Preparation process of BOPP film

(1) extrusion. PP enters the extruder, and after a certain temperature and time, under the action of the shearing force of the screw rotation, the pellets are melted and metered and then quantitatively enter the filter. The extrusion temperature can be adjusted from 230°C to 280°C, but under the premise that the material can be uniformly plasticized, it is better to control the temperature at a lower level, and it is necessary to avoid material degradation caused by excessive temperature and long heating time.

(2) Filtration. PP fresh materials, side materials, recycled materials, remanufactured materials, recycled materials, etc. all contain impure impurities and infusible materials. These non-melting materials must be filtered out with a filter screen, and cannot enter the die head and be mixed with thick sheets. The filter should be selected properly. Although the mesh number is too high and the mesh is too fine, although it is good for thick film extrusion, it will increase the frequency of replacing the filter and increase the consumption of raw materials; if the mesh is too large, the impurities will not be filtered.

Clean, mixed with thick slices will lead to horizontal stretching and rupture of the membrane. Generally, a slightly higher mesh size should be selected, and the filter screen should be replaced every 7d-10d.

(3) Die casting sheet. The material enters the die head after being filtered, and the die head is divided into two types: flat film T-shaped and tubular film round. To produce different products, it is necessary to use a single-layer die head or a multi-layer co-extrusion die head, and adjust the distribution through the "black box" material channel above the die head, so that different materials flow out from the lip according to different layered flow channels, so that Slabs can be layered differently as desired. Through the "black box" and

The conversion of the die head can produce single-layer, two-layer, three-layer, five-layer and even seven-layer co-extruded composite slabs. The opening of the die head and lip can be adjusted manually or automatically according to the feedback information of the thickness distribution of the thick sheet and the product, so as to obtain a uniform thick sheet.

(4) Rapid cooling and forming. Isotactic PP is a crystalline polymer with a maximum crystallization temperature of 90°C. The molten material is extruded from the lip and flows to the surface of the rotating quenching and shaping roller through a gap of several millimeters. Or air) under the common cooling, the material up to 250 ℃ is quenched to room temperature, and the crystallite is very small, almost transparent amorphous thick sheet. Thick slices

The quality of the film is the key to the film formation, and the film breaking phenomenon often occurs in the transverse stretching process, and the main reason for the film breaking is the poor quality of the thick sheet. An ideal slab should have the characteristics of flat surface, uniform thickness and straight edges, no bubbles, no impurities and "fish eyes", and almost transparent.

A good slab has strong adaptability to the stretching process, high film forming rate and good product quality.

Whether it is quenching in a large-roller water bath or multi-roller alternating cooling process, it is to obtain a good thick sheet to ensure the smooth progress of longitudinal-horizontal stretching.

(5) Thick film thickness measurement. The thickness tolerance of the film is an important technical index, and the thickness tolerance of the thick sheet plays a decisive role in the thickness tolerance of the film.