Sealing and truth – what mistakes can be made when designing seals

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Abbildung-1-Der-Einfluss-der-Farbe-auf-die-Shorehärte-1

For a building sealing we had the order to develop a seal in such a way that the tightening torque of the screws is indicated by the squeezing of the seal during assembly on the construction site.
This is understandable, as construction workers do not usually have a torque wrench in their construction trailer.

Based on rough models, in which we estimated the hyper-elastic (incompressible) properties of the rubber via the Shore hardness 50 Shore A for the first simulation (FEM), the feasibility could be shown.

For the exact design we commissioned various tests to determine the material properties of the sealing material used. For this we received black sheet material, also 50 Shore A, from the customer. We then fitted the results as a material model in our FEM software (in this case Abaqus) and used them in the simulation.

On the base of variants, we have designed the function of the sealing material in such a way that a viewing window has just been filled by the swelling rubber when the desired tightening torque of the screws is reached. The bolt preload force was determined in such a way that the contact pressure of the sealing material between the pipes and the borehole is higher than the pressure of the ground water.

The prototype worked as desired and the product went into series production.

The surprise was great when our customer contacted us with the bad news that the first serial parts did not work because they were leaking.

Image 1 The influence of colour on Shore hardness

We then requested further sample material and were shocked: the delivered plate material was squeaky yellow and soft. For the series, a dye had been added to the previous material to improve the visibility around pinch windows. Unfortunately, this worked like a plasticizer and the Shore hardness dropped to 30 Shore A.

Lessons learnt

This example shows impressively what has to be considered when designing seals with the help of FE simulations: Not all rubber is the same!

A derivation of the material model based on Shore hardness is a rough approach and is suitable for feasibility statements. For a correct design, measured material parameters are required to create the material models for the FE calculation.

Additives change basic properties of the elastomer. Therefore, a rubber calculation must be carried out with a material model which not only corresponds to the correct material, but which is also adapted to the load condition. Therefore, we do not carry out the tests ourselves, but we have them under control.

Apparently similar materials often have fundamentally different properties.

Although the surrounding medium, temperature and possibly frequency under dynamic load did not play a role in the above example, they must be considered if necessary.

A few more basics

In technology, seals are used to prevent or limit unwanted mass transfers from one place to another. Sounds swollen, but it hits the mark quite well.

The substance that should normally be prevented from passing is e.g. water, oil, air, gas or any other liquid or gaseous medium.

In the case of the refrigerator door, for example, cold and warm air should be separated.

The sealing media used are rubber, elastomers, PTFE, but also metallic seals or carbon.

Seals are usually softer than the components to be sealed to compensate for differences. They are pressed together, whereby the contact pressure must be at least as high as the pressure of the medium to be sealed or, in the case of a vacuum, the ambient pressure.

How we can help you efficiently in the design of seals is best clarified in a short and non-binding discussion. Please send us a short mail. We will then get in touch with you.

Your Stefan Merkle

PS: And another rule of thumb that has accompanied me for decades: After 10 years, only 25% of the preload from the assembly line is left. So also take ageing into account. After all, your machine should still be leak-proof when it ages.

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Stefan Merkle

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