Medical Design Briefs: 2021 Technology Leaders - 32

TECHNOLOGY LEADERS Tubing/Extrusion
Critical heat shrink tubing
requirements are defined
DESIGN PHASE
Specifications for raw
tubing are determined
very tight specifications that require a
high level of inspection, the cost of the
final product will increase. Or, if the
tubing has extremely thin expanded
walls, the tubing may need to be inflated
during shipment, or it might collapse
on itself and be difficult to use.
The inflation step is a manual process,
which can also increase the final cost of
the tubing.
Extruded tubing is
irradiative via e-beam
IRRADIATION
Tubing is extruded
Tubing is expanded
FINISHING
Tubing is cut, packaged
and shipped for final
device application
Heat shrink process flow (for reference only).
POLYOLEFIN
Minimum Wall Thickness (in.)
Max Wall Thickness (in.)
Shrink Ratio
Expanded ID (in.)
Shrink Temp (°F)
Sterilization
Surface Lubricity
Hardness
Colors
0.00025
0.200
1.5:1 to 10:1
0.010-1.000
275-350
Gamma or EtO
Good
Soft to Hard
Std. or Custom
design for manufacturability. Look for a
company with extensive experience in
the design and development of successful
polyolefin heat shrink tubing applications,
and request samples of their
work. Ideally, the partner will carry standard
samples with varying diameter
ranges that allow for testing in a particular
application.
The final heat shrink tubing will likely
need to be customized in some way, but
32
PET
0.00015
0.0040
1.1:1 to 3:1
0.008-1.000
250-400
Gamma or EtO
Not Good
Hard (Crinkly)
Std. or Custom
Table 1. A comparison of polyolefin, PET, and PTFE materials for heat-shrink tubing.
using samples is an economical means
for the device engineer to determine
whether a specific type of heat shrink
tubing will work for their application
and what the optimal dimensions should
be. When designing heat shrink tubing,
engineers should consider the following
factors.
Impact of Design on Cost. The final
design of the heat shrink tubing is the
greatest cost driver. If the tubing has
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PTFE
0.00100
0.0050
1.3:1 to 4:1
0.020-0.240
500+
EtO
Excellent
Hard (Smooth)
Std. or Custom
Expanded ID. Larger diameters and
thin expanded walls require greater
expansion ratios. Tolerances for ID vary
based on expansion ratio and wall thickness.
Most of the time the expanded ID
is specified as a minimum. It's difficult to
achieve small expanded ID and thin
walls because that requires the base tubing
to be extruded. The thin walls may
stretch during irradiation, causing
dimensional fluctuations on the tubing.
Recovered ID. Specified as maximum
recovered ID. The tubing will shrink
below the maximum if free shrunk (no
restriction). When free shrunk, the ID
will recover to almost the ID of the base
extrusion but will typically be 0.001-
0.002 in. greater than the ID of the base
tubing. The larger the full recovery ratio
(expanded ID/ free recovered ID) the
greater the compressive force. Com -
pressive force is affected by the actual
recovered diameter, which may be
greater than the fully recovered diameter.
Example: Tubing has a free recovered
ID of 0.020 in. and is expanded to
0.100 in. The smaller the recovered
diameter, the larger the compressive
force.
Length. Length tolerance will vary but
typically will need 0.100 in. Upon
recovery, the tubing will also shrink longitudinally.
Keep in mind the need to
order lengths that are longer than their
final desired length to account for the
longitudinal shrink. Typically, the length
shrinkage will be between 25 and 40 percent
but is dependent on many factors,
such as shrink ratio.
Storage Condition. While most polyolefin
heat shrink tubing is ordered for
immediate use, some may choose to
order product to be used at a future
date. Storage condition is very important
because if heat shrink tubing is not
stored in a temperature-controlled area
(not to exceed 140 °F), the tubing may
start to exhibit some shrinkage.
Medical Design Briefs 2021 Technology Leaders
MANUFACTURING
MANUFACTURING
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Medical Design Briefs: 2021 Technology Leaders

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