给样条和台账——这就是这一对材料选型的全部秘密。
问：反复受力件，两边疲劳寿命差多少？ 齿轮、凸轮这类百万次循环的位置，PA 占优——吸湿带来的韧性在疲劳里反而是缓冲垫，裂纹扩展慢；PBT 刚性高、缺口敏感，缺口根部的应力集中先撑不住。要刚性不要弯折的场合另说，纯疲劳件 PA 是多数答案。
问：零下四十度的环境选谁？ PA 的低温韧性好，PBT 在深冷环境发脆——冷链设备、户外寒区设备里的卡扣和护套，这个温差段要写进图纸。两个都不放心的时候，PA 与弹性体的合金是常用的兜底方案。
结语
有些生意我们不做——选料这件事，越早问越省事。
这类件的选料与试模，可以一起聊。
221 How to choose between modified nylon and PBT
Starting from a complaint about an ignition coil
The year before last, an automotive electronics factory in Chongqing received a rectification notice from the OEM: a certain ignition coil cover made of PA66-GF30 caused electrical performance drift during the plum rain season in the south, with excessive high-voltage leakage current. All tests passed in the dry-state during design, but the problem appeared once in real vehicles under high temperature and high humidity conditions.
The rectification plan was changed to PBT-GF30, the dimension chain was recalculated, and local modifications were made to the mold. The issues were closed three months later. The extra mold and validation costs were more than five times higher than if the right material had been chosen initially.
Interestingly, another clip bracket from the same manufacturer is made perfectly with PA—the clip needs toughness, and the notch sensitivity of PBT actually becomes a drawback. The division of labor between these two materials is hidden in the word 'moisture absorption.' This article goes into detail on PA and PBT: where they differ, their respective territories, and how to determine overlapping areas.
The most essential difference: hygroscopicity
PA66 has a water absorption rate of 2.5-3%, while PBT only has 0.1%. This single figure determines all the subsequent differences: PA swells in size, strength decreases, and dielectric properties deteriorate when absorbing moisture; PBT is hardly affected by humidity, with stable dimensions and electrical performance.
This is the first question to ask when selecting materials: Does this part operate in a high-humidity environment, and are the dimensional accuracy requirements high?
Comparison of mechanical properties
The tensile strength, impact toughness, and wear resistance of PA66 are all superior to PBT. Unreinforced PA66 has a tensile strength of 85 MPa and a notched impact strength of 6 kJ/m²; unreinforced PBT has a tensile strength of 55 MPa and a notched impact strength of 4 kJ/m².
However, the performance of PBT is significantly improved after glass fiber reinforcement, with PBT-GF30 reaching a tensile strength of 130 MPa. The advantage of PA lies in its toughness, and reinforced PBT can catch up in strength.
Heat Resistance Comparison
The HDT (1.8 MPa) of PA66 is about 90°C, and about 250°C after reinforcement; the HDT of PBT is about 60°C, and about 210°C after reinforcement. PA66 has a clear advantage in heat resistance. However, PBT has one advantage—its long-term usage temperature is close to its short-term heat resistance, whereas PA's performance degrades more significantly in hot and humid environments. So for high temperature and dry conditions, choose PA; for high temperature and humid conditions, careful calculation is needed.
Electrical performance is the main field of PBT.
The dielectric strength and volume resistivity of PBT are minimally affected by humidity, and its CTI is generally higher. This is why electrical components such as connectors, coil bobbins, and capacitor housings commonly use PBT. PA's dielectric performance decreases significantly in humid environments—after absorbing 2.5% moisture, its dielectric strength may drop by 25%. When selecting materials for electrical components, PBT is the more reliable choice.
Chemical-resistant and hydrolysis-resistant
PBT has worse hydrolysis resistance compared to PA—PBT's ester bonds can hydrolyze in hot water, which is a fatal shortcoming of PBT. Therefore, PBT cannot be used in situations of long-term contact with hot water (such as water pumps, water meters, and dishwashers). Although PA absorbs moisture, its amide bonds are more resistant to hydrolysis than ester bonds, and with the addition of anti-hydrolysis agents, it can be used in hot water environments. This is counterintuitive—many people think that because PBT does not absorb water, it is hydrolysis-resistant, but actually, it is the opposite.
Machinability Comparison
PBT has a fast crystallization rate, good fluidity, and short molding cycle, making its processability better than PA. The mold temperature for PBT only needs to be 60-80°C, while for PA66 it is 80-120°C. However, PBT has a problem—PBT easily degrades at high temperatures, so the processing temperature cannot exceed 260°C, and it must be thoroughly dried (moisture content < 0.03%, which is stricter than PA). If PBT is not properly dried, it will severely degrade.
The reality of price and application
PBT prices are generally slightly lower than PA66 and it is one of the main competitors of PA66. Typical division of labor: PBT mainly targets electrical and electronic applications (connectors, frames, switches); PA mainly targets structural load-bearing components (gears, bearings, structural brackets).
Overlapping areas (such as automotive electrical components) need to be compared item by item according to the above points. Remember one sentence: choose PBT for size and electrical properties, and choose PA for strength and toughness.
