When designing a custom rubber component, choosing the right material is only part of the process. You also need to determine how the component should be manufactured.
Two of the most common manufacturing methods are rubber extrusion and rubber molding.
Both processes can produce custom rubber components, but they are designed for very different types of parts. Rubber extrusion is generally best suited for continuous profiles with a consistent cross-sectional shape, while rubber molding allows manufacturers to produce complex three-dimensional components.
Understanding the differences between these processes can help engineers, product designers, and purchasing teams determine which manufacturing method is better suited to their application.
Northwest Rubber Extruders (NWRE) provides both custom rubber extrusion and custom rubber molding capabilities for a wide variety of industrial and OEM applications.
Here’s how the two processes compare.
What Is Rubber Extrusion?
Rubber extrusion is a manufacturing process used to produce continuous lengths of rubber with a consistent cross-sectional profile.
During the extrusion process, uncured rubber material is forced through a specially designed die.
The opening in the die creates the basic cross-sectional shape of the extrusion.
The rubber is then processed and cured to establish its final physical characteristics.
Because the extrusion process produces a continuous profile, it is particularly well suited for products such as:
- Rubber seals
- Weatherstripping
- Tubing
- D-shaped profiles
- P-shaped profiles
- U-channels
- Rubber strips
- Hollow seals
- Edge protection
- Custom continuous profiles
The extrusion can then be supplied in continuous lengths or cut and fabricated into individual components.
What Is Rubber Molding?
Rubber molding uses a mold cavity to form rubber into a specific three-dimensional shape.
Rather than producing a continuous profile, molding produces individual components.
This allows molded rubber products to incorporate geometry that would not normally be possible through extrusion.
NWRE provides several rubber molding processes, including:
- Compression molding
- Injection molding
- Transfer molding
- Rubber-to-metal bonding
The appropriate process depends on the geometry, material, production requirements, and application.
Rubber Extrusion vs. Rubber Molding: Quick Comparison
Here is a simplified comparison:
| Factor | Rubber Extrusion | Rubber Molding |
|---|---|---|
| Part Shape | Continuous profile | 3D component |
| Cross Section | Consistent | Can vary throughout part |
| Typical Products | Seals, tubing, strips, channels | Gaskets, bumpers, boots, complex components |
| Production | Continuous | Individual parts |
| Custom Tooling | Extrusion die | Mold |
| Cut Lengths | Common | Not generally applicable |
| Complex 3D Geometry | Limited | Excellent |
| Long Continuous Products | Excellent | Limited |
| Rubber-to-Metal Parts | Limited | Well suited |
| Secondary Fabrication | Often available | Application dependent |
The correct manufacturing method depends primarily on the geometry and function of the finished component.
When Should You Use Rubber Extrusion?
Rubber extrusion is generally appropriate when a product maintains the same cross-sectional shape along its length.
Imagine cutting through a rubber profile at several points.
If each cross section looks essentially the same, extrusion may be an appropriate manufacturing method.
For example, a D-shaped weather seal may be manufactured continuously in long lengths because the same D-shaped profile runs throughout the product.
Common Extruded Rubber Products
Extrusion is frequently used to manufacture:
- Door seals
- Window seals
- Equipment seals
- Weatherstripping
- Rubber tubing
- Rubber channels
- Edge protection
- Bumpers
- Solid strips
- Hollow profiles
- Custom gaskets
- Industrial sealing profiles
The finished extrusion can then be cut, joined, or otherwise fabricated depending on the application.
When Should You Use Rubber Molding?
Rubber molding is generally more appropriate when the component has a three-dimensional shape that changes throughout the part.
For example, imagine a rubber component that includes:
- Different thicknesses
- Mounting holes
- Raised sections
- Recessed areas
- Complex curves
- Integrated features
- Multiple geometric surfaces
These characteristics may be difficult or impossible to create through a continuous extrusion process.
Molding allows the entire component to be formed within a cavity designed around its final geometry.
Common Molded Rubber Products
Molding may be used for:
- Custom gaskets
- Bumpers
- Boots
- Bushings
- Pads
- Seals
- Diaphragms
- Vibration isolators
- Rubber-to-metal components
- Complex industrial parts
The Cross Section Is One of the Biggest Clues
One of the easiest ways to determine whether a part is a candidate for extrusion is to look at its cross section.
