How many sealed units does a window have?
A sealed unit (IG unit) is the assembly of two or more panes with a spacer that goes into one opening of the window. The count depends on the window type, not on the number of panes:
| Window type | Sealed units | Panes (double glazed) |
|---|---|---|
| Picture window | 1 | 2 |
| Single hung / double hung | 2 | 4 |
| Two-lite slider (XO) | 2 | 4 |
| Three-lite slider (XOX) | 3 | 6 |
So a per-pane sticker prints four labels for a single hung that has two units — and the glass bench peels the extra two off and throws them away. A per-window sticker prints one, and a single scan of it marks the window "through glass" while the second unit has not been made.
Pairing panes into units: by bill line, not by part number
To print one label per unit, the system has to know which panes belong together. The tempting rule — group the panes of a window by glass part number — breaks on the most common unit there is. A clear/clear IG is the same part twice, so grouping by part number reads it as two one-pane units, and the roll prints double again.
The rule that works follows how a window's bill of material is written: one line for the outer lite and one for the inner lite, each with a piece per unit. The k-th pane of each glass line, at the same cut size, is one sealed unit. A Low-E/clear unit and a clear/clear unit pair the same way, and two equal sashes of a slider come out as two units, not one.
Two cautions. First, filter out anything that is not glass before pairing: screen mesh is often sized like a lite and will otherwise be counted as a unit. Second, units the plant buys in — tempered or obscure units from the IG supplier — are still the window's units. They get a sticker and a scan too; they just never go to the cutting table.
What the sticker should say
- Which window, and which unit of it — the window's serial plus "lite 2 of 2". Put the count in the barcode itself (for example
SN-30612-1-1-L2OF2), so the bench knows how many units to expect from the sticker in its hand rather than from a second calculation that may disagree. - The cut size and makeup — exactly as the glass list prints them.
- Grids, or none — "Grids: Colonial V1 H2", or "Grids: Colonial (none on this lite)" when the pattern is on the other sash, so a gridless bottom unit is not read as an order with no grids.
- The cart — the batch number the floor uses, so a unit can be matched to its cart.
- Paint or color where it matters at glazing.
Print order and scanning
Print a window's units top to bottom, in the order the glazier needs them, while numbering them in the bill's own order so the numbers never change. At the glass bench, each sticker is its own scan, and a window has passed glass only when every unit it wears has been read. At other benches the same sticker simply identifies the window — one sticker, read at several benches — so assembly can sign the whole window off from any of its glass labels.
Keep one escape hatch: a window scanned on its plain window serial still passes whole, so a damaged sticker never stops the floor.
How Extrusia handles it
Extrusia pairs panes into sealed units by bill line and cut size after applying the plant's own glass filter, prints one sticker per unit with the lite count in the barcode, rolls them off top to bottom, includes bought units, and counts a window through glass only when every unit has been scanned. The glass sheets, the stickers and the remake chooser all read the same count, so a bench never waits on a unit nobody printed. See Glass Optimization.