Emphasis On Processing
MDTmag.com 10 / September/October 2016
Within the MPE process, two or more
lumens can easily be split off a center
lumen, or merge two lumens into a single
lumen—all in a single continuous extruded
tube. The multi-lumen process involves
moving dies and mandrels in sync, reducing cross contamination of fluids in the
separate lumens.
Advantages of Multiple Profile
Extrusion
• Facilitates the extrusion of balloons of
any length
• Eliminates secondary bonding operations
• Allows seams to be eliminated
• Various types of tubing (single lumen,
multi-lumen, transitional Geo Trans®,
etc.) can be produced, as well as rod,
ribbon, and other non-standard profiles
• Suitable for extruding both elastomers
and foams
Disadvantages of Multiple Profile Extrusion
• Material choices limited to HCRs (high
consistency rubber)
• Issues can arise from having to move
dies and mandrels in sync
• Cross contamination of fluids in the
separate lumens can occur
Silicone Dip Molding and Coating
Metal, plastic, fabric, and glass medical
components can all be coated with thin
silicone films that are then vulcanized to
produce a smooth, durable, biocompatible
finish. For example, dip molding can be
used to create silicone coatings on devices
such as needles, cannulas, and syringes to
enhance patient comfort.
Dip molding can also be a cost-effective
alternative to silicone molding processes in
instances involving costly metal molds. It is
ideal for rapid prototyping of complex, thin
walled shapes that can later be scaled up
for large-volume commercial production.
The dip molding process involves producing a mandrel in the shape of the final
part. Mandrels are usually machined from
metal, but can be fabricated from engineered plastics and ceramics. The mandrel
is immersed in a vessel containing silicone
dispersion and then withdrawn. The mandrel, now coated with a thin liquid silicone
film, is set and placed in an oven where the
silicone is vulcanized. Following vulcanization, the silicone rubber is stripped from
the mandrel, creating the finished product.
Wall thickness can be adjusted by varying the number of dips and by adjusting
the percent of solid concentration of the
silicone dispersion. Several variables contribute to the quality and reproducibility of
each part, including evaluation of mandrel
surface finish, immersion, and withdrawal
angle and speed, dispersion viscosity and
temperature, ambient manufacturing conditions, and vulcanization parameters.
Advantages of Dipping/Coating
• Processes include mandrel-type dipping
that produces discrete components
(mammary shells, balloons) as well as
durable coated surfaces (coated surgical
blades, non-slip handles on surgical
tools)
• Costly metal molds not required
• Ideal for prototyping and short runs
Disadvantages of Dipping/Coating
• Choice of silicone raw materials limited
to solvent-based dispersions or low vis-
cosity materials with marginal physical
properties
• Possible variation over long runs depen-
dent on multiple variables
Calendaring and Sheeting
In high consistency rubber (HCR) calendaring, silicone is fed through multiple rollers to produce a film of uniform thickness.
The film is then transferred onto a carrier
sheet and may be cured or left uncured.
Calendared sheeting can be pigmented, produced with a number of surface
finishes and used to create vulcanized, un
vulcanized, reinforced, and non-reinforced
products. The process works particularly
well to create mandrel-wrapped hoses and
ducts, as well as die-cut seals.
Custom sheeting is used in a variety
of medical devices. Discs punched from
vulcanized elastomeric sheets are key
components in various valve assemblies,
mesh-reinforced sheeting is integrated
as sewing rings on artificial heart valves,
and laminated sheeting containing both
vulcanized and un-vulcanized layers is
used to produce seal tissue expanders and
mammary devices.
Advantages of Calendaring
• Used to manufacture silicone sheeting in
a continuous process
• Reinforcing mesh can be added to
increase strength and tear resistance
• Sheets can be die-cut or punched into
components
Disadvantages of Calendaring
• Raw material choice limited to HCRs
Assembly, Packaging and Sterilization
Post-manufacturing, including complete
device assembly, packaging, and sterilization, is a final consideration for silicone
processing. Not all suppliers have a full
range of capabilities, and many silicone
components are part of a larger, more
complex device. Others, such as punctual
plugs and small joint implants, function
as complete medical devices but must be
properly sterilized and packaged before
shipping.
Conclusion
The ability of silicones to be formulated to
attain specific performance, aesthetic, or
therapeutic properties makes them ideally
suited for many medical devices. However, suppliers must demonstrate a strong
understanding of the details involved with
both manufacturing silicone rubber parts
and complex medical devices. With time-to-market such a critical component in the
creation and sale of medical devices, the
ability to produce rapid prototypes, quickly
reach a final design, and consistently produce and deliver high-quality products, are
the keys to success. MDT
Geo Trans(R) tubing allows innovative
shapes in multiple profile extrusions
(Image Credit: Trelleborg Sealing Solu-
tions Tustin Inc.)
Catheter tip produced with liquid silicone
molding (Image Credit: Trelleborg Sealing
Solutions Tustin Inc.)