Paper bag size accuracy control: the core points of paper bag machine calibration and maintenance

In the increasingly competitive packaging industry, the dimensional accuracy of paper bag products is not an optional detail, but a core indicator that directly affects customer satisfaction, production line efficiency, and brand reputation. A paper bag with a size deviation exceeding the allowable range may cause difficulty in filling, automatic packaging line shutdown, and even lead to serious consequences such as batch returns. This article will delve into the core technical points of achieving precise size control for paper bag machines, focusing on the key calibration steps and systematic maintenance strategies of the equipment.

1、 The core position of dimensional accuracy: why is every inch worth fighting for?

Automation adaptability: Modern high-speed loading production lines have extremely strict requirements for paper bag size tolerances. Excessive size deviation can lead to unstable gripping of the robotic arm, failure to open the bag, or displacement of the sealing position, directly causing downtime losses.
Content matching and protection: The size of the paper bag should accurately match the volume and shape of the content. If the size is too small, it is easy to break, and if it is too large, the shaking of the contents will intensify, increasing the risk of damage during transportation.
Cost control: Accurate dimensions mean optimized material utilization. The meaningless enlargement of size directly transforms into waste of raw materials and erodes profit margins.
Brand image: Paper bags with uniform size and excellent workmanship are a direct reflection of brand professionalism, otherwise they may convey negative impressions.
2、 The core influencing factors of size accuracy of paper bag machine

To achieve high-precision paper bag size output, it is necessary to have a deep understanding and control of the following key variables:

Paper material characteristics:
Quantification and thickness: Fluctuations in weight or thickness directly affect the final formed dimensions. Thicker paper has a more significant rebound effect at folding and indentation points.
Moisture content: Paper is highly sensitive to humidity. The change in moisture content leads to paper shrinkage or expansion, which is an important cause of size instability. It is crucial to maintain stable humidity in the raw materials and workshop environment.
Fiber direction: There is a significant difference in the expansion and contraction rate between the longitudinal and transverse directions of the paper, which affects the dimensional accuracy of the bag in all directions. Attention should be paid to the matching between the fiber direction of the roll paper and the bag making process.
Flatness: The problem of "ruffle edges" or "tight edges" on the roll paper can cause unstable paper movement and affect the positioning accuracy of subsequent processes.
Equipment accuracy and status:
Transmission system: The wear, clearance, and concentricity errors of servo motors, gearboxes, transmission shafts, bearings, and synchronous belts/chains will be directly converted into cumulative errors in paper conveying length.
Paper feeding and correction mechanism: The wear and uneven pressure of the paper feeding wheel, the sensitivity of the correction sensor, and the response speed of the actuator determine the initial position accuracy of the paper entering the forming station.
Forming and sealing unit: The processing accuracy, installation position, parallelism, and temperature uniformity of the forming template, folding claw, longitudinal/bottom sealing hot press wheel, and transverse sealing cutting knife are the key factors determining the length, width, and bottom of the bag body.
Cutting system: The sharpness, installation angle, dynamic balance, and clearance between the rotating or flat blade and the anvil roller directly affect the quality of the cut and the accuracy of the bag length.
Process parameter setting:
Tension control: The tension setting and closed-loop control that runs through the entire process from unwinding to winding/stacking are the core of maintaining stable paper transportation and preventing stretching, deformation, or wrinkling. Excessive tension can cause stretching deformation (especially for thin paper), while insufficient tension can cause misalignment.
Temperature control: The heat sealing temperature (longitudinal sealing, transverse sealing, bottom sealing) needs to be accurately matched with the material, machine speed, and pressure. Excessive temperature can cause material shrinkage, deformation, and even burning; If the temperature is too low, it will not seal firmly. The calibration of temperature sensors and the uniformity of heating elements are crucial.
Pressure control: The crease pressure, sealing pressure, and cutting pressure should be moderate and uniform. Insufficient pressure affects the molding effect or sealing strength; Excessive pressure can exacerbate paper deformation and even damage equipment.
Speed matching: Accurate synchronization of the speed of each workstation (paper feeding, forming, vertical sealing, horizontal sealing and cutting) is the key to avoiding paper pulling, deformation or stacking.
3、 The core of achieving precise dimensions: fine tuning of paper bag machines

The calibration of paper bag machines is the cornerstone of achieving high precision and requires systematic and refined operation

