道岔区的塑料件，考的是户外老化和疲劳两条线的叠加。这篇讲清六维工况怎么落数字、三条材料路线各差在哪、判据表怎么读，以及哪几种道岔位置不该用改性尼龙。
去年冬天，一个做道岔配套件的客户把一段断件视频发过来。
断的是道岔垫板上的一个凸台，黑色的玻纤增强改性尼龙件，断口发白、很齐。
视频里他拿手一掰就断了，配的那句话是："夏天好好的，天一冷，这料就跟饼干一样。"
我追问了三句：这个道岔在正线还是站线？是尖轨区还是辙鼻区？当地冬天最低温多少？
他答：站线，辙前段，去年最低 -32℃。
三样答完，方向基本就定了。
这批件的经过，是一条跟着季节走的线。
起点是装车时按常温数据验收合格；潜伏是头一个冬天有几处边缘崩口，被当成个别磕碰；爆发是第二个春天，春检一次查出十几件裂纹；结算是天气回暖后复测，常温冲击居然还过得去——数据对不上，是因为温度对不上。
这篇把道岔材料这笔账讲清：六维工况怎么落数字，三条路线各差在哪，判据表怎么读，以及哪些道岔位置这条路不该走。
一、工况六维：道岔件被什么约束
道岔区的塑料件，多为轨距挡板、道岔垫板、滑床板衬垫和各类绝缘垫片。
它们跟普通线路件最大的不同是：载荷不连续。
列车进道岔的时候要过尖轨、过辙叉，冲击是断续的、有方向变化的。
温度这一维要按极端值算。
东北、西北的冬天能到 -40℃，夏天钢轨附近曝晒下到 60℃ 以上；一天之内的温差也能到 20℃ 以上。
塑料件的麻烦在于：低温让它变脆，高温让它变软，一天里两种状态都要扛。
载荷这一维要看"次数"，不看"峰值"。
重载线路轴重按 25 吨算，道岔区乘务条件更复杂，冲击载荷比区间线还高一些。
一条站线一年通过的列车按数万列算，每列车经过时相关件承受多次冲击，累积到百万次量级。
百万次冲击下，件不需要一次断，它只要每次裂一点点，裂纹就长起来了。
道岔件还有一个别处没有的工况：转辙。
尖轨每次扳动，跟它配合的垫板和挡板要跟着让位、再复位，一天来回几十次，一年上万次。
这个动作的载荷不大，但它让件的受力方向多了一层反复，疲劳的账要一起算进去。
介质这一维是雨雪和油污。
雨水、融雪盐的氯离子、机车滴落油污、道砟粉尘，混在一起会在件表面形成一层膜。
这层膜本身不致命，但它会加速表面老化，也会让件的摩擦特性发生变化。
紫外这一维在露天岔区是长期存在的，十年累积辐照量按 MJ/m² 算是四位数。
寿命按大修周期定，常见十到十五年。
外观在这一行主要是标识色和表面状态；道岔件还要看尺寸——配合公差直接决定尖轨能不能密贴。
合规方面，绝缘类件看绝缘指标，露天件看耐候与力学保留率，隧道段还要看阻燃与烟密度。
| 维度 | 正线岔区 | 站线岔区 | 漏了会怎样 |
|---|
| 温度 | -40℃ 至 60–70℃ | 同量级，温差更突出 | 低温脆断 |
| 载荷 | 高速通过、冲击更强 | 次数相对少 | 裂纹萌生 |
| 介质 | 雨雪、盐、油污、粉尘 | 同左，油污更集中 | 表面老化加速 |
| 紫外 | 十年四位数量级辐照 | 同量级 | 表面粉化 |
| 尺寸 | 配合公差决定密贴 | 同左 | 密贴不良 |
| 合规 | 绝缘 + 耐候 | 绝缘 + 耐候 | 验收卡住 |
把六维摆在一起，会看到一个结论：道岔件的失效有季节性。
它不平均地坏，它集中在冬天、集中在温差大的时段、集中在老化之后再遇低温的时候。
所以按常温数据选料，一定选不准。
二、三条路线，并列摆开
道岔件的路线差别，本质上是"韧性与刚性怎么摆"这个问题。
聚酰胺的韧性和刚性是一对拉力：让件更耐冲击的方向，通常会让件更软、更容易蠕变。
道岔件两头都要——低温下要不断，长期载荷下要不变形。
| 路线 | 饱和吸水率（公开资料典型量级） | 低温冲击表现 | 长期使用 | 适合哪种件 |
|---|
| PA66-GF25/30 + 耐候体系 | 约 8–9% | 靠基材本征，-40℃ 下余量有限 | 刚性好、蠕变低 | 低冲击位置的挡板与垫板 |
| PA66 + 核壳增韧体系 | 约 8–9% | -40℃ 下明显优于普通增韧 | 刚性下降，蠕变要一起核 | 低温、高冲击位置件 |
| PA6 / PA66 共混 + 玻纤增韧 | 约 9–10% | 成本友好，低温余量一般 | 户外长期弱一档 | 非关键位置的结构件 |
