Working Principle of Two-Layer Co-extrusion Blown Film Extruder
Core principle: Two independent extruders feed molten plastic into the two-layer co-extrusion die head for lamination. The combined tubular melt is blown into a film bubble, cooled, pulled, flattened and finally wound up. The whole process completes melting, lamination and film forming of two kinds of identical or different plastics simultaneously.
1. Main Structure
Two sets of extrusion systems: One extruder for the inner layer and one for the outer layer, consisting of hopper, screw, barrel and heating rings.
Two-layer co-extrusion die head: The core component. It is equipped with double internal flow channels to laminate two layers of melt and extrude them through the annular die orifice.
Cooling and bubble stabilizing unit: Equipped with internal and external cooling air rings and bubble stabilizers to control the bubble diameter and cooling speed.
Pulling and winding unit: Comprises herringbone board, pulling rollers, tension control system and double-station winder.
2. Working Process
1. Raw material melting and plasticizing
PE, HDPE and other plastic pellets for inner and outer layers are separately fed into the two extruders. Under screw conveying and zoned heating (150–230℃), the materials are fully melted into uniform melt, with impurities filtered out.
2. Two-layer melt lamination (Key Step)
Two streams of melt flow into the two-layer co-extrusion die head. The inner-layer melt runs through the central flow channel while the outer-layer melt flows along the outer channel. They are concentrically laminated at the die outlet to form a double-layer tubular melt without mixing, presenting a clear interface.
3. Film bubble blowing
Compressed air is delivered from the center of the die head to blow the tubular melt into a film bubble. Combined internal and external air cooling rapidly shapes the film and controls its thickness.
4. Stabilization and pulling
The film bubble is stabilized by the bubble stabilizer, then folded into flat film via the herringbone board. The flat film is pulled at a constant speed by pulling rollers to reach the required thickness and width.
5. Thickness detection and winding
An on-line thickness gauge monitors film thickness and makes automatic adjustments. The finished flat film is processed with tension control and wound up by the double-station winder into roll products.
3. Key Features
Co-extrusion lamination: The two layers of melt remain separated inside the die head without blending, ensuring strong interfacial adhesion.
Synchronized operation: The output and temperature of two extruders, as well as the pulling speed, are precisely matched to keep film thickness deviation within 8%.
Material compatibility: It supports combination of different raw materials (e.g. HDPE & LLDPE, virgin material & recycled material) to achieve excellent mechanical strength, heat sealability and barrier performance.
4. Typical Applications
Food packaging film, liquid packaging film, shopping bag film, agricultural mulch film, composite base film and other plastic film products.
