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改性尼龙失效案例速查表：断口平整，最常见的元凶是未调湿

应用领域 发布时间: 2026-09-15 699 阅读

232 Quick Reference Table of Modified Nylon Failure Cases

Let's start with a live scene

In our office, there is a row of metal filing cabinets, and the bottom compartment holds a dozen or so transparent storage boxes. Each box contains a failed part: cracked gears, shafts worn shiny, bulging connector housings, brittle and crumbling wiring boards. Each box has a handwritten label that notes the customer's industry, the time of failure, and the final identified cause. This cabinet contains the tuition we have paid over the years in component selection and after-sales.

When a new colleague joins, the first thing an experienced colleague shows him is not the product samples, but this row of cabinets. Because the samples tell you what the material can do, while the cabinets tell you the failures the material has experienced. If a person making selections has only seen the former, their judgment will lack a layer of respect; if they've only seen the latter, they'll be hesitant to make recommendations. Only after seeing both can they confidently say over the phone what can be replaced and what cannot be touched.

This quick reference sheet basically puts this row of cabinets onto paper. The eight types of failures are arranged according to occurrence frequency, and each type provides three things: what it looks like on-site, where the root cause usually hides, and what to measure first during troubleshooting. The suggested reading method is to first scan the symptoms, then look at the root cause, and finally go through the troubleshooting action checklist—if you complete all three steps, you can locate 80% of the issues yourself; for the remaining 20%, you can take the checklist to the material supplier, which will also reduce communication costs considerably.

Let's first clarify a premise: failures are rarely caused by a single reason. More than half of the labels in our cabinet list two causes, such as hydrolysis combined with assembly stress, or creep combined with high temperature. Therefore, the inspection actions given in each category below will remind you not to stop at the first cause you find.

Failure 1: Brittle Fracture

Phenomenon: The part suddenly breaks without obvious deformation, and the fracture surface is smooth. Common causes: dry material (not conditioned, most common); low temperature (below embrittlement temperature); molecular weight degradation (processing temperature too high or repeated regrinding);

Aging (becomes brittle after thermal-oxidative or ultraviolet aging). Troubleshooting order: first measure the moisture content, then confirm the usage temperature, and then check the melt index to determine if degradation has occurred.

Failure 2: Creep Deformation

Phenomenon: The part slowly deforms under long-term load, eventually losing function or seizing. Common causes: insufficient glass fiber content; choosing PA6 when the operating conditions require PA66; temperature higher than expected (creep accelerates several times at high temperature);

The structural design lacks sufficient stiffness. Troubleshooting sequence: first confirm the actual operating temperature, then calculate the creep modulus, and finally check the structural design. This type of failure often only becomes apparent six months after assembly.

Failure Three: Environmental Stress Cracking

Appearance: After being used for a period of time, fine cracks appear, usually starting from areas of stress concentration. Common causes: combined effect of residual stress and chemical media; large residual stress caused by injection molding (excessive holding pressure, low mold temperature); assembly stress (overly tight interference fit);

Medium erosion (oil, solvents, detergents). Troubleshooting order: first look at the crack initiation point, then check assembly and process, and finally perform medium compatibility verification.

Failure Four: Hydrolytic Degradation

Phenomenon: The part loses strength, becomes sticky on the surface, and develops cracks under hot water or a hot and humid environment. Common causes: Using a grade not resistant to hydrolysis; temperature exceeding the material's limit; medium pH deviation (alkaline accelerates hydrolysis). Troubleshooting sequence: Measure melt index (melt index significantly increases after hydrolysis), confirm temperature and pH, assess whether PPS or PPSU needs to be replaced. This is a common failure in dishwashers, water heaters, and cooling systems.

Failure 5: Thermal Oxidative Aging

Appearance: The component changes color (turns yellow or brown), becomes brittle, and the surface cracks after long-term use at high temperatures. Common causes: no antioxidant added or insufficient amount; high processing temperature causing initial degradation; long-term temperature exceeding the material's RTI.

Investigation order: Confirm long-term operating temperature, check the antioxidant system, measure melt index and color difference. High occurrence around motors and engines.

Three General Rules for Failure Investigation

After covering the eight types of failures, three additional general rules that are not categorized are added. The first is to preserve the evidence: do not throw away failed components, do not pry them open to see the cause—the break itself is the most honest witness; once pried open and rolled up, the testimony is gone. The second is to ask about the timeline: how long it took to start having problems after installation, and how quickly the problems spread. The root causes are completely different for issues that remained dormant for half a year versus those that occurred on the day of installation.