三条路线没有谁更好，只有哪一条跟你的最低温和载荷方式兜得住。
玻纤那条的优势是尺寸与刚性：件在长期载荷下不容易变形，配合能守住。
代价是低温韧性——玻纤与树脂的界面在低温下更容易成为裂纹起点，缺口敏感性也更高。
增韧那条的优势在低温：核壳结构的弹性体粒子在低温下还能吸能，件在 -40℃ 下不至于一碰就断。
代价是刚性和耐蠕变下降，配合精度要重新核。这两条路线常常需要在一件上取折中，比如局部加厚、局部补强。
共混那条是成本路线，用在非关键位置是合理的，用在低温高冲击位置上就要谨慎。
还有一条经验值得说：路线选定之后，先做的对比试验应该是极限温度下的冲击，不是常温拉伸。
同一批料，常温拉伸的差别可能不到一成，-40℃ 的缺口冲击能差出好几倍。选料时看哪一项，结论就完全不同。
一个常见的误判是：既然是低温脆，那就把增韧剂往上加，加够了就不脆了。
方向只对一半。
普通弹性体增韧在低温下自己先变脆，加再多也补不上来，真正起作用的是核壳结构这条路。
而且增韧剂本身不耐紫外——它先老化，件的韧性就跟着掉。所以"冬天脆"这件事，很多不是初始韧性不够，是经过一个夏天之后，韧性已经掉了一截。
三、选型判据表：这张表决定你验哪几项
把前面的约束落成能核对的指标。下表门限是方向性建议，不是验收标准，实际数值要由你的线路位置、最低温和实测确定。
| 指标 | 方向性门限 | 验证方法 / 标准 | 常见失效 | 通行解法 | 对应助剂体系 |
|---|
| 低温冲击强度 | -40℃ 下按项目档定，看缺口值 | ISO 179 / GB/T 1043 | 冬天一碰就断 | 核壳增韧体系 | —（属增韧体系选型） |
| 老化后冲击保留率 | 紫外或热氧老化后再测冲击 | GB/T 16422.3 / ISO 4892 | 过夏后变脆 | 光稳定体系 + 抗氧体系 | 光稳定剂（受阻胺类）、抗氧剂 |
| 疲劳裂纹萌生 | 循环加载后无可见裂纹 | 件级疲劳试验 | 边缘起裂纹 | 低缺口敏感基材 + 结构圆角 | —（属结构侧） |
| 弯曲模量与蠕变 | 长期载荷下变形按项目档定 | ISO 178 / 蠕变试验 | 配合走失、密贴不良 | 玻纤增强 + 结构余量 | 偶联剂（玻纤界面） |
| 吸水后尺寸 | 关键配合尺寸按调湿态定 | ISO 1110 调湿 + 量测 | 密贴不良 | 调湿态出图与验收 | —（属状态管理） |
| 表面状态 | 老化后不粉化、不龟裂 | 老化后目视与手感 | 表面疏松挂污 | 耐候体系 + 控表面迁移 | 光稳定剂 |
| 颜色标识 | 按标识色板判，写入协议 | 色差仪 Lab 值 | 现场装错位置 | 色母与工艺一起定 | — |
怎么读这张表，先看前两行。
低温冲击和老化后冲击保留率是一对：一个考初始韧性，一个考走过一个夏天之后还剩多少。
很多项目只测初始值，这就是"装车时合格、第二年批量裂"的来源。
第三行到第五行是长期项，考的是件在长期载荷和吸湿之后还守不守得住位置。
第六行最容易被当成外观项，其实是预警项——表面粉化是韧性下降的前兆，不是脏了。
四、四种失效，和它们真正的根因
失效一：冬天一碰就断，断口发白、很齐。
断口发白说明是脆断，不是过载断裂。
根因要分两种：一种是初始韧性不足，选料时就偏了；另一种是初始够、老化之后不够。
区分办法很简单：拿库存未装车的件和线路上换下来的件，在同一个温度下做冲击对比。
失效二：只在棱角和孔位起裂，件本体没事。
棱角是应力集中点，低温下材料的缺口敏感性被放大。
这一类靠选料只能改善一部分，另一半要靠结构：把尖角改成圆角，把孔边加厚。
失效三：同一批件，老化之后有的粉化有的还好。
先别归到"料不稳定"。
更常见的是抗氧剂在混料阶段分散不均，加上紫外防护在件表面的分布不均，件的老化起点就不一样。
看到这种现象，先查混料与母粒化，再查模具排气与流动状态。
失效四：装车时配合刚好，半年后松动。
根因通常不是磨损，是吸水膨胀加蠕变一起走。
尼龙件吸湿后会涨，长期载荷下又会慢慢变形，两个方向叠加，配合就守不住了。
这一类要在出图阶段解决：关键配合尺寸按调湿态定，结构余量留足。
这里还有一条要直说的——道岔件的失效排查，先怀疑季节和工况，最后才怀疑基材。
因为同一个牌号在正线和站线、在夏天和冬天，失效方式是四件不同的事。
五、加工与验证：干燥、模温、熔接线
