An Introduction to Optically Clear Silicone Molding
Optically clear silicone materials open exciting new doors for optical product designers. The latest generation of these materials, provides designers with a material that can be used in almost any lighting application. As the precision optics industry increases, liquid silicone rubber sees its use increasing as product’s demand sophisticated lighting components to achieve desired asthetics and performance. Silicone optics can be thought of as an important enabling technology allowing the successful development of many types of devices. What distinguishes silicone optics from their glass alternatives is the ability to combine optical surfaces with mounting features on an economical basis. Albright specializes in turning your silicone optic design in to prototype and production parts. We will work with you through the design phase to ensure that your optically clear silicone part is designed for manufacturability. Once a final silicone prototype design is molded and accepted, then we can assist you with production molding of your optically clear silicone parts.
Silicone material suppliers, including Shin-Etsu, Wacker and Nusil have developed new high performance optically clear silicone materials, and have new medical grade optically clear silicones presently in development that are expected to be released shortly.
Why Use Optically Clear Liquid Silicone Rubber?
Why are many optically clear silicones used in place of glass or plastics? Reasons include, reflective index at a given wavelength, transmission at a given wavelength, operational temperature, UV resistance, and cost. Albright’s rapid prototyping process allows our customers to try different brands of materials, durometers and grades of clarity; all done during the prototyping process and before the production process, saving you time and money.
Important Optical Silicone Design Information
When creating your optical design it is very important that you know the refractive index of optical silicones when designing parts for index matching. Optical silicones are quoted with the value of refractive index at specified wavelengths. The refractive index measurement should be carried out at the operating wavelength to achieve precise index matching. Most optically clear silicones will experience refractive index reductions with respect to temperature at a rate of 1×10-4 to -5×10-4 per degree Celsius. The thermo-optic coefficient or dn/dt is the term for the rate of change of refractive index corresponding to a given temperature. When designing optical systems, the operating temperature must be known to maximize the refractive index at operating temperature.
Air bubbles can also be a problem when molding optically clear liquid silicones. We require that our customer specify what is acceptable in terms of size, location and quantity of air bubbles within their parts.
Optically Clear Liquid Silicone Benefits
Optically clear liquid silicones allow a greater degree of design freedom when creating new optical lighting designs. Tooling and NRE costs are lower due to the elastomeric nature of the material which can reduce the need for additional components and secondary operations. This grade of material can be overmolded onto optical parts to form seals and gaskets without giving up optical characteristics. Since the material is a thermoset it does not have molded in stresses like thermoplastics do. Liquid silicone rubber enables the ability to integrate mechanical and optical features together with ease. It also provides the ability to reproduce aspheric, freeform and other complex geometric optical surfaces cost-effectively. Colorant dispersions can be added to the liquid silicone rubber material to produce tinted parts. Thermal performance and optical stability are maintained when the optical silicone part is operating at high temperatures (up to 150 degrees C) for long periods of time. LED lighting applications are a great fit for optically clear silicones due to the long-term transmission retention and high-flowability that allows for a micro-structure design in a compact shape. In addition, silicone materials are lighter in weight than glass and offer higher impact resistance.
Applications for Optically Clear Liquid Silicone Rubber
Almost any application that requires an optical component from imagery, scanning, light detection or illumination, is a candidate for optically clear liquid silicone. Optical liquid silicone rubber can be found in bar-code scanners, document scanners, spectrometers and particle counters. Telecommunications products, microlens arrays, diffractive optical elements, LED lights are other applications where optically clear liquid silicone rubber is growing in use. Additional applications include:
- Solar Collectors
- Portable Electronics
- Light Pipes
- Optical Filters
- Beam Splitters
- All Types of Lenses
The Manufacturing Process
Once the optical part design is complete, silicone prototypes will be made. The first step for Albright will be to review the part design for manufacturability. Then a mold tool will be designed taking into account material shrinkage, part geometry, expected annual volume, and other requirements. Next, CNC code is created via our Mastercam software and then transferred to one of our CNC machines. Then multiple blocks of Aluminum are machined to create cavities that are essentially a negative of the original part. After machining, the cavities are polished to meet the customer’s requirements and finally the two mold plates are fitted.
Once the silicone molding tool is complete, engineering samples are molded. After the customer approves the engineering samples, production molding runs using that tool can commence. Typically customers will order any where from 50 to 1,000 silicone prototypes. Our prototype tools are typically good for 20,000 shots. After the prototyping phase and tests are completed, the production stage can commence.
The production molding stage of the manufacturing process involves creating a multi-cavity mold that can handle the annual usage of the silicone part being molded. Albright can provide compression or liquid injection molding tooling to mold your silicone optics based part. We have built compression and injection tools that range in cavity count from a single cavity up to 50 cavities.
We have 15 years experience molding optical silicone materials for lighting and optics based applications. All optical grade parts are molded in Albright’s ISO 7 compliant controlled environment room to minimize the chance of particulate contamination in your parts. We use non-contact visual inspection to check for contamination, air bubbles, scratches and flash. Optical parts are given great care when packaging, to lessen the risk of contamination or scratching. For your next optical silicone project contact Albright Technologies at 978.466.5870 or via email at email@example.com.
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