Rubber parts are generally produced from compounded elastomers that contain a base polymer along with curing agents, fillers, plasticizers, antioxidants, pigments, processing aids, and other ingredients selected to create the required finished properties.
Unlike a thermoplastic part that can often be melted and resolidified, many industrial rubber components depend on a curing reaction that converts the uncured compound into a stable elastic material.
Rubber Molding and Rubber Extrusion Defined
Rubber molding forms elastomer compounds inside shaped tooling using heat and pressure, while rubber extrusion forces uncured compound through a die to create a continuous profile that is subsequently cured into its final elastic condition.
Molding is well suited to discrete three-dimensional components, while extrusion is efficient for products whose cross-section remains substantially constant along their length.
Rubber Molding Methods
Compression Molding
A measured charge of uncured compound is placed directly into an open mold cavity. The mold closes and heat and pressure cause the material to flow and cure.
Transfer Molding
Rubber is placed in a separate chamber and forced through runners and gates into closed mold cavities before curing.
Rubber Injection Molding
Prepared rubber compound is injected into heated closed tooling, supporting automation, repeatability, and production of complex parts.
| Factor | Compression | Transfer | Injection |
|---|---|---|---|
| Material Loading | Directly into cavity | Into transfer pot | Into injection unit |
| Tool Complexity | Often comparatively simple | Includes runners and transfer system | More complex machine and tooling interface |
| Automation | Low to moderate | Moderate | Often strong fit for automated production |
| Typical Fit | Larger, simpler, lower-volume parts | Parts with inserts or multiple cavities | Repeat production and more complex geometry |
How Rubber Molding Works
Elastomer, fillers, curing agents, pigments, and other ingredients are mixed into a controlled rubber compound.
Compound may be cut, weighed, stripped, pelletized, or preformed into charges suited to the molding process.
Material is placed into the mold, transfer pot, or injection system.
Heated mold sections close and apply pressure to the compound.
Pressure forces uncured rubber into the required cavity geometry.
Heat and time activate the curing system and develop the final elastomer network.
The tool opens after sufficient cure has been achieved.
Molded rubber components are manually or automatically removed.
Excess material is removed and the parts are inspected, post-cured, assembled, cleaned, or packaged as required.
Rubber Extrusion
Rubber extrusion produces continuous profiles from uncured elastomer compound. The material is fed into an extruder, worked by a rotating screw, and pushed through a die that establishes the profile shape.
Continuous Cross-Section Components
Extruded rubber is dimensionally unstable before cure. The uncured profile can swell after leaving the die, sag under its own weight, distort during handling, and change size during vulcanization.
How Rubber Extrusion Works
Prepared rubber compound enters the extruder in strips, pellets, or another controlled feed form.
The screw moves, works, and pressurizes the uncured compound.
Material passes through shaped tooling that creates the approximate profile cross-section.
The uncured extrusion is supported or conveyed carefully to limit stretching and distortion.
Heat or another curing system crosslinks the elastomer and locks the profile into its final elastic state.
The cured profile is cooled sufficiently for stable handling.
Controlled conveying maintains line speed while minimizing stretch.
Finished extrusion is cut to length, coiled, spliced, or prepared for later fabrication.
Cross-section, dimensions, cure, surface condition, hardness, and length are evaluated.
Curing and Vulcanization
Curing is the stage that develops the elastic structure of many industrial rubber compounds. The process creates crosslinks between polymer chains so the material can deform and return toward its original shape.
What Controls Final Rubber Properties
Under-cured material may not achieve required properties, while excessive heat or cure time can damage some compounds. Cure conditions therefore need to be established for the specific formulation and part geometry.
Common Rubber and Elastomer Materials
| Material | General Characteristics | Common Uses |
|---|---|---|
| EPDM | Weather, ozone, water, and environmental resistance with broad sealing and outdoor usefulness. | Weatherstripping, seals, gaskets, hoses, roofing, and outdoor components. |
| Nitrile | Good resistance to many oils, fuels, greases, and industrial fluids. | O-rings, gaskets, seals, diaphragms, boots, and fluid-system components. |
| Neoprene | Useful balance of weather, oil, abrasion, flame, and general-purpose properties. | Gaskets, mounts, pads, hoses, seals, covers, and industrial products. |
| Silicone | Wide temperature capability, flexibility, electrical insulation, and specialized clean or medical formulations. | Seals, tubing, gaskets, medical parts, electrical components, and high-temperature products. |
| Natural Rubber | High elasticity, tear strength, fatigue resistance, and dynamic performance. | Mounts, vibration products, rollers, bumpers, and mechanical components. |
| SBR | General-purpose synthetic rubber with useful abrasion and wear characteristics. | Pads, gaskets, seals, industrial sheet, and general rubber products. |
| Fluoroelastomer | Strong resistance to heat, fuels, oils, and aggressive chemical environments. | Seals, O-rings, gaskets, valve components, and demanding fluid systems. |
| Butyl Rubber | Low gas permeability and useful chemical, weather, and damping characteristics. | Seals, liners, diaphragms, protective products, and air-retention components. |
Material selection should also consider hardness, compression set, tensile strength, elongation, rebound, tear resistance, abrasion, temperature range, ozone exposure, weathering, oils, fuels, water, chemicals, electrical requirements, and expected service life.
