How is the shrinkage ratio of the Heat Shrinkable Tube calculated?

In the use of Heat Shrinkable Tube, the shrinkage rate is a very important indicator. We know that the heat-shrinkable tube needs to be heated and shrunk, and then wrapped on the corresponding object, so our objects have different shapes, some are simple tubular structures with the same longitudinal size, and some have very different longitudinal sizes. But we usually require the heat-shrinkable tube to completely wrap the minimum cross-sectional size of the object after being heated and shrunk.

When the longitudinal section difference is small, choosing a heat-shrinkable tube with a small shrinkage rate can not only reduce the cost, but also improve the efficiency and shorten the heating time during the heating process. So how do we calculate our shrinkage?
First of all, no matter whether our Heat Shrinkable Tube is round or flat, in its initial production process, it all came out of a round structure. After the finished product is made, it will go through some subsequent processing processes and become other shapes. However, when we heat the Heat Shrinkable Tube, it will naturally return to a round shape with no resistance. So it's important that our calculations are based on circles. Many customers use flat Heat Shrinkable Tube, so they are used to looking down on flat length and width marks in size.
The shrinkage rate of the heat-shrinkable tube means: measure the inner diameter of the expanded state of the heat-shrinkable tube as R1, and then heat it back to make it in a fully retracted state. At this time, measure the inner diameter R2, R1:R2 is our shrinkage ratio. For double-wall heat-shrinkable tubing, the inner diameter here refers to the inner diameter of the layer with glue, but the glue layer flows into an irregular shape when heated and shrunk, which is not easy to measure
For double-wall heat-shrinkable tubing, there are usually two ways to calculate the retracted inner diameter, one is to calculate the inner diameter of the sheath only, and the other is to calculate the inner diameter of the heat-shrinkable tubing during extrusion. It is also because these two methods have errors, so usually, the actual shrinkage rate of the Heat Shrinkable Tube will be slightly larger than the nominal shrinkage rate in the design.