Staying Connected - March 2014

Silicone Medical Cable Assemblies



Excellent flexibility and sterilization
withstand make silicone cable assemblies
ideal for surgical applications

For medical cable assemblies that are required to withstand a high number of autoclave steam sterilization cycles, silicone is the material of choice for both cable jacket and overmolded bend reliefs.  Silicone is also used for cable assemblies in which extreme flexibility and the highest level of biocompatibility is required.

Silicone Composition

Silicone – with an “e” on the end – is not the same as the crystalline chemical element silicon.  Silicone is an elastomeric polymer composed of silicon, carbon, hydrogen and oxygen.  In various forms silicone can be used as an adhesive, lubricant, insulation and sealant!  Silicone is an inert, synthetic polymer that is both heat-resistant and “rubber-like,” two characteristics beneficial for medical cable assemblies.

Liquid Silicone Rubber Injection Molding

Also known as “LIM”, there are two main silicone molding processes: “LSR” – injection molding of silicone rubber and “F-LSR” – injection molding of fluoro liquid silicone rubber.  Of the two, LIM is the most common method used in the manufacture of medical cable assemblies.

The two components of liquid silicone rubber are pumped through a static mixer by a metering pump.  One of the components contains the catalyst; typically platinum based and is commonly referred to as the “B” component.  Colorant can be added before the materials enter the static mixer section. In the static mixer the components are combined and are then transferred to the metering section.

Once the two components are brought together the curing process begins immediately. To retard premature curing, the mixture is kept chilled until it is injected into the heated mold cavity where vulcanization – curing – takes place.


Equipment to mold liquid silicone
ommonly includes a mixing and
dispensing unit as well as an
injection molding press

Liquid silicone rubber is supplied
in two parts, A and B with B
containing the catalyst

The process of molding silicone rubber is nearly the opposite of that used to mold thermoplastic elastomers.  Where silicone starts as a liquid and it transformed into a solid by a catalyst and heat applied during molding, thermoplastic elastomers start as a solid, are liquefied by heat and then return to a solid state when injected into and cooled by mold tooling.

Advantages of Silicone vs. Thermoplastic Elastomers

With excellent biocompatibility characteristics, stability over a wide temperature range and mechanical resilience, liquid silicone rubbers have a variety of advantages over other thermoplastic resins used in the manufacture of medical cable assemblies.

  • Biologically Inert - Liquid silicone rubber used in medical applications is typically formulated to comply with FDA and ISO 10993 biocompatibility requirements and offers superior compatibility with human tissue and body fluids. These materials are odorless, tasteless and do not support bacterial growth.  Silicone rubbers typically do not corrode or stain other materials.

  • Non-Reactive – Silicone is considered a non-reactive material and molded or extruded assemblies do not react with most cleaning and disinfecting agents.  This characteristic is very beneficial for medical cable assemblies that must be cleaned and disinfected often.
  • Heat Resistance - Molded silicone parts have an operating temperature above the melting temperature of most thermoplastic resins.  While medical cable assemblies are not typically subject to high temperatures during use, this characteristic contributes to the ability to withstand the high temperatures of multiple steam sterilization cycles.


  • The design of the back nut of the
    connector allows liquid silicone
    to flow into and around retention
    features producing a robust assembly

    High Strength – Through vulcanization, the crosslinking of silicone molecules gives silicone rubber products higher tear strength than most thermoplastic elastomers. This allows for overmolds to be designed with relatively thin walls and the subsequent molded parts to be very durable.

    Because silicone enters the mold in a cool, liquid state at relatively low pressure, the stress areas that occur when molding thermoplastic elastomers is largely absent.  The absence of “built-in” stress contributes to the high strength of molded silicone parts.
  • Low Viscosity – liquid silicone rubber has much lower viscosity than most thermoplastic resins.  This allows mold tooling to be designed with thin walls while still allowing the un-vulcanized liquid material to flow completely around the part that is being overmolded, filling cavities and mechanically “locking-onto” any retention features.

Silicone Cable Jacketing

Silicone jacketed cable can be extremely flexible, a desirable characteristic for cables that are manipulated during surgical or endoscopic procedures.  Depending on the method used to jacket the cabled components, even greater flexibility can be achieved.

Silicone cable is commonly jacketed using both extrusion and tubing methods.  In the extrusion process, liquid silicone is injected through a heated die through which single or multi-conductor cable is moving.  The silicone material fully surrounds and conforms to the wire or cable bundle.  For many applications, this construction – in which the cabled components are fully encapsulated – is ideal.


Tubed-on and extruded silicone cable jacket

For applications where increased flexibility is advantageous, cabled components may be inserted into a pre-formed silicone tube. When a “tubed-on” jacket is used, the cable bundle is not held in place by the cable jacket which allows the components to move axially as the cable is flexed.  This, in turn yields an increases the overall flexibility of the cable.

Jacket Coating

Silicone jacketed cable tends to feel sticky, does not glide easily over surfaces and tends to retain debris.  For this reason, silicone jacketed cable often has a conformal coating such as Parylene (Poly-para-Xylylene) applied.


Overmolded strain relief and small
section of uncoated silicone cable with
Parylene coated portion to the right


Parylene is biologically and chemically inert.  It has been approved for human implantable devices.  The thermal resistance of Parylene is similar to that of silicone.  Parylene is applied by vapor deposition and forms a pin-hole free coating that has a smooth, “silky” feel.  The surface of Parylene coated cable has a dry-lubricity feel, but there is no lubricant to wear off.