干燥在尼龙件上是固定动作，道岔件上尤其不能省。
上机前要做到露点 -40℃ 以下，含水率压到 0.15% 以内；水分进去不只是气泡问题，它会在料筒里把分子链切短，件的低温韧性直接受影响。
模温决定表面致密度。
模温低，件表面疏松，紫外更容易打进去，老化更快；同时熔接线强度也低。
道岔件上，模温不要照抄上一支料的档位，要按件的填充情况重新定。
熔接线是多孔件和加强筋件的固定风险。
料流绕过孔或筋之后汇合，汇合处强度与致密度都低，而这类件上，孔位往往正好在受力位置。
排浇口时要把熔接线推离受力区。
调湿这一道，道岔件上比一般件更要紧，因为它考的是配合。
关键配合尺寸按调湿态出图、按调湿态验收；按干态尺寸放行的件，装到线路上吸湿后还会再走一段。
低温这一道还要单独落到工艺上。
车间温度、模具起始温度、件出模后的冷却速度，这三样在冬天都会变；同一套参数在夏天好用，冬天打出来的件内应力不一样。
所以冬夏两季的工艺参数，一般要分别定一版，而不是全年一套。
验证顺序建议这样排，不要换：
1. 材料级：-40℃ 与常温两档冲击，缺口与无缺口都做
2. 老化级：紫外与热氧老化后的冲击保留率
3. 件级：调湿后关键尺寸与外观、熔接线位置切片
4. 疲劳级：件级循环加载，看裂纹萌生与配合保持
5. 线路级：按实际区段的温度与污渍条件做状态复测
顺序为什么不能换？因为低温冲击和老化保留率这两项，任何一项换了状态，数据都要重来。
六、边界：这几种道岔位置，先别走改性尼龙这条路
这一段可能比前面几段更值钱，因为它帮你在开模之前止损。
其一，正线高速岔区的高冲击核心件。速度高、冲击强、位置关键，塑料件的长期数据支撑不足，该回金属。
其二，低温极端区的承力件。要求 -40℃ 以下长期服役、且不允许定期更换的位置，材料侧的余量太薄。
其三，需要现场热作业紧邻的位置。阻燃能过，不代表能扛明火与熔渣。
其四，要求免维护十五年以上且不允许状态复测的位置。这条与铁路扣件那篇是同一个道理。
其五，载荷方向频繁变化且带腐蚀介质的位置。疲劳加腐蚀的组合，塑料件的失效会明显提前。
把这五条写前面不是劝退，是省时间——道岔件的验证周期长，装错一次，回退的代价远比料钱高。
换料风险清单（从原体系换到耐候增韧路线，要动的东西）
| 环节 | 要动什么 | 容易漏的点 |
|---|
| 模具 | 吸水率与收缩率都变了，配合尺寸要重核 | 只换料不核模 |
| 干燥 | 换除湿干燥机，按实测含水率定窗口 | 潮湿季节热风干燥基本无效 |
| 调湿 | 关键配合尺寸按调湿态出图与验收 | 按干态尺寸放行 |
| 料温 / 模温 | 提模温保表面致密度，不照抄上一支 | 模温低导致老化更快 |
| 保压 / 脱模 | 多孔件与筋位保压要重定，熔接线避开受力位 | 熔接线落在孔位汇合处 |
| 色差 | 标识色按功能色板对，写进技术协议 | 只当外观项对待 |
| 验证顺序 | 材料级 → 老化级 → 件级 → 疲劳级 → 线路级 | 前一项没过就往下走 |
一页纸汇报表（给要向上汇报的人）
| 项 | 一句话结论 |
|---|
| 换什么 | 低温高冲击位走核壳增韧，配合优先位走玻纤增强 |
| 动什么 | 干燥换除湿、模温上调、配合尺寸按调湿态出图 |
| 验什么 | -40℃ 冲击、老化后冲击保留率、调湿后尺寸、疲劳裂纹 |
| 什么时候能放量 | 老化后冲击不塌、配合守住、疲劳无裂纹萌生 |
读者常问的三句
问：常温冲击数据很好看，为什么冬天还是断？
因为考的不是同一件事。常温冲击考的是材料当前的韧性，冬天考的是材料老化之后、在低温下的韧性。两项都要有数据，只测前一项等于没测。
问：把增韧剂加上去，是不是低温就稳了？
要看加的是哪一类。普通弹性体增韧在低温下本身会变脆；能扛低温的是核壳结构那一类。而且增韧剂不耐紫外，件的韧性会随户外时间往下走。
问：棱角起裂，是换料还是改模具？
先改结构。低温下材料的缺口敏感性被放大，圆角和加厚能直接降低起裂概率；结构改完还有问题，再回头核韧性体系。
结语
回到开头那三句追问：正线还是站线、哪个部位、最低温多少。
这三样答全了，道岔材料往哪条路线走基本就定了。
配方里的助剂体系按件的工况配——常规助剂常备现货，特殊型号按需配套；你报工况和牌号，料和助剂一次配齐。
料有人卖，判断不一定有人给。
