去年九月，一家做售后冷却系统件的客户寄来两个副水壶。
件是深色半透明的，壶身上刻着最低液位线；沿中缝那一道焊缝上，有一圈发白的渗痕。他用气泡膜裹了两层，寄来的时候还特地标了渗漏的位置。
电话里他说："换了三次料，焊缝还是渗。车主那边说液面忽高忽低，仪表也跟着报。"
我先问了三句：压力盖是 0.9 bar 那一档还是 1.4 bar 那一档？焊缝是热板焊还是振动摩擦焊？液面是靠传感器读，还是靠人眼看刻度？
他答完，方向就转了个弯——这个件的难点根本不在"耐不耐乙二醇"上。
这篇把膨胀水箱材料这条判断链写完整，也说清哪几种情况下这个件不该用改性尼龙。
先把膨胀水箱和副水壶这两个名字理一下。
乘用车上常叫副水壶，商用车与工程机械上常叫膨胀水箱，本质是同一个角色：给冷却系统留一个缓冲容积，让冷却液在受热膨胀时有地方去，冷了再回来。
它不承主载荷，所以很多人下意识觉得这个件"没什么难度"。
恰恰因为它不承主载荷，出问题时谁都不会先想到它，等发现的时候，往往已经渗了不止一个件。
一、这个件的工况，六维里至少四样要落到数字
压力交变。 压力盖的开启压力常见 0.9–1.5 bar。壶体在盖的开启压力与系统负压之间来回，一天冷热循环四五次，十年累计大约一万五千次上下。
这个数看着不大，但它作用在整个壶壁面上：一个 100 毫米见方的大平面，1.4 bar 的差压合上去，等于一百多公斤的力压在那一块上。
温度。 发动机舱内从 -40℃ 的冷启动环境到 110–125℃ 的热浸工况，温差跨度能到 165℃。
这个跨度带来的热胀冷缩量级很实在：一个 200 毫米长的壶体，按玻纤尼龙的线膨胀系数粗算，冷热两端的长度差可以到两三毫米。这个数比很多人以为的大一个量级。
介质。 乙二醇基冷却液，常见配比是 50% 上下，还夹着水蒸气与少量酸性氧化产物。
振动与安装。 支架、卡扣、以及和散热器钎焊或装配在一起的结合位置。
外观与可读性。 这是最容易漏的一条——很多副水壶要看得到液面，也就是要半透明。
寿命与合规。 整车常按十年或二十万公里算；蒸发与回收相关的合规要求，也会牵到材料的选择。
六样里先问齐四样：压力盖压力、温度跨度、焊缝方式、要不要看得见液面。
二、三条材料路线，并列摆开
| 路线 | 典型做法 | 它擅长什么 | 它的代价 |
|---|
| PA66 本体（不加纤） | 靠本体做半透明观察窗 | 液面看得清；韧性好、焊接工艺成熟 | 模量低，平面易鼓胀；尺寸随湿度走 |
| PA66-GF15/30 | 玻纤增强，结构刚性好 | 大平面抗鼓胀、支架根部强度好 | 加了玻纤就不透明；熔接面强度对焊接参数更敏感 |
| PA612 或 PA12 体系 | 长碳链树脂，吸水率低 | 长期高温冷却液环境下保留率更好 | 成本高；耐温上限比 PA66 低一档 |
看这张表的重点不在"哪条更好"，在它们分别牺牲了什么。
不加纤那条路线牺牲的是刚性。 半透明能做出来，但壶体上那个大平面在压力交变下会鼓，长期下来密封面跟着走。
第二条路线牺牲的是透明度。 玻纤和半透明天生冲突——纤维把光散射掉，件就浑了。
所以这个件常见的设计解法是分件：观察窗那一段用本体料保持透明，受力大的一段用玻纤增强料，两段焊在一起。
第三条路线解决的是吸水与长期保留率，代价是成本与耐温上限。
这里要说清一个常被忽略的点：乙二醇对尼龙的作用，是跟着温度走的。
同一份材料，在 90℃ 冷却液里泡 1000 小时（约合 42 天）和在 120℃ 里泡同样的时长，保留率不是一个量级。
所以问这个件该用什么料，先要问的不是"介质是什么"，是"长期温度是多少".
