Specifying Injection-Molded Plastic Optics

William S. Beich

Plastic optics have benefits and drawbacks, and a set of material properties that requires entirely different specifications. Learn them before you start.

The use of plastic optics continues to grow in popularity as optical design engineers discover the many ways these components can manage light. Plastic optical elements and systems appear in a wide spectrum of industrial and medical applications, including surgical instruments such as laparoscopes, arthroscopes and cystoscopes. They also have found application in disposable medical devices such as blood analyzers.

Other examples of the creative use of plastic optics encompass imaging systems for displays, night-vision goggles and various kinds of head-mounted displays. Plastic optics are frequently found in PC peripherals, such as videoconferencing cameras and microscopes and in consumer devices such as compact disc and DVD players.

Plastic optics enable mechanical and optical features to be integrated into one element. This is an advantage to product designs that impose space and weight constraints. Also, in the case of spinning elements in bar-code scanners, plastic optics allow the distribution and balance of weight to be factored into the part itself. Courtesy of NCR.

They also appear in display projectors, laser bar-code scanning systems, biometric security systems, smoke detectors, automated toilet flush valve systems, spectrometers, particle measuring instruments and optical encoders. Plastic optics are useful for certain telecommunications products and are commonly used to replicate diffractive optical surfaces.

This wide array of applications stems from several key advantages that plastic optics have over competing glass solutions; namely, cost, weight and ability to integrate mechanical and optical features.

Tremendous economies of scale are possible through the use of multi-cavity molds. This is especially true for reproducing aspheric and other complex geometric surfaces, which are costly to produce in glass.

For a given volume, plastic optics weigh approximately 2.5 to 5 times less than glass optics and offer higher impact resistance. It is well-established that a polycarbonate window of sufficient thickness can withstand the force of impact of a .22-caliber bullet.

Plastic optics also afford designers the opportunity to integrate mechanical and optical features into one element. This is a key advantage if a particular product design, such as that of a handheld bar-code scanner, imposes space and weight constraints. By combining a reflective aspheric optical surface with integrated mounting features (molded at the same time as the mirror), designers can reduce the overall product weight and element count in the assembly.

However, with these advantages come challenges. No one material, whether it be glass, metal or plastic, can address all optical design problems. Polymers present several challenges to the designer, including thermal effects, stress birefringence and the limited range of available materials.

The thermal differential index of refraction coefficient for optical plastics is approximately an order of magnitude greater than that of glass. This means that high-performance lens systems requiring large-temperature-band operating conditions are better served by hybrid glass-plastic designs.

Most optical plastics also have a fairly low maximum sustained operating temperature, ranging from about 80 °C for styrene to 90 °C for acrylic to about 123 °C for polycarbonate and the cyclic olefin copolymer (Topas) and cyclo-olefin polymers (Zeonex/ Zeonor).

Birefringence is another potential issue when using polymers, which exhibit residual stress — a problem exacerbated by poor part design or poor gate location in the mold. It is extremely valuable to involve a competent optical molder as early as possible in the design phase to ensure that these issues are accounted for in the part and tool design.

Although optical glass catalogs offer a wide selection of materials to choose from in terms of index and dispersion, the range of optical-grade plastics is fairly limited. Overall, plastics have lower refractive indices than glass. The most commonly used optical molding resins and their indices of refraction.

Like glass, plastic optics can be coated using physical vapor deposition, except coatings are applied at much lower temperatures on plastic substrates. It is possible to specify reflective, antireflective, beamsplitter and conductive coatings for a wide variety of plastic substrates. Antireflective coatings can be multilayer, with an average surface reflectivity of less than 1 percent over a range of 450 to 650 nm, or single-layer MgF₂, with an average surface reflectivity of about 1.5 percent from 450 to 650 nm.

Aluminum, silver and gold metals are used as reflective coatings and can be used to create first- or second-surface mirrors. A standard aluminum coating will provide a surface reflectance greater than 88 percent from 450 to 650 nm, and gold coating, greater than 95 percent from about 700 to 1000 nm. A protective overcoat can be applied to metal coatings to improve scratch resistance.

The injection-molding process

Injection molding is a very cost-effective way to reproduce spherical and complex aspheric plastic optics. The effort to produce the optical form is confined to the mold insert. Inserts are fabricated in hardened steel, polished or plated with nickel and then single-point diamond-turned to the negative shape of the final component surface. During the molding process, replicating the surface of the insert creates the optical surfaces of the finished element.

Afterward, all thermoplastics exhibit a shrinkage factor when they cool in the mold. Different for each material, this factor ranges from 0.1 to 0.7 percent. Part design, tool geometry and process considerations affect these shrinkage factors and may require that the toolmaker adjust the mold after completing initial trials. A skilled optical molder will provide assistance in evaluating these issues.

