Common Printing Problems and Solutions of Flexo Printing Machines
Flexo printing is the core process in corrugated carton production, and its printing quality directly determines the market competitiveness of finished products. In view of the characteristics of corrugated board, such as soft material and embossed surface, water-based flexo printing machines are prone to a series of technical problems during high-speed operation. Combining industry standards and frontline production experience, this paper systematically analyzes the five core faults and provides a standardized troubleshooting and solution guide for peers.
Analysis and Troubleshooting of Core Faults
Ink Smearing
Fault Characteristics: Irregular ink spots appear in the non-graphic areas of printed products, or ink trailing occurs at the edges of graphics and texts, resulting in a mottled layout.
Root Causes
● Unbalanced Pressure: Excessive linear pressure between the plate cylinder and impression cylinder causes ink to be squeezed out of the graphic areas.
● Abnormal Rheological Properties of Ink: Low viscosity and excessive fluidity of water-based ink lead to spreading on the crests of corrugated board.
● Uncontrolled Ink Transfer of Anilox Roller: Worn doctor blades or improper angles fail to effectively scrape off excess ink on the surface of the anilox roller.
Solutions
● Pressure Calibration: Follow the "minimum pressure principle", with the benchmark of clearly reproducing graphics and texts without crushing the corrugations.
● Ink Adjustment: Add thickener to increase viscosity, or select high-adhesion and quick-drying ink specially for corrugated board.
● Doctor Blade System Replacement: Check the edge of the doctor blade to ensure no nicks; adjust the doctor blade angle to 30°-45° to ensure clean ink scraping.
Dot Gain
Fault Characteristics
The area of dots on printed products is larger than that on the printing plate, resulting in merging of dark tone levels, loss of highlight dots and blurring of patterns.
Root Causes
● Excessive Plate Compression: Deformation of the flexographic plate under pressure causes dots to spread around.
● Mismatched Anilox Roller Line Screen: The selected anilox roller with an excessively low line screen delivers an over-large ink volume, exceeding the bearing capacity of the corrugated board surface.
● Insufficient Plate Hardness: The use of printing plates with Shore hardness lower than 30° is prone to excessive deformation under the pressure of corrugated crests.
Solutions
● Pressure and Deformation Control: Strictly control the impression pressure and select high-elasticity and low-deformation polyurethane printing plates.
● Anilox Roller Matching: Select anilox rollers with appropriate line screens according to printing accuracy (360LPI-480LPI is recommended for conventional corrugated printing).
● Plate Making Process Optimization: Adopt flat-top dot plate making technology to improve the mechanical stability of dots.
Printing Ghosting
Fault Characteristics
Distinguishable ghost images appear at the edges of graphics and texts, divided into "mechanical ghosting" and "ink ghosting", which is particularly obvious in fine lines and small characters.
Root Causes
● Insufficient Mechanical Transmission Accuracy: Worn bearings of the plate cylinder and excessive gear clearance cause circumferential movement during operation.
● Improper Plate Mounting: Unfirm pasting of double-sided tape or air bubbles cause slight movement of the printing plate during high-speed operation.
● Tension Fluctuation: Unstable tension of corrugated board during conveying leads to slippage.
Solutions
● Transmission System Overhaul: Replace worn bearings and gears, and recalibrate the coaxiality of the cylinder.
● Standardized Plate Mounting Process: Ensure no air bubbles or wrinkles on the double-sided tape, uniform tension of the printing plate, and avoid empty pasting.
● Paper Feeding System Stabilization: Adjust the gap and speed of the paper feeding roller to ensure stable conveying of the board without slipping.
Color Variation
Fault Characteristics
Obvious color shade differences occur in products of the same batch, or significant color deviation from the standard sample.
Root Causes
● Poor Ink Circulation: Sedimentation of ink in the circulation system leads to changes in color paste concentration.
● Anilox Roller Clogging: Long-term production causes ink scale to block the ink cells of the anilox roller, resulting in a gradual decrease in the actual ink transfer volume.
● Substrate Differences: Differences in surface water absorption and pH value of corrugated boards from different batches.