三、选型判据表（这一页值得收藏）
下表门限值是方向性建议，不是验收标准；实际数值必须由具体项目、具体工况和实测确定。
| 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 |
|---|
| 冷却液浸泡后保留率 | 按温度档定，1000h 后 ≥75% | ISO 175 / ASTM D543 + ISO 527 | 内壁起层、发脆 | 换基材 + 稳定化体系 | 抗水解剂 |
| 压力交变疲劳 | 0.2–1.5 bar 交变 10^4 次以上无渗漏 | 压力交变台架 + 爆破试验 | 焊缝渗漏、大平面鼓胀 | 分件设计 + 壁厚与加强筋 | 材料本征 |
| 熔接面强度 | 不低于本体强度的 70% | 拉剪试样 + 总成爆破 | 焊缝开裂、渗漏 | 焊接工艺参数 + 材料匹配 | 材料本征 |
| 低温冲击 | -40℃ 不低于常温值的 40% | ISO 179-1 | 冬季拆装崩边、掉角 | 增韧体系（核壳结构） | 基体配方 |
| 雾度与液面可读性 | 按件定，目视可辨最低液位线 | 雾度仪 + 目视确认 | 液位看不清、加油误判 | 控制晶粒尺寸 | 成核剂 |
| 长期热氧保留率 | 110℃×1000h 后 ≥70% | ISO 527 | 支架附近发白变脆 | 稳定化体系 | 抗氧剂 |
| 干湿态尺寸差 | 差异控制在 0.15% 以内 | 调湿前后实测 / ISO 294 | 盖口密封面渗漏 | 调湿态交付 + 密封结构 | 材料本征 |
怎么用这张表：先看第三行的熔接面强度，再看第二行的压力交变。
绝大多数副水壶的投诉，落点都在焊缝上。 材料本身就位的项目，若焊接没配对，照样渗。
表里"验证方法"那一列，压力交变这一项很多主机厂有自己的企业规范，没有统一国标。按主机厂规范走，比按经验猜稳。
四、四类常见失效，和它们的真实根因
失效一：焊缝渗漏，下意识反应是"把壁厚加厚"。
这是本类件里最容易走偏的一步。壁厚加厚解决的是本体强度，而焊缝渗漏的根因通常在两个地方：熔接面的设计和焊接工艺参数。
热板焊的温度、保压时间、冷却时间，振动摩擦焊的振幅与下压深度，任何一项偏掉，熔接面就焊不透或者过焊。
换料解决不了焊不透这个问题。 先动参数，再看材料。
失效二：液面忽高忽低，仪表跟着报。
车主的描述听起来像传感器故障，但排查下来常常是另一回事：壶体在压力交变下发生了弹性变形，液位基准跟着动；再加上尼龙件吸湿后体积会有变化，刻度线与实际容积的对应关系就漂了。
先测壶体的尺寸稳定性，再谈传感器。 顺序反了，换两次传感器也解决不了。
失效三：同一批件，一批清透一批发雾。
这不是"料不稳定"。半透明件的雾度，主要取决于结晶的晶粒尺寸——晶粒越粗，光散射越强，件就越雾。
从助剂侧看，常见根因是成核剂分散不均或用量不足，也可能是抗氧剂在混料阶段没混匀，造成局部色差。
看到这个现象，先查混料工艺和母粒化，别急着换料。
失效四：冬季拆装时卡扣崩边。
断在低温、断口齐整，这是低温脆断。常温脆断与低温脆断是两件事：普通弹性体增韧在低温下会失效，要核壳结构才管用。
客户常说"要韧性好"，追问一句就能分开：断在常温还是低温？断口有缺口还是齐的？这两个答案指向完全不同的两套方案。
排查顺序上有一条要直说：这个件的失效，先怀疑焊接与结构，再怀疑工艺，最后才怀疑材料。
因为渗漏这件事的落点在界面，界面的问题多半不是换料换得掉的。
五、加工与验证：先验什么后验什么
干燥。 尼龙必干燥。含水超标会在熔融温度下水解降解，件的韧性与长期保留率一起掉。普通热风干燥机对尼龙基本无效，要用除湿干燥机。
焊接前的件状态。 熔接面如果受潮或沾了脱模剂，焊接质量直接打折。上焊机之前，件要按同一状态管理。
模温。 模温不足时表层结晶不完全，焊接时的熔融行为会不一致。同一条产线上两模件的模温差，会被焊机放大成焊缝强度的差。
调湿。 尼龙件吸水后尺寸会涨，1% 的吸水大约对应 0.2–0.3% 的尺寸变化。一个 60 毫米的盖口，就是从 60.00 涨到 60.12–60.18。
密封圈的压缩量要按调湿后的尺寸给，否则装车时紧、用一段时间又松。
验证顺序，建议这样排：
1. 小样浸泡与保留率（按实际温度和介质配比）
2. 焊接工艺窗口试验：温度、时间、压力三参数的组合
3. 熔接面拉剪试样 + 总成爆破压力
4. 压力交变台架（冷热交替 + 压力循环）
