Author: Marjolein de Wit-Blok
Not infrequently, the specialists of NDS-Seals – supplier of mainly rubber seals – receive a design with the statement that ‘we also need a seal in it’. Or you can give some advice. That is always possible. But it is better to take the function and location of a seal into account during the design phase. This often prevents an unnecessarily expensive production process of an exotic seal and also contributes to the quality of the final solution. A set of basic rules for the constructor.
Seals are not treated separately at many technical training courses. Of course mentioned. If something should not leak – whether it is a gas or a liquid – then it is indicated that the engineer in question ‘must apply a seal’. It is left open that the design or construction is crucial for the functioning of a seal and that there are countless different variants of which only a limited number are standardized – including the O-ring. If the design does not take into account the available standard products, then developing a customer-specific seal is the only option.
“In some cases unnecessarily expensive”, Henk Jimmink of NDS-Seals knows. Jimmink has been a specialist and advisor at NDS-Seals for years and knows better than anyone where things can easily go wrong during the design process in the context of rubber seals. “When you know the properties of the material rubber and you understand how a seal should function, then it is not that complicated to make a good design. If this knowledge is lacking, however, you will soon find yourself in a situation in which an application fails. Usually the seal is blamed while ultimately it is the combination of the design and the seal that determines how successfully the function of ‘sealing’ is fulfilled.
Material and process variables
With the above, Jimmink already indicates that the design and choice of a seal involves two main variables: the material and the shape. The designer has to deal with the three application related variables: pressure, temperature and medium. The chosen seal must be able to withstand these three factors and must be optimally adapted to them. For example, silicone is micro porous and therefore gas permeable, while butyl rubbers are extremely gas-tight. There are also many different types of rubber, each with its own maximum temperature resistance and differences in chemical and UV resistance, among other things.
Furthermore, the rubber and the medium that flows along it must be able to endure each other. If it is not a good combination, the rubber can be affected and it is also conceivable that ingredients of the seal will end up in the medium, which is generally not desirable. Sometimes a specific medium can still give a positive spin to the applicability of a seal. For example, the maximum temperature resistance mentioned in the specifications is measured in air. However, when the rubber is in oil, for example, it is better protected against oxidation (reaction with oxygen) and it is possible to work with higher temperatures. With silicones this works the other way around. This material needs oxygen in order not to degrade.
Furthermore, the choice for a certain rubber seal depends on the answer to the question whether the application is static or dynamic, for example Rubber is mainly suitable for static applications where open/closed is probably the best known. This is because rubber has a certain memory which means that it returns to its original form after compression. An ideal property for open/closed constructions.
In short: to make the right choice, the designer must be well aware of the environment and function for which this seal is used. And this information is not always available. Jimmink: “Of course, the question is whether this is a task for the designer, but if these quantities are not known, he would do well to follow them. Without this information, you will have to make certain assumptions that may cause the seal in the final solution to be over- or under-dimensioned or of the wrong type of rubber.
Jimmink notes that the average manufacturer has relatively little knowledge of rubber in relation to seals. “Nevertheless, this is important in order to come to a good solution. Know, for example, that rubber is – compared to plastics – a so-called ‘living material’ and that the qualities differ per supplier. This may seem like an open door, but it means, among other things, that it is not wise to change suppliers without first thoroughly testing the new seals. They may well be satisfactory, but there is certainly a chance that the properties will vary to such an extent that the rubber, for example, will still be affected by the medium or will be less resistant to certain temperatures. In fact: the specifications of a type of rubber can be identical at two suppliers and yet differ from each other. For example, in the life span.
This ‘living’ character has to do with the production process of rubber in which a so-called vulcanization process takes place. This involves creating cross-links between different polymer chains and is in fact the phase in which the properties of the rubber are newly formed. Once the rubber is ready, it must first cool down, causing shrinkage. In short: chemically speaking, the production of rubber is very complex and in that respect the final result cannot be predicted for 100%.
