List fluids, cleaning agents, sunlight and ozone. Separate continuous temperature from short peaks.
ETOL ENGINEERING GUIDE
Rubber materials.
Silicone options.
Shore hardness.
Compare rubber and silicone materials for your part, understand durometer and Shore hardness, and review how the shape changes fit and flexibility.
01 / PERFORMANCE REQUIREMENTS
Which operating condition drives the material choice?
A shortlist helps you begin. The exact compound and assembly conditions determine the final choice.
Need several properties? Review each priority, then assess the combined conditions. A material shortlisted for one property may not satisfy the others.
Compare all eight materials →MATERIALS TO CONSIDER
Compare silicone and FKM for the exposure
Silicone is a candidate where flexibility and dry-heat exposure matter. FKM may be more relevant when elevated temperature is combined with compatible oils or fuels. Hot water and steam require a separate review.
What to evaluate
Read the material guidance →Define continuous and peak temperatures, exposure duration and the contact medium. Compare retained flexibility, hardness change and compression recovery after heat aging.
Define sealing, cushioning, retention or movement. Include load, pressure, stroke and cycle frequency.
Decide what success means: leakage, insertion force, deflection, recovery or life under repeated movement.
02 / MATERIAL FAMILIES
Compare rubber and silicone
material families.
These are family-level selection principles. Fillers, cure systems and formulation change performance within the same polymer family.
VMQ
Silicone
Heat, cold & flexible interfaces
A useful starting point for temperature cycling, soft contact surfaces and flexible molded details. Weathering resistance and flexibility across a broad temperature span make silicone relevant to protective boots, keypads and static seals.
Selection limits
Repeated rubbing, sharp edges and aggressive insertion can damage a silicone part. Standard silicone is a poor starting point for fuel contact; silicone oils can also cause swelling.
Design example
For a flexible keypad, review web thickness and return travel alongside hardness. A softer compound alone will not define the button force.
EPDM
EPDM rubber
Outdoor exposure & water contact
Often shortlisted for sunlight, ozone and water exposure. Typical design opportunities include outdoor cable grommets, enclosure gaskets and water-contact seals. Hot-water or steam duty needs a compound selected for the actual cycle.
Selection limits
Petroleum oils and fuels generally rule out EPDM. Include assembly grease and cleaning residues in the fluid review, even when the main service medium is water.
Design example
For an outdoor cabinet gasket, check latch force, corners and the smallest closed gap. Weather resistance cannot compensate for a section that never makes continuous contact.
NBR
Nitrile rubber
Oil-contact machinery components
Consider NBR for many mineral-oil and grease-contact applications, including machinery seals, protective caps and grommets. The formulation influences oil swelling, low-temperature flexibility and recovery after compression.
Selection limits
Outdoor ozone exposure and sustained heat can limit ordinary NBR. Fuel blends, additives and polar solvents need individual compatibility review; oil resistance is not universal solvent resistance.
Design example
For a plug beside a lubricated housing, identify the lubricant and exposure duration. Compare insertion and retention after fluid exposure, when swelling may change the fit.
FKM
Fluorocarbon rubber
Hot oil & demanding fluid contact
A candidate when an oil-contact seal also faces elevated temperature. FKM families offer useful resistance to many hydrocarbon fluids, but different formulations can behave very differently in mixed chemicals.
Selection limits
Do not assume standard FKM suits steam, ketones, amines or every fuel blend. Low-temperature sealing and repeated flexing also need attention. A higher-cost polymer is not automatically the best choice.
Design example
For a hot oil cover seal, review both operating heat and cold starts. The relevant question is whether the assembled seal recovers and maintains contact throughout that cycle.
CR
Chloroprene rubber
General protective parts
CR offers a useful balance of weathering resistance and mechanical properties for protective covers, boots and cushioning parts. It may suit some occasional oil exposure when the exact fluid and compound are compatible.
Selection limits
Treat CR as a balanced option, not a substitute for a dedicated fuel-resistant material. Do not infer a flame rating from the polymer name; fire behavior is formulation and test dependent.
Design example
For a bellows near moving equipment, allow folds to move without rubbing each other. Review the full stroke and any oil splash before choosing the compound.