道岔件与轨距挡板这两笔账，问清最低温和老化账，比多问三家报价有用得多。
我们做改性尼龙（PA6 / PA66 / PA46 / PA11 / PA12 / PA6T / PA9T 及尼龙合金），也做改性 PPO / PPS 与热塑性弹性体；另长期收尼龙原料、水口回料与各类尼龙废料，有正规处置渠道。
For the plastic parts of the turnout area, the assessment focuses on the combination of outdoor aging and fatigue. This article explains how to apply the six-dimensional working conditions to numbers, the differences among the three material routes, how to read the criteria tables, and which turnout positions should not use modified nylon.
Last winter, a customer who makes turnout accessories sent over a video of a broken part.
What is broken is a boss on the switch pad, a black glass fiber reinforced modified nylon part, with a whitish and very neat fracture.
In the video, he broke it with a snap, and the accompanying line was: 'It's fine in the summer, but once the weather gets cold, this material becomes like a cookie.'
I asked three follow-up questions: Is this switch on the main line or the station track? Is it in the switch rail area or the frog area? What is the lowest temperature locally in winter?
He answered: at the stop line, the front section of the rut, last year's lowest was -32°C.
After answering the three questions, the direction is basically set.
The journey of this batch of pieces is a thread that follows the seasons.
The starting point is that the cargo was accepted as qualified according to normal temperature data during loading; the latent phase is that during the first winter, a few edge chips appeared, which were considered individual bumps; the outbreak is the second spring, when a spring inspection found more than a dozen cracks at once; the settlement is re-testing after the weather warmed up, and surprisingly it still passed the normal temperature impact test—the data didn’t match because the temperatures didn’t match.
