Printing shipping labels: thermal, laser and the settings that ruin them
Which printer, which settings, and how to check the label is really 4x6.
A shipping label has one job: to be read, by a scanner, quickly, possibly in bad light on a wet loading bay. Most printing problems are one of three things, and all three are avoidable.
Direct thermal against laser and inkjet
Direct thermal printers have no ink, toner or ribbon. The label stock is heat sensitive and the print head darkens it. This is what carriers use, what the 4x6 format exists for, and it is the cheapest per label to run because the only consumable is the label itself.
The catch is inherent to the technology: the image is produced by heat, so heat removes it too. A thermal label left in a hot van, in direct sun, or against a solvent will fade, sometimes in hours. Anything stored for weeks before dispatch should not be direct thermal, and thermal labels should never be stored near heat sources or in a car.
Thermal transfer is the related technology that uses a ribbon. It costs more and the image is durable. If your parcels sit in a warehouse for months, this is the answer rather than direct thermal.
Laser is perfectly adequate for shipping labels on adhesive sheets, prints sharp barcodes, and is durable. Toner is heat fused and will not fade.
Inkjet is the weakest option. Ink is water soluble unless it is pigment based, and a rained-on inkjet label is unreadable. If inkjet is what you have, cover the label with clear tape, taking care not to leave the tape wrinkled over the barcode, or use a document wallet.
Black on white, at full contrast, always
Barcode scanners work on the contrast between bars and spaces, and they are calibrated for black on white.
Do not print labels in colour, on coloured stock, on a low-toner cartridge, in draft or economy mode, or with any grayscale setting. A grey barcode that looks fine to a human is a barcode a scanner reads slowly or not at all, and a parcel that will not scan drops out of automated sorting into manual handling, which adds days.
If your printer has an economy setting, turn it off for labels specifically.
The settings that silently break the size
This is the most common failure and the hardest to notice, because the label looks correct.
Scaling. Browsers and print dialogs default to fitting the content to the paper, often as “Fit to page” or “Shrink to fit”. A 100 by 150 mm label printed “to fit” on A4 comes out smaller than 100 by 150 mm. It will usually still scan, but the size is wrong, carrier-supplied labels can be rejected for it, and any alignment against pre-cut label stock is destroyed. Set scaling to 100%, none, or actual size.
Margins. Set margins to none. A default margin forces the printer to shrink the page to fit inside it, which is scaling by another name.
Paper size. Choose the paper you are actually loading. Firefox and Safari ignore the page size a web page requests, so on those browsers the size has to be selected by hand in the dialog.
Headers and footers. Browsers add a URL, a page title and a date by default. Turn them off or they print across the label.
A4 is not Letter, and the difference bites
A4 is 210 by 297 mm. US Letter is 216 by 279 mm: 6 mm wider and 18 mm shorter.
That 18 mm is not a rounding error. Two A6 labels are 296 mm of the 297 mm on A4, and they do not fit inside Letter’s 279 mm. A layout designed four-up on A4 and printed on Letter loses the bottom row off the page.
If you are in the US and a tool offers you A4, it is offering you a page your printer does not have loaded. Pick Letter and let the layout adjust.
How to check a label is actually the right size
Print one and measure it with a ruler. This takes ten seconds and is the only reliable test.
A 4x6 label should measure 101.6 by 152.4 mm, or near enough that a ruler cannot tell. If it measures 95 by 143, scaling is on somewhere, and every label you have printed so far is undersized.
Measure again after any change of browser, printer or operating system, because the defaults reset and nothing warns you.
Quiet zones, and why a barcode needs empty space
Every barcode symbology requires a clear margin either side of the symbol, with nothing printed in it. For Code 128 the requirement is at least ten times the width of the narrowest bar; for a QR code it is four modules on all four sides.
A barcode printed flush to the edge of a label, or with text crowding it, fails to scan even though it looks complete. If you are laying out your own label, leave visible white space around every symbol and do not put the label so close to a box edge that the barcode curves around the corner. A curved barcode cannot be read by a fixed scanner at speed.
Placement on the box
Flat, on the largest face, away from seams and corners. Not folded over an edge, not across the opening where a customs officer will cut it, not on top of tape that will wrinkle.
Cover or remove every previous label and barcode. A scanner has no way to know which of two barcodes is current, and the usual result of leaving an old one visible is the parcel going back where the old label pointed.
Save as PDF as the reliable path
If printing directly gives you trouble, print to PDF first, open the PDF in a reader, and print from there with scaling set to none. PDF readers respect page size far more consistently than browsers do, and the intermediate file lets you confirm the dimensions before wasting label stock.