5. 整车工况叠加：振动、温度循环、装车状态
顺序不能换。 前一项不过就往下走，后面测出来的数据没有解释意义。
一个内行细节：壶体上测量点要避开焊缝和浇口。这两个位置的尺寸受工艺波动影响大，把它们排除掉，测出来的数据才代表件本身。
六、边界：什么时候这件事根本不该谈
以下四种情况，膨胀水箱或副水壶走改性尼龙这条路不建议推进：
其一，系统压力明显高于常见区间，或者要求长期承受更高的工作压力。 尼龙的蠕变特性决定了大平面在长期压力下会持续变形，这类结构要看更刚性的方案或金属。
其二，要求整件高透明、又要高刚性。 这两个要求在同一件上天然冲突，分件设计解决不了、又不接受分件的，要换路线。
其三，长期工作温度稳定超过 125℃。 常规体系在这个区间的长期数据支撑不足，要看更高耐温的体系。
其四，焊接工艺本身还不稳定。 熔接面是这类件的头号风险点，工艺没铺稳就先谈材料，等于把钱花在最不容易出成果的地方。
把这四条写在前面不是劝退，是省时间。 样品阶段很顺、卡在批量验证上再回退的项目，回退成本比一开始就不做高得多。
还要说清一句：膨胀水箱和散热器水室不能打包做决定。 水室贴在本体上、长期承受冲刷和高温介质，膨胀水箱是缓冲容积件，判据的重心在压力交变与焊接上。
七、自产能力位：我们能陪到哪一步
我们做的事很具体：把 PA6、PA66、PA46、PA11、PA12、PA6T、PA9T 这些树脂，改成某个件真正能用的样子。
膨胀水箱这类件的打样，我们按轮次走。
先出小样做浸泡与保留率，再上焊接工艺窗口试验，然后做压力交变与爆破。
每一轮的样件按批留样，出了偏差，能倒回去查是哪一轮动的什么。
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。
换料风险清单（从 PP 或通用 PA66 换过来，要动的东西）
| 换料要动的项 | 要关注什么 | 容易漏的点 |
|---|
| 模具 | 收缩率差异随基材与玻纤含量变，盖口与焊缝台阶要重算 | 只按通用收缩率给，不按件做补偿 |
| 干燥 | 尼龙必干燥，含水超标会水解降解，保留率与韧性一起掉 | 用热风干燥机顶替除湿机 |
| 焊接 | 换料等于换一套熔接参数，要做工艺窗口试验 | 沿用原来的焊接参数直接上量 |
| 料温与模温 | 模温决定表层结晶，直接影响焊缝一致性 | 照抄牌号推荐值，不看件 |
| 焊接前状态 | 受潮与脱模剂残留会拉低焊缝强度 | 件在车间放几天再焊，状态没管 |
| 调湿 | 盖口密封面按调湿后的尺寸给压缩量 | 按平均壁厚估时间，厚壁处没吸透 |
| 验证顺序 | 小样浸泡 → 焊接窗口 → 拉剪与爆破 → 压力交变 → 整车 | 前一项没过就往下走 |
一页纸汇报表（给要向上汇报的人）
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项目：膨胀水箱 / 副水壶 · 材料路线评估
结论方向：改性尼龙可作候选路线，能否落地取决于四项前置条件
一、必须守住的三条
1. 浸泡数据按实际温度与介质配比做，常温表不作依据
2. 换料同步做焊接工艺窗口试验，参数不沿用
3. 盖口密封面按调湿后的尺寸给压缩量
二、前置条件（任一不满足则建议暂缓）
· 系统压力落在常见区间内，大平面有加强筋方案
· 焊接工艺已铺稳，有工艺窗口试验数据
· 长期工作温度 ≤ 125℃ 量级
· 需要半透明观察窗时，接受分件设计的成本
三、下一步动作
1. 取实际冷却液，按工况温度做 1000 小时浸泡
2. 做三组焊接参数试样，测拉剪强度
3. 总成爆破 + 压力交变各跑一轮
风险提示：本路线的主要不确定性在熔接面与压力交变，不在耐乙二醇这一项。
`
读者常问的两句
问：副水壶的液面看不清，能不能把壁厚减薄一点？
减薄能改善透明度，但同时会降低大平面的抗鼓胀能力。更稳的做法是把观察窗做成独立一段，用本体料，受力段保留玻纤增强。
问：压力盖换一个开启压力更低的，是不是对壶体更友好？
方向上是，但压力盖的选择要先满足系统要求，不能为了壶体让。壶体这边该做的是加强筋与分件设计，而不是把压力盖往下调。
结语
膨胀水箱与副水壶的塑化，说到底是一道界面题叠加一道状态题。
判断链只有三条：
长期温度定体系 → 焊接与结构定成败 → 调湿态交付定一致性。
回收开头那三句追问——压力盖是哪一档、焊缝是哪种工艺、液面靠读还是靠看——它们分别指向压力交变、熔接面与外观三条线。
