Medical gets a new material: Reviewing optical LSR

Here is a sentence you don’t see everyday: A new material has hit the streets and it could change the rules of medical manufacturing. Optical liquid silicone rubber (LSR) is a transparent, flexible thermoset material that has been replacing PC, acrylic and occasionally glass in many optical applications. The characteristics of optical LSR may also have some benefit to the medical industry.

To explore these options, Proto Labs Inc. provided Medical Design and Outsourcing with some insight into the attributes that make this clear LSR appropriate to replace PC, acrylic and glass for optical applications:

• Optical LSR is almost as transparent as the best glass across both visible and UV spectra.
• It does not discolor or lose transparency with age or with exposure to heat and UV like PC and acrylic
• It is significantly lighter than glass.
• LSR can be made flexible or firm, with a reduced chance of breakage. When the material has a durometer (hardness) of 40 to 50, it is floppy (think silicone bakeware). However, optical LSR has a durometer of 70, but can still withstand vibrations better than plastic or glass.
• The material is scratch and crack resistant, which helps preserve both its physical integrity and optical properties. You could cut a part made of optical LSR with a razor, but if you were to drag it across the ground or other rough surface it would most likely conform to the surface on which it was dragged and not be marred.
• LSR is optically and mechanically stable at temperatures up to 302°F (150°C), which means it can be used near a light source.
• Flexibility of the material also allows designers and engineers the opportunity to combine a lens with a seal to reduce assembly cost, inventory control and seams in the assembly.
• In its liquid state, optical LSR can be mixed with colorants or phosphors to extend its lighting capabilities (not available at Proto Labs, yet).
• LSR molding is an optimized process that allows for prototype and low-volume production runs.

In addition to its performance characteristics, optical LSR has several traits that simplify design and molding:

• Its low viscosity allows easy flow within a mold, so the material can move readily through thin areas and fill small voids. This gives the designer greater freedom when creating fine features for function or cosmetics. It allows molding of thinner walls than could normally be produced in other resins. Note: Low viscosity also increases the likelihood of flash where mold halves meet. Chances of flash can be reduced by designing parts that can have clean, flat parting lines.
• The material cools without creating significant sink or internal stresses, and its dimensional stability allows accurate production of lenses. Reduced likelihood of sink also permits designs with thicker walls than would be acceptable in other resins such as PC or acrylic.
• The material can be molded within a polished mold without the use of secondary polishing processes required to polish individual parts, saving time and money in production. Some tooling can support optical surface geometry and finishes.
• Due to the material’s somewhat rubbery consistency, shapes with small undercuts and negative draft that would be unacceptable in a more rigid material may be able to be safely unmolded. These can be treated as bump-offs or pickouts.

In the medical field, designers are only just becoming familiar with optical LSR’s molding attributes, as well as the material characteristics that could enhance medical devices. Products such as wearables, surgical guides, and optical components of minimally invasive tools could all benefit from a look at this exciting material technology development. And that is probably just the beginning.