John Freedman, business unit director, biomaterials, and Alex Santayana, application engineer, both from NuSil – part of Avantor, explains important factors to consider when using High Consistency Rubber (HCR) silicone in medical device extrusion processes.
HCR silicone is a versatile material with a long history of use in medical devices and other industries. Used to fabricate various articles, such as tubing, balloons, sheeting and moulded parts, HCRs consist of a high-molecular-weight polymer combined with silica to produce a silicone that has a clay-like consistency in its uncured form. Medical device manufacturers often choose HCR because of its demonstrated biocompatibility, various processing techniques and excellent physical properties.
When compared to other silicones, one of the most unique properties of HCRs is ‘green strength.’ Uncured HCRs can be shaped into a given dimension or form and will hold that shape until cured. In addition, due to their relatively low overall cost, HCRs provide an excellent material for low-volume devices or parts production, using different fabrication methods, such as calendaring, compression or transfer moulding and extrusion.
HCRs in extrusion processes: Key considerations
Tubing for catheters and other medical devices are the most common products produced via HCR extrusion, although extrusion can also be used to produce forms, such as ribbon and rod. HCR silicone parts produced through extrusion can have wall thicknesses ranging from less than 0.01 inches up to three inches. Extruded HCR offers a wide range of elastomeric performance, with most having a durometer in the range of 30 to 80 type A.
There are several factors to consider when extruding medical device components using HCRs. The first has to do with curing the HCR, the process by which the gum-like, rubbery material is formed and hardened into a final shape. Obviously, the material needs to remain soft enough to be extruded through a die, so curing occurs after extrusion.
Most HCRs are supplied using either peroxide-catalysed or platinum-catalysed curing systems. They can be supplied either catalysed, where the catalyst is included, or uncatalysed, where the catalyst is sold separately and requires combining prior to extrusion.
A key advantage of peroxide-catalysed systems is that their curing mechanism is not initiated until the HCR is exposed to heat. This translates into a very long work time which is beneficial for moulding or extrusion. It should be noted that peroxide-catalysed systems often require a post-curing process to remove residual byproducts.
Platinum-catalysed systems typically consist of two components: One contains the platinum catalyst, and the other contains hydride functional crosslinkers and cure inhibitors. When combined, the HCR retains its pre-cure consistency for one to two hours and no undesirable byproducts are produced in the curing process. A key advantage of platinum-catalysed HCRs is the ability to heat-accelerate the cure for increased throughput.
Before extrusion, HCRs need to be processed with a two-roll mill to soften the material and to blend two-part systems. This also modifies its consistency so it can flow more easily through the die. This processing step, as well as extrusion itself, generates heat; if too much heat is generated, the HCR can begin to cure, so it is common for this equipment and processing area to be cooled to prevent curing prior to or during extrusion.
Once the HCR has been milled, it is fed into the extruder. Extrusion is a steady-state process designed to continually produce a high volume of finished parts with the exact same dimensions and properties. Many extrusion systems use a screw mechanism to force the HCR through the die since this provides a highly controllable feed system. This is important to ensure uniform production of the finished extrusion and to control factors like wall thickness.
After extrusion, the HCR parts need to be vulcanised or heat cured. A variety of heating systems, including hot air ovens and infrared heating can be used. Processors can adjust temperatures to control the speed of curing on platinum-catalysed HCRs if needed to manage production cycles and throughput.
Working with HCR suppliers
Since no two medical devices or extrusion processes are exactly the same, silicone providers will have solutions that provide greater flexibility for the manufacturing process. There are also advantages to working with a silicone supplier that has experience with providing HCRs for medical devices. Suppliers with robust-quality systems and ISO 9001 certification, deep knowledge of ISO 13485 quality requirements for medical systems and experience with the U.S. Food and Drug Administration (FDA) MAster File submissions (MAFs) can help ensure that the medical devices you produce using HCRs can be more easily and readily supplied to the marketplace.