Heat-shrink tubing!

Here is something about heat-shrink tubing that I bet you never thought about.  If well designed, it shrinks only circumferentially and not axially.  Yeah!  That’s the part I never thought about until somebody told me.  And then my reaction was, yes of course.  You would never want it to be any other way!   Meaning that the material, although visually undistinguished in one direction or another, is extremely anisotropic as to the two directions, the circumferential direction on the one hand and the axial direction on the other hand, in terms of what it does when you heat it.  So how does the manufacturer make that happen? 

The answer turns out to be, the maker of the tubing heats it up and stretches it circumferentially while not stretching it axially, and then quenches it (cools it extremely quickly).  What you finish with is plastic material with a strong memory in it, a stored energy that is released when you warm it up again, and it reshapes itself to move toward a low-energy state that is the original “shrunk” dimensions.

If you go to a hardware store and buy heat-shrink tubing it will usually be black, and this usually means it has a UV blocker in it.  The plastic has a sort of built-in sunblock lotion with a high SPF.

I guess the main point of this blog article is my sense of wonder that of course the material from which heat-shrink tubing is made must be extremely anisotropic, and yet until just now I had never thought about that requirement.

Have you thought about heat-shrink tubing?  If so, what have you thought about it?  Please post a comment below.

5 Replies to “Heat-shrink tubing!”

  1. I LOVE heat shrink tubing! I have a box of clear stuff for all different uses (including making cat toys) and doing some electrical wiring. You should also take a look at the butt end connector things that are crimp and then heat shrink too… they are some that are marine grade… which also means that you do not need an electrical box to house a connection (under most, if not all, electrical codes). I’ve also used this for in-ground perimeter wiring for my Husqvarna automoter perimeter wire when I accidentally cut the wire or when I wanted to modify the perimeter… what I do is use the butt end connectors PLUS the heat shrink tubing (for extra sealing/protection). I am currently using the heat shrink tubing for making in-ground low voltage lights (i.e., I’m extending the distance between each light… and since I want to bury the wire in the ground… well, there you go, HEAT SHRINK TUBING!

  2. The black stuff is usually just good old polymer-grade carbon black.
    The manufacturing process is fascinating. First make your polymer blend, which is usually based on polyethylene and contains an additive designed to promote radiation-induced cross-linking of the polymer chains; extrude the blend it into tubing; then irradiate the tubing – typically with a high intensity electron beam – to cross-link the polymer; that gives you cross-linked tubing. Then heat the tubing above the melting point of the polymer – it won’t melt because of the cross-linking, but it will soften – and inflate it; and cool the inflated tubing (all done as a continuous process). If the inflation process is done right, meaning that the speeds on the reels over which the tubing passes during the process are properly controlled, there is no linear stretching, just axial inflation. Cut to length, package, and sell.

  3. I have used heat-shrink tubing since the late 1960’s or early 1970’s.

    You are all wrong about what it is, how it works and how it is manufactured. It is simply magic stuff that knows what I want it to do, and does it.

    (Except that once in a while, when I was younger, the last assortment of black tubing I had purchased from Radio Shack no longer had the correct small diameter tubing for the job then at hand. In such cases, rather than admit defeat, I would try to use what I had. Sometimes the tubing would begin to look (and smell) a bit burnt before it was as small as I wanted it to be (or it would just never get as small as I wanted it to be — just another excuse to go to the shack and buy some more small parts (if I could convince mom to drive me there).

    Also, everyone who is anyone knows that heat-shrink tubing is made by Keebler elves who live in hollow trees. That’s why the tubing is hollow.

    Who else has held the stuff above a hot soldering iron to make heat-shrink tubing perform its magic? Or a match? It is only in the last 15 years or so that I have owned a real heat gun. Until then I was a manly man who got ‘er done by misusing a tool.

  4. No, but I’ve spent enough time cursing cheap kitchen cling wrap, which is anisotropic the wrong way, so it’s almost impossible to tear cleanly across the roll. (That is made by extruding a strip and then stretching it thin, and it’s easier to stretch lengthways than sideways. I once had a case about making high-end synthetic paper, and the machine to stretch it in both directions was fairly incredible.)

  5. Biaxially oriented polymers are as we say “old in the art”. 1000s and 1000s of applications for them are known. Related to early usages, polyethylene, which was discovered in Britain between WW I and WW II, was found to be an excellent insulator. During the same time period that radiation sources were being built and researched related to materials. It was found early polyethylene materials, which had relatively poor mechanical properties, made in pressure reactors, yielded improved utility when subjected to ionizing radiation. In the late 1950’s a Chem. Engineer, Paul Cook, discovered that if one took a tubular member of polyethylene, irradiated it, subsequently heated it and “stretched” it axially, held the dimension and cooled, on subsequent reheating it shrank to about the original diameter. He founded a company called ‘Raychem” dedicated to the application of radiation to materials. They had almost 1800 issued patents at one point. Today the company is named “TE-Connectivity”. Today, besides cable connectors, “PEX” tubing, often crosslinked by various means (peroxides, etc.) is used in all manner of applications, for example, heating systems. Mediocre “shrink tubing” is made by lack of quality control in expanding radially and the linear speed of the process, the molecular weight of the base material, the catalysts used to polymerize ethylene, etc. etc.

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