Differences and similarities between the manufacturing processes of square-bottom valve bags, heat-sealed bags, and other bag types and cement-sealed bottom valve bags.

In today's increasingly segmented packaging industry, different bag types offer unique advantages for different material characteristics and filling scenarios. Square-bottom valve bags, heat-sealed bags, and traditional cement-sealed bottom valve bags share some similarities in their manufacturing processes, yet also exhibit fundamental differences. As someone who has worked in packaging material production for many years, I would like to outline the process characteristics, key technical points, and differences between these bag types from a practical production perspective.

I. Core Manufacturing Process of Square Bottom Valve Bags

Square bottom valve bags are small bulk packaging containers made primarily of polypropylene (PP) or polyethylene (PE). After being filled with products, they form a square, three-dimensional shape. They are convenient, neat, and have good sealing properties, making them one of the most popular packaging materials internationally.

From a process chain perspective, the production of square-bottom valve bags involves multiple steps, including drawing, weaving, laminating, printing, welding, and packaging. The drawing process is crucial for controlling the stretch ratio, generally requiring a stretch ratio between 5.0 and 5.4, a stretching temperature maintained at around 255℃, and a machine speed controlled between 380 and 500 meters per minute. The weaving process requires strict control of warp and weft density and fabric weight to ensure a defect-free fabric surface. In the laminating process, the thickness deviation of the laminated film must be controlled within ±0.0025 mm, and the lamination strength must be greater than 3 N/30 mm.

The welding (bag making) process utilizes a square-bottom valve-mouth bag making machine, achieving a bag-making speed of over 120 bags per minute. The bags are then processed using specialized bag-making equipment, including multi-color printing, center seam bonding, needle punching, valve attachment, bottom attachment, and compaction. The finished bags feature ultra-micro ventilation holes on the surface, and the entire bag body is seamless with needle holes, significantly reducing dust leakage. Strict environmental requirements govern the production process, with temperature controlled between 18℃ and 28℃ and humidity between 40% and 60%.

II. Characteristics of the Cement-Filled Bottom Valve Bag Manufacturing Process

Cement-sealed valve bags are commonly known in the industry as "sealed-bottom bags." They are produced on a highly integrated automated production line that combines printing, cutting, folding, sealing, valve making, drying, and counting functions.

The manufacturing process of cement-lined bags generally consists of seven key steps. The first is unwinding and tension control. Large rolls of substrate are placed on an unwinding frame, and the unwinding resistance is adjusted in real time using systems such as magnetic powder brakes and pneumatic brakes to ensure the substrate maintains constant tension as it enters subsequent units, preventing loosening, wrinkling, or excessive stretching and deformation. If branded packaging bags are to be produced, the substrate will go through a printing unit, typically using flexographic or letterpress printing. The graphic areas are coated with ink and then dried and cured using infrared or hot air.

Next comes the longitudinal cutting and sheet separation. A high-speed rotating circular blade cuts the wide roll of material into individual bag blanks of a predetermined width. Then, it enters the hemming and pre-treatment process. The two sides of the sheet strip are folded inward by about 1 to 2 centimeters through a complex hemming device, which not only hides the rough edges but also provides an overlapping surface for subsequent center seam bonding.

Forming, gluing the center seam, and cutting to length are the most crucial steps. After pretreatment, the U-shaped sheet enters the bag forming machine, where it is further rolled up so that the folded edges on both sides accurately overlap to form a cylindrical shape. After being coated with quick-drying latex or hot melt adhesive, it is pressed firmly by pressure rollers. Finally, based on the length measured by the detection system, a transverse cutting blade is triggered to cut the bag into individual bag blanks.

In the final bottom gluing and valve sealing process, a bottom gluing machine is used to complete a valve bag through automatic glue application, bag tube conveying for bottom opening, valve sealing strip sealing, and bag bottom flipping.

It is worth noting that the new standard GB/T 9774-2020 "Cement Packaging Bags," which officially came into effect on April 1, 2022, marks a complete shift from sewn to glued bottoms for cement packaging bags. Sewn-bottom bags are now entirely obsolete, and only square-bottom valve bags are permitted. The new standard requires that the bag-making process be changed from the previous glued-bottom and sewn-bottom bags to square-bottom valve bags using adhesive bonding and heat sealing, with a lamination weight greater than 18 grams per square meter.

