When engineering a new product, it’s easy to treat gaskets and seals as an afterthought. Some engineers default to the same grade of silicone rubber for every application, operating under the assumption that “tougher is better”. However, it is this misconception that can often lead to failing enclosures or seals that cannot compress significantly under low pressure, compromising the integrity of the entire device.
When selecting materials for sealing applications, understanding the structural difference between silicone sponge and silicone rubber can be the deciding factor between success and the need for a costly manufacturing redesign.
The Structural Difference Between Silicone Rubber and Sponge
To understand why seals fail, it’s important to understand how different grades of Silicone materials behave under pressure.
Solid Silicone Rubber
The structure of solid Silicone is dense and incompressible, meaning that it requires a significant force in order to provide a successful seal. When squeezed between two surfaces, the material’s volume does not shrink, it simply displaces and bulges out at the sides. If the surrounding hardware cannot handle the outward force of the rubber’s displacement, the hardware may bend, bow or break, leading to failure.
Silicone Sponge
In contrast to solid Silicone, sponge materials have a cellular structure that features bubbles of trapped gas. As a result, when pressure is applied to a closed-cell Silicone sponge gasket, the trapped gas actually compresses, allowing the gasket to absorb the force and decrease in volume without pushing back against the surrounding hardware.
When to Select Solid Silicone
Whilst it requires considerable force to act as an effective seal or gasket, solid Silicone is incredibly durable. Its flexibility is measured on the Shore hardness scale of 0-100, with higher values indicating harder rubber. General purpose Silicone rubber features a shore hardness of 60 Shore A, whilst MF775 Silicone is even more rigid, with a Shore hardness rating of 73 shore A.
This high level of hardness means that rubber forms of Silicone feature a high tear strength, and an excellent resistance to wear over time. These characteristics make it an ideal gasket material for:
- High-pressure environments: Ideal for use as a gasket for heavy industrial flanges where significant clamping force is used.
- Underwater environments: Silicone rubber is highly resistant to water, making it an ideal choice when high pressure water sealing is non-negotiable.
- Precision O-rings: Applications needing precision cut O-rings set into specifically engineered grooves where material displacement is accounted for in the design.
When to Select Closed-Cell Silicone Sponge
For applications featuring imperfect hardware, lightweight casings or delicate assemblies, sponge is a good choice. Whilst rubber gaskets are chosen for applications where Shore hardness is important, Silicone sponge is chosen for its low compression set, meaning that it reliably returns to its original shape after compression, even after years of prolonged pressure.
The best applications for sponge gaskets include:
- Uneven mating surfaces: When compressed, the Silicone sponge fills gaps where surfaces aren’t perfectly flat or flush.
- Low-closure force: Plastic enclosures would likely crack when forced closed against solid rubber gaskets, so the low compression set of sponge is a much better choice.
- Weatherproofing: Like Silicone rubber, sponge provides reliable protection against wind, rain, and dust.
Simply put, rubber is well suited to applications where high tensile strength and pressure resistance are a must, whereas Silicone sponge provides a solution for applications where the gasket must conform to uneven surfaces or fill irregular gaps.
Precision Gasket Materials
Choosing the right gasket material is only the first part. Once the best suited material is selected, precision manufacturing is the next step to ensuring an effective gasket or seal. Whether your project requires the heavy-duty dealing capabilities of solid rubber or the compression set of a closed-cell sponge material, working with an experienced rubber and plastics converter ensures your gaskets meet the exact tolerances of your design.
With excellent knowledge of material science and an extensive catalogue of high performance gasket materials, combined with many decades of manufacturing experience and innovation, CB Frost & Co help ensure all industry specifications and gasket specifications are met.