Consider a piece of rubber tubing.
If you cut the tubing at 12 inches, 24 inches, or 36 inches, the cross-sectional shape remains essentially identical.
That makes tubing a natural candidate for extrusion.
Now imagine a rubber component that is narrow at one end, wide at another, contains several holes, and includes a raised mounting surface.
The geometry changes throughout the component.
That type of part is much more likely to require molding.
A useful starting question is:
Does the component maintain the same cross-sectional shape throughout its length?
If yes, consider extrusion.
If no, molding may be more appropriate.
Extruded Rubber Seals
Sealing applications are one of the most common uses for custom rubber extrusion.
Extruded seals can be manufactured in profiles such as:
D-Shaped Seals
These typically feature a rounded sealing surface and flat mounting surface.
P-Shaped Seals
These combine a compressible section with an extended mounting leg.
Bulb Seals
Hollow bulb profiles can provide significant compression with relatively low force.
Channels
U-shaped and other channel profiles may be used around edges or mounting surfaces.
Tubing
Hollow tubing can be used for sealing, protection, fluid-related applications, and other industrial purposes.
Custom Profiles
When standard profiles won’t fit the assembly, custom extrusion tooling can create a profile around customer specifications.
Molded Rubber Gaskets and Components
Molding offers more flexibility when a gasket or component requires geometry in multiple dimensions.
A molded gasket might incorporate:
- Bolt holes
- Variable thickness
- Complex perimeter geometry
- Raised sealing surfaces
- Mounting features
- Reinforcement
- Inserts
These features would be difficult to produce through extrusion alone.
Molding can therefore be useful when the rubber component needs to integrate precisely with other mechanical components.
Can an Extrusion Be Turned Into a Finished Gasket?
Yes.
This is where the distinction between extrusion and molding becomes especially interesting.
An extruded profile can sometimes be cut and joined to create a finished gasket.
For example, a continuous extrusion may be:
- Manufactured in long lengths
- Cut to specific dimensions
- Joined or spliced
- Fabricated into a finished gasket
This approach can be useful when the gasket requires a consistent cross-sectional profile around its perimeter.
The manufacturing method should be evaluated based on the gasket geometry, performance requirements, production volume, and other considerations.
What Materials Can Be Extruded or Molded?
Many elastomers can be used for both manufacturing processes.
Common materials may include:
EPDM
Frequently used for weather, water, ozone, and outdoor applications.
Neoprene
Provides balanced environmental and physical performance.
Nitrile
Commonly selected when resistance to petroleum-based oils and fuels is important.
SBR
A general-purpose rubber with useful mechanical and abrasion-resistant properties.
Silicone
Often considered for demanding temperature environments and specialized applications.
NWRE also manufactures custom silicone extrusions for a variety of industrial markets.
Material selection should be based on the operating environment rather than manufacturing process alone.
Additional material information is available through NWRE’s Material & Engineering resources.
Compression Molding
Compression molding is one method used to manufacture custom rubber components.
In this process, rubber material is placed into a heated mold cavity.
The mold closes and applies pressure, causing the material to conform to the shape of the cavity.
Heat and pressure help cure the rubber into its finished form.
Compression molding can be useful for a variety of component sizes and geometries.
It is a well-established process for manufacturing custom molded rubber products.
Injection Molding
Rubber injection molding introduces material into a closed mold cavity.
This process can be advantageous for certain geometries and production requirements.
Injection molding may offer efficient production for higher-volume components depending on the design, material, and manufacturing requirements.
Transfer Molding
Transfer molding combines characteristics of compression and injection molding.
Rubber material is transferred into a mold cavity where it forms and cures.
This process can be useful for certain complex components and applications involving inserts or other design requirements.
Rubber-to-Metal Bonding
Some components require rubber to be permanently bonded to a metal substrate.
Examples may include:
- Vibration-control components
- Industrial mounts
- Bumpers
- Isolation components
- Machinery parts
Molding is often well suited to these applications because the rubber can be formed around or bonded to the metal component during manufacturing.
How Production Volume Affects the Decision
Production volume can influence manufacturing decisions, but it shouldn’t be considered independently from part geometry.
A component that fundamentally requires a molded shape cannot simply be extruded because the production quantity is high.