三条对完，"这个件能不能用改性尼龙"自然就有答案了。
如果你手上有个副水壶或膨胀水箱要定料，把三样东西发过来就能给方向：系统压力与压力盖规格、长期工作温度、焊接方式与要不要看得见液面。
这三件事我们从不猜：耐温、寿命、用量。
没给长期温度，不猜；没给寿命年限，不猜；没给年用量，也不猜。猜出来的方案，最后都要用返工和索赔还回去。
我们做改性尼龙（PA6 / PA66 / PA46 / PA11 / PA12 / PA6T / PA9T 及尼龙合金）、改性 PPO / PPS / 热塑性弹性体，也经营各大化工巨头的尼龙树脂、副牌料与大包料。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。
Last September, a customer who makes after-sales cooling system parts sent in two auxiliary water tanks.
The piece is dark and semi-transparent, with the minimum liquid level line engraved on the body of the pot; along the seam in the middle, there is a circle of whitish seepage marks. He wrapped it in two layers of bubble wrap and even specifically marked the leaking spot when sending it.
On the phone he said: 'We changed the material three times, but the welds are still leaking. The car owner said the liquid level fluctuates, and the gauge reports accordingly.'
I first asked three questions: Is the pressure cap set to 0.9 bar or 1.4 bar? Is the weld a hot plate weld or a friction stir weld? Is the liquid level read by a sensor, or by a person looking at the scale?
After he finished answering, the conversation took a turn—the difficulty of this part has nothing to do with 'resistance to ethylene glycol'.
Complete this article's judgment chain for the expansion tank material, and also clarify in which few situations this part should not use modified nylon.
First, sort out the names of the expansion tank and the auxiliary water tank.
In passenger cars, it is often called an auxiliary water tank, while in commercial vehicles and construction machinery, it is commonly called an expansion tank. Essentially, it serves the same role: to provide a buffer volume for the cooling system, giving the coolant a place to go when it expands due to heat, and allowing it to return when it cools down.
