(1) Extrude PP into the extruder, and after a certain temperature and time, under the action of the rotating shear force of the screw, the pellets are melted and measured before quantitatively entering the filter. The extrusion temperature can be adjusted between 230-280 ℃, but on the premise that the material can be uniformly plasticized, the temperature control should be lower. It is advisable to avoid material degradation due to excessive temperature and prolonged heating time.
(2) Filtered PP fresh materials, edge materials, recycled materials, recycled materials, etc. all contain impure impurities and unmelted substances. These non fusible substances must be filtered out with a filter and cannot enter the mold head, mixed with thick sheets. The filter screen should be selected appropriately. Although having too many mesh sizes and too fine mesh holes is beneficial for thick film extrusion, it will increase the frequency of replacing the filter screen and increase raw material consumption; The mesh size is too large, impurities are not filtered thoroughly, and mixing with thick sheets can cause transverse rupture of the membrane. Generally, a slightly higher mesh size should be selected, and the filter should be replaced every 7-10 days.
(3) The casting material of the mold head is filtered and enters the mold head. The mold head is divided into two types: flat film T-shaped and tube film circular. To produce different products, single-layer or multi-layer co extrusion die heads need to be used separately. The distribution is adjusted through the “black box” material channel above the die head, so that different materials flow out of the lip according to different layering channels, so that thick sheets can obtain different layering as needed. Through the transformation of the “black box” and the die head, single-layer, two-layer, three-layer, five layer, and even seven layer co extruded composite thick sheets can be produced. The mold head and lip can provide feedback information based on the thickness distribution of thick sheets and products, and the opening can be manually or automatically adjusted to achieve uniform thick sheets.
(4) Quenching molded PP is a crystalline high polymer with a maximum crystallization temperature of 90 ℃. The molten material is extruded from the lip and flows through a few millimeters of gap to the surface of the rotating quenching and shaping roller. Under the joint cooling of compressed air knife, quenching and shaping roller cold surface, and water bath (or air), the material up to 250 ℃ is suddenly cooled to room temperature, forming amorphous thick sheets with very small and almost transparent microcrystals. The quality of the thick film is the key to film formation, and the phenomenon of film breaking often occurs during the transverse stretching process, and the main reason for film breaking is the poor quality of the thick film. The ideal thick film should have a flat surface, uniform thickness, straight edges, no bubbles, no impurities and “fish eyes”, and almost transparent characteristics. Good thick sheets have strong adaptability to stretching processes, high film-forming rate, and good product quality.
