The Shore A Trap: Why Specifying “Silicone Hardness” Isn’t Enough for Custom Parts

Look at almost any 2D engineering drawing for a custom silicone component, and you will see a specification for material hardness—typically written as “50 Shore A” or “70 Durometer.” While hardness is a critical baseline, treating it as the *only* necessary material specification is a common trap that leads to product failures, leaking seals, and delayed manufacturing timelines.

At Reemane, we frequently receive drawings where the specified hardness actively contradicts the part’s intended function or geometry. True Design for Manufacturing (DFM) requires looking beyond the Shore A scale. Here is a guide to understanding the complex relationship between silicone hardness and real-world mechanical performance.

1. The Hardness vs. Tear Strength Trade-off

There is a common misconception that softer silicone is always more stretchable and harder silicone is always tougher. In reality, the chemistry of silicone compounding is a balancing act.

• Low Hardness (10 to 30 Shore A): While highly compressible and excellent for sealing uneven surfaces, very soft silicones often have poor tear strength. If your design includes sharp internal corners or requires aggressive stretching during assembly, a low-durometer part might tear like paper.
• High Hardness (70 to 80 Shore A): These compounds offer excellent abrasion resistance and withstand high pressures (preventing extrusion in O-rings). However, they lack elasticity and can become brittle. If forced to stretch over a large plastic housing, a high-hardness silicone band will likely snap.

Our engineers custom-formulate the silicone base by adjusting silica fillers and cross-linking agents to maximize tear strength at your specific target hardness.

2. Compression Set: The Hidden Enemy of Sealing Force

For gaskets, O-rings, and valves, the most critical metric is not hardness; it is Compression Set. This measures a material’s ability to return to its original thickness after being compressed for a long time at high temperatures.

You might specify a 40 Shore A silicone because it feels perfect out of the box. But if that specific compound has a poor compression set, it will permanently flatten out (lose its “memory”) after a few months of heat cycling in an engine bay or fluid pump, resulting in a catastrophic leak. We match your Shore A requirement with advanced low-compression-set formulations and rigorous post-curing profiles to ensure decades of reliable sealing force.

3. The Illusion of Geometry: Why Thickness Changes “Feel”

A durometer gauge measures the resistance of a material to indentation. However, the physical geometry of your component drastically alters how the hardness “feels” in practical application.

A solid block of 30 Shore A silicone will feel incredibly firm. Conversely, a thin-walled diaphragm made from 70 Shore A silicone will feel flexible and highly responsive. If a procurement manager tests a thick prototype and decides to lower the durometer for the final thin-walled mass-production part, the resulting component will likely collapse under pressure. This is why Reemane provides DFM feedback on wall thickness and structural ribbing before locking in the final Shore A specification.

4. Manufacturing Tolerances: The ±5 Shore A Reality

Silicone compounding is a physical and chemical mixing process. It is impossible to guarantee an absolute, unwavering hardness of exactly 50.0 Shore A across millions of parts. The industry standard tolerance is ±5 Shore A (e.g., a spec of 50 Shore A means the acceptable range is 45 to 55).

Inexperienced factories often drift wildly within or beyond this range due to poor mixing mill controls or inconsistent curing temperatures. At Reemane, our incoming quality control (IQC) tests every batch of raw silicone with calibrated durometers. By strictly controlling the vulcanization temperature and time in the press, we minimize this drift, ensuring your final products perform consistently from the first shot to the millionth.