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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Secondary Sizing Cages

Secondary bubble cages are important, and especially critical on tall blown film towers, to keep the bubble centered in place before it enters the haul-off unit. Secondary cages are typically configured with 8 arms per level, and 2-3 levels high for a cost-effective solution to securely hold the bubble in place.

These secondary cages maintain the same robust construction and soft roll style rollers as found on the primary cage. Motorized diameter adjustment is controlled by a UL certified control box, to easily adjust bubble diameter as necessary. Bubble diameter support range is matched to the primary sizing cage for maximum versatility

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