In the soft packaging production chain,Bag Making ProcessThe final step that determines the appearance, sealing reliability, and user experience of the finished product. The same roll of composite film can become three-sided, four-sided, or eight-sided bags through different slitting, heat sealing, and forming methods, resulting in significant differences in performance and application scenarios. This article, from the perspective of factory frontline operations, clearly explains the principles, key parameters, and forming methods of common bag types, helping procurement and product teams understand the processes and select the right bag type.
Three Core Steps of Bag Making Process
Bag forming is essentially processing printed composite films into three-dimensional containers with an opening and a sealable closure. The complete process can be divided into three steps:
The first step is unwinding and slitting.The roll film is unwound from the unwind reel with stable tension, and is cut into neat film strips according to the width of the finished bag. Unstable tension will directly cause variations in bag length and wrinkles.
The second step is heat sealing and forming.The inner layer (usually PE or CPP heat-seal layer) of two or more layers of film is heated and softened by a heat sealing knife, then pressed together to form longitudinal seal, transverse seal, and bottom seal. Heat sealing temperature, pressure, and time are the three key parameters that determine the sealing strength.
The third step is cutting and inspection.The formed integral bags are separated into individual bags by a cutting blade, then undergo quality control processes such as leak testing and dimensional sampling. Specific standards can be referred to as follows:End-to-End Post-Packaging Process Flow。
Three-side sealing bag: the most basic and cost-effective forming method
Three-side sealing is the basic form of bag-making: after the film is folded, the two longitudinal seals on the sides and one transverse seal at the top are sealed, while the bottom remains open. Its advantages include a simple structure, minimal material usage, and low unit cost, making it suitable for packaging small-sized food, daily chemical products, and accessories. The disadvantage is that the bag stands unstable, so it is mostly used for flat display or as an outer bag. Three-side sealing has relatively low requirements for heat sealing accuracy, but the flatness of the bag opening and the width of the sealed edge must still be strictly controlled within tolerance ranges.
Quadruple Sealed Bag: Balancing Flatness and Premium Feel
Quadruple-sealed bags are formed by fully sealing the four sides of two film layers, resulting in a flat bag surface with symmetrical edges. The large front visual area makes it suitable for products requiring large-scale printing and display, such as hanging coffee, tea bags, wet wipes, and sample packaging. The main challenge in the bag-making process lies in the consistency of the four-sided heat sealing. Any uneven pressure on a sealing edge can lead to curling or false sealing. Therefore, quadruple-sealed bags rely more on servo tension control and balanced temperature of multi-station heat sealing knives.
Octagonal Sealing Bag: The Technical Challenges Behind the 3D Shelf Display Effect
An eight-sided bag features an M-shaped bottom seal folded into the bag body, forming eight edges together with the pleated sides. After being placed on a shelf, it can stand upright, has a large capacity, and provides a three-dimensional display surface. Its bag-making process is the most complex: it requires first completing the bottom seal and then combining it with the side seals. It demands high precision in bottom seal alignment, consistency in folding edges, and precise temperature control for heat sealing. Therefore, its unit price is significantly higher than that of a three-sided bag. For a more systematic understanding of bag structures, you can read:Three-Side Sealing Bag Explanation。
Key Parameters and Quality Control in Bag Making Process
The quality of bag forming processes depends on several quantifiable parameters regardless of the bag type: heat sealing temperature (set according to the inner layer material, typically 120 to 160°C for PE, and up to over 180°C for CPP), heat sealing pressure and time, sealing width (usually 5 to 10 mm), and tolerance of bag length and width (typically ±1 to ±2 mm). Common defects in the factory include leakage, wrinkled sealing edges, bag length deviation, and false sealing, which correspond to adjustments in tension, temperature, cutting tools, and pressure systems. Fixing these parameters into a bag forming operation manual is the foundation for ensuring batch stability.
How to Choose the Right Bag Type and Bag-Making Process
The logic for selecting bag type is straightforward: choose three-side sealing for low cost and small specifications; choose four-side sealing for a flat surface suitable for large-area printing; choose eight-side sealing or stand-up bags for a three-dimensional appearance on the shelf. After determining the bag type, the film structure is then determined based on the weight of the contents, barrier requirements, and sterilization method, and finally the bag-making process parameters are set. It is recommended to first create a sample to confirm the sealing width, bag stiffness, and opening method before proceeding to mass production, which can help avoid large-scale rework.
Welcome to get more suitable bag-making process solutions and bag type recommendations.Contact Huashu PackagingWe will provide structural recommendations based on the contents, sterilization method, and shelf life requirements. You may also visit our sister site.Om PackagingLearn more about packaging selection resources.