Selection of right rubber hardness is critical to the sealing application. The shape and size of extrusion profile and sealing pressure factors in while deciding on the right hardness of silicone. 

Shore Hardness

Shore A hardness is tested using a durometer. The Shore A hardness measurement is often referred “duro” or “durometer”. The hardness test is performed on cured rubber as per ASTM D2240.

The device consists of a hardened steel rod with a truncated cone at the tip. The steel rod is spring-loaded and actuates a dial with a scale of 0 to 100. The test specimen (a silicone extrusion) is placed directly underneath the truncated cone, and the device is pressed down onto the part until the flat metal plate on the bottom is flush with the rubber specimen  as shown below[1].

 

Testing hardness on a Shore durometer – Jehbco Silicones

Fig 1.0 Testing hardness on a Shore durometer

 

The more the cone deforms the rubber material, the lower the hardness measurement. The less the cone deforms the rubber material, the higher the hardness measurement.

Gaskets

Gaskets are made from joining lengths of extrusion in a closed or open loop arrangement.  Gaskets could be used for simple contact prevention between two parts or sealing. In the latter case they could be referred as seals.

askets (joined extrusions) are designed for deformation. Where extrusion undergoes one-time deformation – such as façade joint seals, the extrusion is designed to stay in place and form a seal after being compressed by the gland or gap initially. Where extrusion undergoes cyclic deformation – such as in oven door seals, extrusion is designed for withstanding alternating loading and unloading over a period of time.

Seal failure can be predominantly attributed to loss of compression resistance or tear[2].  Our engineers are mindful of the magnitude of deformation every application requires. The proportions of the extrusions are designed keeping in mind the hardness – for critical applications Jehbco has in-house ability to carry out hyperelastic simulations to find the right extrusion profile dimensions in relation to the hardness or vice-versa. Alternately, Jehbco provides an option of making test samples of various hardness upon request.

O-rings

O-rings are extensively used across mechanical sealing application involving any cylindrical geometry of parts. An effective seal is made by creating a zero clearance between parts that blocks fluid or gas[3]. Figures 2, 3 and 4 illustrate how a half model section of an O-ring looks before any application of pressure and how it reaches maximum deformation. If the customer provides us with the sealing pressure or the minimum and maximum clearance to be maintained between the faces, we would be able to find the right hardness for a given diameter of the o-ring.

 

O-ring at zero compression – Jehbco Silicones

Fig 2.0 Initial state at zero compression

 

O-ring at 50 percent compression – Jehbco Silicones

Fig 3.0 50% compression

 

O-ring at 100 percent compression – Jehbco Silicones

Fig 4.0 100% compression


If you would like to learn more about Jehbco’s o-ring design and hyperelastic simulations please click here.

For any further enquiries please contact us.

References

  1. http://blog.parker.com/why-is-shore-a-hardness-important?src=Twitter&cm_mmc=Twitter-_-Enterprise_Social_EMG_OES-_-Blog-_-number4_most_popular_blog
  2. https://www.prepol.com/solutions/why-do-o-rings-fail-a-brief-guide-to-o-ring-failure-modes
  3. https://promo.parker.com/promotionsite/oring-ehandbook/us/ehome/ci.How-an-O%E2%80%93Ring-Works,EN.EN