Article 3 is about asking about changes: during this period, whether the supplier was changed, whether the mold was modified, whether the process parameters were altered; most failed switches are involved in some change. These three rules cost nothing, but they can save more than half of the detours in troubleshooting.

Failure Six: Ultraviolet Aging

Appearance: The surface of outdoor parts shows chalking, loss of gloss, discoloration, and decreased strength. Common causes: absence of weather-resistant system; migration and precipitation of weathering agents; pigments not lightfast. Troubleshooting sequence: check the degree of surface chalking, review the weather-resistant system formula, and perform xenon lamp aging comparison tests. Outdoor parts must include carbon black or the weather-resistant three-piece set.

Failure Seven: Wear and Abnormal Noise

Manifestations: Wear of sliding parts, increased clearance, abnormal noise. Common causes: No wear-resistant modification; PV value exceeds the material limit; surface roughness of friction parts is too high; abrasive particles enter the friction surface (sand and dust).

Check sequence: calculate the actual PV value, inspect the roughness of the grinding surface, and confirm whether there are abrasives. Wear-resistant modification should select filler according to the type of wear.

A review of a three-line simultaneous check

In the filing cabinet, there was a box with a label that had two lines indicating the cause: hydrolysis combined with assembly stress. It was a water purifier three-way fitting. The customer insisted that the material was not water-resistant, while the supplier claimed that the customer had installed it incorrectly, and both sides were at a stalemate for two months. After we stepped in, we investigated along three lines. Evidence line: we took photos of the fracture of the failed part and sent it for inspection. The fracture showed obvious signs of hydrolytic embrittlement, and silver streaks were also found at the stress concentration areas.

Timeline: Leakage started three months after installation, matching the gradual pace of hydrolysis; if it were assembled with excessive stress, it would have cracked immediately. Change line: That batch of material was put into use three months earlier than the previous batch, coinciding with the rainy season, and storage humidity increased. The three lines converge, and the conclusion is that both factors contributed equally: the material used a standard grade without hydrolysis protection, and storage humidity was not controlled.

The final rectification was also done individually; each person made their own changes, switching to a hydrolysis-resistant grade and adding dehumidification in the warehouse. There were no incidents for three years. This case later became our internal teaching material: looking at a single line, it was someone else's fault, but only by investigating across three lines could the full picture be seen.

Failure 8: Dimension Out of Tolerance

Manifestation: Cannot be installed, excessive gap, deformation after assembly. Common causes: moisture absorption and expansion (most common); incorrect shrinkage rate calculation; post-crystallization shrinkage; batch-to-batch shrinkage rate fluctuations. Troubleshooting sequence: first measure the moisture content, then verify the shrinkage rate data, and finally check the mold and process. About 80% of dimensional issues with PA parts are related to moisture absorption.

Engineering field measurement: 4 mandatory tests

Test 1: Brittleness. First measure the moisture content—the most common reason is that it has not been conditioned.

Test 2: Hydrolysis. After hydrolysis, the melt index increases by more than 50% — the melt index is a quick criterion.

Test 3: Thermal oxidative aging. Discoloration, embrittlement, melt index changes — a must-check for high-temperature components.

Test 4: Size. PA absorbs moisture 2.5%, size increases 0.35% - eighty percent of size problems come from this.

Boundary Declaration

Operating conditionRecommended materials
Suddenly brittle and breaksMeasure moisture content → Check temperature → Check melt index
Slow deformationCore temperature → Calculate creep → Check structure
fine cracksResidual stress Medium → Perform compatibility verification
Strength decreases and becomes stickyHydrolysis: measure melt index, check temperature and pH
Discoloration and brittlenessThermal-oxygen or ultraviolet aging
Can't installMeasure moisture content, check shrinkage rate

Engineering memo

Failure Quick Lookup: For brittle fracture, first measure moisture content; for hydrolysis, check melt fingers; size issues are mostly due to moisture absorption. Using appearances to reverse the cause is the fastest way to troubleshoot.

Practical Case: Common pitfalls and correct answers

Pitfall 1: Failure quick search finds a conclusion but not applicable conditions, so applying it directly leads to errors. Every quick check table has premises; conclusions that deviate from these premises are wrong. Correct answer: When you see a conclusion, first look for the conditions that hold it—temperature, medium, time, load type. Only use it when all four are complete.

