Before an O-ring, a sealing profile or a silicone tube reaches your assembly line, a rarely discussed process takes place: compounding.
Compounding is the formulation and mixing of the elastomer. This is where it is determined whether that seal will withstand 200°C or 300°C, whether that profile will comply with EN 45545-2 or not, whether that membrane will endure one million flexure cycles or fail after ten thousand.
When an engineer specifies '60 Shore A silicone', they are specifying a property. But that property can be achieved with radically different formulations: one optimised for tear strength, another for chemical resistance, another for low compression set. Same hardness, entirely different in-service behaviour.
This article explains what occurs during HCR silicone compounding, which formulation decisions determine the final properties, and why understanding this process helps to correctly specify seals, profiles, membranes and technical silicone components.
1. What is compounding and why does it matter
HCR silicone does not arrive ready to use. It arrives as a polymer base — a very high viscosity material, similar to a dense putty — which on its own has poor mechanical properties.
| Property | Unreinforced base | Formulated compound |
|---|---|---|
| Tensile strength | < 1 MPa | 6-10 MPa |
| Tear strength | < 5 kN/m | 15-55 kN/m |
| Elongation | Variable | 100%-1000% depending on formulation |
Compounding transforms this base into a useful material through the incorporation of fillers, additives and curing agents. If the formulation is incorrect or the mixing deficient, no subsequent adjustment in extrusion or moulding can compensate.
2. Anatomy of an HCR formulation
2.1 Polymer base
The base determines the fundamental chemistry of the elastomer:
| Base | Temperature range | Typical application |
|---|---|---|
| VMQ (vinyl-methyl-silicone) | -60°C to +200°C | General purpose, food contact, medical |
| PVMQ (phenyl-vinyl-methyl-silicone) | -110°C to +200°C | Cryogenics, liquefied gases |
| FVMQ (fluorosilicone) | -60°C to +170°C | Contact with fuels, oils, solvents |
The choice is not interchangeable. A seal for a fuel circuit requires FVMQ; a standard VMQ would swell and fail.
2.2 Reinforcing fillers
Silica is the primary filler, responsible for mechanical properties:
- Fumed silica: Nanometric particles, high specific surface area (150-400 m²/g). Maximum mechanical reinforcement. Standard for high tear strength formulations.
- Precipitated silica: Lower specific surface area, less reinforcement, more economical. Suitable for general purpose formulations.
2.3 Functional additives
| Additive | Function | Application |
|---|---|---|
| Heat stabilisers | Protection against oxidative degradation | Oven seals, engine components (+300°C) |
| Flame retardants | EN 45545-2 compliance | Railway profiles |
| Conductive fillers | ESD dissipation, EMI shielding | Electronics |
| Radio-opaque additives | X-ray visibility | Medical tubing |
| Plasticisers | Hardness reduction, process improvement | Property adjustment |
Railway
Manufacturing of silicone components and seals with EN 45545 certifications and fire resistance for railway applications.
Explore sector →2.4 Cure system
| System | Advantages | Limitations | Typical application |
|---|---|---|---|
| Peroxide | Versatile, economical, suitable for extrusion and moulding | Requires post-cure, generates by-products | General industrial |
| Platinum (addition) | No by-products, clean vulcanisation | Sensitive to contamination, higher cost | Medical, high-specification food contact |
The cure system affects certifications, processing and cost. Implantable medical applications: platinum cure mandatory.
Medical
LSR parts, tubes and components certified ISO 13485 and USP Class VI for medical devices.
Explore sector →3. The two-roll mill: where mixing takes place
The two-roll mill is where the final mixing of the HCR compound is carried out. Two steel cylinders rotate in opposite directions at different speeds (typical friction ratio 1:1.2), creating a high-shear zone.
3.1 What occurs during the process
- Breaks up silica agglomerates and disperses them uniformly
- Incorporates additives into the polymer matrix
- Homogenises the mixture to uniform consistency
The operator controls the nip gap, temperature and mixing time. They cut, fold and re-introduce the material repeatedly until complete dispersion is achieved.
3.2 Why dispersion matters
Poorly dispersed silica means inconsistent properties. Within the same component there may coexist zones with high filler concentration (hard, brittle) and zones with low filler content (soft, weak).
Consequences in the finished component:
- Hardness variation within the same component
- Weak points where fracture initiates
- Mechanical properties below specification
- Premature in-service failure
3.3 Temperature control and scorching
The rolls can be heated or cooled depending on the process stage. Scorching (premature vulcanisation) is the principal risk: if the temperature is excessive when the peroxide is added, crosslinking begins prematurely and the compound loses processability.
A partially scorched compound does not flow correctly in extrusion nor fill mould cavities properly.