It does not bear the main load, so many people subconsciously think that this part is 'not difficult'.
Precisely because it does not bear the main load, when problems occur, no one thinks of it first; by the time it is discovered, often more than one part has already leaked.
1. For the working condition of this part, at least four of the six dimensions need to be quantified.
Pressure fluctuates. The opening pressure of the pressure lid is commonly 0.9–1.5 bar. The pot body moves back and forth between the lid's opening pressure and the system's negative pressure, cycling four to five times a day with hot and cold changes, accumulating about fifteen thousand cycles over ten years.
This number doesn't look big, but it acts on the entire pot wall: a large flat surface of 100 millimeters square, with a differential pressure of 1.4 bar applied, equaling more than a hundred kilograms of force pressing on that area.
Temperature. Inside the engine compartment, the temperature can range from -40°C during cold start conditions to 110–125°C under hot soak conditions, with a temperature difference span of up to 165°C.
The thermal expansion and contraction caused by this span is quite significant: for a 200-millimeter-long kettle body, roughly calculated based on the linear expansion coefficient of fiberglass nylon, the length difference between the hot and cold ends can reach two to three millimeters. This figure is an order of magnitude larger than what many people expect.
Medium. Ethylene glycol-based coolant, commonly mixed at around 50%, also containing water vapor and a small amount of acidic oxidation products.
Vibration and installation. Brackets, clips, and the joint positions where they are brazed or assembled with the radiator.
Appearance and readability. This is the easiest one to overlook—many secondary water kettles need to have a visible water level, which means they need to be semi-transparent.
Lifespan and compliance. The whole vehicle is often calculated based on ten years or two hundred thousand kilometers; compliance requirements related to evaporation and recovery will also affect the choice of materials.
Among the six items, first ask about four: pressure of the pressure cover, temperature range, welding method, and whether it is necessary to see the liquid level.
Second, three material routes, arranged side by side
| Route | Typical practices | What is it good at? | Its cost |
|---|
| PA66 body (without fiber) | Use the main body to make a translucent observation window | The liquid level is clearly visible; good toughness and mature welding process | Low modulus, surface easily bulges; dimensions change with humidity |
| PA66-GF15/30 | Glass fiber reinforced, good structural rigidity | The large surface resists bulging, and the base of the support has good strength | Adding fiberglass makes it opaque; the strength of the welded joint is more sensitive to welding parameters |
| PA612 or PA12 system | Long carbon chain resin, low water absorption | Better retention in long-term high-temperature coolant environments | High cost; temperature resistance upper limit one level lower than PA66 |
The focus of looking at this table is not on 'which one is better,' but on what each of them sacrifices.
Without adding fiber, the sacrifice on that route is rigidity. It is possible to make it translucent, but the large flat surface on the body of the kettle will bulge under alternating pressure, and over time the sealing surface will follow.
The downside of the second route is transparency. Glass fibers and translucency are inherently in conflict—the fibers scatter light, making the part cloudy.
So the common design solution for this part is to divide it into sections: the section with the observation window uses the main material to maintain transparency, and the section that bears more force uses glass fiber reinforced material, with the two sections welded together.
The third route addresses water absorption and long-term retention, with the cost being expense and the upper limit of temperature resistance.
Here, one point that is often overlooked needs to be clarified: the effect of ethylene glycol on nylon follows the temperature.
The same material, when soaked in a 90℃ coolant for 1000 hours (about 42 days) and soaked for the same duration at 120℃, the retention rate is not of the same order of magnitude.
So when asking what material should be used for this part, the first question is not 'what is the medium,' but 'what is the long-term temperature.'
