Understanding Shore Hardness: A Guide to Silicone and TPE Overmolding

In the contemporary landscape of product design and manufacturing, the interface between user and machine has transcended mere functionality to become a defining element of brand identity and user experience. This evolution is most visibly manifested in the widespread adoption of "soft touch" surfaces—ergonomic, tactile interfaces that communicate quality, safety, and comfort. At the heart of this manufacturing revolution lies the process of overmolding, a sophisticated injection molding technique where a soft elastomeric material is chemically or mechanically bonded to a rigid substrate. Whether it is the vibration-dampening grip of a power tool, the biocompatible seal of a medical device, or the waterproof gasket of a consumer electronic, the success of these components hinges on a precise understanding of material hardness.

For manufacturers and engineers, particularly those leveraging the capabilities of industry leaders like HYM Plastic, the metric of "Shore Hardness" serves as the common language translating subjective tactile desires into objective engineering specifications.¹ However, Shore hardness is far more than a simple number stamped on a material datasheet; it is a complex viscoelastic property that dictates processing parameters, wear resistance, sealing efficacy, and the fundamental mechanics of adhesion.

This comprehensive report provides an exhaustive analysis of Shore hardness within the context of rubber injection molding and silicone overmolding. Drawing upon the technical competencies of HYM Plastic’s Xiamen manufacturing facility—which specializes in high-precision multi-material molding—we will dissect the physics of indentation, the rheological distinctions between Thermoplastic Elastomers (TPE) and Liquid Silicone Rubber (LSR), and the intricate design guidelines required to achieve robust, delamination-free bonds. By synthesizing theoretical polymer science with practical shop-floor realities, this guide aims to empower engineers to optimize their designs for manufacturability, ensuring that the final product meets the rigorous demands of automotive, medical, and industrial applications.

To effectively utilize TPE shore hardness in design, one must first understand the underlying physics of how hardness is defined and measured in polymeric materials. Unlike metals, where hardness is often a static predictor of tensile strength and scratch resistance, the hardness of elastomers is a dynamic, time-dependent response to applied stress.

Polymers, including the TPEs and silicones used in soft touch molding, exhibit viscoelastic behavior. This means they possess characteristics of both viscous liquids and elastic solids. When a force is applied to an elastomer, the polymer chains uncoil and stretch (elastic response), but they also slide past one another (viscous flow). This duality creates a phenomenon known as "creep" or stress relaxation.

When a durometer indenter is pressed into a rubber sample, the material initially resists with a certain force. However, if the force is maintained, the polymer chains slowly rearrange to accommodate the stress, causing the reading to drift lower over time. This is why ASTM D2240 and ISO 868 standards specify precise dwell times for readings.³ A reading taken instantly might be "Shore 60A," while a reading taken after a 15-second dwell might drop to "Shore 55A." For engineers specifying materials for sealing applications, understanding this relaxation is critical; a seal that relaxes too much over time will lose its compression set and fail to prevent fluid ingress.

The wide spectrum of polymeric stiffness—ranging from the gelatinous softness of a shoe insole to the structural rigidity of a hard hat—cannot be accurately measured by a single scale. The Shore hardness system, therefore, employs multiple scales, each utilizing a specific indenter geometry and spring force to target a distinct range of materials.

The Shore A scale is the workhorse of the rubber injection molding industry. It is designed for measuring soft to semi-rigid elastomers, covering the vast majority of TPE and silicone overmolding applications.⁵

• Indenter Geometry: The Shore A durometer uses a truncated cone with a 35° angle and a flat tip diameter of 0.79 mm.³ This blunt tip prevents the indenter from piercing soft materials, allowing it to measure resistance to compressive deformation rather than puncture resistance.

• Spring Force: The device applies a force of approximately 8.06 Newtons (822 g).⁵

• Application Range: This scale is appropriate for materials ranging from soft rubber bands (Shore 20A) to automotive tire treads (Shore 60A) and hard skate wheels (Shore 90A).⁷ In the context of HYM Plastic's production, Shore A is the primary metric for specifying soft-touch grips on handheld devices and flexible seals in medical assemblies.⁹

As materials become harder, the blunt Shore A indenter no longer penetrates sufficiently to provide a readable resolution. The Shore D scale is introduced for these harder, semi-rigid to rigid plastics, such as the Polypropylene (PP) or ABS substrates often used as the base layer in overmolding.⁵

• Indenter Geometry: The Shore D indenter is a sharp point, a 30° cone with a purely spherical tip radius of just 0.1 mm.³ This sharpness allows it to concentrate stress and penetrate rigid thermoplastics.

