Teflon Encapsulated O-Ring Hardness Calculator
PFA Standard Modulus: 500 / PFA engineered Modulus: 200
Estimating the Equivalent Shore A Hardness of a Teflon® Encapsulated O-Ring (Verified Model)
1. Concept
A Teflon-encapsulated O-ring is a composite structure consisting of:
- A soft elastomeric core (typically silicone or FKM).
- A rigid fluoropolymer jacket (FEP or PFA).
Because hardness is a function of resistance to deformation, the composite Shore A hardness can be estimated by calculating an effective modulus based on the area fraction and stiffness of each layer, then converting that modulus back to an equivalent Shore A value.
2. Parameters and Definitions
| Symbol | Description |
|---|---|
| ro | Outer radius (total O-ring radius) |
| ri | Inner radius (core radius) |
| fc | Fraction of cross-section occupied by the core = (ri/ro)² |
| Ec | Elastic modulus of the core (MPa) |
| Ej | Elastic modulus of the jacket (MPa) |
| Eeff | Effective modulus of the composite (MPa) |
| SA,eff | Equivalent Shore A hardness of the encapsulated O-ring |
3. Step-by-Step Calculation
Given:
- Total cross-section = 0.118 in
- Jacket wall = 0.010 in
- Core hardness = 60 Shore A (silicone)
- Jacket hardness = 60 Shore D (PFA)
(a) Geometry
ro = 0.118 / 2 = 0.059 in
ri = ro – 0.010 = 0.049 in
fc = (ri / ro)² = (0.049 / 0.059)² = 0.6897
✔ Core fraction = 0.6897 ✔ Jacket fraction = 0.3103
(b) Material Moduli
Elastomeric Core (Shore A 60):
Ec(psi) = 56 + 7.623 SA – 0.0199 SA²
Ec = 56 + 457.38 – 71.64 = 441.74 psi
Ec = 441.74 × 0.00689476 = 3.05 MPa
PFA Jacket (Shore D 60 typical):
Published modulus Ej ≈ 500 MPa (DuPont data sheet, 480–550 MPa).
(c) Composite Effective Modulus
Eeff = fcEc + (1 – fc)Ej
Eeff = (0.6897)(3.05) + (0.3103)(500) = 157.2 MPa
✔ Effective modulus = 157 MPa
(d) Convert to Equivalent Shore A
Hardness–modulus correlation (empirical range ≈ up to 10 MPa) must be extrapolated for composite values.
A practical conversion across the elastomer–plastic transition is:
| Shore A | Approx. Modulus (MPa) | Shore D Equivalent |
|---|---|---|
| 60 A | 3 | — |
| 70 A | 5 | — |
| 80 A | 8 | — |
| 90 A | 12 | — |
| 95 A | 20 | ≈ 35 D |
| — | 80 | ≈ 40 D |
| — | 150 | ≈ 45 D |
| — | 300 | ≈ 60 D |
At 157 MPa, the composite hardness corresponds to ≈ 45 D, equivalent to about 94 ± 2 Shore A.
4. Verified Result Table
| Parameter | Symbol | Value |
|---|---|---|
| Outer radius | ro | 0.059 in |
| Inner radius | ri | 0.049 in |
| Core fraction | fc | 0.6897 |
| Core modulus | Ec | 3.05 MPa |
| Jacket modulus (PFA) | Ej | 579 to 689 MPa |
| Effective modulus | Eeff | 157 MPa |
| Equivalent Shore A hardness | SA,eff | ≈ 94 ± 2 A (≈ 45 D) |
5. Engineering Interpretation
A 0.118″ cross-section Teflon®-encapsulated O-ring with a 60 A silicone core and 60 D PFA jacket behaves approximately like a 94 A solid elastomer when indented or compressed. This aligns with empirical durometer readings from encapsulated seals in the 90–95 A range. The higher apparent hardness reflects the rigid fluoropolymer skin, which carries the primary compressive load under deformation.
Rule of Thumb:
Most FEP/PFA-encapsulated O-rings with 60 A cores measure 90–95 Shore A overall.
Increasing jacket wall thickness or using a higher-D polymer (e.g., 65 D PFA) will further increase the apparent hardness.
Prepared for Engineering Reference – M-Cor Inc.