Rimless & Semi-Rimless Glasses: Why 1.56 & 1.74 Crack & Why MR-8 is Mandatory
The Mechanical Challenge: Drilling Holes into Brittle Plastics
In full-rim frames, the metal or acetate rim supports the lens securely around its entire perimeter with zero localized stress. In Drilled Rimless Frames (Three-Piece Mounts), however, holes are drilled directly through the lens blank, and temple hinges and nasal bridges are secured using metal screws, pressure compression bushings, or plastic rivets.
Every time you take your glasses off with one hand, flex the temples, or clean the lenses, dynamic torsional bending moments are transmitted directly into the drilled hole. In materials science, a drilled hole creates a severe Stress Concentration Factor () that multiplies mechanical load at the hole border by 300% to 400%.
Tensile Yield Strength & Notch Sensitivity Comparison
| Ophthalmic Polymer Material | Tensile Yield Strength | Notch Sensitivity | Rimless / Drilled Durability |
|---|---|---|---|
| Standard 1.56 Acrylic Resin | 35 – 45 MPa | Extremely High (Brittle) | Fails within 2 to 6 months (Cracks around screws) |
| Standard CR-39 (1.50) | 40 – 50 MPa | High (Prone to chipping) | Poor (Requires oversized washer bushings) |
| Ultra-Thin 1.74 Resin | 55 – 65 MPa | Moderate-High (Hard & Brittle) | Not recommended for rimless (Chips along perimeter) |
| Mitsui MR-8 (1.60 Polythiourethane) | 100 – 115 MPa | Extremely Low (High Elastic Recovery) | Certified 100% Crack-Free Rimless Life |
Why Semi-Rimless Nylon Cord Frames Chip Along Edges
In Semi-Rimless (Grooved / Supra) Frames, an optician cuts a microscopic 0.6mm continuous groove along the perimeter of the lens, holding it in place with a clear nylon monofilament wire. If fabricated from brittle 1.56 acrylic, microscopic micro-fractures develop along the groove edge, leading to unsightly white chips (clam-shell fractures) within weeks.
MR-8 polythiourethane possesses unmatched intermolecular cross-linking elasticity. When grooved, it deforms elastically without chipping, holding the tension cord firmly for years of active wear.
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