Detailed Explanation of Valve Bag Manufacturing Process: Precision Manufacturing from Raw Materials to Finished Product

Valve bags, widely used in industrial packaging for powdery and granular materials, involve a manufacturing process with a precision and complexity far exceeding common expectations. This seemingly simple packaging product actually integrates technologies from multiple fields, including materials science, mechanical engineering, and printing technology. This article will delve into the complete manufacturing process of valve bags, taking you behind the scenes to discover the secrets of this modern industrial product.

Raw material selection and pretreatment

The first step in valve bag production is the selection and preparation of raw materials. Currently, most valve bags on the market are made of plastic materials such as polypropylene (PP) or polyethylene (PE), although paper valve bags and composite material valve bags are also available. Different materials are suitable for packaging materials with different properties, therefore, the selection of raw materials is crucial.

Polypropylene granules undergo rigorous testing before entering the production process, including physical property tests such as melting point index, density, and tensile strength. Qualified raw materials are then sent to a drying system to remove moisture at 80-100℃. This step is crucial for ensuring the quality of subsequent extrusion molding. Drying time typically takes 2-4 hours, depending on ambient humidity and the initial moisture content of the raw materials.

The pre-treated raw materials are automatically fed into the next process through a vacuum conveying system. The entire process adopts a closed-loop control system to ensure the stability and continuity of raw material conveying, laying the foundation for subsequent production.

Extrusion Molding and Thin Film Preparation

Pretreated polypropylene granules enter the extruder barrel and gradually melt into a viscous flow state under electric heating and screw shearing. The extruder typically employs a single-screw or twin-screw design with precise temperature zone control, creating a gradient temperature increase from the feeding section to the homogenization section, with a maximum temperature reaching 200-230℃.

Molten polypropylene is extruded through a die to form a thin film tube. In this process, the rationality of the die design directly affects the uniformity of film thickness. Advanced production equipment uses an automatic die lip adjustment system, which adjusts the die lip gap in real time via hot bolts to ensure that the film thickness deviation is controlled within ±2%.

In the blown film process, compressed air is introduced through the center hole of the die head to inflate the molten plastic bubble to a predetermined size. The coordinated control of the inflation ratio and traction speed determines the ratio of longitudinal and transverse tensile strength of the film. Operators precisely control the stability of the film bubble by adjusting the cooling air velocity and air volume of the air ring.

The formed film is folded by herringbone plates, pulled by traction rollers, and enters the winding system to form a master roll. The entire extrusion process is centrally controlled by a computer, which monitors parameters such as temperature, pressure, and rotation speed in real time to ensure consistent product quality.

Printing process and color management

Valve bags typically require printing of company logos, product information, and instructions for use, making the printing process an indispensable part of the manufacturing process. Modern valve bag production often utilizes flexographic printing technology, which is more environmentally friendly and suitable for printing on plastic films.

Before printing, a printing plate needs to be made, which involves creating an embossed photosensitive resin plate based on the design documents. During the plate-making process, the control of exposure time and cleaning solution concentration directly affects the quality and lifespan of the printing plate. After the plate is made, it is mounted on the printing press rollers and precise registration adjustments are made.

Ink selection is a key factor in print quality. Valve bag printing typically uses water-based or alcohol-based inks; these environmentally friendly inks not only reduce VOC emissions but also improve the safety of the packaged products. Ink viscosity needs to be monitored and adjusted in real time; too high a viscosity will result in blurry prints, while too low a viscosity will affect color saturation.

During the printing process, the film passes through multiple printing units, each applying a different color. Registration accuracy is a crucial indicator of print quality; advanced equipment employs photoelectric registration systems, achieving accuracy within 0.1mm. After printing, the film immediately passes through a drying system to rapidly cure the ink, preventing smudging and sticking.

Bag making and valve design

After printing, the film enters the bag-making process, which is the core step in valve bag production. First, the two sides of the film are folded inward by a folding device to form the basic structure of the bag. Then, a hot knife slitting device cuts the continuous film into individual bag pieces, with the cutting length accuracy controlled within ±0.5mm.

The valve is a key feature that distinguishes valved bags from ordinary packaging bags, and its manufacturing precision directly affects the filling efficiency and sealing performance of the packaging machine. The valve is typically made using a special heat-sealing process, precisely adhering a thin film to a predetermined position inside the bag. The position, size, and adhesion strength of the valve must be strictly controlled to prevent breakage or leakage during the filling process.

Heat sealing the bottom of the bag is a crucial process for ensuring the load-bearing capacity of the valve bag. The proper ratio of heat sealing knife temperature, pressure, and residence time determines the sealing strength. Excessive temperature will cause material degradation and brittle seal; insufficient temperature will result in a weak seal. Modern production equipment uses a PID temperature control system to ensure that the heat sealing temperature fluctuation range is within ±1℃.

After the side sealing is completed, punching is required to create the venting function of the valve bag. The position and size of the punch need to be precisely designed according to the bag type and usage requirements. If the holes are too small, it will affect the venting efficiency; if they are too large, it may affect the strength of the bag.

Quality inspection and performance testing

Finished valve bags require rigorous quality inspection to ensure they meet usage requirements. Inspection items include dimensional accuracy, sealing strength, printing quality, and valve performance, among other aspects.

Dimensional inspection uses a laser measuring instrument to accurately measure the bag's length, width, and valve position. Sealing strength is tested using a tensile testing machine to simulate stress conditions in actual use, ensuring the seal can withstand the weight of the contents and impacts during handling.

Valve port sealing performance testing is a unique inspection item for valve bags. Air pressure is applied to the valve port using specialized equipment to test its sealing performance and opening pressure. A well-designed valve port should be easy to open during filling and reliably seal after filling to prevent material leakage.

Printing quality inspection includes tests for color difference, registration, and adhesion. A spectrophotometer is used to measure the color difference from the standard sample, and the tape test is used to check the ink adhesion strength. In addition, drop tests and temperature resistance tests are conducted on the bags to simulate actual transportation and storage environments.

Automated Packaging and Warehouse Management

The inspected valve bags proceed to the packaging stage. Modern production lines employ fully automated counting and packaging equipment, which neatly stacks the bags according to a predetermined quantity and secures them with stretch film. Packaged products are labeled with information such as product specifications, production date, and batch number.

Warehouse management utilizes a WMS system to achieve first-in, first-out (FIFO) inventory control and precise stock management. Special attention must be paid to the storage conditions of valve bags in the warehouse, avoiding direct sunlight and high temperatures to prevent material aging and deformation.

The valve bag manufacturing process is a complex one, integrating materials science, mechanical engineering, and automation control. From raw material selection to finished product delivery, every step requires precise control and rigorous inspection. With technological advancements, valve bag manufacturing processes are evolving towards greater intelligence and environmental friendliness, providing safer and more efficient packaging solutions for various industries.

By gaining a deeper understanding of the valve bag manufacturing process, we can better appreciate the technological sophistication and manufacturing expertise behind this everyday packaging product. Only by continuously optimizing production processes and improving product quality can we meet the ever-increasing market demands and drive the sustainable development of the packaging industry.