This article clarifies the accounts of turnout materials: how the six-dimensional working conditions are quantified, where the three routes differ, how to read the criterion table, and which turnout positions this route should not take.
1. Six-dimensional working condition: What constrains the switch components
The plastic parts in the turnout area are mostly gauge guards, turnout pads, slide bed plate cushions, and various insulating gaskets.
The biggest difference between them and ordinary circuit components is that the load is intermittent.
When a train passes through a turnout, it goes over the switch rail and the frog, and the impact is intermittent and changes direction.
The temperature dimension should be calculated based on extreme values.
In the northeast and northwest, winter can reach -40°C, and in summer, near the steel rails under exposure to the sun, it can exceed 60°C; the temperature difference within a single day can also reach over 20°C.
The trouble with plastic parts is that low temperatures make them brittle, and high temperatures make them soft, and they have to endure both states in a single day.
The load dimension should consider 'frequency', not 'peak value'.
The axle load on the overloaded line is calculated as 25 tons, and the turnout area has more complex operational conditions, with impact loads slightly higher than those on the mainline.
A track section that sees tens of thousands of trains passing through in a year subjects its components to multiple impacts each time a train passes, accumulating to the order of millions of impacts.
Under millions of impacts, the piece does not need to break all at once; it only needs to crack a little each time, and the crack will grow.
The turnout component also has an operating condition that is not found elsewhere: switching.
Every time the switch rail is operated, the corresponding tie plates and stop blocks have to yield and then reset. This happens dozens of times a day, tens of thousands of times a year.
The load of this movement is not large, but it adds an additional layer of repetition to the direction of force on the part, and the fatigue account needs to be calculated together.
The medium dimension is rain, snow, and oil stains.
Rainwater, chloride ions from melting snow salts, locomotive oil drips, and ballast dust mix together to form a layer on the surface of components.
This layer of film is not lethal by itself, but it accelerates surface aging and also changes the friction characteristics of the part.
The ultraviolet dimension has long existed in the open-air junction area, with a ten-year cumulative irradiation amount in MJ/m² being in the four digits.
The lifespan is determined according to the major overhaul cycle, commonly ten to fifteen years.
Appearance in this context mainly refers to the identifying color and surface condition; for switch components, you also need to look at the dimensions — the fit tolerances directly determine whether the switch rail can sit closely.
In terms of compliance, for insulation components, look at insulation indicators; for outdoor components, look at weather resistance and mechanical retention rate; for tunnel sections, also consider flame retardancy and smoke density.
| Dimension | Main line turnout area | station turnout area | What will happen if it leaks? |
|---|
| Temperature | -40°C to 60–70°C | Same magnitude, more pronounced temperature difference | Brittle fracture at low temperature |
| Load | High-speed passage, stronger impact | Relatively few times | Crack initiation |
| Medium | Rain and snow, salt, grease, dust | Same on the left, the grease is more concentrated | Accelerated surface aging |
| Ultraviolet | Ten years of four orders of magnitude irradiation | of the same magnitude | Surface chalking |
| Size | Determine tight fit according to tolerance | Same as the left | Poor adhesion |
| Compliance | Insulation Weather Resistance | Insulation Weather Resistance | Acceptance stuck |
Putting the six dimensions together, one can see a conclusion: the failure of turnout components is seasonal.
It doesn't deteriorate evenly; it concentrates in the winter, during periods of large temperature differences, and after aging when it encounters low temperatures.
So if you choose materials based on room temperature data, you will definitely pick the wrong one.
Two or three routes, placed side by side
The route difference of turnout components is essentially a question of 'how to balance flexibility and rigidity'.
The toughness and rigidity of polyamide are a pair of opposing forces: in the direction that makes the part more impact-resistant, the part usually becomes softer and more prone to creep.
Both ends of the switch parts must—remain intact at low temperatures and not deform under long-term load.
| Route | Saturated water absorption rate (typical magnitude in public data) | Low-temperature shock performance | Long-term use | Suitable for which type of part |
|---|
| PA66-GF25/30 Weather-Resistant System | About 8–9% | Based on the intrinsic properties of the substrate, the remaining amount is limited at -40°C | Good rigidity, low creep | Baffles and pads in low-impact positions |
| PA66 Core-Shell Toughening System | About 8–9% | Significantly better than ordinary toughening at -40℃ | Rigid decline, creep needs to be checked together | Low-temperature, high-impact positional parts |
| PA6 / PA66 Blending Glass Fiber Reinforcement | About 9–10% | Cost-friendly, average low-temperature margin | Outdoor long-term weak gear 1 | Non-critical structural components |
None of the three routes is better; it all depends on which one can accommodate your minimum temperature and load conditions.