III. The Manufacturing Process Principle of Heat-Sealed Bags

The core of heat-sealed bag manufacturing lies in heat-sealing welding technology. Unlike other bag types, heat-sealing is a heat transfer process. While the process flow is not complex, many factors affect its quality. It involves heating the material at the sealing point to a viscous state and then applying pressure to bond it, thereby achieving a seal.

The heat sealing head, the actuator in the heat sealing process, can be categorized into various methods based on its structure and heating method, including ordinary heat-press sealing, fusion sealing, pulse sealing, and ultrasonic sealing. Three main parameters—temperature, pressure, and time—jointly determine the heat sealing effect. Too low a temperature will result in weak adhesion, while too high a temperature may cause the material to melt through or shrink and deform. The pressure should ideally be controlled at approximately 3 mm at the heat sealing blade tip. Insufficient pressure will make it difficult for the heat sealing surface to fuse, while excessive pressure may squeeze out the semi-molten heat sealing material, leading to thin and brittle sealing edges.

The heat-sealing time is also crucial. The longer the sealing time, the more fully the heat-sealed layer fuses. However, if the time is too long, the film is prone to shrinkage and wrinkling, affecting the appearance. A balance needs to be found while ensuring the sealing strength. The cooling process is also not to be ignored. A lower temperature is used to shape the weld after molten heat sealing. Insufficient cooling will result in inadequate shaping, affecting the heat seal strength and appearance.

In practical applications, heat-sealing welding is an advanced method developed from traditional adhesive sealing. Hot air sealing utilizes hot gas welding technology, resulting in high bonding strength and a welding coefficient of up to 90%. It is also unaffected by dust or other contaminants on the bag surface, unlike adhesive bonding which has stringent requirements for surface cleanliness.

IV. Comparison of the similarities and differences in bag manufacturing processes for the three bag types

(a) Differences and similarities in equipment structure

The three bag types differ significantly in terms of equipment. The bag-making equipment for square-bottom valve bags focuses on the weaving and lamination of plastic flat yarns, using a square-bottom valve bag-making machine to complete the welding seal. The core equipment for cement-sealed valve bags involves a bottom-sealing machine and a tube-making machine working together. The tube-making machine is responsible for unwinding and printing the bag body, while the bottom-sealing machine is responsible for sealing and forming the bottom of the bag. This is accomplished through a series of coordinated processes, including automatic glue application, bag tube conveying for bottom opening, valve strip attachment, and bag bottom flipping.

Heat-sealed bags use heat-sealing welding equipment, whose basic structure is similar to the bottom-gluing machine for paper bags, but the bottom-gluing part is replaced with heat-sealing welding. First, the coated tubular cloth is printed and cut using heat-sealing equipment, and then the top and bottom openings are folded. The valve and reinforcing sheet are welded with hot air at about 400℃.

Sealing machines typically employ integrated photoelectric, mechatronics, and pneumatic technologies, controlled by a photoelectric PLC automatic control system. This allows for automated feeding, gluing, and conveying, significantly improving production efficiency. In contrast, heat-sealing welding equipment relies more on precise temperature and pressure control to achieve the welding and sealing of different parts of the bag.

(ii) Differences in sealing processes

This is the most fundamental difference between the three bag types.

Cement-sealed valve bags utilize an adhesive-bonded bottom process. This involves applying adhesive to the bottom or sides of the bag and drying it to seal the bottom. The valve opening is created by attaching valve paper. The entire bag-making process, including feeding, bottom opening, adhesive application, drying, valve attachment, shaping, bottom bonding, pressing, and stacking, is completed in one step. The advantages of this adhesive bonding process are its relative maturity and stability, and moderate equipment investment. However, it requires a high degree of cleanliness of the bonding surfaces, and the environmental friendliness of the adhesive remains a challenge for the industry.

Square bottom valve bags utilize both adhesive bonding and heat sealing during manufacturing. The new national standard explicitly stipulates that cement packaging bags should use square bottom valve bags that combine adhesive bonding and heat sealing. Therefore, large-scale square bottom valve bag manufacturing equipment needs to be equipped with both adhesive bonding capabilities and heat sealing welding units.

Heat-sealed bags utilize heat-sealing welding technology throughout the entire process. The hot air sealing process uses hot gas welding, unaffected by dust contamination on the bag surface, and does not require highly clean bonding surfaces to achieve good adhesion strength, with a welding coefficient reaching 90%. Heat-sealed bags offer significant strength advantages—sufficient overlap allows their strength to be comparable to the base material itself, making them the most popular and strongest bag type internationally.

