Düsseldorf, Germany — Cleaning liquid silicone rubber molded parts can be difficult, but it is required for some applications. At the recent Silicone Elastomers World Summit in Düsseldorf, two speakers gave presentations on new cleaning methods.
Leopold Pühringer, product engineering manager at Starlim Sterner Spritzguss GmbH, talked about a "unique market-ready" SCD Starlim Carbon Dioxide cleaning process. Starlim is a Marcher, Austria-based silicone rubber molder with global operations that makes 14 billion parts per year.
The company uses SCD-Cleaning during production of silicone components that need to meet very specific requirements regarding the residual content of volatile components. It works by using the very low surface energy of liquid CO2 so that it acts like a solvent that extracts volatile and leachable nonpolar, low molecular weight polar, nonchemically cross-linked siloxane and silicone oils from molded silicone rubber parts.
The CO2 is used under 50 bar pressure at a temperature between 0° C (32° F) and 15° C (59° F) when it is a subcritical liquid between solid, liquid and gaseous phases.
Pühringer said SCD is a low-cost process in a dedicated "washing machine" that reduces volatile and leachable low molecular substances on LSR moldings down to an undetectable level of below 20 parts per million within 30 minutes. This would otherwise need four hours by post-curing at 200°C, and even then not get from 1,500-3,000 ppm down to below 200-400 ppm.
Pühringer said SCD has no effect on initial as-molded properties such as elongation at break, tensile strength, compression set and hardness. Density increases marginally by around 0.003 kilograms per liter and oil resistance is higher by a single-digit percentage.
Rainer Konrad, who has worked for 10 years in the Starlim research and development team and now assists customers from product development through to finished parts, stresses that SCD is not a complete replacement for post-curing, but it is considered a "parallel technology."
SCD can clean both single- and multicomponent moldings to eliminate or reduce stickiness without heat deformation of LSR secondary or more heat-sensitive thermoplastic primary components to which LSR is overmolded, which may occur with high-temperature post-curing.
Similarly, there is less risk of fine molded-in functional slits sealing up by post-polymerization "healing," which is something post-curing does not always entirely eliminate. Relatively large moldings and moldings with large wall thickness can be SCD cleaned in "acceptable time," Pühringer said.
Unlike high-temperature curing, there is no sterilization of moldings with SCD cleaning, due to its low operating temperature, other than of pressure-sensitive bacteria.
Cleaning of LSR primary optics lenses with complex geometries is effective where it may be impossible by post-curing, ensuring low volatile organic content (VOC) that can lead to fogging on secondary optics. Relatively high productivity mass post-curing by tumbling would not be acceptable anyway, as optical surfaces could adhere to each other and post-polymerize.
Pühringer showed a "RMJ Mixing Dome" primary optics LSR lens produced by Starlim for Aldrans, Austria-based lighting system specialist Bartenbach GmbH as a good candidate for SCD cleaning, as it needs to retain its high transparency despite being exposed to high heat, due to being in close proximity to LEDs.