GASKETS - THE CONCEPT & THE MATERIALS

Wherever two uneven and unyielding surfaces come together in a system, and it is necessary to keep what is inside the system in, and what is outside the system out, then a suitable gasket will be required.

The gasket is needed to provide a yielding but resilient interface to take up the anomalies in the hard surfaces and thereby effect a seal.

In order to perform, the gasket will need to satisfy several criteria:

Most obviously the gasket will need to conform to the flange to which the gasket is to be fitted. The gasket itself will need to be made from a suitable material able to withstand the physical conditions such as temperature and pressure which are present both inside and outside the system to be sealed, and the material from which the gasket is made will also need to be capable of withstanding potential attack by the chemical medium being contained.

All this has to be achieved within the parameters of the design of the flange in particular and the whole system in general.

The following brief outlines show how these criteria may be satisfied from a range of available gasket materials. There are, of course, many more gasket materials available than can be discussed here, but our customers can rest assured that we will continue to maintain our awareness of new materials and use the unique blend of experience, enthusiasm and training within our organisation to keep our products up to date.

RUBBERS:

Rubber is an ideal gasket material in many ways. It is easily cut to profile from sheet form and can also be moulded into the most complex of 2 or 3 dimensional shapes. It is soft and highly resilient. Chemical, temperature and pressure resistance vary dependant on grades and design of the gasket or seal. Nitrile and Neoprene gasket grades generally have good oil and fuel resistance up to maximum temperatures of 120oC. Silicone gasket material grades will exhibit similar or enhanced properties up to a maximum of 180-220oC dependant upon the quality of the compound. Viton Rubber is an expensive grade for high temperature gasket applications where a high degree of chemical resistance may also be required. Other grades are available for use in more specific applications, e.g. against ozone.

FIBRE BASED MATERIALS:

Fibre Based Gasket Material likely to be encountered will fall into one of three categories. Calender produced gasket materials are made by curing a blend of fibrous materials into a matrix of rubber compound whilst under constant pressure. Typically the resultant gasket materials will consist of around 70% fibre by volume. Prior to current legislation the fibres were almost invariably asbestos, but modern materials utilise a range of fibres including glass, cellulose and aramid fibres. The rubber matrices may be Nitrile compounds with high oil resistance, or Styrene Butadiene grades which will swell in contact with oil. Blending of these rubber grades combined with a calculated under cure result in the controlled swell grades common for gaskets used in the motor trade which swell to a perfect fit in situ on initial contact with oil before the heat in the system completes the cure. Beater addition gasket materials contain similar ingredients to calender produced materials but are made on a continuous process similar to the paper making process. The fibres used are generally shorter and the cure is not achieved under continuous pressure. The resultant materials are generally less expensive and more compressible, but physically less robust than for calendered gasket materials. The physical characteristics of both calendered and beater addition gasket materials are largely dictated by the type of fibre included whilst the compound chosen for the matrix will be largely responsible for the chemical resistance of the resultant material. Paper is one of the least expensive of all gasket materials. It can be treated with a glycerine based mixture of compounds to enhance resistance to oils and glycol. It is easily cut and exhibits excellent sealing properties in a wide range of applications. The use of paper in the production of cut gaskets is severely restricted by the inherent instability of the material in storage. If the moisture content of the paper is allowed to change then the gasket will grow or shrink accordingly to such an extent that it cannot be fitted.

CORK:

Cork is one of the more traditional gasket materials which is being used less frequently in modern systems. It is an ideal gasket material being highly compressible yet resilient and almost totally inert. Low tensile strength and the inherent instability of the material under climatic changes render pure cork impractical in most cut gasket applications. Cork is sometimes stabilised and strengthened by laminating with aluminium, but this improvement in physical characteristics is more commonly achieved by bonding the cork granules in a rubber matrix. The resultant materials are then dependant upon the matrix used for their temperature and chemical resistance. The matrices will normally be Nitrile, Neoprene or Silicone compounds. Cork granules are also used occasionally as a filler to improve the economy and compressibility of moulded rubber gaskets.

LAMINATES:

The physical properties of relatively low strength beater addition materials can be dramatically improved by laminating them to either side of a steel core which has been pierced in such a way as to leave tangs which key into the soft face. The pressure resistance of the resultant laminates are massively enhanced and the ability of the steel core to dissipate heat also adds to the temperature resistance of the material. In applications where flange movement is likely the laminate can be made using two steel facings on either side of a soft core. The steel facings allow the flanges to slip over the surface of the gasket without disturbing the seal facilitated by the soft core. Our manufacturing company is one of only a few companies in the UK with the facility to produce tanged steel and fibre laminates for Manifold and Cylinder Head Gasket applications. Our laminating line produces fibre faced or fibre core material 500mm wide on a continuous process. The fibre faced material is available in rolls or sheet. The fibre core material is only sold in sheet form as it cannot be re-rolled after the second steel face is applied. Both materials are also available as gaskets which are cleanly cut and well finished.

OTHER MATERIALS:

There are so many materials available from which gaskets may be cut or moulded that it would take a volume of encyclopaedic proportions to discuss them all. Only the more common or traditional materials are covered above. There are materials which make use of the plastic flow characteristics and heat resistance of pure graphite to produce gaskets capable of superb flange conformability at extreme temperatures. Even higher temperatures can be achieved by gaskets made from mica based materials, whilst the chemical resistance of some more inert plastics, most notably PTFE, is widely used in gaskets which are intended to operate in the most aggressive of environments. The list goes on and on, but all these materials and more are available to us from highly reputable suppliers, most of whom continue to manufacture in Europe or North America.

SILICONE BEADED GASKETS (MAGSIL):

Silicone beading is applied to finished gaskets to enhance sealability in critical or low bolt load areas, or is applied to an inexpensive substrate in order to utilise the excellent sealing properties of silicone without incurring the high cost associated with this material. Using a unique mix of modern and traditional technologies we are able to produce gaskets from a variety of substrates subject to the composition of the material with a flood coat or bead of silicone applied. Some gasket materials however contain compounds which will not enable this process to be used. We recommend discussion with our technical staff at an early stage of design.

CYLINDER HEAD GASKETS:

Cylinder Head Gaskets are made using our own laminates and are supplied finished in a variety of surface treatments, applied using the silicone beading and coating technology described above. Our engineers have developed a tooling system for the manufacture and fitting of bore reinforcing eyelets which enables us to consider the introduction of new designs for quantities as low as 500 pieces.

CORRUGATED STEEL GASKETS:

The restrictions on cost and weight in modern bolted systems and the state of fastener technology mean that bolts are being designed to operate at the absolute limit of their elastic properties. In order to achieve the correct bolt loading, which is critical to some bolted systems, it is necessary to be able to obtain accurate torque readings when the bolts are being fitted. The introduction of an elastic layer i.e. a traditional soft gasket into the system can cause false torque readings and impair the performance of the bolts due to incorrect loading. To ensure proper bolt loading the gasket needs to depart from traditional ideas on sealing. A gasket is required which will begin to seal as the flanges are tightened and stop compressing completely as the optimum seal is achieved. One way to do this is by crimping a sealing bead into a gasket made from a single steel sheet. The crimped bead supplies the seal as it is crushed and the steel sheet provides the necessary solid stop. We have again made use of our experience and expertise to develop a tooling system for these gaskets which is inexpensive but effective and produces good quality economical gaskets.