Preventing Gasket Failure Part 2: Durometer
Proper material selection ensures the longevity of a gasket and the major parts a gasket supports. Improper design or incorrect material specifications can cause a gasket to fail, which reduces the life, safety, and effectiveness of the entire product. In part one of this series, we examine the importance of considering compression set when selecting gasketing material. Today, we’ll highlight Durometer.
What is Durometer?
Durometer is a unit of measure used to convey a material’s hardness or, for a foam, its firmness. This article discusses the aspects of Durometer as a factor to consider when choosing a foam or rubber for your gasket application.
How is Durometer measured?
Durometer is measured using a spring-loaded “Durometer” testing machine originally produced by a man named Shore. The machine forces a spring-loaded metal rod or probe into the surface of a material while a dial graduated from 0-100 indicates the amount of force needed to achieve a certain level of indentation. The result is recorded as Durometer. Due to variations of material hardness, Shore created different testers with different probe shapes and spring loads to create the specific amount of indentation.
The three Shore testers are used for soft foam, rubber, and plastic-like materials. Shore OO scale is used for extremely soft materials like foam and cellular sponge. Shore A scale is used for solid rubber materials used for O-rings and rubber sheeting. The most common and widely used durometer in this category is 70 Durometer, referenced as 70d.
Is there a standard or common Durometer for gaskets?
When not specified, it is an industry practice that rubber is 70d. Hard plastic materials are tested using the Shore D scale. There is no linear relationship between the three scales. If a Shore OO or Shore A tester is used on plastic, the dial will be maxed out because these testers do not have enough spring force to indent the hard plastic.
Another consideration is that standard published hardness properties are measured using a minimum material thickness of .25”. As a gasket’s thickness is reduced, the significance of Durometer is less of a factor. This is especially true with rubber and cellular gaskets less than 1/16” thick.
Effect of Durometer on sealability
To have an effective seal, the pressure created by compressing the gasket must be greater than the external or internal pressure trying to be contained or excluded. Hard gasket materials can generate high pressure with minimal deflection. If you are using an open-cell sponge or foam, however, the material must be compressed enough to effectively close off all the internal passageways to prevent leakage. A gasket must also conform and fill in all surface imperfections, machining marks, and discontinuities.
Sometimes a gasket sealing system is comprised of thin metal or plastic flanges that can distort when bolted together. A hard gasket material may be able to better support load around the area of the bolt but may not be soft enough to compress and fill potential leak paths.
On the other hand, using too soft a material can have an adverse effect on sealing when the components are not rigid enough to prevent deflection in the area between bolts. If a leak is encountered in this situation, generally the first action is to tighten the bolts even more. This can cause more flange distortion and less gasket compression in the area between the bolts—actually increasing leakage. In a situation like this, the gasket material and shape must be designed to provide a proper seal within the mechanical limitations of the mating components.
A common, yet often overlooked, solution is to make the gasket narrower, (not thinner). By doing this, more sealing pressure can be generated for a given load because the amount of gasket area supporting the bolt load is reduced. Another often overlooked trick is to create a raised “sealing bead” on one of the flange faces. A raised bead as little as .030” radius will create significantly higher pressure along the bead path creating a highly effective sealing barrier.
The Right Partner Can Make All the Difference
There are many sealing nuances involving tradeoffs between durometer, thickness, gasket shape, flange design, and material choice that can result in a robust sealing system or a marginal design. Utilizing JBC’s years of gasketing and sealing experience is available to work with customers to prevent problems down the road. A simple call to our sealing experts is free and can potentially make the difference between failure and success. Contact our team today.