Mind the gap - a guide to custom rubber
Given the plethora of rubber seal, gasket and moulding companies around, it's easy to see why engineers leave selection until late in the design cycle. All too often, this forces engineers to choose an out-of-the-catalogue product that compromises product performance, or into eleventh-hour development of a specialised moulding, delaying market entry for the new product.
The time and cost pressures on design engineers today mean compromise is inevitable. With seals and gaskets there is a tendency to leave the choice of material, profile and manufacturing process until too late in the design cycle. To plagiarise a phrase from childhood, it's all to easy to spoil the product for a ha'p'orth of rubber.
Rubber mouldings turn up in innumerable applications, from exotic F1 Grand Prix engines to humble electric kettles. Often, the seal is seen as a simple O-ring designed to keep something in or out and the only real decisions are on material and cross-sectional profile. When the moulding is a major part of the end product, such as the rubber boot for this sub-sea fibre optic cable connector, its design is high on the engineer's agenda.
Occasionally, the introduction of new regulations will extend the role of the seal; for example, a seal originally designed to prevent ingress of dirt and fluids into a ruggedised electronics unit can be enhanced to shield against EMI. This highlights our first two application-specific 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? True; but in most cases there is no need to go to full injection mould tooling and its attendant high process control costs: transfer and closed cavity moulding are capable of creating a wide variety of complex 3D shapes and offer similar precision without the high tooling cost.
At DP Seals we transfer mould small rubber balls to a very high accuracy for car suspension units, and mushroom-shaped safety seals incorporating a very fine membrane designed to rupture at a specific pressure for use in electrolytic capacitors - both in very high volumes.
Having identified what, it is important to know why the product needs a rubber seal or moulding: simple feet, boot seals, drive wheels bonded to a shaft in a printer or copier, or to keep liquids and/or gases separate. Rubber comes in a wide variety of forms and compounds, all with different performance characteristics and this helps to make initial material choices.
Unsurprisingly, the softer the rubber (the lower the Shore or I.R.H.D. number) the more a seal or gasket will deform under pressure and fill the groove when the parts of a unit come together. It is worth remembering however all normal rubber materials behave as solids and need space to displace into. They are not foam like in performance!
Natural rubber for example, offers good low temperature performance, down to -50°C, and high tensile strength with a wide 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 similar solvents.
Synthetic rubbers can offer a wider temperature range and are resistant to attack from acids, mineral oils and petroleum solvents, while silicone rubbers offer an even wider temperature range, from -100°C up to 300°C, and excellent protection against water and gas permeation - making them ideally suited for food and drink manufacturing installations - but are poor performers in the presences of oil, petrol and similar solvents. But what do you do if you want a seal in a fuel system that runs very hot?
Know your product
Specialised synthetics such as a hydrogenated nitrile rubber offer a better combination of temperature range, solvent resistance and durability, making them ideal for, say fuel systems in rally or Le Mans car engines but not for the extremes of F1 engines. The key to the F1 situation is 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.
This clearly demonstrates how it is important to understand all the parameters that might influence your seal or moulding design.
With the increasing variety of rubber compounds available, and the ability to tailor them to specific requirements, engineers are continually finding new and innovative uses for rubber mouldings. In some applications it is the environment that is demanding. In others it is the precision required, such as this novel ink-jet print head bulkhead designed to carry signal cables and ink lines.
The key is to challenge your custom moulding supplier to come up with a solution that meets your needs in terms of performance, timescale and cost. But be prepared to answer the key questions: what is it for, why it's needed, and where is it going to go - and don't leave it late on in the design cycle. The more time you give your supplier, the better the solution.
At DP Seals, we specialise in providing customised solutions to customers seeking extreme performance without an extreme price tag. As a result we have mastered the production parameters surrounding these exciting new materials, putting us at the forefront of custom rubber moulding.
A version of this article was published in DPA Design Products and Applications.
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