Likewise, a long continuous profile is naturally suited to extrusion regardless of whether it is needed in moderate or high quantities.
However, production volume can influence:
- Tooling decisions
- Manufacturing efficiency
- Secondary operations
- Packaging
- Cutting
- Automation
- Overall production strategy
Providing estimated annual usage when requesting a quote can help the manufacturer evaluate the appropriate approach.
Tooling: Extrusion Dies vs. Molds
Both custom extrusion and molding generally require tooling.
Extrusion Tooling
Custom extrusion uses a die designed around the profile’s cross section.
Because the profile continues through the die, the tooling primarily controls the shape of the cross section.
Molding Tooling
A mold creates the complete three-dimensional geometry of the component.
Depending on the part, a mold may be significantly more complex than an extrusion die.
Tooling design can affect:
- Part geometry
- Tolerances
- Production efficiency
- Upfront project cost
- Manufacturing repeatability
This is another reason to involve a manufacturer early in the design process.
What About Tolerances?
Rubber is flexible and behaves differently from machined metal or rigid plastic.
Both extruded and molded rubber components require realistic tolerances based on:
- Material
- Geometry
- Durometer
- Manufacturing process
- Part dimensions
- Functional requirements
Engineers should identify dimensions that are genuinely critical to fit or performance.
Applying extremely tight tolerances to non-critical dimensions can increase manufacturing difficulty without providing a meaningful benefit to the finished product.
Which Process Is More Cost-Effective?
There isn’t a universal answer.
The cost of a rubber component depends on numerous factors, including:
- Material
- Tooling
- Geometry
- Part size
- Production quantity
- Tolerances
- Secondary operations
- Quality requirements
- Packaging
Extrusion may provide an efficient way to manufacture long continuous profiles.
Molding may be the only practical way to manufacture a complex three-dimensional component.
Rather than choosing a process based solely on perceived manufacturing cost, the first priority should be determining which process can reliably create the required part.
Questions to Ask When Choosing Between Extrusion and Molding
If you’re uncertain which process is appropriate, ask:
Does the part maintain the same cross section along its entire length?
If yes, extrusion may be appropriate.
Does the geometry change throughout the part?
If yes, molding may be required.
Does the part need holes, recesses, or integrated features?
Molding may offer greater design flexibility.
Does the product need to be supplied in long continuous lengths?
Extrusion is generally better suited for this requirement.
Does the component need to be bonded to metal?
Molding may be the more appropriate process.
Can a continuous extrusion be cut or joined to create the finished component?
If so, extrusion plus secondary fabrication may provide a solution.
Extrusion and Molding Can Complement Each Other
Rubber extrusion and molding shouldn’t necessarily be viewed as competing manufacturing processes.
They solve different problems.
Some assemblies may even use both extruded and molded rubber components.
For example, a piece of industrial equipment might use:
- An extruded rubber door seal
- A molded vibration isolator
- An extruded protective channel
- A molded rubber bumper
Each component is manufactured using the process best suited to its geometry and function.
NWRE works with customers across a broad range of industries and applications where different rubber manufacturing processes may be required.
Rubber Extrusion or Rubber Molding: Which Is Right for Your Project?
As a general guideline:
Choose rubber extrusion when:
- The profile has a consistent cross section
- Long continuous lengths are required
- You’re manufacturing tubing, strips, channels, or seals
- The finished component can be cut or fabricated from an extrusion
Consider rubber molding when:
- The part has complex three-dimensional geometry
- The shape changes throughout the component
- Integrated holes or features are required
- Rubber-to-metal bonding is needed
- The component cannot be created from a continuous profile
The final decision should account for material, geometry, tolerances, quantity, secondary operations, and the operating environment.
Talk to Northwest Rubber Extruders About Your Project
Northwest Rubber Extruders provides custom extrusion and molding capabilities for customers across a variety of industrial markets.
Whether your project requires a continuous custom profile or a complex molded rubber component, providing a drawing and information about the application can help determine the appropriate manufacturing process.
Explore our Rubber Extrusions, Rubber Molding, Silicone Extrusions, Material & Engineering, and Industries Served capabilities.
Have a drawing or specification you’d like reviewed? Request a Quote from Northwest Rubber Extruders.