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Home > MacroLetter > 1.1- Autumn 1995 > Selective Heating & Cooling
More About Selective Heating & Cooling
The more rigid the material, the more difficult it is to stretch or compress. Therefore, when it comes to collapsing, wrinkles are likely to be created on both the sides and the centre of the tubing. This drag and wrinkling effect, caused by the variation in the distance the film travels around the circumference of the collapsing tube, can be overcome by a new innovation that selectively heats and/or cools the bubble in the areas of varying length.
During collapsing, the area of longer travel is heated and the area of shorter travel is cooled, to compensate for the distance the film must travel. Afterward, either heating or cooling may only be necessary.
When the film is collapsed, the material cools to the same temperature across and, as a result, the material is rendered flat. This outcome can be accomplished by several methods: forced hot air, infrared heaters, contact heating or cooling. The heat profile is identical to that of the length profile. The heating itself is done in the area of the collapsing frame. The profile depends on the bubble diameter and the angle of collapsing. Therefore, this heat is variable and can be adjusted.
Considerations: During collapsing, the film is heated or cooled or both, in collapsing area of the bubble or prior to collapsing (on the circumference of the bubble), in order to compensate for the different lengths of travel. For different material and bubble diameters, the profile should be adjustable.
Guidelines for optimal film flatness:
- Maintain bubble roundness
- Use low-angle collapsing
- Provide uniform heat history (side stabilizers)
- Allow differential collapsing surface speeds (segmented rollers or air)
- Maintain maximum bubble stability
- Make use of selective heating
- Collapse rigid material when it is still flexible (hot)
- Select the optimum layer combination for co extruded structures
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