Working Principle of LDPE Blown Film Machine
The LDPE (Low-Density Polyethylene) blown film machine processes plastic pellets into films through extrusion, blowing, cooling, and winding. Its core working principle can be divided into the following steps:
1. Raw Material Pre-treatment
LDPE pellets (or mixtures with masterbatches/additives) undergo drying (if moisture content is high) via a hopper dryer (60–80°C). This prevents bubbles caused by vaporized moisture during extrusion, ensuring film quality.
2. Extrusion & Plasticizing (Core Step)
Equipment: Single-screw extruder (screw, barrel, heaters).
Pellets enter the extruder from the hopper and move toward the die via screw rotation. Three stages occur:
Feeding Zone: Pellets are conveyed and initially compacted.
Compression Zone: Screw channel depth decreases, compressing material and generating shear heat (aided by barrel heating). LDPE softens and melts.
Metering Zone: Molten LDPE is homogenized, forming a stable melt that flows to the die at constant pressure and output.
3. Die Molding
Equipment: Annular die (spiral or mandrel type).
Molten LDPE enters the die and flows through an annular channel (die gap: 0.5–2 mm), extruding uniformly to form a tubular parison (unblown film precursor). Die design ensures uniform flow to avoid thickness deviation.
4. Blowing & Haul-off
Key Parameters: Blow-up Ratio (BUR: bubble diameter/die diameter, typically 1.5–4:1); Draw Ratio (haul-off speed/extrusion speed, determining longitudinal stretch).
Blowing: Compressed air inflates the parison into a bubble (expanding diameter), stretching LDPE molecules transversely (radially) to enhance transverse strength.
Haul-off: Nip rollers pull the bubble upward. The draw ratio controls longitudinal stretch, affecting longitudinal strength and thickness (higher ratio = thinner film).
5. Cooling & Shaping
Equipment: Air ring (external cooling) + internal cooling (optional).
The hot bubble (160–200°C) is rapidly cooled by 360° airflow from the air ring to below LDPE's glass transition temperature (~80–100°C), solidifying its shape. Internal fans or water cooling may optimize efficiency.
6. Winding & Post-processing
The cooled bubble passes through collapsing frames (flattening), thickness gauges, and is wound into mother rolls by winders. Mother rolls are slit into finished films for printing, bag-making, etc.
Key Parameters & Impacts
Temperature Control: Zones (Feeding: 150–180°C; Compression: 170–200°C; Metering: 180–220°C) must avoid degradation (>250°C) or uneven melting.
BUR & Draw Ratio: Determine thickness, strength (biaxial stretching improves mechanics), and transparency (stretching refines crystals).
Cooling Efficiency: Insufficient cooling causes sticking/thickness variation; over-cooling induces stress, reducing toughness.
Summary
The LDPE blown film machine continuously converts pellets into biaxially stretched films via "extrusion → die molding → blowing/haul-off → cooling → winding." It leverages screw extrusion, blowing, and traction to tailor film thickness, strength, and optics for packaging, agriculture, and industry.