3. Selection Criteria Table (This page is worth saving)
The threshold values in the table are directional recommendations, not acceptance standards; the actual values must be determined by the specific project, specific working conditions, and actual measurements.
| Indicator | Directional Threshold | Verification Method / Standard | Common Failures | Common solution | Corresponding auxiliary agent system |
|---|
| Retention rate after coolant immersion | Based on the temperature setting, after 1000 hours ≥75% | ISO 175 / ASTM D543 ISO 527 | Peeling and becoming brittle on the inner wall | Change substrate Stabilization system | Anti-hydrolytic agent |
| Cyclic stress fatigue | 0.2–1.5 bar alternating, no leakage after more than 10^4 cycles | Pressure Cyclic Test Stand Explosion Test | Weld seam leakage, large surface bulging | Parting Design Wall Thickness and Ribs | Material intrinsic |
| Weld joint strength | Not less than 70% of the body strength | Tensile and shear specimen Assembly burst | Weld cracking and leakage | Welding process parameters Material matching | Material intrinsic |
| Low-temperature shock | -40℃ not lower than 40% of the room temperature value | ISO 179-1 | Chipping and corner dropping during winter assembly and disassembly | Toughening System (Core-Shell Structure) | Substrate Formula |
| Foginess and liquid level readability | Set per item, minimum liquid level line visible to the eye | Haze Meter Visual Confirmation | The liquid level is unclear, resulting in misjudgment when refueling | Control grain size | Nucleating agent |
| Long-term thermal oxygen retention rate | 110°C×after 1000h, ≥70 % | ISO 527 | Whitening and brittleness near the bracket | Stabilization system | Antioxidant |
| Dry-wet dimensional difference | Difference controlled within 0.15 %. | Measured before and after humidity control / ISO 294 | Lid-sealed surface leakage | Moisture control delivery + sealing structure | Material intrinsic |
How to use this table: first look at the weld surface strength in the third row, then look at the pressure alternating in the second row.
Most complaints about auxiliary kettles focus on the weld seam. For items where the material itself is in place, if welding is not paired, leakage will still occur.
In the table under "Verification Methods," many OEMs have their own enterprise standards for pressure alternating and do not have unified national standards. Following OEM standards is better than guessing based on experience.
Four, four common types of failures and their real root causes
Failure 1: weld leakage, the instinctive reaction is to "thicken the wall thickness."
This is the most common mistake in this category. Wall thickness thickening addresses the main body strength, while the root causes of weld leakage usually lie in two areas: the design of the weld joint and welding process parameters.
If any of these are off, the temperature, holding pressure, and cooling time of hot plate welding, or the amplitude and pressing depth of vibration friction welding, the weld surface will not penetrate or be over-welded.
Material change won't solve the problem of incomplete welding. First, adjust parameters, then look at the material.
Failure 2: Liquid level fluctuates between high and low, and the instrument responds accordingly.
The owner's description sounds like a sensor failure, but after investigation, it's often a different matter: the kettle body undergoes elastic deformation under alternating pressure, and the liquid level reference moves accordingly; Plus, after the nylon part absorbs moisture, its volume changes, so the correspondence between the scale line and the actual volume drifts.
First, test the dimensional stability of the kettle, then talk about the sensor. If the order is reversed, changing the sensor twice won't solve the problem.
Failure 3: Same batch of parts, one batch clears through, another produces fog.
This is not "unstable material." The haze of semi-transparent parts mainly depends on the grain size of the crystals—the coarser the grains and the stronger the light scattering, the more foggy the parts become.
From the additive side, common causes are uneven dispersion or insufficient amount of nucleating agents, or insufficient mixing of antioxidants during mixing, causing local color differences.
If you see this phenomenon, first check the mixing process and masterbatch; don't rush to replace the material.
Failure 4: Snap edges during winter disassembly.
Fracture occurs at low temperature with uniform fractures; this is low-temperature brittle fracture. Normal temperature brittle fracture and low-temperature brittle fracture are two different things: ordinary elastomer toughening fails at low temperatures, and the core-shell structure is only effective.
Customers often say "good toughness," but a single question can tell the difference: fracture at room temperature or low temperature? Is the fracture gap or uniform? These two answers point to completely different solutions.
The inspection order states directly: for this part's failure, first suspect welding and structure, then the process, and finally the material.
Because leakage issues focus on interfaces, interface issues are usually not something that can be replaced by material replacement.