Products Made by Rubber Molding and Extrusion
Rubber Molds and Extrusion Dies
Design for Rubber Molding and Extrusion
Very thick rubber sections require longer curing time and can create greater variation between surface and internal cure.
Rounded transitions improve material flow and reduce stress concentration in molded elastomer components.
Mold splits determine where flash appears and can influence sealing surfaces, cosmetic areas, and trimming.
Rubber molding naturally produces some excess material at tool interfaces, so critical edges should be identified clearly.
Deep pockets and enclosed regions should allow practical venting during mold filling.
Molded rubber can be bonded to metal or rigid inserts when surface preparation and bonding systems are engineered correctly.
Seals and gaskets should be designed around the required compression, deflection, load, and service environment.
Rubber extrusion is best suited to profiles that remain substantially constant along their length.
Extruded seals may be cut and joined into rings or frames, so splice location and strength should be considered.
Rubber Tolerances, Shrinkage, and Dimensional Control
Rubber dimensions are influenced by compound formulation, cure, tooling, part thickness, mold temperature, pressure, flash, cooling, post-cure, hardness, and the flexible nature of the material itself.
Factors That Influence Rubber Part Size
Soft rubber can deform while being measured, so inspection methods should avoid excessive contact pressure. Functional gauges and assembly checks may be more meaningful than unusually tight dimensional specifications on flexible features.
Common Rubber Molding and Extrusion Defects
Secondary Operations for Rubber Components
Deflashing
Cryogenic, mechanical, manual, tumbling, or trimming methods can remove excess material from molded components.
Post-Curing
Selected elastomer systems may require additional controlled heat after molding to develop final properties or remove volatiles.
Cutting & Splicing
Extruded seals can be cut to length and joined into rings, frames, corners, or custom assemblies.
Bonding
Rubber can be bonded to metal, plastic, fabric, or other elastomer components using suitable systems.
Marking
Printing, colored compounds, labels, molded identification, and other methods support part identification.
Cleaning
Mold release, dust, processing residue, or contamination may be removed before final assembly or packaging.
Rubber Inspection and Testing
Rubber quality cannot be evaluated by dimensions alone. Material properties, hardness, compression set, tensile behavior, elongation, cure, appearance, adhesion, environmental resistance, and sealing performance may all matter.
Characteristics Commonly Evaluated
Testing may also evaluate aging, heat resistance, fuel resistance, oil resistance, ozone resistance, low-temperature flexibility, tear strength, abrasion, fluid immersion, and other service-specific properties.
What Drives Rubber Molding and Extrusion Cost?
Elastomer type, fillers, additives, cure system, color, certifications, and specialty performance affect material cost.
Mold cavities, inserts, runners, gates, vents, extrusion dies, fixtures, and expected production life influence upfront investment.
Longer curing cycles reduce machine throughput and increase recurring processing cost.
Thick rubber sections can require additional compound and longer heat exposure to achieve proper cure.
Multi-cavity tooling increases mold investment but can improve production output.
Manual trimming, cryogenic deflashing, inspection, and finishing add recurring labor and equipment time.
Metal preparation, adhesive systems, insert handling, placement, and bonding add complexity.
Material certification, laboratory testing, dimensional inspection, leak testing, and special documentation increase cost.
Related Rubber and Elastomer Resources
Rubber parts often work directly with metal, plastic, hydraulic, pneumatic, electrical, and mechanical components. Material selection and manufacturing should therefore be coordinated with the operating environment and the mating hardware.
Rubber, Sealing & Production Research
These manufacturing references correspond with elastomer products and related processes used alongside molded and extruded rubber components.
How to Select a Rubber Molding or Extrusion Supplier
Rubber suppliers should be evaluated against compound capability, molding or extrusion method, part dimensions, hardness, annual volume, tooling, curing, secondary operations, testing, and final service requirements.
Confirm experience with the required EPDM, nitrile, neoprene, silicone, natural rubber, SBR, fluoroelastomer, or other compound.
Review compression, transfer, injection molding, extrusion, splicing, bonding, and insert-molding capability.
Mold design, die development, tooling maintenance, repair, venting, inserts, and engineering changes should be supported.
Mold temperature, cure time, extrusion vulcanization, post-cure, and batch traceability should be controlled consistently.
Confirm practical tolerance capability for the selected compound, hardness, geometry, shrinkage, and flexible part condition.
Manual, cryogenic, tumble, and other deflashing methods should match the part's edges and cosmetic requirements.
Hardness, tensile, elongation, compression set, aging, fluid resistance, dimensions, and functional tests may be required.
Mixing, molding presses, extruders, cure systems, tooling, trimming, testing, and staffing should support expected demand.
Rubber Manufacturing Depends on Both Shape Formation and Controlled Cure
Compression, transfer, injection molding, and extrusion can produce seals, gaskets, O-rings, profiles, tubing, boots, vibration mounts, bumpers, and other elastomer components. Successful rubber production depends on compound selection, material flow, mold or die design, cure conditions, shrinkage, flash control, hardness, dimensional requirements, secondary operations, testing, and the chemical, thermal, mechanical, and environmental conditions of final service.