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When purchasing
needle tubing, what is the best choice
for your specific application(s)? Welded
or unwelded tubing?
If welded tubing is preferred, should it
be laser welded or TIG (Tungsten Inert Gas)
welded?
If your choice is unwelded tubing, seamless
tubing can be specified. Be aware that this
selection carries several drawbacks: higher
costs, limitations in available sizes, wider
tolerances, and longer delivery times. In
the mid-1950s, these disadvantages caused
most of the industry to look for alternatives
to seamless tubing. (Refer to the article
"History of Needle Tube Manufacturing"
in Volume 1, Number 1 of the National
Needle News, January 1993.)
For many applications, laser and TIG welded
needle tubing are equally acceptable. Both
processes are capable of meeting industry
specifications (GG-N-196 and the Draft ISO
9626). Differences do exist between laser
welded and TIG welded tubing, and should
be clearly understood so that the most appropriate
tubing is selected for the given application.
Utilizing a laser to weld tubing requires
significant differences in the manufacturing
process. The smaller weld zone of the laser
compared to TIG, demands much tighter control
over the manufacturing process. For example,
conventional forming mills do not usually
have the necessary degree of precision required
for laser welding. Also, the raw material
used in laser welding must be of a higher
quality. In addition, the equipment must
be capable of consistent production of long,
continuous lengths of tubing. Successfully
controlling the laser welding process will
yield a consistently high quality product
with a lower cost, due to the process’s
automation and low level of scrap. (Refer
to the article "History of Needle Tube
Manufacturing" in Volume 1, Number
2 of National Needle News, April
1993.)
There are differences between the end products
of laser welding and TIG welding that might
influence which manufacturing process is
specified. TIG welding creates a heat affected
zone that is much larger than that produced
by the laser. Consequently, TIG welded tubing
does not have the consistency in magnetic
permeability, hardness, and (possibly) corrosion
resistance of laser welded tubing. The laser,
unlike TIG, is a constant and clean heat
source which produces a very small heat
affected zone on the tubing.
When tubing is TIG welded, the large weld
bead "sags" into the ID requiring
a mandrel drawing operation of either mandrel
rods or floating plugs. Plug drawing with
oil creates residual contamination which
can be difficult to remove by normal cleaning
procedures. Laser welding does not require
plug drawing because the tiny weld bead
does not "sag" into the ID. The
"sag" of the TIG weld bead also
creates a less concentric ID and OD than
is seen with laser welded tubing because
it pushes the mandrel against the opposite
wall, making it thinner than the nominal
wall thickness. This thin spot is normally
so slight that it is not noticed in the
point geometry of the cannula, but it could
create a weak spot in certain centerless
grinding applications.
If special fabrications are to be applied
to the needle tubing that require the tubing
to have extra expansion or contraction strength,
or if mandrel drawing is undesirable, laser
welding is always preferred because of improved
weld bead control. With laser welding, the
shape and penetration depth of the weld
bead can be precisely controlled, which
improves the performance of the tubing in
these special applications. TIG welding
cannot offer these capabilities.
In conclusion, for many applications, tubing
produced by laser welding and TIG welding
will both perform adequately. If your application
requires special controls of tubing characteristics
or high dimensional consistency, then laser
welding should be specified.
Laser welding has been in use for more than
20 years, producing nearly ONE BILLION feet
of tubing for the needle industry.
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