July 2011 Design Tip

Shining Some Light on Lenses

Last month we talked about visual characteristics of parts from a cosmetic, as opposed to functional, standpoint. This month we address some visual characteristics that actually are the function. We’re referring to the optical properties of parts that are made of clear resins like polycarbonate and used as light covers, lenses, and light pipes. The ability of these parts to perform optically depends on a combination of material, shape, and internal clarity. To determine whether they can be made using the Protomold process, you’ll need to understand our capabilities and limitations.

Our rapid injection moulding process is specifically designed to produce very good injection moulded parts, quickly and cost-effectively. It is uniquely 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 functionality 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 mouldability, 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 mould 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 analysed will have an optical function, so while the design analysis included in ProtoQuote® can identify moulding problems, it may accept parts with optical flaws as perfectly mouldable. ProtoFlow®, another Protomold analytical tool, considers choice of resin in detecting potential mould-fill problems; however, like ProtoQuote, it cannot anticipate potential optical flaws.

High quality optical production requires specialised 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, we do not offer special packaging to protect optical-quality surfaces in transit. Nonetheless, we do successfully produce optical parts for clients who understand the limits of our process.

Despite its limitations, rapid injection moulding 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 moulding may be suitable for producing some optical parts (and cost as much as 90 percent less than traditional tooling).

This part was moulded in polycarbonate with F1 finish on one mould half, F0 finish on the other and texture around the rim. (F1 has most cutter marks polished out.) The mould was designed with an unbalanced edge gate, and 3,400 pieces were produced with no significant issues.

This part was moulded in polycarbonate with A2 finish on both mould halves. (This is the highest polish but done by hand at Protomold.) The part has uniform, nominal thickness of 2.7mm and was moulded in a run of 750 pieces with no issues.

If you have questions about the suitability of the Protomold process for your optical parts, contact Protomold Customer Service Engineers at +44 (0) 1952 683047.

Read past Design Tips

List of available standard finishes