Manufacturing
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BUMP
TUBING FROM CONCEPT TO REALITY
-Charles
Sparacino, Davis-Standard Corporation, USA
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Abstract
Medical tubing is used throughout the world in many different
applications, from simple intravenous feed tubes to complex multi-lumen
catheters. Producers of tubing used in medical applications are faced with
diminishing tolerances, increasing production costs and the continuing need for
process validation. Although the principles covered in this discussion can be
applied to tubing in general, we will focus on Bump, Tapered or Flare tubing.
Topics of Discussion
A brief history and description of dual or increasing
diameter tubing as well as some examples of the product.
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The manufacturing process.
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An overall view of the extrusion line used in the
manufacturing process.
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Highlights of the important features in the individual
components of the system.
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Description of the controls available to insure
consistency in day to day operation.
Introduction
Extruded medical tubing dates back to the 1930’s. At this
time, useof catheters was limited. Most catheters were made by the tedious
method of wrapping nylon, cotton or silk over a music wire mandrel, then coating
and baking it one layer at a time to produce the desired thickness. As the uses
for catheters and tubing in general increased, so did the demand for more
production. This need accelerated the medical tubing business.
Tubes at this time were fitted with connectors to join them
to the supply or suction device or pump that motivated whatever was to pass
through them. The process of joining the tube to the connector introduced other
possible flaws in the overall product such as steps or flashing. It became
desirable to eliminate or limit the use of connectors and extrude the tube with
the diameter required for the connector incorporated within its length.
The first such tube made in the United States was developed
in a small factory in Argyle, New York in the late 1950’s by Mr. Dave
Sheridan. Mr. Sheridan discovered that by stopping the internal airflow in a
tube while it was being extruded he caused a bump in the outside diameter. He
went on to develop further the process by controlling the airflow and at the
same time controlling the extrudate speed to develop the size and shape of the
bubble required.
Sheridan’s discovery led to an ever increasing amount of
catheters and other types of tubes such as :
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Tubes for IV sets for continuous medicating.
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Continuous taper tubing for ease of catheter insertion.
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Inset Bump tubes for use as suction devices.
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Continuous tubing with multiple bumps that can be cut to
length on site as needed.
The Manufacturing Process
To a point the overall process for manufacturing Bump tubing
is similar to any extruded tubing process.
Bump tubing adds to the process the complexity of forming and
sizing more than one diameter as the part is being continuously extruded. The
larger diameter is formed as the extrudate leaves the die, in the gap between
the die and the cooling device. This is done by controlling the internal air
pressure while varying the speed at which the product is being extruded. Simply
put, the Puller speed is lowered
while the internal air pressure is increased to create the larger diameter, and
then the reverse brings the tube back to the smaller diameter. It is evident
that in order to maintain a consistent size of either or both diameters, the
rate at which this is done must be accurately controlled.
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The tube can be cooled in a simple temperature controlled
water bath, however, the addition of vacuum helps in the overall process. While
the pressure on the inside of the tube is above atmospheric, the vacuum has
little or no effect on the size of the tube. The vacuum tank enhances the
process by holding the water in the cooling vessel as the diameters coming
through the sizing device change. Also the fact that there is no pressure
exerted on either diameter by the cooling water helps in holding the part round.
The cutter in the process must also be a part of the control
system in order to position the cut at the precise part of the bump to create
the required shape, and then the overall length.
Important features of the System Components include:
Extruder, Drive and Motor, Gearbox, Feed Section, Screw and Barrel, Temperature
Control, Base, Melp Pump, Die, and Internal Air control.
Sizing and Cooling
The preferred method of sizing is with the use or assist of a
vacuum. The reason this method is preferred is repeatability. There are two
methods of vacuum sizing. Contact sizing, where the extrudate expands to contact
a sizing tool that is calculated to produce the desired diameter; or for soft
sticky materials, non-contact sizing where the vacuum causes the tube to be
surrounded by a layer of water to lubricate and keep the tube a calculated
distance from the sizing tool. Both allow the producer to establish a repeatable
set of process parameters.
Due to the fact that Bump tubing requires a controlled source
of positive air pressure to create the bump and that there is no constant
diameter, vacuum sizing does not aid in the sizing of the tube in this system.
In a modern day vacuum sizer the entire system both water and air are a part of
the vacuum system, this makes the cooling water weightless as compared to the
tubing passing through, which aids in keeping both or all the diameters in the
part round. The vacuum also holds the cooling water in the cooling tank, to
allow control of the distance between the outlet of the die and the entrance of
the cooling tank.
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