The advantage of the fiberglass one is its size and rigidity: the part is not easily deformed under long-term load, and it can maintain proper alignment.
The cost is low-temperature toughness—the interface between glass fiber and resin is more likely to become a crack initiation point at low temperatures, and notch sensitivity is also higher.
The advantage of the toughening one is at low temperatures: the elastomer particles with a core-shell structure can still absorb energy at low temperatures, so the part will not break instantly at -40°C.
The cost is a decrease in rigidity and creep resistance, and the assembly accuracy needs to be re-checked. These two approaches often need to be compromised on a single part, such as local thickening or local reinforcement.
The blending approach is a cost-effective route; it is reasonable to use it in non-critical positions, but caution is needed when used in low-temperature, high-impact locations.
There is one more piece of advice worth mentioning: after the route is chosen, the first comparative test should be impact at extreme temperatures, not tensile testing at room temperature.
For the same batch of material, the difference in tensile strength at room temperature may be less than 10%, but the notch impact at -40°C can vary by several times. Depending on which property you look at when selecting the material, the conclusion will be completely different.
A common misconception is: since it is brittle at low temperatures, just keep adding more toughening agents, and once enough is added, it will no longer be brittle.
The direction is only half correct.
Ordinary elastomer toughening becomes brittle on its own at low temperatures, and adding more cannot compensate for it; what really works is the core-shell structure approach.
Moreover, the toughening agent itself is not resistant to UV — it ages first, and the toughness of the part decreases accordingly. So the issue of 'brittle in winter' is often not due to insufficient initial toughness, but because after a summer, the toughness has already dropped.
3. Selection Criteria Table: This table determines which items you will test
Turn the previous constraints into verifiable indicators. The thresholds in the table below are directional suggestions, not acceptance criteria; the actual values need to be determined by your line location, minimum temperature, and actual measurements.
| Indicator | Directional Threshold | Verification Method / Standard | Common Failures | Common solution | Corresponding auxiliary agent system |
|---|
| Low temperature impact strength | Check the gap value according to the project file at -40℃ | ISO 179 / GB/T 1043 | Breaks at the slightest touch in winter | core-shell toughening system | — (Belongs to toughening system selection) |
| Impact retention after aging | Test impact after ultraviolet or thermo-oxidative aging | GB/T 16422.3 / ISO 4892 | Becomes brittle after summer | Light-stable system Antioxidant system | Light stabilizers (hindered amine types), antioxidants |
| Fatigue crack initiation | No visible cracks after cyclic loading | Component-level fatigue test | Edge crack | Low-gap sensitive substrate Structural fillet | — (possessive structure side) |
| Bending Modulus and Creep | Deformation under long-term load is determined according to project specifications | ISO 178 / Creep Test | Failure to comply with loss prevention and poor adhesion | Glass fiber reinforced Structural margin | Coupling agent (glass fiber interface) |
| Dimensions after water absorption | Key mating dimensions are determined according to the conditioned state | ISO 1110 Humidity Control Measurement | Poor adhesion | Drawing and Acceptance in Conditioned Humidity | —(Belongs to state management) |
| Surface condition | Does not chalk or crack after aging | Visual and tactile feel after aging | Surface is loose and soiled | Weathering system Control surface migration | Light stabilizer |
| Color identification | Determine according to the identification color code, write into the protocol | Colorimeter Lab Value | Installed in the wrong position on site | The color masterbatch is determined together with the process | — |
How to read this table, first look at the first two rows.
Low-temperature impact and aging impact retention rate are a pair: one tests the initial toughness, and the other tests how much is left after going through a summer.
Many projects only test the initial values, and this is the source of 'qualified at loading, cracking in mass production the next year'.
Lines three to five are long-term items, testing whether the component can still hold its position after long-term load and moisture absorption.
