Food Packaging Bag Materials Production in China by Leading Manufacturers

The Production Processes of Food Packaging Bag Materials-Discover how our manufacturers ensure top-notch quality and innovation to meet diverse packaging needs.



Polyethylene (PE)

- Raw Material Preparation: Ethylene monomers are used as the main raw materials to produce polyethylene resins through polymerization reactions. Depending on the type of polyethylene required (such as low-density polyethylene LDPE, high-density polyethylene HDPE, etc.), appropriate polymerization methods and process conditions are selected. For example, in the high-pressure method for producing LDPE, the pressure is usually 100 - 300MPa and the temperature is 160 - 300℃, with a trace amount of oxygen or organic peroxides as initiators. For the production of HDPE, the low-pressure method (including slurry method, gas-phase method, etc.) is often used, with polymerization occurring under the action of Ziegler-Natta catalysts or chromium-based catalysts. The reaction pressure is generally 1 - 10MPa and the temperature is 60 - 100℃.

- Extrusion Molding: Polyethylene resin pellets are added to the hopper of an extruder. The rotation of the screw pushes the resin to be heated and melted uniformly in the barrel, forming a homogeneous melt. The extruder screw is usually divided into a feeding section, a compression section, and a metering section, with the temperature gradually increasing in each section to ensure the resin is fully melted and plastified evenly. Generally, the temperature of the extruder barrel from the feeding section to the die exit is between 150 - 250℃. Then, it is extruded through a die with a specific shape to form a primary product in the form of a tube or sheet, namely blown film (used for making the main body of plastic bags) or sheet (which can be further processed into various packaging products).

- Blow Molding Process (for Films): At the same time as extrusion, a certain pressure of air (generally 0.1 - 0.3MPa) is introduced into the tubular film blank to expand it laterally and stretch it longitudinally under the action of a traction device, thus obtaining a polyethylene film with a certain thickness and width. By adjusting the blow-up ratio (generally between 2 - 5) and the traction speed, the thickness, width, and tensile properties of the film can be controlled to meet the needs of different food packaging. For example, for LDPE films used to package bread and other foods, a smaller blow-up ratio and a moderate traction speed may be selected to ensure the flexibility and transparency of the film. For HDPE films used to package heavier items, the blow-up ratio and traction speed may be appropriately increased to improve the strength and stiffness of the film.

- Printing and Lamination (Optional): If it is necessary to print patterns, text, or brand logos and other information on polyethylene packaging bags, gravure printing, flexographic printing, or digital printing methods can be used. Gravure printing has the advantages of bright colors, rich layers, and high printing accuracy, and is suitable for high-quality food packaging printing. Flexographic printing has the characteristics of low cost and good environmental protection, and is often used for some food packaging with relatively low requirements for printing accuracy. After printing, if it is necessary to enhance the barrier properties of the packaging (such as preventing the entry of oxygen and water vapor and extending the shelf life of food), the polyethylene film can be laminated with other materials with barrier properties (such as aluminum foil, EVOH, etc.). Commonly used lamination methods include dry lamination and extrusion lamination. Dry lamination involves coating an adhesive on the surfaces of two materials and then bonding them together by heating and pressing. Extrusion lamination is to extrude a molten plastic (such as polyethylene) through an extruder and combine it with another material when it is in a molten state to form a multi-layer packaging material.

- Bag-Making Process: The polyethylene film after printing and lamination (if any) is cut, folded, and heat-sealed by a bag-making machine to make food packaging bags of various shapes and specifications, such as three-side seal bags, four-side seal bags, stand-up pouches, zipper bags, etc. The heat-sealing temperature and pressure are key parameters in the bag-making process. Generally, depending on the thickness and type of the polyethylene film, the heat-sealing temperature is between 120 - 200℃, the pressure is between 0.2 - 0.5MPa, and the heat-sealing time is between 0.5 - 2 seconds to ensure that the heat-sealed part is sealed firmly to prevent food leakage and the entry of external gases and moisture.



Polypropylene (PP)

- Raw Material Polymerization: Propylene monomers polymerize under the action of a catalyst to produce polypropylene. Common polymerization methods include the slurry method, the bulk method, and the gas-phase method. In the slurry method, a diluent (such as hexane) is usually used, and Ziegler-Natta catalysts or metallocene catalysts are used for polymerization at a relatively low temperature (usually 60 - 80℃) and pressure (1 - 4MPa). In the bulk method, no diluent is used, and the polymerization temperature is relatively high (generally 70 - 100℃), with a pressure of about 2 - 4MPa. The gas-phase method is to polymerize propylene in the gas phase using a catalyst, with a reaction temperature of 70 - 100℃ and a pressure of 2 - 3MPa. Through these different polymerization methods and process conditions, polypropylene resins with different performance characteristics can be produced, such as homopolypropylene (PPH), random copolymer polypropylene (PPR), and block copolymer polypropylene (PPB), to meet the different performance requirements of food packaging for material transparency, hardness, toughness, heat resistance, etc.

