blown film extrusion

Plastic Bag Production Line with a Focus on Blown Film ProcessA typical plastic bag production line consists of four core stages: raw material preparation, blown film extrusion (the most critical step), custom printing (optional), and automated bag making. Among these essential procedures, the blown film process fundamentally determines the overall quality, uniform thickness, softness and flexibility of the plastic film. As the foundational raw material for all types of daily and industrial plastic bags, high-performance film output makes blown film technology the absolute heart and core link of the entire production and manufacturing chain.

The complete blown film process officially starts with standardized raw material preparation. High-density polyethylene (HDPE) or low-density polyethylene (LDPE) resin pellets are carefully selected according to actual application scenarios and targeted bag strength, toughness and oil-resistant requirements. These qualified plastic particles are first steadily fed into a storage hopper for stable feeding. Before entering the high-temperature extruder, raw pellets will pass through a professional drying and dehumidifying device to thoroughly remove internal moisture and impurities. This pretreatment step effectively avoids air bubbles, spots, cracks and other visual defects in the finished film, ensuring stable and consistent surface quality.

Next, the fully dried and purified plastic pellets enter a high-efficiency single-screw extruder, where materials are evenly heated and melted at a constant temperature ranging from 160 to 220°C. The internal rotating screw, running at a stable speed of 20 to 30 rpm, fully stirs, shears and homogenizes the plastic materials, converting solid particles into a smooth, flowing and viscous molten mass. The fully blended molten plastic is then continuously and evenly pushed through a precision circular die head, an indispensable core mechanical component equipped with a central compressed air inlet. As the molten plastic extrudes out from the die’s annular opening, stable compressed air is injected from the center to inflate the hot material into a large, thin-walled cylindrical structure commonly known as the “film bubble”.

The freshly formed film bubble requires immediate and uniform cooling to rapidly solidify the hot plastic structure. A circular air ring cooler surrounding the bubble sprays balanced cool air across the entire surface, realizing rapid shaping and solidification. Scientific and controlled cooling treatment maintains stable film shape, enhances surface transparency, glossiness and overall tensile strength, and effectively prevents wrinkling or deformation. After sufficient cooling and shaping, the inflated film bubble is slowly flattened by sets of guide and pressing rollers, completing the collapsing process to form a neat double-layered flat film.

The flattened film is continuously pulled forward by adjustable-speed puller rolls, and the traction speed acts as a key adjustment factor for film thickness: faster traction creates thinner film, while slower operation produces thicker, more durable material. After traction and shaping, the finished film is neatly wound into compact master rolls by an automatic winder. These finished film rolls are fully prepared for subsequent processing, including customized logo printing, color pattern customization, fixed-size cutting and heat sealing for finished bag products.

In summary, the entire blown film process demands strict and precise control over heating temperature, internal air pressure, cooling airflow and traction speed. Only with refined parameter management can manufacturers produce high-quality, flawless plastic film, which directly affects the structural durability, load-bearing capacity, waterproof performance and overall appearance of finished plastic bags for global market supply.