• Spring Force: To overcome the modulus of rigid plastics, the Shore D durometer applies a significantly higher force of approximately 44.5 Newtons (4536 g).⁵

• Application Range: This scale measures structural plastics like hard hats, PVC piping, and the rigid housings of electronic devices.¹¹ While the overmolded skin is usually Shore A, the substrate is almost always Shore D, and the interaction between these two distinct hardnesses is where the engineering challenge lies.

At the opposite end of the spectrum lie ultrasoft gels, foams, and cellular rubbers. These materials are so compliant that the spring force of a Shore A durometer would fully compress them, yielding a useless "0" reading. The Shore OO scale utilizes a spherical indenter and a very low spring force (~1.11 N) to measure these delicate materials.¹¹ While less common in structural overmolding, Shore OO materials are occasionally used in specialized cushioning applications where shock absorption is the primary function.

A frequent source of confusion in material selection is the overlap between scales. The scales are not continuous; rather, they represent different stress-strain interrogations of the material. A material that registers as Shore 95A is essentially measuring the same hardness as a material registering Shore 45D.¹¹

However, relying on conversion charts can be dangerous for critical applications. The sharp Shore D indenter induces plastic deformation and high localized stress, whereas the blunt Shore A indenter induces elastic compression. For a precise engineering specification, particularly when dealing with the high-precision requirements of clients like those of HYM Plastic, the scale must be explicitly stated. It is insufficient to say "Hardness 50"; one must specify "Shore 50A" or "Shore 50D" to avoid catastrophic manufacturing errors.

Table 1: Comparative Analysis of Shore Hardness Scales

The selection of the overmolding material is the single most consequential decision in the product development process. While both TPE and Silicone can be formulated to achieve identical Shore hardness values (e.g., Shore 40A), their chemical backbones, processing requirements, and long-term performance characteristics are vastly different. HYM Plastic offers capabilities in both silicone overmolding and TPE injection, allowing for an unbiased evaluation based on application needs.¹

TPEs represent a class of copolymers or physical mixes of polymers that combine the mechanical properties of thermoset rubber with the processability of thermoplastics. This unique behavior is derived from their phase-separated morphology.

TPEs typically consist of "hard" segments (crystalline domains) and "soft" segments (amorphous domains). The hard segments act as physical cross-links at room temperature, providing strength and determining the Shore hardness. The soft segments provide elasticity and flexibility.⁶ By adjusting the ratio of hard to soft segments during polymerization, manufacturers can precisely tune the TPE shore hardness from a gel-like Shore 20A to a rigid Shore 70D.⁶

The primary advantage of TPE in rubber injection molding is its thermoplastic nature. Unlike thermoset rubber, which undergoes an irreversible chemical reaction (vulcanization), TPEs simply freeze when cooled and melt when heated.

• Cycle Time: Because no curing time is required inside the mold, TPE cycles are generally faster, limited only by the cooling rate of the part.

• Recyclability: Sprues, runners, and defective parts can be reground and mixed with virgin material, significantly reducing material waste and cost—a key factor for high-volume production at facilities like HYM Plastic.¹⁶

• Bonding: TPEs are available in diverse chemistries (Styrenic, Olefinic, Urethane) that can be matched to the substrate for natural chemical bonding. For example, a TPE-S (Styrenic) naturally bonds to Polypropylene (PP) without the need for adhesives.¹⁷

LSR is a two-component thermoset material typically based on a siloxane backbone (alternating Silicon and Oxygen atoms). It is supplied as Part A and Part B, which are mixed in a 1:1 ratio immediately before injection.

The Si-O bond is significantly stronger and more stable than the C-C bond found in TPEs. This gives silicone exceptional resistance to temperature extremes (-60°C to +250°C), UV radiation, ozone, and chemical attack.¹⁹ While a TPE grip might soften and become tacky if left on a car dashboard in the summer, an LSR grip will remain unchanged.

LSR undergoes platinum-catalyzed hydrosilylation, an irreversible cross-linking reaction. This requires the mold to be heated (often 150°C to 200°C) to accelerate the cure. This fundamental difference in processing—a hot mold for LSR versus a cold mold for TPE—requires dedicated LSR injection tooling and machinery, a capability that distinguishes advanced manufacturers like HYM Plastic.¹⁴

LSR is inherently inert and biocompatible, making it the material of choice for medical devices and food-contact applications. It meets stringent USP Class VI and ISO 10993 standards more easily than many TPE formulations. HYM Plastic’s expertise in medical device shells suggests a strong competency in leveraging these properties for healthcare clients.¹

When choosing between TPE and LSR for a soft touch molding project, the following factors must be weighed: