There are some key, fundamental considerations when manufacturing optical components: designing molded optical parts, designing and building the molds to produce them, injection molding, and handling components that manage light. Some are obvious, others less so. But one thing is certain. It is not for the faint of heart, and is, thus, best left to the experts. Complexities, including geometry, surface specifications, and material selection, are vitally important to avoid potential pitfalls along the way to designing, and ultimately producing, a flawless optical part.

Fig. 1 – Assay test trays and vials used in pharma and pathology photo spectroscopy analysis.
Optical part design is complicated—not only the outside dimensions and surface finish, but the internal dimensions, structure, and type of clear material selected. Those factors will ultimately direct and manipulate light for the part’s intended purpose. And, unlike other injectionmolded parts, light energy is moving around inside and outside the part. Most people are familiar with concave and convex lenses, (like a magnifying glass) usually round, disc-shaped looking objects. Often these lenses are internal to microscopes and telescopes. But in reality, “plastic” optical parts, including testing trays, vials, light pipes, and guides, are infinitely more complex and can be manufactured in a way that ground, polished glass or crystal could never be.

Polymer optical components can be configured into complex irregular shapes that each perform a specific function. They allow light (usually LED or laser source, but even fluorescent or incandescent) to move from one location to another, or even multiple points. Examples of these parts are light pipes behind dash displays, radio, AC/heater controls found in cars, to medical test vials and trays.

Like the biology teacher you had in high school, the slides and petri dishes you most likely used in class have largely been replaced now. In their place are 3" × 5" 96-well assay test trays, typically used in pathology labs and by pharmaceutical companies. (See Figure 1)

Manufacturing these parts out of glass, with internal prism features that have surface finishes to angstroms (millionths of an inch), would be next to impossible.

Other optical components direct, amplify, reflect, focus, diffuse, or otherwise “manage” light.

Manufacturing parts with Fresnel, micro prism structures like those found on headlights and taillights in cars for example, to pillow lens optical features— and most everything in between—can be individually problematic, each with its own challenges, but it can be done. However, it is necessary to design the part, design the mold, and mold the part to highly disciplined levels.

Component Design

Of primary consideration is: how will the part be used? Secondly, what are the outside and inside space constraints? Next, in what physical environment will the part be used? Will it be exposed to extreme temperatures, chemicals, moisture? Will it need to withstand vibration or impact? How will the part be used?

Fig. 2 – A light pipe, prism. and a test vial.
Usually, light will need a source (in the solar arena, it’s the sun, naturally) and an exit point or points. Hard, sharp edges, corners, and rough or textured surfaces, become exit, or light-loss, spots. Like electricity, light energy has a travel distance somewhat dependent upon the amount of energy, or in this case lumens or wattage going into it (vs. volts or amps).

Obviously, the further from the source, the brighter the light source will need to be. However, there are exceptions and contradictions. For example, if a magnifying lens feature were somehow incorporated into the part design, it might be possible to reduce the amount of light input, because it would now be amplifying light. Most optical design software programs take many of these factors into account.

It is critical to understand that surface finish, or “polish” per se, is not as important as the absolute dimension. This is particularly true with micro Fresnel structures. Light inside the part is not only going to exit at predesigned points in the geometry, but it will also reflect back into the cross-section of the part, and create a totally new light source that needs to be considered. (See Figure 2)

In some cases, this light may need to be dammed, or blackened out, to contain or manage it. So, while from one perspective, light design management may seem simple, it can be very complex, because it is necessary to shield the stray, reflective internal light, while also preventing external, extraneous light from randomly entering into the equation. Such is the case when the sun coming in from behind can wash out digital displays.

Designing the Mold

A key concept to remember is that an optical part will never be better than the mold that created it. At best, it will replicate and be equal to it, in a mirror image. For this reason, the type of steel will determine the optical finish. Most popular is pre hard stainless steel—like STAVAX, for example. There may be instances when the steel needs to be polished. This requires the part to be plated with copper, then polished, then flash chrome over that, to achieve a smooth surface quality and prevent the grain of the steel from showing.

The cavity/core designs are critical, but so too are the gate and runner type and size for proper cavity fill. The gate size, style, and location can make or break prospects of success. As a rule of thumb, optical parts demand BIG gates and BIG vents. The material needs to enter the cavity with as little turbulence as possible, otherwise it could show up as stress or flow lines, all of which can destroy light management. Not only will this affect performance but, with polarized sunglasses, you can see the stress of the structure from the mold, which will appear almost like oil on the surface.