The sixth line is most easily mistaken as an appearance issue, but it is actually a warning — surface chalking is a sign of decreased toughness, not dirt.
4. Four types of failures and their real causes
Failure 1: Breaks easily in winter, with a white and very even fracture surface.
A white fracture surface indicates a brittle fracture, not an overload fracture.
There are two types of root causes: one is insufficient initial toughness due to biased material selection; the other is sufficient initially, but inadequate after aging.
The method of differentiation is very simple: take the parts that are in stock but not yet loaded onto vehicles and the parts removed from the line, and conduct an impact comparison at the same temperature.
Failure 2: Cracks only appear at edges and holes; the main body of the part is fine.
Corners and edges are stress concentration points, and the notch sensitivity of materials is amplified at low temperatures.
This type can only be partially improved by material selection; the other half depends on the structure: changing sharp corners to rounded corners and thickening the edges of holes.
Failure three: For the same batch, some pieces powder after aging while others are still fine.
Don't attribute it to 'unstable materials' just yet.
More commonly, antioxidants are unevenly dispersed during the mixing stage, and combined with uneven distribution of UV protection on the surface of the part, the aging starting points of the parts are different.
Upon seeing this phenomenon, first check for material mixing and masterbatching, then check the mold ventilation and flow conditions.
Failure Four: Fit was just right during loading, became loose after six months.
The root cause is usually not wear, but water absorption expansion combined with creep occurring together.
Nylon parts will swell after absorbing moisture and will slowly deform under long-term load. With the combination of these two factors, the fit can no longer be maintained.
This type needs to be addressed during the drawing stage: key fit dimensions are determined according to humidity adjustment, with sufficient structural allowance.
There's one more thing that needs to be said directly—the failure investigation of the turnout parts initially suspects the season and operating conditions, and only eventually suspects the base material.
Because the same grade fails in four different ways on the main line and the branch line, in summer and in winter.
5. Processing and Verification: Drying, Mold Temperature, Weld Lines
Drying on nylon parts is a standard procedure, and it must not be skipped, especially on switch parts.
Before going online, the dew point must be below -40℃ and the moisture content must be reduced to within 0.15%; if moisture gets in, it's not just a bubble problem, it will shorten the molecular chains in the barrel, directly affecting the low-temperature toughness of the parts.
Mold temperature determines surface density.
With a low mold temperature, the surface of the part is loose, ultraviolet light can penetrate more easily, and aging occurs faster; at the same time, the weld line strength is also low.
For the turnout parts, the mold temperature should not simply copy the setting of the previous piece of material; it should be re-determined according to the filling condition of the part.
Weld lines are a fixed risk for porous parts and ribbed parts.
After the material flow bypasses holes or ribs, it converges, and the strength and density at the convergence point are low. In such parts, the hole positions often happen to be right at the stress points.
When arranging the gate, the weld line should be pushed away from the stressed area.
The step of adjusting humidity is even more important for switch components than for general parts, because it tests the fit.
Critical mating dimensions are drawn and inspected in the conditioned state; parts released according to dry dimensions will absorb moisture when installed on the circuit and go through another stage.
The low temperature step still needs to be applied separately to the process.
Workshop temperature, initial mold temperature, and the cooling speed of the part after demolding—all three change in winter; the same set of parameters works well in summer, but the internal stress of the parts produced in winter is different.
Therefore, the process parameters for the winter and summer seasons usually need to be set separately, rather than having a single set for the whole year.
It is recommended to arrange the verification sequence like this, do not change it:
1. Material grade: Impact tests at -40°C and room temperature, both with and without notches.
2. Aging Level: Impact Retention Rate after UV and Thermal-Oxidative Aging
3. Part level: After moisture adjustment, slice for key dimensions, appearance, and weld line position
4. Fatigue level: component-level cyclic loading, observing crack initiation and fit retention
5. Line level: Re-measure the status according to the actual temperature and contamination conditions of the section
Why can't the order be changed? Because of low-temperature impact and aging retention rate, if either of them changes state, the data must be redone.
6. Boundaries: For these types of turnouts, don't go down the path of modified nylon for now.
This section may be more valuable than the previous few sections because it helps you cut losses before opening the mold.
First, the high-impact core components in the high-speed turnout area of the main line. High speed, strong impact, and critical position; plastic parts lack long-term data support and should be returned to metal.