Pitfall 2: Blindly copying others' selection without considering your own process capability. The same material produces different results depending on equipment and mold. Correct answer: Selection should be based on your own process level; avoid materials beyond the equipment's capabilities. Pitfall 3: Use the quick reference sheet as the final reference, without actual verification. Correct answer: The quick reference table is used to narrow down the scope; final sample verification is necessary.

Extended judgment: Three things to confirm before selecting

Failure Quick Check Before selecting materials, there are three things to clarify; if the order is wrong, rework everything afterward.

First: What is the long-term operating temperature? Short-term peak temperature and long-term operating temperature are two different things; the thermal deformation temperature on the physical property table is a short-term indicator, and long-term operating temperature should generally be discounted by 70%.

Second: What medium should it be contacted? Oil, water, cleaning agent, sweat, electrolyte—each will change the selection of part numbers. The list of media is more important than the temperature chart.

Third: Are there certification requirements? For parts with certification requirements—flame retardant, CTI, food contact, water hygiene, safety regulations—if there are certification requirements, the replacement number must be reverified, and the cost is much higher than the material price difference. If you clarify these three things, the material selection is half done.

Write out these three things in a form and send it to the supplier—it's more effective than making ten phone calls—the cost of communication for the failure check is basically spent on these repeated confirmations.

Troubleshooting Self-Inspection Q&A

Question: What should you do first when you get a failed part? Take photos first, seal the data, record the discovery scene—do these three things before you get started. We've seen too many scenes where the fracture is split open to look, sanded with sandpaper, or even put back and run again. Once the chain of evidence is broken, all conclusions can only be guessed.

Question: Customers and suppliers each have their own views—who should arbitrate? Qualified third-party testing. But make sure to think carefully about the inspection items first: fracture analysis, composition comparison, thermal analysis. If you choose the wrong item, even if you spend money, the conclusion is still vague. Bring the previous inspection checklist before placing the order.

Question: After the failure responsibility is clarified, should all products from the same batch be recalled? Check whether the failure mode is discrete or systemic. Batch mixing and process drift are systemic, and all batches carry risks; Single-point assembly damage is discrete; after sampling and confirming, there is no need to expand it. The judgment is still based on timelines and change records.

Question: How detailed should the investigation report be written? Detailed enough that even half a year later, outsiders can understand it without explanation. Clearly state the phenomena, evidence, conclusion, and rectification, along with photos and key data. The inspection output is not for this time, but to save money next time.

Four steps to sending samples of failed parts

Sending samples for inspection after discovering defects, the four steps to send samples. Step one is photography: overall photos, close-up of the fracture surface, and assembly position photos—all three angles are essential. The photos are the firsthand view of the inspector's scene. Step two: protect the fracture: do not touch the fracture joints, do not use tape to seal them, do not touch them repeatedly, and wrap the entire unit with bubble wrap before shipping.

Step 3: Attach information: installation time, downtime, usage environment, recent changes—write it all down on one page, which is more effective than making up for it three times over the phone. Step 4: Keep a backup: Before shipping, take photos of the entire damaged item for archiving, and if possible, keep a small sample. We've received many packages before; when opened, they were just a handful of fragments with no information at all, and half of the inspection was entered before the investigation even started. None of these four steps cost anything, and the savings in inspection cycles are calculated weekly.

Establish your own failure record

Quick Reference Sheet When you use the second item, it's time to create your own failure record. The format is simple: one case per line, seven fields listed—date, product, material grade, failure phenomenon, root cause conclusion, rectification actions, verification results. The value of the ledger becomes apparent after half a year: if similar phenomena recur, it means the rectification didn't hit the root cause;

If different products have the same root cause, it means there's a common loophole in the selection criteria; The ledger keeps getting thicker, and when reviewing new projects, you won't have to go through many pitfalls again. Our row of metal cabinets plus electronic ledgers has been maintained for years. At first, we just took random notes, but now they've become living textbook for model selection training.

I suggest starting with the first failed item you receive today. It's better to be detailed than simple, especially in the root cause section—write the specific reason. If you don't write the three words 'bad material', it's not good.

Conclusion

What material is used for this item—the earlier you ask about material selection, the easier it is.

For material selection and mold trials for this type of piece, you can chat together

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