Benchmark calibration:
Equipment level calibration: Use a high-precision level to ensure that the entire machine and key units (such as the forming seat and the transverse sealing cutting knife holder) are in a strictly horizontal state. This is the physical foundation for all subsequent adjustments.
Centerline alignment: Accurately set and run through the "mechanical centerline" or "photoelectric reference line" of the entire machine. All paper feeding rollers, cardboard guides, forming templates, sealing wheels, and cutting blades must be strictly aligned with this reference line. Use a laser collimator or precision mandrel for calibration.
Paper feeding and correction system calibration:
Paper feeding wheel pressure: Ensure even and moderate pressure on paired paper feeding wheels to avoid slipping or damaging the paper. Use pressure-sensitive paper or specialized pressure gauges to check the pressure distribution.
Correction calibration: Accurately set the detection position of the correction sensor (usually the edge of the coil or printed color code). Adjust the sensitivity, response dead zone, and actuator gain of the sensor to ensure fast and stable correction without oscillation.
Precision adjustment of molding station:
Formwork installation: The template must be firm and free of looseness, with its center strictly aligned with the machine reference line. Ensure that the surface of the template is smooth and free of burrs, and the curvature meets the design requirements.
Folding claw/board position: Adjust the angle, height, and timing of the folding claw/board to ensure that the folding line is accurate, straight, and free of scratches or improper folding. Fine adjustments should be made considering changes in paper thickness.
Sealing unit calibration (vertical sealing, horizontal sealing, bottom sealing):
Parallelism and clearance: Use a feeler gauge to accurately adjust the parallelism and working clearance of the upper and lower heat sealing wheels/plates. The gap should be uniform and consistent, usually slightly smaller than (single-layer paper thickness x 2). Parallelism error can lead to uneven pressure, uneven sealing line width, and even local instability.
Temperature uniformity verification: Use a multi-point surface thermometer or thermal imaging device to detect the actual temperature distribution on the sealing surface, ensuring that the lateral temperature difference is extremely small (such as within ± 3 ° C). Clean or replace heating elements/thermocouples as necessary.
Pressure testing and balancing: Check whether the pressure regulating mechanism is sensitive and reliable, and use pressure-sensitive paper or specialized pressure measuring film to confirm that the pressure distribution on the sealing surface is uniform. Uneven pressure can lead to differences in sealing strength and local deformation.
Alignment of meshing lines: For rotary sealing wheels, ensure that their meshing lines (sealing lines) are perfectly aligned at the contact points, without any misalignment or "fighting" phenomenon.
Cutting system calibration:
The gap between the knife and anvil: This is the lifeline of cutting quality and accuracy. According to the thickness and type of paper, use a precision feeler gauge to strictly set the gap between the rotating knife and the anvil roller (or the flat knife and the cutting board) (usually 0.03-0.08mm). Tightness accelerates wear, looseness leads to continuous cutting or burrs.
Cutting blade angle and sharpness: Ensure that the cutting blade is installed at the correct angle (usually with a slight inclination) and the cutting edge remains absolutely sharp. Blunt knives require greater pressure, which can easily cause paper deformation, fraying of the incision, and even fluctuations in cutting length.
Cutting position (bag length): Accurately set the cutting trigger point (photoelectric eye position or encoder pulse number), and repeatedly adjust the bag length by measuring the sample cutting until it meets the tolerance requirement (such as ± 0.5mm). Consider the slight stretch of paper at the moment of cutting.
Calibration of tension control system:
Sensor calibration: Regularly calibrate the zero and range of the tension sensor to ensure accurate signal.
PID parameter optimization: Based on the characteristics of the paper used and the machine speed, optimize the proportional (P), integral (I), and derivative (D) parameters in the closed-loop tension control system to achieve fast response and stable tension without overshoot or oscillation.
4、 The cornerstone of maintaining accuracy: systematic preventive maintenance

The precision status of finely tuned equipment will drift over time, wear, and production load. A rigorous preventive maintenance (PM) system is a long-term guarantee for maintaining dimensional accuracy:

