Tubing Expansion Process
Heat shrink-able plastic tubing is used in a variety of industries and can be made from many types of plastic. Basically, the tubing is heated with internal pressure and then cooled while maintaining the pressure. Once cooled the diameter remains expanded. If the tubing is then heated at or above the temperature that it was expanded at, it will shrink back to its original diameter.
The basic heat-shrink tubing we are all familiar with is polyolefin based and is used to cover electrical wires and form an insulating layer. This is available from many sources and is somewhat generic. The following link is one of many companies that make it:
The generic industrial heat-shrink tubing is relatively easy to make from a cross-linked polymer or any material that gains strength when stretched. A rubber band is easy to stretch until it reaches its limit and then it gets much stiffer. Many polymer materials are in this category and I call them “FREE-EXPANDING”, because they can be expanded without any external diameter constraint during the process. Some other common polymer materials in this category are: Polyester, polyethylene, PVDF (Kynar), etc.
Machines can be purchased to make this “free-expanding” heat-shrink tubing. One company in Holland and EBD are the only companies that I am aware of that sell these machines. While many companies make heat-shrink tubing, they typically make their own expanding machines. A big one is Raychem (now Tyco) that basically invented the process years ago.
A variant of the free expanding process involves cross-linking the polymer tubing before it is expanded. This is done my subjecting it to an electron beam which breaks the polymer string bonds and they reform to connect to other strings. This cross-linking makes the material stretch up to a point and then gain more strength than the amorphous material because the cross-linked web reaches it stretch limit. Material that has been cross-linked can be stretched more with thinner walls and is great for insulating aircraft cable that must be light weight. This is another Raychem invention.
Medical catheters also make use of heat shrink tubing to form the hollow shafts that get snaked into the body through arteries (usually starting in the femoral leg artery). The catheter can pass through the heart and then to every area of the body where the catheter can fit. Even very small arteries in the brain can be accessed by means of very small catheters to perform neurovascular procedures.
Small diameter catheters are made by layering tubing with a wire mesh embedded between layers so that it remains flexible but can be steered around the corners inside the body vascular system. This bending flexibility and torsional stiffness can be obtained in a very small diameter catheter shaft if the walls are thin. This is the same requirement as for aircraft cable.
Part of the process for making a catheter small and thin uses fluoropolymer tubing outside of the catheter to squeeze the outer layer even tighter and thinner. Fluoropolymers (Teflon and the like) melt at a much higher temperature than the typical plastic used for catheter tubing. This allows the shrinking process to be done at a temperature high enough to almost melt the inner layer and get it to stick well to the wire layer and form around the wires. This makes the catheter wall even thinner and makes the wire layer an integral part of the shaft. When used for this process the fluoropolymer layer is cut off and discarded after the catheter outer layer is finished.
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