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• Extrusions • Complex catheters
• Components • Medical devices
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extrusion process, a
design is often selected
for economy when
validating an extrusion
process. Considering a
full factorial design, the
number of individual
extrusion runs becomes
sive. For example, a
DOE fractional design
could result in eight
runs to investigate the variable interaction effects of barrel temperature, screw
speed, air pressure, puller speed, and air gap distance. Whereas in the full facto-
rial design, the number of extrusion runs would be thirty two.
Of all the variables, the first consideration in the design of multilumen extrusions is the particular resin that will be extruded. The reason for the overlying
importance of the material choice is that the multilumen extrusion tooling will
direct the material to form the part geometry by forcing the molten material
through and around the tooling set, which is known as a tip and die. With
challenging multilumen designs where there are exceptionally tight orifices or
sharp corners, some materials can accommodate being forced into tight specific geometry and others will not. Even within the class of peba resins, which are
known to have favorable processing characteristics, certain durometers will not
produce conforming parts without the use of specific tooling to obtain identical
multilumen geometry from different durometers of the same resin type. During
the design phase, it can be beneficial to initiate early discussions even prior to
having formal drawings ready for quote with the extrusion processor who can
provide guidance on dimensioning and tolerancing schemes based upon tool
design and melt flow considerations.
An example of an area of application where this understanding becomes
important is in the creation of ultra-thin wall, deflectable tip catheters. These
types of catheters typically have wall thicknesses in the range of 0.008 to 0.012
inches depending upon the French size and may incorporate a PTFE liner for
lubricity on the main internal diameter. Additionally, there may be one, two,
three, or four lumens within the wall of the catheter, each of which may also
be lined with PTFE and contain a pull wire to actuate the deflection. One approach to constructing such a design is to assemble the component extrusions
that are multilumen liners of various durometers along with the other component materials and fuse the assembly together with the outer jacket extrusions in one or more post extrusion processing steps, also known as reflow or
lamination. The resultant product tolerances can be as tight as ±0.001 inches
on the ID and OD and, as such, are highly dependent upon holding close tolerances of the extruded multilumen liners and jackets in a variety of durometers.
Of the various material properties that are measured and can influence a material’s ability to “run well” with a given tool set are the melt flow index, the melt
range, and the molecular weight. Since these material properties do not consider
the shearing and polymer chain shortening that takes place during the extrusion
process, the information is not predictive to the success of obtaining a desired
result for extrusion designs with a given material. Success remains dependent
upon the actual conditions of the run to be attempted.
Closed loop continuous control system integrating laser
micrometer in-line with puller speed and conveyor
collection (not shown).