Sizing Cages

In the blown film extrusion process, the sizing cage (aka, sizing basket, calibration cage or calibration basket) is used to stabilize the blown film bubble directly over the die and air ring. Keeping the bubble stable over the die and air ring improves film quality by ensuring the air ring and internal bubble cooling effects are uniformly applied to the entire circumference of the bubble.

As production rate increases, and in particular, when internal bubble cooling is used, the sizing cage is a required feature. The higher production rates require more air flow on the bubble which results in bubble movement and sometimes complete instability. A six level cage, like the one shown here provides an excellent bubble stabilizer.

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Typical Sizing Cage - Courtesy Gammatec, Italy
By necessity a sizing cage must come into contact with the blown film bubble in order to stabilize the material. Depending on the film properties, different styles of contact surfaces can be specified. This and a variety of other features all need to be specified when a cage is purchased.
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The key element regarding installation of a sizing cage is the height of the cage above the blown film die and the proper alignment over the die. For sizing cages that do not have a vertical height adjustment, the height position is critical to proper operation of the line.

Improper cage alignment will cause gauge problems, which can lead to roll conformation problems.

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