When designing a custom rubber seal, gasket, or profile, one of the decisions you may need to make is whether to use solid rubber or sponge rubber.
Although both materials can be manufactured into custom extruded profiles, they behave very differently when compressed. Solid rubber is generally denser and more resistant to deformation, while sponge rubber contains a cellular structure that allows it to compress more easily.
Choosing between the two can affect sealing performance, compression force, durability, installation, and the overall performance of the finished component.
Northwest Rubber Extruders (NWRE) manufactures custom rubber extrusions for a wide variety of industrial and OEM applications. Understanding the differences between solid and sponge rubber is an important step when determining the appropriate material and profile for a custom extrusion.
What Is Solid Rubber?
Solid rubber is a dense elastomeric material without the cellular structure found in sponge rubber.
When pressure is applied, solid rubber can deform and compress, but it generally requires more force than a comparable sponge rubber material.
Solid rubber can be manufactured from several elastomers, including:
- EPDM
- Neoprene
- Nitrile
- SBR
- Natural rubber
- Silicone
- Other specialty compounds
The appropriate material depends on the operating environment and performance requirements.
Solid rubber extrusions are available in numerous configurations, including strips, tubing, channels, D-shaped profiles, custom seals, gaskets, and complex geometric profiles.
What Is Sponge Rubber?
Sponge rubber contains a cellular structure that changes how the material responds to pressure.
The cells within the material allow a sponge rubber profile to compress more easily than a similar solid rubber profile.
This characteristic makes sponge rubber useful for applications where a seal must conform to surfaces or fill a gap without requiring excessive compression force.
Sponge rubber can be produced using several elastomer materials, depending on the required environmental and physical properties.
Common applications include:
- Door seals
- Enclosure seals
- Weather seals
- Equipment gaskets
- Automotive seals
- HVAC components
- Industrial equipment
- Protective cushioning
- Vibration control
Solid vs. Sponge Rubber: Quick Comparison
The primary difference between solid and sponge rubber is their physical structure.
| Characteristic | Solid Rubber | Sponge Rubber |
|---|---|---|
| Structure | Dense | Cellular |
| Compression | Lower | Higher |
| Compression Force | Generally higher | Generally lower |
| Rigidity | Generally higher | Generally lower |
| Gap Filling | Moderate | Excellent |
| Cushioning | Good | Excellent |
| Sealing Irregular Surfaces | Application dependent | Often very good |
| Abrasion Resistance | Often higher | Application dependent |
| Common Uses | Profiles, gaskets, strips, seals | Weather seals, enclosure seals, cushioning |
These are general comparisons. Performance varies based on the rubber compound, density, durometer, profile design, temperature, and operating environment.
The Biggest Difference: Compression
Compression is usually one of the most important considerations when deciding between solid and sponge rubber.
Imagine a rubber seal positioned between a door and its frame.
When the door closes, the rubber must compress enough to create a seal.
If the rubber is too hard or requires too much force to compress, the door may become difficult to close or the surrounding hardware may experience unnecessary stress.
Sponge rubber can be useful in these situations because its cellular structure allows it to compress using relatively low force.
Solid rubber can also create effective seals, but profile design and durometer become particularly important.
The correct solution depends on how much compression is available within the assembly.
When Should You Use Solid Rubber?
Solid rubber may be appropriate when the component requires greater density, structural resistance, durability, or resistance to deformation.
Applications may include:
- Industrial gaskets
- Protective strips
- Bumpers
- Spacers
- Edge protection
- Machinery components
- Wear-resistant profiles
- Sealing applications
- Custom channels
- Custom tubing
Solid rubber may also be preferred when the component experiences significant physical contact or mechanical stress.
The specific elastomer should then be selected based on environmental requirements.
For example, EPDM may be considered for outdoor weather exposure, while nitrile may be more appropriate when petroleum-based oils are present.
NWRE’s Material & Engineering resources provide additional information about materials used for custom rubber products.
When Should You Use Sponge Rubber?
Sponge rubber is particularly useful when easy compression and conformability are important.
Typical applications include:
- Weatherstripping
- Door seals
- Access-panel seals
- Equipment enclosures
- Electrical cabinets
- HVAC equipment
- Automotive sealing
- Construction products
- Vibration isolation
- Protective cushioning
A sponge profile can compress to accommodate variations between two surfaces.
This can help maintain contact when the dimensions of an opening aren’t perfectly uniform.
For example, an equipment enclosure may have slight dimensional differences along the perimeter of a door. A compressible sponge seal can help compensate for those differences.
Open-Cell vs. Closed-Cell Sponge Rubber
Not all sponge rubber is the same.
An important distinction is whether the material has an open-cell or closed-cell structure.
Open-Cell Sponge
In an open-cell structure, many of the individual cells are interconnected.
This can make the material soft and highly compressible.
Open-cell materials may be useful for cushioning, vibration absorption, and applications where maximum compressibility is important.
However, the interconnected cellular structure can allow water or air to move through the material more easily.
Closed-Cell Sponge
Closed-cell sponge contains cells that are largely separated from one another.
This structure generally provides better resistance to water absorption and air movement than an open-cell material.
Closed-cell sponge is therefore frequently considered for:
- Weather seals
- Outdoor equipment
- Enclosures
- Automotive applications
- HVAC systems
- Construction products
The appropriate cellular structure depends on what the seal needs to accomplish.
What Materials Are Available for Sponge Rubber Extrusions?
Like solid rubber, sponge rubber can be manufactured from different elastomers.
Material selection should be based on environmental exposure.