Daily maintenance:
Cleaning: Thoroughly remove paper scraps, dust, glue stains, and hot melt adhesive residue from all parts of the equipment, especially the paper guide roller, forming area, sealing wheel, and cutting area. The accumulation of dirt is the main cause of scratches, slippage, and uneven heat transfer.
Lubrication: Strictly follow the requirements of the equipment manual and use designated grades of lubricating oil/grease to lubricate the bearings, guide rails, gears, camshafts, chains and other moving pairs in a timed and quantitative manner. Avoid under lubrication (exacerbating wear) and over lubrication (adsorbing dust).
Visual and auditory inspection: Observe for any abnormal vibrations, noises, overheated components, or air or oil leaks during operation. Check if the air pressure in the circuit is stable and if the trachea is aged or damaged. Check if the key fasteners (anchor bolts, bearing seats, tool holders) are loose.
Key dimension sampling: Sampling finished bags at regular intervals (such as every 1-2 hours), measuring key dimensions (length, width, bottom depth, edge width) using precision calipers, tape measures, or specialized gauges, recording data, and observing trends.
Regular maintenance (weekly/monthly):
Transmission system inspection: Check the tension and wear of the synchronous belt/chain, and adjust or replace it if necessary. Check the alignment of the coupling. Detect bearing clearance and abnormal noise.
Maintenance of heat sealing system: Clean the surface of the heat sealing wheel/plate (using a dedicated copper brush or non-metallic scraper), check the status of heating elements, thermocouples, and temperature control modules. Verify the consistency between the actual temperature and the displayed value.
Cutting system maintenance: Check the sharpness of the cutting blade and perform professional sharpening or replacement. Re verify the clearance and parallelism between the knife and anvil. Check the status of the cutting drive mechanism.
Pneumatic system maintenance: Drain the accumulated water in the air path filter, check the working status of the pressure reducing valve, solenoid valve, and cylinder, and lubricate the cylinder rod (if applicable).
Electrical system inspection: Clean the dust in the electrical control cabinet, check the tightness of the wiring terminals, and inspect for signs of overheating. Backup key parameters.
Deep maintenance (quarterly/annual):
Precision detection and restoration of key components: Use professional measuring tools (dial indicators, laser interferometers, etc.) to detect the straightness, concentricity, and runout of key shaft systems, as well as the parallelism and straightness of guide rails. Scratch, adjust or replace components as necessary.
Servo system calibration: Adjust the gain and origin of the position and velocity loops of the servo driver, motor, and encoder system to optimize dynamic response performance.
Comprehensive lubrication system replacement: Replace the lubricating oil in the gearbox and hydraulic station (if any).
Comprehensive performance testing: Using standard testing materials, comprehensively test the equipment's registration accuracy, sealing strength, cutting quality, and dimensional stability at different speed levels, generate reports, and compare historical data.
5、 Dealing with dimensional deviations: rapid diagnosis and correction

When size deviation occurs, it is necessary to systematically investigate:

Confirming Deviation Mode: Is it a systematic bias towards large/small? Or random fluctuations? Is it a problem with a single size (such as bag length) or multiple sizes? Does it occur on specific products or on all products?
Traceability of process changes: Have there been any recent changes in paper batch and supplier? Have the equipment speed, temperature, and pressure parameters been adjusted? Has the mold or tool been replaced?
Check the raw materials: immediately measure the quantity, thickness (multi-point measurement), and moisture content (if conditions permit) of the roll paper currently in use, and compare them with the standard values.
Key inspection of relevant workstations:
Inaccurate bag length: Priority should be given to checking the position setting of the cutting blade, the gap/wear of the anvil, the triggering photoelectric eye/encoder for cutting, the stability of the paper feeding tension, and whether the horizontal sealing wheel slips.
Inaccurate bag width: Focus on checking the installation/wear of the forming template, the parallelism/gap/temperature of the longitudinal sealing wheel, the position of the guide board, and whether the correction system is stable.
Inaccurate bottom depth: Check the position of the bottom sealing molding mold, the pressure/temperature/parallelism of the bottom sealing wheel, and the timing of the bottom pushing mechanism.
Review equipment status: Check the wear and looseness of relevant transmission components (bearings, gears, synchronous belts), whether the air path pressure is stable, and whether the temperature sensor is accurate.
6、 Tools and personnel: the twin wings of precision assurance

Precision measuring tools: invest in high-precision digital calipers (0.01mm resolution), micrometers, tape measures, feeler gauges, angle gauges, level gauges, laser rangefinders, surface thermometers, and even online size monitoring systems (such as laser scanners). Regularly send measuring tools to metrology institutions for calibration.
Professional maintenance tools: equipped with complete mechanical, electrical, and pneumatic maintenance tools, specialized disassembly and assembly fixtures, and high-quality lubricating oil/grease.
Skilled team: Operators need to understand the principles of craftsmanship and the significance of parameters; Pilots need to have solid knowledge in mechanical, electrical, and pneumatic fields, as well as sharp tactile and observational skills; Maintenance engineers need to be proficient in equipment structure, preventive maintenance processes, and fault diagnosis techniques. Establish a continuous internal training and external learning mechanism.
The control of paper bag size accuracy cannot be achieved overnight or through a single process. It is a systematic engineering that integrates materials science, precision machinery, automatic control, and scientific management. From a deep understanding of influencing factors to implementing meticulous equipment tuning; From establishing and strictly implementing a preventive maintenance system to cultivating a high-quality technical team; Every step requires a rigorous attitude and professional skills. Only by integrating the concept of "precision" into every moment of the equipment lifecycle can we win the trust and orders of customers in the fierce market competition with stable and high-quality paper bags, making every paper bag a silent endorsement of the brand's exquisite craftsmanship. The control of precision is essentially a never-ending battle of subtle details.