Sealing problems - Solutions in product design and choice of material
Jaap Havinga, TNO Department of Polymer Technology
Seals are often seen as a necessarily bad thing in constructions. Therefore, many times, the design of the seal is delayed until the rest of the construction is developed. As a result the boundary conditions for the seal are such that a proper and easy design is no longer possible. In cases where insufficient sealing is discovered during testing of the prototypes it is difficult to get a solution within the time limits set for the product development. Also there are several cases known were the malfunctioning of the seal was discovered even after the product was taken in practical use. What also means after making the molds and taking the product in production. At that time only costly solutions are possible because they are likely to lead to adjustments of the total product design. Insufficient knowledge of the product designer of rubber properties, especially the long term behavior, and not enough asking about the real application of the producer or supplier of the rubber seals, is mostly the basis of these problems. This means that such problems are, in almost every case, possible to avoid. In case of discovering the problems during prototype testing it might be possible by redesigning the seal and its material to avoid adjustments to the other product characteristics. This means that sometimes it is possible to stay within the time limits for first delivery. Although in this stage small adjustments to the product might well be possible in order to improve the solution. In case the malfunctioning is discovered after the product is actually taken in use, problems become more complicated. Especially were sophisticated machines are involved, the solution should fit in the same space were the original, wrong, seal was located. Otherwise all machines marketed already have to be replaced. Failure at this stage is mostly the result of underestimating the long term behavior of rubber, like stress relaxation properties.