- Extrusion and Molding: Similar to polyethylene, polypropylene resin pellets are melted and plastified by an extruder and then extruded. The temperature of the extruder barrel is generally between 180 - 280℃ and is adjusted according to the specific grade and processing requirements of polypropylene. After extrusion, it can be made into different forms of packaging products by blow molding, injection molding, or extrusion of sheets. In blow molding, the extruded tubular blank is blown and stretched to make polypropylene films, which can be used for food packaging, such as the packaging of potato chips, biscuits, and other foods. Injection molding is used to produce various polypropylene plastic containers, such as food storage boxes and beverage bottle caps. Extruded sheets can be further processed into blister packaging trays, etc., used for packaging fresh fruits, vegetables, meats, and other foods.

- Oriented Stretching (Optional): For some food packaging applications that require improved strength, transparency, and barrier properties, polypropylene films can be subjected to oriented stretching treatment. Usually, a biaxial stretching process (BOPP) is adopted. First, the extruded thick sheet is stretched longitudinally (the stretching ratio is generally between 4 - 8), and then stretched transversely (the stretching ratio is between 5 - 10), so that the polypropylene molecular chains are oriented and arranged in two directions, significantly improving the physical properties of the film. The BOPP film after biaxial stretching has high transparency, high gloss, good barrier properties (having a certain barrier effect against water vapor and oxygen), and mechanical strength, and is widely used in the field of food packaging, such as the packaging of various snack foods, candies, tea, etc.

- Printing and Post-Treatment: Polypropylene materials have good printing performance and can be surface decorated by various printing methods, such as gravure printing, flexographic printing, screen printing, etc., to meet different design and brand promotion needs. After printing, some post-treatment processes can also be carried out according to the functional requirements of the packaging, such as aluminum plating treatment (a layer of aluminum is plated on the surface of the polypropylene film by vacuum evaporation and other methods to further improve its barrier properties and decorative effect, and is often used for high-end food packaging), coating treatment (such as coating an antifogging agent to prevent the packaging surface from fogging in a refrigerated or humid environment, affecting the display effect of food, and is often used for the packaging of fresh fruits and vegetables), etc., and then the final food packaging bag products are made through slitting, bag-making, and other processes.



Aluminum Foil

- Aluminum Ingot Melting and Cast Rolling: First, high-purity aluminum ingots are put into a melting furnace for heating and melting. During the melting process, an appropriate amount of alloy elements (such as iron, silicon, manganese, etc., to improve the strength, hardness, corrosion resistance, and other properties of the aluminum foil) are added, and the temperature, time, and atmosphere in the furnace are strictly controlled to ensure that the aluminum liquid has a uniform composition, is pure, and the impurity content meets the requirements. After melting, the aluminum liquid is refined, degassed, and filtered, and then enters a cast rolling mill. Through the cooling and rolling action of the cast rolling rolls, the aluminum liquid is continuously cast and rolled into aluminum foil slabs with a certain thickness (usually 6 - 10mm). The casting temperature is generally around 650 - 750℃, and the rolling speed is about 1 - 3m/s.

-Cold Rolling and Annealing: The aluminum foil slabs are further cold-rolled in a multi-stand cold rolling mill to gradually reduce the thickness to the required level. Each pass of cold rolling reduces the thickness by a certain amount, and intermediate annealing treatments are carried out at appropriate times to eliminate work hardening and improve the plasticity and formability of the aluminum foil. The annealing temperature is usually between 300 - 500℃, and the annealing time is about 2 - 6 hours. The final thickness of the aluminum foil can be adjusted according to the specific application requirements, such as 0.006 - 0.2mm for common food packaging aluminum foil.

-Slitting and Finishing: The cold-rolled and annealed aluminum foil is slit into different widths according to customer requirements using a slitting machine. Then, it undergoes surface treatment processes such as cleaning and coating (if necessary). The coating may be a protective layer to prevent oxidation or a layer with certain functional properties, such as a heat-sealing layer to improve the heat-sealing performance of the aluminum foil when used in composite packaging. After these finishing processes, the aluminum foil is ready for use in food packaging, such as being laminated with other materials to form composite packaging materials for packaging chocolates, coffee, and other food products.