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:
- Speed of
operation.
- Maintenance
of electrodes.
- Ease of use.
- 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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