Recent advancements in silicone manufacturing, such as color and scent dispersions, antimicrobial additives, and multishot molding, were presented at the recent Liquid Silicone Injection Molding Symposium hosted by Shin-Etsu Silicones of America and Arburg, Inc., at Arburg's American Technology Center in Irvine, Calif. Six keynote presentations highlighted the significance of today's silicone technologies for medical-device manufacturers.

Why silicone? That question was posed by Eric Bishop of Shin-Etsu Silicones of America, Inc., Akron, Ohio (shinetsusilicones.com), who went on to list silicone's useful physical properties that include:

• Broad thermal stability, ranging from -65 to 400F. Additives can bump it to 500F

• Electrically and thermally nonconductive

• Naturally elastomeric

• Wide hardness range, 5 to 80 Shore-A, lets manufacturers tailor the feel of the material. This ability to tailor the material's feel is effective for applications such as respiratory masks for treating sleep apnea. Also, silicone seals well against the face without discomfort.

Bishop also discussed liquid injection molding with automated equipment that minimizes the risk of contamination. He noted that multicomponent parts can be molded in a single two-shot injection press, which improves consistency and increases productivity. The flash-free parts eliminate need for any post operations.

Molding the right color and even the right smell in silicones was discussed by Bob Bradley, co-owner of Gayson Silicone Dispersion, Inc., Barberton, Ohio (gayson.com).

Pigments are not typically designed for silicone applications because they require heat and pressure that affect color and curing. But the medical-device market needs color materials. For example, oxygen tubes for babies are color coded for quick recognition in OR and ICU applications.

Gayson creates additives to modify silicones for applications that require liquid silicone rubber (LSR). Bradley said his company has created more than 10,000 custom color variations and does its USP Class IV Biological Testing in accordance with ISO10993.

Bradley hinted at other possibilities for dispersions in medical devices such as a peppermint scent for oxygen masks.

An antimicrobial additive for liquid silicones was the subject of a presentation by Business Development Manager Aric Axness, Milliken, Spartanburg, SC (milliken.com). Milliken's AlphaSan CW I2 is an inorganic, soluble powder that uses silver ions as its active antimicrobial ingredient. Properties include high thermal stability, low moisture content, and a small particle size.

The material is available in a concentrate version that can be added as a third stream LSR additive. When used with the Shin-Etsu medical grade LIMS, up to 10% of CW 12 can be dispersed into silicone without affecting material properties needed for medical applications.

Material properties for LSRs used in multishot molding systems was discussed by Juergen Giesow, a regional sales manager for Germany-based Arburg (arburg.com). “The end goal in any multicomponent system is to improve the final product's properties and reduce its overall cost with engineering-grade materials,” he said.

Giesow also stressed that a spring-loaded check ring was vital for consistent shot size in a repeatable process.

“Project planning is of great importance when it comes to maximizing productivity and cost-effectiveness in any silicone-based molding,” said Kipe Molds Owner George Kipe during his presentation. “The key to a successful project is building the mold around the part and its specs.” Kipe Molds, (kipemolds.com) is based in Placentia, Calif.

Kipe stressed the importance of a final analysis of material properties before the material goes into the mold. Those properties include compressibility, shear capabilities, pot life, expansion during curing, and shrinkage.

“The main objective of meter-mixing dispensing is to provide a consistent ratio of material to the injection-molding machine feed throat at a repeatable pressure,” said Bob Pelletier, vice president of sales for Fluid Automation, Wixom, Mich. (fluidautomation.com). Silicone molds differ from molds for plastics. Therefore, the key is to understand the geometry of the mold and its application, he said.

Typical injection molding machines use 5 to 55 meter-mixing models capable of using 5 or 55-gal. containers of LSR and third and fourth stream additives for medical and industrial models. Medical applications combine wetted components with stainless steel and Teflon-lined hoses to meet medical clean room requirements, he added.

The material comes in a 1:1 ratio for fixed-ratio units, Pelletier said. Variable-ratio machines have self-adjusting mechanisms that can simplify changeovers, from 1:1 to 5:1 drums. This lets users make adjustments for specific gravity or durometer in the finished product.