Cast film extrusion uses Vaccum box in its operational form. The vacuum box is used to ensure intimate contact between the molten polymer web and the casting roll. However vacuum box relies on a negative pressure, not a positive one, for pinning the web to the roll. A good designed vacuum box creates a true vacuum between the molten polymer and the roll, hence sucking the film to the roll.
If a true vacuum is created, air will not be trapped between the web and the roll. An even pinning force across the width of the vacuum box is required to prevent film imperfections. The vacuum box does little for tooling the film. The vacuum box does, however, allow you to adjust the exit angle of the film from the die. A true vertical exit places the web on the chill roll faster where more efficient cooling can take place compared to ambient air.
There are two types of vacuum boxes used in the industry today a single chamber and dual chamber design. Each has its own advantages and disadvantages. Regardless of design a vacuum box consists of four major parts: the body, the plenum, the blower and the seals. The body is constructed of stainless steel and has the reciprocal shape of the die/casting roll geometry. It is mounted to the die and supports all but one of the four seals-the last which is created by mounting the body to the die.
If a true vacuum is created, air will not be trapped between the web and the roll. An even pinning force across the width of the vacuum box is required to prevent film imperfections. The vacuum box does little for tooling the film. The vacuum box does, however, allow you to adjust the exit angle of the film from the die. A true vertical exit places the web on the chill roll faster where more efficient cooling can take place compared to ambient air.
There are two types of vacuum boxes used in the industry today a single chamber and dual chamber design. Each has its own advantages and disadvantages. Regardless of design a vacuum box consists of four major parts: the body, the plenum, the blower and the seals. The body is constructed of stainless steel and has the reciprocal shape of the die/casting roll geometry. It is mounted to the die and supports all but one of the four seals-the last which is created by mounting the body to the die.
