What heat shrink tubing is, and why crosslinking makes it work

The second number is not a calculation. It is where the tube came from.

Read GSC 100/50.0 backwards. The 50.0 mm is the bore the tube was formed to, with a 1.17 mm wall; the 100.0 mm is the state it is supplied in. The manufacturer's cable-accessories catalogue defines the term outright. Heat shrinkable covers extruded and moulded polymeric materials with a shape memory effect. Heat them past the crystalline temperature and they return to their original shape and profile. Nothing is manufactured to 100 mm. That figure is storage.

Crosslinking is what makes the memory hold. The manufacturer's technology note describes controlled crosslinking of the molecular chains, induced either chemically or by electron-beam irradiation, fixing each molecule's position relative to the molecules around it. Below the crystalline melting point that structure resists flow under heat, which is why a warm store does nothing. Above it the crystals disappear, the material softens, and the only geometry left is the formed one.

There is no single recovery temperature, and treating 125 °C as one is the common error. GMW and GHW carry 125 °C to IEC 216 on their own sheet, and the 5:1 sheet repeats that figure for GMW 5x. The double layer tube GDLT prints 135 °C in its property table. Its feature list separately names 100 °C as the minimum to start shrinking and 130 °C as the minimum for full shrink. It is the only sheet here that separates the temperature where the tube starts moving from the one where it finishes. Every thin wall sheet prints no recovery figure at all. The number belongs to the compound, not to heat shrink.

The four jobs the memory is bought for

Shape memory is a mechanism. These are the purposes the manufacturer lists against it, and most part numbers are built for one.

• Sealing water and dust out of a splice or a cut end. Water is kept out by an adhesive, not by the closing wall. That is hot melt co-extruded with the polyolefin on dual wall GDW, or the optional lining GMW, GHW and GMW 5x list against IP68.
• Insulating against heat and chemicals. The GSC thin wall sheet publishes a dielectric strength above 20 kV/mm at 0.28 mm to 1.27 mm of wall. GHW at the other end of the range carries 2.4 mm to 4.5 mm and is doing a different job — mechanical protection and outer seal on a buried straight-through joint to 36 kV.
• Identification. GSC is published in black, white, yellow, red, blue and green, plus a dual yellow-and-green stripe reserved for earth leads. The function is read off the tube rather than traced back along it.
• Strain relief where a conductor leaves a terminal, so the bend happens along the wire and not at the joint. The GSC sheet lists it as an application in its own right, alongside light-duty harnessing and busbar insulation for switchgear.