Failure Analysis of Rubber Seals

Failure Analysis of Rubber Seals

There are four common reasons for failure of rubber seals: design errors, material selection errors, seal quality problems and improper use.

1. Design error

Design errors are usually caused by the designer''s lack of understanding of the product. For example, the pressure of sealing parts is not estimated enough, the understanding of contact stress distribution on sealing face is wrong, and the groove design of sealing parts is unreasonable. Finite element analysis (FEA) is often used to assist seal design and failure analysis. We once made a seal for an American customer. The seal was made of plastic and partially covered with rubber. In the process of testing the parts, the customer found that the plastic parts were easy to break in the test, and it was concluded that the plastic parts were in the secondary forming (that is, rubber) The glue was coated on the plastic parts) was damaged. After our analysis, we found that plastic parts are all broken in one place. Through finite element analysis, we find that the damaged part of plastic part is actually the place where the seal is subjected to the maximum stress, and the stress here is far more than the plastic can bear.

If the customer analyzes the product with finite element method at the time of design, it can not only avoid similar mistakes, but also save time and money. Of course, in order to successfully analyze and predict the performance of rubber seals, it is necessary not only to have suitable finite element analysis software, but also to have rich material experience, modeling experience and long-term data accumulation.

2. Material selection error

The commonly used rubber sealing materials are ethylene propylene diene rubber (EPDM) (EPDM), acrylonitrile butadiene rubber (NBR),) silicone rubber (VMQ), fluororubber (FKM or FPM) and Chloroprene rubber (CR) and so on. These rubber have different properties and different applications.
The selection of materials should be considered from a variety of aspects, such as use of temperature, material resistance to media, hardness, compression permanent deformation and wear resistance and other factors.
Material selection errors are often due to the designer is not familiar with the performance of various materials. An experienced rubber seal supplier can point out the problem of material selection from the beginning.
We have a domestic customer who doesn''t like the O circle in use because it''s easy to break. We examined the replacement sample and found that the surface of the sample was cracked and the grain was much like ozone aging. We also asked about the O-circle environment and found a lot of mechanical equipment and electric motors around. The answer is: the spark of the electric motor can produce ozone, resulting in high ozone concentration in the local environment; and the materials chosen by the customer are nitrile rubber, which is not ozone tolerant. To verify the findings, we tested them in the laboratory ozone chamber and found that the new O-ring surfaces provided by the customer were similar. A crack in the Because the seal is only air contact for a long time, no mineral oil and other substances, we eventually recommended EPDM to replace the customer''s existing products.
Sometimes using a complex environment, or a completely new design, it is not very easy to choose materials. In addition to the careful screening of various factors, there is also a need for functional testing.

3. Seal quality

The production quality of sealing parts is closely related to the reliability of the final product. Common problems include unstable quality of raw materials, misfeeding of raw materials in rubber mixing, improper storage of raw materials or blends (cross-contamination), uneven mixing of rubber materials, vulcanization conditions (temperature, time, etc.) Improper pressure, improper sealing products, improper use of dies, etc. These problems often involve quality control in the production process. The ordering party should inspect, investigate and test the sealing parts during the selection of the factory. During the supply process, the manufacturer of the seal may also be required to provide the truth. Accurate inspection report.

4. Improper use of sealing parts

A good seal, if used improperly, will also cause the entire product to fail, such as lubricating oil use error. One of our customers feedback that the O-ring parts size and requirements vary greatly. When we analyzed the sample, we found that the customer had used the wrong lubricating oil. The O-ring is made of EPDM, the material itself is not resistant to mineral oil lubricating oil. The customer applies this lubricating oil, will cause the product volume swelling. Later, the customer switch to silicone oil is not a problem.

Another common type of problem is an installation error. For example in the O-ring assembly process produced distortion; due to improper installation of the seal due to uneven pressure; seal lubrication is not enough and so on. If the failure is caused by the manufacturer of the seal, it is under the control of the production quality. If caused by the order party or a third party, then it is improper use.

With the rapid development of science and technology, many new sealing materials and engineering designs appear in the market to meet all kinds of harsh requirements. At the same time, most seal manufacturers also adopt a scientific quality management system.
Unfortunately, none of these efforts can stop seals from failing. Therefore, failure analysis is more important.
Failure Analysis of Rubber Seals
Failure analysis of sealing parts not only requires technicians to have comprehensive knowledge of materials, but also has rich engineering experience, and must be good at using some analytical instruments and equipment. Therefore, complex failure analysis often requires a team to complete.
When the customer''s product seal failure, they often think of the first reason is rubber seals, they most often asked the question is: has the supplier changed materials? In fact, as can be seen from the above, material replacement is only one of the possible factors. Therefore, when the sealing failure occurs, we should grasp the data from the following four aspects, and draw the conclusion based on the data rather than the conjecture.

