Working principle of valve bag paper bag machine: the secret of efficient and accurate bag making

Valve bags, with their unique bottom-filling valve design, hold a central position in the packaging of powders and granular materials in the chemical, building materials, food, and pharmaceutical industries. The core equipment supporting their large-scale, high-efficiency production is the valve bag machine . This article will delve into its operating principles and, through a detailed process breakdown, demonstrate the precision and efficiency of modern industrial automated bag-making equipment.

1. Core features of valve bags and the mission of bag making machines

• Valve bags  feature a specially designed paper valve on the bottom or side. During filling, the filling tube is inserted into the valve and withdrawn after the material is filled. A special flap structure inside the valve automatically closes under the pressure of the material and its own rebound, achieving a seal. This eliminates the need for sewing or heat-sealing the bottom, significantly improving filling efficiency and providing a leak-proof and moisture-resistant seal.
• The bag-making machine's mission is  to automatically complete a series of complex processes, including paper feeding, forming, longitudinal bonding, quantitative cutting, bottom folding and bonding (forming the valve port structure), valve port installation and bonding, pressing and curing, on rolled kraft paper, composite paper and other base materials, and ultimately output valve bag products with complete structure and precise dimensions.

2. A complete analysis of the core structure and workflow of the valve bag paper bag machine

A typical fully automatic valve bag paper bag machine is mainly composed of the following core systems, and its workflow is closely linked:

1. Paper unwinding and tension control system
   • Function:  Loads and smoothly releases rolled paper. Tension control is key to ensuring the paper does not sag, stretch, or deviate during subsequent high-speed operation.
   • Workflow:
     • The base paper roll is installed on the unwinding shaft.
     • Tension detection devices (such as floating rollers, tension sensors) monitor paper tension in real time.
     • The control system (such as magnetic powder clutch, servo drive) accurately adjusts the braking force or driving speed of the unwinding shaft according to the feedback signal to maintain constant tension.
     • The Electronic Paper Processing (EPC) detects the edge of the paper and automatically fine-tunes the lateral position of the web to ensure that the paper is centered.
2. Paper feeding and pre-folding system
   • Function:  convey the flat paper forward and perform necessary pre-crease processing before entering the former, so that the subsequent forming is smoother and the creases are clearer.
   • Workflow:
     • The drive roller (usually a rubber roller) rotates under the drive of the motor and cooperates with the pressure roller to clamp the paper and provide a stable forward conveying force.
     • A pre-crease device (such as a crease wheel or a creasing wheel) is set on the paper path to press out shallow crease lines at specific positions on the paper (such as the longitudinal bonding edge, the bottom fold line, etc.).
3. Forming Shoulder/Tube Former
   • Function:  This is the "heart" component of paper bag forming, shaping the flat paper into a continuous cylinder or flat cylinder.
   • Workflow:
     • The flat sheet of paper is guided into a gradually narrowing former (usually a triangular plate or arc plate structure).
     • The paper slides on the surface of the former, and the edges on both sides are forced to bend and close towards the center.
     • Finally, the paper is wound into a continuous tube with overlapping edges (lap edges). The width and position of the overlapping edges form the basis for the subsequent longitudinal gluing.
4. Longitudinal lap bonding system
   • Function:  firmly bond the overlapping edges of paper tubes to form a sealed cylinder.
   • Workflow:
     • The overlapping area of ​​the paper tube passes through a gluing device (such as a glue wheel, a nozzle).
     • Glue (usually hot melt or cold glue) is evenly applied to the inner or outer layer (depending on the design) of the overlap area.
     • The overlapping area is immediately fed into a pressing device (e.g. pressing belt, pressing wheel).
     • The pressing device applies continuous and uniform pressure to make the overlapping edges fit tightly together. The glue quickly penetrates the paper fibers and solidifies to form a strong longitudinal sealing seam.
5. Quantitative cutting system
   • Function:  Cut the continuous paper tube accurately according to the set bag length.
   • Workflow:
     • Photoelectric sensors or encoders accurately measure the length of the paper tube during operation.
     • When the preset bag length is reached, the control system issues a command.
     • The high-speed rotating cutter (circular or guillotine) moves instantly, severing the paper tube to form the open cylindrical bag body. The cutting is usually precisely synchronized with the subsequent bag-pulling action.
6. Valve bag bottom forming and bonding system (core technology)
   • Function:  This is the key difference between valve bags and ordinary paper bags. The bottom of the bag tube is folded into a specific "valve" structure after being cut and sealed with glue. At the same time, the valve patch is installed and fixed.
   • Detailed workflow (this is the most complicated part):
     • a. Bag tube grasping and transfer:  A robot or suction cup device grasps the cut bag tube and transfers it from the continuous production line to the intermittent motion bottom forming station (such as a turret or chain clamp system).
     • b. Bottom pre-opening and valve plate installation:
       • The bottom of the bag tube is opened (e.g. using suction cups or braces).
       • The valve port sheet (pre-cut special-shaped paper sheet) is accurately picked up by an automatic feeding system (such as a vibrating plate, a suction nozzle).
       • The valve plate is precisely placed and pre-fixed at the designated position (valve position) on the inside of the bag bottom. This step is crucial for the subsequent valve function.
     • c. Bottom fold:
       • Complex folding mechanisms (such as folding forks, folding plates, and forming molds) begin to move.
       • The bottom of the bag tube is folded multiple times in a strict sequence:
         • Start by folding over the two side corners (to form the plug).
         • Then fold the front and back pages.
       • The key to the folding process is forming the internally concealed "valve tongue" structure . This valve tongue is typically formed by folding the rear leaf, and will be the core component of the future filling channel and automatic closure. The front leaf is folded over the valve tongue, leaving a valve opening.
     • d. Gluing:  Accurately apply glue (mostly quick-drying hot melt adhesive) to the folded areas that need to be bonded (such as the junction between the plug and the bag wall, the front leaf and the plug/bag wall, and the edge of the valve plate).
     • e. Pressing:  A powerful pressing mechanism (e.g., a pressure plate or airbag) presses the folded bottom together, ensuring full contact and a secure bond between all bonding surfaces. Pressing typically involves a dwell time. The valve flap is securely bonded to the interior of the bag, with its opening precisely aligned with the valve channel formed by the fold.
     • f. Molding and shaping:  After pressing, the bottom is kept in shape for a while in the mold or shaping station, waiting for the glue to initially solidify and shape, ensuring that the bottom is flat and square and the valve port shape is standard.
7. Conveying and counting/stacking systems
   • Function:  Output, count and stack the finished valve bags in order.
   • Workflow:
     • The finished bag is released from the bottom forming station.
     • The conveyor belt delivers the bags.
     • The counting sensor counts the bags that pass through.
     • The stacking device (such as a robotic arm or a push plate) neatly stacks the bags in a set quantity on a pallet or conveyor line.

