Medical silicone acts as a protective cover against dust, water, impact, heat and electrical shock, as well as a biofriendly implantable barrier, between device and patient.
Liquid silicone overmolding, also known as two shot molding is a process in which uncured liquid silicone rubber is applied to a base material and formed to shape while in direct contact with the substrate. The number of medical silicone applications today are rapidly increasing.
Liquid silicone overmolding operations take advantage of mechanical bonding, chemical bonding, or ideally both. A mechanical silicone bond takes place physically by creating undercuts and interlocks for the liquid silicone material to “grab” hold of the substrate with. Chemical silicone bonding is typically stronger and more reliable than mechanical silicone bonding, as it takes place on a molecular level, essentially welding the liquid silicone to the base material.
Selection of liquid silicone and substrate can be difficult, not every base material will bond well to liquid silicone on a chemical level and many possible substrate options can release silicone cure inhibitors. Testing should be done to determine the ideal pair of plastic and liquid silicone, typically this testing consists of lap shear tests, tensile strength tests and cyclic fatigue tests. It should also be noted that many plastic substrates have glass transition temperatures lower than that required of the liquid silicone to properly cure. If possible, it is best to select a substrate material with a glass transition temperature above 300°F.
The three most common substrates to bond medical silicone to are, silicone, metal and thermoplastics. The liquid silicone molding temperature in the range of 300°F (149°C) limits the selection of thermoplastics for overmolding liquid silicone onto. Typically liquid silicone overmolding onto silicone and metal substrates is not an issue. Albright has done development work on liquid silicone overmolding at low temperature for temperature sensitive substrates.
Typically, applications in medical silicone overmolding are gaskets, seals, fluidics, catheters and implants. The use of overmolding of medical silicone reduces assembly costs.