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Hypo Tube Manufacturing

The following article is reprinted with permission as it appeared in the March 1999 edition of the Tube and Pipe Journal. The author Michael E. Ferreri is the president of Medical Tube Technology, Inc.

Producing Stainless Steel Tubing for the Medical Industry

A primer on the process

Producing tube for the medical industry requires a great deal of attention to welding, material, and surface quality.

Tubing for the medical industry is produced primarily from 304 and 304L (low-carbon) welded stainless steel. The 304 stainless steel has relatively low carbon content (0.08 percent maximum), and it resists corrosion better than 302 stainless.

Its counterpart, 304L stainless, has even lower carbon content (0.04 percent maximum) and offers similar corrosion resistance. However, 304L also offers high resistance to intergranular corrosion following welding.

 

Strip Stock

One of the leading challenges for the medical tubing manufacturer is securing strip stock with a good-quality edge condition. The walls of tubing tend to be relatively thin in the medical market and, as a result, alignment and edge condition are critical.

Weld mill operators are well advised to keep all strip stock as clean and protected as possible. This includes careful and proper storage and handling all stock with clean cotton gloves to prevent body oils from contaminating the edges.

The producer must work closely with the strip slitter to ensure the incoming strip meets edge condition requirements. Most mills have one or two strip suppliers that furnish good-quality strip that meets the mill's requirements.

Welding

Manufacturers generally use one of three different means for welding 304 and 304L stainless steel strip for the medical market.

The first is gas tungsten arc welding (GTAW). This is the oldest of the popular methods and is still widely used. The second, plasma welding, is a variation on GTAW. The last, laser welding, is the newest of the common methods. Each of these weld methods is similar in basic design, with the differences coming in the respective weld heads and power supplies. Each also has a variety of strengths and weaknesses.

Some primary items to consider in selecting a weld mill process are:

  1. Speed of operation.
  2. Maintenance of electrodes.
  3. Ease of use.
  4. Cost of weld head and power supply.

Most weld mills use eddy current test equipment after the final fin pass to verify the integrity of the weld seam. Some also add a video camera focused on the weld seam to observe the alignment of the seam and the weld head electrode. These two welding aids allow the mill to produce larger continuous coils with a fairly high level of confidence in the weld seam at speeds ranging from 20 to 30 feet per minute (FPM).

Size Ranges

The majority of medical tubing is made to a gauge scale that is loosely based on the United States military GG-N-196 specification. Modern requirements and technology advances have greatly expanded this scale. Incremental gauges from 6 (0.203 inch) to 36 (0.004 inch) with varying walls (0.015 to 0.0015 inch) are now generally accepted as standard.

Three Basic Products

Producers supply original equipment manufacturers (OEMs) and distributors with three basic types of finished tubing-welded followed by several sinks [reductions without inside diameter (ID) plug or mandrel] to finish; welded, plug-, or mandrel-drawn (to smooth the weld and to set the wall thickness), followed with several sinks to finish; and welded followed by several plug or mandrel draws to finish. Each of these methods produces a sound product, and each product has a specific area of application.

Some tube producers offer all three types of products while others specialize in one or two. They offer medical OEMs a variety of weld seams and surface finish qualities and tolerances, as well as varying costs.

Welded and Finished by Sinks. The strip stock in welded and finished-by-sinking tubing is formed and welded at or very close to the finished wall size with an outside diameter (OD) larger than the desired finished size. The desired OD of the welded tube is reached by sinking the tube to the finished size.

The sinking process is performed by lubricating the tube on the OD and pulling it through a die. This process is repeated with gradually smaller dies until the desired OD is obtained. This process does not require any intermediate annealing and as a result can produce tubing efficiently and inexpensively. In this process, however, the ability to achieve high levels of work hardening is usually hampered because the amount of cold work is limited by a lack of wall reduction in the process.

The process does not require intermediate annealing, so the weld zone remains intact and does not homogenize. As a result, the weld zone remains visible and is not integrated into the base metal. Reducing the tube without an ID mandrel or plug renders a rough surface on the ID of the tube. If a tube finished by many sinks is cut to expose a transverse section, mounted, polished and then viewed through a microscope, severe peaks and valleys along the ID are visible.

The peaks on the ID wall caused by this process make it difficult to clean the ID thoroughly because dirt and contaminants can collect in the valleys of the peaks. These peaks also make the ID erratic and can cause difficulty in measuring ID size consistently.

This type of tube is generally used for basic hypodermic needle applications such as an injectable device. Because of the erratic condition of the ID, this tube is usually not used for processes that require the ability to pass a stylet, wire, or tube through the ID. Also, this method generally is not used for drawing blood because the rough ID can cause blood to build up and coagulate, slowing or eventually stopping blood flow. Last, because of severe cold work on a non homogeneous grain structure in the weld zone, the tube may fail during a fabrication process that may be performed on the finished tube.

Welded, Plug-, or Mandrel-Drawn Followed by Sinks to Finish. With welded, plug-, or mandrel-drawn followed by sinks-to-finish tubing, the strip stock is welded and formed as with the tubing that has only sinks, but the starting wall size is larger than the desired finished wall size.

The OD and ID are lubricated, and ID plug or mandrel is inserted, and the tube is pulled through a die. Depending on the amount of reduction, this process can be repeated using gradually smaller dies and ID plugs or mandrels until a work hardness rate that requires an intermediate anneal has been reached. After annealing, the process may continue with the ID plug or mandrel or it may switch to a sink process as described previously.

The product is then finished with sinks continually to the desired finished size. The addition of the ID plug or mandrel and the intermediate anneal increases product costs, and the product efficiencies tent to decrease. Because this process has only one intermediate anneal, the weld zone, while homogenized, is still visible.

Higher work-hardening rates are achieved more easily with this process because of the amount of cold work that is required for reduction. The surface condition quality, both ID and OD, tends to be high because of the ID plug or mandrel supporting the ID wall during reduction. In addition, cleaning the ID is almost no problem, ID size is consistent, and the tube can hold tighter tolerances than the tube finished by the sinking method.

A transverse view of this tube through a microscope shows less ID wall distortion and a smoother appearance. This tube generally is used in applications that require a higher-quality tube with tight tolerances. It is normally not used as an injectable device.

Welded and Plug- or Mandrel-Drawn to Finish. In manufacturing welded and plug- or mandrel-drawn-to-finish tubing, the strip stock is formed and welded as with the other methods, and the starting wall and OD are larger than the desired finished size.

After the OD and ID are lubricated and an ID plug or mandrel is inserted, the tube is pulled through the dies. Depending on the amount of reduction and the desired finish size hardness requirement, this process is repeated with gradually smaller dies and ID plugs or mandrels until work hardness requires an intermediate anneal. After annealing, the process can start again until the desired finished size is achieved.

The product is drawn with an ID plug or mandrel from start to finish. This, in conjunction with at least two intermediate anneals, allows the weld zone to homogenize fully into the base metal. The ability to hold very tight OD and ID tolerances as well as ID cleanliness are key features of this products. A transverse view of the drawn-to-finish tube through a microscope shows a smooth ID wall and no visible weld zone.

Some uses for this product include blood donor needles, products that require tight tolerances and superior surface conditions, and products that require the ability to pass other devices through the ID.

To be sure, not all stainless steel tubing is manufactured to the same specifications. Medical device OEMs and distributors have a variety of products and product qualities to choose from when ordering stainless steel tubing for the medical marketplace. Most important, and OEM or distributor must have a good understanding of the intended use of the tube to ensure the proper product is ordered.

 

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