NR
Natural rubber
Elastic recovery & dynamic movement
Natural rubber is relevant where resilience, repeated deformation and mechanical durability matter. It can be considered for indoor bumpers, feet and vibration-isolating components in suitable environments.
Selection limits
Petroleum oils, ozone and prolonged outdoor exposure are important constraints. Vibration isolation requires dynamic stiffness and damping assessment; a soft hardness reading does not establish isolation performance.
Design example
For a machine mount, identify load per mount, available deflection and excitation frequency. Evaluate the loaded assembly instead of selecting hardness from the total machine weight alone.
IIR
Butyl rubber
Reduced gas permeation
Butyl is worth considering when gas transmission through the material is a design concern. Its relatively low gas permeability can be useful in suitable closures, diaphragms and sealing components.
Selection limits
Low permeability is different from leak-tightness at the joint. Petroleum oil and fuel exposure can be unsuitable, while compression recovery and the actual gas must be considered separately.
Design example
For a gas-retaining closure, assess the wall thickness and sealing interface as well as the compound. Leakage around the edge can dominate permeation through the material.
SBR
Styrene-butadiene rubber
General cushioning & wear
SBR is a practical candidate for general mechanical cushioning, protective pads and selected wear applications. Blends can adjust processing and mechanical behavior, so identify the actual formulation when comparing parts.
Selection limits
Ordinary SBR is not a good starting point for petroleum oils, fuels or ozone exposure. Similar appearance and hardness do not make SBR interchangeable with natural rubber or EPDM.
Design example
For an indoor protective pad, compare thickness, contact area and marking on the finished surface. Increasing hardness may reduce movement but also reduce conformability.
SOLID OR CELLULAR?
Foam changes the structure.
The polymer still matters.
Foam contains cells that deform under load. This can make it useful for low-force cushioning and gap filling, but it does not describe the chemical family. Silicone foam and rubber foam must still be selected for the environment.
Compare open or closed cells, density, thickness, compression-force deflection and recovery. Closed cells can help limit water uptake, but cut edges and the assembled joint still need evaluation. A foam hardness value alone cannot establish sealing pressure or water resistance.
03 / HARDNESS, EXPLAINED
Rubber durometer
and silicone hardness
Shore hardness measures resistance to indentation. Higher values mean a harder response within the same scale, under comparable test conditions.
Solid silicone
20–80 Shore ACustomization range for soft interfaces through firmer support.
Solid rubber
30–90 Shore ACustomization range across rubber families; the achievable range depends on the compound.
Foam selection reference
Shore C Separate scale20–3035–4550–65
Soft cushioning · Balanced support · Firmer feel
These are customization selection references, not measured values for photographed products. The Shore A bars show position on a 0–100 scale. Foam bands use ETOL's Shore C reference and must not be converted to Shore A or substituted for another foam test scale. Agree the instrument, method and sample conditions.
WHAT THE CHOICE MEANS
A discussion starting point, not a finished-part specification. Compare samples with the same geometry and relevant operating conditions.
| Selection direction | What it may help | What to check |
|---|---|---|
| Lower Shore A | Conformity to mating surfaces; easier flexing of a comparable section. | Over-compression, lip folding, pull-out resistance and damage at edges. |
| Middle of the available range | A starting balance between compliance and shape support. | Actual assembly force, contact pressure and recovery after dwell. |
| Higher Shore A | Greater resistance to local deformation in a comparable design. | Higher closing or insertion force; reduced conformity. Hardness alone does not establish wear life. |
| Foam firmness | Adjusting cushioning and support within a selected cellular material. | Force at working compression, bottoming out, cell structure and permanent set. |
Same hardness. Different feel.
A thin lip bends more easily than a thick block made from the same compound. Contact area, cavities, ribs and bonded inserts all change the force required to deform the part.
Softness is not recovery.
Compression set describes residual deformation after a defined compression and recovery cycle. Stress relaxation describes loss of force while held at a fixed deformation. Both matter in long-term sealing.
Measure consistently.
Specify the scale, target and tolerance, reading time, specimen thickness, conditioning and method. A curved or thin finished part can give a different reading from a suitable flat test specimen.
04 / PRACTICAL DESIGN EXAMPLES
Material and hardness
in practical assemblies
Illustrative design situations, not tested performance claims for a particular ETOL product.
