Custom silicone rubber parts are used across medical, automotive, consumer
electronics, and industrial applications where flexibility, environmental
resistance, and long-term reliability are critical. For engineering teams,
material selection is often the most important decision influencing part
performance, manufacturing feasibility, and total cost of ownership.
This article provides a practical guide to selecting silicone materials for custom silicone rubber parts, covering hardness, temperature resistance, sealing performance, regulatory requirements, and how material choice interacts with mold design and molding process.
Silicone rubber is not a single material but a family of elastomers with widely varying properties. Different formulations offer distinct combinations of hardness, tensile strength, elongation, compression set, temperature range, and chemical resistance. Selecting the wrong grade can lead to premature failure, excessive compression set, poor sealing performance, or inability to meet regulatory requirements.
Material selection must therefore be driven by application requirements rather than cost alone. Key questions include: What loads and deformations will the part experience? What temperature and chemical environment will it face? Are there biocompatibility, food-contact, or flame-retardant requirements? The answers determine the appropriate silicone family and specific grade.
Several silicone material families are commonly used in compression molding, each with distinct performance characteristics.
The choice among these families depends on the operating environment and performance priorities. For a broader view of how these materials are processed, see Silicone Compression Molding: A Complete Process Guide for Engineers.
Silicone hardness, typically measured in Shore A, is one of the most visible material parameters and has a direct impact on part behavior. Soft silicones (e.g. 20–40 Shore A) conform easily to mating surfaces and are ideal for sealing and cushioning, but may be more prone to extrusion under high pressure. Harder silicones (e.g. 60–80 Shore A) offer better structural support and resistance to deformation, but may not seal as effectively on rough or uneven surfaces.
Hardness also affects compression set, rebound, and feel. For soft-touch components such as buttons or grips, a lower hardness combined with good rebound is often desired. For static seals in high-pressure environments, a higher hardness with low compression set is typically preferred. The optimal hardness must balance sealing performance, mechanical support, and assembly requirements.
One of silicone's key advantages is its wide operating temperature range. General-purpose silicones can typically operate continuously from -50°C to +200°C, with short-term exposure to even higher temperatures. However, prolonged exposure to extreme temperatures accelerates aging and can increase compression set.
Environmental factors beyond temperature also influence material selection. Exposure to oils, fuels, solvents, ozone, UV radiation, and steam can degrade certain silicone formulations. Fluorosilicone offers superior fluid resistance but at higher cost. For outdoor applications, UV and ozone resistance must be considered. In sterilization-intensive medical applications, resistance to repeated autoclave cycles or other sterilization methods is critical.
For sealing applications, compression set is often the most critical property. Compression set measures the permanent deformation remaining after a silicone part has been compressed for a defined time and temperature. A high compression set means the seal does not fully recover its original shape, leading to leakage over time.
Low compression set formulations, combined with appropriate post-cure treatment, are essential for long-term sealing reliability. Material selection must consider not only initial hardness and tensile properties but also how the material behaves after prolonged compression and thermal exposure. This is particularly important in automotive, industrial, and medical applications where seal failure can have serious consequences.
Many silicone part applications are subject to regulatory or industry standards. Medical devices may require biocompatibility testing per ISO 10993 and documentation of material composition. Food-contact applications must comply with regulations such as FDA 21 CFR 177.2600 or EU 10/2011. Automotive and aerospace applications may have additional requirements for flame retardancy, outgassing, or material traceability.
When specifying custom silicone rubber parts, engineering teams should clearly communicate all compliance requirements to the silicone molding service provider. Material certifications, batch traceability, and documentation should be agreed upon before production begins.
While performance is paramount, appearance also matters in many applications. Silicone parts can be produced in a wide range of colors using pigments that are stable at curing temperatures. Transparency or translucency may be required for visual inspection of fluid paths or for aesthetic reasons. Surface finish, from high gloss to matte or textured, affects both appearance and functional behavior such as friction and cleanability.
Color consistency across batches is important for brand and product identity. This requires careful control of pigment dispersion, curing conditions, and material handling. For applications where appearance is critical, sample approval and color matching under defined lighting conditions are recommended.
Material selection cannot be considered in isolation from mold design and molding process. Different silicone grades exhibit different flow behavior, shrinkage rates, and cure characteristics. A material that works well in one mold geometry or process window may perform differently in another.
For example, high-hardness silicones may require higher compression pressures to fully fill intricate features. Materials with fast cure rates may be more prone to trapped air if venting is inadequate. Post-cure requirements also vary by formulation and must be factored into cycle time and equipment capacity. Close collaboration between material suppliers, mold designers, and molding engineers is essential to optimize the entire system.
For more detail on how process parameters and mold design influence part quality and defect formation, refer to Silicone Rubber Compression Molding: Defects, Quality Control, and Process Optimization
Consider a medical device requiring a silicone sealing diaphragm that must withstand repeated flexing, exposure to bodily fluids, and periodic sterilization. The design team initially selected a general-purpose silicone based on cost and availability. However, early testing showed excessive compression set after autoclave cycles, leading to leakage.
By switching to a medical-grade silicone with low compression set formulation and optimized post-cure treatment, the team achieved stable sealing performance over thousands of cycles. The material change increased unit cost slightly but significantly reduced field failure risk and warranty costs. This example illustrates how material selection directly impacts both performance and total cost of ownership.
To streamline material selection and avoid common pitfalls, engineering teams can follow a few practical guidelines when specifying silicone materials for custom parts.
By following these guidelines and working closely with an experienced molding partner, engineering teams can select silicone materials that deliver reliable performance, meet compliance requirements, and support efficient manufacturing across prototype and production stages.
