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Blown film extrusion produces tubular film by extruding a plastic melt through an annular die and inflating it into a bubble with internal air pressure. The bubble is cooled by external air rings and collapsed into flat tube, then wound onto rolls. Blown film is the dominant process for LDPE/LLDPE bags, agricultural mulch film, heavy-duty sacks, and multi-layer barrier film. Advantages: no edge waste (tubular film gives a bag directly), good biaxial orientation improving toughness, suitable for multi-layer co-extrusion with up to 9 or more layers. Limitations: slower output compared to cast film; thicker gauge control is harder below 15 microns. Cast film extrusion produces film by extruding a flat sheet through a T-die and quenching it on a chilled casting roll, then winding. Cast film is faster, produces better optical clarity (for stretch film, cling film, overwrap), and achieves thinner and more consistent gauge (down to 10 microns or below). Used for stretch wrap (LLDPE), cast polypropylene (CPP) film for lamination, and food wrap. Limitations: edge trim waste; less biaxial orientation limits toughness for heavy-duty applications. For agricultural film, refuse sacks, and heavy duty bags: blown film. For machine stretch wrap, CPP lamination film, and cling film: cast film.

Key specifications for a plastic film extrusion line: die diameter and layflat width (blown film) or die width (cast film) — determine maximum film width; typical blown film die diameters: 300–2,000mm giving layflat widths from 300mm to 6m+. Output rate (kg/hour) — depends on screw diameter, screw speed, and film thickness; a 55mm extruder produces approximately 80–150 kg/hour on a single-layer blown film line. Number of layers: single-layer for commodity bags and simple film; 3-layer for improved barrier or asymmetric structures (ABA or ABC); 5, 7, or 9 layers for high-performance barrier packaging (EVOH, tie layers, PE skins). Gauge range — film thickness from approximately 10 microns to 200+ microns. Wind-up system — single-station or dual-station winder; surface wind or centre wind; trim and edge handling. Automation and recipe control — modern lines include centralised recipe management, automated gauge control (IBC — internal bubble cooling — for blown film), and gravimetric dosing for multi-layer lines. Chinese film extrusion line manufacturers (Reifenhauser-equivalent Chinese brands, Zhejiang Zhuxin, Ruian Polystar, and others) offer competitive pricing on standard 3-layer and single-layer lines.

Standard blown film lines are designed for LDPE, LLDPE, and HDPE. Multi-layer lines also process PP (for CPP), EVA, EVOH (barrier layer), and nylon (PA). Confirm that the screw and barrel design is appropriate for your target resin — HDPE film requires a different screw design than LDPE due to its higher melt viscosity and processing temperature. For lines processing recycled materials, confirm compatibility of the screw and barrel with the contamination levels expected in the recycled feed. Production capacity planning: estimate annual output in tonnes, divide by planned operating hours (typically 6,000–7,500 hours for a continuously operated line), and size the line to meet peak demand with 15–20% headroom. Factor in product changeovers — switching film width or gauge takes time; minimising changeover frequency through production scheduling reduces lost output.