Solutions
● Ink Circulation Management: Keep the ink circulation pump running continuously and stir the ink tank regularly to ensure uniform color paste.
● Anilox Roller Cleaning: Clean the anilox roller with a special cleaning agent at the end of daily production, and conduct regular ultrasonic deep cleaning.
● Substrate Preprocessing: Conduct sample test printing in advance for boards of different batches, and compensate for color deviation by fine-tuning the pH value of water-based ink.
Registration Error
Fault Characteristics
In multi-color overprinting, the positions of graphics and texts on each color plate are misaligned, resulting in "ghosting" or "white exposure".
Root Causes
● Registration System Drift: Unlocked parameters of the photoelectric eye or servo motor lead to inaccurate target tracking; decreased sensitivity of the photoelectric eye fails to accurately identify registration marks.
● Board Deformation: Warpage and expansion of corrugated board due to changes in water content, or uneven thickness of the board itself, make it unable to pass through the printing unit smoothly and cause displacement deviation.
● Inconsistent Tension Between Units: Asynchronous speed of guide rollers between each printing unit; or large tension fluctuation of unwinding and rewinding leads to stretching deformation and slippage of the board.
● Plate and Transmission Problems: Improper plate mounting and positioning, inconsistent benchmarks, or worn gears and bearings, deviated cylinder coaxiality, which cause circumferential movement during operation and affect the synchronization of overprinting.
Solutions
● Registration System Calibration: Reset the color group phase, lock the servo parameters to ensure accurate mechanical zero position.
● Board Water Content Control: Control the workshop temperature and humidity within the standard range (temperature 23±2℃, humidity 55±5%) to prevent the board from absorbing moisture and deforming; preprocess warped boards before printing and reject severely deformed substrates.
● Transmission Speed and Tension Synchronization: Check and calibrate the main transmission chain or belt of each unit to ensure complete synchronization of linear speed; adopt an automatic tension control system to stabilize the unwinding and rewinding tension and avoid board stretching and slippage.
● Standardized Plate Mounting and Transmission Overhaul: Paste the printing plate in strict accordance with the standard to ensure uniform positioning benchmark and no empty pasting; regularly overhaul transmission components, replace worn gears and bearings, and calibrate cylinder coaxiality.
Preventive Maintenance Specifications
For corrugated flexographic printing equipment, "prevention" is far better than "maintenance". Establishing a standardized maintenance process is the key to ensuring the stability of printing quality and reducing the failure rate of equipment.
Daily Cleaning
● Thoroughly clean the printing plate, anilox roller, ink tank and ink delivery pipeline to prevent ink from drying and hardening.
● Remove paper lint and scraps from the paper feeding and delivery sections to prevent foreign matters from entering the printing unit.
Regular Inspection (Weekly/Monthly)
● Pressure Detection: Use pressure test strips to detect the uniform pressure distribution of each printing unit.
● Wear Inspection: Focus on checking the wear of vulnerable parts such as doctor blades, rubber rollers and bearings, and establish a replacement record.
● Accuracy Calibration: Calibrate the overprint accuracy and cylinder levelness to ensure the equipment is in the best mechanical state.
Standardized Operator Operation
● Strictly implement the Standard Operating Procedure (SOP) and prohibit arbitrary changes to technical parameters.
● Establish a first article inspection system; the quality inspector must confirm and sign before mass production for each batch.
Summary
Flexo printing of corrugated board is a dynamic balance process of "pressure, ink volume and tension". When facing faults, peers should follow the principle of "first observe the phenomenon, then analyze the mechanism, and finally make precise adjustments". Among them, registration error, as a core fault affecting the quality of multi-color overprinting, requires focused attention on substrate status, registration system accuracy and transmission synchronization.
The solutions described in this paper are all based on the common problems of standard water-based flexo printing machines. In actual production, flexible adjustments should be made in combination with specific equipment models, ink brands and board characteristics.
Contact: Leo
Phone: +86 18875791688
Email: info@depack.cn
Add: Hezhuang Village, Chengdong Industrial Zone, Dongguang County, Cangzhou City, Hebei Province, China