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Fluoropolymer Tubing Expansion Process

Fluoropolymer Tubing Expansion Process

Overview:

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:

http://www.heatshrinktubingdirect.com/polyolefin_shrink_tubing.html?gclid=COqTuJ_Hi7MCFYl7QgodryQAlg

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.

Common fluoropolymer Teflon-like materials are Polytetrafluoroethylene (PTFE) and Fluorinated ethylene propylene (FEP). While they all melt at higher temperatures, they all act different when expanding and then recovering (shrinking) in a typical process. They are difficult to make because they lose strength when expanded and must be constrained during the expansion and heating process. If not, they will simply blow up. I call this “CONSTRAINED EXPANSION”. For the purpose of further discussion, I will refer to FEP as being the fluoropolymer that is the subject of this new invention that constrains the tubing while heating and expanding it to make heat-shrink tubing.

The common process method for constraining FEP is to pass it through a glass tube that is heated (by various means) and injected with a high-temperature lubricant (usually silicone oil) to keep the tubing from sticking inside the glass tube. This process is slow and messy and leaves a silicone residue on the expanded tubing. Silicone oil is hydrophobic and spreads across any surface it contacts. It is hard to clean off and tends to contaminate any area where it is present. This is why Armor-All makes the tires and dash look good. But it must be cleaned off carefully before painting a car or the finish will be covered with small pits where the paint tries to get away from it.

In a catheter manufacturing area any amount of silicone oil can spread through the process environment and is very difficult to remove. The way it repels water (and paint) also interferes with any gluing or fusing process that is used to make catheters.

The EBD process involves injecting hot air into the constraining glass tube to act as an air bearing and keep the tubing from sticking. The many difficulties include:

  1. Balancing the externally injected air pressure with the internal expansion air.
  2. Controlling to injected air temperature so that it maintains the expansion process.
  3. Adjusting the pressure and temperature in multiple injection areas to maintain the process flow.
  4. Connecting the system of pressure, temperature, flow rate and line speed controls to keep the processes moving at production speeds.

This process has been made to work on an expanding machine using manual controls. Constantly adjusting several independent temperature controllers, flow controllers, pressure regulators and line speed can be made to work up to about 1 foot per minute (FPM). Production rate using silicone oil is expected to be around 10 FPM, so automated controls are needed to make this a useful process.

This basic control process is as follows:

The pressure sensors opposite the hot air input points on the chamber indicate whether the forming bubble is blocking the inlet gap. Air is introduced into the small crack between tubes at the three points. It is assumed that the bubble will cover at least the last downstream crack so we know where it is. If it covers the middle crack we need to make process adjustments to get it back to only covering the last crack. If it covers the first crack we need to make different process changes to avoid a completely stuck tube as once all 3 cracks are covered there is no entry point for the injected hot air and the expansion will soon stop completely (which of course will open all of the cracks and start a process discontinuity).

We have data that indicates temperature for expansion can be anywhere from 250 to 650 deg F. This is related to line speed and internal pressure.  The faster the line, the higher the temp needs to be to soak through the wall. The higher the internal pressure the more it overcomes the tube strength and forces it to expand.  We want the line speed maximized but there will be an upper limit we can get to given the 16 inch oven hot zone. Current plan is to locate the expansion chamber about 2/3 of the distance through the oven so the tube can preheat close to the expansion temp in the oven before the injected hot air pushes the temp over the expansion threshold. If the max speed is inadequate, we can make the oven longer in the next machine and/or move the expansion chamber farther downstream to give the tube more time to heat up in the oven.

The injected air will need to be at a slightly higher pressure that the internal pressure to keep the tube from sticking. Since the pressure at each of the three injection points is individually adjustable, we will vary each one to get the air flowing the right direction. As the air flows more it brings more heat which triggers more expansion and moves the bubble to cover more injection points.

The flow will change in response to the bubble location. As the flow changes due to injection points being covered, the injected pressure can be increased to move the bubble away and maintain flow. The last injection point will generally be covered with minimal flow and less heat as this is mostly there to keep the tube from sticking and start cooling the expanded tube. The middle injection point is the main location where the pressure will be varied to keep the bubble from covering the inlet and stopping flow. The first injection point is more of a startup option and a means of controlling the preheat and preventing the middle point air flow from going freely out the inlet end.

If the first injection point is covered the process has a serious problem and quick adjustment of line speed and injected pressures (and maybe injected air temperatures) must be made to get it uncovered again.

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