This processing method involves creating fluidity by heating thermoplastic polymer material, extruding it into a film in the thermal molten state from the T-Die, cooling and solidifying it through the cooling roll, and subjecting it to corona treatment to yield a product.

■ Process

CPP Film Molding Device

The CPP film molding device consists of 4 units: extrusion, cooling, pulling, and winding.

Extrusion Unit

The extrusion device consists of a single screw extruder, with the screw diameter of 60-200mm and the head with widths of 800-2400mm.

Cooling Unit

Cooling is performed by a water cooling cylinder with an inner diameter of 500-750mm. The extruded film is cooled by an air knife and adheres to the cylinder.

Pulling Unit

The pulling unit is an integral device of the CPP molder. The film is stretched due to variations in the speed of the pulling roll, and the polymers in the film are arranged in a specific direction.

Winding Unit

After the pulling process is completed, the film passes through the thickness adjustment device to be adjusted into a product with uniform thickness, then goes through the corona discharge device for post-process and is wound around the winder to become the final product.

CPP Film Composition

Domestically, 3-layer co-extrusion film comprises most CPP films and a 5-layer film is also being attempted.

- SKIN: By use (general HOMO-PP, for deposited skin, Dry Lami, printing)

- CORE: Rigidity, transparency (general HOMO-PP, Retort CPP)

- SKIN: mostly for heat sealant

■ Directions for CPP Film molding

As for transparency, resin MI, extrusion temperature, cooling temperature, etc., are influencing factors. In the case of CPP, MI is normally 6-10 and the higher it is, the higher the transparency.

In extrusion, high temperatures are better, but caution must be used to not raise it excessively as blocking and slipping properties are affected in opposite ways.

Blocking: When polymer film is molded into a sheet, low molecular weight compounds within the polymer are diffused to the surface, which then becomes integrated due to the impact of temperature and pressure. This phenomenon is called blocking.

Slipping Property: It refers to good and bad slipping

Polymer film mainly undergoes a high-speed revolution molding process during processing. High tension is loaded to the film around the roll to increase productivity, in which case high energy is transmitted between the oil and polymer film or between films wound around the roll. Thus, if slipping property is insufficient, the film under processing may break apart or cause problems in the post process.