Seal selection is often left until late into the design cycle,
sometimes compromising product performance and even delaying market
entry. However, seals moulded from natural and synthetic rubbers can be
customised early on in the cycle - and at low cost, as Andrew Piper
explains
Rubber mouldings turn up in innumerable applications, from exotic F1
Grand Prix engines to humble electric kettles. A seal may be a simple
O-ring, in which case the only real decisions a design engineer needs to
make concern material choice and cross-sectional profile. However, when
the moulding forms a major part of the end product, design considerations
move higher up the engineer's agenda. When tackling a sealing problem,
the design engineer should ask two questions: what needs sealing and why?
Asking what needs sealing helps point the way towards choice of
manufacturing process. For example, the gasket seal on a radiator cap is
flat and easily die-cut from sheet, whereas the rubber drive wheels for a
printer or copier need to be moulded and bonded. However, there are
pitfalls in assuming that any flat rubber moulding can be simply cut from
sheet; most notably, accuracy tends to drift and the cut edge to
'apple-core'. In demanding applications, neither of these is acceptable.
Moulding the component ensures greater accuracy but isn't there a penalty
in tooling costs? To some extent, yes, but in most cases there is no need
to go to full injection mould tooling. Transfer and compression moulding
methods are capable of creating a wide variety of complex 3D shapes and
offer precision without the high tooling cost. At DP Seals, for example,
we transfer-mould small rubber balls to a very high accuracy for car
suspension units, and a mushroom-shaped safety seal for electrolytic
capacitors incorporating a very fine membrane that ruptures at a specific
pressure - both in very high volumes.
Having identified what, it is important to know why the product needs a
rubber seal or moulding. Rubber comes in a wide variety of forms and
compounds, all with different performance characteristics and this helps
to make initial material choices. The softer the rubber - i.e. the lower
its Shore number (See box story)- the more a seal or gasket will deform
under pressure and fill its seating when mating parts come together.
Natural rubber, for example, offers good low temperature performance, a
high tensile strength and a hardness range from 30 to 90 Shore A, making
it ideal for a wide range of simple applications. However, it does not
perform well at high temperatures and offers poor resistance to
hydrocarbons, eliminating it from many environments involving fuel oils
and solvents. Synthetic rubbers offer a wider temperature range and are
resistant to attack from acids, mineral oils and petroleum solvents.
Silicone rubbers offer an even wider temperature range (-100°C up to
300°C) and excellent protection against water and gas permeation, but are
poor performers in the presence of oil, petrol and solvents. So, what do
you do if you want a seal in a fuel system that runs very hot?
Know your product
Specialised synthetics, such as hydrogenated nitrile rubber, offer a
better combination of temperature range, solvent resistance and
durability. This will satisfy the endurance requirements of, say, fuel
systems in rally or Le Mans car engines but not the extremes of F1
engines. Where the F1 is concerned, it is important to remember that the
seal only has to perform at its peak for less than two hours, not the 24
hours demanded by a Le Mans engine. An application-specific blend of
silicone rubber can be tailored to be petroleum resistant at the upper
end of its temperature specification for the duration of a Grand Prix
race. This application demonstrates how important it is to understand all
the parameters that might influence your seal or moulding design.