Engineering field measurement: 4 mandatory tests
Test 1: Water absorption. PA66 2.5-3%, PBT 0.1% - the most fundamental difference.
Test 2: Dielectric. After absorbing moisture, the dielectric strength of PBT remains almost unchanged, while PA66 decreases by 25%—choose PBT for electrical components.
Test 3: Hydrolysis resistance. PBT retains 45% strength after 1000 hours in 80°C hot water, hydrolysis-resistant PA66 retains 85% — for hot water, choose PA.
Test 4: Heat resistance. PA66-GF30 HDT 250℃, PBT-GF30 210℃ — choose PA for high temperatures.
Boundary Declaration
| Operating condition | Recommended materials |
|---|
| Electrical and electronic components | PBT |
| Structural load-bearing member | PA66 |
| Precision-sized parts | PBT |
| Long-term exposure to hot water | Hydrolysis-resistant PA66 (PBT not suitable) |
| High temperature and dry environment | PA66 |
Engineering Memo
PA vs PBT: Choose PBT for dimensions and electrical properties, choose PA for strength and toughness. But PBT is not resistant to hydrolysis from hot water, which is counterintuitive.
Practical Case Study: Common Pitfalls and Correct Solutions
Pitfall 1: Concluding based on the strength comparison between nylon and PBT alone. Material selection should focus on weaknesses—PA's weaknesses are water absorption and acid resistance, PBT's weaknesses are heat resistance and impact resistance, and metals' weaknesses are weight and cost. Correct approach: Make a comparative table of weaknesses and see which one's weaknesses are not fatal under the given working conditions.
Pitfall 2: When replacing metal with plastic, directly making the plastic part in the shape of the metal part. Correct approach: The design logic for plastic and metal is different; plastic relies on ribs and wall thickness distribution, while metal relies on section moment of inertia, so a redesign is necessary. Pitfall 3: Changing materials without recalculating costs.
The material is cheaper, but the wall thickness needs to be increased, or additional post-processing steps are added, which may actually result in higher total costs. The correct approach: calculate the cost for the entire piece, not the price per kilogram.
Extended Judgment: The Hidden Variable Most Likely to Be Overlooked
In mass production accidents involving nylon and PBT, half are not due to selecting the wrong material, but because hidden variables were not controlled.
The first variable is moisture content. The factory moisture content of PA series materials, drying conditions, and storage time before injection molding together determine the actual moisture content. If the moisture content is incorrect, both strength and appearance will be affected.
The second variable is mold temperature. If mold temperature is 20°C lower, the surface float fiber and weld mark strength may differ by half.
The third variable is the time after assembly. The torque, dimensions, and seal compression amount are different at 24 hours after assembly and 30 days after assembly.
These three variables are not listed on the material properties table, but they are all included in the failure report.
Write these three things into a table and send it to the supplier; it’s more useful than making ten phone calls — the communication cost for choosing between nylon and PBT basically all goes to repeatedly confirming these few items.
The underlying logic of a material selection decision table
Compress the content of the previous sections into a single decision table; the logic is just three rows.
First line asks about humidity. Do parts absorb moisture during use? In engine compartments, outdoors, and in steam environments, PA's moisture absorption is an unavoidable variable—for precision parts with dimensional chains, a 0.1% dimensional drift after PA absorbs moisture can consume tolerances. PBT is almost unaffected, which is its ticket to electronic parts.
Second line asks about the medium. Does it come into contact with hot water, water, ethylene glycol, or brake fluid? PBT's ester bonds are sensitive to hot water, which is a weak spot; PA can actually withstand hydrolysis after adding anti-hydrolysis agents. Many people fall for the assumption that "not absorbing water equals being water-resistant."
The third line asks about toughness. Are there clamps, hinges, or assembly loads? PA's toughness and wear resistance are its main strengths; PBT can keep up with strength after enhancement, but the notch impact is always lower. For thin-walled clips that are repeatedly assembled, PBT is prone to cracking. After
finished asking in three lines, the material selection direction was basically decided. For overlapping orders (automotive electrical parts, motor end covers, etc., which have dual electrical and structural status), scoring each of the three and summing them makes the trial-and-error cost much lower than blindly swapping materials.
PA High-Frequency Q&A with PBT
Q: When making flame-retardant parts, which is more worry-free? PBT's flame-retardant system is more mature, with brominated and halogen-free routes available in many ready-made formulas. Plus, with a high CTI basis, you can make voltage-resistant components like connectors in one step. PA flame retardant is also mature, but attention must be paid to the combined effect of moisture absorption on leakage marks — the CTI of flame-retardant PA should be retested as wet, and dry report is insufficient.
Question: Which part should be used for hot water contact? The counterintuitive answer is PA. The ester bonds in PBT continuously hydrolyze in high-temperature water; after soaking in 80°C hot water for 1,000 hours, the strength drops by more than half; After adding anti-hydrolysis agents to PA, the same conditions can maintain 85%. Factories that chose PBT have all been reworked for locations like water pumps, water meters, and dishwashers.
