Frequent black-and-white lines appearing on cigarette packs? Here are 4 easy ways to fix it!
At the end of 2022, the National Tobacco Monopoly Administration issued regulations requiring all cigarette packaging to be printed with QR codes to achieve full-process traceability and anti-counterfeiting control. This policy imposes stricter requirements on the quality of inkjet printing for cigarette pack printing companies: the symbol quality requirements for QR codes must comply with the national standard GB/T 33993-2017 "Commodity QR Codes" ≥1.5/XX/660 (where 1.5 is the reference number for measuring aperture size, and the selection varies depending on the application environment; 660 is the measured light wavelength, unit: nm, with an allowable deviation ±10nm), to ensure that QR codes can be stably and accurately read on high-speed cigarette pack lines.
However, in the early stages of policy implementation, many cigarette pack printing companies encountered severe quality bottlenecks. High-speed sheet-fed inkjet printers, as mainstream QR code assignment equipment in the cigarette packaging industry, frequently exhibit quality defects such as black lines, white lines, and plate paste during actual production, resulting in a high decoding failure rate and seriously affecting production efficiency and product delivery timeliness.
This article is based on the technical research practices of a large cigarette pack printing enterprise. In the early stages of production, when using high-speed sheet-fed inkjet printers, issues such as white lines and paste were particularly prominent. Almost every plate paper production process frequently stopped due to quality defects, with an effective production time per shift less than 50%, and a defect rate far exceeding industry acceptable levels.
Through continuous tracking and data collection, the technical team focuses on two of the most representative defects-white lines, black lines, and the resulting dirty paste issues. This article systematically summarizes the causes, handling measures, and long-term control mechanisms of these issues, aiming to provide reference for peers.
High-speed inkjet printers face issues with white lines, black lines, and pasted plates
Cause analysis
In-depth analysis of the causes of quality issues is a prerequisite for formulating effective countermeasures. Quality defects in cigarette bag QR code printing are usually not caused by a single factor, but are the result of the coupling of multiple factors such as print head status, ink performance, equipment parameters, and environmental conditions.
01/ White Line Problem
(1) Phenomena and Effects
White lines refer to white lines running through or partially vertically appearing in QR code patterns, caused by specific nozzles failing to properly eject ink droplets, resulting in missing image information. In mild cases, it only affects aesthetics; in severe cases, module loss directly leads to decoding failure and results in the product being deemed defective.
(2) Key cause analysis
(1) Physical blockage of the printhead: This is the most common cause of white line issues. The nozzle pore is extremely small, and after prolonged use, solvent evaporation easily forms a dry conjunctival film on the nozzle surface. Tiny pigment particles and environmental dust can enter the nozzle, causing localized blockage. The blocked nozzle cannot eject ink droplets, resulting in continuous or intermittent white lines along the paper flow direction.
(2) Abnormal ink supply system: Bubbles mixed into the ink supply line are a hidden but common cause. Bubbles in the ink circuit are compressible; when pressure waves are transmitted to the nozzle, the bubbles absorb energy, causing ink droplets to fail to be ejected normally, manifesting as random white lines. Additionally, if the filter is not replaced for a long time, more impurities are retained, causing the ink supply pressure to decay and also triggering the white line problem.
(3) Ink rheological mismatch: Ink viscosity and surface tension are key factors affecting droplet fracture behavior. When viscosity is too high, filamentous fractures at the nozzle are not brittle, and the ink tends to accumulate at the nozzle mouth, hindering subsequent droplet ejection; Abnormal surface tension causes unstable meniscus morphology, disrupting the continuity of ink droplet ejection, thus forming white lines.
(4) Environmental temperature and humidity shock: When the ambient temperature is too low (e.g., in winter workshops below 20°C), ink viscosity rises sharply and flowability drops significantly; When humidity is too high, the paper moisture content increases, which may also affect the spreading and drying process of ink droplets on the paper surface, indirectly worsening the white line phenomenon.
02/ Black line problem
(1) Phenomena and Effects
Black lines refer to unnecessary black fine lines, trailing lines, or brushed marks in the QR code pattern, as shown in Figure 1. These excess ink marks can connect adjacent modules, alter the size and shape of modules, lower symbol levels, and in severe cases, directly damage fixed patterns and decode errors.