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Silicone Rubber Material Benefits and Fabrication Advantages

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Silicone Rubber Material Benefits and Fabrication Advantages Silicone Rubber Material Benefits and Fabrication Advantages A Lubrizol Company The Facts About Silicone Rubber Selecting a high-quality elastomer for critical today, it is often difficult to recognize the optimum applications, particularly medical devices, can be solution. Further, the implications of selecting an a challenge. Designers, engineers and managers inferior material may cause a project to fail. must carefully evaluate a wide array of material properties and processing possibilities in order To make the most informed decision, it is helpful to meet demanding performance specifications to gather as much data as you can about each and budget requirements, so the performance material. This white paper is intended to provide criteria must be clearly defined. For example, you with factual information on silicone rubber durometer, elongation, modulus and tear must be elastomers, their properties, fabrication methods determined to specify the correct material. With so and advantages. many materials and fabrication methods available Silicones: Their History and Definition CH3 Since the 1960s, silicone rubber has found widespread use in medical, aerospace, electrical, construction and industrial applications. Flexibility over wide temperature Si O ranges, good resistance to compression set, a wide range of durometer selections, and inert and stable compounds are some of the reasons for its use. O CH3 CH3 Si CH3 Silicone rubbers are synthetic polymers with an unusual molecular structure — CH Si CH CH O 3 3 3 a giant backbone of alternating silicon and oxygen atoms. This linkage is similar to the O Si linkage found in quartz, thus silicones have superior heat resistance compared to other elastomers. There are two popular catalyst systems used to cross-link silicon polymers: CH3 the platinum (addition cure) systems and peroxide (free radical). Alternating siliconcyclic and silo oxygenxane atoms contribute to the superior heat resistance offered by silicone rubbers. Properties of Silicone Rubber Gumstocks R R The strong silicon-oxygen chemical structure extremes far higher or lower than those in are routinely extruded, as Si they have excellent green O Oof silicone gives the elastomer its unique which conventional insulating materials are R R strength and will retain their performance properties. Examples include: able to perform. shape uncured prior to Si Si vulcanization. R O TemperatureR Resistance Biocompatibility Silicones withstand a wider range of temperature In extensive tests, silicone rubbers have exhibited cyclic silcone extremes than nearly all other elastomers, superior compatibility with human tissue remaining stable through a range of -75°F to and body fluids, and an extremely low tissue 500°F. They may be sterilized by ethylene oxide response when implanted, compared to other Liquid Silicone Rubber (ETO), gamma, e-beam, steam autoclaving and elastomers. They do not support bacteria growth (LSR) various other methods. and will not stain or corrode other materials. flow is readily achieved Silicones are odorless, tasteless and are often with little shear stress. Mechanical Properties formulated to comply with biocompatibility This flow characteristic Silicone rubbers have high tear and tensile guidelines for medical products. makes the liquid silicones strength, good elongation, great flexibility and a ideal for molding. They are durometer range of 5 to 80 Shore A. The softest Chemical Resistance normally run on semi- or fully durometers available are reinforced gels. Silicones resist water, oxidation and many automated liquid injection molding machines. chemicals, including some acids and alkali Electrical Properties solutions. Concentrated acids, solvents, oils and Silicones exceed all comparable materials in fuels have a negative effect on silicone rubber their insulating properties as well as flexibility in and should not be used with silicone. electrical applications. They are nonconductive and maintain dielectric strength in temperature Overall Benefits of Silicone Rubber Silicone rubbers are unmatched by other elastomers in many important specification categories. Outstanding benefits of silicone include: • Unsurpassed biocompatibility • Ability to be pigmented and radiopacity • Ability to be sterilized by many methods • Ability to retain softness indefinitely • Low compression set • Long shelf life • High tear strength • Processing versatility • High elongation • UV, moisture and steam resistance • Natural translucence or high clarity • Ease of cleaning Fabricating Silicone Rubber Molding LIM Steps: Silicone elastomers are typically molded by three Liquid injection molding, due to its closed process, main