Secondly, load-bearing parts in extreme low-temperature areas. Requirements: long-term service below -40°C, with positions where regular replacement is not allowed; the material margin is too thin.
Third, it is necessary to be in close proximity to the site of hot work. Flame retardancy alone does not mean it can withstand open flames and molten slag.
Fourth, it requires locations that are maintenance-free for more than fifteen years and do not allow status re-testing. This is the same principle as in the article about railway fasteners.
Fifth, in positions where the load direction changes frequently and is exposed to corrosive media. The combination of fatigue and corrosion will noticeably accelerate the failure of plastic parts.
Writing these five points at the beginning is not to discourage, but to save time — the verification cycle of switch parts is long, and if installed incorrectly once, the cost of rollback is much higher than the cost of the materials.
Material change risk checklist (switching from the original system to the weather toughening route, items to be moved)
| link; segment; part | What needs to be moved? | Points that are easy to overlook |
|---|
| Mold | Both water absorption rate and shrinkage rate have changed, so the matching dimensions need to be carefully checked. | Only replace materials without checking the mold |
| Dry | Replace the dehumidifying dryer and set the window according to the measured moisture content. | In the humid season, hot air drying is basically ineffective |
| Humidity control | Key fitting dimensions are drawn and inspected according to the conditioned state | Release according to dry-state dimensions |
| Material Temperature / Mold Temperature | The demolding temperature maintains surface density, do not copy the previous one | Low mold temperature causes faster aging |
| Pressure Holding / Demolding | The porous part and rib position need to be re-pressurized, and the weld line should avoid stress areas. | The weld line falls at the intersection of the hole positions |
| Color difference | The identification colors should be matched according to the functional color palette and written into the technical specification. | Treat it only as a cosmetic item |
| Verification order | Material level → Aging level → Component level → Fatigue level → Circuit level | If the previous item fails, just move on. |
One-page report form (for people who need to report upwards)
| item | A one-sentence conclusion |
|---|
| Change what | Low temperature high-impact positions use core-shell toughening, combined with priority positions using glass fiber reinforcement. |
| Move what | Drying and dehumidification replacement, mold temperature adjustment upward, coordinate dimensions to produce drawings according to the moisture-adjusted state |
| Test what | -40℃ impact, impact retention after aging, dimensions after humidity adjustment, fatigue cracks |
| When can the volume increase? | No collapse after aging impact, cooperation maintained, no fatigue crack initiation |
Three questions readers often ask
Question: The data for room temperature impact looks good, so why does it still break in winter?
Because they are not testing the same thing. The room temperature impact test measures the material's current toughness, while the winter test measures the toughness of the material after aging, at low temperatures. Data for both tests are required; only measuring the first one is equivalent to not measuring at all.
Question: If we add the toughening agent, will it be stable at low temperatures?
It depends on which type is added. Ordinary elastomer toughening becomes brittle at low temperatures; the type that can withstand low temperatures is the core-shell structure. Moreover, toughening agents are not resistant to UV, and the toughness of the part will decrease over time outdoors.
Question: If the edges crack, should we change the material or modify the mold?
First, modify the structure. At low temperatures, the notch sensitivity of the material is amplified, and rounding corners and adding thickness can directly reduce the probability of crack initiation; if there are still problems after the structural modifications, then go back to check the toughness system.
Conclusion
Going back to the three questions at the beginning: main line or platform, which part, and the lowest temperature.
If these three questions are all answered correctly, the route for the switch material is basically determined.
The auxiliary system in the formula is matched according to the working conditions per item — conventional auxiliaries are kept in stock, and special models are matched as needed; you report the working conditions and grade, and the materials and auxiliaries are prepared together at once.
There might be people selling information, but it’s not certain that anyone will provide a judgment.
For the accounts of turnout parts and gauge blocks, asking clearly about the minimum temperature and aging accounts is much more useful than asking for quotes from three more companies.
We produce modified nylon (PA6 / PA66 / PA46 / PA11 / PA12 / PA6T / PA9T and nylon alloys), and also manufacture modified PPO / PPS and thermoplastic elastomers; additionally, we regularly purchase nylon raw materials, sprue regrind, and various nylon waste, with formal disposal channels.
