The first step in developing a viable manufacturing process for any silicone component involves considering the component itself.  Is the component a length of tubing, a valve, or a balloon?  Certain components can only be manufactured by a specific process while other components can be manufactured by several techniques.  Silicone tubing for example is always extruded while silicone valves are always molded.  Silicone balloons on the other hand can extruded, molded, dipped or sprayed. At times the process will be determined solely by the nature of the finished component while at other times several candidate materials along with several possible processes must be considered.

Most silicones are processed in one of three ways; extruding, dipping/spraying, or molding.

Only high consisitency rubber (HCR’s), can be used for extrusion.  In the unvulcanized state, this type of silicone has the clay-like consistency necessary for the extrusion process. 

Dispersions, low consistency elastomers, and fluids can be dipped or sprayed.  Only materials with sufficiently low viscosity can be used in these processes.

Liquid silicone rubber or high consistency rubber are both used for molding although the molding equipment and tooling differ considerably for the two types of silicones.

Some processes such as extrusion are material specific while other processes such as molding can accommodate different types of silicone.

At times the silicone article you’ve manufactured is the device, a finger joint for example, while at other times its one of several components that will be assembled as part of the device. The added time and expense of assembly steps, such as adhering two components together, can and should be avoided.  Overmolding for example, where a component is vulcanized while in contact with another component, eliminates subsequent tedious adhesion steps.   Design a process that minimizes downstream assembly.

Deciding which general process to employ in manufacturing a component is the easy part.  Regardless of the process chosen, additional decisions must be made regarding material handling, mixing, and curing.  For the common two component systems, how long should the material be mixed?  What type of mixers work best with a particular type of silicone? What pressure is needed to push HCR’s down the barrel of an extruder or to fill a mold?  What combination of time and temperature is needed to properly vulcanize a molded part?  Is a post cure necessary?

But there’s more.  Physical properties of any particular silicone material vary lot to lot.  Also, properties such as viscosity and the cure rate of any given lot of silicone will drift as the material ages.  All processes used in the handling and curing of silicones must specify parameters for variables such as pressure, temperature, and time.  Because of the inherent variability of silicones, processes should be developed and validated with this variability in mind.  Processing parameters should be sufficiently narrow to consistently produce conforming components while at the same time broad enough to accommodate material variability. 

Most medical devices and certainly all implantable medical devices are supplied sterilized.  Medical devices, including those with silicone features, can be sterilized by several different methods including gamma radiation, dry heat, steam autoclave, and ethylene oxide.  Each method offers advantages and disadvantages.  It’s important to keep in mind that sterilization can affect the properties of silicone components.  Gamma radiation for example can cause rearrangement of the siloxane polymer while dry heat sterilization can increase polymer-filler interaction. In both cases the bulk modulus of the material increases.  In most cases the change is sufficiently small and the ultimate performance of the component is within the design specification.  There are cases however where even a small change in the physical property of a critical component can cause the device to fail.  Consider a valve that is designed to open at a certain pressure.  Here even a slight increase in the bulk modulus of the material can cause the valve to open only at higher pressures and outside the design specification.  The choice of sterilization method should not be an afterthought that is “bolted on” at the end of the process.  It is part of the process.  Indeed the choice of sterilization method and its potential effect on silicone components should be considered when first selecting a material.