Failure analysis of the hottest rubber seal

2022-08-08
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Failure analysis of rubber seals

rubber seals are widely used in all walks of life. Once the seal fails, the equipment will not work normally and cause economic losses; Serious accidents may result in casualties. For example, on January 28, 1986, the O-ring of the solid rocket booster on the right side of the space shuttle Challenger failed, causing it to disintegrate 73 seconds after liftoff, killing all seven astronauts on board

failure analysis of rubber seals can not only help find out the problem and take corresponding preventive measures in the future, but also help customers and seal manufacturers improve existing technology and achieve a win-win situation

four failure causes

there are four common failure causes: design errors, material selection errors, seal quality problems and improper use

1. Design errors

design errors are usually caused by designers' insufficient understanding of products. For example, the pressure on the seal is underestimated, the distribution of contact stress on the sealing surface is incorrectly understood, and the design of the groove for placing the seal is unreasonable

finite element analysis (FEA) is often used to assist in the design and failure analysis of seals. We once made a seal for an American customer, which is mainly made of plastic and partially covered with rubber. During the inspection of parts, the customer found that the plastic part was easy to crack during the test, so it was concluded that the plastic part was damaged during the secondary molding (that is, the rubber was wrapped on the plastic part). After our analysis, we found that the plastic parts broke in one place. Through finite element analysis, we found that the damaged part of the plastic part is actually the place where the seal is subject to the maximum stress, and the technology can be used as a warning, where the stress has far exceeded the plastic can bear

if the customer has analyzed the product with finite element method at the time of design, it can not only avoid similar errors, but also save time and money. Of course, to successfully analyze and predict the performance of rubber seals, we should not only have appropriate finite element analysis software, but also have rich material experience, modeling experience and long-term data accumulation

2. Wrong material selection

common rubber sealing materials include ethylene propylene diene monomer (EPDM), nitrile rubber (NBR), silicone rubber (VMQ), fluororubber (FKM or FPM) and neoprene (CR). These rubbers have different characteristics and applications. The selection of materials should be considered from many aspects, such as the use temperature, whether the material can withstand the medium, the hardness of the material, compression set, wear resistance and other factors

the wrong material selection is often because designers are not familiar with the properties of various materials. An experienced supplier of rubber seals can point out the problem of material selection at the beginning

one of our domestic customers doesn't like the O-ring being used because it is easy to break. We checked the replaced sample and found that the surface of the sample was cracked, and the texture was very similar to ozone aging. We also asked about the operating environment of O-ring and found that there were many mechanical equipment and electric motors around. Here's the answer: the spark of the electric motor can produce ozone, causing a high concentration of ozone in local small environments; The material selected by the customer is nitrile rubber, which is not resistant to ozone. In order to verify the conclusion, we tested it in the ozone aging box in the laboratory, and the results showed that similar cracks appeared on the surface of the new O-ring provided by the customer. Because the seal is only in long-term contact with air and there is no mineral oil and other substances, we finally recommend EPDM to replace the existing products of customers

sometimes the use environment is complex, or it is a new design, so it is not very easy to choose materials. In addition to carefully identifying various influencing factors, functional tests are also required

3. Seal quality

the production quality of seals is closely related to the reliability of the final product. Common problems include: unstable quality of raw materials, wrong feeding of raw materials during rubber mixing, improper storage of raw materials or rubber compounds (cross contamination), uneven mixing of rubber compounds, improper vulcanization conditions (temperature, time, pressure, etc.), improper storage of seal products, improper use of molds, etc. These problems often involve quality control in the production process. When choosing the seal manufacturer, the orderer should go through many inspections, investigations and product tests. In the process of supply, the manufacturer of the seal can also be required to provide a true and accurate inspection report

4. Improper use of seals

a good seal, if used improperly, will also cause the whole product to fail, such as the wrong use of lubricating oil

one of our customers reported that the size of O-ring parts was very different from the requirements. When we analyzed the sample, we found that the customer used the wrong lubricant. The O-ring is made of EPDM, and the material itself is not resistant to mineral oil lubricants. If the customer applies this kind of lubricant, the volume of the product will swell. Later, it was no problem for the customer to use silicone oil

another common problem is installation error. For example, distortion occurs during the assembly of O-ring; Uneven compression of seals due to improper installation; Insufficient lubrication of seals and other errors. If these errors are caused by the manufacturer of the seal, it is within the scope of production quality control. If it is caused by the ordering party or a third party, it is improper use

reasonable design is one of the important factors to ensure the effective use of seals

(the picture shows the umbrella check valve seal produced in Minnesota.)