(III) Differences in material selection

The three bag types also differ in their material usage. Square-bottom valve bags can be categorized by material into PP valve bags, PE valve bags, paper-plastic composite valve bags, kraft paper valve bags, and multi-layer kraft paper valve bags. Cement-sealed bottom valve bags traditionally use multi-layer kraft paper, but laminated woven fabric or paper-plastic composite materials can also be used, with specific requirements for lamination quality exceeding 18 grams per square meter. Heat-sealable bags have high requirements for the heat-sealing performance of the materials. The peel strength of the composite film, the heat resistance of the printing ink, the precipitation of additives in the heat-sealing layer, the width of the heat-sealed edge, and the uniformity of the composite film thickness are all closely related to the final quality and need to be comprehensively considered during the production process.

(iv) Appearance, structure and valve port type

From a structural perspective, square-bottom bags (including square-bottom valve bags and cement-bottom valve bags) and heat-sealable bags differ significantly in their bottom design. Square-bottom bags have their bottoms aligned, folded, and glued together, forming a rectangle after filling. The flat bottom ensures stability during filling and reduces the risk of spillage. Heat-sealable bags, also known as stepped-opening hot-melt adhesive bags or flip-top heat-sealable bags, utilize a multi-layered kraft paper staggered-cutting technique. The stepped opening is named for its design; heat-sealing adhesive is sprayed onto the stepped opening, and the bag is then folded and sealed.

In terms of valve design, bottom-sealed bags come in various types, including tubular, reinforced, top-attached, and pocket-type valves. To accommodate the packaging of certain ultrafine powders, the valves can be heat-sealed or made into double-valve designs to enhance sealing, in addition to self-sealing. Square-bottomed valve bags, on the other hand, prioritize self-sealing performance, automatically closing under the action of the internal material after filling.

(v) Comparison of sealing performance and strength

According to actual test data, the tensile strength of woven fabric/paper composite sewn bags is relatively low, ranging from 50 to 150 N/5 cm, regardless of whether the edges are folded, rolled, or bound. The strength of woven fabric/film sewn bags is improved, with TA type bags reaching 200 N/5 cm and A and C type bags reaching 350 N/5 cm.

The adhesive strength of the square bottom valve-sealed bag shall comply with the requirements of GB/T 8947. The strength of the TA type bag shall be 300 N/5 cm, while the strength of the A, B and C type bags shall reach 350 N/5 cm and 400 N/5 cm respectively. The strength is slightly higher than that of woven fabric and woven fabric/membrane sewn bags.

The square-bottom valve heat-sealed welded bag boasts the best strength, with sufficient overlap making its strength comparable to the base material, making it the strongest among current mainstream bag types [16†L15]. From an environmental perspective, pure PE heat-sealed bags can achieve glue-free heat sealing, are 100% recyclable, have strong and sturdy welds, good sealing performance, and an attractive appearance, making them increasingly popular among export-oriented enterprises.

V. Usage Scenarios and Selection Recommendations

Square-bottom valve bags have the widest range of applications, mainly used for packaging powdery or granular solid materials in industries such as building materials (cement, mortar, plaster), chemical raw materials (carbon black, titanium dioxide, resin), and food additives. Especially in the cement industry, they have become a standardized product under the mandatory requirements of the new national standards.

Cement paste-bottom valve bags, as traditional powder packaging bags, are the most maturely used in the building materials packaging field, especially suitable for materials such as cement and dry mortar. Operators need professional training to operate the equipment proficiently, and a higher degree of coordination with the filling machinery is required.

Heat-sealed bags excel in applications requiring high sealing, high strength, and high environmental protection standards, making them particularly suitable for packaging powdered materials for export-oriented companies. They avoid the problems of dust leakage and strength loss caused by pinholes in adhesive processes, while also mitigating the adverse effects of adhesives on human health and the environment; therefore, they are known in the industry as eco-friendly bags.

Overall, square-bottom valve bags, cement-bottom valve bags, and heat-sealed bags each have their own unique technological characteristics and application scenarios, presenting a complementary and mutually beneficial situation in the bag-making industry. With increasingly stringent environmental protection requirements and product quality standards, the advantages of heat-sealed bags are gaining wider recognition, but their requirements for equipment investment and process control are also correspondingly higher. Packaging companies should comprehensively consider factors such as material characteristics, filling requirements, environmental standards, and budget when selecting bag types to choose the most suitable technological approach for their specific needs.