Articles posted by Jehbco Silicones

Jehbco manufactures custom silicone extrusions in our Sydney-based factory, carrying out all steps in-house, from product design and development to die cutting, material preparation and manufacture.  This gives us fine grained control over the properties and quality of the final product, allowing us to tailor each product to the requirements of our varied customer base.

Jehbco’s manufacture of silicone extrusions is a process with several steps.  The raw silicone material is first mixed and then fed to an extruder.  The extrusion that comes from the extruder is passed through several stages of heating and then boxed or coiled.  Finally, extrusions are placed in an oven for several hours to post-cure.  The post-cure process reduces the stickiness of silicone, as well as removing volatiles and improving other properties.

In its raw form, silicone is quite sticky.  While some applications require a high degree of stick and friction, such as gaskets which must be retained in joints without additional adhesive, many applications require lower friction than silicone naturally has.  For example, o-rings must slide easily against machine parts and some seals must slide easily into a joint.

A problem with silicone’s natural stickiness is the tendency of some silicones to self-adhere when coiled.   The coils of silicone extrusion may stick to each other, which can lead to tearing of thin sections during uncoiling.   As extrusions are coiled prior to post-curing, the post-curing process does not solve the issue of self-adherence.

To reduce the stickiness of our extrusions, both for low friction applications and to eliminate the risk of self-adherence, we use a talcing process.  After curing and before coiling, a thin layer of talc is applied to the surface of the extrusion.  This layer of talc acts as a lubricant and prevents the extrusions from self-adhering when coiled.  The talc does not react with the silicone and has no effect on the material properties of the extrusion; it only serves to reduce surface friction.

While the talcing process has previously been done by hand, Jehbco are currently implementing an automated talc application process.  This will allow quicker application of a more even and consistent layer of talc on our extrusions.

Figure 1: An extrusion passing through Jehbco’s new talc applicator

On the other side of the coin, some applications require higher friction than our standard extrusions can offer.  Not a problem! Jehbco can increase the coefficient of friction of our silicone in a number of ways: specifying softer (lower durometer) silicone, different grades of silicone and removing the post-cure process.

Jehbco makes every effort to ensure our product fits your application, from profile design to material properties.

For help matching our products with your application, please review our website www.Jehbco.com.au and contact us with any questions.

Jehbco manufactures custom engineered silicone extrusions to suit your application’s requirements.  Our engineers take every effort to ensure that our silicone’s properties match your application perfectly.

An important property of silicone is the coefficient of friction (CoF), which describes how hard it is to slide the silicone along a surface.  The CoF affects many areas: examples include the forces in moving seal systems (for example, o-ring seals on pistons); the ability of a seal to remain in place without adhesive; and the compression of a gasket between two hard surfaces.

In a previous article, we discussed how the coefficient of friction is measured.  In this article, we’ll look at what factors influence the CoF and how it can be tailored to your application.

The coefficient of friction is a property of the entire system, not only the silicone.  In general, the coefficient of friction will be higher (that is, sliding will be more difficult) when silicone is in contact with a rough surface than when silicone is in contact with a smooth surface.  The material itself will also affect the coefficient of friction – silicone will slide less against a smooth piece of aluminium than against a smooth piece of Teflon.

Figure 1: Measuring coefficient of friction.

One important system property that can affect the coefficient of friction is temperature.  As the temperature decreases, the silicone becomes slightly less soft.  This causes the coefficient of friction to decrease very slightly.  In addition, lower temperatures may produce condensation.  A thin film of water on the surface of the silicone will act as a lubricant and further lower the coefficient of friction, causing the silicone to slide more easily.

Silicone is naturally quite tacky, with a coefficient of friction of approximately 1.0 in many cases.  On many of our products, processes are applied to reduce the coefficient of friction.  These processes include applying a small amount of talc to the silicone surface and post curing the silicone in an oven for several hours.  In general, harder silicones with a higher durometer have a lower coefficient of friction, and our platinum silicone material has a lower coefficient of friction.

Jehbco’s engineers are able to tailor the properties of our silicone to produce a coefficient of friction that meets your requirements, and we have in-house testing capabilities that allow us to measure the coefficient of friction.  For any help with your application please review the Jehbco website www.Jehbco.com.au, and contact us with any questions.

Jehbco’s pure silicone extrusions are used in a wide range of applications, from aircraft seals to sea water piping gaskets.  These applications present a wide range of operating conditions that our silicone must work under, and Jehbco works with our customers to ensure the best silicone product is chosen for your application.

In order to determine the best silicone for your application, we must be able to measure the performance of our silicone.  A range of material properties are tested to describe how our silicone will perform under different conditions.  Several of these properties are measured using a stress/strain curve.

The stress/strain curve

The stress/strain curve is created by stretching a piece of silicone and measuring both the force required to stretch the silicone (tensile force) and how much it stretches (elongation).  The silicone is stretched until it breaks.  Since the silicone samples being stretched can be different sizes, the tensile force and elongation measurements are corrected.  Tensile force is divided by the cross sectional area of the silicone to get stress and elongation is divided by the original length of the silicone to get strain.  Stress is plotted on the y-axis and strain is plotted on the x-axis to get a stress/strain curve.   An example stress/strain curve is shown  below.

From the stress/strain curve, we can determine several important mechanical properties that tell us how the silicone behaves.  The modulus of the silicone tells us how stiff the silicone is, or how hard it is to stretch.  This is measured by calculating the slope of the first part of the stress/strain curve.  Stiff silicones will have a very steep curve, while softer silicones will have a shallower curve.

The tensile strength of the silicone is the amount of stress, or normalised force, needed to break the silicone.  This is easily read off the stress/strain curve – it is the stress at the very end of the curve, where the silicone breaks.

To ensure our product works as required, it is important that properties such as modulus and tensile strength are right for the application.  Jehbco has in-house facilities for generating stress/strain curves, to help us tailor the right product for your application.

For any help with your application please review the Jehbco website www.Jehbco.com.au, and contact us with any questions.

Seals are used to prevent the ingress of fluid or particles in a wide range of mechanical applications.  Applications vary from traffic light enclosures to hydraulic mining equipment, fluids range from seawater to DOT brake fluid and pressures range from a couple of kilopascals to hundreds of megapascals. While some applications have been standardised, many seals require individual design.  Jehbco manufactures custom silicone extrusions for a range of non-standard sealing applications.

Seal performance is influenced by a number of factors.  These include:

• The nature of the fluid being sealed, whether it be liquid or gas;
• The condition of the sealing surfaces and presence of any grooves or roughness;
• Materials;
• Geometry;
• Force exerted on the sealing surfaces.

This last factor, sealing force, is useful not only in estimating performance, but is essential for proper design of the sealing system.  For example, in a gasket system (Figure 1), sealing force is exerted on the gasket by a series of bolts.  To design the bolting system, the designer must know the force required to compress the gasket and form a seal.

Figure 1: A gasketted flanged pipe joint.

Sealing force is often determined through guesswork and expensive in-field testing.  Jehbco are breaking this mould by developing custom tooling to measure force on our sealing products.  With these tools, Jehbco will be able to provide our customers with accurate estimates of sealing force required under specific conditions – for example, the force required to compress an o-ring by 20%, or the force exerted by a seal on an expansion joint when the joint is 10 mm wide.  Jehbco aims to use these tools to enhance your design processes so we can more quickly produce a silicone extrusion that meets your sealing requirements.

For more information on silicone seals and assistance, please contact us at Jehbco.