Why does a barcode that scans on the proof fail at the till? Because a proof is read under office light at close range, and a retail scanner reads at speed, at an angle, through a plastic bag, on a carton that has been handled. Print quality grading exists to close that gap. It measures the symbol as printed rather than as designed, and it produces a grade that predicts how the code will behave across a whole print run.
How is a barcode graded? Under the ISO/IEC 15416 standard for linear symbols, a verifier scans the symbol at several points and scores parameters including edge determination, symbol contrast, minimum reflectance, edge contrast, modulation, defects and decodability. Those combine into a single grade from A to F. Retail specifications usually ask for a C or better on every symbol, and a run where some symbols grade C and others grade D has a distribution problem rather than an average one.
What causes the common failures? Bar gain – ink spreading so bars grow wider and spaces shrink – is the classic. It comes from too much ink, too soft a plate, too much impression, or ink that has not dried. Contrast failures come from printing a dark barcode on a coloured or kraft background, or from a varnish over the top that greys the spaces. Defects come from voids, spots and scuffing. Each has a different fix, which is why grading before the full run saves reprints.
Does the substrate matter? It does, and heavily. Corrugated board has a rough surface that scatters light and produces uneven edges, so a barcode on brown kraft will grade lower than the same artwork on coated white board at identical settings. Where a code has to sit on a rough face, print it on a white panel or label rather than directly on the liner. Quiet zones and placement rules are covered in the guide to barcode and QR size and placement.
What about the printing process? Flexo gives heavier bar gain than offset, and digital toner sits differently again. Fine bar widths and small symbol sizes are where the differences show first. If a code is generated at the minimum permitted size for a SKU, switching print process without re-checking the symbol is asking for a failed scan. Screen ruling and dot structure affect edge sharpness too, as the article on printing dots and screen ruling explains.
How should quality be checked during a run? Verify a symbol from the start, middle and end of each pallet, not from one sheet. Record the grade and keep the record with the job, because retail chargebacks often arrive months later and a grade record is the evidence. Where the buyer requires it, verification data can travel with the shipment. The article on AQL inspection before shipment explains how barcode checks sit inside a wider inspection plan.
What should a buyer send with a barcode requirement? The symbology and data, the printed size, the substrate and print process, the retail specification including the minimum grade, and the placement drawing. If the retailer supplies a barcode file, use it rather than regenerating the symbol – bar width adjustments made to fit a panel are a frequent source of unreadable codes. Colour and registration around the symbol are checked in the article on registration and colour accuracy.
Chengtop prints retail cartons, folding cartons and labels in Wuhan, and barcode verification can be run on the finished pack before shipment. Send the barcode file and the retailer’s grading requirement and we will confirm what the substrate can hold. Message us with the detail.