EPDM Sponge Rubber
EPDM is commonly considered for exterior sealing because of its resistance to:
- Weather
- Ozone
- UV exposure
- Water
- Environmental aging
EPDM sponge can therefore be useful for outdoor weather seals and enclosure applications.
Neoprene Sponge Rubber
Neoprene provides balanced environmental properties and may be considered where moderate weather and oil resistance are required.
Silicone Sponge Rubber
Silicone sponge can be useful when temperature performance and environmental stability are important.
NWRE also manufactures custom silicone extrusions for specialized applications.
The specific compound should always be evaluated against the actual operating environment.
What Is Durometer?
When working with solid rubber, durometer is commonly used to describe the hardness of the material.
A lower-durometer rubber is generally softer, while a higher-durometer rubber is generally harder.
Durometer affects:
- Compression
- Flexibility
- Sealing pressure
- Installation
- Resistance to deformation
However, sponge materials are also heavily influenced by their density and cellular structure.
Two materials made from the same general elastomer can behave very differently if one is solid and the other is sponge.
This is why simply specifying “EPDM,” for example, isn’t always enough information to properly define a rubber component.
Profile Design Matters as Much as Material
Another important consideration is the geometry of the extrusion.
A solid profile doesn’t necessarily have to be extremely difficult to compress.
For example, a hollow solid-rubber extrusion can use its geometry to provide additional compressibility.
Common compressible profile designs include:
- D-shaped seals
- P-shaped seals
- Bulb seals
- Hollow tubing
- Hollow rectangular profiles
- Custom hollow geometries
The cavity inside the profile allows the rubber to deform when pressure is applied.
This means engineers may sometimes achieve the required sealing characteristics using a hollow solid-rubber extrusion rather than sponge rubber.
Material and profile geometry should therefore be evaluated together.
Solid Rubber vs. Sponge Rubber for Weather Sealing
Both materials can be used for weather-sealing applications.
The better option depends on the assembly.
Sponge rubber may be advantageous when:
- Low closing force is important
- Significant compression is required
- The mating surfaces aren’t perfectly uniform
- The seal needs to conform easily
Solid rubber may be advantageous when:
- Greater physical durability is required
- The profile encounters abrasion
- Less compression is needed
- The seal requires greater resistance to deformation
EPDM is commonly considered for outdoor versions of either design because of its excellent resistance to weather and ozone.
Solid vs. Sponge Rubber for Industrial Equipment
Industrial applications can introduce additional factors such as:
- Oils
- Chemicals
- Abrasion
- Vibration
- Heat
- Repeated opening and closing
- Mechanical stress
A machine enclosure door, for example, may benefit from a compressible sponge seal.
A protective bumper on the same machine may be better suited to solid rubber.
There is no requirement that every rubber component within a product use the same material or construction.
Each component should be selected according to its function.
NWRE serves a wide range of industries and applications with different environmental and performance requirements.
How Much Should a Rubber Seal Compress?
There isn’t one correct compression percentage for every rubber seal.
The appropriate amount depends on:
- Material
- Profile geometry
- Sponge density
- Durometer
- Required sealing force
- Available space
- Expected movement
- Environmental conditions
Over-compressing a seal can potentially shorten its useful life or place unnecessary stress on the assembly.
Under-compressing a seal can prevent it from maintaining sufficient contact with the mating surfaces.
The extrusion and surrounding assembly should therefore be designed so the seal operates within an appropriate compression range.
Compression Set Is Another Important Consideration
Compression set describes a rubber material’s tendency to remain permanently deformed after being compressed for a period of time.
Consider a door seal that remains compressed whenever the door is closed.
Ideally, when the door opens, the rubber should recover toward its original shape.
If the material experiences excessive permanent deformation, its ability to create an effective seal may decrease over time.
Material selection, temperature, compression level, compound formulation, and service conditions can all influence compression-set performance.
For applications involving continuous compression, this should be considered during material selection.
Questions to Ask Before Choosing Solid or Sponge Rubber
Before selecting a material, consider the following:
How much compression is available?
Determine how much space exists between the mating surfaces.
How much closing force can the assembly tolerate?
Doors and access panels may require relatively low compression force.
Will the seal be exposed to water?
Cell structure and material selection become particularly important.
Is the application outdoors?
Weather, UV, and ozone resistance should be evaluated.
Will the rubber encounter oils or chemicals?
The base elastomer needs to be compatible with those substances.
Will the component experience abrasion?
Physical durability may influence the decision.
How frequently will it be compressed?
Repeated compression can affect long-term performance.
What temperatures will it experience?
Both high and low operating temperatures should be considered.
Custom Rubber Extrusions for Your Application
Choosing between solid and sponge rubber isn’t simply a matter of deciding which material is “better.”
The correct choice depends on how the component needs to perform.
Solid rubber may be preferred when density, durability, abrasion resistance, or structural resistance is important.
Sponge rubber may be preferred when easy compression, cushioning, gap filling, or conformability is the priority.
In some applications, a hollow solid-rubber profile may provide another option by combining the characteristics of a solid elastomer with a highly compressible profile design.
Evaluating the material, cellular structure, durometer, profile geometry, and operating environment together can help determine the most appropriate solution.
Talk to Northwest Rubber Extruders About Your Project
Northwest Rubber Extruders manufactures custom rubber profiles for customers across a variety of industrial markets.
Whether you need a simple rubber strip, hollow sealing profile, tubing, D-shaped extrusion, or a completely custom geometry, NWRE can manufacture profiles based on customer drawings and specifications.
Learn more about our Rubber Extrusions, Silicone Extrusions, Material & Engineering, and Industries Served capabilities.
Have a drawing or specification for an upcoming project? Request a Quote from Northwest Rubber Extruders.