methods: liquid injection molding (LIM), minimizes contamination. Additionally, because 1. Meter Mixing transfer molding and compression molding. The it employs a single automated step, it provides injection molding process, an excellent choice consistent part quality with less chance for A B for high-volume applications, employs lower material mix variation. A B pressures and higher temperatures than the other molding methods — 250 to 2,000 psi injection The principal advantages of LIM include: pressure and temperatures of 245°F to 485°F. By contrast, transfer and compression molding • Cleanliness AB Mixer operate at pressures of 2,000 to 8,000 psi and • No material preparation labor mix AB Mixer temperatures of 200°F to 370°F. In designing for • Lower injection pressure mix HEA the molding process, designers should take into • Faster cycle rates T HEA AB T HEA account the material shrinkage rate, which can • Availability of fully automated systems T mix HEA AB range from 2% to 4%, depending Aon the material.B T mix Transfer and Compression Molding Using HEA T During molding, the three variables that must be High-Consistency Rubber (HCR) Silicones HEA T 2. Mold Forming and controlled are temperature, pressure and time. Transfer and compression molding are widely Vulcanization The temperature must be optimized to ensure accepted and are in use today. Unlike the LIM AB sufficient crosslinking in a minimum cureMixer time process, transfer and compression molding mix but low enough to prevent scorching the require separate pre-mixing of the HCR silicone HEA T elastomer. The pressure must allow complete rubber on a two-roll mill. The material is then cut HEA AB T filling, maintain dimensional stability inmix the part, into predetermined shot size and fed into the tool prevent voids and flash. Timing of all functions via transfer or a stuffer box. HEA is critical for the production of consistently T acceptable, fully cured parts. These processes must be operated at lower temperatures, requiring longer operating cycles. Liquid Injection Molding (LIM) This is why it’s not uncommon to see large molds 3. Final Part Liquid injection molding has many benefits in the with 100 cavities or more on diaphragm, bottle fabrication of silicone rubber, including cleanliness closure, O-ring seal and many other applications. and speed. In the LIM process, pumping systems deliver the two-part liquid silicone When transfer molding, a hydraulic ram displaces (catalyst and crosslinker) directly into a mixer for HCR silicone through the sprues and gates into homogenization, then directly into the mold cavity the cavities. Compression molding differs. The in a completely closed process. Molding and HCR silicone is manually placed into the cavities, vulcanization (curing) occur rapidly within the mold and it is compressed in the mold to complete the cavity at a range of temperatures. fill during the closing action of the press. Overall, injection can take as little as .05 to 10 seconds, while molding and vulcanization take 10 to 90 seconds or more, depending upon shot weight and cross-section thickness. A B AB mix Extrusion A B HEAT Extrusion Steps: The HCR silicone extrusion process yields tubing in aAT variety of properties and HE a broad range of tubing and profiles. The diameters. Other extrusions include: 1. Mill Blending extrusion process begins with the two-part AB HCR silicone (catalyst and crosslinker) being • Profiles or non-round cross-sections, for mix blended on a two-roll mill. The blending such applications as instrument stands, A B yields a homogeneous compound that is clips, gaskets, seals, ties and markers formed into strips and fed continuously AB into the extruder. A feed screw maintains • X-ray opacity in stripe or opaque form to HEAT mix AT proper pressure at the pin and die. Once HE provide the doctor with a visual aid AB extruded, the tubing passes through hot-air mix vulcanization ovens (HAVs), where radiant • Reinforced tubing to provide added heat cures the product. During the extrusion strength, electrical conductivity, kink process, laser micrometer checks are resistance or stretch resistance 2. Extrusion Feed performed to continuously ensure proper HEAT AT HE AB dimensional control. Examples of extruded silicone products mix include catheters, drain and fluid path tubes, The extrusion process is able to produce gasketing, ribbon, sheathing, balloon cuffs single-lumen, multilumen and coextruded and coextruded electrical conductors, with fluid path lumens. AB mix Assembly 3. Pin and Die Profile Forming There is almost no limit to the configurations Other methods include tipping, reinforcing, TUBE CROSS in which two or more silicone rubber dipping and cuffing. SECTION components may be joined to create assemblies for special functions. Silicone rubber assemblies can include A B airways, shunts, occluders,
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