Components Seals The very highest levels of precision, even with large diameters A special endless vulcanisation production method makes it possible to produce precision O-rings, even in extreme special sizes in accordance with ISO 3601. Dipl.-Ing. (FH) Michael Krüger The O-ring is the world's most frequently installed seal. But what should the user do if large-scale installations need to be sealed? In various sectors there is demand for "XXL O-rings", and the market generally has only technically limited solutions to offer here. It can be very unsatisfactory for a design engineer or user when standard precision O-rings can no longer be used in an application over a particular size because the market simply has no high-quality technical solutions to offer. Technical downgrading is a bad choice in these cases because installing a technically inferior seal will either not satisfy the requirements of the application or mean a drastic reduction in the service life of the seal and a shorter replacement interval. In sealing technology, a distinction is made between O-rings made of extruded cord and precision O- rings. Both are used across a very wide range of industrial sectors. But only precision O-rings are capable of satisfying the most stringent requirements. However, economically viable production has to date only been possible up to a maximum diameter of approx. 1,400 mm. An innovative method now means that it is possible to produce larger diameter O-rings at a fair market price. Precision O-rings are seals with a circular cross-section which are made in special tools that use compression or injection methods to vulcanise carefully calibrated rubber mixtures. This makes it possible to produce O- rings within relatively narrow production tolerances and with good surface properties. The precision O-rings made in this way are graded in accordance with the ISO 3601 only this, but it is also extremely difameter of more than 1,400 mm. Not standard with the appropriate grading, which is either N or S. Based on and hard to carry out the subsequent ficult to handle such big tool moulds previously defined vulcanisation parameters, which can be exactly ad- diameter in a manner consistent with work on the O-ring across its entire hered to, compression and injection the standard. This is why most manufacturers do not offer O-rings with a methods can be used to manufacture O-rings which demonstrate invariably diameter of more than 1,400 mm. high mechanical quality levels across their entire circumference. It is only Growing demand with this high quality level that good sealing values can be achieved over a The demand for high-quality precision O-rings with internal diameters long period in actual use. However, this method does not over 1,400 mm has, however, steadily increased in recent years. The mar- make it possible to produce O-rings of any size economically. This is because ket is essentially dominated by two of the enormous amount of work and methods: to create O-rings of the required size, extruded round cords are associated expense required to make a tool which would bear no relation joined, either through gluing or shock to the reasonable market price that vulcanisation of the ends of the cords the manufacturer of this kind of precision O-ring would be able to ask the In the case of glued O-rings, the cord in a process that is mostly unreliable. customer to pay. This generally applies to any O-ring with an internal di- shock vulcanised O-rings, the cord ends are joined using adhesive. With Fig. 1: O-ring made using the endless vulcanisation method (Photo © : COG) 106 PROCESS TECHNOLOGY & COMPONENTS 2022
Components Seals either not at all or only to a very limited extent. Innovative production method Fig. 2: 4000 l vacuum chamber (type: process chamber for PECVD) from EDIS Anlagenbau GmbH (Photo © : EDIS Anlagenbau GmbH) The independent producer C. Otto Gehrckens has established an additional production method alongside compression and injection processes. This special endless vulcanisation production method makes it possible to produce precision O-rings with an internal diameter of up to 3,000 mm. These comply with the specifications of the ISO 3601 standard for precision O-rings. The company is currently offering O-rings produced using endless vulcanisation in various FKM, HNBR and NBR qualities with internal diameters ranging from 1,400 to 3,000 mm. Greater cord thicknesses or larger internal diameters are also possible by arrangement. In this way the company has already succeeded ends are held together in special devices and then hot vulcanised with a suitable adhesive mixture. The disadvantages of these two processes are the significantly inferior physical properties of the shock vulcanised/glued area. For instance, the adhesive never has the same physical or chemically resistant properties as the seal material itself. With shock vulcanisation, this area of the material likewise has properties that differ from, and are not of the same high quality as, the rest of the O-ring. In technically challenging applications, this area is declared, as it were, to be a predetermined breaking point. A further disadvantage which is not insignificant for the user lies in the significantly greater tolerances of the round cords which are glued together or shock vulcanised. Compared to compression-moulded precision O-rings in accordance with ISO 3601, these round cords reveal clear differences in dimensional stability and surface composition. This is because the process used in the production of round cords necessarily entails higher tolerances, due to the fact that the cord thickness increases at the point of exit from the extrusion jet and shrinkage and a certain resultant amount of deformation generally Fig. 3: Steam steriliser (Photo © : iStock_TotoRuga) occur during the subsequent vulcanisation process. This results in inferior sealing properties, particularly in those seals which are intended to have a longer service life. Moreover, in producing an FKM O-ring with an internal diameter of 6,000 mm. The O-rings manufactured using this endless vulcanisation method thus correspond to precision O-rings with the shock vulcanised or glued points smaller dimensions manufactured do not have the same properties as the material itself, and the tolerances are even greater at these points. In many areas of application with extreme demands on the O-rings, such as high vacuum applications, or in the case of gaseous media or dynamic sealing, they can be used using conventional techniques. Compared to glued or shock vulcanised round cords, these O-rings have noticeably improved mechanical properties without exception around the entire circumference of the O-ring, properties which one otherwise only gets with compression-moulded O-rings. PROCESS TECHNOLOGY & COMPONENTS 2022 107
