How to Make Supermarket Plastic Shopping Bags

From Resin to Retail: Unveiling the Industrial Manufacturing Process of Supermarket Plastic Shopping Bags

Abstract: The supermarket plastic shopping bag, an iconic symbol of modern retail, belies a complex and precise industrial production system behind its seemingly simple appearance. This article will systematically elaborate on how to professionally produce a qualified supermarket plastic shopping bag from multiple dimensions, including raw material science, production processes, quality control standards, and sustainability challenges.

Chapter 1: Raw Material Selection and Formulation – The Foundation of Manufacturing

The production of plastic bags does not begin on the production line but in chemical laboratories and petrochemical plants. The core raw material is polyethylene, primarily divided into two categories:

High-Density Polyethylene: Typically used for making tear-off roll bags (vest bags). Its molecular structure is highly linear and orderly, resulting in plastic bags with high tensile strength, rigidity, and puncture resistance, and a slightly crisp and hard feel.

Low-Density Polyethylene: Often used for making thinner, softer flat-mouth handle bags. Its molecular chains are highly branched and irregular, resulting in good toughness, high transparency, softness, and easy heat sealing.

In actual production, manufacturers often blend HDPE and LDPE in specific ratios according to requirements to balance the bag's strength, softness, and cost. Furthermore, various additives are incorporated to meet different functional and visual needs:

Color Masterbatch: Provides various colors.

Calcium Carbonate: Acts as a filler, reducing costs and increasing the stiffness of the bag body, but excessive addition compromises its strength and toughness.

Slip Agent: Such as silica, prevents film layers from sticking together due to static electricity during winding.

Antioxidants and UV Stabilizers: Delay the aging and degradation of plastic during processing and use.

The formulation of raw materials is the first step in manufacturing and is crucial in determining the final product's properties.

Chapter 2: Core Manufacturing Process – Film Blowing and Bag Making

The industrial production of plastic bags is mainly divided into two major steps: plastic film manufacturing (film blowing) and bag forming (bag making).

Step One: Film Blowing Process – From Pellets to Film

Feeding and Plastification: Polyethylene resin pellets and various additives are vacuum-fed into the hopper of an extruder.

Heating and Extrusion: The material is heated and melted inside the extruder through electrical heating and frictional heat generated by screw shear, becoming a viscous melt. The screw continuously rotates, conveying, compressing, and homogenizing the melt forward.

Filtration and Metering: The melt passes through a breaker plate (screen) to filter out impurities and unplasticized particles, ensuring film quality. Subsequently, the homogenized melt is steadily conveyed to the die head.

Blowing and Forming: This is the core step. The melt is extruded vertically from an annular die head, forming a tubular parison. Simultaneously, air at a certain pressure is blown into the tube from the center of the die head, inflating it like a balloon, forming a bubble tube. The blow-up ratio (the ratio of bubble diameter to die diameter) is a key parameter for controlling film width and thickness.

Cooling and Hauling Off: The expanding bubble tube is rapidly cooled and set by cold air blown from a cooling ring above the die head as it rises. Subsequently, it is flattened and gathered by nip rolls, becoming a "film tube."

Winding: The flattened film tube is wound onto large rolls, becoming "film rolls," ready for the next bag-making step.

Step Two: Bag Making Process – From Film to Finished Product

Using the most common "vest bag" as an example, the bag-making process is highly automated:

Printing (Optional): If the bag requires patterns or text, the film roll first enters a gravure printing machine. Ink is transferred to the film surface via engraved printing rollers and cured through a drying system.

Folding and Bottom Sealing: The film tube passes through a set of guide rollers, is precisely folded to form the basic shape of the bag, and then the bottom end (the future bottom of the bag) is heat-sealed with a hot knife.

Punching Vest Bag Handles: This is the iconic step in vest bag formation. The high-speed running film tube passes a precise punching device where two circular die cutters simultaneously punch out the bag's handle sections. The resulting waste is immediately recycled.

Side Sealing and Cutting: Immediately after, the film tube with handles passes through another set of hot knives. Side seals are made at preset length positions, while simultaneously cutting individual bags from the continuous film. This process integrates heat sealing and cutting, making it extremely efficient.

Stacking and Counting: Finished bags are conveyed via a belt, automatically stacked into fixed quantities (e.g., 100 bags per stack), and then packaged for storage.

Chapter 3: Quality Control and Performance Testing

Professional plastic bag manufacturing relies on a strict quality control system. Key test items include:

Dimensional and Thickness Deviation: Using calipers and thickness gauges to ensure length, width, and thickness meet standards.

Tensile Strength and Elongation at Break: Using a universal material testing machine to test the bag's load-bearing capacity and extensibility.

Dart Drop Impact Test: Determines the energy required for a free-falling dart to puncture the film under specific conditions, evaluating its puncture resistance.

Heat Seal Strength Test: Inspects the strength of the sealed edges to ensure they do not split under load.

Printing Quality Inspection: Includes color difference, adhesion, abrasion resistance, etc.

Chapter 4: Industry Trends and Sustainability Challenges

Amid the global wave of "plastic restriction" and "plastic bans," the manufacturing technology of supermarket plastic bags is also evolving:

Lightweighting: Through process optimization, reducing the thickness of bags while ensuring performance, thereby decreasing plastic consumption per bag.

Application of Degradable Materials: Using bio-based or biodegradable plastics such as Polylactic Acid (PLA) and PBAT as raw materials. However, it is important to note that these materials require specific industrial composting conditions to degrade, and their large-scale application still faces challenges related to cost and disposal infrastructure.

Recycling and Reuse: Increasing the recycling rate of plastic bags and using post-consumer recycled (PCR) plastic pellets for production is currently the most pragmatic and mainstream environmental approach.

Functional Innovation: Developing plastic bags with special functions such as higher strength, antibacterial properties, and anti-fogging to enhance product added value.

Conclusion

Making a supermarket plastic shopping bag is far more complex than simply "making a bag from plastic." It is a comprehensive industrial product involving polymer chemistry, mechanical engineering, automation control, and quality management. From the ethylene monomer obtained through petroleum cracking, to the polymerized polyethylene resin, and then through precise film blowing, high-speed bag making, and strict quality testing, it ultimately becomes the seemingly ordinary yet exquisitely crafted carrier we hold in our hands. Facing the future, the industry's challenges and opportunities coexist, and its development trajectory will inevitably focus more on material innovation, process optimization, and environmental friendliness throughout the entire lifecycle.