It’s possible that hot water, hot oil, or electric heaters in the mold plates will be necessary. Sometimes both heat and cooling lines are needed in both the A and B plates, in what is often referred to as the hot/cold cycle method.

Next, ejection is an important factor, and it will probably not work to put ejector pins on any optical surfaces without compromising the part. Thus, ejector blades, or a stripper bar or ring on the outermost edges may be necessary. In some cases, ejector pads outside the part area are necessary and will need to be trimmed off, post-molding. Incidentally the gates may need to be so large and thick, like a fan gate, that they require a post-secondary operation to be removed. All of this requires an experienced mold maker, who has expertise with optical molds. Remember that every clear resin type has its own processing requirements, corresponding to certain types of geometries.

Spending money on a quality mold will prevent headaches, and ultimately reduce costs. Opt for quality over price, remembering that light is infinitesimally small, so imperfections will show immediately and cuttings corners on costs may end up being more expensive in the long run.

Molding the Part

Fig. 3 – Although parts look different in overall outside geometry, there is little difference in optical structures between some automotive parts—such as this automotive instrument gauge cluster, which has back lights, diffusors behind graphics, pointer light pipes and guides—and some medical devices, including surgical lamps, ophthalmic lighting, and patient monitoring devices.
Material selection is a big part of the part design and mold design process. Acrylic, polycarbonate, styrene, PVC, and TOPAS all have performance/cost tradeoffs. No single resin is better than another. Each may be appropriate for particular reasons. Polycarbonate is not the best for light transmission, or UV stability since it tends to yellow, but it is the strongest and so it is often used for optical parts where this is not an issue, such as automotive parts. And, while automotive and medical technology manufacturing may seem like opposite ends of the optical parts spectrum, oftentimes the lenses are very similar from a manufacturing standpoint. (See Figure 3)

In fact, automotive lenses are quite akin to surgical, ophthalmic, lighting lenses and reflector can structures—they have the same prismatic/Fresnel micro features and polish requirements, for example, and many mold makers make both types.

Other materials, like acrylic, are not only optimal for light transmission providing between three to five percent more light, but also for bio-compatibility, providing blood fluid protein and uric acid resistance. Regardless of the resin chosen, it will probably need to be processed hotter than manufacturer’s specifications suggest in order to increase the melt flow, and improve surface replication and part finish, from the mold.

If the resin has too long a dwell time in the machine barrel, in the mold, or in the cycle, the material will begin to degrade. Degraded material specks, black or otherwise, are almost always a cardinal sin in optics. The barrel and screw must be clean. The most effective way to do this is to dedicate equipment to clear optical parts. Additionally, the cycle time will need to be increased dramatically—from seconds in the world of conventional plastic parts, to minutes. To prevent sink, the process requires that the part be injected, and held with pressure forward. Other, more demanding optical parts require a special modified molding machine, otherwise known as compression molding. Additionally, to achieve maximum clarity, surface finish and density, many optical parts weigh more than they should for their standard, scientific volume of material. Only an experienced, optical-molding process engineer will understand and be able to execute this specialized technique, without a steep and costly learning curve.

Once the part has been manufactured, it needs to be protected until it is incorporated into an assembly. Packaging is needed to cradle the part to the assembly floor, and generally layer packing is the order of the day. Foam sheets, coated paper, or tissue can be used, but it is important to use caution, as some paper types are abrasive and can destroy a highly-polished surface finish. The part may need a liner or may need to be individually poly bagged, then layer packed. Many optical parts need to be molded in a clean room environment as they may get coated with bright aluminum, or masked and painted.

It is important to note that the housing, or cans, that sometimes go with clear components may often be as important as the lenses themselves, since they are all critical parts of the light management system. The dimensional tolerances can be extreme and for good reasons.

Conclusion

Like most things that are often easier said than done, the manufacturing of optical parts may appear to be fairly simple and straightforward, but there are a multitude of pit falls along the way that can ruin any project. To minimize risk, find an experienced, high quality tool maker, with high-speed milling equipment (e.g. 20,000 rpm + spindle speeds) who understands and uses diamond cutters, knows polishing methods, has built several optical molds, and actively works with an experienced optical molding house. Because of added cycle times, light box QA, and other considerations, optical parts will typically be more expensive than standard engineering parts, but, with an experienced manufacturer and some practical guidelines, a potentially costly learning curve can be avoided.

This article was written by Marc Jaker, Sales and Marketing Manager, D&M Plastics, LLC, Burlington, IL. For more information, Click Here .


Medical Design Briefs Magazine

This article first appeared in the May, 2016 issue of Medical Design Briefs Magazine.

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