Silicone and thermoplastic elastomers will not adhere to Parylene.  Because of this, the areas of the cable jacket where adhesion is required are masked-off and not coated.  In some applications, Parylene is applied to finished cable assemblies.  For other applications, Parylene is applied to unmasked sections of cable before being used to fabricate cable assemblies.



A silicone overmolded strain relief
provides protection against moisture
ingress, necessary for sterilization withstand

Silicone Overmolding

Most medical cable assemblies that are designed to withstand a high number of steam sterilization cycles incorporate one or more metal shell connectors and silicone-jacketed cable.  Over-molding silicone provides a stronger bond to metal connector components than thermoplastic resins, creating a moisture-proof mechanical bond which contributes to sterilization withstand.  Typically, the same overmolding captures the silicone jacketed cable and creates a chemical bond between overmolded strain relief and the cable jacket also enhancing sterilization withstand.


Tooling used to mold liquid silicone
are heated to accelerate curing


Silicone Mold Tool Design

The design of mold tooling for liquid silicone rubber is similar to the design of molds for thermoplastic resins. There are, however, some important differences in the behavior of liquid silicone that need to be considered.

Because of the relatively low viscosity of liquid silicone rubbers, the fill-time of the mold cavity can be very short.  In order to avoid entrapping air, good mold venting is required.  Because of its low viscosity, silicone will flow and flash into spaces less than 0.001” requiring parting lines and shut-offs to be precisely tooled.

While tools used to mold thermoplastic elastomers are commonly liquid cooled, silicone mold tools are heated to accelerate curing.  This requires imbedded heating elements and connection to a temperature control unit.



Flexibility, durability and excellent
sterilization withstand are characteristics
of silicone medical cables

Advantages of Silicone Cable Assemblies

Silicone cable assemblies are characterized by excellent flexibility, durability and the ability to withstand cleaning, disinfection and hundreds of steam autoclave sterilization cycles.  These characteristics make silicone a logical material choice for cables used for video endoscopy, surgical power tools, electrosurgical instruments, laparoscopy, ophthalmic lasers and robotic surgery.

Summary

For medical cable assemblies that must withstand a high number of steam autoclave cycles, silicone-jacketed cable and silicone overmolding of strain reliefs are typically required.  The Affinity engineering team can guide in material selection and cable design.

We invite you to take advantage of Affinity’s experience and expertise in designing and manufacturing high quality, reliable silicone medical cable systems.  Please contact us at +1 949-477-9495 or via email to cmccustcare2@molex.com.

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Meet Jason Gambino – Mold Process Technician



Jason Gambino,
Affinity Mold Process Technician


Jason Gambino joined the Affinity team mid 2013 as a Mold Process Technician. 

“When we committed to adding silicone molding as an Affinity capability, we knew that we would have to find a mold process technician who understood and had experience in molding silicone,” said Affinity General Manager, Bob Frank.  “Jason has proven to be that person, helping us get our silicone molding up and running ahead of schedule.”

Asked what attracted him to the company, Jason replied, “I liked the opportunity to join a company that manufactures medical devices.  That was appealing to me.  I was also excited to part of the new silicone molding project at Affinity.  I knew that my experience and knowledge would be help and would be appreciated.”

After nine months at Affinity Jason was asked about his impressions of the company.  “I really like the friendly atmosphere at here and the manufacturing support that is available.  Everyone works as a team.  When I need help, I get it and when I can help others, I am happy to provide it.”

“One of my goals is to train all of the molding associates on the more scientific and technical aspects molding.  We have a great team of mold technicians, and the more they know, the better they will be able to apply their skills for the benefit of our customers.  I also want to learn all aspects of manufacturing medical cable assemblies from product concept through design and through the manufacturing process,” commented Jason.


Jason molding silicone cable assemblies
on customized Autojector mold press


Jason brings over 25 years’ experience in molding to Affinity.  One of the reasons he was hired was his nine years of experience molding silicone.  He also has eight years’ experience being a supervisor.  “I have worked with many different types of injection molding machines, have experience in automation and have experience with many different manufacturing processes,” said Jason.

Jason graduated from Redondo Union High School in Redondo Beach California.  After graduating, he attended Injection Molding Certificate courses at Cerritos College.  And, wanting to expand his knowledge further, took courses offered by RJG, an international leader in mold training and technology.   Courses included Process and Controls, Theory of Injection Molding, Decoupled Process 1 & 2 and Scientific Molding.

Speaking about his biggest challenge since joining Affinity, Jason commented, “Most of my experience has been injection molding components.  Here at Affinity we primarily overmold cable assemblies.  It took me a few months to really be able apply my experience to what we do at Affinity.  Now, I feel that I really have a lot to contribute.”

Jason is a “local,” having grown up in nearby Hermosa Beach California.  He still enjoys outdoor activities.  “I love to ride bikes and usually ride from Hermosa Pier along the ocean to Marina Del Rey, about 12 miles each way,” said Jason.  “I also like to body-surf the big waves at The Wedge, skim board by Hermosa Pier, skateboard and play soccer with the kids.

Jason and his wife Marisa live in Paramount, California.  They have 4 children Anthony, Emily, Michelle and Brian.  Jason says, “They are a blessing and a joy to be part of our life.”

Asked what he and his family like to do for vacations, Jason replied, “My family and I like to take long road trips to different places.  Last year we visited Sequoia National Park and Morro Bay, both in California.

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