with the rapid development of science and technology, there are many new seal materials and engineering designs in the market to meet various demanding use requirements. At the same time, most seal manufacturers also adopt scientific quality management system. Unfortunately, these efforts cannot prevent seal failure. Therefore, failure analysis is more important

failure analysis of rubber seals

failure analysis of rubber seals requires technicians not only to have comprehensive material knowledge, but also to have rich engineering experience, and to be good at using some analytical instruments and equipment. Therefore, complex failure analysis often needs a team to complete

when a customer's product has sealing failure, the first reason they think of is often the rubber seal. The most common question they ask is: has the supplier changed the material? In fact, it can be seen from the above that material replacement is only one of the possible influencing factors. Therefore, when encountering sealing failure, we should start from the following four aspects. The relevant person in charge of the National Standards Commission recently said that he had a comprehensive grasp of the information and made conclusions based on the data rather than guesswork

1. Choose a suitable organization to conduct failure analysis of seals

in reality, customers often choose the supplier of the rubber seal to do failure analysis, or find a well-equipped University Laboratory (or professional testing company) to do it. However, the failure analysis of rubber seals requires technicians not only to have comprehensive material knowledge, but also to have rich engineering design experience, and to be good at using some professional analytical instruments and equipment. Only senior rubber and plastic component manufacturers have such technical strength. In addition, some customers' technical secrets may be involved in the analysis, so it is best to find a reputable company to analyze

2. Collect background information

when encountering sealing failure, the following background information should be collected first to preliminarily determine whether the failure is related to the material:

· who is the manufacturer of the seal; What is the name of the compound (the customer may make a mistake about the supplier)

· the batch number or order number of the seals used in the failed products, which can be easily queried in the future. If the seal failure is only related to a certain batch, it is likely to be caused by some changes in the production of the seal

· how was the seal failure found? What are the conditions for the use of seals (temperature, pressure, medium, frequency of use, etc.)? If the structure is special, drawings or photos of assemblies may also be required

· what proportion of failed products (%)

· if possible, please send back some samples: invalid seals, seals without failure, and seals not used in the same batch number. In this way, comparative tests will be conducted in the future. It is worth noting that some samples may contain toxic substances. Customers and relevant personnel must be reminded to take protective measures and send samples properly

after obtaining this information, analysts should be able to establish corresponding files. First, follow up the investigation in the future; Second, it can provide clues for similar cases in the future

3. Preliminary analysis

if the failure analysis is carried out by the seal supplier, it is best to do some simple material analysis after obtaining the sample. The distance of the corresponding angle is not less than 10% of the original design value (such as hardness, tensile strength, permanent compression deformation, specific gravity and other physical property tests and dimensional inspection), so as to reconfirm that the product is indeed its own product. Because many rubber parts look the same (such as O-rings)

we have such experience. Customers have repeatedly emphasized that it is our product XYZ. However, after material analysis, the composition and performance of the samples are quite different from our XYZ products. At the same time, these test results can also be used to judge whether there is a problem with the material

sometimes some features can be found on the failed seals. Experienced technicians can preliminarily determine the cause of failure based on these characteristics. In addition, in the vicinity of failure features, it is also necessary to observe whether there are abnormal conditions such as foreign bodies, bubbles, flow marks or burrs. These fiber stresses - the stress characteristics of a point on a part with uneven stress distribution are often related to the production quality of seals. It can be observed by naked eye, magnifying glass, microscope and electron scanning microscope (SEM)

according to the above clues, we can preliminarily identify the general cause of failure, and if necessary, we can further arrange other analysis and testing

4. In depth analysis

if the preliminary analysis cannot solve the questions about materials, then some more precise chemical analysis methods need to be used

common thermal analysis techniques include differential scanning calorimeter (DSC), thermogravimetric analyzer (TGA), dynamic mechanical analyzer (DMA) and thermomechanical analyzer (TMA) and spectral methods such as Fourier transform infrared spectrometer (FTIR). Most of these analytical instruments are expensive, but generally the analytical results can be obtained within a few days

differential scanning calorimeter (DSC) is often used to test the glass transition temperature of rubber materials, and can be used to check the low temperature resistance of rubber materials in failure analysis

thermogravimetric analyzer (TGA) is often used to study the components of materials, so that we can quickly and indirectly see whether there is a problem with some components

dynamic mechanics analyzer (DMA) is often used to test the stiffness and damping of materials under certain conditions, so as to judge whether the seal is suitable in the actual use environment (such as impact resistance)

Fourier transform infrared spectroscopy (FTIR) is often used to determine the molecular composition and structure. In failure analysis, it can be used to determine what the rubber itself and some components are, the foreign material composition on the seal, and the seal hair in use

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