Edge deletion
A Low-E coating runs right out to the edge of the glass, which is a problem, because that is exactly where the IGU seal needs to grab bare glass. Edge deletion is the quiet little step that grinds that coating off a thin border so the seal can actually stick. Skip it or do it badly, and the unit fogs up years later.
Why edge deletion is necessary
Soft-coat (MSVD, magnetron sputtered vacuum deposited) Low-E coatings are stacks of ultra-thin metal layers laid down on the glass after the float line. They cling to the glass just fine, but here is the catch: they do not play nicely with the sealants that hold an IGU together.
The primary seal in an IGU is polyisobutylene (PIB), and it only does its job as a moisture barrier if it bonds to bare glass. Let it land on that metallic coating instead and the bond turns weak and leaky, moisture creeps in, the desiccant fills up early, and one day the customer is staring at a foggy window that can only be fixed by replacing the whole unit. All from a few millimetres of coating that should have been removed.
Edge deletion removes the coating from the perimeter band, exposing bare glass for the PIB seal to bond to.
Deletion widths
The required edge deletion width depends on the spacer system being used and the sealant type. The general principle: the deletion must extend far enough inward that the PIB primary seal lands entirely on bare glass, with a small margin.
| Spacer / system type | Typical deletion width | Notes |
|---|---|---|
| Standard aluminum spacer, PIB primary seal | Spacer width + 2–3mm | Deletion must extend past the inner face of the spacer |
| Warm-edge spacer (stainless / thermoplastic) | Spacer width + 2mm | Same principle; spacer width varies by manufacturer |
| Structural sealant glazing (SSG) | Additional deletion for secondary sealant contact | Secondary seal in SSG is a structural bond, must also contact bare glass or primer |
| SGCC / IGMA requirement (general) | Minimum 6mm; typically 8–12mm in practice | Follow the coating manufacturer's specific requirements |
Deletion methods
Mechanical grinding
A rotating abrasive wheel or belt removes the coating by abrasion as the glass passes through a dedicated edge deletion machine. This is the standard production method for most IGU fabricators. The grinding head traverses all four edges in a single pass. The result is a clean band of bare glass with visible grinding marks, acceptable for functional performance but not optically refined.
Laser ablation
A laser beam vaporizes the coating from the glass surface with high precision and no mechanical contact. Laser deletion is preferred for:
- Complex or curved shapes where mechanical grinding is impractical.
- Coatings where mechanical grinding risks edge damage to adjacent coating layers.
- Tight tolerance applications requiring very precise deletion width.
Laser deletion is more expensive and slower than mechanical grinding, so it is typically reserved for specialty applications.
Inspection and verification
Bare glass and coated glass can look almost identical to the naked eye, so inspectors bring a trick: UV light. Many coatings fluoresce a different shade than bare glass under UV, which makes the deletion boundary jump out clearly. For quality control, fabricators typically inspect deletion width and continuity on a percentage of units per batch. On critical projects, a specifier or third-party inspector may witness the deletion process or review QC records.