Jimmink: “We actually see rubber as a cake: you always bake it in the same way, but in the end it is always different. When choosing a rubber seal, the Constructor must always take into account the tolerances of the final seal of a few tenths of a millimetre into account.
These tolerances also apply to the most common error encountered by the specialists at NDS-Seals: a construction in which the seal has too little space. JaapJan Berghuijs is also a specialist in seals for, among other things, HVAC installations and indicates this: “It’s very simple: Contrary to popular belief, rubber is not compressible. This means that the material must remain somewhere when pressed in and must have the space to do so. If that is not possible, then there is no good sealing. One common mistake in that context is that more sealing material is added when the seal is not satisfactory. Actually the worst solution you can think of.
If a good seal does not occur, it is wiser to first look at the construction of the product and the shape of the seal. There is a good chance that the solution can be found there. Especially when construction is ‘at the cutting edge’. If certain assumptions are made with regard to tolerances that are incorrect, then the seal does not work. Personally, I prefer to work with a view to the worst case scenario. Also with regard to temperature, for example. If you have an application where temperatures reach 90 °C, you can of course choose a seal with a temperature resistance up to 100 °C, but perhaps better to choose a seal with a higher temperature resistance because it simply lasts longer. You should then consider whether the additional cost of the seal outweighs the costs of replacing the seal in a maintenance round.
When constructing the space in which the seal is placed, it is also good to know that with standard mass-produced O-rings, there is often still a membrane in the form of a partial seam on the outside as a result of the mould used. This fleece can possibly be removed by ‘drumming’, but this means additional processing (costs) and sometimes seals are so sensitive that drumming is not possible because it would damage the seal. Seals are now also available where the partial seam is at a different location and the construction suffers less from it, but also there this different design can be taken into account.
Finally, the surface of the room in which the seal must fulfil its function: not too smooth and not too rough is the motto in most cases. A surface that is too smooth can lead to stick-slip effects in dynamic applications. Especially in highly dynamic applications (speeds up to 3,000 rpm) this is an item; however, an oil film can provide relief here (note that the oil and the rubber are compatible). A slightly rougher surface gives the best results with static seals because the rubber is pressed into the pores of the material. A smooth surface is desirable for vacuum applications.
With regard to the construction in connection with seals, it is also wise to look at the method of assembly. For example, by making a construction symmetrical, the seal can often be inserted in different ways, which shortens the assembly times. If, on the other hand, the seal can only be installed properly in one way, it is preferable to make the construction in such a way that this is only possible in one way. Furthermore, assembly times can be shortened by combining certain seals. This sometimes leads to a more expensive seal because it is more complex, but it can outweigh the savings resulting from a shorter installation time.
A point of attention is also the packaging of the seals. Practice has shown that rubber is a difficult material to process automatically. Sometimes the products stick slightly together and measures are taken by adding, for example, silicone oil. It is then important to determine whether this oil is permitted in the production process concerned. In the paint industry, for example, absolutely no silicones are permitted.
Finally, the seal must also fit the material of the construction. Rubber between two aluminium parts is not a problem, but when aluminium and stainless steel are combined, stress corrosion can occur and this can affect the rubber and thus affect the good seal.
When a specific seal is chosen, companies that will use the seals for large series should test regularly. For example on the memory function. This is possibly a compression set in which the seal is pressurised and must return to its original state after this pressure has been removed. The more often and better this is done, the higher the quality of the seal.
Berghuijs: “As a supplier of rubber seals, we test a lot in any case, accurately recording and checking the ‘DNA’ of each type. Even then, you cannot give a 100% guarantee because of the complex vulcanization process explained earlier, but you can go a long way further. This certainly applies when you want to try new compositions. New compounds of rubber are not very common nowadays, but there are many experiments with compounds of different materials. For example, a combination of rubber with plastic or rubber with a certain metal. The metal is then usually the carrier of the final sealing rubber.
Finally, I recommend that designers who have less experience with seals should have their design assessed by specialists. This can save a lot of problems not only in the production phase but also during use.”