1. Selection of appropriate mechanisms for failure Analysis of Seals

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

2. Collect background information

In the event of a seal failure, the following background information should be collected to initially determine whether the failure is related to the material:

Who is the manufacturer of the sealing parts and what is the name of the compound (the customer may get the wrong supplier).

The batch number or order number of the seal used in the invalid product, which can be easily queried later. If the seal failure is only related to a batch, it is likely to be caused by certain changes in the seal production.

How was the failure of the seal found? What are the conditions for sealing (temperature, pressure, medium, frequency, etc.)? If the structure is special, you may also need a picture of a drawing or assembly.

The percentage (%) of failed products.

If possible, please send back some samples: failed seals, unexpired seals, and seals not used in the same batch number. This makes it easier to do comparison tests later. It is important to note that some samples may be toxic and must be alerted to customers and relevant personnel to take precautions and send the samples properly.

After getting this information, the analyst should be able to create the appropriate files. It is easy to follow up the investigation in the future, and can provide clues for similar cases in the future.

3. Preliminary analysis

In the case of failure analysis by the seal supplier, it is best to do some simple material analysis (such as hardness, tensile strength, permanent compression, specific gravity, physical properties and size checks) after obtaining the sample. To reconfirm that the product is indeed its own product. Because many rubber pieces look the same (for example, O circle).

We have the experience that our customers have repeatedly stressed that it is our product XYZ. However, after material analysis, the composition and properties of the sample are very different from our XYZ products. At the same time, these results can also be used to determine whether there is a problem with the material.

Sometimes some features can also be found on the failure seals. Experienced technicians can initially determine the cause of failure on the basis of these characteristics. In addition, in the vicinity of failure characteristics, note whether there are foreign bodies, bubbles, flow marks or margin anomalies. These features are often related to the quality of seal production. It can be observed by naked eye, magnifying glass, microscope and electron scanning microscope (SEM).

Based on these clues, we can preliminarily identify the general causes of the failure and , if necessary, further arrange for other analytical tests.

4. Depth analysis

If preliminary analysis does not solve the material problem, some more sophisticated chemical analysis is needed.

Some common thermal analysis techniques such as differential scanning calorimeter (DSC),) thermogravimetry analyzer (TGA),) dynamic mechanics analyzer (DMA) and thermo-mechanical analyzer (TMA) and Fourier transform infrared spectrometer (FTIR) are used. Most of these analytical instruments are expensive, but the results are generally available within a few days.

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

The thermogravimetric analyzer (TGA) is often used to study the composition of materials.

Dynamic Mechanical Analyzer (DMA) is often used to measure the stiffness and damping of the material under certain conditions, which can be used to determine whether the seal is suitable in practical use (such as impact resistance).

Fourier transform infrared spectroscopy (FTIR) is often used to determine molecular composition and structure. In failure analysis, it can be used to determine what rubber itself and some components are. What chemical changes have taken place in the sealing parts in use, etc.

These technologies were used only for basic scientific research at universities and research institutes. Now, in rubber industry, especially in some high-end technology enterprises in Europe and America, these technologies have been successfully used in the failure analysis of seals. Many of these techniques have been discussed and used in more detail in many literature, which will not be restated here.

In addition to experience and various tests mentioned above, finite element analysis (FEA) is often used to assist the failure analysis of seals. The example given above in "Mechanical Design errors" is a good example.

5. Field visit

An important part of failure analysis is field inspection of the seal production process or the installation and use of the user. Because sometimes a laboratory analysis alone can''t lead to a final conclusion.

For example, after laboratory analysis, sealing materials may be contaminated. To confirm this speculation, it is best for the analyst to visit the manufacturer to see if there are any loopholes in the operation and to sample each possible production link. If the field sample verifies the results of the analysis, then the diagnosis can be made. The results can help manufacturers improve their production processes to avoid similar problems in the future.

For example, we once had a product that failed to live up to its expected life expectancy. After laboratory analysis and inspection of the seal factory, we still can not find out the reason. Finally, after visiting the customer''s assembly line, our technicians found that one of the components used in the assembly process had burrs and the rubber parts were scratched during assembly, which resulted in premature failure of the rubber seals in use. After the customer changes this kind of component, the rubber seal completely meets the use request.

To sum up, failure analysis of rubber seals is often a complex task. Technicians need not only rich experience and knowledge, but also careful attitude, perseverance and logical reasoning ability. At the same time, failure analysis also has certain requirements for hardware, such as analytical instruments and equipment. Of course, a good product can not do without high-quality rubber machine, Qingdao Running Machinery Co., Ltd. can provide you with high quality and efficient rubber mixing, rubber vulcanization machine.