Method for producing BOPP film

The production processes for BOPP film primarily fall into two categories: the tubular film method and the flat film method; the latter can be further subdivided into sequential biaxial stretching (two-step method) and simultaneous biaxial stretching (one-step method). The tubular film method offers advantages such as simple equipment, low investment, a small footprint, and quick returns, but it suffers from low per-unit output, significant thickness tolerances, and high thermal shrinkage. While the simultaneous biaxial stretching variant of the flat film method yields products with isotropic properties, it has failed to gain widespread adoption due to equipment complexity, manufacturing difficulties, and high costs. Sequential biaxial stretching is the method currently employed by most manufacturers; although the resulting products exhibit some differences in properties-such as tensile strength, elongation at break, and thermal shrinkage-between the machine direction (MD) and transverse direction (TD), these differences have a negligible impact on overall product quality. Conversely, this method offers advantages such as high output, high speed, wide film widths, and consistent product quality.

A typical process flow consists of: raw material preparation → melting → plasticizing and extrusion → filtration → longitudinal stretching → transverse stretching → corona treatment → winding → aging → slitting → finished product. Most companies utilize the sequential flat-film stretching process, which allows for the production of monolayer or multilayer composite films. Taking a three-layer composite film as an example: since the material properties of each layer differ significantly, independent processing systems are required for each material prior to the die head; the melt streams for the inner and outer layers converge before the die to form a co-extruded composite sheet. Longitudinal stretching involves stretching the thick sheet from the casting unit by a specific ratio while it is heated. The transverse stretching mechanism is relatively complex, comprising functional sections for film entry, preheating, tentering (stretching), buffering, heat setting, and cooling.