Injection molding produces one or more optics per molding cycle through the use of single- or multicavity molds installed in the press. Economies of scale are possible by increasing the number of cavities in the mold.

Formed in multicavity molds, plastic optics allow significant economies of scale by enabling not only reproduction of aspheric and other complex geometric surfaces, but also fully integrated assemblies, such as this viewing lens for a center punch. Courtesy of Lee Valley Tools.

The injection-molding press consists of a fixed and a moving platen, a clamping unit and an injection unit. Plastic pellets fed into the injection unit are plasticized into a molten state and injected into a mold mounted between the fixed and moving platens, which the clamp mechanism holds together. As the material cools and solidifies in the mold, the optic takes on the shape of the insert and cavity detail. After cooling, the mold opens and ejects the optic runner system from which the optics are removed (degating).

Injection-molding techniques can reproduce optics with a high degree of repeatability. Much of this is due to the precision of the molding press as well as to the precision built into the mold itself. A mold’s construction typically exhibits a tighter set of tolerances than those required of the components it produces. An experienced optical molder, therefore, should dictate how the mold is to be constructed.

If there is uncertainty about how a part will process, the mold can be built steel-safe. That is, it will be built to smaller dimensions than the nominal final dimensions of the part. This will allow the mold maker to make very fine adjustments once initial molding trials have been performed. Part geometry, part size, the choice of material, the overall mold design, the gate scheme and a host of process issues all play critical roles in the quality of the final product.

Rules for specifying

Because of the number of variables inherent in the process, it is difficult to speak in general terms about what tolerances are possible. Each job should be approached on a case-by-case basis with a competent optical molder. Nevertheless, the following serves as a starting point for discussion with the optical molding vendor. These tolerances should be considered state-of-the-art. As always, the tighter the tolerance demanded, the more costly a part will be.

First, a basic specification is the optical forms. The ideal plastic optic shape is a nearly uniform wall thickness. The overall part design should be as symmetrical as possible to optimize the melt flow in the mold. Strong meniscus, biconvex and biconcave shapes should be avoided. Extreme variations in part thickness can cause uneven flow characteristics. Large, thick or uneven parts may require detailed three-dimensional flow analysis to model how the part will fill in the mold. This exercise usually is undertaken early on in the design cycle.

A thinner optic will have fewer shrinkage-compensation issues and shorter cycle times, translating into less costly parts. Thicker optic cross sections not only increase cycle times, but also make it more difficult to hold a tight surface figure. Because flat surfaces have a tendency to sink as they cool in the mold, you should have a surface of power on both sides of the optic whenever possible.

Other specifications include:

• Focal length: ±0.5 to 1.0 percent.

• Radius of curvature: ±0.5 to 1.0 percent.

• Optical power (for diameters of up to about 4 in.): 2.0 to 5.0 fringes per inch.

• Irregularity (for diameters of up to about 4 in.): 1.0 to 2.0 fringes per inch.

• Scratch/dig: 40/20.

• Centration: ±1 arc min.

• Center thickness (up to about 1 in.): ±0.0005 in.

• Diameter (for diameters up to about 4 in.): ±0.001 in.

• Repeatability (lens to lens over a molding run): 0.3 to 0.5 percent.

Selecting the right vendor

Injection molding of optics is a complex interaction among the design of the part, the design of the mold tools and the processing of the molds in the press. Designers must find an optical molder who thoroughly understands the engineering issues. The optical molder should be involved in the process as early on as possible.

It is in the designer’s best interest to visit the vendor’s manufacturing site to check on its capabilities. It is important to realize that the parts produced will be no better than the tools in which they are molded.

However, good tooling alone does not guarantee that good parts will be molded. Complete understanding of the optical molding process is the driving factor in producing precision plastic optics. The optical molding company should have experience with a variety of optical forms and materials.

Finally, it is important that the molder have the metrology capability in-house to perform all of the necessary measurements for the components it manufactures. It is safe to say that you cannot manufacture what you cannot measure.

Meet the author

William Beich is business development manager for G-S Plastic Optics in Rochester, N.Y.

Rules of Thumb

The design of injection-molded plastic optics involves more than specifications. The following are a few guidelines:

• The desired ratio of the diameter to center thickness is 5:1 or less.

• The desired ratio of the center thickness to the edge thickness is 3:1 or less.

• For an optic without a flange, the clear aperture should be no closer to the edge than 1.5 times the edge thickness.

• All walls should be tapered for a draft angle of at least 0.25°. However, on elements with tight tolerances, it is not uncommon to see 5° to 10°.

• Gate type should be discussed with the optical molder. Many configurations are possible.

• Shrinkage varies according to the material but falls into the range of 0.1 to 0.7 percent.

• Holes and sharp corners can be a source of localized stress in the part. This is a contributing factor in stress-birefringence and can be the source of weld lines.