Preparation process of biaxially oriented polypropylene film

日期:2022/2/9 9:00:44 / 閱讀: / 來源:本站

Preparation process of biaxially oriented polypropylene film
      (1) Extrude PP into the extruder. After a certain temperature and time, under the action of the rotating shear force of the screw, the pellets are melted and metered and then quantitatively enter the filter. The extrusion temperature can be adjusted from 230 to 280 °C, but on the premise that the material can be uniformly plasticized, the temperature control should be lower, and the material degradation caused by the excessive temperature and the long heating time of the material should be avoided as much as possible.
     (2) Filtration of PP fresh materials, edge materials, recycled materials, remanufactured materials, and recycled materials all contain impure impurities, infusible materials, etc. These infusibles must be filtered out with a filter screen and cannot enter the die and mix into the thick sheet. The filter should be properly selected. Although the mesh size is too high and the mesh is too fine, although it is good for the extrusion of thick sheets, 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 cleanly, and will be mixed into the thick sheet. It will cause the membrane to be broken by transverse pulling. Generally, a slightly higher mesh should be selected, and it is advisable to replace the filter screen once every 7 to 10 days.
     (3) Die head The casting material enters the die head after filtration, and the die head is divided into two types: flat film method T-shaped and tubular film method round. To produce different products, it is necessary to use a single-layer die or a multi-layer co-extrusion die respectively, and adjust the distribution through the "black box" material passage above the die head, so that different materials flow out from the lip according to different layered flow passages, so that the The slabs can be layered differently as desired. Through the transformation of "black box" and die head, single-layer, two-layer, three-layer, five-layer and seven-layer co-extruded composite thick sheets can be produced. The die head and lip can be adjusted manually or automatically according to the thickness distribution feedback information of the thick sheet and the product, so as to obtain a uniform thick sheet.
     (4) 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 setting roller through a gap of several millimeters. Under the co-cooling of a cold surface and a water bath (or air), the material up to 250°C is quenched to room temperature to form a thick, almost transparent amorphous sheet with very small crystallites. The quality of the thick sheet is the key to film formation. The film rupture phenomenon often occurs in the transverse stretching process, and the main reason for the film rupture is the poor quality of the thick sheet. The ideal thick sheet should have the characteristics of flat surface, uniform thickness, straight edges, no bubbles, no impurities and "fish eyes", and almost transparent. A good thick sheet has strong adaptability to the stretching process, high film forming rate and good product quality.

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