A grommet that is difficult to install
A harder wall may resist insertion through the panel; a very soft lip may fold or pull out.
Review: Compare candidate hardnesses within a compatible material family, keeping the same geometry. Check panel thickness, hole finish, groove width and pull-out resistance.
A gasket that leaks despite being soft
Softness helps conformity, but a thin section or uneven flange can leave an open path.
Review: Check the closed gap and continuous contact first. Review compression set and stress relaxation, then validate sealing after the intended dwell time.
A foot that feels firm but leaves a mark
Hardness does not specify staining, friction or compatibility with a painted surface.
Review: Evaluate the actual compound on the finished surface under load and heat. Compare contact area and any screw or insert that concentrates the load.
A suction cup that releases too soon
A soft lip may conform well but still leak on a textured surface. A rigid lip may fail to seat.
Review: Match lip geometry to surface roughness, then check vacuum retention, lift load and release behavior. Compare parts at the same diameter and wall thickness.
A keypad with an inconsistent feel
Button force depends on the web shape, travel and support as well as silicone hardness.
Review: Compare complete molded samples for actuation force, return and repeated cycling. Changing the whole pad hardness can affect every key.
A foam strip that bottoms out
A soft foam may close easily but lose useful cushioning travel under sustained load.
Review: Compare force at the intended compression, recovered thickness and long-term set. Increasing thickness or changing cell structure may be more effective than increasing hardness.
A plug that swells in service
A correct initial fit can become excessively tight after contact with an incompatible fluid.
Review: Address the compound and fluid pairing before changing dimensions or hardness. Check removal force and sealing after exposure, not only immediately after molding.
A mount that still transmits vibration
A very soft mount can deflect excessively, while an unsuitable stiffness can place the system near resonance.
Review: Review static load, dynamic stiffness, damping and operating frequency together. Confirm stability and clearance in the mounted equipment.
05 / TURN THE CHOICE INTO A SPECIFICATION
Describe the job your part must do.
ETOL is the direct manufacturing factory for custom silicone and rubber parts. We can develop custom-shaped components around your mating interfaces and requirements.
A useful selection brief includes
- Part and fit: drawing or sample, dimensions, tolerances, mating surfaces and available space.
- Exposure: named fluids and concentrations, cleaning agents, contact duration and outdoor conditions.
- Temperature: minimum, continuous maximum, brief peaks and the frequency of thermal cycles.
- Forces and motion: load per part, pressure, displacement, assembly force and repeated movement.
- Material target: preferred polymer, solid or foam structure, hardness scale, target and agreed tolerance.
- Acceptance: functional checks, appearance, estimated quantity and any end-use requirements.
Material selection questions
What does a rubber durometer reading tell me?
It measures resistance to indentation using a specified scale and method. Compare readings taken under equivalent conditions. Durometer does not directly specify grip, sealing force or load capacity, because the part's thickness and geometry also affect its response.
How should I compare silicone and rubber for a gasket?
Compare the actual contact medium, temperature and compression conditions. Silicone, EPDM, nitrile and other material families have different selection limits. Choose a compatible compound, then review its hardness and the section needed to maintain contact around the joint.
Can I specify only 60 Shore?
Name the scale, material, nominal hardness and agreed tolerance. Include the measurement method and sample conditions so the sample and production checks compare equivalent readings. A Shore C foam reference should not be treated as a Shore A value.
Should I copy the hardness of an existing part?
Use it as one input. A used part may have aged, swollen or hardened, and its shape can affect the measurement. Review the original function, dimensions and environment before setting the replacement target.
Does choosing silicone establish food-contact or medical suitability?
No. Polymer identity and color do not establish end-use suitability. The exact formulation, processing and intended contact conditions must be addressed for that project. This guide does not assign food-contact or medical approval to any material.
How should a material sample be evaluated in the assembly?
Check dimensions and installation first, then the agreed load, fluid and temperature sequence. Look for changes relevant to the part, such as leakage, removal force, recovered shape or loss of support. Compare the results with the acceptance criteria for that design.
FROM SELECTION TO A CUSTOM PART
Bring us the application.
We will help define the part.
Share the fit, environment and performance you need.