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This is the customizable version of the original High-Tech Soda Can Cooler. It is exactly the same design (with a few refinements), but now you can generate models in Customizer for any size of can or bottle, as long as its lower part is a perfect cylinder.

If Customizer is broken yet again on the Thingiverse website, here are instructions to run it on your own computer.

As explained on the original Thing's page, the idea behind this cooler or ‘koozie’ is that it has no inner shell, instead it relies on the can or bottle itself to seal the air inside. This has the benefit of minimizing the material needed hence lighter weight, as well as a minimal contact area with the plastic such that it maximally relies on air for insulation: air is a better thermal insulator than most if not all plastics. To further improve insulation, the zone between the can and the outer walls is divided into horizontal compartments to reduce convection.

When creating your own model in Customizer, the most important parameters are the diameter of the can and its height you want to be covered by the koozie. To get a tight fit without having to push or tug on the can, make sure to either accurately measure the diameter or just look it up in specification sheets from can or bottle manufacturers. The JSON file contains a few presets with accurate diameters that can be loaded in the customizer interface inside OpenSCAD.
The air gap determines the distance between the outside of the can/bottle (plus tolerance) and the outer wall. I usually set this to between 4 and 5 mm. More will obviously offer better insulation, but also makes the thing fatter and unwieldy.

This works equally well in rigid and flexible filaments. A flexible filament is recommended for robustness, for instance the koozie for the SIGG bottle in the photos was printed with rigid.ink flexible PLA.

Depending on what material you print in, to compensate for shrinkage of the material when it cools down after printing, you may need to slightly scale up the model or generate it with either a larger tolerance or larger diameter. Flexible filaments tend to shrink considerably but since they're flexible, it usually also isn't too bad if the model is slightly too tight. To avoid spending hours on a print and then noticing it is too tight or sloppy, the customizer has an option test_slice that will generate a minimal model that can be quickly printed for testing the fit.