Extrusion
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Molding
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View capability →3.4 The human factor
The two-roll mill is not a press-button-and-wait process. The operator works the compound manually: cutting the sheet with a blade, folding it, re-introducing it between the rolls, and repeating. They observe the material appearance, detect undispersed agglomerates, and adjust the nip gap on the fly.
An experienced operator knows when the silica is fully dispersed by touch and the sheen of the sheet. They know that certain pigments require additional passes. They know that in summer, with the shop floor at 35°C, the roll temperature must be reduced to avoid scorching. They know that the peroxide is added last, with the compound already cooled, and incorporated rapidly to minimise exposure time.
This knowledge does not appear in any technical data sheet. It is acquired through years of work at the mill.
Automation has its limits in this process. An internal mixer (Banbury) controls time, temperature and energy with precision. But the visual inspection of dispersion, the fine adjustment according to how the compound behaves on that particular day, the decision to run two additional passes — that remains the operator's judgement.
4. Formulations by functional requirement
| Requirement | Formulation | Key properties | Typical products |
|---|---|---|---|
| General purpose | Standard VMQ | 10-90 ShA, -60/+200°C, tear 10-23 kN/m | O-rings, profiles, industrial tubing |
| High tear strength | Reinforced VMQ | 40-80 ShA, tear 26-55 kN/m | Inflatable seals, bellows, membranes, diaphragms |
| High temperature | Stabilised VMQ | 40-70 ShA, up to +300°C continuous | Oven seals, engine gaskets |
| Low temperature | PVMQ | 50 ShA, down to -110°C | Cryogenic equipment, liquefied gases |
| Chemical resistance | FVMQ | 40-70 ShA, hydrocarbon resistant | Fuel circuit seals, hydraulic applications |
| Railway | VMQ + flame retardants | EN 45545-2 HL1-HL3, Ds max 45-85 | Door profiles, window seals, cable grommets |
| Medical | Platinum-cured VMQ | USP Class VI, ISO 10993, 25-80 ShA | Aspiration tubing, catheters, implantable components |
| Food contact | Certified VMQ | FDA, BfR, EC 1935/2004 | Food processing seals, transfer tubing |
| Low compression set | Optimised VMQ | CS 11-18% (70h/150°C) | Static O-rings, covers, flanges |
| Electrically conductive | VMQ + conductive fillers | Resistivity 4-12 Ω·cm | EMI shielding, ESD dissipation |
The in-service difference is significant: a pump membrane with a standard formulation (tear strength 15 kN/m) may fail at 100,000 cycles; the same geometry with a high tear formulation (45 kN/m) may exceed one million cycles.
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View product →5. Specification errors related to compounding
Specifying hardness alone: '60 Shore A silicone' is not a complete specification. There are dozens of formulations at that hardness, each optimised for something different.
Requesting 'food-grade silicone' without specifying the standard: FDA, BfR and EC 1935/2004 are not equivalent. And certain pigments invalidate certifications that the base compound does hold.
Copying the specification from another component: A formulation for an extruded profile may not be suitable for a moulded part in the same application. The vulcanisation process is different.
Ignoring the cure system: Peroxide and platinum are not interchangeable. Implantable medical: platinum mandatory. Overmoulding onto certain plastics: platinum may be inhibited.
Not considering minimum order quantity: Special formulations require minimum batch sizes. If your consumption is 50 kg/year and the minimum batch is 500 kg, there is a stock and shelf-life issue.
6. Quality control
From each compound batch, a sample is vulcanised and properties tested to ISO standards:
| Test | Standard | Purpose |
|---|---|---|
| Hardness | ISO 7619-1 | Specification verification |
| Tensile strength and elongation | ISO 37 | Mechanical properties |
| Tear strength (Type C) | ISO 34-1 | Crack propagation resistance |
| Density | ISO 2781 | Formulation control |
| Compression set | ISO 815 | Elastic recovery (where applicable) |
Each batch is identified with a unique number enabling traceability of raw materials, manufacturing conditions and destination. For medical (ISO 13485) and aerospace applications, full traceability is a requirement.
ISO 8 Cleanroom
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View capability →Conclusion
Compounding is the invisible step that determines whether an O-ring, a sealing profile, a membrane or a silicone tube will fulfil its function or fail in service.
Specifying correctly means understanding that 'silicone' is not a single material, that the same hardness can be achieved with radically different formulations, and that each special property requires a specific formulation.
Most silicone specifications focus on the finished component: hardness, temperature, certification. But the finished component is a consequence of the compound. And the compound is a consequence of formulation decisions and the expertise of the operator working the material on the mill. Whoever controls the compounding controls the outcome.
Need advice on formulations?
Our engineering team will help you select the optimal compound for your specific application.
Consult engineering →Las especificaciones indicadas corresponden a valores típicos de ensayo. Las propiedades finales dependen de la geometría, el proceso de fabricación y las condiciones de servicio. Validar requisitos específicos con el departamento técnico.
