Cast film, also known as cast film. The casting method is a method of producing a thin film by applying a plastic solution of a certain viscosity onto a continuously rotating carrier using casting dies, scraper coating machines, or reverse rollers, and removing the solvent by heating.
Compared with other methods of producing plastic films, the prominent advantage of the casting method is that it can produce films with small thickness (5-10 μ m), small thickness deviation (± 10%), and uniform and pure texture. The product is highly transparent, has good glossiness, and has no internal stress. It is often used to manufacture plastic films with high optical performance requirements. The disadvantages are slow production speed, high solvent consumption, high cost, and low product strength.
The plastics used for producing cast film include cellulose triacetate, polyvinyl alcohol, vinyl chloride acetate copolymer, polytetrafluoroethylene, polycarbonate, etc. The film-forming process is shown in Figures 11-9 and 11-10. The prepared plastic solution is cast onto a polished circular stainless steel strip or a large-diameter drum, and dried to obtain a cast film.
Casting equipment and operation
(1) The main equipment of the casting method
① Metal drum; ② Circular stainless steel (or large-diameter drum); ③ Casting machine head; ④ Entering and leaving the system; ⑤ Heating and drying system; ⑥ Coiling device; ⑦ Raw material pretreatment, dissolution, filtration, and defoaming equipment.
(2) Operation precautions: The thickness of the cast film can be controlled by changing the carrier operating speed or the solution casting rate. It is generally achieved by changing the rotational speed of the stainless steel strip and the distance between the casting nozzle seams. It is generally achieved by changing the rotational speed brought by stainless steel and the distance between the casting nozzle seams. The main problems that may be encountered with cast films include resin solubility, solvent toxicity, and carrier peelability. Taking the production of cellulose triacetate film as an example, the recovery of its solvent is an issue that cannot be ignored in production.
The entire casting device is sealed in a drying room. Blow 65 ℃ hot air in reverse from below the stainless steel strip to gradually dry the solution. When the coated film is wound from the upper part of the front roller through the upper and lower parts of the rear roller to the lower part of the front roller, the solution has preliminarily dried and formed a film. At this point, it is peeled off from the stainless steel strip and sent for further drying. The gas containing solvent is discharged from the upper exhaust port and sent to the recovery device for solvent recovery. Two types of devices are commonly used for solvent recovery: cryogenic recovery and adsorption recovery.
The thin film peeled off from the stainless steel strip usually contains 15-20% solvent and needs to be dried again. There are two methods for further drying: drying and ironing, both of which need to be done in a rectangular drying room. Drying is achieved by dividing the drying room into several parts and adjusting the temperature of each part. Ironing is the use of a heating roller to directly iron the film to achieve the purpose of drying. The volatile matter content of dry and thin films should be less than 1%.