3. Key Process Points and Quality Control

1. Tension control:  Stable tension throughout the entire process is essential for ensuring paper flatness, clear folds, accurate color registration (if printed), and dimensional accuracy. Tension fluctuations can lead to problems such as wrinkles on the bag, poor gluing, and inaccurate cutting.
2. Bonding quality:
   • Glue selection and application:  Choose the appropriate glue based on the paper material, production speed, and storage environment (hot melt glue is the fastest, cold glue is the least expensive). Glue quantity, coating location, and temperature (for hot melt glue) must be precisely controlled. Insufficient glue can cause delamination, while excessive glue can easily overflow and contaminate equipment or the bag surface.
   • Lamination pressure and time:  Sufficient pressure and appropriate holding time ensure that the glue fully penetrates the paper fibers and forms a strong bond. Uneven pressure can cause localized debonding or an uneven bottom.
3. Forming accuracy:  The design and processing precision of the former, folding mechanism, and mold, as well as the synchronization of the various actuators, directly determine the shape and size of the bag (especially the bottom width and height) and the accuracy of the valve structure. A skewed valve or dimensional deviation can lead to filling difficulties or seal failure.
4. Cutting accuracy:  The consistency of bag length depends on high-precision length measurement and cutting mechanisms. Length errors will affect the matching of subsequent packaging lines and the customer experience.
5. Valve plate installation and bonding:  The valve plate must be positioned with extreme precision, perfectly aligned with the channel formed by the bottom fold. Secure and reliable bonding is crucial to prevent material from leaking from the edge of the valve plate during filling. The quality of the valve plate itself (strength and stiffness) is also crucial.
6. Speed ​​and stability:  Modern high-speed valve bag paper bag machines can reach 60-80 bags per minute or even higher. Maintaining precise synchronization of all movements and long-term stable operation at high speeds is a testament to the quality of the equipment's manufacturing. Good lubrication, a sturdy frame, and reliable servo drives and control systems are crucial.

IV. Summary

Valve bag paper bag machines exemplify the high degree of automation and precision machinery employed in the modern packaging industry. From rolls of base paper to finished, intricately structured, reliably sealed valve bags, the entire process integrates precise mechanical transmission, stable tension control, efficient bonding technology, complex folding and forming logic, and precise electrical control. Understanding its operating principles helps operators better use and maintain the equipment, process engineers optimize production parameters, procurement personnel evaluate equipment performance, and equipment manufacturers pursue continuous technological innovation to meet the growing market demand for more efficient and higher-quality valve bag production. At its core, this process precisely replicates and enhances the key steps of manual valve bag folding through a series of highly coordinated automated actions, enabling scalable, standardized, and highly efficient industrial manufacturing.