5. Processing and Verification: Inspect what needs to be tested before
Drying. Nylon must be dried. Excessive moisture content will hydrolyze and degrade at melting temperature, and the toughness and long-term retention of the part will decrease. Ordinary hot air dryers are basically ineffective for nylon; a dehumidifying dryer is needed.
Condition of the part before welding. If the weld surface is damp or contaminated with release agent, welding quality will be directly reduced. Before being soldered, the part must be managed in the same state.
Mold temperature. If the mold temperature is insufficient, the surface crystallization will be incomplete, resulting in inconsistent melting behavior during welding. The temperature difference between two molds on the same production line can be magnified by the welding machine into a difference in weld strength.
Humidity control. After absorbing water, nylon parts increase in size; 1% of water absorption corresponds to about 0.2–0.3% of the dimensional change. A 60mm cap increases from 60.00 to 60.12–60.18.
The compression of the sealing ring should be adjusted according to the moisture adjustment size; otherwise, it will be tight during installation and loose after a while.
Verification sequence, recommended arrangement as follows:
1. Sample soaking and retention rate (based on actual temperature and medium ratio)
2. Welding process window test: combination of temperature, time, and pressure parameters
3. Weld joint tie-cut specimen + assembly bursting pressure
4. Pressure alternating test frame (alternating hot and cold + pressure cycle)
5. Overall vehicle operating conditions stacked: vibration, temperature cycle, loading status
cannot be changed in sequence. The first item is just a step down; the data measured later have no explanatory value.
A knowledgeable detail: the measurement points on the kettle body should avoid welds and gates. The dimensions at these two positions are greatly affected by process fluctuations; excluding them reveals the measured data representing the part itself.
6. Boundaries: When should this matter be discussed at all ?
In the following four situations, it is not recommended to pursue modified nylon for expansion tanks or auxiliary kettles:
First, the system pressure is significantly higher than usual, or it requires long-term higher working pressure. The creep characteristics of nylon mean that large planes will continuously deform under long-term pressure. For such structures, more rigid solutions or metals are needed.
Second, the entire piece must be highly transparent and rigid. These two requirements naturally conflict within the same piece; if the part design cannot solve or does not accept parts, the route must be changed.
Third, long-term operating temperature consistently above 125°C. Conventional systems lack long-term data support in this range; you need to look for systems with higher temperature tolerance.
Fourth, the welding process itself is still unstable. The weld surface is the number one risk point for these parts; discussing materials before the process is stable means spending money on the least likely results.
Putting these four points in front is not to discourage you, but to save time. Projects that go smoothly during the sample stage but are stuck in batch validation before rolling back have much higher costs than if they were not done at the start.
One more point: the expansion tank and radiator water chamber cannot be bundled together for decision-making. The water chamber is attached to the main body, endures long-term scouring and high-temperature media, and the expansion tank is a buffer volume. The focus of the criterion is on pressure alternating and welding.
7. Self-production capability: How far can we go ?
What we do is very specific: modify the resins PA6, PA66, PA46, PA11, PA12, PA6T, PA9T into a truly usable part form.
For prototyping of parts like expansion water tanks, we proceed in turns.
First, produce samples to test soaking and retention rates, then test the welding process window, followed by pressure alternating and blasting.
Each round of samples is retained by batch; if there is a deviation, you can go back to check which round moved what.
The additive system in the formula is matched according to the working conditions of the parts—regular additives are always in stock, special models are matched as needed; You report the working conditions and grade, and the materials and additives are all matched at once.