Question: How do you match the reinforcement ratio? Conventional structural parts like GF30 are the universal ranges on both sides. The difference lies in thin-walled parts: PBT flows well, and the fiberglass distribution at thin-walled areas is more uniform; PA thin-walled fibers tend to float, so pairing with low warpage and surface improvement formulas increases costs.
Q: How do prices and delivery times match on both sides? Raw material chain is unsynchronized—PBT is linked to PTA and butylene glycol, PA is linked to caprolactam, and price differences often reverse within half a year. For large orders, each side locks in a quarter's volume, which is safer than betting on one side.
Q: Why are PBT drying requirements so strict? Because when the ester bond meets water vapor in the barrel, it directly hydrolyzes and breaks the chain—once the moisture content exceeds the standard, the molecular weight loses more than just a layer of skin, resulting in brittle, yellowing, and data impact halved. The rule is to bake with hot air at 120°C for three to four hours, then use the baked material immediately after opening; if exposed for more than half an hour, it must be re-baked. PA drying discipline is already strict enough, but PBT needs to be raised even higher.
Question: How does the temperature difference between the two molds affect production scheduling? PBT mold temperature is 60 to 80 degrees Celsius with a short cycle; PA mold temperature is 80 to 120 degrees Celsius with long cycles. When changing material with the same mold, a 20% cycle difference is normal. Factories running two parallel lines must record cycle time differences in their capacity plans—scheduling single orders and misaligned material lines during peak season leads to delivery delays.
Four must-test items for new material acceptance
PA and the acceptance checklist used in PBT, these four items can block the vast majority of delivery risks.
Wet dielectric re-testing. For electrical components, dry state reports are just a ticket — after treating the spline for moisture absorption under working conditions, dielectric and CTI are tested; wet data is the real state of the actual vehicle and machine.
Water absorption rate splines. Each batch retains one set of splines for water soaking and weighing; batch with abnormal water absorption rates often indicates formula changes, so early detection and handling are correct.
Thermal aging comparison. Two sets of spline ovens are kept at 150 degrees for 100 hours; batches with strength retention rates not matching the rated values are suspended — thermal aging data is a mirror for substrate quality control.
Batch viscosity tracking. Viscosity values from five consecutive batches are recorded and stable suppliers are worth long-term cooperation; single batch qualification does not prove much.
Each batch takes less than an hour, but the risk of mass returns is often prevented—the cost of acceptance is always lower than the cost of firefighting.
Failure review: electrical boxes in the plum rain season
An outdoor meter factory in East China uses flame-retardant PA66-GF30 for the electrical box cover, with a dry CTI report of over 400 yuan, and all inspections passed at the factory. The first year was peaceful, but the second year saw a string of complaints: a carbonized conductive channel appeared near the box cover terminals, causing leakage current alarms.
Submitted for re-testing was a wet sample: after moisture absorption, the surface leakage trace index dropped by a level, and the creepage distance margin at the terminals was completely depleted. Rectification and replacement with flame-retardant PBT, mold unchanged, three-week switch completed, two rainy seasons with zero complaints.
The review meeting accounted for worth noting: material replacement and rectification cost over a hundred thousand. If they had done a wet CTI retest during material selection, the cost would have been several thousand yuan. Electrical parts must be certified under the most humid usage conditions — this rule was later written into the factory's material selection process, and whoever signed was responsible.
Boundary Notes on Two Types of Parts
This pair of materials has been arguing for decades; the boundaries can actually be shorthanded in three sentences.
Humidity plus electrical, PBT—junction box, coil frame, sensor housing. Wherever moisture absorption and drift are feared, PBT's stability is a real treasure. Stress and toughness, PA—gears, clips, brackets; repeated assembly and impact loads are PA's main domain.
Hot water plus medium, hydrolysis resistance PA—PBT's ester bonds are a fatal spot in hot water, and this is non-negotiable.
It's even more intuitive in real life: PA is applied to the gear shaft in the transmission, PBT is used for the insulating cover of the battery management box, PA with anti-hydrolysis agent is used in the engine water chamber—on the same machine, each of the two types of materials serves their own role, and neither can claim both.
What procurement needs to do is to expand the three shorthand lines into actual test data in the overlapping area. Shorthand saves you 80% of the arguments, and the remaining two are sold to splines and ledgers—this is the whole secret to material selection.
Question: For repeatedly stressed parts, how much is the fatigue life difference between the two sides? For gears and cams like gears and cams, where millions of cycles are used, PA has the advantage—the toughness from moisture absorption actually acts as a buffer pad in fatigue, causing crack propagation to grow slowly; PBT has high rigidity and sensitive notches, so stress concentration at the base of the notch can't hold up first. If rigidity is needed without bending, that's another matter; pure fatigue parts like PA are the most common answer.
Q: Which one should you choose in environments as low as minus 40 degrees? PA has good low-temperature toughness, while PBT becomes brittle in deep cold—for buckles and sheaths in cold chain equipment and outdoor cold region equipment, this temperature difference range should be written in the drawings. When neither is reliable, alloys of PA and elastomers are commonly used as a safety net.
Conclusion
There are some businesses we don't do — the earlier you ask about material selection, the easier it is.
For these kinds of pieces, you can discuss material selection and mold trial together
