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Alloy blocking is a method used in optical labs to temporarily secure a lens blank to a metal holder (the block) using a specialized, low-melting-point alloy. The purpose of this step is to firmly hold the lens during the surfacing process where the back curve is ground and polished.
read more about alloy blocking in lens surfacing ...The alloy is important because it hardens quickly, holding the lens extremely rigidly during high-speed grinding. Once surfacing is complete, the entire assembly can be heated slightly, causing the alloy to melt and release the finished lens without damage.
Alloy blocking is a traditional method known for its superior grip and stability, which is helpful when grinding heavy, thick glass or polycarbonate lenses. Adhesive blocking uses a sticker-like pad and is a cleaner, more modern method often used for softer lens materials.
After the lens is surfaced, it goes through a deblocking process. The lens assembly is heated, the alloy melts away, and the lens is removed. The alloy is then cooled and collected for reuse, making it an environmentally friendly process.
Surfacing determines the accuracy of your lens prescription. High-quality alloy blocking makes sure your backup glasses have perfectly ground curves. This accuracy prevents distortion, giving you immediate, clear vision when you switch from your contacts.
No, alloy blocking is less common now. Many labs use adhesive or "tacky" blocking pads for modern plastic and thin lenses.
The alloy used in blocking is safe. It is a mixture of metals that melts at a very low temperature and is generally non-toxic.
No, the alloy does not contact the lens surface directly. A protective coating or pad is placed between the alloy and the lens.
If you see "scuffs" or circular marks in the center of your new lenses, the blocking alloy may have been too hot or the protective pad failed. These marks can cause light scattering and eye strain. Return the glasses to your provider for a lens replacement.
The Vision Council. Lens Surfacing Procedures (thevisioncouncil.org). 2024.
ABDO. Manufacturing Spectacle Lenses (abdo.org.uk). 2023.
Review of Optometry. Coating Damage in the Lab (reviewofoptometry.com). 2024.
Optical Journal. The History of Lens Blocking (opticaljournal.com). 2024.