Material change risk checklist (items to be moved when switching from PP or general PA66)
| Items to be moved for material change | What to pay attention to | Points prone to leakage |
|---|
| Molds | Shrinkage differences vary with the substrate and glass fiber content, and the cover and weld step need to be recalculated. | Compensation is given based only on the general shrinkage rate, not per piece. |
| Dry | Nylon must be dry; excessive moisture will cause hydrolytic degradation, reducing both retention rate and toughness. | Use a hot air dryer instead of a dehumidifier |
| Welding | Changing the material is equivalent to changing a set of fusion parameters, and a process window test needs to be carried out. | Directly increase the output using the original welding parameters |
| Material Temperature and Mold Temperature | Mold temperature determines surface crystallization and directly affects weld seam consistency | Copy the recommended value from the brand, without looking at the part |
| Pre-welding condition | Moisture and release agent residue can reduce weld strength | Leave the parts in the workshop for a few days before welding, without caring about their condition. |
| Humidity control | The sealing surface of the cover is given a compression amount according to the size after moisture adjustment. | Based on the average wall thickness to estimate the time, the thick-walled areas are not fully soaked. |
| Verification order | Sample soaking → Welding window → Tensile shear and blasting → Pressure alternation → Whole vehicle | If the previous item fails, just move on. |
One-page report form (for people who need to report upwards)
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Project: Expansion Tank / Auxiliary Water Tank · Material Route Evaluation
Conclusion direction: Modified nylon can be a candidate route, and whether it can be implemented depends on four prerequisite conditions
1. Three Rules That Must Be Followed
1. Soaking data is based on the actual temperature and medium ratio, and the normal temperature table is not used as a reference.
2. Conduct welding process window testing simultaneously when changing materials, without using the previous parameters.
3. The sealing surface of the cover should be compressed according to the size after moisture adjustment.
2. Precondition (It is recommended to postpone if any are not met)
· System pressure falls within the common range, and the large flat surface has a rib reinforcement plan
· The welding process has been stabilized, with process window test data available
· Long-term operating temperature ≤ 125℃ range
· When a translucent observation window is needed, accept the cost of component design
3. Next Steps
1. Take the actual coolant and soak it for 1000 hours according to the working condition temperature.
2. Make three sets of welding parameter samples and measure the tensile-shear strength
3. Assembly blasting Each run one cycle under alternating pressure
Risk warning: The main uncertainty of this route lies in the fusion interface and alternating pressure, not in resistance to ethylene glycol.
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Two questions readers often ask
Question: The liquid level of the auxiliary kettle is not clear. Can the wall thickness be reduced a bit?
Thinning can improve transparency, but at the same time it will reduce the flat surface's resistance to bulging. A more stable approach is to make the observation window as a separate section, using the base material, while retaining fiberglass reinforcement in the stress-bearing section.
Question: If I replace the pressure cap with one that opens at a lower pressure, will it be more friendly to the kettle body?
In terms of direction, yes, but the choice of pressure cover must first meet the system requirements; it cannot be compromised for the sake of the kettle body. What should be done on the kettle body side is the design of reinforcing ribs and separate parts, rather than lowering the pressure cover.
Conclusion
The plasticizing of the expansion tank and auxiliary water tank, ultimately, is an interface problem coupled with a state problem.
There are only three judgment chains:
Long-term temperature determines the system → Welding and structure determine success or failure → Humidity-controlled state delivery determines consistency.
Revisiting the first three follow-up questions — which setting is the pressure cap, what type of process is the weld seam, and whether the liquid level is read or observed — they correspond respectively to the lines of pressure fluctuation, the fusion surface, and appearance.
After the three items are matched, whether this piece can use modified nylon naturally has an answer.
If you have a spare water tank or expansion tank and need to determine the specifications, sending over these three things is enough to give guidance: system pressure and pressure cap specifications, long-term operating temperature, welding method, and whether the liquid level needs to be visible.
There are three things we never guess: temperature tolerance, lifespan, and dosage.
If the long-term temperature is not given, don't guess; if the service life is not given, don't guess; if the annual usage is not given, don't guess either. Solutions that are guessed will eventually have to be returned with rework and claims.
We manufacture modified nylon (PA6 / PA66 / PA46 / PA11 / PA12 / PA6T / PA9T and nylon alloys), modified PPO / PPS / thermoplastic elastomers, and also distribute nylon resins, second-brand materials, and bulk materials from major chemical companies. Additionally, we have long-term procurement of nylon raw materials, sprue regrind, and various nylon waste, with formal disposal channels.
