The ability of optical parts made of clear resins like polycarbonate to perform optically as light covers, lenses, and lightpipes depends on a combination of material, shape, and internal clarity. Not all rapid prototyping processes are suitable for working with this material in this type of application. For the Protomold process, it’s important to understand the capabilities and limitations.

Protomold’s rapid injection molding process produces very good injection molded parts quickly and cost-effectively. It is suited to most prototyping applications and many production applications as well. In some optical applications, however, performance can be impaired by small flaws that would not ordinarily be a problem. Small imperfections like internal bubbles, minor surface irregularities, flow lines in thin sections, suspended particles, gate vestiges, and slight knit lines would have negligible impact on the function and even the cosmetics of most parts. But since they can affect the passage of light through clear resins, they can seriously impact the function of optical components.

Some of these imperfections may be preventable through careful design, but others may be unavoidable. For example, a widely used clear resin (like a polycarbonate) shrinks significantly as it cools. This increases the likelihood of internal bubbles and can disrupt the passage of light through a part. Similarly, while uniformity in part thickness helps moldability, lenses often require variations in thickness and these can create flow problems or lead to sink as the part cools. And because Protomold does not offer precision grinding on mold finishes, part surfaces may be less than optically perfect.

While the Protomold process includes several steps designed to identify and head off problems, they are not designed to identify optical issues. In fact, Protomold has no way of knowing that the part being analyzed will have an optical function, so parts with optical flaws may be accepted as perfectly moldable. ProtoFlow®, a Protomold analytical tool, considers choice of resin in detecting potential mold-fill problems; however, like ProtoQuote, it cannot anticipate potential optical flaws.

High quality optical production requires specialized facilities and handling. At Proto Labs, parts are produced in an open manufacturing area where there is potential for minor particle contamination that could impact optical performance. Also, the company does not offer special packaging to protect optical-quality surfaces in transit. Nonetheless, it successfully produces optical parts for clients who understand the limits of the process.

Despite its limitations, rapid injection molding can be useful in prototyping optical components even if the prototypes may not have all the optical properties of production parts. Optical properties are generally fairly predictable and may not need prototyping, but those parts may still need to be tested for fit and structural integrity. And, for those who can work within its limitations, rapid injection molding may be suitable for producing some optical parts, and at a cost that can be as much as 90% less than traditional tooling.

Proto Labs, Protomold
www.protomold.com