CAST FILM

摘要

The cast film process, unlike the blown process, is one that quenches the molten extrudate on a chilled steel roller after it exits the die. This paper discusses the process and physical variables and how they affect film properties and quality. Most polyfin films are made by either the cast or blown process. The major differences are the basic resins and the method of cooling the extrudate once it leaves the forming die. The blown film process extrudes the molten poly from a annular die, chills the tube with air, usually chilled, and forms a bubble, with a pair of nip rolls holding the bubble pressure as cooling occurs. In the cast film process, the other popular method of thin film manufacture, the polymer is forced through a slot die and chilled on a chilled roller, solidifying the molten plastic and forming a sheet of film. Fig. 1shows a diagram of this process. Fig. 2 shows some of the variables. The two different processes requires resins that have properties that support the process and give different properties to the finished film. This paper will discuss the cast film process, how the film is made, properties of the film and how varying the process effects the film properties. The extruder, a major player in cast film has been discussed and the output of the extruder is the feed for the cast film process. Most cast film lines manufactured today are coextrusion lines and as many of seven extruders may feed product to a common die by means of a coextrusion adapter. This adapter aligns the various polymers in a form required for the finished sheet and supplies a "plug" of polymer, correctly proportioned to the entrance of the die. The output of the multiple extruders, the combining adapter and the die determine the layer distribution of the coextruded film. In this discussion we will assume the extrusion line to be coextrusion, however, if the line is mono, the same principles apply except for layer distribution.